There are two main ways to monitor absolute shaft position: encoders and resolvers. Resolvers are the older technology, but their ruggedness allows them to survive where other devices could not. Encoders, being inherently digital, have become the method of choice for most applications, but they can't survive where a resolver can. This article will look at both technologies, explain how they work, and give some pointers on where they're best applied.
Encoders
An optical encoder uses a scale with a pattern of lines deposited or engraved on its surface. A light source shines light on the scale, and the transmitted or reflected light passes through a grating to a photodetector.
Encoders are available with both incremental and absolute outputs. An absolute encoder has as many tracks as it has output bits. An incremental encoder outputs a stream of pulses as the shaft rotates, and incremental encoders are specified in pulses per revolution. Incremental encoders can be optical, using a scale or disk with just one or two rows, or magnetic, using several different technologies.
The major difference between an absolute encoder and an incremental one is that the absolute encoder keeps track of its position at all times, and provides it as soon as power is applied. An incremental encoder does not provide any position information at startup, but merely keeps track of how far it has moved. The only way to determine the absolute position of an incremental encoder is to set the equipment to a known reference position and then zero the counters. (The Dynapar ACURO absolute optical encoder, shown here, has a resolution of up to 17 bits.)
Advantages: The biggest advantage of an encoder is that it's inherently digital, which means it can interface easily to modern control systems. An encoder sends digital-quality signals back to the computer. There is no need for an engineer to get involved in the wiring and integration of signal electronics.
In fact, many engineers are unfamiliar with resolver electronics -- it is much easier to buy an encoder with a digital signal that is ready to go. An encoder is also fast: some 12-bit optical encoders can provide a reading of absolute position on a shaft rotating at 12,000 rpm, and there are 12-bit magnetic encoders that can run at 30,000 rpm.
As mentioned before, encoders are available with absolute or incremental output. Incremental encoders find a great deal of use as tachometers, because their spot-on digital output allows more accurate speed control than is available from an analog tachometer, and because, since they lack the analog tach's brushes, they have a longer life.
Disadvantages: The biggest limitations of encoders are that they can be fairly complex and contain some delicate parts. This makes them less tolerant of mechanical abuse and restricts their allowable temperature. One would be hard pressed to find an optical encoder that will survive beyond 120ºC.
Optical encoders can be harmed by contamination -- their fine-pitch scales, LEDs and photodetectors can be put out of action by oil, dirt, or dust.
Probably the biggest drawback to an optical encoder is its reputation for mechanical fragility. The heart of most optical encoders is a thin glass disk that can be broken by excessive shock or misaligned by shock or severe vibration. Encoders traditionally have been complex electronic devices that contain integrated circuits as well as LEDs and photodetectors, and a severe electrical disturbance could damage them.
Advances: This situation has changed considerably over the last ten years. Just as incandescent indicator lamps have been replaced by long-life LEDs, the glass disks have been replaced by steel and plastic disks, and the electronics are more integrated and thus more durable.
All of the advancements mentioned are making encoders more reliable and popular for heavy duty applications. In addition, encoders are now available that use magnetic technology. These heavy duty magnetostrictive encoders can stand environments just about as hostile as a resolver can, and still provide the easy integration of an encoder. However, they are incremental encoders, whereas resolvers provide absolute position information.
Resolvers
A resolver (sometimes called a coordinate transformer) is an electromechanical device with a mechanical design similar to a motor. It contains a rotor with one or two orthogonal primary windings and a stator with two orthogonal secondary windings. (Illustration: The voltage in one stator winding varies as the sine of the shaft angle and the other varies as the cosine. This resolver uses a rotary transformer to excite the primary, so no brushes are needed.) An ac voltage is applied to the rotor and the voltage induced in each stator winding depends on the position of the shaft. The voltage in one stator winding is Er Cos theta, where Er is a certain fraction of the input voltage and theta is the shaft position, while the voltage in the other stator winding is Er Sin theta. A resolver-to-digital converter (generally mounted in the equipment to which the resolver is connected) compares the two voltages to give a highly accurate value of theta.
