Wireless, hand held monitoring device for Medical device application

I was going to suggest a Linear Variable Differential Transformer (LVDT) for piston travel - we use them on the Eurofighter Typhoon Flight Control System - they are very reliable and accurate. Alas, they are also rather pricey . . . . . . .

I suppose that if you're trying to keep a £75M fighter aircraft from crashing though you're willing to spend a bit!

Not sure if there are items available in your price bracket, if not then finding a way of turning a rotary encoder with linear stroke motion (think rack and pinion) could be an option to explore.

As for pressure, John has helpfully suggested one option, another is a strain gauge system (simple wheatstone bridge setup with one arm of the bridge a resistive strain gauge sensor - at rest the bridge will be balanced (a variable resistor will be needed on the opposite arm of the bridge to null it out). Flexing that sensor changes the resistance - this unbalances the bridge and gives an output voltage proportional to the amount of stress measured.  (R1 and R4 match, and are the same as strain gauge at R3, R2 is a multi-turn trimpot in the same range (close value). The supply MUST be stable, any fluctuations will cause the output (at V) to vary.  Set R2 to give a zero output at rest, thats it set up. Measuring that output can then be done easily.

A quick edit to the above diagram, the balancing trimpot would actually be in the place of R4 and NOT R2 as I drew (and stated)!

The idea is the 2 arms of the bridge act as 'Potential Dividers'. If R1 and R2 are the same value then the voltage measured at their junction would be exactly 1/2 supply voltage. In reality there will be a small difference.

The combination of unstressed strain gauge and the trimpot should also be approximately equal, their junction will ALSO be 1/2 supply.

The trimpot's function is to balance the bridge (effectively negating any variance in resistors) so both outputs to the voltmeter are identical.

As you are using the meter to measure the potential difference between both legs of the bridge, balance = 0V

Change the resistance of the strain gauge and you get a voltage out.

Thanks a ton for the recommendations and options.  Since my device will be mostly targeted to the consumer market incorporating this type of spendy device is obviously overkill and financially impractical.  Your second options sounds much more feasible.  Any links to the OEM or distributors who sell these units? I will definitely to look into the " strain gauge system (simple wheatstone bridge setup with one arm of the bridge a resistive strain gauge sensor. Flexing that sensor changes the resistance - this unbalances the bridge and gives an output voltage proportional to the amount of stress measured. 

But any more suggestions, ideas, or links would be fantastic.  My realm is more mechanical engineering, structure and design rather than wireless, electronical devices for monoritoring purposes.  I really appreciate your help as i'm very much a newby in this category of product development.

jack r ellenberger

President/SoftSports, LLC

For translating linear motion into rotary - I know of nothing available, I was thinking more along the lines of having a 'rack' on the sliding portion of the chair turning a gear (pinion) on a rotary encoder.

Rotary encoders are a standard electronic component and are relatlively easy to interface to any microcontroller. Newark / E14 / CPC / eBay / RS all will stock them.

The strain gauge is also easy to get online, what you DO need is a way to VERY ACCURATELY measure small voltage changes. The variance in resistance is small, or does depend on how much the device it is attached to will flex.

Some sort of amplification (very low noise) will be needed to turn potentially a mV reading to something useable.

Here is a link to some simple info about strain gauges (the system I teach uses the 'Full Bridge' setup.

One thought is to use a sprung spool with a wire running to the fixed point.

On the end of the spool wire is an encoder.

As the wire is pulled by the top going up it rotates and the encoder measures the amount of rotation and displays the distance.

You could do something simiar with a wheel that gets rotated as the upper section moves.

This would allow distance and time to be recorded at any stage.

mark

Awesome idea Mark!!!. thank you, thank you, thank you!!   Anyway possible you could send me a pic or link or drawing of how this "spring spool" would work with my device????  I got the basic idea but a pic is worth a million words. Where would they be available? Thank you so much.

jack ellenberger

Yes, absolutely.  I like the "rack and pinion" idea on a rotary encoder. 

I'm not sure of the strain gauge and a way to measure small amounts of current.  The material i used on the metal chair is HDPE or High Density Polyethylene which is flexible and will give substantially if too much pressure is exerted via the actuator and i like this because it is better to have the integrated flex in the HDPE so as to bend itself and "give" instead of exerting excessive traction pressure and thus creating potential liability.  It's not cheap but it should last almost forever in this application. Again, our medical disclosure information should handle this issue if necessary. The unit should be easily capable of exerting 50lbs plus  of upward traction pressure which only a large, heavy person might need.  Typically 10-30 lbs of traction pressure would suffice with most adult users.  Duration periods would range from 2 to 8 minutes depending on the individuals needs.  We recommend starting out with only high pressure working upward.  Longer periods are possible but we are still in my testing stages. The unit works great as it but having these three additional digital read outs would be a giant step in the right direction.

I assume all these ideas would be compatible with bluetooth/wireless technology?

Thank you again very much.  Your help is appreciated beyond words.

jack ellenberger

Apparently there is a wiki page (that I din't know about before suggesting it)

https://en.wikipedia.org/wiki/String_potentiometer

The concept is explained here.

Draw Wire Encoders | Technology Overview | Dynapar

Control Valve Positioner Performance Diagnostics

more frightening is when you type " wire encoder " into google images, you get a lot of VERY expensive looking devices.

Mark

Apparently there is a wiki page (that I din't know about before suggesting it)

https://en.wikipedia.org/wiki/String_potentiometer

The concept is explained here.

Draw Wire Encoders | Technology Overview | Dynapar

Control Valve Positioner Performance Diagnostics

more frightening is when you type " wire encoder " into google images, you get a lot of VERY expensive looking devices.

Mark