I've been tasked to design a circuit which measures the peak impact force of a crimping press on a terminal. I am using a load cell under the press as a sensor. The load cell outputs 20mV (differential) at full scale (4000 kg force). The impact is quite fast – an entire crimp is over in about 20-30 ms!
The approach I've come up with is as follows: feed the load cell's differential output to an instrumentation amplifier (gain ~ 200) which in turns a peak detector and that in turn is fed to a 10 bit ADC on my AVR. I've tried this approach and while it works quite well it's not repeatable enough. The noise in the system makes repeatable measurements difficult. Please note: I've only tried this on a veroboard and haven't yet tried it out on a PCB. I intend to do that soon.
The 2nd method which occurred to me was that I could use a differential ADC with built-in gain of, say, 200x+ and directly obtain the result that way. The only downside is that I'm not reading the peak value – but I can always do some processing and find the peak value. Is the second approach inherently better?
I am also open to other ideas. The goal is just to measure the peak force and then compare that with a programmed range in the MCU to see if the force was too high or low.
Best Answer
A 20mV output should not be too noisy so I reckon it's the layout issues you have and a proper PCB should cure that. I'd definitely be wanting to analyse the peaks inside the MCU as it gives you many more options.
Whether you use an ADC or instrumentation amplifier you should consider low-pass filtering the signal before it goes into an ADC input to improve the noise situation as much as you can. You'll also need to consider putting an anti-alias filter inline too. Both filters are easily combined.
If you do sample at a much higher rate than what you actually need you should also consider an averaging filter in software too.
What load-cell excitation voltage are you currently applying? Maybe this can be increased to get a bigger signal?