All closed loop systems use feedback to control speed and or position. This plays an important role in keeping equipment operating accurately and smoothly. When using feedback for the best benefits in an application, it is important to understand how feedback works, because a variety of devices as well as models is available for the purpose. The most popular among them are tachometers, Hall sensors, encoders and resolvers.
Tachometers are rotating electromagnetic devices. Typically, these are connected to the shaft of a motor, rotating when the shaft rotates and generating a voltage as a signal. The faster a tachometer shaft rotates, the larger is the magnitude of the voltage output. Therefore, the output signal is directly proportional to the speed of the motor shaft. The polarity of the output voltage indicates the direction of rotation, clockwise or counter clockwise.
Usually, analog or DC tachometers provide direction and speed information. When fed to a meter, this information can be used in servo control for stabilization. DC tachometers are the simplest of feedback encoders.
Hall sensors are solid-state electronic devices and they can sense or detect magnetic fields. The output of the sensor changes or flips whenever a magnet comes close to a Hall sensor. Therefore, a Hall sensor provides a digital output as either a high or a low voltage.
Hall sensors are used for brushless motor applications, providing information about rotor position. This works as an electronic commutation, with the controller using the information to turn on or off specific power devices applying power to the stator windings.
Encoders are simple mechanical-to-electrical conversion devices and turn mechanical rotary motion into velocity or position information for systems controlling motion. Encoders can be rotary, digital, optical or incremental types.
In its most basic form, and encoder consists of a light source, a mask, a coded disk and a photo sensor along with related electronics. After passing through the mask and the coded disk, light from the source is detected by the sensor. As the encoder shaft rotates, light is alternately passed through or blocked, making an alternating light and dark pattern.
The associated electronics converts this into an electrical signal representing high or low corresponding to light passing through or being blocked. The resolution desired for the application governs the number of lines etched on the coded disk. By counting the number of pulses, the position of the shaft relative to its starting position is known.
There are two types of encoders, classified as incremental and absolute. Absolute encoders generate a specific address for each shaft position throughout the 360-degree rotation of the shaft.