Encoders, Resolvers and Feedback

Why feedback matters

A feedback device tells the drive and controller where the motor really is and how fast it is turning. Without it the controller can only assume. With it the drive can commutate a brushless motor correctly, close the speed and position loops (Servo Systems and Tuning), detect a lost step or a stall, and report the true position to the machine. The quality of the feedback (resolution, accuracy, noise, delay) sets a hard limit on the quality of the motion, so it deserves as much attention as the motor.

The feedback devices

Device	Output	Strengths	Limits
Incremental encoder (optical or magnetic)	Pulses in quadrature (A, B), index Z	Cheap, simple, high resolution	Position lost at power-off; needs homing
Absolute encoder (single-turn)	Position within one revolution, serial or parallel	Knows position at power-up within a turn	More costly; serial protocol to match
Absolute encoder (multi-turn)	Turn count plus position	No homing at all after a power loss	Battery or gear/Wiegand-wire counting; the most costly
Resolver	Analog sine and cosine from a rotating transformer	Very rugged, high temperature, vibration-proof	Lower resolution, needs a converter
Hall sensors	Three digital signals, six states per electrical cycle	Cheap commutation for BLDC motors	Coarse: not for position control
Tachometer	A voltage proportional to speed	Simple speed feedback	Largely replaced by encoders
Linear scale (glass, magnetic)	Pulses or serial, along the axis	Measures the load directly, ignoring screw errors and backlash	Costly, needs protection from dirt and coolant
LVDT, potentiometer, magnetostrictive	Analog voltage or digital	Simple absolute position over a short stroke	Limited accuracy or resolution (see Sensors)

Incremental encoders and quadrature

An incremental encoder has a disc with a pattern of lines, read by two sensors placed a quarter of a line apart. As the disc turns they produce two square waves, A and B, 90° out of phase (quadrature), plus an index pulse once per revolution. The controller counts the edges: the order of A and B shows the direction, and counting all four edges per line (4× decoding) quadruples the resolution.

Absolute encoders and Gray code

An absolute encoder reports the position directly: every shaft angle has a unique code, so there is nothing to home. A disc with several tracks is read in parallel; the patterns are in Gray code so that only one bit changes between neighboring positions, which prevents a large error if the read head straddles two sectors. Modern encoders convert it to a serial protocol (SSI, BiSS, EnDat, Hiperface, or an industrial fieldbus such as EtherCAT or PROFINET).

Resolution, accuracy and frequency

Term	Meaning	Typical figure
Resolution	The smallest change the device can report (counts per turn or per mm)	Thousands to millions of counts per turn
Accuracy	How far the reading may be from the true position	Rotary: ±20 to ±120 arc-seconds; linear scale: ±1 to ±5 µm per meter
Repeatability	How closely the same position is reported again	Much better than the accuracy
Maximum frequency or speed	The highest signal frequency the device or interface can handle	100 kHz to several MHz; check the cable length
Latency	The delay between the shaft position and the value the drive sees	Microseconds; matters for the velocity loop bandwidth

A linear scale with a 1 µm resolution gives 1 000 counts per millimeter. Many “high-resolution” encoders are limited by the signal noise, not the number of bits: the last two or three bits are often noise.

Resolvers, Hall sensors and tachometers

• A resolver is a rotating transformer: an excitation signal of a few kHz on the rotor winding induces sine and cosine signals in two stator windings whose amplitude depends on the shaft angle. It has no electronics or optics on the shaft, so it survives shock, heat (150 °C and more) and oil. A resolver-to-digital converter in the drive gives 12 to 16 bits. It is the choice for harsh environments.
• Hall sensors in a brushless motor give three digital signals that change every 60° electrical, enough to commutate the motor but not to control position. Many servo motors have both Halls and an encoder.
• A tachometer (a small DC generator) gives a voltage proportional to speed. It was the standard speed feedback; encoders now do the job and also give position.

