The drive system/mechanism consists of an individual drive drum for each wheel ... speed was fed to the controller then that caused the vehicle to brake ...
7th ESA Workshop on Advanced Space Technologies for Robotics and Automation 'ASTRA 2002' ESTEC, Noordwijk, The Netherlands, November 19 - 21, 2002
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ELASTIC LOOP MOBILITY SYSTEM : THE CONCEPT AND FUTURE PROSPECTS FOR ROVER MOBILITY ON MARS Nildeep Patel, Alex Ellery, Chris Welch, Andy Curley, Michael Van Winnendael* Astronautics and Space Systems Group, School of Engineering, Kingston University, London,U.K. * ESA-ESTEC, Noordjwik, Netherlands. ABSTRACT The Elastic Loop Mobility System (ELMS) is a novel mobility system developed as a backup mobility system for the U.S. Lunar Roving Vehicle (LRV) programme which operated on the Moon during the Apollo 15, 16 and 17 missions. The ELMS has been assessed in and under a variety of testing modes and terrain conditions and its performance has indicated that in a low gravity environment it significantly outperformed both the LRV and the two unmanned Soviet Lunokhod I and II rovers. There were subsequent studies which showed that the ELMS, if attached to an atmospheric entry lander, could sustain a free-fall landing on Mars and enable the lander to traverse around 500 kilometers over the Martian surface for a period of two years without the refurbishment of its consumables. With the renewed interest in UK-led Martian exploration, we have focussed our attention primarily on the ELMS as a potential mobility system for a Mars micro-rover, “Endurance” which is part of the Vanguard mission concept. This work is partly funded by the European Space Agency under the Aurora programme. 1. INTRODUCTION The Elastic Loop Mobility System or the ELMS is a novel mobility system invented by an English inventor, Dr. J.G.K. Kitchens about 65 years ago. The design predates the Space Age by a quarter century, and at one point might have gone to the moon. The original design as documented in the 1933 patent by Kitchens describes the system as an “Endless traveller track band”. It was proposed that using a continuous, elastic track to move the vehicles over the rocky or the loose muddy soil, will enable them to ride much comfortably over the irregular terrain. The track would curl across its width so the section between the wheels will flatten out and hold the track taut. But the problem with this design was that it used to large wheels, one at the front and the other at the back. These wheels tend to jam the rocks and other debris between the wheels and the track, so the design never progressed beyond the test models, despite the great promise of eliminating several moving parts. The whole mobility concept being very attractive in terms of eliminating majority of the moving parts and being simple to operate autonomously at the same time, encouraged Dr. Nicholas Costes, a senior research scientist at NASA’s Marshall Space Flight Center to make further efforts to make the ELMS suitable for Lunar Rovers. In this effort, along with W. Trautwein of Lockheed Missiles & Space Co. and Dr. Stein Sture of University of Colorado, he made some major changes in the system. The variation was to raise the main wheels and add an additional load wheel next to each main wheel. This turned the track into a spring, which elevated the vehicle and let the rocks and dirt fall off before they could jam the main wheels. This design also made it possible to spread the vehicle load over a larger area and thus, providing the vehicle with better traction with a much compact size as compared to the wheeled systems. As a part of the ELMS project, there were several tests conducted regarding the performance of the system on a variety of terrain with varying slopes and soil configurations at the U.S. Army Waterways Experiment Station. These tests showed that the ELMS performed better than the Lunar Rover Wheels. The vehicles equipped with ELMS were capable of climbing 35 degree slopes whereas the Lunar Rovers could negotiate 18 degree slopes only. Also the ELMS had obstacle negotiation capacity twice than that of the conventional tracked vehicles and with a good ride quality because of the spring-like damping provided by the tracks themselves.
