Inductive and Capacitive Position Sensors – Key Terms
Provides position change information only so the actual position is unknown at startup. A once-per-rev index/marker signal defines the zero position or null of the device. It is detected during a homing routine. For commutation of a brushless motor the motor typically has three magnetic Hall sensors to provide coarse absolute position information for preliminary alignment of the magnetic fields. Incremental sensors are typically small, accurate and cost-effective.
Provides the actual physical position within one revolution or within the range of linear travel. The motor does not require Halls and homing is only necessary for rotary applications if the range of movement exceeds one revolution. The sensors are usually bigger and more expensive than incremental devices.
Defines the smallest position increment that can be moved or measured and is typically expressed in “counts”. High resolution is required for high performance servo systems. A positioning system “dithers” between two counts so the higher the resolution the smaller the dither. Resolution also has a significant impact on velocity ripple at low speed. Since velocity is derived from position feedback, if the resolution is low there may be insufficient data in a sample to accurately derive velocity. At high speeds, high resolution devices can generate data rates beyond the tracking capability of the controller or servo drive.
As will be seen, many sensors generate sine and cosine signals. The period of these signals is defined by the inherent “pitch” of the device. With sin/cos information it is theoretically possible to have infinite resolution by computing the ratio of the signals. This technique is known as interpolation. In practice, the fidelity of the sin/cos signals and signal to noise ratio limit the realizable resolution.
Defines how close each measured position is to the actual physical position. Accuracy is very much a system issue and can be dominated by mechanical errors such as eccentricity, straightness and flatness. Sensor errors include non-accumulating random variations in pitch (linearity), accumulating pitch errors (slope) and variations in fidelity of internal sin/cos signals. Precision machine builders typically calibrate out errors via a lookup table of offsets. More detail can be found in TN-1005.
Defines the range of measured positions when the system is returned to the same physical position multiple times. Repeatability can be more important than absolute accuracy. For system inaccuracies to be effectively calibrated it is important for each position reading to be consistent. Sensor hysteresis (different readings depending on direction of approach to measure position) is an important factor in repeatability.