Servo Motors
A servo motor is not just a motor. It is a motion system that uses feedback to drive position, speed, or torque toward a commanded value. Small hobby servos hide the motor, gearbox, feedback sensor, and controller in one package; industrial servos separate the motor, encoder, drive, and controller for higher performance.
Learning Objectives
By the end of this lesson, you should be able to:
- distinguish hobby servos from industrial servo axes;
- explain closed-loop position control;
- estimate torque, speed, and power requirements;
- understand PWM command timing for hobby servos;
- recognize common servo faults such as hunting, overload, and poor grounding.
Servo System Blocks
The controller compares command and feedback. The error drives the motor until the measured position matches the target. This feedback is what separates a servo axis from an open-loop motor.
Hobby Servo Commands
Most hobby servos use a repeated pulse signal. A typical frame is about 20 ms; pulse width sets target angle.
| Pulse width | Typical meaning |
|---|---|
1.0 ms |
near minimum angle |
1.5 ms |
center |
2.0 ms |
near maximum angle |
Exact limits vary. Do not force a servo mechanically against its end stops; current rises quickly and gears can fail.
Torque, Speed, and Power
Torque must exceed load torque with margin:
$$
T_\text{required}=F r
$$
Rotational mechanical power is:
$$
P=T\omega
$$
where T is torque in newton-metres and omega is angular speed in rad/s. Servo datasheets often use kg-cm; convert carefully:
$$
1\ kgf\cdot cm \approx 0.0981\ N\cdot m
$$
Power and Grounding
Servos draw burst current during acceleration, stall, and direction reversal. Power them from a supply sized for peak current, not from a weak MCU pin. The control signal and servo power usually need a shared ground unless an isolated interface is used.
Add local bulk capacitance near groups of servos and route high-current servo returns away from sensitive sensor references.
Tuning and Behavior
A closed-loop axis can overshoot, oscillate, or hunt if the controller gain is wrong or if the mechanism has backlash. Industrial servo drives expose tuning parameters; hobby servos hide them, so mechanical load, supply stiffness, and linkage geometry matter even more.
Common Mistakes
- Powering a servo from a microcontroller regulator with no current margin.
- Treating pulse width limits as universal.
- Ignoring stall torque and startup current.
- Using loose linkages that create backlash and hunting.
- Forgetting common ground between controller and servo signal.
Summary
Servo motors combine actuation and feedback. Choose them by torque, speed, travel, feedback quality, duty cycle, and power supply requirements. Verify motion under real load, not only on the bench with no linkage.
Further Reading
- Pololu, hobby servo control and power guides.
- Kollmorgen, servo motor sizing basics.
- Texas Instruments, motor-control feedback application notes.