Servo drives: how they differ from VFDs

What makes a servo

Cascaded control loops

position setpoint → [position loop Kv] → speed setpoint → [speed loop Kp, Tn] → current setpoint → [current loop] → motor
                         ↑ position feedback          ↑ speed feedback (encoder)      ↑ current measurement

Inner loops are faster than outer loops. Tuning starts from the inside: current loop (usually fixed by the drive), speed loop (gain and integral time against load inertia), then position loop. Many drives offer one-button or online auto-tuning.

Encoders

Incremental encoders need homing after power-up; absolute encoders (single- or multi-turn) know the position immediately. The servo motor's encoder also gives the drive the rotor position for commutation — an encoder fault stops the axis immediately (see the V90 fault list in the Diagnostic Database).

Following error

The difference between commanded and actual position. A drive monitors it against a tolerance: too small tolerance gives nuisance faults during acceleration, too large hides mechanical problems. A growing following error usually means insufficient torque, wrong tuning, mechanical binding or an encoder problem.

Control interfaces

• Pulse/direction or analog ±10 V from a motion controller (older or simple systems).
• Fieldbus telegrams: PROFIdrive (e.g. telegrams 3, 102, 105, 111 on SINAMICS), CiA 402 over CANopen/EtherCAT, CIP Motion.
• Internal positioner (basic positioner / indexing): the PLC only selects a target or a motion block.