We need to measure motor speed in our dry food packaging machines to close the loop for more precise control. We narrowed our choices to either an encoder or a resolver, but we’re having trouble deciding which is best. What are the advantages and drawbacks of each technology?
—from October ’07 Control Design
Resolvers Are Rugged, But Encoders Are Everywhere
Resolvers are inherently more rugged than encoders and less susceptible to electrical noise. Those are the only advantages. Not all resolvers are built the same way. You have to match your resolver to your input carefully. Though resolvers theoretically have an infinite resolution, in practice, they’re roughly equivalent to 1,000 ppr encoders. Resolvers are rarer and more difficult to obtain.
Encoders have a glass disk that can shatter. They also contain electronics susceptible to heat and possibly vibration. However, encoders are used all over the place, so it’s not like they’re super fragile. You can have encoders with very high resolution, but around a 1,000 ppr is common. Lots of places have encoders in stock, though this variety means you can accidentally pick an oddball, so work with your vendor and make sure you pick something common.
I recently went through this same issue and ended up picking an encoder.
I disagree with the comment that sufficient maintenance can make it more worthwhile to use a resolver [see Scott Gee’s response below]. There is no maintenance on either one. If there’s a problem, replace it like any other sensor. If you’re getting out a multimeter or o-scope to check a less than $500 sensor while production is down, then you’re wasting your time.
Support, Flexibility and Reliability
Resolvers had a brief heyday as the feedback device of choice. Tough and durable, their simple motor-like construction let them survive huge temperature extremes and rough environments.
However, resolvers today primarily are relegated to a small corner of the motion control market, namely, units built into the back of servo motors for feedback. Even in this application, digital encoders are squeezing them out.
All major drive and PLC manufacturers support encoder feedback, but only a handful support resolver feedback. This motivates motor manufacturers to offer encoders, not resolvers. There are just a few resolver manufacturers left.
Encoders can be fitted to many form factors. There are shafted models, hollow shaft, modular bearingless, and others. Customers can mount encoders anywhere, eliminate unreliable couplings, and fit encoders into unusual spaces.
Finally, encoder reliability now can exceed resolvers. Some encoder designs eliminate all moving, wearing parts. With magnetic sensors and fully sealed electronics, they can operate in the dustiest or wettest washdown environments and provide 100% uptime.
For your dry food packaging machines, we would recommend a magnetic encoder fitted to the back of the motor. Depending on the style or brand of motor, we would recommend either a modular, a C-face-mount, a wide-gap model, or a hollow-shaft unit. In either case, we would recommend a no-solder style of industrial connector to make your machine even more reliable.
BRIAN W. WINTER, encoder product manager
Avtron Manufacturing, avtronencoders.com
Resolvers: Tuning Required
The phrase “more precise control” would generally drive the solution toward an encoder over a resolver. The application of dry food packaging, however, might tend to drive it back toward a resolver. Consider these characteristics to help make the decision:
Encoders and resolvers can be packaged in basically the same ways, so there’s no advantage to one or the other as far as environmental protection.
The resolver is sturdy and possibly more suitable for a severe environment application with extreme continuous temperatures or very high vibration. However, an encoder can be very robust as well, so the only real advantage would be the ability of the resolver to withstand higher continuous temperatures. If your application requires the feedback device to operate in more than 125 °C environments, you probably go with the resolver.
Most motion control systems can work with resolvers or encoders, so there is no real difference in wires and interconnects. However, resolvers are analog devices, and they require a converter to format the measurement for processing by a digital computer. This conversion is done by a resolver to digital converter (RDC) or by a DSP and suitable input filtering circuitry. For example, something like Analog Devices’ AD2S80A can be used.
This is a tracking converter, which is implemented using a type two servo. A type two servo is a closed-loop control system characterized as having zero error for constant velocity or stationary inputs. Conversely, this type of system will demonstrate errors in all other situations, and the magnitude of these errors must be controlled through optimized tuning of the converter. The fact that the converter itself has dynamics becomes an important part of the system design. Being type two, the converter can introduce up to 180° of phase lag into the system. For a 12-bit converter using a 400 Hz reference , the RDC bandwidth (-3 db point) will be less than 100 Hz. Using the same reference, a 14-bit converter will have a bandwidth of 66 Hz, and a 16-bit converter will have a bandwidth of 53 Hz. A 100 Hz, -3db bandwidth means there will be approximately 3 db of peaking and 45° of phase shift at 40 Hz. As many servos attempt to close position loops near these frequencies, an added 45° phase shift would be undesirable.