Back to Basics 1: Modification

When I started my first company in 1996, I would take just about any job that came my way. At first, these were generally small jobs modifying PLC code or adding features to machines.

Two of these jobs stick out as typical. They both allowed me to build small control assemblies in my garage and install them on existing machinery:

Modified Zero Speed Switch

This was installed on a small conveyor that removed metal slugs from underneath a press. Punched metal slugs would fall onto the conveyor and be carried out to a bin. Sometimes the slugs would build up and jam the conveyor, and sometimes the conveyor would stop running for other reasons. While zero speed switches can be bought as a device, this system needed to interface with the press itself and alert the operator to remove the bin when full, so a small MicroLogix 1000 PLC was used.

The logic for a zero speed switch is pretty simple. A spring-loaded rubber wheel with a screw in the side of it was placed against the bottom of the running conveyor belt. An inductive proximity switch was set to read the screw head when it passed. Two On-delay timers are used with the proximity switch as the driving contact; one NO and one NC contact. The timers will only be “done” if the wheel stops turning, either with the prox stuck in the on state or in the off state.

A red light was mounted in the door of the small enclosure, it would come on when the wheel stopped turning, unless the press wasn’t operating. A green power light and a reset button were also mounted in the door.

2. Divot Detector

This plant made small compressors for refrigerators. A metal dome was welded to the top of the compressor, it had an indentation or divot pinged into it for orientation. The domed piece was loaded onto a rotating plate by the operator and a sensor was supposed to detect the divot. The company had tried a variety of photoelectric sensors and proximity switches, but the dome had color variations and was not perfectly centered, so it had never worked correctly.

The solution was to use an analog photoelectric and take readings as the dome rotated. When two adjacent values had a large difference or “delta”, there was the divot. Once again, I used an analog capable MicroLogix for the job.

Now, almost 30 years later, I am “Back to Basics”. After building a company into a small machine builder/integrator, working for a much larger machine builder, contracting to a high-tech engineering firm and teaching all over North America, I find myself looking for small jobs and projects like I started with. If you have any projects, no matter how small or off the wall, hit me up!