How can human finger pressure be measured?

Question

I am dealing with a situation where a patient can only very lightly press his fingers / hand to signal what he / she wants. Speaking or eye tracking is not possible.

The person is also not able to lift his hand. The only movement possible is a slight press with the hand / fingers.

I was therefore thinking about using a capacitative device that would be calibrated to detect how much pressure is a willing / intended press and which amount pressure is simply a resting position of the hand on the device.

I guess a capacitative device is what is required here. I saw a solution of a custom device here on YouTube, but that looked so custom-made and so rigid that I was wondering if what I need actually already exists.

I would therefore like to ask how I could measure the pressure of a human finger / hand on a device in a reliable way.

Thank you.

Edit: Nobody so far suggested to use a precision balance with USB output option. May I ask if that would not work?

Answer 1

There is a component called a "force-sensitive resistor" that I'd suggest you try. You can get them in a small, round configuration like this one from Adafruit force-sensitive resistor.

They are easy to read as they just appear as a variable resistance; as the finger pressure is increased, the resistance decreases. Make a voltage divider with the FSR as one leg, then most any microcontroller with an analog input can measure the pressure.

You can also make these yourself using a sandwich of piezoresistive sheet between two layers of conductive fabric, but the ready-made FSR is easier.

A capacitive sensor may also work, but detecting variation in pressure in a consistent way is trickier. The capacitance will increase with finger pressure, but the capacitance will also be affected by the skin's hydration, surface moisture, etc.

Answer 2

I've tried out a bunch of different pressure sensing technologies for robotic manipulation (specifically robot prosthetics). There are many that are easy to manufacture with limited resources (3d printing + cheap PCBs + off the shelf, room temp cure silicone rubber). One of the best sensors I ever made was based on this paper. It's quite literally a blob of silicone rubber that suspends a magnet over a 3-axis hall effect sensor (the one in the paper is the MLX90393, but there are many such devices on the market that would work equally well). It's extremely simple and cheap to make, highly reliable, sensitive, and can measure shear forces as well.

I ultimately wasn't able to use this for my robot manipulator, because magnetized objects (i.e. screwdrivers, any steel object with light magnetization) caused false positive pressure readings. However, since you'll be interfacing only with a patient's finger, stray field lines from other objects won't really be an issue. The extra dimensions of sensitivity also may allow you to maximize whatever range of motion is left in your patient. You also should be able to 'tune' the sensitivity by changing the stiffness of the silicone blob. You'll want to purchase platinum cure silicones that are 'skin safe' for your application as well.

Link to video showing off 'feather touch' and light finger touches.

Answer 3

You could take a look at fiber optic laser whiskers. What you do is you get a laser diode and send it down a cheap, plastic, crappy, step-graded fiber optic loop (you don't want the expensive continuous graded or glass kind) with a photodiode on the other end.

Deforming the loop, even slightly changes the the interference pattern read by the photodiode or phototransistor. The output should correspond with how badly the loop has been deformed.

It might be possible to use one of the laser diodes with an integrated photodiode to send the signal down a straight fiber with a reflector of some sort on the other end. That way you wouldn't need a loop. I don't really understand the way the decoherence and interference pattern works in fiber so I don't know if light taking the same path down the wire both ways would still produce the changes you need (they might cancel out and eliminate the changes you need or they might double up and make things more sensitive, I don't know).

You can then put it in any cushion or substrate you want, and a hard plate on top of that if you wish. Or make a glove or put it in a silicone or plastic tube that the guy can squeeze, or whatever other form factor you want.

I've never used this for anything myself but it is written about in Gordon McComb's book Robot Builder's Bonanza but you could probably also find other search results about it. In the book he says they are very, very sensitive. Feather touch is how it is described.

There are also Fiber Bragg Grating sensor which are smaller and simliar but not identical principles. You'd need to buy these as is since they are made with a specially constructed fiber. These are what is used on commercially available optical strain gauges (as opposed to more conventional metal alloy resistive strain gauges). They sound really cool.

Answer 4

I would try making device where is LED shining at some angle to photoresistor. When the LED (or photoresistor) changes the angle, the amount of light on the photoresistor decreases or increases.

LED-moving variant:

Photoresistor-obscuring variant:

Similar device can be also made with fixed LED and photoresistor, when some opaque part of the device partially obscures the photoresistor.

Such devices should not have any problems with signals from the environment (such as magnetism, electrical charges etc.). The device should be lightproof and the LED should have stable power supply.

Answer 5

An alternative pressure sensor is the Quantum Tunnelling Composite materials from Peratech which changes its resistance from megohms through kilohms to ohms according to the applied pressure.

A good article about the technology here

Small "pills" of this material used to be available here or here (but apparently no more) Maplin in the UK also used to sell them, but no more.

It might be easier to scan a 2D array of these looking for low resistances than detecting voltage from a large array of piezo sources.

Wikipedia mentions licensing deals with another manufacturer but their website appears to be dead atm..

Answer 6

An alternative is to measure the patient's muscle instead of their hand movement. You can get muscle sensors that connect to electrodes that you'd stick to the patient's bicep, brachioradialis, or whatever muscle they're using to apply pressure with. When they press with their hand, you detect the muscle flexing instead of the pressure. Depending on how sensitive your sensors are, you might even be able to detect movements in other muscles that are controllable by the patient, but are too small to have any visible effect on hand movement/pressure.

Answer 7

This would probably require testing and calibration, but pressing the finger does deform it slightly, increasing contact area, which in turn increases the measured capacitance.

I am not sure whether this difference is enough to be detectable and would most likely require per-patient calibration, but it is something you might want to check.

Answer 8

The classic solution here is a strain gauge bonded to the side of a metal or hard plastic grip. They're accurate, cheap, and easy to interface to. They're used in all manner of sensors for exactly this kind of application.

Capacitive sensors are very, very accurate, but by orders of magnitude more than you need for this. (I actually work for a company which makes capacitive sensors - some kit made by my company back in the day went into the Canadarm2 on the ISS for strain measurement). They aren't cheap, and nor is the electronics to drive it.

Touchscreens do use capacitive sensors too, of course, but they're designed as a binary sensor. Although you can do some stuff with them for analogue measurement, they're not a great place to start.

Answer 9

Look at a strain gauge, they make some very small ones that should work. You could also adjust the sensitivity over time or a learning method and as the patient improves so does the force required.

Answer 10

There are lots of ways to measure pressure, but it seems like it might work better to just use a leaf-type microswitch, or some similar switch that clicks when it engages and disengages. This won't distinguish degrees of pressure, but it seems likely that a patient in this state wouldn't be able to reliably control multiple pressure levels.

If the patient has any sense of touch left, haptic feedback from a click-switch can make a huge difference in usability. The amount of pressure required can be tuned by adjusting lever length or position.