Instrumentation · Lesson 5 of 10

Encoders 2: absolute, mounting and faults

An incremental encoder counts, but if the power goes off it forgets where it is. An absolute encoder knows its position at any moment without counting. Here you will understand the Gray code inside it, the protocols, how to mount an encoder properly, and how to find its faults like a sharp maintenance technician.

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1Incremental or absolute?

	Incremental	Absolute
Outputs	A, B and Z pulses	A number (code) for each position in the turn
After a power cut	Forgets: needs homing	Knows its position straight away
Connection	Counter card / HSC	SSI, BiSS, Profinet, EtherCAT, CANopen…
Price	Cheaper	More expensive
Examples	Motor speed, cut-to-length machine	Hoist, dam gate, robot, press, crane telescope

Single-turn: knows the position within one turn only (4096 positions, for example). After a full turn it comes back to the same number.
Multi-turn: also counts the number of turns (4096 turns, for example) using internal gears or a counter powered by the motion itself (Wiegand), or a battery in some types. This is what goes on hoists and cranes.

Resolution in bits: a 12-bit single-turn encoder = 2¹² = 4096 positions per turn ⇒ each step 360 ÷ 4096 ≈ 0.088°. A 25-bit multi-turn is usually 13 bits for position (8192) + 12 bits for turns (4096 turns).

2Gray code: why only one bit?

An absolute disc has several rings, one bit per ring, with a sensor under each ring. The reading at any position gives a number. The problem is the moment of transition: the sensors never switch at exactly the same instant. In plain binary, going from 7 (0111) to 8 (1000) changes 4 bits, so for a split second you could read 15 or 0 or any number! And the machine goes crazy.

The fix: Gray code is arranged so that between any two neighbouring positions only one bit changes. So the worst that can happen is reading the old or the new position, never a strange number.

Gray = Bin XOR (Bin >> 1)

4-bit absolute disc

Position	Binary	Gray
6	0110	0101
7	0111	0100
8	1000	1100
9	1001	1101

The encoder or the card converts Gray to binary before sending it on, and with SSI you can choose in the parameters to receive it as Gray or binary. Choose wrong and the reading jumps around strangely.

3Protocols and the resolver

Interface	The idea
SSI	The card sends clock pulses and the encoder answers with the bits one after another (RS-422). Simple and common. You must set the number of bits and Gray/binary.
BiSS-C	Like SSI but faster, with a check (CRC) and diagnostics. Common on servos.
Profinet / EtherCAT / EtherNet/IP	The encoder is a device on the network with an address, configured with a GSDML or ESI file. It sends position, speed and diagnostics.
CANopen / Profibus	Older fieldbus networks, common on cranes and mobile machines.
Resolver	Not a digital encoder: a rotating transformer that outputs two signals, sin and cos. No electronics inside, so it withstands severe heat and vibration. Common on older servo motors and in harsh areas.

4Homing with an incremental encoder

With an incremental encoder, the machine does a homing run at start-up: it moves slowly until it hits a reference switch, then carries on to the first Z pulse after it and zeroes the counter there. Why both? The switch is only accurate to millimetres and depends on speed, while Z is very accurate but repeats every turn, so the switch says "we are in this turn" and Z marks the exact point.

5Mechanical mounting

Solid shaft: connected to the machine shaft with a flexible coupling (bellows, jaw or disc). The flexible coupling absorbs misalignment. Never use a rigid coupling: any slight misalignment will break the bearing.
Hollow shaft: slides straight onto the motor shaft and is fixed with a clamping ring, and its body is held by a flexible torque arm (stator coupling) that stops it turning and allows slight runout.
Never hammer the encoder when fitting it: the glass disc inside will break.
Put a pen mark across the shaft and the coupling after tightening: if you later find it has moved, something is slipping.

6Cable, shield and noise

A shielded cable with twisted pairs (A with A̅, B with B̅). For fast encoder signals the shield is usually grounded at both ends with a 360° clamp on the connector body and the panel, together with a heavy equipotential bonding conductor between motor and panel (follow the maker's recommendation: this differs from the 4–20 mA rule).
Keep it away from motor and VFD cables: a separate tray or a metal divider.
Cable length: single-ended TTL (5V) manages only a few metres, an RS-422 line driver up to about 100 m, and HTL also covers long distances but the frequency drops with length. The datasheet gives a length × frequency table.
Voltage drop: a 5V encoder on a long cable may receive only 4.5V. Some encoders have sense lines for the supply.

7Fault table

Symptom	Likely cause	What to do
Extra counts when the VFD runs	Noise, shield, cables run together	Ground the shield, separate the cables, line driver
Counts the wrong way	A and B swapped	Swap them or reverse with the parameter
Half the count / no direction	A channel is open	Measure each channel with a scope or multimeter
Lost at high speed only	Frequency above the card or encoder limit	Calculate f, faster card, less filtering
Slow position drift	Coupling or measuring wheel slipping	Pen mark, tighten, replace the wheel
Noise and a failed bearing	Rigid mounting / misalignment	Flexible coupling, torque arm
The absolute reading jumps around	Wrong Gray/binary, wrong SSI bit count	Check the parameters against the datasheet

Game: find the encoder fault

Safety: the encoder is usually on a running motor. Before undoing the coupling or touching the shaft: lock out the motor and make sure it cannot turn. On a hoist, a suspended load can turn the motor even when it is switched off: secure the load first.

?Lesson quiz

Get at least 5 of 6 right to mark the lesson as done.

Shield grounding and cable length rules vary between makers; follow the installation manual of the encoder and the drive. Your progress is saved on this device only.
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