The rotary encoder is a proven and popular solution for measuring the speed, direction of motion or position of a rotating shaft. Several different types are available, the main two being the absolute encoder and the incremental encoder. How do they work? What are their differences? And how do you choose the right type for your application?
Encoder operating principles
As the name suggests, an absolute encoder directly outputs the exact position of the shaft it is measuring. Every point of rotation has a unique position value, or data word, which is encoded on a disc that rotates with the shaft. The number of unique codes on the disc determines the precision with which the position can be expressed. The encoder reads the code using an optical, capacitive or magnetic sensor as soon as it is turned on and generates a valid output. There is also no need to establish a reference or turn the shaft to let the sensor determine the position, and it can keep track of the position even if power is lost temporarily.
Figure 1: The absolute encoder’s disc presents a unique code for each position, which enables the output to be valid immediately and determines the encoder resolution. (Image source: CUI Inc.)
The encoder’s resolution is expressed in terms of bits that correspond to the number of unique data words over one revolution. Absolute encoders are available as single-turn or multi-turn types, with single-turn versions providing position data over one full revolution of 360°, which repeats for every revolution of the shaft. The multi-turn type has a turns counter that enables the encoder to output not only the shaft position but also the number of revolutions.
Consider incremental encoders: these operate by generating pulses as the shaft rotates. The output is usually two square waves that are 90° out of phase and additional circuitry is needed to track or count these pulses.
Figure 2: An incremental encoder generates pulse waveforms 90° out of phase. (Image source: CUI Inc.)
The resolution of an incremental encoder is expressed as the number of pulses per revolution (PPR), which is equivalent to the number of high pulses from either of the square-wave outputs. You can read CUI’s blog post about PPR for more information on this subject.
By examining Figure 2, you can see that there are only four distinct, and repeating, output states. For this reason, an incremental encoder must be referenced to a known fixed location to provide meaningful positioning information. This “home” location is the encoder’s index pulse. The absolute position of the shaft is then calculated by tracking the relative incremental change in rotation from the index pulse. This referencing process is necessary each time the encoder is turned on, or following temporary power loss, so the shaft must be rotated before the position can be known. This process is slower than obtaining position from an absolute encoder which requires no initial rotation.
Absolute vs. incremental: selection criteria
Absolute encoders are more complex than incremental types, and hence, typically, more expensive. Although the price difference is narrowing, an incremental encoder is usually preferred for simple monitoring of speed, direction or relative position. On the other hand, there are some situations where an absolute encoder is a better choice.
A major strength of the absolute encoder is that the position of the shaft is maintained, so that position data is available immediately without waiting for a homing or calibration sequence to be completed. This allows the system to start-up more quickly, or recover from a power failure, even if the shaft position has changed while the encoder is off.
Another reason to choose an absolute encoder is when position information is needed immediately at startup before any mechanisms are activated or moved. This could be a scenario where rotating the shaft in the wrong direction from the start position could damage the equipment or present a danger to the user.
In addition, because an absolute encoder gives the true position in real time, a digital system can poll the encoder via a central communication bus to capture the position with minimal latency. Continuously tracking position is more difficult with an incremental encoder because external circuitry is usually needed to track all the pulses using quadrature decoding, which adds overhead to the host system, especially in cases where multiple encoders must be monitored.
Figure 3: Absolute encoders generate a unique, digital “word”, equivalent to the stated resolution, for each position of the code-wheel. (Image source: CUI Inc.)
A further benefit is that using an absolute encoder helps reduce the system’s susceptibility to electrical noise. Unlike pulse-counting incremental encoders, absolute encoders allow the system to read an error-checked code from a binary output, or digitally over a serial bus, to calculate the position.
Furthermore, combining more than one absolute encoder in the same system is also easier than with incremental encoders. Typical examples include factory automation or multi-axis robots. Monitoring the outputs of multiple incremental encoders can become complicated and require significant processing power, whereas the readings from individual absolute encoders are more easily interpreted, particularly when they can be connected to a central communication bus.
Opportunities for absolute encoders
You should by now have a good understanding of the key differences between absolute and incremental encoders. Let us consider some application areas in which absolute encoders are typically used.
Among these, robotics is a rapidly expanding area that is penetrating numerous sectors from healthcare – such as remote surgery, which relies on large quantities of precise positional information to monitor and control surgical-robotic arms – to industrial use cases such as automated assembly, welding, paint spraying, and more. Looking ahead, the possibilities for home-assistance robots are particularly exciting and will benefit from the speed and ease of use provided by absolute encoders.
As businesses continue to pursue digital transformation, and as the price gap between incremental and absolute encoders becomes less, an almost endless variety of applications are possible for absolute encoders. There are many opportunities in consumer markets as well. Whether they are used to control mechanisms like automated gates, camera gimbals, smart HVAC controls, factory automation or motorized automotive subsystems, absolute encoders represent a high-performing and increasingly budget-friendly option for equipment designers.
The engineer’s choice for position feedback
The differences in performance, price and user experience between absolute and incremental encoders mean it is vital to choose the right type of encoder for any new product design. As the price gap narrows, and technologies continue to change, the advantages absolute encoders offer compared to incremental types could make them the designer’s first choice for position-feedback duties in a growing number of sectors.