Crack the Code on Encoders and Motor Feedback
Many motor and drive applications require some type of motor feedback to provide our controls with a direct reflection of what the motor’s shaft is actually doing, such as rotating direction, speed and shaft position. Understanding the difference in encoder types is key to troubleshooting motor problems in the field. I am going to provide you with a brief explanation of the most popular types.
Tachometers. These devices sense SPEED and DIRECTION. They are usually directly coupled to the motor shaft either in tandem or offset with a pulley and belt. As the shaft spins an Analog voltage or signal is generated at the tachometer outputs. This signal will grow in magnitude as the shaft spins faster. There will be, on the tachometer’s name plate or in its corresponding manual, a ratio of rotation speed to voltage output. A true Tachometer will NOT provide a position signal and can only be used for speed and direction control.
Pulse Encoders. Pulse encoders provide a digital count as they rotate and controls can determine SPEED, DIRECTION and POSITION from these digital signals. If your Pulse Encoder is of the 1024PPR (Pulse Per Revolution) variety that means for every shaft rotation there will be 1024 pulses. These encoders usually have inputs and outputs to power the encoder and for multiple channel feedback. Incremental pulse encoders provide a steady count for speed control and with the use of an Index pulse (Sometimes referred to as a Z pulse) your controls can count to the PPR every shaft rotation to obtain reasonably accurate shaft position feedback. An Absolute Pulse Encoder provides a unique number for degrees of rotation to obtain extremely accurate position sensing.
Resolver Encoders. Used in maximum duty environments where reliability is of the utmost importance. Resolver Encoders act essentially as a rotating transformer and provide an amplitude modulated sinusoidal analog signal to sense SPEED, DIRECTION and POSITION over a specific speed rating.
DYNAPAR has written up an industrial white paper on this subject that, just like their products, is fantastic. I have included the PDF here.