Hi:

I am trying to measure force on a string. I was told and strain gauge will be suitable for this. The thing is that I do not know how to do so.

I know strain gauges give a voltage based on a resistor which is bent (not quite sure this is how it works). But the main question is how would you know the force applied with just a voltage given?

Can anybody help me with this?

 

I've built strain gages for a previous job, so I guess I can be called familiar with them. Basically they are a 4 resistor bridge similar to what is shown in http://www.allaboutcircuits.com/vol_1/chpt_8/10.html . Every resistor in the Wheatstone bridge is a strain gage resistor, you measure the differential between the two legs, which is proportional to the force applied.

They use a excitation voltage (feeding the bridge), and have a differential out. They are perfect for op amps, but that isn't the only way to go about it.

The are extremely repeatable, but you will need to calibrate them. Apply a known amount of force and take a reading.

 

ok I see so where would be the best place to get them from ?

You know cheap and good

 

Omega Engineering. Take the time to read up on them and the ways to place them to read strain on the fixture. You will need to use known weights to calibrate the readings.

 

Force on a string with strain gauges?

That's going to be kinda tough.

Usually, they're put on a beam or cylinder. If you take two matched strain gauges and put one on two opposing sides, you wind up with a reasonably linear response.

I still have a few strain gauges that were made by the Baldwin Locomotive Works back in the 40's; my grandpa had them in his machine shop.

 

You don't need two matched gages on each end, just one. The ones I'm familiar with are web type, which use the 4 resistor setup, think in terms of a set of scales for weighing things. My old job has marked me for life though, I'm always saying it is better to buy premade, and here I'm thinking of making one from scratch (DOH!). I know better too, tain't easy making them, not if you're doing it right.

Again, you'll need an excitation voltage, usually around 5V, and a set of weights if you're going to calibrate them to a DVM / Computer interface.

 

You mount the gauges on a cantilever beam that the string pulls on. It bends with the stress applied.