Both incremental and absolute rotary encoders are electromechanical devices that provide motion feedback about a rotating shaft.
Rotary encoders sense changes in the position of a rotating shaft, then generate signals that send speed, direction, and position information to a receiving device such as a counter, drive, PLC, etc. Rotary encoders come in two basic varieties: absolute encoders report the actual position of the shaft at a specific time, while incremental encoders indicate relative changes in the shaft’s position.
Depending on the system’s feedback requirements, either type of encoder will work. But when the system powers down, absolute encoders retain their shaft position information and can pick right back up where they left off. Incremental encoders, however, must restart after power-off by establishing a starting point, or index, against which to report changes in position. This little difference becomes very important when operating conditions aren’t ideal.
Both types of encoders are somewhat simple electromechanical devices, and they operate on similar principles. Each time the shaft rotates past a certain position, a sensor in the encoder registers this movement and sends either a digital signal or an electrical pulse through an output channel.
Absolute rotary encoders assign a unique code to each indicated position on the shaft, allowing them to identify the position of the shaft at any given time. For example, an absolute encoder with resolution of 8-bits (1024), will report 1024 unique shaft position values with each rotation. Incremental encoders, on the other hand, produce an output signal, or pulse, each time the shaft passes a specified angle – the number of pulses in a given time span is enough to measure the shaft’s change in position and its speed but does not specify the shaft’s position at any given time.
To identify the shaft’s position, absolute encoders can use magnetic or optical sensors. Either way, the encoder typically includes a magnet or patterned disk mounted to the shaft and a fixed-position sensor. As the shaft rotates, the unique disk pattern is read by the signals fixed sensor. The absolute position is generally represented as a binary word 2n, where n is the number of bits. The higher the number of bits, the greater the resolution, or more precise measurement of the shaft’s position and speed.
Incremental encoders typically have one or two signals, along with an index, or Z index signal. With one signal, the encoder can count the number of rotations the shaft makes over time, providing feedback on the speed of rotation but cannot determine direction. With a second signal, offset in phase electrically of 90 degrees, the encoder can provide information on the direction of shaft rotation. An encoder with two signals that have a phased electrical offset is called a quadrature encoder.
If the system loses power, an absolute encoder can resume functionality immediately upon powerup. An incremental encoder, though, cannot begin relaying usable information until the shaft has made at least one turn. For some applications, this isn’t good enough. For those cases, incremental encoders can include an additional signal, or Z index pulse, which serves as a location index of the shaft’s position as it occurs once during the 360-degree rotation of the shaft.
For some applications, like the precise orientation of radar or other sensing systems, there is no real debate: absolute rotary encoders are a must. For other purposes, including the simple measurement of shaft rotation speed, you may have an honest choice to make. The difference can often be measured in stability and cost.
Absolute encoders are uniquely able to begin operation immediately when a system is powered up. Absolute encoders provide more information beyond speed and direction as they can let an operator know the exact location by supplying the position of the shaft in the 360-degree rotation and, in the case of multi-turn encoders, how many revolutions the shaft has performed. For systems that use incremental encoders to determine location, there is typically additional software and programming needed to be able to calculate the same information that absolute encoders can instantly provide.
That’s just one of the ways that incremental encoders may cost more in the long run than their absolute counterparts. The upfront cost typically favours incremental encoders by a wide margin. Absolute encoders simply contain more components to ensure higher resolution, and this means a higher up-front investment. But maintenance issues and downtime, along with the need to reinitialize incremental encoders each time the system regains power, may lead to performance losses which result in a higher overall cost option.
When evaluating the differences between absolute and incremental encoders, it is crucial to consider the specific applications and operational environments. Absolute encoders provide a precise position reading at all times, eliminating the need for recalibration after power loss. In contrast, incremental encoders, while generally less expensive and simpler in design, may encounter issues with accuracy and reliability, particularly in systems where power interruptions are frequent.
Ultimately, the decision should align with the operational demands, budget constraints, and desired performance characteristics of the project at hand. In applications where continuous operation is paramount, the choice of encoder can significantly impact overall system efficiency. For environments with frequent power outages, investing in absolute encoders may prove to be more cost-effective in the long run.
Additionally, the integration of advanced diagnostic tools can help monitor encoder performance and preventively address maintenance concerns. As technology evolves, the development of hybrid systems combining both types of encoders may also present a viable solution, offering the advantages of both designs. Careful consideration of these factors will ensure optimal performance tailored to the specific needs of each industry.
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