Manufacturers’ needs with respect to encoders are diverse. Measurement accuracy, precision, resolution, size, durability and cost are all factors that affect the purchasing decision. Taking time to select the appropriate encoder solution for the system’s intended purpose produces the best results. The goal of this technical brief is to summarize the types of encoders available and basic pros and cons of each. There are two main categories of encoders: linear (responds to motion along a path) and rotary (responds to rotational motion). This brief will focus on rotary encoders.
ABSOLUTE VS. INCREMENTAL
All encoders, rotary and linear, can be divided into two subcategories: absolute and incremental. Incremental encoders work by generating pulses as a shaft rotates. These pulses must then be counted by an external counting mechanism. Most incremental encoders use quadrature encoding, which produces two channels of pulses that are offset, or out-of-phase, by 90 degrees, which then allow the encoder to determine not only position but also direction.
Conversely, an absolute encoder uses an optical, magnetic, or capacitive sensor to read a unique code off a disc that rotates with the shaft. This code translates to the shaft’s specific position. Resolutions with absolute encoders are represented in terms of bits (binary digits) that correspond to the number of unique data words over one revolution, while with incremental encoders, resolutions are represented by the number of pulses per revolution (PPR) and represent the number of high pulses that an incremental encoder detects from either of its square wave outputs.
The primary difference between incremental and absolute encoders is that an incremental encoder must go through some type of calibration step upon wakeup to know its position, whereas an absolute encoder knows exactly where it is upon wakeup without need for a reference point. Some manufacturers are now marketing “pseudo-absolute” encoders; however, these are probably more accurately described as incremental encoders because they still must go through some routine on startup (albeit limited), in order to determine absolute angle.
To learn about the pros and cons of Absolute vs Incremental encoders and about Optical vs. Magnetic encoders, click here.
This white paper was contributed by Kyle Hanley, Product Solution Specialist, Kaman Automation.
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