Analog inputs on most programmable logic controllers (PLCs) just aren't designed to handle the speeds involved in weighing the contents of hoppers on packaging lines.
- Leland Teschler
"In the PLC world, analog inputs are typically for thermocouples," says Phillip Harrison of Metro Weighing and Automation Inc. (MWA), Taylor, Mich. "Most PLC analog I/O has a 12-bit resolution at most and is slow. In weighing applications, anything under 20 bits is unacceptable and 60 Hz is the minimum useful speed."
For these and other reasons, MWA worked with Opto 22 in Temecula, Calif., to define an I/O module specifically for handling weigh scales. The module, soon to be an off-the-shelf product from Opto 22, puts the cost of hopper weighing in the $200 per/channel range.
MWA uses Opto 22 modules in a control systems which incorporate a panel-mounted scale readout doubling as a filter for scale output signals. The indicator, in turn, feeds filtered outputs via RS-232 to a Snap Ultimate I/O system, also from Opto22. The Snap system acquires data from different areas of the conveying and weighing apparatus, tasks most PLCs are not designed to do, Harrison says.
"Conventional PLCs are not a good fit for modern control. They are slow when operating over networks because of their proprietary networking protocols. And PLC makers still have an outlook where everything happens in the controller, unlike the distributed model in common use today," he explains.
He adds that PLCs don't support the right data structures or the deterministic processing that packaging applications need. Specifically, PLC software couldn't adjust feeder speeds deterministically on MWA systems. Software had to pause for up to 300 msec when the I/O queue got too large. The controller couldn't see new scale data coming during this intermission.
Twenty to 50 msec was a typical I/O call time with a PLC. The Opto 22 controller that MWA uses, in contrast, can give response times of 0.5 to 1.5 msec for some tasks. And communication takes place over 10 and 100-Mbps Ethernet lines to facilitate data exchange with enterprise databases.