Various feedback devices are often offered as options on rotary servo motors. Feedback devices include optical incremental encoders, absolute incremental encoders and resolvers. Choosing the right feedback device for your application can be tricky but we're here to help with a three-part series designed to help you choose the correct device for your servo motor application.
This blog lays the foundation with basic operational theory of three feedback devices for servo motors.
Optical incremental encoder
An incremental encoder is made up of a disc, light source and photo detector (figure 1). It is designed to be attached directly to the servo motor shaft.
The encoder disc can be made from glass or steel with transparent lines incorporated around it. A light source projects to the disc and the resulting optical pattern is read by a photo detector. The optical pattern read by the photo detector produces a series of electrical pulses that are then read by the servo controller. The output can be a single line of pulses (“A” channel) or two lines of pulses (“A” and “B” channels) that are offset ninety electrical degree out of phase. This offset is used by the servo motor controller to determine, and if necessary, change, the direction of motor shaft rotation. A third channel, typically called “Z”, can be provided, which will produce one pulse/rev to act as a home.
To increase encoder resolution, the servo motor controller can count the leading and trailing edges of the pulses generated by channels “A” and “B” (figure 2). This is defined as quadrature. Counting both the leading and trailing edges of both channels will quadruple (x4) the number of pulses per revolution. So, 5,000 pulses/rev can now be 20,000 pulses/rev. The increase in resolution allows for more precise control of your motor shaft rotation and can also make it easier to tune your servo.
Optical absolute encoder
An optical absolute encoder is similar to an incremental encoder as it is made up of a disc, light source and photo detector (figure 3).
The difference with an absolute encoder vs. the incremental is that the patterns read by the photo detector are unique. It will maintain position information when power is removed from the system. The position of the encoder is available immediately on applying power: There is no need to return to a home position. Instead of defining resolution relative to pulses, the absolute encoder generates a binary code, defined as a bit, for each of its unique disc patterns. The resolution on these encoders is typically much higher than their incremental counterparts, with a 16-bit encoder providing 65,536 counts/rev. and a 20-bit encoder providing 1,048,576 counts/rev. As mentioned earlier, higher resolution results in more precise motion control and a stiffer, easier to tune system.
A resolver is a rotary transformer that is made up of a reference winding, a sine and cosine winding, and a transformer (figure 4).
The reference winding (also called the primary winding) is located on the resolver rotor that will be rotated by the servo motor shaft. As the primary winding is energized, a magnetic north and south pole are created. When rotated, the voltage induced into the secondary windings is varied. The subsequent sine and cosine signals are fed into a controller, compared to each other to calculate shaft position, and then converted to a digital signal. The analog to digital converter within the drive determines the resolution (typical standard is a 4096 converter, which equates to 12 bits of resolution).
Watch this video to discover the features and benefits of our MPP servo motor. Optical incremental encoders, optical absolute encoders and resolvers are standard options for this product line.
Look for our next blog in this series, which will discuss how to choose the best feedback device to meet your application requirements.
Article contributed by Jeff Nazzaro, Gearhead and Motor Product Manager, Electromechanical and Drives Division, Parker Hannifin Corporation.