Proceedings of the 1999 IEEWASME lnternational Conference on Advanced Intelligent Mechatronics September 19-23.1999 Atlanta, USA

Compact Servo Driver for Torque Control of DC-Servo Motor Based on Voltage Control

' Hitoshi Maekawa Mechanical Engineering Laboratory, AIST-MITI

Namiki 1-2, Tsukuba, Ibaraki 305-8564 Japan

Abstract A new torque contro. methoi Tor DC-servo motors based on voltage control is proposed and investigated. Through analyzing the torque control system consisting of DC- servo motor, reduction gear and torque sensor, it is shown that torque control by voltage control is feasible. It is also possible to eliminate the static error of the output torque and the interference of the disturbance torque such as the friction at the reduction gear.

According to the analysis, a prototype servo driver is developed. Since only the voltage control capability is needed for the servo driver, the volume and mass ofthe developed servo driver are around 30% of the conventional values. Through experiments using the developed servo driver, it is conjrmed that accurate torque control is feasible by the proposed method,

1. Introduction In order to achieve delicate motion by a mechanical system such as grasping an object by fingers, contacting a surface by a manipulator with desired force and so on, force-torque control is indispensable. On the other hand, a compact and light servo driver is needed for the integration of the mechanical system providing many degrees of freedom.

In this paper, as a solution for these requirements, a torque control based on voltage control for a DC-servo motor with reduction gear that is one of the most widely used actuators is proposed. Also, a simple and compact servo driver is developed for torque control. The developed torque control is experimentally evaluated using the developed servo driver.

Reviewing the conventional technology, several torque control methods for DC-servo motors have already been established.

Direct current control Valid when the output torque of the motor is proportional to the input current such as the direct drive motor. Although the open-loop current control that does not require the current sensor [SI is also proposed, the accuracy of the torque control is

degraded by disturbance torques in the actuator such as the friction at the reduction gear.

Current control using disturbance observer [3] Since the torque is estimated from the motion, the force sensor is not needed. However, precise model of the friction that is very difficult to identlfy is required.

Velocity control using torque feedback The desired velocity proportional to the torque error is set to the servo driver [1][2]. The servo dnver is complicated and bulky since it contains both the velocity controller and the internal current controller.

Examining the conventional methods, accurate torque control that is robust against disturbance torque is needed. Additionally, simple and compact servo driver is also needed since all methods contain a current controller that results in complicated and bulky servo driver.

2.Analysis of torque control of DC-servo

In order to investigate the feasibility of torque control based on voltage control, the torque control system consisting of DC-servo motor, reduction gear and the torque sensor shown in Fig. 1 is analyzed.

The DC-servo motor is first modeled as a series of resistance and back-emf (electro motive force). According to this model, the voltage e,, current i , supplied to the motor and the velocity of the rotor em are related as follows:

motor

e, = R,i, + K,S, (1) where R, is the resistance of the armature and K, is the back-emf constant of the motor. On the other hand, the torque KJ, generated at the rotor is distributed to its acceleration and to the reduction gear. This relation is written as:

K,im = I,S, + Z, (2) where I , is the sum of the moment of inertia for the

0-7803-5038-3/99/$10.00 0 1999 IEEE 34 1

rotor and the input stage of the reduction gear, and z, is the torque applied to the reduction gear from the motor.

Concerning the reduction' gear, the position a, and torque tg at the output are determined by the reduction ratio K , according to the position e, and the torque r, at the input as follows:

1

In Eq. (4), the disturbance torque rd due to the friction and so on is considered. The torque Z, at the reduction gear is distributed to its acceleration and to net output torque as follows:

zg = z,e, + '5, (5) where I , is the sum of the moment of inertia for the output stage of the reduction gear and the torque sensor, and 7, is the output torque. At the torque sensor, the applied torque is proportional to the torsion of the sensor:

where K, is the stiffness of the torque sensor and 0, is the output position.

