International Carpathian Control Conference ICCC’ 2002 MALENOVICE, CZECH REPUBLIC May 27-30, 2002
COMPUTER-ASSISTED MEASUREMENTS IN THE SERVO-DRIVES DIAGNOSTICS BASED ON THE FEEDBACK SIGNALS EVALUATION Wojciech BLACHARSKI Department of Manufacturing Engineering and Automation Technical University of Gdańsk Gdańsk, Poland,
[email protected] Abstract: The paper concerns diagnostics of servo-drives. The proposed computer-assisted way of the feedback signals measurements can be applied as a simple and low cost solution useful for inspection and on-line monitoring of the servo-drives motional accuracy and also data collecting for later analysis. Despite the fact that the feedback devices installed in the servo-drives can be different the square wave signals are commonly used in position and commutation loops as their output signals. This kind of signals can be applied for quite simple in realisation digital measurements of both velocity and position increments. Lack of limitations in the distance and trajectory of the inspected movement as well as possibility to perform and repeat the tests on an operating machine without any interrupting its working process seem to be their valuable features. However, it is necessary to have at disposal quite simple but properly prepared equipment and right software instruments when a manufacturing machine with the servo-drives has to be inspected with different static and dynamic tests by this way. Key words: measurements, diagnostics of servo-drives, commissioning.
1 Demand The Servo-drives Diagnostics Automated drives are more and more important subsystem of the integrated automation used in the modern manufacturing systems. Numerous manufacturers of the drives are forced to improve their products in order to satisfy high requirements of the very competitive market. The requirements are placed on the motional accuracy, dynamics, communication possibilities and other electromechanical performances. On the other hand the list of requirements includes also non problematic and time saving planning, sizing, commissioning and diagnostics of the drives. Especially commissioning and effective diagnostics are a serious problem because a number of the adjustable parameters of a drive 39
Axis
Position controller
p*
Reference speed filter
Speed controller
n*
Actual speed filter
p
M
-
-
-
Output stage
Torque controller
m*
Inertia, load, accuracy, elasticity, play
m
Processing of signals and calculation of actual torque, speed, rotor and axis position
n
FS
ϕ T
FS
Figure 1. Structure of a brushless servo-drive with one feedback device installed on the motor : M- brushless servomotor FS- positioning feedback signals accessible for external measuring devices, ϕ- angle of rotation of the rotor, T-transducer of the rotor position, p-actual position, p*- position reference, n- actual speed, n*- speed reference, m- actual torque, m*-torque reference Axis
Position controller p*
Reference speed filter
Speed controller n*
m*
p n
Output stage
M
-
-
-
Torque controller
Inertia, load, accuracy, elasticity, play
Actual speed filter
FS
m
Processing of signals and calculation of actual torque, speed and rotor position Processing of signals and calculation of actual axis position
ϕ T1
FS T2
FS
Figure 2. Structure of a brushless servo-drive with two feedback devices installed on the servomotor: FS- feedback signals accessible for external measuring devices, ϕ- angle of rotation of the rotor, T1- primary transducer generating commutation and velocity signals, T2- secondary transducer generating positioning signals (also ve-locity if needed), p- actual position, p*- position reference, n- actual speed, n*- speed reference, m- actual torque, m*-torque reference 40
Processing of signals and calculation of actual axis position
FS
p p*
-
Position controller
Reference speed filter
Speed controller
n*
T2 Torque controller
m*
inertia, load, accuracy, elasticity, play
Output stage
M
-
n
Axis
FS
Actual speed filter
m
Processing of signals and calculation of actual torque, speed and rotor position
ϕ T1
FS
Figure 3. Structure of a brushless servo-drive with one feedback device installed on the servomotor and with the second one installed on the axis: FS- feedback signals accessible for external measuring devices, ϕ- angle of rotation of the rotor, T1- primary transducer generating commutation and velocity signals, T2- seconddary transducer generating positioning signals, p- actual position, p*- position reference, n- actual speed, n*- speed reference, m- actual torque, m*- torque reference tends to be between some hundreds and about two thousands [4,6]. The set of parameters includes among others a numerous group of the parameters used for tuning of the servodrive. The tuning is usually made only one time during commissioning of a machine whereas selecting of the parameters values is based on experiments and previous experiences. However, actual operating