VR Sensor Fault Detection - Page 1
Forum section: electronics / projects

author: rvalente
author_rank: Frequent Contributor
timestamp: September 06, 2020, 04:13:37 pm
content: 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 MAX9926Some 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 100VAs for the sensor, need to get some statusSensor OK, in 900 to 2000 ohm rangeSensor NOT OK, do not need to know the fault precisely but to know its not fine.Sensor disconnected/open circuitSensor 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 VbatSo 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 ampI'm not sure what the best approach would be, to have a permanent fault monitoring or a switched and check every in a while?

author: rvalente
author_rank: Frequent Contributor
timestamp: September 08, 2020, 08:20:33 pm
content: 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?

author: max_torque
author_rank: Super Contributor
timestamp: September 09, 2020, 07:53:43 pm
content: 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!

author: rvalente
author_rank: Frequent Contributor
timestamp: September 10, 2020, 01:58:41 pm
content: Quote from: max_torque on September 09, 2020, 07:53:43 pmIt is function critical, if any sensor fail, I need to detect and disable the system throw a MIL and CODE, thats allTake 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.

author: max_torque
author_rank: Super Contributor
timestamp: September 10, 2020, 04:54:27 pm
content: 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.

author: rvalente
author_rank: Frequent Contributor
timestamp: September 10, 2020, 08:22:04 pm
content: I wish I could say what the system do but, unfortunately, due to NDA restrictions I can notThe sensor is the very same used in trucks ABS system. For truck ABS in case of failure the pneumatic the brakes keeps working but, without abs assistance.If my system fails the vehicle would keeping working just fine but, would loss an extra feature, just like the ABS. By the other hand, if the system acts due to sensor a false positive/negative information this could lead to serious problems. One of the supervisions is making sure the frequency read by the AD and the MAX9926 are similar, as wheel as the voltage/frequency being generated by the tone wheel.Unfortunately due to commercial reasons I'm tied to a VR sensor, the sensor it self is very very reliable, since wabco, knorr bremse and haldex still uses them as today.Bosch has moved from VR sensor to hall current sensors for ABS generations ago but not trucks.So, I'm still looking for a way to detect if the sensor is connected, measuring its resistance.I can detect short to ground or Vbat with the diff amp, as the average signal would change with a offset.I would like to know the resistance without feeding a bias current or, if I do so, I believe it must be switched.