Using a load cell on belt conveyor idler

As you all know there are suppliers who sell belt conveyor weight measuring systems that come with the electronic modules(integrators), load cell and weigh frame and speed sensor. I have an application where this would work for me but I don't need that much accuracy and budget is too high.

I was thinking of having one of the idlers mounted and suspended one a load cell with its output going to a load cell to 4-20ma converter. At the head pully shaft I would like to use and existing proximity sensor that would be used to calculate the RPM or speed based on the revolution of counts. From this I can also use a simple volumetric rate calculation for material going over the conveyor. Prior to using the scale the PLC would "zero" the scale read-out. I would also add to the calculations some way of calibrating the full range measurement value.

If I could get an weight accuracy of 5-10 lbs that would be fine. The head pulley speed is about 60-90 rpm, 2' wide belt and volume rate is about 1200lb/min of concrete.

I also forgot to add that the belt is driven by a hydraulic motor. The tail end of the pulley is fed by a hopper and it seems that the more full the hopper is the belt travels slower due to the weight of the concrete in the hopper

Thoughts on this?

I have done a basic set up where I use a transducer (With a 4-20mA output, approx £40) to measure the motor load current. This goes straight into the plc and gives a readout of 0 to 80 tonnes/hr.
I ran the belt empty and used this as the "Zero" and then ran the belt full and set it at maximum. It seems to scale reasonably linear.

My belt runs at a constant speed though, but you may find the flow/speed/load increases in a linear fashion.

I forgot to add but revised the original post that the belt is driven by a hydraulic motor

D'oh! What about an ultrasonic level sensor? The taller the product(for a given speed) = more flow

And what do you intend to do with the weight value once you get it

Dan Bentler

I suggest you check out the single idler scale from Siemens, formerly Milltronics. I have installed many of those in mining applications. If you don't have the budget at least it will give you some ideas. You should measure the belt speed at a non-drive pulley (no slip), usually the tail pulley. The better you can align the belt the better the accuracy the weigh idler should be within a mm of the ones on either side.
Good Luck