In my physics class we used a electronic force reader hooked up to a CBL to find the coefficient of friction of pulling a block of wood. What we tied the string leading from the block to the instrument was a hook that didn’t visibly move at all no matter how much force was acting upon it. What I am wondering is how it worked?
This is all of the top of my head so I don’t remember the names for it, but there are substances whose properties change under stress. For example some crystal might have electical conductivity that changes with stress.
So, it’s a chunk of stuff that when you squeeze it, some other fundamental property changed besides length. Imagine a solid chunk of ‘magic’ wood and when you stand on it it quickly but gradually turns red, when you get off it turns blue. If you were careful, you could calibrate the color to get an accurate measure of your weight. Same idea except you are measuring resistivity or conductivity or resonance frequency.
It probably used either a peizoelectric element or a strain gauge, probably a strain gauge.
A piezoelectric element is built around a crystal. As the crystal is stressed, a voltage can be read across it. howstuffworks.com has a good description of piezoelecricity. It doesn’t exactly describe a pressure gauge, but it’s pretty clear about the principles.
A strain gauge (pic) changes resistance as it is stretched or compressed. I think that as the gauge is stretched, the metal traces lengthen and narrow, increasing the resistance across the element (I couldn’t find a description online and I can’t find my electronics dictionary, but I think I’m right.)
cornflakes is correct. The device mongrel_8 used (please call it a “force transducer” rather than a “force reader”) is almost certainly based on a strain gauge.
Piezoelectric force transducers are also manufactured, but these are basically used in dynamic application because they have a quicker response time than the strain-gauge type. For a constant-load application, like the OP describes, piezos are not used because the charge they produce when strained bleeds off over a short time, meaning that the voltage you read across the material quickly drops to zero, even though you keep a constant force on the transducer.
The strain gauge does indeed change resistance as the transducer strains (the traces lengthen and narrow, as cornflakes said), and this change in resistance is read (via a bridge circuit) as strain. The strain gauge is pasted onto a metal block that has a known force-to-strain relationship, and voila! apply force->stretch block->stretch strain gauge->change resistance->alter bridge circuit->output voltage->calibrate to a force number.
Thanks all for answering my question. Zut, I figured it wasn’t called a force reader but I couldn’t remember exactly. Please forgive me for my stupidity.
Sorry, mongrel, I wasn’t intending to denigrate your intelligence or come across as snippy. Upon rereading, though, my statement (please call it a “force transducer” rather than a “force reader”) does seem…intolerant, for lack of a better word. That was not my intention; my apologies.