Analog torque sensor - Page 1
Forum section: electronics / projects

author: Fiorenzo
author_rank: Regular Contributor
timestamp: October 19, 2023, 07:53:41 am
content: Maybe a year ago I've posted here some question about the sensor I am going to discuss but after that I have got quite ill so everything was abandoned.But now I am keen to resurrect such project.So, If you watch the attached photos you see the magneto elastic sensor I was referring to.It is from a Panasonic e-bike motor made in the late 2000.It is made of 2 identical coils with a common point as in the schematic.I think the common point is for ground.Now I am trying to understand how to drive it in the best way but no documentation exists.For what I know the simplest way is to inject a pulsating signal on one of the two coils and catch the output from the other coil.What is frustrating for me is that I do not have strong knowledge on this kind of applications and I am not used to work with analogic signals etc...Ofcourse I am studying all I can find about It but many things remain confusing for me.For example:The coils has the same resistance of about 75 ohm, so what could be the best peak voltage to apply on the primary coil? I am working with 12 volts peak to peak but I am not sure if it could damage the coil.I aim for a high voltage beacuse, on the secondary coil I get, as output, a signal that has an amplitude of about 0,7volts with 12volt input, so it is a bit faint I think.Now another thing I cannot understand is why the input signal gets very distorted when it is applied to the coil.I have two signal sources: one is a signal generator by Siglent and the other is an lm555 used in a configuration that gives an adjustable pulses generator from 1 to a 100 khz.With a square wave 10khz signal and duty cycle of 50% the pulses of output from the Siglent becames ramped down like a discharging capacitor, on the other hand the signal from the lm555 becames also distorted but a lot more, the frequency goes down by half and the duty from 50% goes down to less than 20%, also the pulses becames deformed.I don't understand why all these happens so this is the point where I am stuck now.After "solving" this situation I have to understand how to get a reliable and good output from the secondary coil and how to read it in the best way (I know that some people rectify it and someone instead read the peak of the output wave with an ADC).Thank you!

author: SeanB
author_rank: Super Contributor
timestamp: October 19, 2023, 09:54:26 am
content: Normally you use around 1V peak to peak to drive the coil, using as good a sine wave as possible. the output coil has a few AC coupled amplifier stages, then a synchronous rectifier to sample the output on the peak of the waveform. This voltage is then proportional to torque, as the coils are coupled by the tube, and the torque on it varies the amount of signal coupled between the coils.

author: Fiorenzo
author_rank: Regular Contributor
timestamp: October 19, 2023, 10:29:15 am
content: Thank you for your reply, but why a so low voltage of 1 volt?

author: Fiorenzo
author_rank: Regular Contributor
timestamp: October 20, 2023, 06:32:08 am
content: I would like to ask another question also: why is a sinusoidal signal better than a square wave one?Thanks

author: Fiorenzo
author_rank: Regular Contributor
timestamp: November 10, 2023, 11:47:59 am
content: Ok I am asking crappy questions

author: Terry Bites
author_rank: Super Contributor
timestamp: November 10, 2023, 01:18:45 pm
content: You can generate a nice enough sine wave by lowpass filtering a 50% pwm output from a GPIO.Some MCUs have uncomitted opamps making life very easy. eg PIC16F1779The simple peak detector will poop out at very low signal levels but that might not be a bad thing in your applcation.

author: Paulo Peres
author_rank: Contributor
timestamp: November 10, 2023, 11:32:24 pm
content: Are you able to measure the winding wires diameter?You can find the wire max ampacity in the AWG table and use the ohms law to calculate the maximum DC voltage value for that current (since you have the impedance).Knowing the maximum, use a good margin to avoid that current (I would start with values to generate no more than some tens of miliwatt). A sine wave will be enough to know if the value you adopt can transfer a good amount of energy.The problem with square wave is the core saturation. If you put too much DC in to the core, it will not transfer energy as it would with AC.But I don't know this core. So, you need to test.Last hint. Uses a low impedance generator and manage to obtain an internal resistance of same value of the coil you are exciting.I will guess a frequency value too: about 400Hz (but, again, I don't know the core).