# Load cell millivolt signal conditioner

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#### Simple68

I am trying to connect load cell to PLC analog input module with millivolt input capacity. But the variations of received values are beyond acceptable limits. Can someone please tell me how I can make this value steady so that it can be used in a PLC program?

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#### Roy Matson

By "the variations of received values are beyond acceptable limits" I assume you mean the mV range is greater than what your PLC will accept.

There are a few different things you can do:

1/ Drop the mV across a resistor network

2/ Lower the cell's excitation Voltage, a Typical cell has a mV/V rating meaning that the output will be so many mV per excitation Volt at a certain load. This is a linear relationship so if you supply half the volts you will get half the mV.

Note: Bear in mind that your PLC mV input is probably referenced to ground so you likely need a floating power supply dedicated to the strain gauge in order to use the PLC mV input.

3/ Use a transmitter that will give 4-20 mA output which you can then feed into your PLC. If you go that route try to find a transducer that also provides the source of excitation Voltage. If you do a search on "Strain Gauge Transmitter" you will find lots.
There may be some others as well, if you need further you will have to post some more details on your hardware.

Hope this helps
Roy

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#### Roy Matson

Oops, on re-reading your post do you mean the signal is changing too fast for the PLC?

If this is the case you need some filtering, perhaps a 4-20mA with filtering might be the easiest way to go.

Roy

#### LeGa

Of course load cell isn't connected to PLC directly. Except Strain Gauge Transmitter with outputs 0-5V or 0-10V, check if your PLC has an Analog Input module with load cell option.

D

#### Don Lupo

If you're connecting the millivolt output of a load cell directly to a
PLC millivolt input card, then some possible sources of noise or
variation are:

1. Noise found on the load cell excitation power supply will show
up on the millivolt output signal. Thus, you need a clean power supply

could have ground noise going thru the system. You'll want to use an
isolated or good differential input card on the PLC.

3. If the PLC has other millivolt or voltage input channels that
are not being used, you may be experience switching noise from the use
of multiplexers on the analog input card. You'll want to short out
unused inputs on the PLC input card.

4. You might have a bandwidth issue. If the bandwidth on the PLC
input card is too wide, then you might be picking up noise as stated
there are filtering or averaging options on the input card then you
could try those.

In short, I agree with a previous post that stated you should try
checking if the PLC has a good Strain Gage/Load Cell Input card. If so, many of the issues above should be addressed. Otherwise, there are very good Signal Conditioner Amplifier solutions on the market that deliver high level voltage or current outputs (i.e. 0-5, 0-10, +/-5Vdc, 4-20mA, 0-20mA, 0-10mA, 0-1mA, etc...).

Acromag offers two very good Strain Gage/Load Cell Amplifier Solutions. Moreover, Acromag offers a white paper on Strain Measurement for reference. See the links below.

IntelliPacks, PC Programmable/Pushbutton Field Adjust, Model 851T:
http://www.acromag.com/parts.cfm?Model_ID=72&Product_Function_ID=17&Category_ID=8&Group_ID=2

Low Cost, Fixed Range, Model 8B38 Back-panel System:
http://www.acromag.com/parts.cfm?Model_ID=366&Product_Function_ID=60&Category_ID=32&Group_ID=2

White Paper: Strain Measurement
<http://www.acromag.com/authentication.cfm?thepage=manuals/SC_whitepapers.cfm&group_id=2> , PDF 95 KB

Good Luck,

Donald Lupo
Director of Sales & Marketing - Process
Corp. Tel. 248-624-1541 x 1860
Group Tel. 248-295-0880
Corp. Fax 248-624-9234
Corp Web: http://www.acromag.com

Acromag is an ISO9001:2000 and AS9100 certified manufacturer.

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#### S.Gordon

How could you be connecting MV signal directly to the PLC card?? You need a conditioner & amplifier device in between before signal reaches for PLC. Generally load cell output will be fed to a amplifier board which usually connects to a weighing card in the PLC.

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#### Dave Zimbrich

You might want to check out the KL3356 from Beckhoff Automation:
http://www.beckhoff.com/english/bus_terminal/kl3356.htm?id=1096711761

A load cell such as the Transducer Techniques LSP-x can be connected directly to this terminal and the I/O can be connected to almost any fieldbus via a bus coupler or controlled directly with a BX or CX controller.

-Dave Zimbrich

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#### Roy Matson

S Gordon,

There is no reason why you cannot connect the load cell to the PLC provided there is no conflict between the excitation supply and the signal input. For example, if the excitation is floating, most thermocouple input cards also accept mV. Inputting the signal directly eliminates one source of error, the mV/4-20 converter.

Roy

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#### bhushan

Hello Mr. Donald,

Thanks for your suggestion, but still i need one help from your end.
As per point no.4 actually we are having a high speed tablet press machine for that we are using Load cell of capacity 100KN to measure the tablet compression force with that we connected MP55 device to give the signal to PLC of Omron make (CJ2H CPU64) but it is not suitable for high speed i.e. more then 50rpm.

So can you suggest that which instrument/device is suitable for to run the machine till 100rpm.

Kindly let we know if you required any more clarification/information from our side.

Thanks and Regards,
Bhushan

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#### Sandeep Bhise

Dear Mr. Bhushan

Can you let me know your contact number. We are a company based in Pune supplying Precision Load cell conditioners to dynamic Packaging machine manufacturers with damping time less than 250ms. I am pretty sure we can cater to your requirements.

Regards

Sandeep Bhise
Partner
Versa Controls
+91-9822648495

#### Carl

Plot the data out in the PLC there is a feature on Omron where you can plot data......I've used this feature in the past very handy. This will show you what the plc software is "seeing".
If you are saying that this is a hardware issue...maybe the load cell has some hysteresis in it and it's taking time to normalize....but a plot of the data( raw ) should show this scenario.
Failing that you could just simply measure the signal with a scope.
I usually find that noisy signals are filtered in the plc via some averaging method ( and sometimes an average of an average etc )...obviously, this takes time and this then has a detrimental effect on the speed of measurement, I'm not saying this is your case, but this is usually what I see.

I remember a particular case where we had a load cell measuring the torque on an electric power steering system where we had to measure the torque of the unit via a load cell. It had been giving measurements that had a large spread on it ...due to noise. I simply put a 10pF cap on the PLC AD card.

Hope this helps? unless you have concluded that it is a hardware issue