Resolvers are available with accuracies of 7 minutes of angle (equivalent to 1 part in 3,085, or 11 bits of resolution), and some are good to 3 minutes of angle (1 part in 7,200, or a better than 12 bits resolution). Some resolver-to-digital converters can interpolate results to give 16 bit output. Resolvers are available for speeds of 10,000 rpm, and are often used as the position sensing element in brushless DC motors.
This technology is used mostly in three areas:
- Aerospace and military -- The military likes resolvers because the sensing portion of a resolver is so simple. Resolvers got their start during WW II to point artillery guns.
- Servos -- Servos already involve a lot of electronics, integration, and customization, so the additional work to convert the resolver signal is not a burden.
- Industry -- Heavy duty applications such as mills use resolvers, although as encoders have become more durable they are replacing resolvers in this area.
Advantages: The biggest advantage of a resolver is that it's very rugged. It can survive anywhere an electric motor can survive, and will ignore shock and vibration that would knock out even a heavy-duty encoder. Resolvers are available with shock ratings to 200 g and vibration to 40 g. They're also very resistant to both nuclear radiation and electrical disturbances. Compare this to one of the more rugged optical encoders (the Danaher ACURO Drive-M series), which is rated for shock of 100 g and vibration of 10 g from 10 to 2000 Hz. Heavy magnetostrictive incremental units are available that can stand vibration of 18 g.
While it takes a pretty tough optical encoder to survive at 120ºC or more (and temperatures that high tend to shorten the life of the electronics inside the encoder), resolvers are available that will operate at 220ºC. They also have long lives. The rotor can be excited using a rotary transformer, which means there are no brushes, so the resolver's lifetime is limited only by its bearings. (Harowe Size 11 resolver shown here is available with either rotor or stator as primary and accuracy as close as 6 minutes of angle.)
Since a resolver has no fine-pitch gratings through which light must pass, it can tolerate dust and dirt that would put an optical encoder out of action.
Resolvers have one more advantage: they're inexpensive. Since they're built very much like motors, they can be made with automated equipment that keeps the price down.
Disadvantages: Resolvers are not without their own drawbacks. The biggest is the need for interfacing electronics. The conversion of a resolver output to digital is far more complex than an analog to digital conversion. First, an oscillator sends a sine wave interrogation signal to the resolver. Second, the sensor modifies the signal, which then has to be interpreted into a series of sine waves. Finally, the signal is interpolated into a digital signal. The cost of the electronics to do all this can be three to five times the cost of the resolver itself.
Another downside of resolvers is their limitations in speed and speed range. While many resolvers can run at 10,000 rpm, there is some falloff in position accuracy during rapid changes in speed. Depending on the resolver-to-digital converter used, a resolver can give a speed range of 200:1, whereas an encoder can provide a speed range of better than 100,000:1. Speed range may be a concern when using resolvers as the feedback element to a servomotor.
Comparison of Optical Encoders and Resolvers
Optical encoder
- Temp range: To 120ºC
- Shock: 100g
- Vibration: 10g
- Contamination: Fair to good
- Output: Digital
Resolver
- Temp range: To 220ºC
- Shock: 200g
- Vibration: 40g
- Contamination: Excellent
- Output: Analog
Conclusion
Both optical encoders and resolvers are highly accurate feedback devices; the choice of which to use in a particular application depends to a large extent on the environment -- where conditions are reasonable and ease of integration is important, an optical encoder works well. When things get rough or hot or dirty and durability is the most important factor, a resolver is often the best bet. The decision is also sometimes a personal one. Some engineers prefer resolvers and others prefer encoders, but resolvers now account for less than half of the heavy-duty market for feedback devices, mostly because of improvements in encoder reliability.
About the Author: Jeff Christensen is Director, Feedback Business for Danaher's Industrial Controls Group. Prior to joining Danaher he spent 16 years with General Electric, the last ten with GE Fanuc Automation -- first as Marketing Manager in the Control & I/O Business and then as President of GE Fanuc Automation's General Motion Control Business. He has a degree in Electrical Engineering from Baylor University.
