How feedback turns a motion command into controlled movement

Feedback gives a servo system a way to compare commanded motion with measured motion. An encoder or resolver reports position to the drive, which adjusts its commands to reduce the error. The measurement may refer to the motor shaft or to the load itself. Velocity is commonly derived from position changes over time or supplied by the feedback-processing circuitry.

What the controller does with the measurement

In position control, the difference between the target and measured position is following error. A 90° target and an 88° measurement give a +2° error. A cascaded controller can turn that error into a velocity request and then a torque-producing current request, updating the calculation as the motor moves. Position, velocity and torque loops handle different parts of this process. The encoder or resolver does not directly measure motor current. The drive's current sensors provide separate feedback for speed and torque control. Rotor angle also supports commutation: applying phase currents in the correct relationship to a brushless motor's rotor.

An encoder converts position into electrical information

An encoder measures rotary or linear position. Optical and magnetic designs use different sensing principles, while the output can take the form of digital pulses, analog sine/cosine signals or a serial position value. Sensing technology, interface and absolute versus incremental measurement are separate choices. A magnetic encoder, for example, is not necessarily an absolute encoder.

Reading incremental A/B signals

In a common quadrature encoder, A and B are displaced by one quarter of a signal cycle. The leading channel identifies direction; counted transitions indicate movement. An encoder with 2,000 cycles per revolution yields 8,000 counts with x4 decoding, so a 90° move represents 2,000 counts. Check the manufacturer's PPR/CPR definitions. A Z index typically marks one location per revolution, rather than defining the machine datum by itself. Match the output interface and line-driver signals to the receiver.

Absolute feedback and position after power-up

An absolute encoder reports a unique position value within its measurement range. A single-turn device identifies angle within one revolution; a multi-turn device also identifies turns within its specified range. How turn information survives or updates during a power outage depends on the design. Saving an incremental count cannot automatically account for unpowered movement. Absolute feedback simplifies startup, but datum definition, offsets or a homing procedure may still be required by the machine.

A resolver carries angle in two analog channels

A resolver uses magnetic coupling to measure angular position. The drive supplies an AC excitation signal, and two output amplitudes vary with the sine and cosine of rotor angle. Electronics demodulate the carrier and recover angle, commonly using a tracking converter. In a simplified normalized example, both components are about +0.707 at 45°. At 135°, sine remains +0.707 while cosine becomes −0.707. Using both channels distinguishes positions that one channel alone cannot identify.

A single-speed resolver identifies angle within one mechanical revolution. Multi-speed designs repeat the signal pattern within that revolution and need additional information to resolve the sector. A resolver alone does not store multi-turn position. Its construction can suit demanding environments, but achievable accuracy and tracking speed depend on the sensor and conversion electronics together.

Measure the load when motor position is not enough

Motor-mounted feedback measures the shaft. The controller can estimate load travel using the transmission ratio and screw lead, but compliance, backlash or slip can make actual load position differ from the estimate. A linear encoder measures linear load position directly. In a supported dual-feedback arrangement, motor feedback can serve commutation and velocity control while load feedback closes the position loop. This requires appropriate drive support and loop tuning.

Resolution describes the steps, not the total error

A 17-bit single-turn measurement provides 131,072 position values, corresponding to a nominal angular increment of about 0.00275°. It does not guarantee that measurement error or machine positioning error is that small. Resolution, accuracy and repeatability describe different properties. Alignment, temperature, sensor errors and processing delay all matter. Electronic gearing changes command scaling; it does not improve the physical accuracy of the feedback device.

Make compatibility part of commissioning

During motor and drive selection, match feedback protocol, supply requirements, pinout and resolution. Resolver compatibility also includes excitation, transformation ratio and pole information. Confirm direction, scaling and commutation offset through the manufacturer's configuration procedure. Good shielding and signal filtering protect measurement quality; auto-tuning cannot fix an incorrectly wired feedback device. Validate actual load travel, following error and feedback diagnostics together.