PLC + Load Cells

[Bob O]

Load cell I am attempting to use and application

I am getting back to a programming a scale system that doesn’t work as sold to our customers. The individual that created this has left our company to pursue other interests. I am using the Hardy load cell with C2 calibration along with the HI 1746-WS Weigh Scale Module for the SLC 500. http://www.hardyinst.com/
Our customers are experiencing weight drift (Not sure about this) and erratic machine behavior (I believe this to be programming).
The scale is in a cold wet environment on a piece of equipment that is filling food containers.

Bob

 

[JesperMP]

Deviations that seem to follow changes in the temperature can enter the system in 2 ways:

1. The load cells must have some kind of temperature compensation internally. But it is difficult to achieve 100% immunity against temperature drift.

2. There can be mechanical tensions in the stands or supports that transfer the load to the loadcells. These tensions can vary with the temperature. The load cells should be placed so that they are pivotted and the axis loadcell/load should be 100% vertical. In that way they should only be able to transfer the vertical load force.
I have seen the combination where the loadcell is fixed to a surface, and the load is pivotted on the loadcell, that horisontal forces in the surface can alter the signal of the loadcell, even if it should only measure forces in the vertical direction. This behavior was minimized by simply cutting the surface between the loadcells so that some flexibility was allowed.

 

[mjamil]

One thing confuses me too much, as I mentioned earlier, this application I'm talking about is a 'Bailing press', this means tonnes of pressure, which also means either I install a Load cell that can withstand tons and tons of pressure (rather than a load cell of my required maximum capacity of 250kgs.) or I have to free the load cell prior to the the press being activated.

this may not be a plc question, but rather a tricky mechano-stuff. the other press they are using and requesting us to manufacture 100% with possibly using same load cells, etc.
they are using pneumatic pistons to detach the load cells from the bottom of the platform.

question rises, what would be the effect (and/or deviations) of these pistons incharge of placing the load cell accurately, what would happen if they for some reason exert more pressure on the load cell, this could result in the load cell telling me there is 100kgs of product whereas actually it would be empty, this means I need a mechanical overload???? and a bit higher range of loadcell, just in case.

since there are too many questions now, I'm off to visit the site (+500miles from my hometown) will see the stuff in working and come back with more questions. ofcourse after reading the specs of the existing system. will try to post some pictures so that you guys can see the stuff yourself too, the more you see the more you learn.

 

[elevmike]

On elevatoros we use a weight measuring device that measures for a predetermend load that is pre-set. This is for the purposes of bypassing hall calls if the load weighing device is tripped to indicate a full load. The unit has a digital readout, and is adjusted by the user to close a dry contact when a full load is detected. The dry contact provides a simple on/off signal for the PLC. We have found this much easer, and maintenance free over using a strain gauge, and analog input, programming etc. Not to mention the cost is some what less than a load cell & analog input module. If you only need to measure full load / no full load, this may be a easer option for you. If you need to measure the exact weight all the time, than listen to the other guys.

 

[mjamil]

Thanks for the information, thats something I did'nt know!

Can you mention a website so that I can check for specs and adv./dia-advantages for the stuff.

I think the stuff may work, 'coz, I just need to close the chute door when I've reached the preset value.

thanks

 

[elevmike]

mjamil,

Below I listed a number of websites I found using a Google search for: Load weighing switch.

The 1st link is what we typically use on elevators. It comes with a strain guage that is mounted on the elevator crosshead beam, and a control unit to set the load setpoint, and provide a relay output. I understand that this control unit has been used in other applications with load cells etc.

On hydraulic elevators we commonally use a pressure switch on the elevatror hydraulic line.

another device I saw in the field simply used a large compression spring under a load plate. a photo sensor was set in a position so that when the spring became compressed due to the load, a vane on the plate would trip the photo sensor, providing a contact output.

http://www.ktechonline.com/product/kweigh.html
http://www.ron-crane-scales.com/overload/1000c.asp
http://www.ipandc.com/overloadweigh.html
http://www.scalebank.com/SM-10 15 Indicator.htm
http://www.danaloadcell.com/Dn3100.pdf