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I need to measure 0-200kg using a Baumer SAS PA418 process indicator. The manual is in french, I tried to use google translate but understand very little. A load cell is attached with the indicator which is PST-101 (200kg). The problem is I have no background on these device or process, I am a CSE background. Do you have any suggestion where I should look for or how to set the device step by step?

Thanks for your any help.

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this should be the datasheet in english, but it's pretty crappy – clabacchio♦ Feb 28 '12 at 9:28
yes. no programming instruction in English :( only the specs. – newcomer Feb 28 '12 at 9:30
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You could ask for better specifications from the manufacturer's support – clabacchio♦ Feb 28 '12 at 9:32
btw, what is hysteresis? – newcomer Feb 28 '12 at 9:38
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It's a double threshold used to filter noise; suppose that you have a comparator (and you have many, since it's an ADC): it will give '1' over a certain threshold and '0' below. But with noise, if you are near the threshold you could see the output flipping; the hysteresis separate it in 2 rails, one for increasing signals and one for decreasing; so the signals will be more stable. Sorry for the short answer but the space is limited, ask if it's not clear. And wiki is your friend :) – clabacchio♦ Feb 28 '12 at 9:50
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1 Answer

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The indicator that you have has a 0-10V input (or 0-20mA as alternative, but it's easier to work with voltages as the specs of the load cell are for voltages).

The load cell is simply a resistive load (a Wheatstone bridge, to be exact), and requires to be excited with 10-12V (so we can work with these supplies easily). The problem is that load cells use strain gauges, which are resistances which value change with deformation, so the change in their value respect to the nominal value is small: they give a strong common mode signal.

What you need is an amplification circuit that:

  • Removes the common mode signal, giving 0V for 0 kg

  • Amplifies the useful signal, such as for 200kg you have a ~10V output

These characteristics (the first for sure) are met by Instrumentation Amplifiers (InAmps), like the AD620. To eventually increase gain (and eventually filtering some noise) you may use a multi-stage amplifier, using an OP-AMP active filter to remove some noise and split the gain between the two circuits.

Probably for load cells there are industrial amplifiers, embedding all these things in one rough box, but that goes out of my knowledge.

Update

As I was thinking, there are such things, like this guy, which gives an 1VDC (a bit small for your output, usable but you lose a digit of accuracy) or 20mA full scale output; the latter is the best for your requirements, as I think that the output in current improves noise robustness.

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Thanks, Yes my wiring is working fine with the process indicator PA418. Excitation voltage is from the indicator itself to the load cell and output from load cell (mV output) is fed into the indicator. The indicator also showing different values depending upon the load on the load cell. Just don't understand different options in the indicator as can't read the manual in English. viz. There are two input values need to be entered in the PA418 for which two display values etc. Also what is TARE in the indicator etc. But by trial and error I am able to measure some value... need to be more exact – newcomer Feb 28 '12 at 9:21

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