Fig.1: Operating Principle of ELMS
2. THE CONCEPT The ELMS employs a continuous and elastic track/band wrapped around the front and the rear wheel thereby forming two 180 degrees bends for each loop. This is the major design advantage of the ELMS as these two 180 degree bends offer the damping when they roll over the uneven ground and serve the same purpose as that of the spring dampers. As seen from the figure, the idea of employing a continuous and endless track eliminates several sources of internal friction and mechanical complexity as no bogie wheels or linked tracks are required. The tight fit between the rollers and track provides all the friction needed to drive the tracks through the wheels. As evident from the figure, the contact between the drive drums and the elastic loop is restricted to very small sections in the “clean” upper part of 1
7th ESA Workshop on Advanced Space Technologies for Robotics and Automation 'ASTRA 2002' ESTEC, Noordwijk, The Netherlands, November 19 - 21, 2002
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(Courtesy: NASA- MSFC) the loop wheel which is practically free from all the dirt, dust or any other debris picked up by the loop during its contact with the ground. This allows sufficient time to allow the debris fall off the track before it reaches the drive wheel and eliminates any chance of jamming of the rocks between the tight fitting loop and the drive wheel. This was the major problem in the very first design of the ELMS as developed by Kitchens. But raising the wheels by employing a more elastic track by using the modern materials to make it stiffer without any change in the required elasticity of the loop helped to solve the problem. Now there are negligible chances of ELMS failure due to jamming of the rocks between the loop and the drive wheel. Each drive wheel is suspended from the chassis by means of a suspension arm. There is a damper between the chassis and each wheel. The suspension arm follows the loop as it extends under the applied load. The suspension arm is the connecting point for the damper. Hence it serves to retract the wheel and hence the loop after they have moved out to take up the load. In principle there is a double damping effect as the loop and damper both provide damping to ensure a smooth ride for the experimental instruments onboard the vehicle. It can be seen from the overall vehicle mobility design that a good quality ride can be obtained with a very simple mechanism with reduced number of driven wheels. The system also eliminates the need of separate wheel suspension and steering systems as employed by most wheeled systems like “Rocker-Bogie” and results in a major saving in the energy consumption. 3. DRIVE MECHANISM The drive system/mechanism consists of an individual drive drum for each wheel equipped with a brushless d.c. motor, a planetary spur gear combination with an appropriate speed reduction. The power supply to the motor is controlled by the electronic controller to prevent the overloading of the system. This drive system has very high performance rating with a combined efficiency of 70% and above which varies over a wide range of torques and speeds. 3.1 MOTORS: The motors were developed for the original single loop ELMS unit which had the loop dimensions as follows: Width: 38.8 cm Length: (circumference) 3.68 cm Weight: 148 N Each motor weighted about 11 kg and were capable of producing max. torque of 217Nm per pair, but only 38% of that torque was used by that ELMS unit. The current drain from the battery for both motors is less than 200mA. The max. current drain at max. torque of 57.6Nm (40ft-lb) at 120 rpm was approximately 30 amps. The motors operated with the gear reduction ratio of 80:1. The motor-gear combination had two torque-speed ranges and with a nominal battery voltage, it was capable of developing 115.2Nm (80ft-lb) at drum speeds up to 25 rpm and 28.8Nm (20ft-lb) at speeds up to 120 rpm. The change in torque-speed range was performed electronically by changing taps on the motor winding. As the drive system accelerated through 25 rpm, the controller automatically switched to the highspeed tap and correspondingly changed the current in the motor so that the output torque remained the same before and after the switching. The