lifetime. It is suitable to use a brushless DC (BLDC) motor for this application rather than a ..... a framed BLDC motor with double ended shaft was considered.
DEVELOPMENT OF CONTROL MOMENT GYROSCOPES FOR ATTITUDE CONTROL OF SMALL SATELLITES
By VIVEK NAGABHUSHAN
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2009 1
© 2009 Vivek Nagabhushan
2
To my parents, Arundathi and Nagabhushan
3
ACKNOWLEDGMENTS This thesis was an outcome of a challenge posed by my advisor Prof. Norman Fitz-Coy when I was seeking for him to be my advisor. I would like to thank him for the opportunity. I am indebted to his motivation and guidance. I would like to thank my colleagues at the Space Systems Group for their comments, criticisms and support during the course of my work. I received a lot of support from John Hines and the nano-satellite team at NASA Ames Research Center in building the CMG prototype and testing it. I am grateful to them for their help. I finally thank my parents Arundathi and Nagabhushan, sister Namitha, and Mini for their moral support and encouragement.
4
TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................................... 4 LIST OF TABLES................................................................................................................................ 8 LIST OF FIGURES .............................................................................................................................. 9 ABSTRACT ........................................................................................................................................ 11 CHAPTER 1
INTRODUCTION....................................................................................................................... 13 Motivation.................................................................................................................................... 13 Control Moment Gyroscope ....................................................................................................... 16 Types of Control Moment Gyroscopes ...................................................................................... 17 CMG Design Specifications ....................................................................................................... 18
2
CMG DYNAMICS ..................................................................................................................... 19 Equations of Motion.................................................................................................................... 19 Nomenclature ....................................................................................................................... 19 Derivation ............................................................................................................................. 20 CMG Configurations................................................................................................................... 21 Four - CMG Pyramid Configuration .......................................................................................... 22 Coordinatized Equations of Motion and Torque Analysis ....................................................... 22
3
CONTROL MOMENT GYROSCOPE DESIGN ..................................................................... 27 Design Iterations and Hardware ................................................................................................. 27 Design Iteration 1 ........................................................................................................................ 28 Flywheel Assembly ............................................................................................................. 28 Flywheel motor............................................................................................................. 28 Flywheel........................................................................................................................ 30 Bearings ........................................................................................................................ 31 Flywheel housing ......................................................................................................... 32 Gimbal Assembly ................................................................................................................ 32 Gimbal motor................................................................................................................ 33 L-bracket and gimbal motor housing .......................................................................... 33 Bearings ........................................................................................................................ 34 Slip ring......................................................................................................................... 35 Inductive sensor ............................................................................................................ 35 Mass Budget ......................................................................................................................... 36 CMG Performance ............................................................................................................... 36 Issues with Design Iteration 1 ............................................................................................. 37
5
Design Iteration 2 ........................................................................................................................ 43 Flywheel Assembly ............................................................................................................. 43 Motor selection ............................................................................................................. 44 Flexible coupling .......................................................................................................... 44 Motor drivers ................................................................................................................ 45 Gimbal Assembly ................................................................................................................ 45 Prototype Development ....................................................................................................... 46 Mass Budget ......................................................................................................................... 47 CMG Performance ............................................................................................................... 47 Design Iteration 3 – Hybrid Design ........................................................................................... 53 4
CONCLUSIONS AND FUTURE RESEARCH ....................................................................... 54
APPENDIX A
CMG EXPLODED VIEWS ....................................................................................................... 55 CMG Exploded View – Design Iteration 1 ............................................................................... 55 CMG Exploded View – Design Iteration 2 ............................................................................... 55 CMG Exploded View – Design Iteration 3 ............................................................................... 56
B
CMG DRAWINGS ..................................................................................................................... 57 CMG Drawings – Design Iteration 1 ......................................................................................... 58 CMG Drawings – Design Iteration 2 ......................................................................................... 68
C
MOTOR SPECIFICATION SHEETS ....................................................................................... 74 CMG Flywheel Motor Specification Document ....................................................................... 74 CMG Flywheel Motor Specification Document ....................................................................... 77
D
CMG HARDWARE DATASHEETS........................................................................................ 79 Kollmorgen Flywheel/Gimbal Motor Datasheet ....................................................................... 79 Minebea Flywheel Motor Datasheet .......................................................................................... 80 Micromo Gimbal Motor Datasheet ............................................................................................ 81 Micromo Integrated Encoder Datasheet .................................................................................... 82 Jinpat Slip Ring Datasheet .......................................................................................................... 83 Inductive Sensor Datasheet ........................................................................................................ 84
E
CMG HARDWARE TEST REPORTS ..................................................................................... 85 Slip Ring Test Report.................................................................................................................. 85 Flywheel Running Test Report................................................................................................... 89 Flywheel Assembly Vacuum Test Report ................................................................................. 94
F
PROVISIONAL PATENT ......................................................................................................... 98
6
LIST OF REFERENCES ................................................................................................................... 99 BIOGRAPHICAL SKETCH ........................................................................................................... 100
7
LIST OF TABLES page
Table 2-1
SwampSat CMG parameters ................................................................................................. 23
3-1
Flywheel bearing load cycle profile ...................................................................................... 37
3-2
Gimbal bearing load profile ................................................................................................... 38
3-3
Mass budget – design iteration 1 ........................................................................................... 38
3-4
Mass budget – design iteration 2 ........................................................................................... 48
8
LIST OF FIGURES page
Figure 1-1
Application requiring attitude control of satellite – stereo imaging for 3D view of object ....................................................................................................................................... 14
1-2
Layout of 3U satellite with ½ U CMG based ACS .............................................................. 15
1-3
Probable realization of 3U satellite with ½ U CMG based ACS ........................................ 15
1-4
Illustration of a CMG ............................................................................................................. 17
2-1
Pyramidal CMG ..................................................................................................................... 24
2-2
Spherical angular momentum (normalized) envelope of the pyramidal CMG .................. 25
2-3
CMG coordinate frames......................................................................................................... 25
2-4
3D torque span for SwampSat pyramidal CMG .................................................................. 26
3-1
CMG design iterations – assembled views ........................................................................... 39
3-2
Exploded view of flywheel assembly (iteration 1) .............................................................. 39
3-3
Housed and frameless BLDC motors .................................................................................. 40
3-4
Motor rotor integrated with flywheel .................................................................................... 40
3-5
Flywheel (back EMF based) and Gimbal (hall effect sensor based) motor controllers; (www.atmel.com ) .................................................................................................................. 40
3-6
Sectional view of flywheel housing with gimbal motor rotor (iteration 1) ........................ 41
3-7
Exploded view of gimbal assembly (iteration 1) ................................................................. 41
3-8
Sectional view of L-bracket and gimbal motor housing (iteration 1) ................................. 42
3-9
Slip ring................................................................................................................................... 42
3-10
Inductive sensor ...................................................................................................................... 42
3-11
Exploded view of flywheel assembly (iteration 2) .............................................................. 48
3-12
Sectional view of flywheel and flex coupling assembly...................................................... 49
3-13
Flywheel motor driver board assembly and realized driver board ...................................... 49
3-14
Exploded view of gimbal assembly (iteration 2) ................................................................. 50
9
3-15
Prototype gimbal and flywheel assemblies .......................................................................... 51
3-16
Exploded view of the CMG prototype .................................................................................. 51
3-17
CMG prototype – assembled views ...................................................................................... 52
B-1
CMG assembly ....................................................................................................................... 58
B-2
Flywheel housing (1/3) ......................................................................................................... 59
B-3
Flywheel housing (2/3) .......................................................................................................... 60
B-4
Flywheel housing (3/3) .......................................................................................................... 61
B-5
Flywheel and endpiece (1/2).................................................................................................. 62
B-6.
