Differences Between Absolute and Incremental Encoders
How Do Incremental and Absolute Encoders Work?
A- Incremental Encoder
Incremental encoders generate a specific number of pulses per inch or millimeter of linear movement.
An incremental encoder has two outputs, commonly called A and B, which produce two signals, theoretically square waves, 90 degrees out of phase when movement occurs. Industrial encoders often have a third output called index (Z), which is used to indicate that one complete revolution has been made and to verify that the encoder is counting properly.
This output allows us to monitor processes that may reverse direction or that must maintain a clear position when stopped or oscillating mechanically.
The bidirectional square-wave output is recommended for most position, speed, and distance applications.
An incremental encoder output only shows movement. To determine position, its pulses must be counted by a controller or counter. Pulse counting occurs continuously, while the Z signal provides one reference pulse per shaft revolution. Counting may be affected by a power outage or electrical noise. When the equipment starts, it must usually be moved to a reference or home position to re-establish the correct position count.
Advantages of Incremental Encoders
Some of the advantages of using incremental encoders are the following:
- They are inherently digital, so they can be easily connected to modern devices.
- They are inexpensive and easy to use.
- They can provide good resolution in relation to their cost.
- Their differential digital output makes them more immune to electrical noise than analog devices.
Disadvantages of Incremental Encoders
Some of the disadvantages of using incremental encoders are the following:
- An incremental encoder without commutation signals provides speed and position feedback, but not commutation feedback.
- If the unit’s power supply is interrupted, the position is lost.
- Compared with other feedback technologies, the working temperature range may be lower because the electronics are built into the encoder.
B- Absolute Encoder
An absolute encoder generates digital messages that indicate the encoder’s current position, as well as its speed and direction of movement. If power is lost, the position information is recovered as soon as power is restored. Unlike an incremental encoder, it is not necessary to return to a reference position.
An absolute encoder’s resolution is defined by the number of bits in each output message. This output can be in binary code or Gray code, which changes only one bit at a time to minimize errors.
It emits a unique digital code for each shaft angle.
An absolute Gray code encoder may use multiple tracks. In this type of encoder, the housing contains the sensing system, the code disc, and the power supply connections.
The resolution of a sin/cos encoder depends on the frequency of the sinusoidal signal and the encoder’s peak-to-peak differential voltage.
Just like conventional rotary sin/cos encoders, linear sin/cos encoders are also available. The linear scale is fixed to the machine structure, while the sensor head is mounted on the moving unit.
Advantages of Absolute Encoders
Some advantages of absolute encoders are the following:
- High resolution can be achieved with interpolation inside the unit.
- They can also provide incremental signals.
- They retain position information even after a power cycle.
Disadvantages of Absolute Encoders
Some disadvantages of absolute encoders are the following:
- They are usually more expensive than incremental encoders.
- Installation and integration can be more complex depending on the interface.
- Sin/cos feedback signals can be sensitive to electrical noise due to their analog nature.
