People who know me (especially my family) will tell you that I have an impatient streak. This sometimes comes out when I am driving. I am not a candidate for road rage or anything like that but I do sometimes marvel at the amount of time it takes the driver at the front of a line of cars waiting at a red light to begin moving once the light turns green. The signal has been given so why aren’t they moving?!
Reaction time is very important when driving. It is important to maintain a safe following distance so when the car in front stops, you have enough reaction time to also stop. It takes the human brain time to react to a signal and then take another action.
So what does all of this have to do with PLCs and industrial automation? Well, just as the human brain controls the body, PLCs are the ‘brain’ of key industrial processes. And like the human brain, PLCs also have a set reaction time, or scan time as it is called. Once a signal is received (whether from an input module or an internal flag), they need to process logic and take action. This can happen no faster than the PLC’s scan time, and many designers wisely bank on two PLC scans when they calculate the effect this delay has on industrial machines.
High volume discrete manufacturing plants have processes which consist of multiple steps which, together, make up a cycle. It might take six sequential steps, each of which must be completed before a cycle is complete, one part is made, and the process can then get on with the business of making the next one. The time it takes for a cycle to complete governs how many parts can be produced in a day. Lower cycle times mean more parts and, of course, more profits.
Here’s where the PLC controlling the process can greatly affect plant productivity. Remember that two-scan reaction time? The PLC must ‘realize’ that each step is complete and start the next step. Much like our driver waiting at the red light, the PLC can’t start the next step until it first realizes it has the green light, i.e. the previous step is complete. It then needs to ‘step on the gas’ or start the next step in the cycle.
Consequently, in these applications, a faster PLC will increase production as it very slightly reduces the wait time between steps and decreases scan time. The difference is small. Usually, so small you can’t see it by watching the plant. But it adds up over time. Consider the following:
• A six-step, 30-second cycle time machine running eight hours a day, controlled by a PLC with a 30mS scan time, can make 2250 more parts per year if the PLC is upgraded to one with a 6mS scan time. For a part costing $1000, that’s over two million dollars a year in additional productivity!
• Following an engine plant’s recent upgrade from older PLCs to one of our latest models, the plant was pleasantly surprised to find they were making over 120 more engines per day!
This was all thanks to the lower scan time of the newer PLC.
The moral of this story is that if you want more production from your existing machinery, don’t overlook your PLCs. They are the ‘brains’ of your systems and can be the ‘green light’ to some serious productivity gains.
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