Flywheel and endpiece (2/2).................................................................................................. 63
B-7
Flywheel bearing spacer ........................................................................................................ 64
B-8
Gimbal assembly .................................................................................................................... 65
B-9
L-Bracket ................................................................................................................................ 66
B-10
Gimbal Housing ..................................................................................................................... 67
B-11
CMG assembly ....................................................................................................................... 68
B-12
Coupling.................................................................................................................................. 69
B-13
Flywheel.................................................................................................................................. 70
B-14
Flywheel housing ................................................................................................................... 71
B-15
Gimbal housing ...................................................................................................................... 72
B-16
L-bracket ................................................................................................................................. 73
10
Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science DEVELOPMENT OF CONTROL MOMENT GYROSCOPES FOR ATTITUDE CONTROL OF SMALL SATELLITES By Vivek Nagabhushan August 2009 Chair: Norman Fitz-Coy Major: Aerospace Engineering Small satellites are becoming increasingly popular due to their low cost of development and shorter realization time. The cost of putting these satellites into orbit is also cheaper as they can be launched as secondary payloads or multiple satellites can be launched from the same launch vehicle. As a result, there has been a lot of effort to push satellite technology to smaller sizes and mass. This would enable small satellites to accomplish missions to complement the larger satellites. Examples of such missions include imaging, remote sensing, surveillance, disaster management and blue force tracking. These missions are achieved by payloads which demand pointing capabilities from the satellites. This requires an attitude control system (ACS) with small actuators that can fit into the volume and mass constraints of small satellites. The work presented in this thesis describes the development of a control moment gyroscope (CMG) – an actuator that would enable three-axis attitude control of small satellites whose mass is about 10Kg. The actuator was developed to serve as a part of the ACS of the SwampSat, a pico-satellite being developed at the University of Florida and is designed to occupy a small volume of 100 ´ 100 ´ 100mm 3 and has a mass less than 500grams. The dynamics of the CMG are developed and the magnitude of torque that can be produced by CMG is determined. The work includes the various iterations of the mechanical design of the actuator 11
and the description of the involved hardware. The work also exhibits a prototype of one of the iterations and performance tests of the prototype. The CMG designs explained in the content of this thesis are protected under a provisional patent (Appendix F).
12
CHAPTER 1 INTRODUCTION Motivation Small satellites are becoming increasingly popular due to their low cost of development and shorter realization time. This will make the access to space more responsive. The cost of putting these satellites into orbit is also cheaper as they can be launched as secondary payloads or multiple satellites can be launched from the same launch vehicle. As a result, there has been a lot of effort to push satellite technology to smaller sizes and mass which would enable small satellites to accomplish missions to complement the larger satellites. Examples of such missions include imaging, remote sensing, surveillance, disaster management and Blue Force Tracking. These missions are achieved by payloads which demand pointing capabilities from the satellite (e.g. pointing a camera towards a particular point on the Earth; direct an antenna towards a ground station on the Earth). A representation of one such application in which it is required to observe the same point on the Earth at different positions along the orbit (stereo imaging) is shown in Figure 1-1. To accomplish these missions the satellites need a 3-axis attitude control system to control the orientation of the satellite. The two major components of the attitude control system are the actuator and the control algorithm. Various types of actuators include the reaction wheel, magnetic rods, torque coils, thrusters, momentum wheels and control moment gyroscope. The focus of this thesis is the design of one such actuator - the control moment gyroscope suitable for missions on pico and nano-satellites. A trade study between various actuators based on their performance and feasibility for use on small satellites has been done in [1]. The reference clearly identifies the CMG as being a better actuator amongst others for a small satellite mission requiring 3-axis attitude control because of their high torque to power ratio and lower mass per unit torque. The
13
CMG has been used on large spacecrafts and even on International Space Station; but there are currently no CMGs developed that can be used in a pico or a nano-satellite. The work presented in this thesis concentrates on the development of a miniature CMG which, along with the ACS electronics can be accommodated in a volume of 100 ´ 100 ´ 50mm 3 and has a total mass less than 500g. The CMG is being developed as a part of the SwampSat mission, a University of Florida pico-satellite for on orbit validation of rapid retargeting and precision pointing. The ACS can now be packaged in half the volume, ½ U of SwampSat (a pico-satellite of volume
100 ´ 100 ´ 100mm 3 and mass less than 1Kg). The rest of the volume of the satellite can be used to package the electronics including the electrical power system (EPS), communication system (COMMS), command and data handling system (CDH) and the attitude determination system (ADS). In a 3U configuration of the CubeSat, the remaining 2Us can be used to package the payload. An illustration of this concept and one such realization are shown in Figure 1-1 and Figure 1-2.
Figure 1-1. Application requiring attitude control of satellite – stereo imaging for 3D view of object
14
1U
½U
½U
Payload
2U 300mm
Figure 1-2. Layout of 3U satellite with ½ U CMG based ACS
Figure 1-3. Probable realization of 3U satellite with ½ U CMG based ACS
15
100mm
Payload
ACS
Control Moment Gyroscope The control moment gyroscope is a mechanism that produces torque by a combination of two motions – spinning a flywheel about an axis referred to as the flywheel axis and the rotation of the spinning flywheel about an axis perpendicular to flywheel axis referred to as the gimbal axis. The two main components of a gyroscope are the flywheel and the gimbal. The flywheel is a spinning rotor with inertia sufficient to provide the desired angular momentum; the gimbal is a pivot about which the flywheel assembly can be rotated. The magnitude of torque produced is directly proportional to the inertia of the flywheel, the angular speed of the flywheel and the rate of rotation of the gimbal. In a control moment gyroscope the inertia of the flywheel and the speed of the flywheel is constant and the torque output is controlled by changing the rotation rate of the gimbal. The direction of the torque produced is perpendicular to both the flywheel and the gimbal axes per the right hand rule. This torque acts on the satellite structure to change its attitude. A combination of gyroscopes is used to produce a net torque in the desired direction and magnitude. There are various combinations of gyroscopes that can be used depending upon the mission requirements (box configuration, inline configuration, roof top configuration, pyramidal configuration). The ACS on the SwampSat uses the pyramidal configuration for 3-axis control which produces a near spherical momentum envelope described in section Chapter 2. Apart from the gyroscopic torque produced by the CMG, there are other torques that arise from the motion of the flywheel and gimbal that contribute to the dynamics of the satellite. •
Reaction torque due to friction in the flywheel bearings
•
Reaction torque due to the acceleration of the gimbal; this torque depends on the angular acceleration and the inertia of the gimbal.
•
Reaction torque due to the friction of the gimbal bearings and slip ring
The effect of these torques on the satellite dynamics are explained in section Chapter 2.
16
The motion to the flywheel and gimbal is provided by flywheel and gimbal motors. There are feedback devices (ex. encoders, hall effect sensors) for sensing the angular speed and position. A slip ring is provided for continuous power supply to the flywheel motor for endless rotation of the gimbal. All these hardware are assembled together with structural components.
Figure 1-4. Illustration of a CMG Types of Control Moment Gyroscopes The control moment gyroscope shown in Figure 1-4 is in its basic form and called the single gimbal control moment gyroscope. The torque output of this CMG is in a unique direction for every orientation of the gimbal and flywheel axis. The torque span of this type of CMG lies in a plane (for 360° rotation of the gimbal axis). The second type of CMG is the double gimbal control moment gyroscope (DGCMG). In this type there are two gimbals about which the flywheel assembly can rotate. The output torque direction of this CMG is determined by the angular positions of both the gimbals and since these gimbals are in two different orthogonal planes, the torque output is in 3D space and not confined to a plane as in a SGCMG. One of the drawbacks of this type is the phenomenon of gimbal lock which occurs when the flywheel and gimbal axes align. In this situation the CMG cannot produce any torque. The mechanical construction of the DGCMG is more complex.
