Jeremy Harris wrote:Nick,
These sensors do appear to latch when used with the sort of magnetic field strength we're using. In practice, you find that the sensor changes state when a certain magnetic flux threshold is passed, with a given magnetic polarity. The sensor output then remains "latched" in that state until exposed to a similar magnetic flux with the opposite magnetic polarity.
This is a good characteristic for use as a motor sensor, where the controller is relying on accurate edge-sensing, as it means that the sensors are relatively immune to noise.
My experience, having converted three motors now, is that the output from these sensors, when used in this application, is very reliable. I was surprised at just how accurate the zero-crossing point of the three sensor outputs were, when compared to the trapezoidal waveforms from the motor being driven by a sensorless controller. With the motor delta wired, these were spot on, as close as I could measure on the 'scope (probably better than 5 electrical degrees).
If you think in terms of a latching relay, then that does sound just like latching. But in terms of circuit operation, its hysteresis. I imagine what's inside is an analogue field sensor followed by a Schmidt trigger. In that particular case its debatable whether there is any practical difference between latching and hysteresis. Normally, though, I'd expect the term latching to be used where the input is digital and hysteresis where the input is analogue, so I still claim its "hysteresis". The latching relay I mentioned at the beginning has digital inputs.
It looks like the "latching" versions just have more hysteresis, so its a bit naughty of Honeywell to call some latching and some not. They would be better off calling some "high sensitivity" and some "high field strength". That would be less misleading.
Anyway, I'm going to be running some of these motors with sensored controllers, which is why my head is full of these details. I'm probably going to try both fitting Halls and electronic synthesis of the Hall signals.
BTW, it looks like most of the prior art reported in this thread is in the UK.