Project Description:
One of Applied Sciences Group's customers requested integrating two automated stamping units onto a single rotary station, in order to provide synchronized nests at which the product could be stamped. Each nest contained a different stamping die. The customer wanted the downstream stamp operation inhibited if the upstream stamp operation failed or if there was no product in the downstream nest. Additionally, the stamp machine cycles had to be synchronized so that the table could be rotated as soon as both stamp heads had raised enough for the product to clear the heads.

The machine did not use a PLC but rather, a Hitachi 8-bit microprocessor on a custom motherboard. The original program was written in assembler with no provision for diagnostics or in-circuit emulation. The program had to be burned into EPROM and tested as a black box. There was no provision for external communications other than digital I/O, and there were only two I/O lines available to meet the machine-to-machine communications requirement.

Approach:
Applied Sciences Group decided to use a pulse width modulation (PWM) scheme on one I/O line to convey information from the upstream to the downstream machine. Pulse durations varied from a few milliseconds to several hundred milliseconds and indicated various machine states (cycle started, head down, head clear, cycle ended). The second I/O line was used to send back acknowledgment strobes from the downstream machine.

Assembler code modules were added to deal with the timing issues associated with the PWM signals. Additional functionality had to be added so that the downstream stamp cycle could be triggered on signal from the upstream machine.

Results:
There was no solution to the "black box" problem, so considerable effort was spent during integration testing, using trial and error techniques, to work out any bugs (moral: Always include low-level diagnostic functions in any custom-built computer). Applied Sciences Group decided to insert the new code as separate modules so that their functionality could be switched off easily, which allowed us to quickly isolate any problems. This also minimized the iterative process needed to fully integrate and test the new logic.

In today's world, the availability of low cost microcontrollers and PLCs should weigh heavily into any determination of custom versus off-the-shelf solutions. In the machine control environment, our recommendation is to avoid custom hardware, wherever possible.