I am currently constructing a dynamometer for a project in school. I will be using a hydraulic disk brake connected to a moment arm, which will compress a load cell to record the force. It has gotten to the point where I need to construct the wheatstone bridge for the load cell to amplify the signal. I have hit a wall here as I am not sure how I should proceed in constructing this bridge. The load cell is a compression style box shaped cell, with 2 strain gauges, one on each side. It is rated for 1300 pounds. I plan on purchasing a signal conditioner to give the necessary voltage gain, but must first construct the bridge. If anyone has any advice on how to construct the wheatstone bridge or what my next step should be, I would greatly appreciate it.

 

If you are starting with just strain gauges I suggest you give this a read. I assume you will be calculating the micro-strain and doing all the work. You are not using an off the shelf compression type load cell but making your own? Got a link to what you have?

Ron

 

I don't have a link it's all on paper. Basically what i have so far is a load cell and a signal conditioner in mind. All i need to do with the conditioner is determine the appropriate gain needed. I have been looking for an equation or some formula to determine the size of the resistors needed for the bridge. From what i have been told so far i will need to make a quarter wheatstone, with 3 resistors, preferably of the same resistance value. The load cell was made, not purchased so there are no specs on it. I need to test it to determine how i reacts to various loads, and the resistance values it will experience.

 

OK, that actually tells quite a bit. You will be constructing your own load cell, quite the project. You will start with the actual strain gauge. The actual strain gauges have several manufacturers, years ago I did quite a bit with Micro-Measurments who were since acquired by Vishay. They have plenty of informative information out there so you may want to start here and read. Among the more common strain gauges are 350 Ohm gauges so to make a bridge you would have 1 EA 350 Ohm strain gauge and 3 EA 350 Ohm precision resistors. You have a mountain of data to learn as to bonding the gauge to your substrate, that alone is a science unto itself. There isn't much to the electronics of the bridge or the subsequent Instrumentation Amplifier, learning the bonding techniques takes time and practice and generally several wasted gauges. You need to figure how the forces will be applied and act upon the gauge to determine the best gauge to choose. The link I gave you will lead to dozens of links and reading.

This is the easy part:


Vin needs to be absolutely stable, typically 10 volts so Google 10 Volt Reference. Vout will be passed to your IA (Instrumentation Amplifier) and you will not know the gain till you do all the micro-strain math calculations and understand the gauge itself. Anyway, start with the link and subsequent links.

If your load cell is already made you need to know the gauge resistance and build the bridge around what you have. If it is made much of the difficult stuff is done.

Ron