forward and reverse motions of the vehicle were actuated by means of feeding a single input command with a particular polarity to the motor through the electronic controller. A given polarity if corresponds to the forward motion of the vehicle then the opposite polarity of the input signal corresponds to the reverse motion. The braking was by proportional regenerative braking i.e. when the command for reducing the speed was fed to the controller then that caused the vehicle to brake proportional to the magnitude of the speed reduction. This can be controlled up to zero speed. A power supply which can absorb energy (battery) was used to absorb the energy of regenerative braking. 3.2 DRIVE DRUM: The drive drum consists of conical magnesium discs which are flanged also to the inner cones which are also made of magnesium and a cylinder of rolled magnesium sheet (Fig.2). At the interface of the discs and inner cones, a sprocket ring is mounted. Sprocket ring, disc and inner cone are held by the same array of cap screws and can be readily disassembled after the dust cover is removed. This also gives access to the flexural pivot (Fig.3). There are two rows of conical drive lugs mounted on the inside of the elastic loop and an array of rollers at the drive drum circumference. The rollers are mounted on the drive drum using the flexural pivots which have a limited angular movement about its axis of rotation. Fig.2: ELMS Drive Drum 3.3 PLANETARY ROLLERS: (Courtesy: NASA-WES) Array of planetary rollers is mounted on the drive drum circumference. These rollers have a limited angular rotation and are mounted in frictionless flexural pivots. This greatly minimises the internal losses of the drive train. 2
7th ESA Workshop on Advanced Space Technologies for Robotics and Automation 'ASTRA 2002' ESTEC, Noordwijk, The Netherlands, November 19 - 21, 2002
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Flexural pivots are frictionless bearings of limited angular travel, having no backlash and require no lubrication. The bearings are made of pairs of flat, crossed springs supporting the rotating sleeves. Such pivots are available in variety of sizes with various pivotal angles. Each roller is capable of taking 249N (56lb) of radial load. The possibility of reduction of internal losses by using frictionless bearings is the major advantage of the system. However, the available sizes at the time of study had very limited impact load resistance which under adverse conditions (one roller takes all impact) may be as low as 249N. Under a more likely condition, the impact load would be distributed among at least three rollers which then would increase the radial load resistance to over 500 N. In an alternative design for the drive system, conventional freely rotating rollers with friction-type radial and axial bearings were used. “Conventional” Rollers: • Advantages: (a) Lower development risk (b) High structural strength (c) Simple installation (d) Lower cost • Disadvantages: (a) Friction proportional to load (b) Lubrication required (c) Good sealing from dust required (d) Higher weight of rollers and housing “Limited Rotation” Rollers with Flexural Pivots: • Advantages: (a) No friction Fig.3: Drive Drum with Rollers (b) No lubrication required (suitable in hard vacuum) (Courtesy: NASA-WES) (c) Minimum sealing required (d) Low weight of rollers, pivots and housing • Disadvantages: (a) Higher developmental risk (b) Low safety margin against dynamic loads of uncertain magnitude (c) Higher cost 3.4 TORQUE CALCULATIONS: 3.4.1 Slope Climbing: Tmax = re W sinα η Where, re = Effective radius of the loop (~0.16m for original loop) W = Vehicle weight (=557 N) η = Estimated Efficiency (~70%) α = Max. slope angle (=35o) Tslope ≤ (0.16 x 557 x 0.574) 0.62 = 82 Nm (60 ft-lb) i.e. the maximum torque required to climb the slope of 35o is 82 Nm. 3.4.2 Step Obstacle Climbing: Tstep = µ (1 + µ) re W 1 + µ2 Where, µ = co-efficient of friction between the loop and the ground. (=0.5) Tstep ≤ 0.6 re Wmax = 64 Nm (47 ft-lb) T ≤ 82 Nm (60 ft-lb) 3
7th ESA Workshop on Advanced Space Technologies for Robotics and Automation 'ASTRA 2002' ESTEC, Noordwijk, The Netherlands, November 19 - 21, 2002
