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Weighing without a scale controller

Weighing without a scale controller

Weighing without a scale controller

(OP)
A typical scaling system includes one or more load cells, a summing board if more than one load cell, and a controller to convert the load signal to an engineering unit. I am wondering if I cannot build a scale where the load cells connect directly to PLC analog inputs. A simple routine would take care of all of the signal processing, calibration, fault monitoring, and any other needs. I am well versed on the functionality of the scaling equipment. I see this design as a way to eliminate not only a summing board and scale controller but also eliminate communication traffic of the scale values to our SCADA system. I am certain that it will work, and I'm certain it won't meet the 'legal for trade' requirement, but I should be able to use it everywhere else.
Does anyone have experience with this and if so, what are the do's and don'ts?

Brad Waybright

The more you know, the more you know you don't know.

RE: Weighing without a scale controller

I have often seen load cells connected in series so that their signals automatically add or subtract. This is particularly useful when measuring load on cantilevered shafts, where there is a load cell under the forward (closest to the load) bearing and another over the rear bearing. The rear bearing load is subtracted from the front bearing load to measure the load accurately, regardless of load position on the cantilever.

RE: Weighing without a scale controller

So you will still use the load cell conditioners (with the amplifiers and excitation)?

RE: Weighing without a scale controller

Seems like a lot of re-invention required. Not sure what your PLC includes, but many load cells require analog conditioning before digitization, such as amplifiers, excitation, Wheatstone bridges, etc. Then, you need to calibrate all of this stuff and take hit on drift and thermal compensation. I really don't see a great upside for this.

TTFN (ta ta for now)
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RE: Weighing without a scale controller

Most load cells contain the bridges and temperature compensation. Strain gauges are just components of the load cell. Are you working with load cells or strain gauges?

RE: Weighing without a scale controller

(OP)
The load cells we use are described as 350 ohm wheatstone bridge load sensor/cell. The output is linear millivolt signal with a range described as mv/v. If I apply a 10 volt excitation voltage to a 2 mv/v load cell then the output range is 0-20 mv. That is well within the range of my standard analog input modules.

Brad Waybright

The more you know, the more you know you don't know.

RE: Weighing without a scale controller

If you have the bits in the A/D (resolution) then I don't see a problem.

RE: Weighing without a scale controller

Be aware that Ratiometric measurements (mV/V) need really stable DC power supply excitation. Any drift in the power supply shows up as proportional error in the 'signal'.

RE: Weighing without a scale controller

(OP)
danw2--
I thought of that. With a ratio of mv/V, I can read supply voltage into the PLC to use for the calculation to compensate for that.

Brad Waybright

The more you know, the more you know you don't know.

RE: Weighing without a scale controller

I just brought up a 20 ton flour tower that had four separate load-cells one in each sawed off I-beam leg. They all read independently and came up with their individual weights summed for the total. I think it will work fine Brad.

Keith Cress
kcress - http://www.flaminsystems.com

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