Wednesday, October 01, 2008

What is a Servomotor?

Servomotors are used in many radio-controlled model airplanes, cars, boats, and helicopters. Because of this large hobbyist market, servomotors are readily available in a number of stock sizes. Servomotors are used in a few of our robots.
Primarily, servomotors are geared dc motorswith a positional feedback control that allows the rotor to be positioned accurately. The specfications state that the shaft can be positioned through a minimum of 90° (±45°). In reality we can extend this range closer to 180° (±90°) by adjusting the positional control signal.
There are three wire leads to a servomotor. Two leads are for power +5v and GND. The third lead feeds a position control signal to the motor. The position control signal is a single variable width pulse. The pulse can be varied from 1 to 2 ms. The width of the pulse controls the position of the servomotor shaft.
A 1-ms pulse rotates the shaft to the extreme counterclockwise (CCW) position (-45°). A 1.5-ms pulse places the shaft in a neutral midpoint position (0°). A 2-ms pulse rotates the shaft to the extreme CW position (+45°).
The pulse width is sent to the servomotor approximately 50 times per second (50Hz). Figure 1 illustrates the relationship of pulse width to servomotor position.

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