Stepper Motor or Servo feedback to measure the pressure?

@homerna

I assume you are thinking of a mechanism to do the same thing as a physiotherapist?

I would say that would require the input from someone who knows what is required.  Any solution may have legal implications if done incorrectly?  If you can't say how much pressure to apply how can you build it in the first place even if you knew how?  If you use a pressure sensor you would still have to know how much pressure to apply.

The current a motor draws will vary with the load. If there is resistance to the turning of the motor shaft the current will go up. I saw in a recent post someone using a ACS712 Current Sensor.

@homerna

I would suggest you look at using pneumatics versus steppers/servos for the RoM device.  You'll need to calibrate the device to use proper amount of force and then monitor for variances and vent if necessary.  

@huckohio

Thanks. Can I ask why are you suggesting pneumatics? Is it because it's easier to get feedback or regulate the force? 

@homerna

Another thought was an adjustable torque slip clutch.  You will see them used on portable drills.

@robotbuilder

Thanks so much. "acs712" was what I was looking for. I am pretty rusty when it comes to electronics but I will find a way to write an arduino or Pi code based on acs712 and control the pressure. I also searched for acs712 on youtube and found some videos.

Thanks again for your pointer.

@homerna

Mainly for safety reasons.  Air compresses when you apply resistance.  For example, if you're doing RoM on an arm, and there is a muscle spasm/twitching, you want the unit to 'give' or vent when the measured pressure exceeds a variance you can program in.  The air solution will be bigger than using a motor/servo, but in my opinion it will be significantly safer.      

@homerna

Think of any/all possible failure/fault conditions and if your solution enters into that condition will it cause harm to your son.

@homerna

I assume here we are talking about a continuous passive motion (CPM) machine?
I knew someone with some kind of nerve disease who didn't get therapy and his joints all froze up. Out of curiosity I did an online search to see what works.
https://www.medgadget.com/2015/10/a-low-cost-3d-printed-finger-continuous-passive-motion-device-video.html
https://www.medicalexpo.com/prod/kinetec/product-103834-825008.html
https://www.medicalexpo.com/prod/qal-medical/product-93399-817339.html

@robotbuilder

You guys are wonderful. Yes. The devices you have found on medicalexpo are the ones I am trying to build (a mini version of it). I can't really afford to buy any of these devices and insurance won't cover them.

So far acs712 seems to be the way to go.