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VR Sensor Fault Detection

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rvalente:
Hello Mates,

i'm working in this automotive design, 28V, need to monitor a shaft speed. The design is based around a VR sensor (pick-up), which is a requirement (easily sourceable, certified, cheap and robust for harsh enviroments). The resistance is 900 to 2000Ohms and inductance 400mH. The sensor is mounted on a pole wheel, the signal is handled by the MAX9926

Some fault monitoring is required:

As for the pole wheel its necessary to monitor it run-out and air gap, the run-out is easy to catch as the voltage would sweep over one revolution and the air gap is a ratio of voltage x frequency, so I'm thinking of use a differential amp with a attenuation, as the voltages can go as high as 100V

As for the sensor, need to get some status

Sensor OK, in 900 to 2000 ohm range

Sensor NOT OK, do not need to know the fault precisely but to know its not fine.

Sensor disconnected/open circuit
Sensor shorted (under 900?)
Sensor open (>>2000ohm)
One or two leg(s) shorted to Vbat (28V)
One or two leg(s) shorted to Ground (gnd)

It would be very rare but, could happen to have one leg shorted go gnd and the other to Vbat

So I was trying to imagine a circuit that could detect all this, be simple, robust (as auto designs must pass a lot of bad situations and survive).

The first idea was to add some current biasing around 1mA (through resistors or a constant current supply), and check the voltage in the op-amp, but I was afraid it would reduce the circuit performance.

Thought about adding a little switcher to bring each sensor leg to Vbat/1mA and ground, to do some measurements with the diff amp

I'm not sure what the best approach would be, to have a permanent fault monitoring or a switched and check every in a while?

rvalente:
So, I did some tests injecting a 1mA current in the VR sensor and the MAX9926 was blinded for low voltages, which was expected..

So I'm thinking of a chopper circuit, to test it every now and them, but what switches would be suitable turn on an off this 13K8 resistor over 28V? (I know the rail voltage so) or should  make a 1mA switchable current source?

max_torque:
The first question is "why"?

What does the diagnostics actually provide? Is it safety critical? Is it functional critical? What happens if it fails?

Engine management systems only tend to do a basic "sensor present" check for crank sensors (looking for the presence of the bias voltage) and that's it. If the signal fails the EMS either stops or these days reverts to using the CAM input with reduced angular resolution on the crank.

I suspect, it it's that critical, you'll want to simply fit two sensors!

rvalente:

--- Quote from: max_torque on September 09, 2020, 07:53:43 pm ---The first question is "why"?

What does the diagnostics actually provide? Is it safety critical? Is it functional critical? What happens if it fails?

Engine management systems only tend to do a basic "sensor present" check for crank sensors (looking for the presence of the bias voltage) and that's it. If the signal fails the EMS either stops or these days reverts to using the CAM input with reduced angular resolution on the crank.

I suspect, it it's that critical, you'll want to simply fit two sensors!

--- End quote ---

It is function critical, if any sensor fail, I need to detect and disable the system throw a MIL and CODE, thats all

Take for example, Wabco and Knorr Bremse ABS systems (this is not a ABS system) they use similar sensors but, can sense the failure and the ABS is disabled, here the criteria is similar. Ack, signal, disable.

max_torque:
ABS is SAFETY critical, not function critical!  What does your system do, and what happens if it doesn't do it?

If your system is realy SAFETY critical, then you'll need more than diagnostics, you may need redundancy!  That's a different ball game entirely!

(also note, that ABS/DSC despite being SAFETY critical, is able to go "non operational" in the event of sensor failure, because the hydraulic system will remain operational and the vehicle can still be braked (just without the ABS/DSC function, which means it does not require sensor redundancy)

In any case, id suggest not using VR sensors, and move to active sensors (such as modern ABS sensors) that use a current loop so basic data can be exchanged with the sensors and be highly noise resistant and easy to diagnose (because output current is not in anyway proportional to sensed state, so diagnostics can occur without movement!  Ie your ABS system can do a key-on diag BEFORE you drive away and warn the driver as necessary.

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