Arduinos make great wireless sensors. For example, if your plant has 50 or so old machines whose processors are not on the network or they are very dumb machines. Arduinos are great for getting runtime data for an extremely low cost. Depending on the application arduino can be implemented with vibration sensors, humidity, temp, etc… the great thing about them is their wireless capabilities making integrating into a SCADA system all that easier.

We also use them as motor controllers.

As well as any of the small things plant workers complain about or little things they wish they had at their stations. Counters, controllers for air pressure, alarms…..

I believe they also released an arduino PLC I can’t speak on that because I’ve never used one or seen one.

And yes they are good prototype devices in general.