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4. CHASSIS AND SUSPENSION 4.1 CHASSIS: The main chassis structure provides the housing for the essential scientific equipment as well as for the batteries used for operation during the darker periods of operation. It also supports the loop suspension system. An ELMS chassis was originally made of aluminium in the form of welded rectangular frame (fig 3). The skin of the box was made of 0.5 mm magnesium alloy sheet. There is a provision of a curved bumper underneath the chassis in order to support the loop when it buckles temporarily under any excessive local loads. Friction in the buckled state is reduced to a minimum by a Teflon coating covering the bumper. 4.2 ELASTIC LOOP SUSPENSION: Fig.4 ELMS Chassis Structure As mentioned earlier, the ELMS has an advantage of the dual function (Courtesy: NASA-WES) of its elastic loops namely: (a) Provides support and traction over a larger ground contact area (b) Provides spring suspension using the 180-degree bends of each loop as suspension springs. The load is transferred from the chassis to the loop by the upper load wheels which are usually passive. These load wheels are mounted on the suspension arm and can pivot about the pivoting point. The loop is wrapped around the drive wheels which are suspended at the lower end of the suspension arms. When the loop is stressed then it undergoes deflection in forward and the rear directions. Under the influence of the torque (resulting from the vehicle weight and the eccentricity of the pivot point), the arm and drive drum rotate outwards by an angle α. A spring is installed to increase the contact pressure between the drum and the loop. Under the effect of additional torques resulting from the reaction to the drive torques, the front suspension arm rotates outwards causing an increase in the loop-wheel contact pressure. On the other hand, the rear suspension arm rotates inwards causing a reduction in the loop-wheel contact pressure. This results in a very desirable “nose-down” rotation of the chassis thus eliminating some of the “nose-up” pitching due to the load shifts and digging-in of the tracts delivering high torques on soft soil. 4.3 SUSPENSION ARMS: The suspension arms of the ELMS were made of 3.2 mm magnesium alloy sheet with a honeycomb core closed section. The honeycomb core(6.35mm thick) with 0.5mm magnesium alloy panels resulted in 300% increase in torsional stiffness and a 15% weight reduction over the previous open section design. This design was safe even when ELMS unit was under high side loads of about 650N. 4.4 DAMPERS: The dampers/shock absorbers were required to dampen the loop oscillations resulting from the pitching and dipping through the coupled suspension arm oscillations. These dampers were specially designed to operate over a wide range of oscillations with only one way damping (i.e. inward only movement of the torsional dampers) to prevent the separation of the drive wheel from the loop during the outward rotation of the suspension arms. The elastomer cap seals were used for the piston to obtain the desirable one-way damping action. The dampers employed lowviscosity oil as a damping medium. Having determined the damping requirements and damping characteristics for the ELMS unit, there are chances of developing a rotary damper which can be incorporated as an integral part of the suspension arm bearing. Such kind of dampers will not be vulnerable to contamination or damage by soil, dust or rocks. 4.5 ELASTIC LOOP: 4.5.1 MATERIAL SELECTION: Due to the higher load capability desired for the ELMS-second generation loops, very high fatigue life is required in future loops after welding and cold forming operations. Several materials with high strength-to-weight ratio were evaluated (mainly titanium alloys) having improved cold formability and higher ultimate tensile strength than the material under test (Ti-6Al-4V) in the annealed condition. Two materials were selected namely, “BETA III” (Ti11.5Mo-6.5Zr-4.6Sn) and “Transage 129” (Ti-2Al-11.5V-2Sn-11.3Zr) with the standard Ti-6Al-4V alloy for 4
7th ESA Workshop on Advanced Space Technologies for Robotics and Automation 'ASTRA 2002' ESTEC, Noordwijk, The Netherlands, November 19 - 21, 2002