conditions can change what in turn can significantly affect the real accuracy of the servo-drives. This is a problem that limits possibilities of the drives implementation into integrated automation and also can increase costs of building modular manufacturing machines for individual tasks. As an answer for this demand more and more powerful software tools are offered for planning, sizing, commissioning and diagnostics. Also new advanced algorithms of adaptive auto-tuning have been implemented into controllers by some leading manufacturers [7]. It does not change the fact that operational principles of the most popular types of the servo-drives still are based on the traditional PID control. Additionally, there is a great variety of the servodrives in use that are not supported by any appropriate diagnostic software tools whereas the existing tools usually are able to operate with servo-drives from only one manufacturer. What is more the operational abilities of their monitoring and diagnostic functions are usually limited only to some the most typical tasks. Finally, probability of untypical tasks to solve during design, commissioning and diagnostics of a complex multi-axes machine should be considered. It involves a need to dispose an elastic, computer assisted and independent of the control methods of measurement, monitoring, data collecting and storage them for further analysis. Evaluation of freely selected different aspects of motional accuracy and other dynamic behaviour of the servo-drives operating in one or more axes should be possible as a result. 41
2 Computer Assisted Measurements of The Feedback Signals There is a variety of diagnostic methods that are based on assembly an external transducer on one or more moving elements of the tested axes in order to measure and evaluate course of the movement during a test run. Despite the obtained accuracy the tests have essential limitations concerning the possible shapes of the resultant trajectory, the possible to measure distance of the movement and also the necessity to prepare a right setup of the experiment before [1]. Diagnostics based on measurements of the feedback signals [1,2] can be an alternative way possible to apply in many cases as a simple and low cost solution, that is useful for inspection and on-line monitoring of the servo-drives motional accuracy. The square wave signals, that are often used in position and commutation loops, allow to measure small increments of actual velocity and position. The block diagrams in the Fig.1, Fig.2 and Fig.3 show three types of brushless servo-drives with differently solved feedback loops [3,4,5,6]. Possible places of the feedback signals measurements for diagnostic purposes can differ in particular cases dependently on the structures of the commutation, velocity and position loops. In a servo-drive with only one feedback device installed on a brushless servomotor (Fig.1) some different kinds of the transducer can be applied whereas its output signal have to be used for commutation, velocity and position loop simultaneously. The transducers typically met in such structures are as follow: rotary incremental encoders with square wave outputs (TTL or HTL), rotary incremental encoders with sine wave outputs and external digitalisation of the signals, rotary absolute value encoders; resolvers. In a structure with two feedback devices installed on the brushless servomotor (Fig.2) the primary transducer usually has to generate only the commutation signals but sometimes also velocity data whereas the secondary one generates the positioning signals and usually velocity data. The following types of the transducers are used as the primary one: resolvers (transmitter type), tachogenerators with an additional output of the commutation signals and Hall sensors (for commutation signals only). Resolvers (transmitter or reciver type) and incremental or absolute rotary encoders are commonly used as the secondary transducer. In the structure presented in the Fig.3. the primary transducer is installed on the servomotor and generates commutation signals and velocity data whereas the secondary transducer is connected with a moving element of the axis and realises direct measurement of its position. A linear encoder is usually applied as the second one in this case. By means of the incremental square wave signals used in the position feedback loops not only digital reversible counting of the position is possible but also digital measurement of the velocity. Actual value of the velocity can be calculated by two ways with different features [1,2]: by means of the feedback signal frequency or by means of the time measured between selected pulses of the signals (Fig.4). Also the commutation signals from the Hall sensors can be used to measure the velocity by the second way. A servo-drive with a resolver in the position loop usually includes a device for conversion of the encoder output analogue signals to the encoder-like square wave signals. Therefore also in this case the digital measurements of the position and velocity increments are possible. The described ways of the computer assisted diagnostic measurements can be carried out by means of a typical DAS card and a properly built virtual instrument with general structure as it is shown in the Fig.5. The environment of a standard DAQ software, such as for example DasyLab or LabView, is quite enough to built the instrument successfully. 42
