Would like to split a Loadcell mV signal to 2 separate controllers ?

I have a Vishay Precision Group Damped Loadcell 50 Kg going into a Vishay Precision Group Dynamic High Speed Digital Interface. I would like to take this 6 wire loadcell and share this with a Rice Lake SCT20-AN and covert this to 4-20 ma signal , so I can share this loadcell value with 2 controllers. What negative effect would this have on the mV signals from the loadcells ?

Thanks in advance

Your current load cell is giving a mV signal out. This is a voltage so just wire the same signal into both units.
This is similar to wiring thermocouples into two controllers to read the same value.

Regards,
Garry
http://www.accautomation.ca

It may not be quite that simple. I haven't done it, so if Garry has then perhaps his input is more valuable than mine, but...

Different transmitters use different excitation voltages. Let's assume that both transmitters are set to a load cell rating of 2mV/V. So if your full scale is 50kg, then at 50kg, both transmitters will expect the mV signal to be 2mV multiplied by the excitation voltage. If transmitter #1 uses 10V excitation, and transmitter #2 uses 5V excitation, then with a 50kg test weight, #1 will expect to see 20mV (2[mv] * 10[V]), whereas #2 will expect to see 10mV (2[mV] * 5[V]).

If both transmitters use the same excitation voltage, then that problem is eliminated. But there's more.

With 6 wire load cells, in addition to the signal and excitation, you have sense wires. The idea of these is that they are joined to the excitation wires at the closest possible point to the load calles (in the actual load cell for 6 wire devices, or at the summing box or connection box for 4 wire load cells). The reason for this is that while your transmitter may be designed to put out 10V of excitation voltage, it may not be exactly 10V. It may be, say, 9.7V, which isn't a lot you might say - but all of a sudden it's a 3% error in your measurement. Plus, if there are long cable runs involved, you also have voltage drop to contend with. By the time your excitation reaches the load cells, it could have dropped to 9.5V. The sense wiring in effect tells the controller "this is what the excitation voltage is AT THE LOAD CELLS". So now your transmitter realises that the excitation voltage is not 10V, it's 9.5V. So it knows that 50kg is not 2*10=20mV, it's 2*9.5=19mV. And adjusts accordingly.

So. If you joined the sense wires to both transmitters, and just used the excitation from one transmitter to actually provide the excitation voltage, would it work? Maybe. You'd definitely have to be powering the two transmitters from the same power supply, but even so, I don't know how happy a transmitter would be about getting a "sense" voltage from a different transmitter. You'd have to try it out and see.

And finally, you have temperature compensation to consider. You'd have to, at the least, make sure the two transmitters were right next to each other and at the same temperature, but even then I don't know enough about how this works to be sure how they would react.

I guess the long and short of it is, have a play with it and see what results you get - but be careful of what happens to your accuracy!

Rob S.,

ASF has a pretty good idea of what's going on in my opinion.

The excitation provided by one signal conditioner will never be an exact match to the second. In addition to that, each unit adds a load to the circuit and will cause an error.

if I am wrong, someone please step in and point out my mistake.
regards,
james

The two units will not be the same unless everything is exact. You should be able to calibrate to achieve the same readings.
Check the impedance of the two devices the load cell will see and the maximum amount of current that each will draw.
I agree with ASF and James and you sometimes have to play with it and understand the result you get.
I have done this before with two Omron displays from a load cell.
Regards,
Garry
http://www.accautomation.ca

The DHS seems to have a Modbus or ASCII interface available, maybe you would be better sharing the data that way.

You should not try to "split" a load cell signal. What exactly do you need each controller to do?

Use the ASCII output of the Vishay Precision Group Dynamic High Speed Digital Interface to one of these depending on what controller you have. http://www.rtaautomation.com/products/. You will not be happy with the results if try to share the load cell inputs with 2 scale interfaces.

Thanks for all your responses. So TWS , you are saying that I can take the ASCII signal from the Dynamic High Speed Digital Interface and put a "splitter" from RTA Automation and be able to use that to share with another controller ?

Thanks again.

I am assuming by controller you mean PLC. The RTA device is a protocol converter. If your PLC is for example a ControlLogix, it will convert the ASCII output of the high speed digital interface directly into the tag database of the ControlLogix processor.
There is this: http://www.laurels.com/transmitter-serial2analog.htm. I have never used one of these. I have used the RTA device to import scale data into a ControlLogix processor

http://www.omega.com/pptst/DRSL-SP2.html

Perhaps you can find something here.