In need of a very short range displacement/distance sensor with mm accuracy

So for my thesis project i'm measuring fruit firmness without damaging the fruit (so no penetrometers). My way of approaching it is to use a force sensitive resistor attached to an actuator which will press against the fruit with a big enough surface area that the fruit will not get damaged while a distance sensor placed strategically to tell me how far the actuator has moved from it's original position so i can plot a distance vs force graph. Hence my need for a short range displacement/distance sensor. So far I can't find any decent cheap ones. Suggestions please?

How about something like the instrument an optometrist uses to measure intra-ocular pressure?

I think you should look into using an Arduino to read a digital caliper, they will provide mm accuracy for you if the caliper doe not have jaws deep enough you could extend the jaws or buy linear scales that can also be interfaced to Arduino. Here is a link that will probably get you rolling interfacing arduino with a digital caliper

Google is your friend if that link is not enough.

Use a mechanical lever to amplify the movement so that mm becomes cm. Use an array of cheap reed switches to sense the position of a magnet attached to the end of the lever.

Why not use an servo or stepper motor to press your device against the fruit? Then you can increase the distance while monitoring the pressure applied.

You should stay away from force sensitive resistors – they’re just not very precise nor accurate and you get lots of problems with drift from temperature or duty cycle. A four-wire load cell and an HX711 amplifier combination is inexpensive (<$10) and will be much, much more precise.

For a linear distance sensor I would sugguest a rotary encoder and then match it with any type of rotary-to-straight-line linkage (scotch yoke, rack and pinion, etc.).

Load cells for sure.

I’m guessing that this isn’t for a production environment. You will be place the fruit into some kind of holding gig and then start the test.

You will have to control the force that the actuator uses to push against the fruit, otherwise you will stall against the fruit (not enough force) or smash into it (too much force).

For a linear distance measurement, I used LVDT’s. Pretty bullet proof. You could also use linear pots. Just get one the has good repeatability. Need to make sure they will handle the mess of a fruit explosion.

I think you want a durometer, google "Shore durometer".