How servo systems work
A servo system moves an axis to an exact position at an exact speed, again and again: cutting material to length, placing parts, driving robot joints. Its secret is a closed loop: it constantly measures where the motor really is and corrects any difference.
The three parts
| Part | What it does |
|---|---|
| Servo motor | usually a permanent-magnet synchronous motor: compact, fast, high torque at every speed |
| Encoder | mounted on the motor shaft, reports the exact position thousands of times per second (often 20-bit or more: over a million counts per revolution). Many are absolute multi-turn, so no homing is needed after power-off |
| Servo amplifier (drive) | powers the motor and runs the control loops, using the encoder feedback |
Three control loops inside the amplifier
- Current (torque) loop: the fastest, tens of kHz. Controls the motor current, which is the torque.
- Velocity loop: compares the commanded speed with the encoder speed and asks the current loop for more or less torque.
- Position loop: compares the target position with the actual position. The difference is the following error; the loop turns it into a speed command.
Tuning
The loop gains decide how hard the amplifier reacts. Too low and the axis is sluggish with a large following error; too high and it overshoots, oscillates and buzzes. The most important setting is the load inertia ratio: how heavy the load is compared with the motor. Most modern amplifiers have auto-tuning that measures it while the axis moves.
Important terms
- In position: the following error is inside a small window (e.g. ±0.01 mm). The PLC waits for this signal before the next step.
- Following error limit: if the error gets too big (crash, jam), the amplifier stops with a fault.
- Homing: finding the zero position with a home sensor or the encoder's index pulse (not needed with absolute encoders).
- Software and hardware limits: stop the axis before it hits the mechanical end.
- Holding brake: needed on vertical axes so the load does not fall when the power is off.
- Electronic gear: converts motor revolutions into machine units (mm, degrees).
Servo vs stepper vs VFD
| Servo | Stepper | VFD + induction motor | |
|---|---|---|---|
| Feedback | always (encoder) | usually none | optional |
| Positioning | very precise | good at low speed, can lose steps | basic |
| Speed range | wide, high speed | torque drops at high speed | wide |
| Cost | highest | low | low to medium |
| Typical use | pick and place, axes, robots | small positioning | conveyors, pumps, fans |
Next: controlling a servo from a PLC. The PLC Sim Lab Playground has a servo amplifier and a linear axis to try.
Try it yourself. Wire sensors, contactors and a PLC, write ladder logic and press RUN in the free PLC Sim Lab Playground. Nothing to install.
Open the Playground →Guided lessons →