Scales and Load Cells
Last week I described some of the vibratory equipment used in the system I was working on. I wrapped up most of the work on that system and have moved on to a different type of machine, but I thought I’d discuss weighing systems.
The picture above shows a couple of different approaches to weighing products. The diagram at the top illustrates the type of components used on the packaging system described previously, while the bottom diagram shows a much simpler – yet I believe more flexible – way of accomplishing the same thing.
The Mettler-Toledo system is a self-contained system that allows the user to send ASCII strings to acquire weigh values, zero the scale and calibrate the system. Unfortunately, PLCs are not the best choice for formatting and sending serial strings. In the case of the Allen-Bradley SLC5/05 PLC that was used to control this system, a special card made by ProSoft was needed to interface with the scale.
This machine was a duplicate of a system that was installed a couple of years ago, so the hardware choices were somewhat cast in stone. It still begs the question of why a SLC system was used rather than the newer CompactLogix system, but that is not the point of this discussion. I am not sure that communicating serially with a CompactLogix would have been any simpler.
The diagram at the bottom shows a simpler system using load cells and a summing board. My old company Automation Consulting Services in Knoxville built a batching system for Alcoa-Howmet in 2004 that used this technique.
This system performed nearly the same function as the Mettler-Toledo solution described above, however instead of having to communicate using serial strings weigh values were obtained using a much simpler analog signal. Calibration of the system was performed in the same way, development time was much less, and accuracy and repeatability were the same. For the analog system three load cells were used along with a summing board. The best part was, hardware cost for the analog system was only about 20% of a similar packaged solution!
I am sure that a scale manufacturer would try and convince a purchaser that their system is more accurate, easier to use and more flexible. This might be true in a non-industrial application such as a laboratory, but when a control system already exists and values have to be obtained into the PLC for process purposes, the simpler analog version would be my choice every time!