Absolute vs incremental encoder – how to choose

What is an encoder?

A rotary encoder sits on a motor shaft, a wheel or a roller and turns the movement into electrical signals. The control system uses them to regulate speed, measure length or position an axis.

There are two main types. An incremental encoder tells you that the shaft has moved and by how much. An absolute encoder tells you where the shaft is. The difference is most obvious when the power goes off.

Incremental or absolute – how they work

Incremental encoder

An incremental encoder gives a set number of pulses per revolution, for example 1,024. The control system counts the pulses from a reference point. Channels A and B are shifted by a quarter period, 90°, from each other. From the order of the channels, the controller can tell the direction of rotation, and by counting every edge it gets four times the resolution: 1,024 pulses become 4,096 steps per revolution. Channel Z, the index pulse, comes once per revolution and serves as a reference.

The drawback is that the count is lost when power is interrupted, and interference can add or drop pulses. The machine must then move to a reference position before it knows where it is.

Absolute encoder

An absolute encoder reads a unique code for every angular position. Resolution is given in bits: 12 bits give 4,096 positions per revolution, 16 bits give 65,536. The position is available immediately at start-up, even if the shaft was turned while the power was off.

A singleturn encoder starts over every revolution and is enough when the movement is one turn or less, such as a valve or a rotary table. A multiturn encoder also counts revolutions, often 4,096 or more. You need it when a spindle, winch drum or rack makes many turns. Revolutions are counted with gears, with a battery or with a so-called Wiegand wire, which takes its energy from the movement itself and needs no battery.

Interfaces: HTL, TTL, SSI, BiSS and fieldbuses

Many modern encoders are programmable, so pulse count, output type and counting direction can be set with a tool. That means fewer variants in your spare parts store.

Shaft or hollow shaft, optical or magnetic

A solid shaft encoder is connected to the machine shaft with a flexible coupling that takes up misalignment. A hollow shaft encoder slides straight onto the motor shaft and is held by a torque arm. A hollow shaft saves space and a coupling, while a solid shaft encoder is easy to replace and protects its bearings from movement in the machine shaft.

Optical encoders read a code disc with light and give very high resolution and accuracy, but are sensitive to condensation, dirt and heavy shock. Magnetic encoders read a magnet with a Hall or magnetoresistive sensor. They cope better with vibration, moisture and dirt and are often the first choice for mobile machines and outdoor use.

Which one should I choose?

1. Incremental where the position must be kept. After a power cut the machine must home again, which is not always possible with a moving load. Choose absolute.
2. The wrong signal level. A 5 V TTL encoder gives no reliable signal on a 24 V input, and an HTL encoder can damage a TTL input.
3. A rigid coupling. Misalignment that a flexible coupling does not absorb wears out the encoder bearings.
4. Long unshielded cable. Interference adds or drops pulses. Use shielded twisted-pair cable and differential signals.
5. Singleturn for several turns. The encoder starts over every revolution and cannot tell the turns apart.

Where are encoders used?

• Factory and packaging – length measurement on conveyors, cutting, labelling and synchronising rollers.
• Cranes and lifting – hoist height on winches, slewing angle and position of overhead cranes.
• Mobile machines – steering angle, boom angle and position of attachments.
• Energy – blade pitch and yaw in wind turbines, position of solar trackers.
• Infrastructure – lock gates, movable bridges, stage machinery and gates.
• Medical technology – positioning of patient tables and X-ray equipment.
• Defence – rotation of antennas, masts and sensor platforms.

Encoders from SCN

We carry incremental and absolute encoders from Posital, with interfaces such as CANopen, EtherCAT and IO-Link, programmable incremental encoders with HTL or TTL output, and versions with IP68 and IP69K for mobile machines.

Examples from our range

Frequently asked questions

What is the difference between an absolute and an incremental encoder?

An incremental encoder gives pulses counted from a reference and loses its position when power is lost. An absolute encoder gives a unique code for every position and knows where the shaft is at start-up.

What does PPR mean?

PPR stands for pulses per revolution. With quadrature (4x) decoding of A and B, 1,000 PPR gives 4,000 counts per revolution.

What is the difference between HTL and TTL?

HTL works at the supply voltage, usually 10–30 V, and suits PLC inputs. TTL to RS422 is a differential 5 V signal that suits drives and high frequencies.

Does an absolute encoder need a battery?

A singleturn encoder never needs a battery. Multiturn encoders count turns with gears, a battery or a Wiegand wire, and gear and Wiegand designs work without a battery.

What is SSI?

SSI (Synchronous Serial Interface) is a simple point-to-point interface where the controller sends clock pulses and the encoder replies with its position bit by bit. It is common on absolute encoders.