Linear and Rotary Encoders
An encoder is an electrical mechanical device that can monitor motion or position. A typical encoder uses optical sensors to provide a series of pulses that can be translated into motion, position, or direction. Figure 10-109 shows a diagram and picture of rotary encoders. The diagram in Fig. 10-109b shows that the disk is very thin, and a stationary light-emitting diode (LED) is mounted so that its light will continually be focused through the glass disk. A light-activated transistor is mounted on the other side of the disk so that it can detect the light from the LED. The disk is mounted to the shaft of a motor or other device whose position is being sensed, so that when the shaft turns, the disk turns. When the disk lines up so the light from the LED is focused on the phototransistor, the phototransistor will go into saturation and an electrical square wave pulse will be produced. This figure shows an example of the square wave pulses that are produced by the rotary encoder. This type of disk was used in early applications but the size of the holes in the metal disk limited the amount of accuracy that could be obtained. As more holes were cut in the disk, it became too fragile for industrial use.
FIGURE 10-108 Example of Hall-effect switches. These switches are used for positioning applications such as indicating when a door is open or closed or if a safety guard is in place. (Courtesy of Honeywell Micro Switch Division.)
Incremental Rotary Encoder
An encoder with one set of pulses would not be useful since it could not indicate the direction of rotation. Most incremental encoders have a second set of pulses that is offset (out of phase) from the first set of pulses, and a single pulse that indicates each time the encoder wheel has made one complete revolution. Figure 10-110 shows 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 is 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.
(a) Rotary Incremental Optical Encoder
FIGURE 10-109 (a) Internal operation of incremental optical encoder. (b) Examples of optical rotary encoders. (Courtesy of Dynamics Research Corp.)
FIGURE 10-110 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.
This pulse is called the command pulse, which is used to count revolutions of the shaft where the encoder is connected.
Since the incremental encoder provides only 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. The early encoder wheels that were made from metal were not too useful as more resolution was needed. Today encoder wheels are made from clear glass that 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 to 6000 segments. This means that 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 would be impossible to drill hundreds of holes in the encoder wheel to get the higher amounts of resolution because the wheel would not have enough material remaining to give the wheel strength. For this reason 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. Since 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.
Figure 10-111 shows an example of the etched glass encoder and a diagram of the light source and receiver. From this figure you can see 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 from this figure 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.
One of the major drawbacks of the incremental encoder is that the number of pulses that are counted are stored in a buffer or external counter. If power loss occurs, the count will be lost. This means that if a machine with an encoder has its electricity turned off each night or for maintenance, the encoder will not know its exact position when power is restored. The encoder must use 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 is relative to fixed positional points.
Incremental Encoder Internal Construction
FIGURE 10-111 Example of an etched glass incremental encoder wheel. (Courtesy of Analog Devices.)
Pulse Train for Incremental Encoder
The absolute encoder is designed to correct this problem. It is designed in such a way that the machine will always know its location. Figure 10-112 shows an example of an absolute encoder. From this figure you can see 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 dartboard. 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 10 rings, its outermost ring will have 512 segments, and if it has 16 rings it will have 32,767 segments.
Since 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 will be a light source and receiver for every ring on the encoder wheel. This means that 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 is that 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 10 inches and its encoder has 16-bit resolution, the resolution of the machine will be 10/65,536, which is 0.00015 inch. If the travel for the machine is longer, such as 6 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 foot. This means the coarse encoder can be geared so that it makes one revolution over the entire 6-foot distance, while the fine encoder is geared so that its entire resolution is spread across 1 foot (12 inches).
Absolute Optical Encoder Internal Construction
FIGURE 10-112 Example of an absolute encoder wheel that shows the pattern of concentric circles. This diagram 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. (Courtesy of Analog Devices.)
Since the absolute encoder produces only one distinct number or bit pattern for each position within its range, it knows where it is at every point between the two ends of its travel, and it does not need to be homed to the machine each time its power is turned off and on.
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