Digital Signal Processing Reference
In-Depth Information
the left in Figure 13.2), built-in control circuitry, and a gear reduction system.
They are small, produce a relatively large amount of torque for their size, and
run at the appropriate speed for a robotics drive motor.
Figure 13.2
Left: Radio Control Servo Motor and Right: Servo with Case and Gears Removed.
The control circuitry of the servo uses a potentiometer (variable resistor) that is
used to sense the angular position of the output shaft. The potentiometer is the
tall component on the right in Figure 13.2. The output shaft of a servo normally
travels 180-210 degrees. A single control bit is used to specify the angular
position of the shaft. The timing of this bit specifies the angular position for the
shaft. The potentiometer senses the angle, and if the shaft is not at the correct
angle, the internal control circuit turns the motor in the correct direction until
the desired angle is sensed.
The control signal bit specifies the desired angle. The desired angle is encoded
using pulse width modulation (PWM). The width of the active high pulse varies
from 1-2 ms. A 1ms pulse is 0 degrees, 1.5ms is 90 degrees and a 2 ms pulse is
approximately 180 degrees. New timing pulses are sent to the servo every 20
ms.
13.3 Modifying the Servos to make Drive Motors
Normally, a servo has a mechanical stop that prevents it from traveling move
than half a revolution. If this stop is removed along with other modifications to
the potentiometer, a servo can be converted to a continuously rotating drive
motor. Modifications to the servo are not reversible and they will void the
warranty. Some robot kit vendors sell servos that are already modified.
To modify the servo, open the housing by removing the screws and carefully
note the location of the gears, so that they can be reassembled later. The
potentiometer can be replaced with two 2.2K ohm ΒΌ watt resistors or
disconnected by cutting the potentiometer shaft shorter and setting it to the
