The feedback behind precise motion

A servo motor works with a suitable drive and feedback system to deliver controlled position, speed or torque. The defining feature of servo systems is the continuous comparison between commanded and measured motion. The motor produces mechanical power; the control calculations and power electronics usually reside in a separate drive. Integrated designs can combine these components in one assembly.

What do the motor, drive and feedback device do?

The basic components of a servo system perform complementary tasks. The motor contains stator windings, a rotor, shaft and bearings. Common industrial AC servo motors use a permanent magnet rotor. Controlled winding currents create a magnetic field that applies torque to the rotor. A rotary motor transfers motion through a coupling, gearbox or screw mechanism; a linear servo motor produces linear force directly.

The servo drive generates the required phase currents, processes feedback and runs the control loops. A higher level controller can define the motion target, but a separate PLC is not mandatory for every servo axis. Some drives provide internal positioning tables or built-in motion functions.

An encoder or resolver reports motor position so that the drive can monitor actual motion. Feedback devices differ in resolution, signal format and whether they provide absolute position information. Matching a motor to a drive requires more than checking power ratings: the feedback interface and motor parameters must also be compatible.

How does a command become actual motion?

In a typical position-controlled servo, the position loop is the outer loop, the velocity loop sits inside it and the current loop is innermost. Position control uses the difference between target and measured position to generate a corrective motion demand. Speed and torque control regulate the speed and motor torque required for that motion. In torque mode, the position loop may not be active in the same way; the control structure depends on the selected operating mode.

Depending on the model, a PLC or motion controller can send commands through Pulse/Direction, an analog reference or a network such as EtherCAT. With pulse inputs, scaling defines the number of command pulses per motor revolution. The drive's PWM switching of the motor windings belongs to the power stage and serves a different purpose.

Application example: For a directly coupled screw with a 5 mm lead, moving a slide 20 mm requires four motor revolutions. If electronic scaling is set to 10,000 command pulses per revolution, the move requires 40,000 pulses. At a motor speed of 600 rpm, the theoretical travel speed is 50 mm/s. This calculation excludes slip, mechanical errors and acceleration or deceleration time. The command pulse count does not have to equal the encoder resolution.

What does high resolution actually guarantee?

Resolution describes the smallest distinguishable measurement increment; accuracy describes closeness to the true target; repeatability describes the spread when returning to the same target. A high bit count does not automatically remove screw errors, a loose coupling or gearbox backlash. Feedback measured at the motor shaft cannot reveal every load-side error. A suitable control architecture can use a linear encoder on the load as a second feedback device. The factors affecting servo precision must be assessed across the complete machine.

Select the motor using more than its power rating

Servo motor selection considers continuous torque, short-duration peak torque, speed range, cycle time, load inertia reflected to the motor and the transmission. Peak torque is not a continuous rating. Frequent starts and stops also require assessment of thermal loading and braking energy. Dynamic performance depends on correct sizing and tuning rather than simply fitting a larger motor.

Power and torque example: For a motor rated at 0.75 kW and 3,000 rpm, torque at that rated operating point is approximately 2.39 Nm, calculated using T = 9550 × P / n. P is in kW and n is in rpm. This calculation does not determine standstill torque or permissible peak torque; check those values against the manufacturer's torque–speed curve.

Which commissioning details affect the result?

Start by checking motor–drive compatibility, wiring, direction of travel, homing and mechanical limits. Set speed, acceleration and torque limits to suit the load. Automatic tuning is useful, but it cannot repair mechanical backlash or loose connections. Excessive gain can cause vibration, noise and heating. Begin test moves at low speed with a restricted travel distance.

Power and feedback cable routing, shielding and protective earthing affect measurement reliability. Select an EMC filter in accordance with the drive manufacturer's instructions. Power supplies serving the control electronics or brake must provide the required voltage and current. Signal noise and filtering problems can produce symptoms similar to mechanical vibration; investigate the conditions that trigger an alarm instead of only resetting it.

Where is servo motion useful?

Industrial servo applications include CNC axes, packaging feeds, labelling machines, winding systems and robot joints. The deciding requirements are positioning time, changing loads, synchronization and motion quality. A simple mechanism running at constant speed may not need a servo. Brushed DC, brushless synchronous and linear solutions should each be evaluated against the application's requirements.