A hobby servo is a compact actuator that turns an output shaft to a commanded angle and then holds that position. It is widely used in robotics, radio control vehicles, animatronics, and small automated mechanisms because it combines a motor, gears, sensor, and controller in one package. Understanding its anatomy helps students see how electrical signals become precise mechanical motion.
The cutaway view reveals that a servo is not just a motor, but a closed loop control system.
Key Facts
- A hobby servo contains a DC motor, gear train, output shaft, feedback potentiometer, and control board.
- The control signal is usually a repeating PWM pulse, often about every 20 ms.
- A pulse width near 1.0 ms often commands one end of travel, 1.5 ms commands the center, and 2.0 ms commands the other end.
- Gear ratio = motor speed / output shaft speed, so a larger gear ratio gives more torque but less speed.
- Torque = force x lever arm distance, so τ = F r for a perpendicular force.
- Closed loop error is the difference between commanded angle and measured angle: error = θcommanded - θmeasured.
Vocabulary
- Servo motor
- A servo motor is an actuator that uses feedback to move an output shaft to a commanded position.
- PWM
- Pulse width modulation is a control method in which the width of a repeating electrical pulse carries information.
- Gear train
- A gear train is a set of meshing gears that changes the speed, torque, and direction of rotation.
- Potentiometer
- A potentiometer is a variable resistor that can measure shaft position by changing resistance as it rotates.
- Feedback control
- Feedback control compares a measured output to a desired input and corrects the system when there is an error.
Common Mistakes to Avoid
- Treating a hobby servo like a bare DC motor is wrong because a servo has internal electronics that expect a control pulse and try to hold a position.
- Confusing PWM pulse width with motor power is wrong because in a standard position servo the pulse width commands angle, not speed or voltage level.
- Ignoring the gear train is wrong because the output shaft turns much slower than the motor while producing much larger torque.
- Forcing the output horn by hand is wrong because it can strip plastic gears or damage the feedback potentiometer alignment.
Practice Questions
- 1 A servo maps 1.0 ms to 0 degrees and 2.0 ms to 180 degrees linearly. What angle is commanded by a 1.25 ms pulse?
- 2 A servo output arm is 0.030 m long and pushes with a perpendicular force of 12 N. What torque does it produce at the shaft?
- 3 A robot arm jitters when commanded to hold still. Explain two servo anatomy related causes that could produce this behavior.