When tracking the turning of shafts, it is usually necessary to generate digital position and motion information. The most popular way of doing this is by using rotary encoders. They may be incremental or absolute, optical or magnetic, but they are used extensively in industrial and commercial designs in myriad applications. You will find rotary encoders being used on motors paired with automated machinery and drivers for nearly everything from robotics, position control and conveyor speed monitoring on automated industrial machines, elevators and consumer electronics.
Incremental encoders, mostly used for industrial applications, output the shaft’s relative position compared to a reference. In contrast, absolute encoders provide a different binary output for each position that defines a shaft’s position absolutely. Where incremental encoders define resolution as counts per turn, absolute single turn encoders define it as positions per turn, and express it as a multi-bit word. There are multi-turn encoders that track over multiple 360-degree turns and they specify resolution as positions per input-shaft turn along with the number of internal gear ratio turns.
Rotary optical encoders are the most widespread designs used. They typically consist of an LED light source, light detector, a code disk and a signal processor. Although the precision of the mechanical pattern on the code disk defines the measurement precision, there are other factors as well. For example, a quadrature encoder has several opaque regions that produce four distinct reference points. Two of these points correspond to the leading and trailing edges of the region itself. Another two additional points correspond to the leading and trailing edges of a second detector. Apart from providing higher resolution – four times of the code disc – it also indicates direction of turn depending on which detector responds first.
Incremental encoders are named after their outputs, which consist of two square waves, each corresponding to one increment of rotation. A convex lens focuses the light rays from the LED into a parallel beam. This passes through grid diaphragm, whose sole purpose is to produce a second beam of light 90-degrees out of phase to the original. Light from both channel A & B pass through a rotating disc onto the photodiode or photovoltaic array. The rotating disc creates a light & dark pattern as the clear and opaque segments interrupt the beam.
The absolute encoder has a nearly similar structure, except for multiple detectors and multiple unique tracks on the rotating disc. This produces a Gray Code output, which is a binary numeral system where the successive values differ by one bit. One advantage is this information is available even if the encoder has been temporarily shut down.
Although several methods are used to boost the resolution of direct-read encoders, the electric interpolation method is the most widely used. A voltage divider circuit subdivides the raw analog signal into the number of interpolation steps desired. The interpolation is actually a function integrated into the encoder logic and is transparent to the designer. This method allows for boosting the direct-read encoder resolution by about twenty times.
