Timer Functions - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 439

Figure L8.3a, b ■ Photos of DC and servomotors
Servos are constructed from three basic pieces: a motor, a potentiometer that is connected
to the output shaft, and a control board. The potentiometer allows the control circuitry to
monitor the current angle of the servomotor. The motor, through a series of gears, turns the
output shaft and the potentiometer simultaneously. The potentiometer is fed into the servo
control circuit, and when the control circuit detects that the angle is not correct, it turns the
motor the correct direction until the angle is correct. Normally a servo is used to control an
angular motion between 0 and 180 degrees. It is not mechanically capable (unless modified) of
turning any farther due to the mechanical stop build on the main output gear.
Servos are controlled by sending them a pulse of variable width. The control wire is used to
send this pulse. As shown in Figure L8.3c, the pulse has a minimum pulse, a maximum pulse,
and a repetition rate. Given the rotation constraints of the servo, neutral is defined to be the
position where the servo has exactly the same amount of potential rotation in the clockwise di-
rection as it does in the counterclockwise direction. It is important to note that different servos
will have different constraints on their rotation but they all have a neutral position, and that
position is always 1.5 ms.
20 ms
Pulse width 1 ms (min.) to 2 ms (max.)
Figure L8.3c ■ Pulse pattern of a servomotor
The angle (indicated by the pointer on the white plate of the servomotor) is determined by
the duration of a pulse that is applied to the control line. The servo expects to see a pulse every
20 ms. The length of the pulse will determine how far the motor turns. For example, a 1.5-ms
pulse will make the motor turn to the 90-degree position (neutral position).
When a servo is commanded to move, it will move to the position and hold that position.
If an external force pushes against the servo while the servo is holding a position, the servo will