Automation 101: Selecting the Right Feedback Option

Automation 101: Encoders, Resolvers & Tachometers

What is my motor’s position? What is its speed?

These questions are the heart of every application involving all types of electric motors. As applications do vary, there is no one right approach to answering these questions. When these questions need to be answered in an application an engineer will look to feedback. Three common types of feedback are encoders, resolvers, and tachometers. Which of these feedback methods might be the most appropriate for an application? This question has no universal answer. However the roadmap to choosing a feedback type is the application itself.

How do these different feedback options work?

Both magnet and optical encoders utilize onboard electronics to provide a clean square wave output. Resolvers, on the other hand, provide only analog signals, requiring all conditioning to be done elsewhere. Tachometers can provide a digitized or analog signal. Optical encoders pass a disk with dark portions between a light source and a light sensitive circuit, while incremental encoders include a disk with only one set of lines. Absolute encoders have so many of these lines that the line pattern never repeats around the circumference of the disk. Both magnetic encoders and resolvers use one or more pairs of poles of magnetization and sensors. In a magnetic encoder these pairs are taken from their sine wave signal into a square wave.

Before a feedback method can be chosen, a few things need to be known about your specific application. Ask yourself:

  1. To what precision would I like know the position of the motor?
  2. Should rotor position be retained on power cycle?
  3. What feedback resolution does the application require?
  4. Will other hardware in the application influence my choice? If so, can the hardware change to accommodate a different feedback option?
  5. In what kind of environment will the application be operating?
  6. Is cost or reliability the driving factor?

After answering these questions, your selection becomes far less difficult. The table below summarizes the strength and weaknesses of different feedback options.

 

Encoders
Position Feedback Information on Power Cycle Resolution Hardware Compatibility Environmental Factors Cost vs. Reliability
Optical & Incremental Yes No Up to ~40,000 Counts Per Revolution Nearly All Mostly Clean Environment Necessary Low Cost
Good Low Noise Immunity Reliable When Maintained
Terrible High Noise Immunity
Optical & Absolute Yes Yes Up to ~4,000,000 Counts Per Revolution Most Mostly Clean Environment Necessary Medium Cost
Good Low Noise Immunity Reliable When Maintained
Terrible High Noise Immunity
Magnetic & Incremental Yes No Up to ~8,000 Counts Per Revolution Nearly All Harsh Environment Medium Cost
Good Low Noise Immunity Reliable When Mounted Correctly
Poor High Noise Immunity
Magnetic & Absolute Yes Yes Up to ~4,000 Counts Per Revolution Most Harsh Environment Medium Cost
Good Low Noise Immunity Reliable When Mounted Correctly
Poor High Noise Immunity
Resolvers
Position Feedback Information on Power Cycle Resolution Hardware Compatibility Environmental Factors Cost vs. Reliability
Resolvers Yes Yes Near Limitless Usually Special Option, if Available Extremely Harsh Environment Higher Cost
Poor Low Noise Immunity Extremely Reliable
Best High Noise Immunity
Tachometers
Position Feedback Information on Power Cycle Resolution Hardware Compatibility Environmental Factors Cost vs. Reliability
Digital No No N/A Most Mostly Clean Environment Necessary Lowest Cost
Good Low Noise Immunity Reliable When Maintained
Terrible High Noise Immunity
Analog No No N/A Older Hardware Harsh Environment Low Cost
Good Low Noise Immunity Reliable When Maintained
Terrible High Noise Immunity

It is important to understand the accuracy requirements of your application when choosing your feedback type. More accurate hardware requires careful and precise mounting, but as accuracy of the product increases, the hardware and installation costs also increase. For this reason, it isn’t always necessary to incorporate a high-end product into a design, as it may be not required for the application and may increase costs.

Picking the right components for your application can be the overwhelming with all the different options. For help with component selection please contact your nearest Kaman Industrial Technology distributor.

For more information on this topic, please visit the Kaman Automation, Control & Energy Info Center by clicking here.

Estimated Reading Time: 3 minutes

Spencer Hepworth

Application Engineer at Kaman Automation
As an Application Engineer, Spencer plans and implements technological designs for the automation, control and energy solutions offered by Kaman Automation.
Spencer Hepworth
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