Eqs. (1)-(6) are fundamental formulae that describe the dynamics of the torque control system. Through Laplace transform, these equations are combined to give the following relation of the output torque z0 (s) , the voltage e,(s) supplied, to the motor, disturbance torque zd(s) and output position 8, (s) :

Desired output torque ' 0

Output torque ' 0

TFque sensor

Servo driver DC-servo motor torque rd

Fig. 1 Torque control system

In order to control the output torque, the voltage supplied to the motor is controlled according to the torque error:

(9) e,(4 = r, (SI[ .;o(s) - r , (41 where .3, is the desired output torque and T , ( ~ ) is the transfer function of the compensator.

On the other hand, the dynamics of the load, namely the relation between the output torque 7, and output position 0, are described by the transfer function T,(s) as follows:

H ( s ) =[l+K,To(s)]G(s)+K,KgK,T,(s)+R,Ks (14)

Transfer functions T ( s ) and T,(s) determine the dynamic response of the output torque 7, when the desired torque io and the disturbance torque 7, vary. From the viewpoint of accurate torque control, it is desirable that IT,(jw)l=l and LT,(jw) = O over a wide frequency range w. Also, for eliminating the interference of the disturbance torque on the output torque. it is desirable that IT,( jw) ( = 0 .

In this paper, as a primary evaluation of the proposed torque control, the static characteristics are examined. For static analysis, the static gain of the transfer function is . obtained by setting s=O. For evaluating the static gain T(0) and T, (0) , G(0) is first derived according to Eq. (8) as:

G(0) = 0 (15)

Next, considering the model of the load as a mass- damper-spring system for general situations, the transfer function of the load T, (S) is given by:

where I o , Bo , KO are the inertia, viscosity and stiffness of the load respectively. In such a case, the static gain T, (0) is given by:

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Eq. (17) is set to a finite value if stiffness of the load K, exists. This is satisfied in the case that the load contacts the external environment. Therefore, according to Eqs. (14)-(15), (17), the static gain of H(s) is derived as follows:

(18) H(0) =[l+KJT,,(0)]G(O)+KmKgKJT,(O)+RmKs

From Eqs. (12)-( 13), (18), the static gain of I;@) , r, (s) is finally derived as:

It is desirable that q (0) = 1 and r, (0) = o because the output torque is accurately set to the desired value while the disturbance torque does not interfere. This is satisfied when the static gain of the compensator is infinite ( T , ( o ) = w ) . One of the compensators to satisfy this requirement is a PI-controller whose transfer function is given by:

Ki T,(s) = K, +- S

K,s + K, - -- S

By implementing the above PI-controller, both the static error of the output torque and the static interference of the disturbance torque are eliminated. Since the transfer function of the PI-controller in Eq. (21) contains factor Us, the proposed compensation method exactly corresponds to the strategy of control design that multiplies the open-loop transfer function by l/s in order to eliminate the static error.

Through the analysis of the torque control system, it has been shown that the voltage control with PI-feedback is capable of controlling the output torque of a DC-servo motor even if the disturbance torque caused by the friction exists. In such a case, both the static error of the output torque and the static interference of the disturbance torque are eliminated.

3. Compact servo driver for torque control A prototype servo driver was developed for the torque control of the DC-servo motor. Since only the voltage control capability is needed for the driver according to the previous analysis, the prototype servo driver is designed as an open-loop P W M controller that can be realized with simple and compact hardware.

Fig. 2 shows the structure of the prototype servo driver. The driver is capable of controlling two DC-servo motors according to the PWM rate sent from the MPU (Micro Processing Unit). The 8254 PIT (Programmable Interval Timer) that contains 16bit programmable counters (counter #0, #1, #2) is used for the timing generator. The system clock is supplied to all counters and counter #O is programmed as a base counter to generate constant frequency pulses by dividing the system clock. Counters #1 and #2 are programmed as a variable one-shot monostable multivibrator triggered by the output of counter #O.

As shown in Fig. 3, when the counter #O triggers counter #1, counter #1 sets the PWM signal while counting the system clock for the preset value. After counter #1 ends counting, the PWM signal is reset until the next trigger. The PWM rate varies according to the preset value of counter #1 sent from the MPU. Counter #2 for the other motor operates in the same manner as counter #l. The frequency of the base counter #O is 2.4kHz and the resolution of the PWM signal is 1/4096.

