By Dan Hebert, PE, Senior Technical Editor
Encoders and resolvers often are used to sense motor shaft speed in machines and robots. Resolvers have been around longer, but encoders are more widely used in modern designs because of higher accuracy and superior digital communication interfaces.
Incremental rotary encoders work on either of three technologies: optical, magnetic or capacitive. Each technology has its drawbacks and advantages, and the right choice typically is determined by the application.
"Optical encoders can be found in office applications such as copiers, and in industrial applications such as automated guided vehicles," says Cory Mahn, senior specials engineer at Dynapar (www.dynapar.com). "Magnetic encoders typically are needed in harsh conditions where optical encoders could show significant performance decrease. These applications can be overhead cranes, paper mills and steel mills."
Some applications are too rugged for optical encoders, and others have inherent strong magnetic fields that preclude use of magnetic encoders. "Our capacitive encoders are designed and built to meet the challenges of difficult applications as they don’t have bearings or optics, and are well-suited for direct-drive applications," notes Joanna Suresh, product manager for motor feedback and absolute encoders at Sick (www.sickusa.com).
The vulnerable component in optical encoders is the glass disk, and various designs are available to provide protection. "Our Safety-Lock bearing design incorporates two mechanically interlocked bearings that have the maximum possible distance between them," says Tony Udelhoven, sensors division director at Turck (www.turck.us). "This design helps protect the internal optical disk system from shock and vibration that would damage traditionally designed encoder bearings."
Bearing design is critical for optical encoders because the bearings are subject to contact and wear. Magnetic encoders eliminate bearings from their design, and can also offer higher performance.
"Our bearing-less, incremental rotary magnetic encoder offers the simplicity of just two pieces: a magnetic wheel that attaches to a rotating shaft, and a sensor assembly that mounts in proximity to the rotating magnetic wheel and produces encoder outputs," explains Don Ecker, encoder product manager at Pepperl+Fuchs (www.pepperl-fuchs.us).
According to Ecker, magnetic encoders are more reliable and can accommodate higher speeds. Most optical encoders with bearings are limited to 6,000-10,000 rpm, but magnetic encoders can typically operate at up to 30,000 rpm.
"Magnetic encoders are less affected by ambient conditions, so they reliably operate in dirty, dusty, wet and extended temperature environments in the range from -40 to 100 °C," Ecker says. "Magnetic encoders also function well in applications where shock or vibration would destroy the glass disk used in most precision optical encoders."
No matter the type of encoder selected for a particular application, it requires an interface to other components in the automation system. Sick, for example, includes a range of electrical outputs to interface to the motor controller, including transistor-to-transistor logic (TTL), high transistor logic (HTL), synchronous serial interface (SSI), sin/cos and Hiperface. "The future roadmap for our encoders will include EtherNet/IP and Hiperface DSL interfaces, the latter a new digital communication protocol that communicates with just two wires that are part of the motor cable," Suresh says. As with many automation components, digital interfaces provide high-speed two-way communications, enabling high-resolution outputs and diagnostics.
Although resolvers are not as widely used as encoders, they can be a good fit in certain applications. "Resolvers lend themselves to maximum-duty applications because of their similarity to electric motors as both components have windings, laminates, bearings and a carrier," Mahn explains.
No solid-state electronics, as well as optics and their precision alignment requirements, makes resolvers more suitable for applications in extreme temperatures, applications with high shock and vibration, and in high-radiation environments, Mahn notes. "Resolvers have been time-tested and proven, but the analog output limits connectivity options," he adds.