a)
d) angle of rotation of the rotor (ϕ)
one revolution of the rotor (3600) A
A
B
B Z
C
b)
∆ϕ = 90
0
∆ϕ = 90
∆ϕ = 1/k * 360
0
0
∆ϕ = 1/k * 360
c)
0
e) f) clock pulses
clock pulses t1(∆ϕ)
t1(∆ϕ)
t2(∆ϕ)
t2(∆ϕ)
Figure 4. Principle of digital measurements of the velocity small increments with using of the commutation and position feedback signals: a- commutation feedback signals from a Hall sensor, b- pulses obtained from rising and falling edges after dividing, c,f- pulses from a clock generator, d- position feedback signals from an optical incremental encoder, epulses from rising and falling edges of track A after dividing, k- factor of dividing, ∆ϕincrement of the angular position of the rotor, t1(∆ϕ),t2(∆ϕ),…- the measured time of following shifts on the angle ∆ϕ
3 Conclusions The proposed method of the computer assisted tests and data collecting based on measurements of the feedback signals is free of some typical limitations. The possible to measure shape of the resultant trajectory, distance of the movement and also number of the tested simultaneously axes are not limited. There is no need to employ any additional transducers. Experimental setup can be quite simple and the main task to solve is to prepare a set of additional cables and connectors. It is possible to carry out the tests also during normal operation of a machine. The ways of the data collecting and their processing can be freely selected if an appropriate set of the software tools is in disposal whereas similar or the same virtual instruments can be applied for different servo-drives. Main limitations of the method are connected with the obtained resolution that is comparable with resolution of the control. Also the problem of electrical disturbances can turn out to be difficult to solve in practice. If the cases when the transducer is installed on the motor it is not taken into account how the dimensional accuracy, elasticity and play in the axis mechanisms influence the results. There is lack of legal verification of the measuring instruments therefore they can not be used for acceptance testing. Because of the feedback signals can differ from real actual values of the velocity and position, the described way of measurement can be especially useful in the tuning adjustable parameters of a servo-drive.
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b) virtual device for digital measurement of the small increments of velocity
a) virtual device for digital measurement of the small increments of position 7
10
6 5
1
1
8
3
2
7
3
11
2 6
4
12
4 9
5
10
8
9
Figure 5. General structures of the virtual devices for digital measurement of the position (a) and velocity (b) small increments. Legend for a): 1- encoder or a source of encoder-like signals, 2- external reference switch, 3- inputs of a DAS-card, 4- triggering of the counting process, 5- reversible counting and buffering the encoder pulses, 6-programmable frequency divider, 7- digital meter, 8- control of measurement and data acquisition, 9- X/Y or Y/t chart, 10- data storage and analysis; Legend for b): 1- encoder or a source of encoder-like signals, 2- inputs of a DAS-card, 3,4,5– programmable frequency dividers, 6clock pulses generator, 7- counter of the clock pulses, 8-counter of the encoder pulses, 9digital meter, 10- control of measurement and data acquisition, 11- data storage and analysis, 12- Y/t chart,
References [1] Blacharski W.: Independent Measurement of the Feedback Signals – an Alternative in the Servo-Drives Diagnostic. Proceedings of International Carpatian Control Conference ICCC’2001. Krynica, Poland May 22-25, 2001. University of Mining and Metallurgy in Cracov, Machinery Construction Committee Polish Academy of Science. [2] Blacharski W.: Cyfrowe pomiary prędkości i drogi w diagnostyce zautomatyzowanych napędów. I Konferencja AUTOMATYZACJA MASZYN, URZĄDZEŃ I PROCESÓW APRO’99. Katedra Automatyzacji Procesów AGH w Krakowie. Sekcja Mechanizacji Górnictwa Komitetu Górnictwa PAN. Krynica, 19-22 kwiecień 1999. [3] Kissell T.E.: Industrial Electronics Applications for Programmable Controllers, Instrumentation and Process Control, an Electrical Machines and Motor Controls. Prentice Hall. New Jersey, 2000. [4] Lust Antriebstechnik GmbH: Technical brochures and catalogues. [5] Pacific Scientific: Technical brochures and catalogues. [6] Siemens A&D: Technical brochures and catalogues. 44
