The original intention at Rivercircle was to use an ASI-based 2-wire system for the Metzeler machine, so one or two companies were approached for a feasibility study along those lines. Instead, one of the companies, Festo, advised the adoption of a system based on a Programmable Valve Terminal.
Rik Hunnings, assistant design manager at Rivercircle, told Design Engineering that while members of the design team were keen to benefit from the generally acknowledged savings in wiring and programming time that would be accrued from the use of an ASI system, it was pointed out that the functionality of the proposed machine did not warrant the extra cost. On the other hand, the less-costly valve terminal could be supplied as a complete pre-wired and tested monoblock that would include the PLC, I/O and solenoid valves already assembled.
Use of this would eliminate the need for wiring the PLC to the higher modules and the pneumatic valves, while the sensors would be wired to a socket which could be simply plugged into the PLC I/O expansion units. In addition, since the terminal could be mounted directly to the machine without a cabinet, there would also be savings to be made by passing the wires directly to the module instead of using drilled holes and glands.
Engineers at Rivercircle found the PLC used in the valve terminal to be simple, if somewhat unusual, in the way it is programmed. Ray Marshall, electrical manager, explained that it uses a statement-list programming language with which they were unfamiliar. Previous applications had involved the use of ladder logic.
However, the main reason for adopting the valve terminal concept was to eliminate the tedious work involved with terminal blocks, trunking, conduit terminations and so on. It proved to be very successful in that respect and the resulting time saved by adopting the Valve Terminal has been estimated as being between two and three man-hour weeks.