What is A Motor Encoder?
A motor encoder is a rotary encoder mounted to an electric motor that provides closed loop feedback signals by tracking the speed and/or position of a motor shaft. There are a wide variety of motor encoder configurations available such as incremental or absolute, optical or magnetic, shafted or hub/hollow shaft, among others. The type of motor encoder used is dependent upon a number of factors, particularly motor type, the application requiring closed-loop feedback, and the mounting configuration required.
How to Specify A Motor Encoder
When selecting components for a closed loop control system, the motor encoder choice is first determined by the type of motor chosen in the application. The most common motor types are:
AC Motors Encoders
AC induction motors are popular choices for general automation machine control systems as they are economical and rugged. Motor encoders are used for more precise speed control in applications using AC motors, and often times need to have more robust IP, shock and vibration parameters.
Servo Motor Encoders
Servo motors encoders (permanent magnet motor encoders) offer closed loop feedback control systems to applications that require higher precision and accuracy, and are not as robust as AC induction motors. The motor encoder used on servo motors can be modular, incremental or absolute depending on the level of resolution and accuracy required.
Stepper Motor Encoders
Stepper motors are cost effective, precise, and are typically used in open-loop systems. In systems using stepper motors where speed control is required, an incremental motor encoder is often mounted to this motor and will allow the stepper motor system to achieve closed loop feedback. Stepper motor encoders can also be used in some applications to allow for improved control of stepper motors by providing precision feedback of the location of the motor shaft in relation to the step angle.
DC Motor Encoders
DC motor encoders are used for speed control feedback in DC motors where an armature or rotor with wound wires rotates inside a magnetic field created by a stator. The DC motor encoder provides a mechanism to measure the speed of the rotor and provide closed loop feedback to the drive for precise speed control.
Motor Encoder Mounting Options
The next factor impacting motor encoder selection is the mounting option, and the most common options are:
- Shafted Motor Encoders: Uses a coupling method to connect the motor encoder shaft to the motor shaft. The coupling provides mechanical and electrical isolation from the motor shaft but can add cost via the coupling and the longer shaft length required to mount the motor encoder
- Hub/Hollow shaft Motor Encoders: Hollow shaft encoders directly mount to the motor shaft via a spring loaded tether. This method is easy to install and required no shaft alignment, but proper care must be taken to provide electrical isolation.
- Bearingless Motor Encoders: Also known as ring mount, this mounting option is comprised of a sensor assembly in the form of a ring that is mounted on the motor face, and a magnetic wheel which is mounted on the motor shaft. This type of motor encoder mounting configuration is mostly found in heavy duty applications like paper, steel and cranes.
Types of Encoder Technology
The application in which the motor encoder is being utilized will determine the motor encoder technology that needs to be used. The two broad types of motor encoder technologies available are:
- Incremental Encoders: The output of an incremental motor encoder is used to control the speed of a motor shaft. Learn more about incremental encoder technology.
- Absolute Encoders: The output of an absolute motor encoder indicates both the motion and the position of the motor shaft. Absolute motor encoders are most often used on Servo Motors in applications where position accuracy is required. Learn more about absolute encoder technology
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