Servo System

Posted on Feb 4, 2014

This study is made possible by the availability of a motor potentiometer from All Electronics at small cost. The item is catalog number MPOT-10K, and costs $4. 00. It is manufactured by Alps, and the motor is marked MD157320, the body 725t 10BX2. The potentiometer is dual 10k, so one potentiometer can be used for feedback of the position, while the

Servo System
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other serves as the output. What we are going to do here is to make a remote-controlled potentiometer that repeats the setting of the control potentiometer. This is a typical feedback system, but since it involves a mechanical element, it is called aservomechanism, orservo, for short. The output could be the shaft position, as well as the electrical output of the controlled potentiometer. In fact, the output of most servos is mechanical. The motor pot consists of a small DC motor, gearing that drives the sliders, the support for the potentiometer resistances, and a shaft. The connections are shown at the right. We are looking at the motor end; the shaft is at the other end. It is slightly disappointing that the potentiometers are Siamese twins, so they are not completely independent, but that is little matter for our purposes. A clockwise rotation of the shaft, looking from the shaft end, increases the resistance between the common end and the sliders. A voltage applied to the motor terminals with the marked polarity turns the shaft anticlockwise, bringing the sliders toward the common ends. The motor requires a little more than 2 V to begin turning, so that 5 V is a reasonable maximum. The motor draws about 30 mA or a bit more. There are two connection pins (not shown) on the shaft side of the potentiometer connections that light a red LED that shines through the hole in the shaft. As you face the shaft, the anode is at...

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