Signal types, cables and wiring

Signal	Levels	Notes
TTL (5 V, single-ended)	0 to 5 V	Short cables only; picks up noise
HTL (10 to 30 V)	24 V push-pull	Industrial, better noise margin, still single-ended
Differential line driver (RS-422)	A and /A, B and /B, Z and /Z	Preferred: noise cancels, long cables (up to 100 m)
Sin/cos 1 V pk-pk	Analog differential	High resolution through interpolation; sensitive to noise
Serial (SSI, BiSS, EnDat)	Clock and data, differential	Absolute position by a protocol; match the protocol and clock
Open collector (NPN or PNP)	Switches to 0 V or the supply	Needs a pull-up or pull-down to the right level

• Use twisted-pair shielded cable, with the shield bonded at the drive end (and at the encoder if specified). Route it away from motor cables, which carry the high dv/dt (see Cabling, Grounding and Noise).
• Terminate differential signals with the right resistor at the receiver (about 120 Ω) and check that the receiver matches the line driver.
• Keep the supply voltage right at the encoder end. The cable drop in the supply wires can bring a 5 V encoder below its minimum on a long run: use the sense wires or a higher supply voltage (the encoder accepts 5 to 30 V in many cases).
• Check the pin-out twice: reversed A and B reverses the direction, and swapped phases in a servo give a runaway.

Homing and reference

After a power-up an incremental system does not know where it is. Homing moves the axis to a known reference and sets the position counter. Common methods: seek a home switch and then the next index pulse (the best repeatability), drive into a hard stop and detect the torque rise, or use a limit switch. An absolute (multi-turn) encoder holds its position through a power loss and needs a one-time reference only. The home switch must be a reliable sensor (see Sensors and Field Devices) with a repeatable edge, and the approach direction and speed should be fixed.

Motor feedback or load feedback

With the encoder on the motor shaft, the loop controls the motor position and the screw error, backlash and belt stretch between the motor and the load are outside it. A linear scale on the load puts them inside the loop, giving true position at a cost: the loop includes the mechanical compliance, which is harder to tune. Dual-loop control uses the motor encoder for the fast velocity loop and the load scale for the position loop, which gives the best of both.

Troubleshooting feedback problems

Symptom	Likely cause	Check
Position error that grows over time	Lost counts: noise, an input too slow, a slipping coupling	Count rate against the counter limit, shielding, coupling set screws
Runaway or fault on enabling	Reversed feedback direction or wrong commutation offset	Swap A and B or invert the direction; repeat the commutation set-up
Axis position differs after each homing	Index not used, a worn or loose home switch	Home on the index and fix the switch
Occasional jumps in position	Noise on the cable or a poor ground	Use differential signals, bond the shield, separate from motor cables
Encoder fault or no signal	Supply low, a broken wire, a failed encoder	Measure the supply at the encoder; check the signals with a scope
Whine on a servo	Noisy or quantized velocity feedback	Raise the resolution, add a filter, check shielding (see Servo Systems and Tuning)
Absolute position wrong after a power loss	Multi-turn battery flat or a counter fault	Replace the battery and re-reference the axis

Related

• Motion Control and Actuators: the overview of this guide.
• Servo Systems and Tuning: how the loops use feedback.
• Mechanical Transmission: Screws, Belts and Gearboxes: backlash, windup and where the encoder is placed.
• Sensors and Field Devices: limit switches, proximity sensors for homing.
• Serial Communication: the electrical standards behind encoder serial links.

FAQ

How many pulses per revolution do I need?

Divide the smallest move you need by the travel per revolution to get counts per turn, and aim for several counts per smallest move. Then check that the highest speed gives a signal frequency within the counter’s limit.

Incremental or absolute?

Incremental is cheaper and perfectly adequate if the machine can home at start-up. Choose absolute for axes that cannot easily home (a robot joint, a long travel), or when a power loss must not lose the position.

What is the difference between resolution and accuracy?

Resolution is the smallest step the device reports; accuracy is how close the reading is to the true position. A high-resolution encoder can still be inaccurate over a turn because of disc and mounting errors.

Why does my encoder count wrongly only at high speed?

Most often the counter input or the cable bandwidth cannot follow the frequency, or noise adds false edges. Check the frequency against the input rating and use differential signals.

References

• Heidenhain, Rotary encoders and Linear encoders technical information; Renishaw and Sick encoder application guides.
• Analog Devices, Resolver-to-digital converter AD2S1210 data sheet and application notes.
• BiSS Interface and SSI specifications; EnDat 2.2 specification (Heidenhain).

Last reviewed: October 2026. Resolutions and accuracies are typical; use the manufacturer’s data for the exact device.