Why should I upgrade to a Program Logic Controller, (PLC)?
That was the question posed to us, by one of our readers, so we hope this helps.
Why automate, why improve? They're all about the same question, so after covering the more general area, we'll zoom in on the specifics of PLCs.
Automation came about to eliminate the manufacturing errors originating from human error. This error was usually caused by boredom and fatigue when doing a repetitive task. Speed in doing that repetitive task, gained through automation, was also a big plus. Automation frees up your human workforce to do quality and management tasks, that only humans can do. I don't believe a computer has been developed that can do what a human can, utilizing their intelligence and their five senses. If there is such a replacement for us, I think it's safe to say... you can't afford it.
"Why improve?" Because your competitors are, and you will not be in competition with them very long if you don't. "More and better" are today's bottom line. With the customers being better educated, and having better availability of the product they seek, you have no choice.
So then the question arises, to what level of automation do we go? And when should we be there? It is projected that by the year 2000, technology will double every 75 days. In order to keep up with the astonishing rate, you must have the foundation now. The foundation is PLCs and computers in your plant. Most of the automation advances will be in how we use these, and how they interact with the manufacturing process. Additions and changes to the programs they use will be of minimum cost, related to the large return in profits, your company will see.
Why PLCs?
Less cost to implement: For the cost of relays and timers to automate about 3 lines for conveying product from finishing machine to packaging machine, you could pay for a PLC to do the same job. In general, a PLC system would make production more flexible and responsive.
Reliability: Relays and Electro-mechanical timers (magnetic control), are susceptible to electrical/mechanical failure. With PLCs, the control logic is non-mechanical, (solid-state). And with the PLC, you can program extra logic to monitor and test itself for possible failure at no extra cost. This would make safety circuits safer, and reduce process variability. With PLC control you could increase compatibility with existing equipment, scalability, improving ease of use, and providing a common look and feel.
Speed: The PLC can speed up the operation of machines that you could not obtain with that old outdated relay logic. Control logic makes decisions more accurate and faster than a human operator could hope to achieve. Timers can be set to hundredths of a second to compensate for external variables, and enhance safety.
Greater functionality: PLCs have the ability to compare real-time values and make decisions based on that comparison. They can do complex mathematical functions, and adjust the machine accordingly. The greater functionality allows you to design logic that can automatically adjust for different machine products, thereby reducing downtime for setups and machine change over.
Little or no cost to modify or upgrade the system: Once you have the basic PLC system in place, most cost of adding circuits and machine control is minimum. For example, let's look at adding a vacuum control that would only come on at a certain time during the machine cycle. You would still need the photo eye to sense the dust, and a motor starter to turn on the vacuum, but the parts you would not have to buy would be: a valid dust sensed timer, a vacuum on a timer, a vacuum fault timer, a fault lamp flash timer, or a vacuum on the relay. You would probably need an extra cabinet to mount these timers and relays in. The cost without a PLC, ( not including the vacuum)would be over $ 1000, and 10 man-hours. With a PLC, it would only take 30 minutes to add the logic to the program, and about 1.5 hours to wire and mount the motor starter and fault lamp. Since you would only have one relay to mount, you should not need another panel. So the total cost would be $ 200 and 2 man-hours (80% savings). Of course, if you were adding a larger project, the savings would climb exponentially.
Safety: In the past with relay control logic, safety circuit implementation was weighed out on a cost versus the likely hood method. In other words, the basic emergency stop button, relying on operators to stop the machine before an accident occurred. Usually, due to distractions and the slow reaction time of humans, the button wasn't pressed until after the damage was done. With PLCs, you can cover 99% of all the possible safety risks, and the only cost is the time it takes to add a few rungs of logic to your program. Safety curtains are about the most expensive item, but well under the cost of an accident. Using the PLC to monitor safety risks is equivalent to having a full-time employee watching, but with quicker reaction time than any human.
The PLC logic should never be a substitute for hard wired safety circuits, just used as an enhancement to machine safety. Safety curtains should be ran in series with hard wired Emergency Stops. Always consult your safety equipment vendor and use a qualified Engineer when designing safety circuits.
Less downtime: Downtime can be broken down into two separate areas. Scheduled and non-scheduled. DuPont has been quoted as saying: "Maintenance is the single largest controllable cost opportunity, representing $100-$300 million per year corporate-wide."
- Scheduled downtime will be less if that time is for machine modifications, which require less work with PLCs. Some of the preventive maintenance can be automated through the PLC to even further reduce downtime.
- Unscheduled downtime can be broken down (no pun intended), further into two groups: Troubleshooting and repair. The more complex our systems become, the more beneficial it is to write logic that will not only isolate failures but also indicate potential failures that may occur in the near future. This allows maintenance to change unscheduled downtime into scheduled downtime. Scheduled downtime, in turn, reduces the amount of time down and loss of production.
The troubleshooting downtime can be reduced by 90%. This is accomplished by utilizing the sensors already in place, to do testing through PLC logic to isolate where the trouble is originating. Repair is usually only a fraction of the time it takes to find the problem. It's not uncommon to spend hours tracking down a limit switch that is bent, or a dirty proximity sensor. The cost to find the problem is more expensive than the cost to repair it. With PLCs the cost to find the problem is lower than the repair cost.
Implementing PLC technology in the ways mentioned above could result in a 43 percent or more reduction in installation costs, maintenance, and variability. And we cannot put a price on human health and well being.
