What the drive regulates when the load changes

A servo motor produces motion; its drive decides how much current to apply. In a closed-loop servo system, feedback lets the controller compare the requested motion with the actual response. Velocity mode holds a speed reference by varying torque. Torque mode regulates turning effort and allows speed to respond to the load. Choosing the mode determines what the machine tries to keep constant.

Feedback and the three control loops

An encoder or resolver reports rotor position, from which the drive derives velocity. An absolute encoder can also retain usable position information in a compatible system. Separate current sensors measure winding current. Position feedback does not directly measure shaft torque. Most drives estimate electromagnetic torque from current and motor data; applications needing accurate load torque measurement may require a torque transducer.

The inner current loop regulates torque-producing current. The velocity loop converts speed error into a torque request. If position is controlled, the outer position loop generates the required velocity command. These cascaded control loops may be divided between the drive and a motion controller, depending on the architecture.

A practical example of velocity control

Consider a feed roller commanded to run at 1,500 rpm. When material drag increases, actual speed starts to fall. The drive responds with more current and more torque to bring speed back towards the reference. With less drag, less torque is needed. This explains the distinction between speed and torque control: a steady speed can require continually changing torque.

If the required effort reaches the configured torque limit, the roller may slow down because the drive cannot supply any more. Also, running at a chosen speed for a chosen time does not guarantee an exact stopping point. Use position control when accurate travel and final location matter.

What a torque command actually means

In servo torque mode, the drive regulates current to produce the requested moment. For many permanent-magnet AC servos within the applicable operating range, T ≈ Kt × Iq, where Kt is the torque constant and Iq is the torque-producing current component. Current conventions matter: RMS and peak values must match the manufacturer's definitions. Supply-side current cannot simply be substituted into this relationship.

A lightly loaded shaft can accelerate under a constant torque command. Configure the drive's supported torque-mode velocity limit and check its overspeed response. Reaching a limit may reduce the torque request or produce a fault, depending on the drive. Winding applications introduce another variable: approximately T = F × r. As roll radius increases, maintaining the same web tension requires more torque; constant torque alone will not maintain constant tension.

Stay inside the motor and drive capability

Continuous torque is sustainable under the specified mounting and cooling conditions. Peak torque is available only for the permitted duration. Check the torque-speed envelope for the actual motor, drive and supply combination. The relationship between torque and power is P = T × ω: at standstill, a servo can produce torque while delivering zero mechanical output power, yet still generate winding heat.

Load and transmission calculations must include friction, acceleration and reflected inertia. A planetary gearbox changes the speed-to-torque relationship and the inertia seen by the motor. An S-curve motion profile softens abrupt acceleration changes and can reduce mechanical shock. Deceleration also returns energy to the drive; check its regenerative capacity and any required braking hardware.

Command interfaces and commissioning

Depending on the drive, commands may arrive as analog references, pulse-and-direction signals or network data. Pulse-and-direction usually represents a position command; the PLC pulse output frequency determines the commanded travel rate through the electronic gearing settings. EtherCAT servo drives and CANopen devices offer modes according to their supported profiles. Feedback wiring must also suit the encoder signal interface accepted by the drive.

Automatic tuning can provide initial settings suited to the connected load. Excessive control-loop gains can cause oscillation and overshoot; conservative settings can leave the axis sluggish. Every loop does not need a full PID controller: PI velocity control is common. Validate the result under realistic loads by recording commanded speed, measured speed, motor current and following error.

Technical references: Kollmorgen: how servo drives work · PMD: control loops and tuning · Kollmorgen: limiting speed in torque mode