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Publisher’s version / Version de l'éditeur:

EDN The Design magazine for the Electronics Industry, 1999

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Simple Circuits Control RC Servos

Korba, Larry

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National Research

Council Canada

Institute for

Information Technology

Conseil national

de recherches Canada

Institut de Technologie

de l’information

Simple Circuits Control RC Servos

*

L. Korba

Copyright 1999 by

National Research Council of Canada

Permission is granted to quote short excerpts and to reproduce figures and tables from this report, provided that the source of such material is fully acknowledged.

*Published in “EDN, The Design Magazine for the Electronics Industry.

Cahner’s Publishing, USA. pp. 100-101. 1999.

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100E D N |AP R I L i 1999 www.ednmag.com

ideas

design

D

EVE LO P E R Ssometimes need to actuate rota-O F R O B O T I C D EV I CE S tional elements between two posi-tions over a range of several tens of de-grees. Such motion control is particularly useful for opening doors, actuating valves, or controlling the movement of small robotic arms or legs. Two alterna-tives to produce this type of actuation are a solenoid and a motor attached to a gear train. Solenoids require careful

mechanical design to effect the conversion from linear to rotational motion and to achieve consistent tion control between two posi-tions. Motor-based systems re-quire careful positioning of limit switches. An alternative to these two approaches, embod-ied in the circuits of Figure 1, involves the use of radio-con-trol (RC) servomechanisms. Common RC servo devices are available in a variety of size, speed, and torque specifica-tions. The range of motion of a typical RC servo is 190

(Refer-ence 1). Because these servos must fit into RC cars, airplanes, and boats, they are usually small—especially considering their torque. The angular posi-tion of the servomotor shaft is related to the pulse width of an input control signal.

A technique for controlling the output of an RC servo be-tween two positions uses a 0 to 5V digital signal (Figure 1a). The circuit requires a 5 to 6V power source. D2, D3, R3, R4, and C1configure the 555 IC to op-erate in astable mode at a fre-quency of approximately 285 kHz. R3 controls the pulse width of the output data stream over 0.3 to 2.5 msec. This set-ting sets the maximum counter-clockwise position of the

servo-motor with the control input set at 0V. When the control input is at 5V, the out-put pulse train contains a 75-msec pulse.

This pulse sets the servomotor at its most clockwise position. To calibrate the ser-vo using the circuit in Figure 1a, you first set the control input at 5V. With this in-put, the shaft of the servo reaches its max-imum clockwise position (which the ra-tio of R2to R2+R6 controls). With the

control input set at 0V, R3controls the ro-tational limit of the actuated assembly in the counterclockwise direction.

The circuit in Figure 1b controls the movement of an RC servo between two preset positions, using an input signal of +12 to 212V. In this case, the input

con-trol signal supplies the current that the servo and control circuit require. The bridge rectifier comprising D1to D4and

Simple circuitry uses pulse-width modulation to control an RC servomotor, using a 0 or 5V control signal (a); a self-powered version (b) uses a ±12V control input.

Simple circuits control RC servos

Larry Korba, National Research Council, Ottawa, ON, Canada

CONTROL GND RC SERVO (a) (b) + WHITE BLACK RED R4 10k R2 33k IC1 555 IC1 555 R1 1k R6 2.2k R5 2.2k D3 1N4001 D2 1N4001 D1 1N4001 T2 T1 R3 100k C1 0.033 mF CONTROL INPUT 0 TO 5V GND TRG OUT RST VCC DIS THR CTL 1 2 3 4 5 6 7 8 6V Q1 2N3904 CONTROL GND RC SERVO + WHITE BLACK RED R4 10k R5 33k R7 240 R6, 1k R2 1k R3 2.2k D2 1N4001 D3 1N4001 IC2 LM317 D1 1N4001 D4 1N4001 D5 1N4001 D6 1N4001 D7 1N4001 T1 T2 R1 100k C1 0.033 mF C2 1 mF C3 1 mF CONTROL INPUT 512V GND TRG OUT RST VCC DIS THR CTL 6V Q1 + +INCOMOUT MPS6520 Fi gu r e 1

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ideas

design

the base-emitter junction of Q1, with the aid of some filtering by C3, provides the input voltage for the three-terminal reg-ulator, IC2. R6and R7bias the regulator to produce a 6V output, which the ser-vom otor requires. Transistor Q1 pro-duces a signal of 0 or 5V, depending on the polarity of the input voltage. The cir-cuit in Figure 1b supports the entry of the Carleton School Board of Ottawa (Reference 2) in the 1996 Canada First

Robotics Competition (Reference 3), in which competitors developed a remote-ly controlled robot that plays a special-ized gam e of basketball. The receiver unit provides control servomotors in the robot with several bipolar outputs.(DI #2338).

RE F E R E N C E S

1. Jones, Joseph, and Anita Flynn,

In-spiration to Implementation, AK Peters,

Wellesley, MA, 1993.

2. Carlton School Board Warriors Web site, www.warriors.ottawa.com.

3. Canada First Web site, www. candafirst.org.

TO VOT E FO RTH I ShE SI G Nn CI RCLEcO. 315

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