Tuesday, 8 January 2013

Amplifying Load Cells

Inside a small kitchen scale
A while ago I pulled pieces out of small electronic scales and found some full bridge load cells along with hidden circuitry. The circuits were applying a time varying excitation signal to the bridge, but without an oscilloscope, I couldn't see what it was. It is possible to operate a bridge in a simple DC circuit, although there can be issues with noise.

Resistive bridge circuits detect small changes in resistance by producing really small changes in output voltage, so the difference between the two measurement points on the bridge will be millivolts for an excitation of multiple volts. Getting this tiny, differential voltage up to something readable requires an instrumentation amplifier like the AD8221. Unfortunately, I could only find the chip in a surface mount package, less than a quarter inch on a side. This challenged my soldering skills getting it onto a DIP carrier so I could work with it.

The 8221 takes a really small difference in voltage between pins 1 and 4 and produces a larger voltage output on pin 7. If there is no differential voltage, the output will be equal to the reference voltage applied to pin 6. If there is a differential, the output will be above or below the reference voltage by the differential voltage, multiplied by the amplifier gain. The gain is determined by the resistance you attach between pins 2 and 3.

Tiny package, huge gain!
G = 1 + ( 49400 / R23)

Setting R23 = 124 ohms gives a gain of 399, so a 1.5 mV differential should give

3.0 + 399 * 0.0015 = 3.60 V

if the reference voltage applied to pin 6 is 3V. The reference voltage has to be somewhere in the middle between the supply (pin 8) and ground voltages (pin 5) and the expected output must be as well. The output should actually drive a small current through some load (like a 10K resistor), and some small capacitors will smooth the signals in and out. The direction the voltage swings about the reference voltage will depend on which direction you push the load cell and which side of the bridge you hooked to pin 1 vs pin 4, so swap until it increases in the right direction.

Hooked it up like this and it worked as advertised. As a mechanical engineer that always surprises me!

The response time of the circuit will be fast, but not infinitesimal. It will depend on the gain and the capacitors you select, and maybe which version of 8221. It will probably be way, way faster than the mechanical system you're measuring, but you will need to wait for it to settle if you are switching the excitation voltage rapidly, rather than sticking with this basic DC configuration.

For my latest project, I want to measure the loads applied by a couple of small linear actuators. I've mounted them so their bases push on 20 kg load cells, and I've hooked up the load cells to AD8221 amplifiers, with nominal 82 ohm gain resistors for a nominal gain of 603. Vref is derived by dividing the supply voltage across a pair of 2K precision resistors. The output load resistors are also 2K each. For now, there are no filter capacitors on Vref or the outputs.

The circuit on the breadboard provides two channels of amplification, with separate inputs, but common connections for supply, ground and Vref. The excitation voltage for the bridges is common to both bridges, but could be separated from the amplifier power supply. That will allow the bridge excitation to be switched to eliminate low frequency noise, or increased to increase the magnitude of the outputs. Even a small push on the actuators generates an easily measurable voltage on the outputs.

When I first hooked them up the green wires went to pin 1 on the amps and the output voltage decreased with increasing load. I switched them around, and now the voltage increase goes with the applied load. If I knew which wires ran to which strain gauges under the white goop on the load cells, then I could predict the sense of the response, but it's easier to measure it!

The load cells are mounted with M5 socket head cap
screws on a block of hard red cherry wood. The bottom
screw doesn't contact the support, so the load cells are
free to flex under applied loads.  
Two realizations of the circuit are smaller
than the diagram describing them. There are
currently no capacitors on the outputs. Common
power comes from the 2.2 mm jack top left.