         |
An encoder with one set of pulses wouldn't be useful because it couldn't indicate the direction of rotation. Most incremental encoders have a second set of pulses that is offset (or out-of-phase) from the first set of pulses, and a single pulse which indicates each time the encoder wheel has completed one revolution. Diagram 1 (below) illustrates an example of the two sets of pulses that are offset. Since the two sets of pulses are out of phase from each other, it's possible to determine which direction the shaft is rotating by the amount of phase shift between the first set and second set of pulses. The first set of pulses are called the A pulses, and the second set of pulses are called the B pulses. A third light source is used to detect a single pulse that appears once per revolution. This pulse is called the command pulse, which is used to count revolutions of the shaft where the encoder is connected.
Above: Diagram. 1: Examples of the A pulse, B pulse, and the command
pulse. If the A pulse occurs before the B pulse, the shaft is turning clockwise,
and if the B pulse occurs before the A pulse, the shaft is turning counterclockwise.
The C pulse occurs once per revolution.
Since the incremental encoder only provides a string of pulses, a home switch must be used with this type of encoder to ensure that the encoder is calibrated to the actual location of the home reference point. Earlier, metal-made encoder wheels were not very useful since more resolution was needed. Modern encoder wheels are made from clear glass which has opaque segments etched in them like bars. As the encoder wheel spins, the opaque segments block the light and where the glass is clear, light is allowed to pass. This provides a pulse train similar to the encoder wheel that has holes drilled in it. Typical glass encoders have from 100-6000 segments. Hence, these encoders can provide 3.6° of resolution for the encoder with 100 segments, and 0.06° of resolution for the encoder with 6000 segments. If the shaft of the encoder is connected to a drive shaft for a motor that is connected to a ball screw or a reduction gear, the number of degrees of resolution can be converted into linear position.
It's not possible to drill hundreds of holes in the encoder wheel to get the higher amounts of resolution since the wheel would not have enough material remaining to give the wheel strength. Therefore, modern encoder wheels with high resolution use etched glass wheels. The glass is etched with chemicals to produce alternating opaque segments.
The second pulse train is developed in this type of encoder by placing a second light source and second light receiver at a different angle from the first set. Due to the fact that the location of the second light source is different from the first, the second pulse train will be shifted from the first just as if two separate sets of holes were drilled. This arrangement allows the encoder wheel to provide both incremental and direction of rotation information with only one set of opaque bars etched in the glass. The second pulse train is used to determine the direction of rotation for the encoder wheel.
Diagram 2 illustrates an example of the etched glass encoder and a diagram of the light source and receiver. Notice that the glass encoder looks as if it has very thin black lines drawn on it. The black lines are the opaque segments that block light. The diagram shows only one light source and receiver. A second identical light source and receiver is mounted on the encoder in such a way that it produces the offset pulse train.
Above: Diagram 2: An etched-glass incremental encoder wheel.
