DIRT CHEAP OPERATOR INTERFACE
Most small PLC projects are under severe cost constraints, and yet many such projects can benefit from some sort of minimal operator interface. Sometimes factors other than cost make selection of an operator interface difficult. Here is an example of a surprisingly capable operator interface using just one discrete input line and one or two discrete outputs. The operator interface consists of a single one or two color illuminated push button.
The customer has a piece of equipment which presently operates manually. An operator manually takes the machine through several process steps by operating various buttons, levers, and such. To improve throughput and process consistency, it is desired to automate the process. But at the same time it is necessary to be able to vary the duration of one main cycle, and possibly also to skip one or more later steps in the process. Since this equipment lives in a Class 1 Division 1 Group D classified location, operator interface selection is somewhat limited. Here is a sequence of operations for this simple and economical solution:
Operator loads the machine and presses the cycle start button. This starts the first cycle of the process, and also a 5 second timer. As long as the timer is timing, the button is illuminated, and the PLC counts the number of button presses. When the timer times out if the count is greater than one, the counter accumulator is used to reference a look-up table with different times for the main cycle. In addition, this count value is saved. The process continues to run, using the value from the look up table to control the duration of the main process. As the programmed process nears the end of a process step where it may be desirable to skip the next step, the button is again illuminated for 5 seconds. If the operator presses the button during this 5 second window, then the next process step is skipped (and a flag is set in the PLC to control skipping that step in future runs). Multiple process steps can be run or skipped in this way. So the operator has selected a specific program sequence using only one button.
If the operator presses the cycle start button at the beginning of the process only once during the 5 second window, then the PLC uses the latest saved program (with the previously selected main process duration value and with steps skipped just as they were selected by the operator in the set up run). Now, as long as the same programmed sequence is needed, the operator can press the button once to start the process and walk away.
At the start of a new run, if the operator holds the button on until the timer times out, then the PLC reverts to the default program sequence, which does not skip any steps. The PLC flashes the light quickly to acknowledge this situation, and then provides another 5 second window for the operator to select the main timing value as before. Again, as each decision step is reached, the button is illuminated to give the operator the choice to skip this step, and again these values are stored for use in the next run.
A sample of a ladder program to implement this will be posted later. E-Mail me if you need it sooner.