Appendix 1

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Model Control Servos

These are cheap hi-tech consumer items made in the millions by several manufactures and consist of an electric motor, gearbox, output-shaft position-feedback-potentiometer, and control electronics. Generally the only information available for them is the size and pullout torque. Often the pullout torque quoted is exaggerated.

In the main they are designed to run from a 5 or 6 volt supply and the standard size used in BigFoot draws between tens of milliamps and 1 amp depending on the load. The size of a standard-size servo is about 40mm x 35mm x 20mm plus output shaft with a pullout torque of between 3.5 and 4.5 Kgcms (torque is sometimes also quoted in oz inches).

Quite often signal specification is unavailable and (when available) different manufactures may specify slightly different figures. However generally the following will work with all servos:- the control signal is a positive going 5volt square pulse of between 1 and 2 milliseconds (ms) repeated every 20 ms.

Repeating the pulse at less than about 8ms will make many servos malfunction and overheat, some malfunctioning permanently, and repeating at longer intervals than 20ms will make operation jerky and power will fall off the longer the interval between pulses.

The output shaft position, within about an 100 degree arc, is proportional to the length of the the 1 - 2 ms control pulse. The output shaft is generally capable of rotating about 200 degrees between mechanical stops and the pulse length can be varied outside the range of 1 - 2 ms to increase the travel of the shaft but driving the shaft to an end stop may result in damage to the motor, gearbox or drive electronics. The direction of rotation for an increasing pulse length is often not known and may vary for different servos from a particular manufacturers range, Hitec are an exception and the direction is standardised throughout their range.

Although the servos are rated at 5 or 6 volts some people run them on much higher voltages, often for small robots powering them from a PP3 9v battery. The author has accidentally run servos from 12 volts with no apparent harm. However most servo manufacturers rather than using discrete components in the control electronics are using smaller and smaller integrated circuits (ICs) and while discrete components may not fail on the higher voltages the same probably will not be true of ones using ICs which may have an absolute maximum rating of about 7 volts.

Some servos, often described as 'indirect drive' may be modified for continuous rotation, where the servo can be driven in either direction continuously depending on the length of the control-pulse. The pulse length at which the motor stops is set by the position of the feedback-potentiometer, if the wiper is set to about the middle of its travel then a pulse length of about 1.5ms will cause the motor to stop. 1ms pulses will cause it to rotate one way and with 2.0ms pulses it will rotate the other way.