17
The other type is the variable speed control moment gyroscope (VSCMG). This CMG controls the acceleration of the flywheel to produce torque in addition to the gyroscopic torque produced by gimbal movement. The output torque direction of this CMG is determined by the acceleration of the flywheel and the orientation of the gimbal. The torque span hence lies in 3D space. Two different control algorithms – one for the flywheel and the other for the gimbal needs to integrated for the functioning of the VSCMG. The SGCMG is popular and widely used for its simplicity in mechanical construction and relatively simpler control logic. The control moment gyroscope discussed in this thesis is the single gimbal control moment gyroscope and shall be referred to simply as CMG instead of SGCMG for brevity. CMG Design Specifications The CMG is designed based on specifications that will enable its use as an attitude control actuator for rapid retargeting and precision pointing of pico-satellites (typically in low earth orbits). The specifications seen below are manifestations of this primary goal. The detailed design follows in Chapter 3. a. b. c. d. e. f. g. h.
Mass of the CMG actuator assembly with electronics shall be less than 500g The volume occupied by the assembly shall be less than– 0.5U (100mm ´ 100mm ´ 50mm) The total power consumption shall be less than 3 W The ACS shall achieve a pointing accuracy within 0.1° of ADS measurement The ACS shall enable a slew rate of 2-3 deg/s for the 1U satellite The CMGs shall produce a maximum torque of – 0.75Nmm The hardware shall conform to the environmental specification as delineated in the NASA GEVS document The design shall make maximum use of commercial off the shelf (COTS) hardware
18
CHAPTER 2 CMG DYNAMICS The working principle and torque generation property of the CMG was schematically explained in Chapter 1. The current chapter discusses the development of the governing differential equation of the CMG. The torque span of a single CMG lies in a plane; to be able to control attitude about all three axes, multiple CMGs in appropriate configurations are required to produce torque in 3D space. Different such configurations of CMGs which produce different torque spans are discussed briefly and the pyramidal configuration which produces a near spherical torque span is considered for development of equations and simulations. The pyramidal configuration is used in the SwampSat as its inertia is approximately the same about the principal axes and requires similar magnitude of torque in all three directions. Results of an analysis for estimating the torque of the CMG used in SwampSat is also presented. Equations of Motion The CMG produces torque by redistribution of angular momentum; it is a device that stores angular momentum in its flywheels and produces a torque by changing the direction of the flywheel axis or the angular momentum vector. The equation of motion that governs this characteristic is developed below. Nomenclature
H C - Total angular momentum of the CMG about the satellite center of mass (cm) f
H C - Angular momentum of the flywheel about the satellite cm g
H C - Angular momentum of the gimbal about the satellite cm f
I - Inertia of the flywheel g
I - Inertia of the gimbal
19
ω f - Angular velocity of the flywheel ω f - Angular acceleration of the flywheel δ - Angular velocity of the gimbal δ - Angular acceleration of the gimbal
τ d - Total dynamic torque produced by the CMG
τ gy - Total gyroscopic torque produced by the CMG τ fa - Torque due to flywheel acceleration τ ga - Torque due to gimbal acceleration τ ff - Torque due to flywheel bearing friction τ gf - Torque due to gimbal bearing friction
τ gf - Torque due to slip ring friction Derivation The total angular momentum of the CMG,
H= HG + HG G f
g
(2-1)
f f g = H G I ω + I δ
(2-2)
From Eulers law, the rate of change of angular momentum is equal to the torque acting on the system.
(
)
(
( )
d d f f d g f f g I ω + I δ + δ × I ω + I δ = (HG ) dt dt dt
(
)
( )
(
d d f f d g f f I ω + I δ + δ × I ω = (HG ) dt dt dt
20
)
) (2-3)
d ( H= G) dt
(
)
f f g f I ω + I δ + δ × I = ωf τd Flywheel Gimbal Acceleration
Acceleration
(2-4)
Gyroscopic (control)
Equation 2-4 is the governing equation for the dynamic torque produced by the CMG. Torque due to flywheel and gimbal accelerations are not used for control and are unwanted consequences which occur during start and stop of flywheel and gimbal motion; it is ideal to have the torques due to flywheel and gimbal accelerations to be zero. These torques are very small compared to the gyroscopic torque in a CMG for large satellites and do not have a considerable effect on the satellite attitude. This is because the torques are very small to affect the attitude of satellite with large inertia. But in a CMG for a small satellite, the torques due to flywheel and gimbal accelerations are considerable and cannot be neglected. Apart from the torques mentioned above there are frictional torques from the flywheel and gimbal bearings, and slip ring which, in a small satellite are of a considerable magnitude to cause disturbance to the attitude of the satellite. The frictional torques along with the torques due to the flywheel and gimbal accelerations are considered as uncontrolled disturbance torques affecting the attitude of the satellite and are considerable in a CMG for a pico-satellite. This poses a challenge to the control system and is complicated. The total torque acting on the satellite due to the CMG is the sum of the gyroscopic and disturbance torques. gy fa ga ff gf sf τ = τ + τ +τ + + τ + τ τ Control torque
(2-5)
Disturbance torques
CMG Configurations A combination of multiple CMGs in different configurations can be used to shape the torque span in 3D space. Various such configurations have been developed and used in many
21
applications; some of the configurations include the roof top configuration, box configuration, pyramid configuration and the inline configuration. The pyramid configuration with four CMGs is used on the SwampSat mission and is discussed in the following section in detail. Four - CMG Pyramid Configuration The geometry of the pyramid configuration (gimbal inclination, φ and angular spacing of 90° between CMGs) is based on achieving a near spherical torque envelope [3]. The spherical torque envelope gives uniform control authority in 3D space. The schematic of the pyramidal configuration geometry and the picture of the CMG pyramid used in SwampSat are shown in Figure 2-1. The angular momentum envelope due to the pyramidal CMG configuration [1] is shown in Figure 2-2. It shows the maximum available momentum in any direction with a combination of all four CMGs. Any attitude maneuver using the CMGs which requires more than the limit of the envelope will saturate the CMGs. Coordinatized Equations of Motion and Torque Analysis The vectorial equations of motion developed earlier in this chapter need to be appropriately represented in co-ordinate frames in order to estimate the value of the torque produced by the CMG. The equations are represented in the co-ordinate frame fixed to gimbals and then transformed into the satellite body frame. The various coordinate frames involved in the transformation are seen in Figure 2-3. The angular momentum of the CMG, H G represented in the coordinate frame ( e x , e y , e z ) attached to the flywheel axis is given by f f g H G = I ⋅ ω + I ⋅ δ = I xxf ω x
0 I zzg δ
T
Equation 2-4 represented in the coordinate frame ( E x , E y , E z ) fixed to the gimbal axis is G
τd = − ( I xxf ω xδ sin (δ ) ) E x + ( I xxf ω xδ cos (δ ) ) E y + ( I zzg δ) E z
22
(2-6)
Transforming equation 2-6 into the co-ordinate frame fixed to the satellite, the dynamic i
torque is given Equation 2-7. C1 is the transformation of the gimbal inclination and C 2 is the transformation from the i th CMG gimbal frame to the satellite frame C
τ di = C i2 ⋅ C1 ⋅ Gτ di
(2-7)
cos ϕ 0 − sin ϕ C1 = 0 1 0 sin ϕ 0 cos ϕ 1 0 0 0 −1 0 −1 0 0 0 1 0 2 3 4 −1 0 0 C2 = 0 1 0 , C 2 = 1 0 0 , C 2 = 0 −1 0 , C 2 = 0 0 1 0 0 1 0 0 1 0 0 1 1
The total dynamic torque produced by all four CMGs represented in the coordinate frame fixed to the satellite is given by C
