Product specifications

R01AN0168ED0101 Rev. 01.01 38
Application Note
Chapter 7 Performing Zero Point Detection
7.1.1 The ZPD Measurement Principle
For all 4 cases, the same measurement principle is applied:
An analogue measurement of induced voltage is performed, while the other
coil moves the anchor. If the anchor is still able to move, it will cause a small
voltage induction in the measurement coil. If the motor has hit the mechanical
stop, the anchor will almost not move any longer, and the induced voltage will
decrease or disappear.
(1) Finding the Measurement Time Window
Typically, the measurement waveforms could look like this:
Figure 7-2 ZPD Measurement Waveform
Caused by the previous movement, the coil also creates a self-induction, when
its current is switched off and it is attached to the measurement input. This self-
induction is not relevant for the ZPD status, and therefore it must be reduced
(also to protect our measurement circuitry) and blanked out from the
measurement window.
Reduction of the self-induction can be performed by creating a short-circuit on
the coil for a limited amount of time.
The measurement window has to be chosen by delaying the measurement
start and limitation of the measurement time.
(2) Determining the Measurement Voltage Level
As a target, a voltage detector (comparator) has to be set up such, that it
shows an indication above its level, if the zero point (ZP) has not yet been
reached (induction was found), and that it shows an indication below its level, if
the ZP has been reached (no induction was found). Like this, the ZP status is
digitized into a single binary state flag.
As shown in the following figure, the difference in the voltage level between a
reached ZP and a still turning motor is quite small.
This is our
measurement target
Self-Induction of measured coil
(must be skipped from measurement)
Induction time: about 500 µs