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comparison which must be used in the annealed condition to avoid water quenching. A detailed description of the physical properties of the alloys can be found elsewhere. 4.5.2 PREDICITION OF SPRING CHARACTERISITICS AND STRESSES FOR ELASTIC LOOP: Assumed Nominal Load: Wo = 557 N (125 lb) Nominal load Fo to be carried by loop under the assumption that the lower half of the loop rests on the ground : Fo ~ Wo – ½* Wloop Where, Wloop = weight of assembled loop (including grousers) = 148 N (33lb) Thus, Fo = 557 - 74 = 483 N (108.5 lb) Beta III Data: Modulus of Elasticity: E=10.3 * 106 N/cm2 (15 * 106 psi) Poisson’s Ratio: v= 0.3 The bending rigidity of the loop: B= Et3 = 3840 N cm. 12(1-v2) The load deflection law is given as: F = 3.14*b*B*Kb(1)2 * {1 - Kb(0) + v( Kb(1) – Kb(0)} Kb(1) Where, Kb is the longitudinal curvature in the bent loop section, Superscript (0) denotes no load, superscript (1) denotes load F. Under no load condition : Kb(0) = 0.0236 cm-1 Thus, F = 4.69 x 105 Kb(1)2 * [ 1- 0.0236 + 0.3 ( Kb(1) – 0.0236 ) ] N Kb(1) 5 (1) (1) = 4.69 x 10 Kb [ (1-0.3) Kb – (1-0.3) 0.0236) ] N F = 4.69 x 105 Kb(1) [ 0.7 Kb(1) – 0.0166 ] N Maximum stress, σmax, can be predicted using: σmax = 6*B [ Kb(1) – Kb© - Kq© ] v2 where, superscript © denotes “loop in circular form” and Kb© = 0.0171 cm –1, Therefore,
Kq© = 0.0002 cm -1
σmax = 8.7 x 105 [ Kb(1) – 0.0173 ] N/cm2
The study indicates that the new loop can well carry loads between 270N and 670 N (60 and 150 lb). The maximum stresses under nominal load (557 N = 125 lb) of 47 kN/cm2 (68,000 psi) would be far too high for long fatigue life of Ti-6Al-4V loops and is even 17% too high if the new Transage alloy were used which has 58,000 psi safe stress levels for infinite fatigue life (106 cycles to fatigue of notched specimen). By an empirical formula for the available spring deflection, “α” of a loop : α = 2/Kb – Dd + 5 cm. Where, Dd = Drive drum diameter, the loop curvature in Fig. 2-6 can be related to vertical spring deflection as shown by the additional scale. 5. PERFORMANCE EVALUATION: The following tests were conducted on the ELMS model by the LMSC for determining the following operating parameters [2]: (i) The performance of an ELMS unit on level and inclined surfaces of fin-grained granular soil. (ii) The maximum slope climbing capability on such soil. (iii) The ability to negotiate rigid, step obstacles and simulated crevasses. 5
7th ESA Workshop on Advanced Space Technologies for Robotics and Automation 'ASTRA 2002' ESTEC, Noordwijk, The Netherlands, November 19 - 21, 2002
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During the first test, the ELMS was tested for the maximum design load of 690 N. The drum speed was varied between 0.5 m/s to 2.5 m/s. The slope angles developed by lifting up the soil bin at one end by a crane were found to lie between 0 to 35 degrees. These tests were carried on the well-graded crushed basalt, having angular grains in the silt to fin-sand size range. The Lunar Soil Simulant (LSS) was prepared [2] at two compositions: one in which the soil was air-dry and place loosely to simulate sandy surface with high compressibility and low strength characteristics. The other composition was with moist and compacted soil to simulate the hard rocky surface with high strength and penetration resistance. 5.1 On Level Ground: The plots show the Pull Coefficient (PC)-vs-slip and Torque Coefficient (TC)-vs-slip plots for the constant slip tests done using the dense LSS mode. It can be observed from the plots that the mobility performance of the ELMS was not affected by either the change in load or by the change in the vehicle speed from 0.5 m/s to 1.6 m/s.The maximum values of torque and pull developed by the ELMS occur at the 20% slip and for the higher values of slip, the pull coefficient (PC) and the Torque Coefficient (TC) remain constant under all the conditions whether they be on loose, air-dry or compact LSS. The energy requirements of the ELMS for a given level of performance on the level surfaces are shown in the fig. 6 (a),(b),(c). The pull coefficient (PC) and the Power Number (PN) are considered to represent the specific energy output to the system and the specific energy input to the system, both normalised with respect to the applied normal load and the distance travelled by the ELMS unit. From Fig. 6(a),(b),(c), it can be seen that if all other factors remain constant then the performance of the ELMS unit improved while operating on the compacted, moist soil. The relative fall in performance while operating on the loose, air-dry soil is marginal but tends to increase with the increasing values of the pull coefficient. Thus, it can be deduced that the difference in ELMS performance caused by the change in soil composition becomes more prominent at higher slope angles. S lo p e A n g le o n C o m p a c t S o il 40 35
