Feedback Sensors Keep Servomotors on Target
- Tuesday, 01 October 2013
Among the simplest and least expensive feedback devices are Hall-effect sensors. These are digital on-off devices that detect the presence of magnetic fields. Made of semiconductor material, they are rugged, can be operated at very high frequencies (equal to tens of thousands of motor rpm), and are commonly used to provide six-step commutation of brushless motors. They are well suited for torque control or coarse speed control, and simplify the drive electronics by directly switching the motor phase power devices.
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 detect the presence of magnetic fields, either by measuring the strength of an electromagnetic or permanent magnetic field. At each pass of a magnetic field, they generate a pulse. Hall-effect devices come in standalone packages that are mounted within the servomotor housing. In brushless servomotors, 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.
In servomotor applications, the most common function for Hall devices is six-step commutation, a type of electronic commutation requiring relatively simple drive electronics. This may not suit some industrial servo applications because it can be less efficient at producing torque, and worse, can generate high torque ripple. In this case, torque ripple results from abrupt current transitions resulting in torque fluctuations, which usually produce minute 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 precision sinusoidal commutation. In servo drives, Hall sensors also function as current sensors to close the current loop. In other industry applications, they sense the position of crankshafts, cams, or other mechanical devices.
Resolvers are rotary transformers that are well suited to harsh environments, where extreme temperatures or vibration and shock are factors. They can also handle motor speeds in excess of 10,000 rpm. These are low to moderate on a cost scale, and provide moderate accuracy and resolution that is suitable for most industrial applications.
Resolvers, along with encoders, handle the majority of closed-loop motioncontrol 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 basically analog devices, they provide relatively clean signals. Their high voltage range 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 multispeed, which refers to the number of electrical cycles per mechanical revolution. 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 tolerate heat, vibration, and shock. However, they require more electronics for signal conversion than is needed for encoder based systems. Additionally, resolvers are generally less accurate than optical encoders, but some versions, known as tooth-wound units, improve on this. The manufacturing techniques for these units keep part-to-part variation to a minimum, which increases their output accuracy by about 50%. Resolvers are commonly rated at 155 ºC, with special models able to withstand 230 ºC, or even be radiation-hardened. Frameless brushless types are commonly used in servomotors due to reduced maintenance needs and a large through bore that can accommodate motor modifications such as hollow shafts and additional shaft extension options.
Encoders are characterized in three basic categories: rotary or linear, incremental or absolute, and by the method of signal generation being optical, magnetic, or contacting. When optical encoders first appeared, they were praised for their ability to offer high accuracy in both low- and high-speed applications.
Today’s versions are more rugged, with better-protected electronics and optics. Even so, most manufacturers still recommend that optical encoders be selected for lighter industrial applications where they are exposed to temperatures below 90 ºC and vibration below 20gs.
Encoders can be contacting or noncontacting devices, with non-contacting optical encoders the most common of the two. These use a light detection unit that reads the on-off pattern as light passes through the coded disc/shutter mechanism, which then sends that data to the drive system.