Electronics 115: Servo Motors & Closed-Loop Control
A practical engineering guide to servo motors and closed-loop systems: how servos differ from steppers and DC motors, how feedback works, and how to design precise, stable motion systems without oscillation, jitter, or burned gearboxes.
1) What is a servo motor?
A servo motor is a motor system designed to move to and hold a commanded position. Unlike steppers, servos actively correct their position using feedback.
2) Open-loop vs closed-loop control
| Feature | Open-Loop (Stepper) | Closed-Loop (Servo) |
|---|---|---|
| Position feedback | No | Yes |
| Error correction | No | Automatic |
| Missed steps | Possible | Corrected |
| Complexity | Lower | Higher |
3) Types of servo motors
| Type | Description | Typical Use |
|---|---|---|
| RC Servo | Integrated motor + control | Robotics, RC, hobby projects |
| Industrial Servo | Motor + external drive | CNC, automation |
| Smart Servo | Digital comms & feedback | Robotics arms |
4) Feedback devices (encoders & potentiometers)
Feedback tells the controller the motor’s actual position.
- Potentiometers: Cheap, limited rotation, wear over time
- Optical encoders: High resolution, durable
- Magnetic encoders: Compact, robust
5) Servo control signals (PWM)
Most RC servos use a PWM-style control signal.
- Pulse width: ~1.0ms to 2.0ms
- Frame rate: ~50Hz
- Pulse width encodes position
6) What happens inside a servo
Internally, a servo continuously:
- Reads desired position
- Measures actual position
- Computes error
- Drives motor to reduce error
7) PID control basics (intuitive)
Most servo systems use PID control:
- P (Proportional): Push harder when error is larger
- I (Integral): Correct steady-state error
- D (Derivative): Damp oscillation
8) Torque, speed & holding force
Servos actively apply torque to maintain position.
- Rated torque defines maximum load
- Stall torque causes high current draw
- Continuous stall overheats servos
9) Power supply & grounding rules
Servos are electrically noisy and current-hungry.
- Never power servos from MCU 5V pin
- Use separate supply or high-current regulator
- Common ground between MCU and servo
- Add bulk capacitors near servo supply
10) Servo selection guide
- Small robotics: Metal gear RC servo
- Precision positioning: Encoder-based servo
- High load: Industrial servo + drive
- Smart robotics: Digital bus servos
11) Common servo problems
- Jitter at rest ? noisy power or bad PID tuning
- Buzzing under load ? near stall condition
- Overheating ? excessive torque demand
- Limited travel ? mechanical stops or signal limits
12) Practical applications
Robotic arm joint
- High torque servo
- Short duty cycle near stall
- Mechanical leverage optimization
Camera pan/tilt
- Smooth motion
- Low jitter
- Quiet operation
