Controlled servo drives are used in many areas of automation technology, converting, printing, handling, and robotics, including production machines and machine tools. The selection of a rotary encoder or encoder technology for use within the system is dependent on the accuracy requirements of the application, whether it is position and/or velocity control. Before making an encoder decision, an engineer should examine this and all the major encoder properties that influence important motor performance.
Positioning accuracy depends solely on the application requirements. Resolvers mostly have one signal period per revolution. Therefore, the position resolution is extremely limited, and the accuracy typically is in the range of ~ ±500” (Arc seconds), assuming interpolation in the drive electronics usually results in a total of 16,384 positions per revolution.
On the other hand, an inductive scanning system, as found in many rotary encoders, will provide significantly higher resolution, typically in the range of 32 signal periods per revolution, resulting in an accuracy of ~ ±280”. The interpolation in this case is internal to the encoder, resulting in 131,072 positions per revolution.
A good fit from the start will provide positive performance in the motor/drive system.
Optical rotary encoders are based on very fine graduations, commonly with 2,048 signal periods per revolution. Therefore, even much higher resolutions are possible with internal interpolation electronics. The output resolution here is 25 bits, 33,554,432 absolute positions per revolution, with accuracies in the range of ~ ±20”.
To ensure smooth drive performance, an encoder must provide a large number of measuring steps per revolution as the first piece of the puzzle. However, an engineer must also pay attention to the quality of the encoder signals. In order to achieve the high resolution required, the scanning signals must be interpolated. Inadequate scanning, contamination of the measuring standard, and insufficient signal conditioning can lead to the signals deviating from the ideal shape. During interpolation, errors then occur when the periodic cycle is within one signal period. Therefore, these position errors within one signal period are also referred to as “interpolation error.” With high-quality encoders, these errors are typically 1-2 percent of the signal period.
The interpolation error hurts the positioning accuracy and significantly degrades the speed stability and audible noise behavior of the drive. The speed controller calculates the nominal currents used to brake or accelerate the drive, depending on the error curve. At low feed rates, the feed drive lags the interpolation error. At increasing speeds, the frequency of the interpolation error also increases. Since the motor can only follow the error within the control bandwidth, its effect on the speed stability behavior decreases as the speed increases. However, the disturbances in the motor current continue to increase, which leads to disturbing noises in the drive at high control-loop gains.
Bandwidth (relative to command response and control reliability) can be limited by the rigidity of the coupling between the motor shaft and encoder shaft, as well as by the natural frequency of the coupling. Encoders are qualified to operate within a specified acceleration range. Values typically range from 55-2,000Hz. However, if the application or poor mounting cause long-lasting resonant vibration, it will limit performance and possibly damage the encoder.
Tom Wyatt is automation division manager at HEIDENHAIN Corp., North America.