Reading a AC signal from an inductive rpm sensor. - Page 1
Forum section: electronics / projects

author: exosdel
author_rank: Newbie
timestamp: December 07, 2017, 08:58:09 am
content: Hello.I have a little issue regarding reading an rpm signal.The rpm sensor is a 2-wire inductive pickup coil that generates a AC sine wave.The sinewave frequency and amplitude vary´s between 600-1500Hz, and 5.5V to 12V. (measured between the two leads.)Readings:1560rpm|5.5V|610Hz2280rpm|7.8V|860Hz3800rpm|12 V|1480HzI´ve tried to read the signal by feeding in one of the leads into an optocoupler and grounded the other leadI tried it and i got some readings but not until the rpm was quite high, so a measured the sine wave againand realized that the amplitude had dropped significant.So i lowered the value on the resistor (R23) , and it got better, but not good.So I´m wondering, how should read this signal the correct way.Maybe i should but it trough i diode and rectify it, then use a clamp diode to clamp it to something like 5 volt?I hope you guys have some good tips.

author: fcb
author_rank: Super Contributor
timestamp: December 07, 2017, 09:03:16 am
content: It's quite possible that you are being limited by the current output capability of the inductive sensor. Not enough current to drive the opto. Look up the CTR of the opto-coupler - can you change it for a low-current opto?Are both sides of the circuit tied to the same ground as indicated? If so, why bother with an opto-coupler at all?

author: Rerouter
author_rank: Super Contributor
timestamp: December 07, 2017, 10:19:34 am
content: an inductive sensor generally has almost no current capacity, with an impedance of a few K ohm, your optocoupler will work, but you will need to do most of the hard work on the output side, e.g. pulling down a 100K pullup would be a reasonable number to aim for. as the conveyed current will be quite small.

author: nfmax
author_rank: Super Contributor
timestamp: December 07, 2017, 11:08:54 am
content: The secret to signal conditioning inductive to pickups is to recognise the signal amplitude is proportional to speed, because the voltage depends on the rate-of-change of magnetic flux through the pickup coil. So if you use an analogue integrator as your input stage, you will get a voltage output which is independent of speed over a wide range. Then use a comparator to drive your onto isolator (if you need one - usually the pickup coils are not connected to case ground)

author: Tomorokoshi
author_rank: Super Contributor
timestamp: December 07, 2017, 06:22:11 pm
content: Try these:https://www.digikey.com/product-detail/en/texas-instruments/LM1815M-NOPB/LM1815M-NOPB-ND/281357https://www.digikey.com/product-detail/en/texas-instruments/LM1815N-NOPB/LM1815N-NOPB-ND/228997It's the "LM1815 Adaptive Variable Reluctance Sensor Amplifier".In my application it was on an AC fan. It detected the little cooling fan blade for the motor, not the main airflow blade or the shaft, so I got 12x or something the RPM. As mentioned above the higher RPM gives a stronger signal.

author: xavier60
author_rank: Super Contributor
timestamp: December 08, 2017, 03:16:57 am
content: If the signal is put into an op amp miller integrator, the output level will be fairly constant. The value R needs to be kept low with respect to the impedance of the pickup. The output current capability of the op amp needs to be higher than that of the pickup.http://s3.amazonaws.com/answer-board-image/20089181425633572989255163750514.jpg