S lo p e A n g le o n M o is t S o il
25 20 15 10 5 0 0 .2 0
0 .5 0 .3
0 .5 0 .4
0 .6 0 .4 5
0 .7 5 0 .5
0 .8 0 .5 5
0 .9 0 .6
0 .9 5 0 .7
1 .1 0 .6 5
1 .4 0 .7 5
P o w e r N u m b e r & P u ll C o e ffic ie n t
40 35 30 25 20 15 10 5 0 0 .2 5 0 .2
0 .4 5 0 .3
0 .5 0 .4
0 .7 0 .5
0 .9 0 .6
1 0 .7
1 .4 0 .7 5
P o w e r N u m b e r & P u ll C o e ffic ie n t
S lo p e A n g l e o n L o o s e S o il
Slope Angle
Slope Angle
Slope Angle
30
35 30 25 20 15 10 5 0 0 .4 0 .2 5
0 .5 0 .4
0 .7 5 0 .5
0 .9 0 .6
1 .2 0 .7
P o w e r N u m b e r & P u ll C o e f f ic ie n t
Fig. 6 (a), (b), (c): Plots for PC, PN v/s Slope Angle for Compact soil, Loose Soil and Moist Soil. Thus, it can be concluded from the above plots that the maximum slope angle that can be negotiated by a single ELMS unit without excessive power requirements is within the range of 31 degrees to 34 degrees for the loose, airdry LSS and between 36 degrees to 38 degrees for moist, compacted LSS. 5.2 On Sloped Surfaces: The same ELMS unit was also tested on the sloped surfaces with the soil placed in the same loose and compact compositions as in the tests on the level surfaces. At each slope, the ELMS was subjected to a series of controlledpull tests ranging from a self-propelled condition to the complete immobilisation of the unit. During each test the applied pull was measured for the calculation purposes. The performance of the ELMS unit on the sloped surfaces is as shown in Fig. 7. It can thus be inferred from the plots that the maximum slope angle that can be negotiated by the ELMS unit is 35 degrees and the corresponding maximum value of the Pull coefficient developed is 0.7 corresponding to the equivalent maximum slope angle of 38 degrees. However, for the Power Coefficient (PC) 6
7th ESA Workshop on Advanced Space Technologies for Robotics and Automation 'ASTRA 2002' ESTEC, Noordwijk, The Netherlands, November 19 - 21, 2002
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values greater than 0.4 to 0.5, there is an apparent increase in the specific energy consumption of the ELMS when operated on the slopes, which becomes more prominent at the increasing values of the Pull Coefficient.
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7th ESA Workshop on Advanced Space Technologies for Robotics and Automation 'ASTRA 2002' ESTEC, Noordwijk, The Netherlands, November 19 - 21, 2002
P e r fo r m a n c e o n S lo p e d S u r f a c e
40 35
Slope Angle
30 25 20 15 10 5 0 0 .3 0 .1
0 .4 0 .2 5
0 .6 0 .4 5
1 0 .7
1 .3 0 .7 5
P o w e r N u m b e r & P u ll C o e ffic ie n t
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5.3 Obstacle Negotiation Tests: The same ELMS unit was also tested for the obstacle or the step that can be negotiated during the traverse on the irregular surface. The results indicate that the obstacle negotiation capability of the ELMS exceeds any other wheeled concept with the wheels of the diameter same as the height of the ELMS loop from the ground. The tests on multiple ELMS units showed even higher obstacle negotiation capability. The results for the single ELMS unit are as tabulated in Table 1.
Fig.7: PC, PN v/s Slope Angle on Slopes Mobility System
Diameter/ Height (cm) 13
Max. Step Obstacle (cm) 20
Max. Crevasse (cm) >40
Any Wheeled System ELMS 13 40 >100 Table 1: Comparison data for obstacle negotiation capability for Rocker-Bogie and ELMS 6. A THIRD GENERATION ELMS MICRO-ROVER – “ENDURANCE” 6.1 “ENDURANCE” SPECIFICATIONS: Dim: 60 x 40 x 30 (cm) Wheel Dim: 6 x 10 x 1.6 (cm) Loop Weight: 1 kg/loop (with grousers) Material: Nitinol Max. weight of vehicle: 12 kg Max. slope climbing cap.: 35o (ELMS) Efficiency: 70% Effective radius of the loop: 0.04 m 6.2 TORQUE REQUIREMENTS: The maximum torque is restricted to 4 N-m (34 in-lb).
