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The biggest servo of the lot: the shoulder

The motors supplied with the kit are very robust. They can be asked to move outside their range, or against too much torque, and they will not be damaged (unless this occurs for a long time in which case the motors can overheat). When the robot is powered down, you can take hold of the thing and move the joints about by hand without breaking any of the gearboxes in the motor casings. Probably the only thing to worry about is crashing the arm hard, causing extreme shocks into the gear trains.

All the motors are built and operated with the same principles. They are all fully sealed cased motors, potentiometers, gearboxes and analogue control units. Control of the motors is by comparing the potentiometer value with the selected position, and turning the motor one way or the other to get the potentiometer in the right direction. This is known as local closed loop control, and unfortunately has its limitations which are shown up admirably in their application to this kit.

The motors themselves are three pole-wound iron cores between permanent magnets. One effect of this is that there are preferred directions the motors like to point, and if the motors are asked to move slowly they will do so with some jerkiness as the poles align and dis-align with their preferred directions.

The servo motors are controlled with carefully timed pulses, fifty per second (thus the spacing between the pulses is 20000µs). A pulse length of 1500µs will put the motor into its central position. Variations around this of 400µs will displace the motor by 40°; the range of movement is (nominally) 90° each way, so the range of input values is from 600µs to 2400µs.

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