Direction control

tor

Fig. 2 Structure of developed servo driver PWM cycle (constant) 2 U Counter #O output (base counter)

T r i F o u n t 1 end T r i r i o y t end

Counter # 1, #2 output (PWM signal) J-U-L - -

Pulse width (variable)

Fig. 3 Timing generation of PWM signal

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The PWM signal is gated in order to control the polarity of the output voltage and finally sent to the full-bridge driver (Toshiba TA8429H, maximum supply voltage: 27V, maximum average current: 3.0A). As a result, the output voltage of the driver is proportionally controlled to the PWM rate. The driver is isolated by the photo-couplers in order to prevent the switching noise from interfering with the other circuits. Fig. 4 shows the developed prototype servo driver (72x47x35mm, 0.084kg, for 2 motors).

In order to evaluate the prototype servo driver, it is compared with one of the typical conventional servo drivers (Harmonic Drive Systems HS-250-2 [6] ) . Examining the comparison in Fig. 5 and Table 1, it is clear that the prototype servo driver is much more coinpact and light; its volume and mass per total output wattage are around 30% of the conventional values. Additionally, the tachogenerator needed by the conventional driver for velocity feedback is no longer necessary.

The developed servo driver is controlled through parallel digital I/O. It is also possible to directly interface the driver to the MPU bus with some additional logic. Therefore, high-speed communication between the MPU and the servo driver is easily realized.

Control mode Number of motors

Total output wattage (maximum average)

Components

This comparison shows the superiority of the developed servo driver in terms of size, mass and interfacing with

Prototype servo driver Conventional servo driver Harmonic Drive Systems HS-250-2 [6]

P W M rate (voltage) Velocity 2 1

28W 38W

A. Servo driver . A. Servo driver

?

. .

B. Power supply B. Transformer

Fig. 4 Developed servo driver (both faces, 72x47x35mm, for two motors)

Fig. 5 Comparison of the developed servo driver (driver and power supply: front) with conventional

one (driver and transformer: back)

Table 1 Comparison of the developed servo driver with conventional one

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the MPU. It is also shown that applying the servo driver designed for velocity control to torque control is quite inefficient.

4. Experimental results The proposed torque control based on voltage control is experimentally evaluated using the developed prototype servo driver. The experimental setup shown in Fig. 6 consists of a DC-servo motor with harmonic drive (Harmonic Drive Systems HT-8-6006), torque sensor (Sinmei Electric SM60-KA-TB) and prototype servo driver. The PI-gain is set as:

1 y P = 5 2 v ~ m , K, = ~ . ~ x ~ o v / N ~ s

The static characteristics of the torque control system are first confhned while setting the desired output torque and measuring the corresponding output torque. Fig. 7 shows the experimental results when the P-feedback and the PI- feedback is employed respectively.

When only the P-feedback is employed (solid circle), although the output torque ro is roughly proportional to the desired output torque io, the static error due to the friction at the reduction gear and so on is seen. In contrast, when PI-feedback is employed (blank circle), the static error is drastically eliminated and the output torque coincides with the desired value as derived through the analysis.

The dynamic characteristics of the torque control are also confirmed by the step response while setting the indicial desired output torque. The output shaft of the torque sensor is rigidly fixed during the experiments ( r, (s ) = o ). Fig. 8 shows the response of the output torque r, when the desired output torque io varies from 0 to 1 .ONm. When only the P-feedback is employed, although the output torque responds rapidly, the static error exists due to the friction. On the other hand, when the PI-feedback is employed, the static error seen at the response with the P- feedback is eliminated within approximately O, 13s. Through these experiments, the accurate torque control capability of the proposed control and the prototype servo driver is confirmed.