4
τ d = ∑ Cτ d
i
(2-8)
i =1
The torque span of the SwampSat CMG (from equation 2-8) for parameters listed in Table 2-1 is shown in Figure 2-4. It shows the maximum torque available in any direction and is a plot of the torque for discrete positions of the CMG gimbals. Table 2-1. SwampSat CMG parameters Parameter
Value
Flywheel Inertia - I xxf
0.8 Kgmm 2
Gimbal Inertia - I zzg
1 Kgmm 2
Flywheel angular velocity - ω x
5000 rpm
Flywheel angular acceleration - ω x
0 rad / s 2
Gimbal angular velocity - δ
1 rad / s
Gimbal angular acceleration - δ
0 rad / s 2
Gimbal inclination - φ
40°
23
Z
Flywheel Axis
φ
Gimbal Axis
Gimbal 4 Gimbal 1
90
Y
Gimbal 3 Gimbal 2 X
(a)
CMG -1 CMG -4
CMG -2
CMG -3 (b)
Figure 2-1. Pyramidal CMG a) Pyramidal CMG geometry b) SwampSat pyramidal CMG assembly ( φ =40°)
24
Z
Y
X
Figure 2-2. Spherical angular momentum (normalized) envelope of the pyramidal CMG
s zi
ϕ
ϕ
s yi
Ez
ey ez
δ
Ey ex
S3
Ex ez
S2 s xi
ey ex
S1
Figure 2-3. CMG coordinate frames
25
Figure 2-4. 3D torque span for SwampSat pyramidal CMG
26
CHAPTER 3 CONTROL MOMENT GYROSCOPE DESIGN Design Iterations and Hardware The final design of the CMG has undergone several iterations in order to meet the mass, volume and power constraints while producing sufficient torque. Three significant iterations leading to the final design have been explained in detail in this thesis. Figure 3-1 shows the assembled views of the three design iterations arranged in the order of their development. These iterations are discussed in detail in this chapter. The CMG assembly is made up of two distinct sub-assemblies viz. the flywheel assembly and the gimbal assembly. The flywheel assembly consists of the flywheel spinning inside a housing called the flywheel housing. This sub-assembly is mounted on another sub-assembly called the gimbal assembly which rotates the flywheel assembly about the gimbal axis. The blue dotted line in Figure 3-1 represents the flywheel axis about which the flywheel rotates. The red line represents the gimbal axis about which the flywheel assembly is rotated. The CMG consists of electrical and structural components. The electrical components are commercial off the shelf (COTS) hardware. It is economical to use COTS electrical hardware and it also expedites the development process. Although there is some compromise in the design and performance, it was decided to use COTS electrical hardware for the CMG development. A fully functional CMG essentially consists of the following electrical components: 1. 2. 3. 4. 5.
Flywheel motor Flywheel angular velocity feedback sensor Slip ring Gimbal motor Feedback sensor for gimbal angular position and speed
Each of the above hardware components are explained in detail as they appear in the design iterations.
27
Design Iteration 1 The exploded view of the first iteration of the CMG is shown in Appendix A. It consists of the following components: a) b) c) d) e) f) g) h) i)
Flywheel housing Flywheel motor (Brushless DC Motor) Flywheel Flywheel bearings and snap rings L-bracket Gimbal motor housing Gimbal motor (Brushless DC Motor) Inductive sensor Slip ring The first four components in the list form the flywheel assembly and the rest form the
gimbal assembly. Each of these components is described in the following sections. Flywheel Assembly The function of the flywheel assembly is to accommodate a spinning flywheel and its motor that provide the required angular momentum to the CMG. It also consists of an interface to the gimbal assembly. The exploded view of the flywheel assembly is shown in Figure 3-2. It consists of the flywheel with the motor rotor (magnet) mounted on a pair of bearings housed inside an aluminum cage called the flywheel housing. The flywheel is driven by a brushless DC motor whose stator (windings) is also located in the same housing. Snap rings are used to axially lock the assembly. The components are assembled from left to right in the order shown in the exploded view. The motor is assembled carefully without possibly stripping the insulation of the electrical wires while routing them between the motor and the housing. The assembly drawing and detailed drawings of all the structural components are shown in Appendix B. Flywheel motor The flywheel in the CMG is spun by the flywheel motor at a constant speed through its lifetime. It is suitable to use a brushless DC (BLDC) motor for this application rather than a
28
brushed DC motor as the brushes would wear out due to increased friction under vacuum conditions. It would also cause additional friction affecting the dynamics of the satellite. Commercial BLDC motors are available in two different forms – framed motors and frameless motors shown in Figure 3-3. Framed motors are in a completely packaged form with their own housing and bearings. There are several disadvantages of using a housed motor for this application–the bearings are designed to sustain the radial load of the motor shaft only and additional load due to the flywheel will lead to failure of these bearings; mounting the flywheel on to the end of the motor shaft will make it cantilevered leading to severe loading of the bearings during launch and hence it is not advisable to use these bearings as primary load carriers; also the housing of the motor is redundant as the motor along with the flywheel is assembled into the flywheel housing. The design would be simplified if the flywheel housing can itself be used as the motor housing. On the contrary frameless motors are supplied with the rotor and the stator as two separate entities. The stator is a coil winding and the rotor is a radial array of permanent magnets. The rotor attaches directly to the flywheel and gives the designer the freedom to select appropriate bearings, design an integrated flywheel (with the rotor) and save on the additional mass of the motor housing. The above argument justifies the selection of a frameless BLDC motor for spinning the flywheel. The RBE 00410 motor from Kollmorgen which meets all the specifications of the desired motor (refer motor specification sheet in Appendix C) was selected as it is the smallest frameless BLDC motor available in the market that meets the motor requirement. The datasheet of the motor can be seen in Appendix D. The motor was tested in the laboratory and the motor consumed about 1W of power which was four times the expected power of 0.25W. Upon consultation with the motor manufacturer it
29
was understood that this issue can be resolved by customizing the motor windings to our requirement. The BLDC motor can be driven by a microcontroller in two different ways – one by using the hall effect sensor feedback to determine the position of the rotor and the second by using back EMF (electromotive force) generated by the coils as feedback to control the speed of the motor. An illustration of these methods of control is shown in Figure 3-5. The second method requires just three electrical connections to run the motor as against eight required by the first. The number of electrical connections to the flywheel motor must be limited as all these connections must be routed through the slip ring to allow endless rotation of the flywheel assembly. The use of the back EMF feedback control method allows the use of just three electrical connections to run the motor. This is advantageous as it substantially reduces the size and mass of the slip ring. The back EMF feedback method is hence chosen to drive the flywheel motor. Flywheel The flywheel is an axisymmetric rotor which is designed to have maximum inertia about its axis of rotation within its mass and volume constraints; it is the momentum storage device of the CMG. As it was seen in Chapter 2, the flywheel inertia directly affects the angular momentum capacity of the flywheel which in turn determines the toque that the CMG can produce. The flywheel is designed to maximize the capacity of angular momentum storage while considering effects of size, mass and vibrations. The flywheel is made of stainless steel to increase the inertia of the flywheel with justifiable tradeoff in the increase of mass. The permanent magnet rotor of the motor is press fit on to the shaft of the flywheel and locked in place by the flywheel end piece which is also press fit on to the shaft. These three components together form the integrated flywheel and rotor assembly. This assembly is balanced on a 30
