Absolute Encoders

A major disadvantage of incremental encoders are the number of pulses, which are counted, are stored in a buffer or external counter. In the event of power failure, the count will be lost. hence, if the equipment with an encoder has its electricity turned off every night or for maintenance, the encoder won't know its exact position when power is restored. The encoder must utilize a home-detection switch to indicate the correct machine position. The incremental encoder uses a homing routine that forces the motor to move until a home limit switch is activated. When the home limit switch is activated, the buffer or counter is zeroed and the system knows where it's relative to fixed positional points.

The absolute encoder is designed specifically to address this issue. It's designed in such a way that the machine will always know its location. An example of an absolute encoder is shown in Diagram 1. Notice that this type of encoder has alternating opaque and transparent segments like the incremental encoder, but the absolute encoder uses multiple groups of segments that form concentric circles on the encoder wheel like a "bull's eye" on a target or dart board.

Above: Diagram 1: An absolute encoder wheel showing concentric-circle patterns. This figure also shows the location of 16 light sources and 16 light receivers that decode the pattern of light as it passes through the 16 concentric circle patterns.

The concentric circles start in the middle of the encoder wheel and as the rings go out toward the outside of the ring they each have double the number of segments than the previous inner ring. The first ring, which is the innermost ring, has one transparent and one opaque segment. The second ring out from the middle has two transparent and two opaque segments, and the third ring has four of each segment. If the encoder has ten rings, its outermost ring will have 512 segments. If it has 16 rings it will have 32,767 segments.

Because each ring of the absolute encoder has double the number of segments of the prior ring, the values form numbers for a binary counting system. In this type of encoder there'll be a light source and receiver for every ring on the encoder wheel. Hence, the encoder with 10 rings has 10 sets of light sources and receivers, and the encoder with 16 rings has 16 light sources and receivers.

The advantage of the absolute encoder: it can be geared down so that the encoder wheel makes one revolution during the full length of machine travel. If the length of machine travel is 12 inches, and its encoder has 16-bit resolution, the resolution of the machine will be 12/65,536 which is 0.00018 inches. If the travel for the machine is longer, such as 12 feet, a coarse resolver can keep track of each foot of travel, and a second resolver called the fine resolver can keep track of the position within 1 feet. This means the coarse encoder can be geared so that it makes one revolution over the entire 6 ft distance, while the fine encoder is geared so that its entire resolution is spread across 1 foot (12 inches).

Because the absolute encoder produces just one distinct number or bit pattern for each position within its range, it knows where it's at every point between the two ends of its travel, and it doesn't need to be homed to the machine each time its power is turned off and on.