The Influence of Encoder Operating Characteristics on Motor Performance
Why are there so many different angle encoders on the market? Why use different scanning and measurement methods? Which solution should design engineers choose? Heidenhain's demonstration unit clearly answers these questions by showcasing four different angle encoders.
The demonstration setup was very simple: four different angle encoders were mounted on ETEL 's TMB+ torque motor.
The Heidenhain RCN 8311 absolute angle encoder is a typical enclosed angle encoder used in rotary tables and oscillating milling heads of precision machine tools.
The Heidenhain ECA 4410 absolute angle encoder is a typical steel drum modular angle encoder used in large-diameter shaft rotary tables and oscillating milling heads of precision machine tools.
The Heidenhain ECM 2410 absolute magnetic encoder is a modular angle encoder with excellent dirt resistance.
AMO 's WMxA 1010 encoder is a typical steel-belt absolute inductive encoder designed for applications requiring compactness, high resistance to contamination, and flexible installation.
The demonstration device uses the Heidenhain TNC 640 CNC system to simulate the positioning motion of encoders at various angles, analyzing the impact of signal quality on dynamic performance and the influence of measurement principles on accuracy. The device also demonstrates the potential to intelligently utilize data from the entire system to improve process reliability; the system includes motors, angle encoders, and sensor junction boxes.
Signal quality: A crucial factor determining surface quality
In direct-drive motors, the encoder signal quality directly affects the magnitude of current noise, thus significantly impacting dynamic performance potential and increasing motor power loss. Noise is a negative effect of microstepping error, affecting the dynamic performance potential of the motion axis. Microstepping error leads to rapid changes in position values and speed calculation errors, which in turn further increase current noise. To avoid drive system instability, the control loop gain must be reduced, lowering dynamic performance to offset the increased noise.
Noise also affects the temperature characteristics of a motor. Low noise reduces power loss, which in turn lowers the motor temperature. Conversely, high noise increases power consumption and thus significantly increases the motor temperature.
Motor temperature characteristics: The left figure shows the temperature distribution. As can be seen from this figure, the motor temperature is lower when the position control loop uses an optical scanning angle encoder. The non-optical scanning angle encoder results in a higher temperature, as shown in the right figure.
The differences in temperature characteristics are clearly visible when comparing different encoders . Optical encoders have low and stable noise, while magnetic grid and inductive encoders have higher noise levels, more non-uniform noise, and the noise remains high even after using a low-pass filter. Therefore, optical encoders are the ideal choice for motors to achieve high performance and high surface quality.
Noise comparison of encoders at different angles in the demonstration device.
Actual position vs. theoretical position
According to ISO 230-2 standard, the positioning accuracy of the rotary table is measured to evaluate the degree of conformity between the actual and theoretical positions of the rotary table. To this end, the rotary table is rotated five times clockwise and five times counterclockwise, with twelve points measured per revolution, and adjacent measurement points spaced 30° apart .
The main metrics for evaluating an encoder are parameter A , which represents the bidirectional accuracy of the positioning motion, and parameter M , which represents the range of the average bidirectional positioning deviation. Parameter A is equivalent to the system accuracy of an angle encoder, and parameter M is equivalent to the indexing accuracy; both parameters take into account application errors.
Bidirectional positioning accuracy and average bidirectional positioning deviation range: The deviation between the actual position and the ideal position of RCN and ECA optical angle encoders is significantly smaller than that of magnetic grating ( ECM ) and inductive ( WMxA ) angle encoders.
To evaluate achievable profile accuracy at a specified maximum motion speed, Heidenhain's accuracy specifications include not only the requirements of the ISO 230-2 standard but also dynamic positioning accuracy ( represented by the letter D ). According to the ISO 230-2 standard, the rotary table is rotated five times clockwise and five times counterclockwise, and then the measurement is performed again. However, this time, the measurement is performed using a continuous scan frequency of 5 kHz and a rotational speed of 20 rpm .
Dynamic position accuracy was measured using achievable contour accuracy, revealing a significant deviation in the WMxA inductive angle encoder. This deviation stems from the inductive scanning method, which causes accuracy to vary with rotational speed. In contrast, the RCN and ECA optical encoders exhibit almost no deviation between theoretical and actual positions. The ECM magnetic encoder shows no severe deviation and falls into the middle performance range.
Dynamic positioning accuracy is one of the achievable contour accuracy parameters, and inductive angle encoders have a relatively large deviation.
The large deviation is due to the inductive scanning method itself, whose accuracy is related to the rotation speed, and there is a serious jump between clockwise and counterclockwise measurements.
Intelligent motor protection improves the reliability of the machining process.
Torque motors, such as the ETEL motor used in the demonstration device , are not only compact but also boast excellent performance. However, under certain machining conditions, if the current distribution in the windings is asymmetrical, the temperature may become excessively high, causing a sudden rise in the temperature of one set of windings. Digitizing the temperature data from a temperature sensor located near the application and providing this data to the CNC system intelligently protects the motor and improves process reliability. In particular, the improved availability of temperature information effectively enhances machining reliability and work efficiency.
The Heidenhain EIB 5200 sensor junction box monitors all three windings of a motor, providing temperature data that is immediately usable. Located near the motor, between the angle encoder and the machine tool's CNC system, this junction box can quickly detect sudden temperature rises if a temperature model of the motor, such as an ETEL torque motor, has been established and saved. This prevents damage to the motor windings and protects the motor from overheating.
During manufacturing, the Heidenhain EIB 5200 sensor junction box intelligently protects the motor. Cable connections are simple and additional data for the motor system is immediately available.
Choosing the right encoder and intelligently utilizing the various data available during the machining process are crucial to the reliability, stability, and accuracy of the machining process. Determining the unique operating characteristics of different encoder models helps design engineers and developers select the correct angle encoder to meet application requirements. Finally, selecting the right encoder not only affects dynamic performance and accuracy, but design engineers and developers must also consider relevant factors such as shaft diameter and mounting method, and of course, economic efficiency.