precision balancing machine (used to balance computer hard disk drives) to minimize vibrations. The vibrations caused due to the imbalance of the flywheel will affect the attitude of the satellite known as attitude jitter [2]. The flywheel and the end piece have machined surfaces for mounting bearings. The integrated flywheel and rotor in assembled form is shown in Figure 3-4. The flywheel assembly is designed to spin between 6000 and 8000rpm. Bearings The bearings in the flywheel assembly support the integrated flywheel and rotor. The bearings should be able to rotate at a continuous speed of about 8000rpm through the lifetime of the satellite. Hybrid bearings with silicon nitride balls (ceramic) and steel races are chosen for this application. A hybrid ceramic bearing is a combination of ceramic rolling elements with steel bearing races. The ceramic balls provide a chemically inert surface at the ball-race contact. The use of ceramic balls in steel raceways has shown to be beneficial in marginal lubrication conditions [4]. The CMG in the SwampSat is not isolated from the rest of the satellite components; the bearings in the CMG are expected to run with marginal or no lubrication to prevent outgassing and contamination (due to debris from lubricant) of electronic equipment in the satellite. The hybrid bearings are chosen as they can operate without lubrication for a longer time and have lower coefficient of thermal expansion and lower coefficient of friction compared to steel bearings. There are two bearings in the flywheel assembly, one on each side of the flywheel. This placement ensures equal distribution of launch loads on the two bearings. A typical load cycle for a flywheel bearing is shown in Table 3-1. The radial and axial clearances are designed considering the thermal expansion effects and are kept to a minimum to avoid axial and radial movements of the flywheel that can affect the satellite dynamics. The bearings chosen are SKF hybrid bearings 618/6-H and 61802-H of ABEC 5P precision degree. The 90% reliability bearing life for a specific load and speed is calculated using the formula, 31
and L10 h L10 = ( C / P ) in millions of revolutions= 3
(10
6
/ 60n ) × L10 in number of hours. The
L10h life for the bearing for each load case is also tabulated in Table 3-1. The overall life of the bearing under the variable load conditions listed in the Table 3-1 is calculated using the formula L10 h =
t1
1 . The calculation shows that the bearings would sustain in tn L10 h1 + L10 h 2 + ..... + L10 hn t2
excess to 1million hours of revolution. Flywheel housing The flywheel housing is an aluminum structure that is designed to house all the components of the flywheel assembly and to serve as an interface between the flywheel assembly and the gimbal assembly. Certain sections of the housing have milled pockets and lightening holes to reduce the mass of the structure. The housing has machined surfaces for mounting bearings and motor windings. Grooves are machined for snap rings. It also has an interface for the slip ring rotor and routing ports for electrical connections through the slip ring. A steel shaft is screwed to the bottom of the housing using a thread locking agent. This shaft is required to be of steel to avoid differential expansion between shaft and the motor rotor which is also made of steel. The rotor of the gimbal motor, similar to the flywheel motor is press fit on this steel shaft. The entire assembly of the aluminum housing and the steel shaft is machined in a single setup to achieve concentricity around the gimbal axis. A sectional view of the flywheel housing is shown in Figure 3-6. The flywheel housing along with the gimbal motor rotor is referred to as the gimbal. Gimbal Assembly The exploded view of the gimbal assembly is shown in Figure 3-7. The main function of the gimbal assembly is to facilitate the rotation of the entire flywheel assembly about the gimbal
32
axis which is perpendicular to the flywheel axis. It consists of two structural components – Lbracket and the gimbal motor housing. Apart from providing a pivot for gimbaled movement of the flywheel housing, the structural components also have interfaces for mounting of the slip ring brushes, gimbal bearings, gimbal motor and the inductive sensor. The detailed drawings of all the components of the gimbal assembly are in Appendix B. The design and purpose of each component in the gimbal assembly is explained below. Gimbal motor The gimbal motor is similar to the flywheel motor in all aspects except its control method and its operational speed. The speed and position of the gimbal which determines the torque output of the CMG is determined by the speed and position of the gimbal motor rotor. The gimbal motor speed varies between 0 and 2 rad/s and the angular position of the gimbal is required to be known at all times. This demands precision feedback control and is achieved by using the Hall Effect sensors which are integrated with the motor stator. The gimbal motor hence is controlled using a technique utilizing the feedback from the hall sensors. The motor has eight electrical connections including the feedback lines from the hall sensors. These connections are directly connected to the motor control board of the CMG. The calculation of the peak torque requirement of the gimbal motor can be seen in the gimbal motor specification document in the Appendix C. The datasheet of the gimbal motor which is the same as the flywheel motor is in Appendix D. L-bracket and gimbal motor housing The L-bracket and the gimbal motor housing are the two structural components of the gimbal assembly that together support the pivoting of the flywheel assembly. They have machined surfaces to mount gimbal bearings and the axis passing through these bearings forms the gimbal axis about which the flywheel housing rotates. The L-bracket and the gimbal housing 33
are bolted together and doweled in position. The L-bracket has two threaded holes on its top face on to which slip ring brush is mounted using screws. The gimbal motor is assembled on to the gimbal motor housing and locked axially by a snap ring. The gimbal motor housing also has provision for mounting an inductive sensor which is required for initializing the angular position of the gimbal. This is required as the gimbal is free to move during launch and will not start from the same angular position it was assembled in; the hall effect sensor also do not function unless they have power supply which is cutoff during launch. Thus the inductive sensor is used to provide information about the initial angular position of the gimbal. The L-bracket also has interfaces for assembly of other CMGs and for mounting on to the satellite structure. The Lbracket and the gimbal housing are made of aluminum and optimized structurally for reducing mass. The structural components are anodized to prevent galvanic corrosion at the bearing interface. A sectional view of the L-bracket and the gimbal motor housing is shown in Figure 38. The detailed drawing of these components is found in Appendix B. Bearings The gimbal bearings are similar to the ones used in the flywheel assembly. They are made of ceramic (silicon nitride) balls and stainless steel races. There are two identical gimbal bearings – one mounted on the L-bracket and the other on the gimbal motor housing. The axis through these bearings is the gimbal axis. The flywheel assembly is mounted on the inner race of these bearing. The bearings run at a maximum speed of about 2rad/s. The typical load cycle for a gimbal bearing is shown in Table 3-2. Two SKF 618/7-H bearings are selected for the gimbal bearings. The 90% reliability bearing life for a specific load and speed is calculated using the formula, L10 = ( C / P ) in millions of revolutions= and L10 h 3
(10
6
/ 60n ) × L10 in number of hours.
