While encoders are the most widely applicable feedback devices for servo systems that require ultimate precision, Hall-effect sensors and resolvers are also appropriate choices for addressing specific challenges. Here, we explore how they work, the available options, and the advantages and drawbacks of each with regard to specific applications.
Hall-Effect Sensors
When a machine doesn’t require precise speed control or high resolution from the motion system, low-cost feedback sensors such as Hall-effect devices are a suitable option. These digital on/off sensors measure the strength of either an electromagnetic or permanent magnetic field, generating a pulse with each pass of the field.
Hall-effect devices come in stand-alone packages that are mounted within the servo motor housing. In brushless servo motors, these sensors are sometimes embedded in the stator windings and switched by the rotor magnets. These devices report the shaft’s position, which can also be converted to speed or acceleration data.
Hall-effect devices are commonly used in six-step commutated servo systems, a type of electronic commutation requiring relatively simple drive electronics that is often used for DC brushless systems. Hall-effect sensors are best used when paired with an incremental encoder, commonly known as a comcoder. The advantage of this pairing is that you can achieve higher resolution than with a Hall-effect device alone, and you do not need to run a “wake and shake” sequence to determine the commutation angle, since that is handled by the Hall-effect sensor.
However, the trapezoidal rather than sinusoidal control that Hall-effect sensors support may not suit some industrial servo applications because it can be less efficient at producing torque than other commutation methods. Worse, this coarse control can generate high torque ripple due to abrupt current transitions, which usually produce small but detectable speed variations. In some cases torque ripple can seriously deteriorate the overall performance of a drive system.
With sinusoidal current drives, Hall sensors may be used in combination with incremental encoder feedback to provide precise sinusoidal commutation. In servo drives, Hall sensors also function as current sensors to close the current loop. In other industry applications, they can be used to sense the position of crankshafts, cams or other mechanical devices.
Resolvers
Resolvers, along with encoders, handle the majority of closed-loop motion-control tasks. A resolver is a rotary transformer with a primary and two secondaries. The primary is fed with an AC voltage. The secondaries couple the input voltage ratiometrically according to shaft position.
The resulting sinusoidal signals, sine and cosine, are converted into digital signals in the drive controller by resolver-to-digital converters (RDCs) or by interpolation software in the drive. A two-pole (single speed) resolver provides an absolute position signal within one revolution of the motor.
Because resolvers are analog devices, they provide relatively clean signals. Their high voltage range also makes them less susceptible to noise. The converted output resolution is generally determined in the drive, and may be up to 16 bits. However, the resolution may be limited by motor speed because of a maximum frequency limitation.
Resolvers can be single-speed or multi-speed, with the number of “speeds” indicating the number of electrical cycles generated per mechanical revolution (not to be confused with physical speed). In other words, the counts per revolution increase by a factor of the resolver “speed.”
Resolvers have many positive attributes: They are rugged devices that are highly resistant to EMI noise, and they are very tolerant of heat, vibration and shock. Resolvers are commonly rated at 155ºC, with special models able to withstand 230ºC, and they can even be radiation-hardened.
However, resolvers require more electronics for signal conversion than is needed for encoder-based systems. Additionally, they are generally less accurate than optical encoders, although some versions, known as tooth-wound units, improve on this. The manufacturing techniques used for these units keep part-to-part variation to a minimum, which increases output accuracy by about 50%.
Frameless brushless resolvers are commonly used in servo motors due to reduced maintenance needs and a large through-bore that can accommodate motor modifications such as hollow shafts and additional shaft extension options.
Learn More
Be sure to read all three of our Feedback Devices blog posts, including articles on high-level selection criteria and linear, rotary and sine encoders, to become conversant in this crucial motion topic. And feel free to contact a Kollmorgen engineer to discuss your specific application and get recommendations for the best servo-loop technology to suit your needs.