Fig.8 ELMS Micro-Rover: Endurance
6.2.1 Slope Climbing: 6.2.2 Step Climbing: Tmax = re W sinα Tstep = µ (1 + µ) re W η 1 + µ2 = 0.04 x 12 x 9.8 x sin 35 = 0.6 x 0.04 x 12 x 9.8 0.7 = 2.8224 Nm =3.424 Nm *N.B.: Stall torque for Maxon REO 16 motors used by Sojourner Rover is 13 N-m(110 in-lb) Sojourner has max. torque of 1.6 N-m/wheel (10 in-lb). 7. DISCUSSION The ELMS presents a very attractive alternative to the wheeled locomotion for autonomous planetary rovers with an identical mass and size limitations and reasonable power consumption. ELMS is far more capable in terms of terrain negotiation than that achievable by the Rocker-Bogie or any other wheeled suspension. It presents a very compact and yet highly efficient system with all the obvious advantages of tracked systems along with low power consumption similar to the wheeled locomotion. Table 2 shows the direct comparison between two vehicles with the same structural dimensions – one has the Rocker-Bogie Suspension and the other has the ELMS locomotion system. It is evident from the that an ELMS suspension is light in weight as compared to the Rocker-bogie in spite of not taking the weight of the Rocker and the Bogie link into consideration. The weight advantage gained by the ELMS over other systems can be used to accommodate increased payload. The ELMS has a higher torque requirement but the same motors can be used as used by the Rocker-Bogie i.e. Maxon REO-16 as the operating torque of ELMS (4 Nm or 30 in-lb.) is much below the stall torque (13 Nm or 110 in-lb.) of the motor. Steering will be accomplished by 8
7th ESA Workshop on Advanced Space Technologies for Robotics and Automation 'ASTRA 2002' ESTEC, Noordwijk, The Netherlands, November 19 - 21, 2002
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“Skid” steering and this can be done very efficiently without any excessive power losses as the ELMS doesn’t have a planetary gear train for driving the tracks on the drive wheel like the conventional tracked vehicles. This greatly minimises the internal energy losses thereby elevating the efficiency of the entire drive/traction mechanism to above 70%. The obvious advantages gained in terms of larger obstacle negotiation capability (2.5 times the wheeled concept) and greater slope climbing capacity (about 1.5 times the wheeled systems), make it an attractive option as a mobility system for future planetary rovers. 8. COMPARISON BETWEEN “SOJOURNER” AND “ELMS” Subsystem
Rocker-Bogie
ELMS
Dimension 60x40x30 (cm) 60x40x30 (cm) No. of wheels 6 4 Wheel diameter 13 cm 6 cm Wheel Width 7 cm 10 cm Wheel rim thickness 1.5 cm 1.5 cm Wheel weight 1.031 kg 0.576 kg No of loops 0 2 Weight of each loop 0 1 kg Total weight of Mobility System 6.2 kg 4.3 kg Motors Maxon REO –16 Maxon REO-16 No. of motors 6 4 Gearing 2000:1 2000:1 Stall Torque 13 Nm (110 in-lb.) 13 Nm (110 in-lb.) Torque/wheel 1.2 Nm (10 in-lb.) 4 Nm (34 in-lb.) Speed 0.4 m/min 0.4 m/min Steering Rate 7 degrees/sec Skid steering Power/motor 14 v (normal operation) 14 v (normal Operation) Operating Range 100 m 1000 m Slope Climbing 21 degrees max. 38 degrees max. Obstacle Negotiation 20 cm max Twice the linear dimension of loop Payload capacity 5 kg 5-8 kg Table 2: Comparison between ELMS and Rocker-Bogie Vehicles having same Dimensions 8. REFERENCES: 1. 2. 3. 4. 5.
Costes N., Trautwein W., “Elastic Loop Mobility System, A new concept for Planetary Exploration” Presented at 8th National Off Road Mobility Symposium of the International Society for Terrain Vehicle Systems, Purdue University, October, 1997. Costes N., Melzer K., Trautwein W., “Terrain-Vehicle dynamic interaction studies of a mobility concept for planetary surface exploration”, AIAA 73-407, 14th Structural Dynamics and Materials Conference, Williamsburg,, March 1973. Lindemann R., “Quasi-Static Mobility Analysis Tool for the design of Lunar/Martian rovers and construction vehicles”, SAE Paper 901650, Sept. 1990. Melzer k., Swanson G., “Performance Evaluation of a Second Gen. ELMS”, Technical Report M-74-7, U.S. Army WES, CE, Vikksburg Patel N., Ellery A., Welch C., Curley A., “Preliminary Analysis of Mobility/Suspension Systems for a Mars Robotic Rover”, 2nd World Space Congress, IAC-02-U.2.08, Houston, TX. October, 2002.
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7th ESA Workshop on Advanced Space Technologies for Robotics and Automation 'ASTRA 2002' ESTEC, Noordwijk, The Netherlands, November 19 - 21, 2002
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