The L10h life for the bearing for each load case is also tabulated in Table 3-2. The overall life of
34
the bearing under the variable load conditions listed in the Table 3-2 is calculated using the formula L10 h =
t1
1 . The calculation shows that the bearing would sustain in tn L10 h1 + L10 h 2 + ..... + L10 hn t2
excess to 1million hours of revolution. Slip ring A slip ring is a device that allows continuous electrical connection between two parts which rotate relative to each other. The slip ring carries electrical signals required to run the flywheel motor, from the stationary part (w.r.t. the satellite) of gimbal assembly to the rotating flywheel assembly. This allows the endless motion of the flywheel assembly about the gimbal axis and enables continuous control of the CMG. The datasheet of the slip ring can be seen in Appendix D. The selected slip ring consists of three channels with gold on gold contacts for low wear characteristics. The three channels are sufficient to drive the flywheel motor through back e.m.f feedback. The rotor (rings) and stator (brushes) of the slip ring are available in separate form providing the designer with the freedom of mounting without having to worry about the additional weight of the slip ring case and bearings. The rings of the slip ring are embedded on a plastic shaft and this shaft is mounted on the interface in the flywheel housing (rotating part) while the brushes are mounted on the L-bracket which is stationary using screws. The wires from the slip ring rotor are routed through the ports on the flywheel housing to the motor. The electrical leads from the brushes are connected to the motor controller. The electrical noise characteristics of the slip ring are evaluated on a slip ring test bed, explained in the slip ring test document in Appendix E. A picture of the slip ring used in the CMG is shown in Figure 3-9. Inductive sensor The inductive sensor is used to set a reference point for the angular position of the gimbal. Knowledge of this position is required to be known precisely for feedback to the attitude control
35
system. The Hall Effect sensors in the gimbal motor provide this information only when the motor is supplied with power. The flywheel housing is mounted on bearings and is free to rotate during launch and handling operations; also the motor is not powered during these operations. Thus the initial angular position would be different from when it was assembled to when the satellite is in orbit. Using an inductive sensor to sense a predetermined high point on the surface of the flywheel housing will give us a reference to bring the gimbal to a known angular position before starting CMG operations. The inductive sensor helps in providing an initial condition and the hall sensors provide information on real time change of angular position. The selected inductive sensor has a sensing range of 0.8mm and there is a projection of 0.7mm on the external surface of the flywheel housing that can be sensed. The data sheet of this inductive sensor is seen in Appendix D. A picture of the inductive sensor is shown in Figure 3-10. Mass Budget The entire ACS unit needs to conform to the mass limit of 500g. Table 3-3 gives a breakdown of the mass of all the components in the ACS. The total estimated mass budget exceeds the specified value. A customized design for a smaller motor and more structural optimization would help mitigate this issue and is addressed in the second design iteration. CMG Performance The performance of the CMG as against the specifications listed in Chapter 1 are compared below. 1.
Mass – The overall mass of the CMG was 580g and exceeded the specification of 500g
2.
Power – The CMG motors consumed about 1W each (8W for all 4 CMGs) which exceeds the specification of 3W
3.
Volume – The CMG cluster could be accommodated in ½ U and thus meets the specification
36
4.
Torque – It was inferred from calculations based on the simulation discussed in Chapter 2 that the CMGs were capable of producing a maximum torque of 0.8Nmm and meets the specification
Issues with Design Iteration 1 The first design iteration had some characteristics that did not conform to the specifications as listed in Chapter 1. The issues and the possible mitigation plans leading to the second design iteration are discussed below. 1.
The design of the CMG in the first iteration used the Kollmorgen brushless DC motor for spinning the flywheel and for gimbal movement. An assembly for testing this motor was built and the motor was run using an off the shelf brushless DC motor controller. The test showed that the motor consumed 1.2W of power when spinning at 5000 rpm and consumed about 1W of power when stalled at no load (very slow speeds equivalent to gimbal speeds). This power was four times over the SwampSat power budget. After a negative response from the motor manufacturer to customize the windings, it was decided to look for a new motor that would best suit the profile of the SwampSat mission
2.
The gimbal motor consumed a lot of power and was also difficult to control at low speeds as the hall effect sensor resolution was not good enough.
3.
Controlling the flywheel speed and maintaining it constant was difficult using back emf control of the flywheel motor. It was necessary to either have a slip ring with more channels or a controller on the rotating part of the assembly so that the hall sensors on the motor can be used to control the speed
4.
The resolution of the hall effect sensors on the gimbal motor was not sufficient for accurate gimbal control, hence there was need to accommodate alternative sensors like encoders
5.
Mass budget exceeded the target value
Table 3-1. Flywheel bearing load cycle profile Accln.Load (g’s)
Duration
Mode
L10h (hours)
1 8000
1g
1hr
Free run test
>1Million
2 8000
1g
4hr
Thermo-Vac test
>1Million
3 8000
1g
30min
Final test
>1Million
4 0
8g
15 min
Launch
-
5 8000
0g
2-3 years
Operational
>1Million
Speed (RPM)
37
Table 3-2. Gimbal bearing load profile Accln.Load (g’s)
Duration
Mode
L10h (hours)
1 5
1g
1hr
Free run test
>1Million
2 5
1g
1hr
speed reversals
>1Million
3 2
1g
4hr
Thermo-Vac test
>1Million
4 2
1g
30min
Speed reversal
>1Million
5 0
8g
15 min
Launch
-
6 2
0g
2-3 years
Operational
>1Million
Speed (rad/s)
Table 3-3. Mass budget – design iteration 1 Item Quantity Unit mass(g) L Bracket 4 8 Gimbal Housing 4 10 Flywheel Housing 4 15 Flywheel 4 15 Bearing - 618/7-H 8 2 Bearing - 61802 4 7.4 Bearing - 618/6-H 4 2 Snapring(CFH-24) 4 0.2 Snapring(CFH-22) 4 0.2 Snapring(CFS-6) 4 0.1 Inductive sensor 4 5 Slipring 4 2 Motor driver 1 65 Motors 8 30 Total
38
Mass(g) 32 40 60 60 16 29.6 8 0.8 0.8 0.4 20 8 65 240 580.600
Figure 3-1. CMG design iterations – assembled views
Flywheel housing Motor Bearing Snap ring Flywheel
Figure 3-2. Exploded view of flywheel assembly (iteration 1)
39
Figure 3-3. Housed (left) and frameless (right) BLDC motors
Motor rotor
End piece
Flywheel
Figure 3-4. Motor rotor integrated with flywheel
Gimbal motor controller
Flywheel motor controller
Figure 3-5. Flywheel (back EMF based) and Gimbal (hall effect sensor based) motor controllers; (www.atmel.com )
40
Figure 3-6. Sectional view of flywheel housing with gimbal motor rotor (iteration 1)
Slip ring brush
L-Bracket
Bearing
Gimbal motor housing
Inductive sensor
Gimbal motor
Figure 3-7. Exploded view of gimbal assembly (iteration 1)
41
CMG pyramid interface
Bearing seat
CMG-Satellite interface
L-Bracket
Gimbal axis Bearing seat
Inductive sensor mount Gimbal motor housing
Figure 3-8. Sectional view of L-bracket and gimbal motor housing (iteration 1)
Figure 3-9. Slip ring
Figure 3-10. Inductive sensor
42
Design Iteration 2 The second iteration was designed to overcome the shortcomings of the first. The exploded view of the 2nd iteration of the CMG design is shown in Appendix. It consists of the following components: a) b) c) d) e) f) g) h) i) j) k)
Flywheel housing Flywheels (2 pieces) Flywheel motor (Double ended BLDC motor) Flywheel bearings Flexible couplings Flywheel motor controller L -bracket Gimbal motor mount plate Gimbal bearings Gimbal motor with integrated encoder Slip ring The first six components form the flywheel assembly and the rest of the components form
the gimbal assembly. The description of all these components and the changes in the design that address the issues discussed in design iteration 1 are dealt with in the remainder of this section. Flywheel Assembly The exploded view of the flywheel assembly is shown in Figure 3-11. The construction of the flywheel housing is similar to the one described in iteration 1 but with some modifications to accommodate the new motor and flywheels. The new design is smaller in size, accommodates two flywheels and has a central plate for the motor mount. The flywheel motor is face mounted inside the flywheel housing using four M1.6 screws. Identical flywheels are mounted on either sides of the motor on the inner races of the flywheel bearings. The motion from the motor shaft to the flywheel is transmitted through flexible couplings. The splined shaft of the flexible coupling is press fit on to the motor shaft. The flywheels are located on bearings and do not impart any load on the motor bearings. The flywheels and the flexible coupling are made of stainless steel. The bearings chosen have the same duty cycle and are similar to the ones in the 43