5. Conclusion In this paper, a new torque control method for DC-servo motors was proposed and examined. The results are summarized as follows:

Fig. 6 Experimental setup (Left: Servo motor with harmonic drive, Right: Torque sensor)

Desired output torque .2b [Nm] Fig. 7 Static characteristics of torque control with

P-feedback ( ~ ~ = 5 2 V / N m ) and

PI-feedback (1yp=52V/Nm, K~ =1.3x103V/Nms)

E ,Desired torque

* Y F !

E / 2 1.0 !

20.5 ! 6 !

- 1

PI-feedback P -feedback

! !

!

!

- CI

0 -

+ 0.1s 4 Time Fig. 8 Dynamic response of torque control for

indicial input ( io = 0 -+ io =l.ONm) with P-feedback (1yP=52V/Nm) and

PI-feedback (1yp=52V/Nm, K~ =1.3x103V/Nms)

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Torque control of DC-servo motors by voltage control is feasible. The static error and the interference of the disturbance torque are eliminated by the PI-feedback. The prototype servo driver is much more compact and light than conventional ones (around 30% of the volume and mass) and is easy to interface with a MPU. The accurate torque control capability of the proposed method was experimentally confirmed.

Although the basic dynamics of the torque control system were derived in this paper, the details remain to be investigated. The influence of the load dynamics will be investigated in order to establish the strategy for optimizing the PI-gain.

The proposed torque control achieved with a compact and light servo driver will allow the development of advanced mechanical systems whose components need to be integrated within limited space and weight. For example, while developing the four-fingered hand mounted on the arm in Fig. 9, twelve servo drivers for finger actuators are integrated in the module shown in Fig. 10 using surface mount technology and programmable logic devices. Since this module is small (200x 12Ox25mm, including MPU and memory), it can be mounted on the arm close to the actuators. Therefore, the cables between the drivers and the actuators are reduced. Additionally, since the proposed torque control does not need velocity

Fig. 9 Four-fingered hand on arm

information, the tachogenerators (total 0.96kg for 12 actuators) can also be omitted.

Several possibilities for improving the servo driver remain. The prototype employs bipolar transistors for the switching device whose turn ordoff speed is limited. Therefore, a loss of power that results in heat occurs while the transient of switching. Also, the PWM frequency is set to a rather low range (2.4kHz) that emits an audible noise. These problems .will be resolved by using FETs which turn ordoff much faster [4].

The servo dnver will be made even smaller by implementing it on a one-chip microprocessor containing AD converters and counters. This design will enable the integration of the controller, PWM driver and the circuits for the torque sensor into a tiny module. Additionally, integration of the regenerative brake for energy consumption will be important especially for mobile robots that are operated by a limited power source.

References Imamura, N., M. Kaneko, K. Yokoi, and K. Tanie, Development of a Two-Fingered Robot Hand with Compliance Adjustment Capability, Proc. of 1990 Japan4J.S.A. Symp. on Flexible Automation, pp.

Maekawa, H., K. Yokoi, K. Tanie, M. Kaneko, N. Kimura, and N. Imamura, Development of a Three- Fingered Robot Hand with Stiffness Control Capability, Mecliatronics, vol. 2, no. 5, pp. 483-494, 1992. Nakao, M., K. Ohnishi, and K. Miyachi, A Robust Decentralized Joint Control Based on Interference Estimation, Proc. of IEEE Int. Conf. on Robotics and Automation, pp. 326-331, 1987. Tokunaga, Y., T. Tsubouchi, and S. Yuta, A P W M Motor Control Circuit with Low Power Consumption for a Middle Size Robot, Proc. of Annual Conf. of Robotics Society of Japan, pp. 669-670, 1997 (in Japanese). Watanabe, G., T. Tsubouchi, and S. Yuta, Feed Forward Current Control on DC Motor by PWM Drive Method, Proc. of Annual Cod. of Robotics Society of Japan, pp. 667-668, 1997 (in Japanese). Harmonic Drive Systems, DC Servo Actuator Super Mini Type RH Series (Technical manual), no. 9507- OR-RH (in Japanese).

997-1004, 1990.

Fig. 10 Integration of 12 servo drivers (20Ox120x25mm, including MPU and memory)

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