Learning about Load CellsFollow article
The journey so far
So far, I’ve done a couple of posts about The Raspberry Pi Starter Kit and using that to create a device that could be used in hospitals to increase patient’s health and overall experience. I have so far made a moisture sensor that could be placed in the beds of bedbound patients. I’ve spoken to a few nurses about what would make their lives and patients’ lives easier, and one of the things mentioned was a way to weigh a patient without having to get them out of bed, as its sometimes painful or just out and out impossible. Enter, the compression load cell.
What are they?
Time to get technical. A load cell is a transducer that is used to create an electrical output whose magnitude is directly proportional to the force being placed upon it. Load cells come in many forms, hydraulic load cells, pneumatic load cells and strain gauge load cells etc. We are looking at a compression load cell, a type of strain gauge load cell. A strain gauge measures the amount of strain, or in our case compression due to the weight placed on it. A strain gauge is actually usually made up of four resistors, arranged internally in a Wheatstone bridge configuration. Other, less common configurations are possible, but the FX1901 I selected uses this configuration, so that’s what we’re on a journey to fathom. The Wheatstone bridge is a great measurement system and is an arrangement of three resistors of known values, used to measure an unknown resistance using their ratios. The unknown resistance is proportional to the impact of some physical phenomenon, in our case pressure. Therefore, if you know the resistance, current etc, you know the pressure. Easy as that.
What makes them a magnificent measurer of unknowns?
We are looking at measuring current in order to tell us the unknown resistance and in turn, how much pressure was put on the device. If we look at current measurement using Ohm’s law can see that the error margins become quite high. Take the diagram below.
If the actual current is measured to the red point, we might measure the current anywhere in the blue region, if we the measure the resistance from the graph, we see a small error margin, however due to the Ohm’s law relationship and curve of the graph, a smaller current can have a much bigger error margin.
So working out the value for the current like this is as useful as a handbrake on a canoe. No wonder Edison, or as I like to call him, the poor man’s Tesla, went through so many filaments before he perfected his stolen invention and gave the world the lightbulb. Enter, the Wheatstone bridge. It is such a successful system because it minimises the error in the measurement of the unknown resistance. Its precision comes from the placement of the resistors. They are arranged as so.
The voltage reading we get across v1-v2 will depend on the ratio of the resistors on each side. If we plot a new graph to show us the voltage we will measure as a function of the actual voltage we see a linear relationship seen in the diagram below.
No matter where we measure we get a small error margin. This is what makes the Wheatstone bridge so great at measuring voltages.
The math behind the magic
Now we know why it’s so great, we can look into how to use is to measure the unknown resistance. Take the diagram below.
It works on the principle that if there is no difference in potential from one side to the other no current will flow. If we adjust variable resistor Rs until we get no current flow, and that is a value of 25 Ω , we know that the sides are equal in potential so we can say that:
Or in simpler terms:
we can then put the values in and say:
Then we can transpose the equasion so we can say that:
The resistance is proportional to the pressure put upon it, therefore if we know the resistance we know the pressure.
Which one is the one?
I had the pleasure of working with the FX1091. This little baby utilises Microfused™ technology. It’s trademarked, that’s how you know it’s good, isn’t that right Edison? This device from TE Connectivity allows for a virtually unlimited life cycle expectancy — always handy — and a superior resolution and high over-range capability when operated at low strains. It was designed principally for OEM use in labs and hospitals in everything from physical therapy devices and chiropractic and exercise equipment, to patient weight monitoring and payload monitoring. To get to grips with the FX1091 I laid it on a breadboard and hooked it up to a bench top power supply and measured the change in output when I applied pressure. Below you can see it in action.
With the power supply set in the picture above, with no pressure I got a reading of 1.29v, seen below.
After applying a little pressure that increased to 1.33v
I’m looking to use this in a patient weight monitoring system. So the FX1091S compact size, the potential for an unlimited life span, great range of operating temperatures and reasonable price make it ideal for what I need it for. Match made in heaven or so you’d think. Sadly, the compression cell is an analogue device and if you try hook up an analogue device to a Raspberry Pi and “computer says no”. Simple solution, add an Analogue to digital converter and that solves that. To solve the problem of the teeny tiny signal I’ll get out of the cell I’m going to employ the use of an amp too. The next post will explore putting the amp, the cell and the ADC together to make my weight monitoring idea into a reality.