first iteration except for their size. Two SKF 61801-H silicon nitride ceramic ball bearings were used as flywheel bearings. The entire flywheel assembly along with the flywheel housing is balanced on a precision balancing machine. The motor driver board is also mounted on the flywheel housing. The motor drivers are built on two separate identically sized PCBs. These two boards are clamped on to the outer cylindrical surface of the flywheel housing through studs. Motor selection The advantage of using a frameless motor was explained in the section on design iteration 1. Since the Kollmorgen motor was the smallest frameless BLDC motor available in the market, a framed BLDC motor with double ended shaft was considered. The specification sheet for the motor is in Appendix C. This motor was selected instead of a single shaft to maximize the inertia by using two flywheels and avoid a cantilever situation. The Minebea double ended BLDC motor was selected. With a double ended shaft design, two identical flywheels supported by bearings are mounted on to both ends of the shaft via flexible couplings. This minimizes the load on the motor bearings caused by misalignment of motor bearing and flywheel axes. The motor has a flexible printed circuit (FPC) lead which mates with FPC connector on the motor driver board. The motor was tested using an off the shelf brushless DC motor driver and it consumed about 0.3W of power at no load. The datasheet of the selected motor is in Appendix D. Flexible coupling The framed BLDC motor for the flywheel has its own set of bearings but cannot support the flywheel during launch, hence the flywheels are supported on an additional set of bearings. This arrangement introduces some misalignment between the axes of the motor shaft and bearings. The misalignment could be detrimental to the motor bearings over a period of time and will also cause additional friction torque on the motor. Hence the motion from the motor has to be transmitted to the flywheels through a compliant medium like a flexible coupling. The 44
sectional view of the coupling is shown in Figure 3-12. The coupling is similar to a claw coupling but with larger spacing between the claws and also has a silicone filling in the gaps. The silicone used was the Nusil CV-1142 which has high shear strength but very low compressive strength and absorbs the effects of misalignment. The silicone is highly viscous and can be injected into the gap by a syringe with a needle orifice diameter of 1mm. Motor drivers Design iteration 1 had the motor drivers for the flywheel motor located external to the CMG and the motor had to be controlled using back EMF feedback as the slip rings had only three channels. Precise speed control without the use of hall sensors was not possible because of noisy feedback in back EMF control. Therefore the flywheel driver board was miniaturized to be mounted on the rotating gimbal assembly itself. By doing so, the motor can be driven with the feedback from the hall sensors. Two channels of slip ring provide power to the controller and the other one is used as a feedback line to measure the speed of the motor. The FPC from the motor is connected to one of the driver boards and there is a wired connection between the two boards routed around the housing. The wire routing is carefully done to avoid interference with any nonrotating parts. A picture of the driver board assembly on to the flywheel housing and the realization of the driver board is shown in Figure 3-13. Gimbal Assembly The exploded view of the gimbal assembly is shown in Figure 3-14. The gimbal assembly in this design is different the first design iteration in using a brushed DC gear motor as opposed to a frameless BLDC motor. The output shaft of the gear motor is press fit into the hollow interface shaft in the flywheel housing. The design also has a simplified gimbal motor housing as the motor has its own frame. Misalignment between the motor shaft bearings and the gimbal 45
bearings is not a concern because of the low rotational speed of the gimbal. This design also eliminates the need to use an inductive sensor as the DC motor has an integrated encoder. The gimbal bearings are the similar to the ones used in the first iteration with an exception in their size. The bearings used for the gimbal are the SKF 618/6-H silicon nitride ceramic ball bearings. The duty cycle of the bearings is as listed in Table 3-2. The slip ring assembly is also the same as in the previous iteration. Motor selection: The specification sheet defining the criteria for selection of the gimbal motor is seen in Appendix C. Since the gimbal speeds are very low, a brushed DC gear motor with an integrated encoder was considered for the gimbal motor. The motor operates in the range of 3-6V and consumes a power of 0.2-0.3W during operation at 5V. The motor has an integrated incremental encoder. The gearbox which is in line with the motor has a gear ratio of 1:33. Due to the high gear ratio the gimbal inertia cannot back drive the motor during launch. This assures that the gimbal will maintain its orientation about the gimbal axis and will not be disturbed by the launch loads. Thus we need not use and inductive sensor to determine the angular position and realign the gimbals. The data sheet of the motor and integrated encoder is in Appendix D. Prototype Development Preliminary tests of the motors confirmed their power characteristics. A prototype CMG was built for further testing and development. The pictures of the prototype are shown in Figures 3-15 through 3-17. The structural components of the CMG were made of aluminum 6061-T6 grade. The free running of the flywheel was tested after the first assembly, but the motor was not able to spin the flywheels due to the drag caused by the grease in the bearings. The flywheel assembly was disassembled; the bearings were washed free of the grease using acetone and assembled again. This rectified the problem and the flywheel performance was further tested for its current draw over a period of 90 minutes. The motor was damaged during the re-assembly 46
and hence started drawing more current (85mA) as opposed to its actual value of 35mA. The total current draw of the flywheel assembly was 101mA, which tells us that the total current drawn by the bearing friction is about 16mA. The test procedure and results for the test can be seen in Appendix E. The performance of the bearing under vacuum conditions needs to be characterized and a procedure for the test is seen in the vacuum test document in Appendix E. Mass Budget The estimated and achieved value of the mass is tabulated in Table 3-4. The total mass of the CMGs and controller is within the target value of 500g. The savings in mass as compared to the previous design were due to a smaller flywheel motor, redesigned gimbal motor housing and an optimized structure with more lightening holes. CMG Performance The performance of the CMG as against the specifications listed in Chapter 1 are compared below. 1.
Mass – The overall mass of the CMG was 437g and meets the specification of 500g
2.
Power – The CMG motors consume about less than 0.4W each (3.2W peak for all 4 CMGs) which is close to the specification of 3W
3.
Volume – The CMG cluster could be accommodated in ½ U and thus meets the specification
4.
Torque – It was inferred from the simulation discussed in Chapter 2 that the CMGs were capable of producing a maximum torque of 0.8Nmm and meets the specification The CMG design in the second iteration meets the entire design criterion. The prototype
has to be tested to quantify its performance and also qualify for operation in the space environment.
47
Table 3-4. Mass budget – design iteration 2 Estimated Item Quantity Unit mass(g) L Bracket Gimbal motor housing Flywheel housing Flywheel Bearing - 618/6-H Bearing - 61801-H Slipring Flywheel motor driver Gimbal motors Flywheel Motors TOTAL
4 4 4 4 8 8 4 8 4 4
8 5 12 15 2 7.4 2 6 26 15
Actual Unit Mass (g) 8.2 4.9 12.3 15 2 7.4 2 6.2 25.5 10.1
Estimated Mass(g) ($) 32 20 48 60 16 59.2 8 48 104 60 455.200
Actual Mass(g) ($) 32.8 19.6 49.2 60 16 59.2 8 49.6 102 40.4 436.800
Slip ring interface Flywheel motor Minibea BLDC15
Flexible coupling
Flywheel Hybrid bearing SKF 61801-H
Flywheel housing Flex connector
Figure 3-11. Exploded view of flywheel assembly (iteration 2)
48
Motor shaft
Splined sleeve Flywheel Region filled with silicone (NUSIL CV 1142)
Figure 3-12. Sectional view of flywheel and flex coupling assembly
Flywheel motor driver board (a) Flywheel motor driver board assembly
(b) Realized driver board
Figure 3-13. Flywheel motor driver board assembly and realized driver board
49
Slip ring stator Slip ring rotor
L-bracket Hybrid bearing SKF 618/6-H
Flywheel housing
Bottom plate
Micromo DC gear motor Series 2619
Figure 3-14. Exploded view of gimbal assembly (iteration 2)
50
(a) Flywheel assembly
(b) Exploded Gimbal assembly
Figure 3-15. Prototype gimbal and flywheel assemblies
Figure 3-16. Exploded view of the CMG prototype
51
Figure 3-17. CMG prototype – assembled views
52
Design Iteration 3 – Hybrid Design The CMG in design iteration 2 was able to meet the requirements of the SwampSat. However a third design iteration was considered to make the CMG capable of producing more torque for using them in larger satellites (nano-satellites). The mass and volume constraints of 500g and ½ U were still considered applicable. But the power constraint of 3W was relaxed assuming more power availability in larger satellites. The Kollmorgen motor used in design iteration 1 was considered again as power consumption, the only drawback of the motor was relaxed. The third iteration was developed as a hybrid of the first and the second design iterations. The hybrid design uses the Kollmorgen RBE 00410 frameless BLDC motor for the flywheel and the Micromo 2619 series DC gear motor for the gimbals. The exploded view of this design is shown in Appendix A. The following are the specifications of the hybrid design: 1. 2. 3. 4. 5. 6.
