Analyzing the positioning behavior of four different angle encoders on a TMB+ torque motor from ETEL reveals the effect of signal quality on dynamic performance and the effect of the measuring principle on accuracy. The demonstration unit contains typical examples of absolute angle encoders used in machine tools.
A comparison of the motor current noise starkly reveals the different motor behavior with different encoders. Optical encoders cause low, steady noise, whereas magnetic and inductive encoders cause higher, much more heterogeneous noise, even with a low-pass filter. Optical encoders are therefore the best choice for making optimal use of a motor’s performance potential and for obtaining the best-possible surface quality. That’s because an encoder’s signal quality has a direct impact on multiple aspects of direct-drive motor behavior:
The WMxA inductive angle encoder exhibits a particularly high level of error in dynamic positioning accuracy when measured at the attainable contouring accuracy. This deviation is a by-product of the inductive scanning method, which causes varying accuracy based on speed. In contrast, the RCN and ECA optical encoders demonstrate hardly any deviation between the desired and actual position. The ECM magnetic encoder exhibits mid-level performance without extreme error.
Torque motors such as the TMB+ from ETEL feature very high performance in a compact design. However, if the current distribution in the windings is asymmetric during certain machining situations, the temperature of a single winding can suddenly surge, causing an overheating risk. The EIB 5200 sensor box from HEIDENHAIN monitors all three windings of the motor, making the temperature data immediately available for use. Intelligent motor protection and high process reliability are attained by digitizing the temperature sensor data close to the application and transmitting it to the control.