Loadcells and plc

thierry2003

Member
Join Date
Oct 2003
Posts
70
Hi all.
I am reprogramming some machinery that has a weigh-hopper built into it. I just haven't got a clue how I must use the analog value from the loadcell transmitter.
On my analog input card (plc) I can set offset and gain, on the loadcell-transmitter there are some dipswitches to set the span.
In the current setup one can set the offset and change the span on the touchscreen. What is the use of all this stuff?

I thought I just had to do the following...
Put 0 kg in the hopper and set the offset of my AD converter to 0.
Put the maximum possible weight into the hopper (let's say 500 kg) and set the gain of the AD converter to the maximum (4000 in my case).
So 0 kg give's a digital value of 0, and 500 kg a value of 4000.
And that's it, I THINK!

I am probably wrong so if anyone can help me...
 
When you say to are "reprogramming some machinery", do you mean you are scrapping the current program and writing something from scratch or are you keeping as much of the original as you can and modifying it to suit you? If you are writing from scratch you can pick the adjustment method you want. The one you describe in your post (#1) will work fine.
I typically don't mess with the scale and offset in the analog input module. It's just a personal preference so don't put much stock in it. I usually adjust the sensor to get the scaling I need.
However, I have started to do scaling from the HMI also. I don't strictly adjust the sensor output. I just make sure it stays in the analog inputs usable range through the full measurement range. Then i use the HMI to trigger sensor reads at two different process values and I scale accordingly. It sounds like you might have that one, too.
If you plan to keep the section of the program dealing with the scale input you will need to figure it out. However, your time might be better spent just erasing the existing scaling section and coming up with your own. That way you know how it works. Just make sure you document it VERY well so the next guy doesn't have to figure out what you did (like you are ding now).

Keith
 
What kind of PLC do you use ?

You are on the right track.
You have to gather some information.
Range of the weigh signal from the load cell transmitter: 0-xxx kg/ 4-20 ma.
Input range of the analog card: 0-16384 / 4-20 mA (example).
Desired signal range in the PLC program 0 - xxx kg (same as load cell range).

You must then scale the input signal so that you get the value from the analog input card scaled to the desired engineering values.
Depending on the PLC, there will be a scaling function allready, or you have to roll your own.
Notice that the analog signal can be less than 4 mA (wheigh signal can drift downwards). So the engineering value must be able to show negative values.

When you have the analog signal as an engineering value, then you can detect the tare value when the vessel is empty.
You then finally have the net value of the contents in the weigh hopper when you subtract the tare value:
Net weight = Raw engineering value - tare value.
 
Thanks Keith and Jesper, I'll check the equipment again and post my solutions so you guys can check it out.
Good point that the analog signal can be less than 4 mA, never thought about that...
BTW Keith, I am using another PLC (from Siemens to Mitsubishi), so I am starting from scratch.
Jesper, who talked about a 'vessel'? :)
But you're right, it uses a vessel, the equipment is a steam peeler for potatoes...
 
"Good point that the analog signal can be less than 4 mA"
Actually, I recommend that you use a 0-20 mA analog input, even if the signal is 4-20 mA.
You can then test for missing signal. Use a limit of say 2.5 mA to generate an alarm. Dont use 3.99 mA as the limit, it will just give you lots of false alarms.
some PLCs have built-in "underrange" alarms for analog inputs. But my experience is that they just give false alarms for 4-20 mA signals, and are useless for 0-20 mA signals.

And, search for "weigh scale" and "load cells", as this topic has been discussed before.
 

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