Mass: < 500g Volume: ½ U ( 100 ´ 100 ´ 50mm 3 ) Power: 6W Flywheel speed: 10000 rpm Maximum gimbal speed: 1 rad/s Torque: 3Nmm The torque produced by this CMG is sufficient for the ACS of a nano satellite whose mass
is less than 12 Kg and occupies a volume less than 300 ´ 200 ´ 100mm 3 . Since the volume and the mass constraints were maintained as per the previous iterations, the design can also be used for a CubeSat.
53
CHAPTER 4 CONCLUSIONS AND FUTURE RESEARCH The thesis explains the detailed development of the CMG for use in pico-and nanosatellites. The CMG design in the second iteration meets all the requirements of the SwampSat and hence a prototype of the same was built for testing. A thorough testing of the prototype will be conducted to qualify the product for space environments. This CMG will enable picosatellites to perform rapid retargeting and precision pointing maneuvers and render them capable of a plethora of missions requiring attitude control. A prototype of the third design iteration will also be built and similar tests will be conducted. An improved design using a backlash free gearbox and high precision encoders will be considered. Intergrated components and customized hardware development will improve and make the design even more compact.
54
APPENDIX A CMG EXPLODED VIEWS CMG Exploded View – Design Iteration 1 Slip ring
L-bracket Bearing (SKF 618/6-H) Bearing (SKF 61802-H) Flywheel Housing Bearing (SKF 618/7-H) Flywheel Motor (Kollmorgen)
Inductive Sensor Gimbal motor housing Gimbal motor (Kollmorgen)
CMG Exploded View – Design Iteration 2 L-bracket
Slip ring assy.
Flywheel housing Flywheel motor Hybrid bearing SKF 61801-H Flywheel Motor driver board Hybrid bearing SKF 618/7-H Gimbal motor mount plate
Flexible coupling
Gimbal motor Micromo 2619 series
55
CMG Exploded View – Design Iteration 3
Slip ring assy. L-Bracket Flywheel housing Flywheel motor Kollmorgen RBE 00410 Flexible coupling Hybrid bearing SKF 618/7-H Gimbal motor mount plate
Hybrid bearing SKF 618/6-H Hybrid bearing SKF 61802-H
Gimbal motor Micromo 2619 series
56
Flywheel
APPENDIX B CMG DRAWINGS
57
CMG Drawings – Design Iteration 1
Figure B-1. CMG assembly
58
Figure B-2. Flywheel housing (1/3)
59
Figure B-3. Flywheel housing (2/3)
60
Figure B-4. Flywheel housing (3/3)
61
Figure B-5. Flywheel and endpiece (1/2)
62
Figure B-6.. Flywheel and endpiece (2/2)
63
Figure B-7. Flywheel bearing spacer
64
Figure B-8. Gimbal assembly
65
Figure B-9. L-Bracket
66
Figure B-10. Gimbal Housing
67
CMG Drawings – Design Iteration 2
Figure B-11. CMG assembly
68
Figure B-12. Coupling
69
Figure B-13. Flywheel
70
Figure B-14. Flywheel housing
71
Figure B-15. Gimbal housing
72
Figure B-16. L-bracket
73
APPENDIX C MOTOR SPECIFICATION SHEETS CMG Flywheel Motor Specification Document
74
75
76
CMG Flywheel Motor Specification Document
77
78
APPENDIX D CMG HARDWARE DATASHEETS Kollmorgen Flywheel/Gimbal Motor Datasheet
79
Minebea Flywheel Motor Datasheet
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Micromo Gimbal Motor Datasheet
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Micromo Integrated Encoder Datasheet
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Jinpat Slip Ring Datasheet
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Inductive Sensor Datasheet
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APPENDIX E CMG HARDWARE TEST REPORTS Slip Ring Test Report
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Flywheel Running Test Report
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Flywheel Assembly Vacuum Test Report
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APPENDIX F PROVISIONAL PATENT
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LIST OF REFERENCES 1. Development of The Spacecraft Orientation Buoyancy Experimental Kiosk, Master’s Thesis, F. Leve, University of Florida, 2008 2. V. Nagabhushan, N.Fitz-Coy, Split Flywheel Design With Attitude Jitter Minimization Through Flywheel Phase Control, AIAA@Infotech, Seatlle,2009 3. Kurukowa,H., A Geometric Study of Control Moment Gyroscopes, PhD Thesis, University of Tokyo, 1998 4. Conley, Space Vehicle Mechanisms, John Wiley & Sons Inc., 1998 5. F. Leve, V. Nagabhushan, and N. Fitz-Coy, “P-n-P Attitude Control System for Responsive Space Missions,”Responsive Space Conference, April 2009. 6. G. Margulies and J. Aubrun, “Geometric Theory of Single-Gimbal Control Moment Gyro Systems,”Journal of the Astronautical Sciences, Vol. 26, No. 2, 1978, pp. 159–191. 7. Leve, F., Tatsch, A., and Fitz-Coy, N., A Scalable Control Moment Gyro Design forAttitude Control of Micro-, Nano-, and Pico-Class Satellites, Advances in the Astronautical Sciences, Vol. 128, Published for the American Astronautical Society by Univelt; 1999, 2007, p. 235. 8. V. Lappas, W.H.Steyn, and C. Underwood, “Design and Testing of a Control Moment Gyroscope Cluster for Small Satellites,” Journal of Spacecraft and Rockets, Vol.41, No.6 9. S. R. Vadali, H. S. Oh, and S. R. Walker, “Preferred Gimbal Angles for Control Moment Gyro,” Journal of Guidance, Control and Dynamics, Vol.13, No.6, 1990, pp.1090-1095 10. Twiggs, B. and Puig-Suari, J., CUBESAT Design Specifications Document, 2003. 11. FUNSAT – IV Preliminary Design Report, Space Systems Group, University of Florida, 2008
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BIOGRAPHICAL SKETCH Vivek Nagabhushan was born in 1981 in the garden city and the silicon valley of INDIA – Bangalore. He completed his early education in different schools in Karnataka and received his Bachelor of Engineering (B.E) degree in mechanical engineering with distinction from B.M.S. College of Engineering, Bangalore in 2003. He was hired on campus by Larsen and Toubro Limited as a design engineer for their weapon systems and sensors division and started working for them on graduation. He was recognized at L&T Limited for his exceptional performance. He filed for two patents at the Indian patent office during the course of stay at L&T Limited. After working at L&T Limited for about four years, he worked at Altair Engineering Inc. as a senior engineer and was involved in dynamic mechanism simulation. He worked at Altair for seven months before moving to University of Florida for his master’s degree in aerospace engineering. At University of Florida, he is a part of the Space Systems Group advised by Prof. Norma FitzCoy. His research interests include spacecraft dynamics and space robotics.
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