Is there anywhere where it is possible to buy a strain gauge, finished with opamp that can easily be connected to arduino? Was thinking about the wii fit board, this has four straingauges i believe.
I am trying to make an automated epoxy mixer. Tired of always getting my hands and stuff full of glue.
Was thinking about two pumps, a mixer, a stepper motor to drive the mixer up and down, and a scale to meassure the correct weight of resin and hardener. And maybe one scale for each can, to detect when they are getting empty.
How much epoxy are you mixing up at a time, as the wii board weight limit is 150kg (I think) and I would imagine that they would have design it to take all that 150 kgs (design weight) on 1 corner/sensor.
Just saying make sure you are not going to be getting 4 gauges with to big a scale to be useful.
Try ebay for strain gauges, or I saw sparkfun had some piezoresistive force sensors.
i can find a lot of cheap load cells on ebay, the problem is then how connect to the arduino. Any tips? Like a breakout board with opamp?
A typical load cell is a 4 wire resistance bridge. Two of the wires are to power the bridge and can easily use ground and +5vdc to do that. The other two wires are the measurement value and usually output some tens of millivolts full scale. So a good instrumentation op-amp is usually used to amplify that range up more usable for the Arduino analog input pins.
Possible issues are:
Is the op-amp used able to drive it's output close to the negative rail if ground is used for the negative rail? It's better if there is a negative voltage available to power the op-amp negative rail.
The +5vdc voltage will vary on a Arduino depending on if USB or external power is being used, and that will result in small calibration error. A more stable Aref voltage or possibly using the internal band gap reference would result in more stable calibration.
These problems can certainly be solved with a good instrumentation grade op-amp that has necessary gain and offset adjustments and able to drive down to zero volts. If offset cannot be corrected at the op-amp then there will be need in software to add span and offset corrections.
Not a cheap solution, but I had a project at work where I used a loadstar analog loadcell ($395) to monitor monitor force in a mechanical test application. Worked really well with my Arduino, infact its over in China chugging away as we speak.
So there seems to be no Opamp available with screw terminals, or arduino shield. So maybe its possible to get a scale from dealextreme and hack the output from opamp? Anyone done this?