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motor pwm

Definition

A Servo Motor is a cheap rotary actuator used for radio control and small-scale robotics.

Colored Wires

A servo motor is normally connected using just 3 wires.

Color Purpose
Yellow PWM Signal
Red VDD
Black GND

Control

The angle of the servo is determined by the width of the PWM signal that is applied to the control wire. Most servos expect to see a pulse every 20 ms. The angle is calculated as the fraction of the pulse width

$$pw = {1.0\,\mathrm{ms}} + {1.0\,\mathrm{ms}} \cdot \left( \frac{\alpha}{\alpha_{\text{max}}}\right)$$

with the pulsewidth $pw$, the current angle $\alpha$ .

For most RC servos the exact refresh rate is irrelevant as long as it is in a range of 40 Hz to 200 Hz.

Example

A pulsewidth of $pw = {1.5\,\mathrm{ms}}$ will turn a servo with a maximum angle $\alpha_{\max} = {180\,\mathrm{{\text{deg}{\,}}}}$ to $\alpha = 90$° (neutral position).

References

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