So when is a PC (personal computer) a PAC (programmable automation controller) or a PLC (programmable logic controller)? They are all the same. They all have microprocessors. They are all programmable. They are all available with hardware that is rated for industrial environments, shock, vibration, temperature extremes and various other requirements. They are all connected to real world devices such as sensors, switches, etc. They all execute control based on a programmed binary model of a real world manufacturing process.
They have differing abilities in terms of data storage and throughput. In previous generations of PLC and CNC, the memory for those systems was very expensive and early memories were hand wound wire and discrete magnet memory. With the advent of mass manufactured memory for the PC, the industrial platforms have had to engineer hardware with different components to take advantage of competitive costs. A hard disk drive module was available for some PLC systems where extensive process information was needed.
In today’s market one vendor offers an industrial computer platform that replaces the PLC, the HMI (human to machine interface) and uses a touch screen interface, using the PC platform to reduce control system costs in many applications by doing several functions with the same hardware. Sounds like a Tablet PC engineered for the plant floor. Nothing unreasonable about that.
Some suppliers suggest that applications which require a lot of information to be acquired and stored at the same time as the control system is executing exceeds the bandwidth of the PLC and requires a different kind of control platform. But I’m not sure how to describe that situation exactly, but I would know it when I see it.
For instance if you are making hard disk drive platters, the exact temperature, pressure, time duration and chemical concentration of each plating step may be required to be tagged to each platter. At millions of platters per year that’s a lot of information.
So what describes the need for data? Sometimes its an external regulatory requirement, like the FDA or Department of Agriculture requiring milk producers to record sterilization of facilities or store batch information on pharmaceutical production. Yeah, that sounds like something you would want to keep tabs on in the event of a problem later on, or even as a means of improving plant productivity and machine utilization.
Is it possible that we can describe the need for different control solutions as a function of the bandwidth of the application? Is the speed of the process and the amount of data needed in given timeframe, a means of defining a different model of control? Some information used for traceability is not needed on a real time basis and could theoretically be streamed in a lower speed thread with a different priority. The control application itself might have a higher priority than the data gathering part of the application. And it can have varying bandwidth within the range of a machine or process operation.
A multi-ton roll of paper for a printing press has much lower bandwidth than chocolate covered candy bars moving down a packaging line at 35 miles an hour. The candy bar wrapper is running so fast, it takes a high speed camera to diagnose problems because it is far quicker than the human eye can discern.
Within the printing operation are registration functions that are required to insure that colored rollers line up in order to make the images from four (or more) different print sources. This registration function is usually far quicker than a normal controller can respond to and is usually handled with a dedicated module that can discriminate at 50 microsecond time frames.
Not only do the two applications have differing bandwidths, but the bandwidth varies from starting conditions to the bandwidth needed at maximum speed. So maybe by looking at the controls world with an eye toward control system and data requirements as bandwidth propositions, we can construct better boundaries for applying the control system hardware that is available. Just a thought.