Hi, I have a project in mind and I've started ordering these 75kg load cells, but I guess the signal needs amplification. I need to read each individual load cell so I can calculate the center of the force applied. Do I need 4x amplifiers, or does it exist a more cost efficient way of amplifying and reading individual values from multiple load cells (lets say 8 cells) than buying and using 8 amplifiers?
The load cells has 3 wires.
Hi, and welcome to the forum.
A HX711 breakout board is commonly used for that.
Very cheap on ebay.
Read this.
Enter "HX711" in the search box on top of this page.
Leo..
Because my load cells has 3 wires, I see its possible to connect two load cells to a single HX711 to form a full bridge, but will I then be able to read both forces applied on each load cell, or is the value retrieved via HX711 a sum of both load cells?
If I use one load cell per HX711 I need to connect a resistor to get the 4th wire right?
If you're using one load cell you need two resistors. Assuming you were "exciting" the load cell with 5V, the load cell will output 2.5V on its sense wire. You then need to use the two resistors (or a multi-turn, cermet potentiometer) to create a divider to create a second, 2.5V output. Then when a force is applied to the load cell that 2.5V will increase/decrease and what you're measuring is the difference (in millivolts) of the outputs on those two sense wires.
With two, three wire load cells wired together you arrange them so the voltage out the sense wire on one of the cells increases with force and the other decreases with force. And again you're measuring the difference between those two voltages.
The HX711 has two channels (two wires from each pair of load cells) so it'll work well with your set of four.
This question comes up so often, maybe there should be a "sticky" about it.
Oh, right. No one reads those...
PS to the OP: the above is a hint that the answers to your questions can be easily found by searching this forum (see the search box in the upper right?) or using Google...
The only reason you would wire two 3 wire load cells to one HX711 would be to either use one unloaded for temperature compensation, or if you are measuring only the difference in load between the two load cells.
Your load cell schematic diagram looks like this:
You just need to add two resistors, or as pointed out two resistors and a trim pot, to get this Wheatstone Bridge configuration:
A trimpot like this:
You can think of this as a zero set.
Use two (selected from the same batch) 1k 1% resistors for the other half of the bridge.
The absolute value is not important. Just select close matching pairs.
Leo..
To answer one of your original questions, if you need to read the output of "n" load cells (to calculate the center of force), then you need an ADC with "n" channels.
As pointed out by @Chagrin, the HX711 has two channels, but beware: the gain of Channel A is 64 or 128, and Channel B is fixed at 32. Maybe you can live with that.
You might be able to use an ADC with just one channel together with a mux/de-mux analog switch like this DP4T example, but that's getting way beyond my experience. Looks like they're only available in surface mount. Using a multiple-channel ADC would be simpler.
If you can find 0.1% resistors even easier, but costlier, they will probably be good to go as is.
Trimming the offset can be done with much large value resistors in parallel to one of the
bridge legs - such parallel resistors don't have such high accuracy requirements.
Its important the resistors are at the same temperature as each other, so mount them closely together.