Ch4rlotte

How to measure fingertip forces?

Hello,

I'm looking for some recommendations about what approach would be for my project.

I am aiming to create an instrumented device that measures the forces exerted by the fingertips during precision grip (between the thumb and index finger). So I would need to measure the force normal to the contact surface and the vertical force applied by the fingertips at the contact point. I need the distance between the thumb and index finger to be below 40mm - preferably less, so the depth of any force measuring sensor would need to be small.

I would need to be able to measure up to 20N with a safe overload capacity up to 50N. Ideally, the resolution of the device would be around 0.01N if possible with high accuracy and repeatability. The instrumented device will need to be wireless and light, so all data must be wirelessly transmitted to a standalone computer for logging and processing. Because of this requirement all amplification of the force signal will need to be very compact and light - I have been looking into some electronic boards, such as the AD7705. I will need a high sampling frequency of at least 200Hz, but ideally 500-600Hz.

I wasn't sure whether a triaxial load cell would be best for this or a configuration of strain gauges and the best approach to the signal conditioning.

Any advice would be appreciated :)

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Boss

September 2, 2020 08:38

I think Brad's approach will provide the most accurate which I think is what you are aiming for, but also consider what is being used in the sports world, particularly for golf and also in the medical world for health. Do an image search
https://duckduckgo.com/?q=measuring+grip+pressure&atb=v102-1&iar=images&iax=images&ia=images to see some of the possibilities.

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BradLevy

September 1, 2020 08:43

I'd look in to sensors like TE Connectivity FX1901-0001-0100-L (RS part stock #893-7398) or FX29K0-040B-0010-L. They are compact, lightweight, and the right maximum force range.
The FX19 model has low-level (~mV / V) analog output from a bridge across a user-supplied excitation voltage, and would need amplification and signal conditioning circuitry for use.
The FX29 sensor is available in several different output types, identified by the two characters following FX29 in the model number. K0 indicates digital output using I2C protocol, easily interfaced to microprocessors. That's the approach I'd go with.
You could use it with small processor boards with wireless built in, like offerings from Adafruit and Nordic, and keep the combined package quite small. The spec sheet for the FX29K0 lists a response time of 3ms if not in sleep mode.

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Ch4rlotte

September 1, 2020 08:43

That sounds great, thank you for the recommendation. I was aiming to measure both the compression and the vertical shear force with one sensor - is there one in the TE Connectivity range that could do that?