PC-Based Control Systems Gives Automated Equipment 

Automation. It can greatly increase quality while reducing labor content. It can save time, increase productivity, and boost profitability. But it does create some potential problems. With the shop floor more dependent on these machines' output, reliability becomes even more critical. The need to reduce or eliminate downtime increases. Employers may find they need highly skilled, highly trained engineers and electricians to operate and maintain their automated equipment. And as the equipment becomes more complex and more highly specialized, changing functions can be a difficult and time-consuming proposition.

The solution: A new generation of PC-based machine controls that delivers all the benefits of flexibility, higher quality and increased productivity, while significantly enhancing reliability. Best of all, these new-generation controls are sufficiently user-friendly to allow trained machine operators to trouble-shoot most problems themselves without relying on the specialized expertise of engineers and electricians. Hall Dielectric Automation Systems, Inc in Rock Hill, South Carolina, has just completed one such PC-driven machine. It is a highly complex, multiple-operation automated system that produces blood pressure cuffs. Automation is Hall Dielectric's stock in trade. They started some 35 years ago as a manufacturer of radio-frequency welders, a specialization which gradually led to their current leadership position as a manufacturer of automated radio-frequency heat-sealing equipment for industrial applications. Their machines are now found across the United States in specialized applications that require precision heat sealing for plastics. They make everything from plastic-covered three-ring binders to swimming pool liners to parts used by the automotive industry. 

 They are also developing significant new applications in the medical equipment field. Hall Dielectric President Bill Hall says that, "Our customer, a manufacturer of blood pressure cuffs, came to us for a system that would give them an edge on their competitors, whose blood pressure cuffs are made in Mexico. They wanted the increased repetition and reduced labor content that automation offers. Their idea is to bring jobs back to the U.S. by putting the system at the heart of their production line. It will turn out thousands of cuffs a shift, and can operate 24-hours a day, seven days a week with a single operator and a part-time material handler."

Hall responded with a wonder of modern automation. The system produces blood pressure cuffs in eight different sizes. The size range allows medical personnel to get an accurate reading from different limbs of the body or from different body types. It makes, for example, a thigh cuff, an adult-sized arm cuff and a large adult cuff.

"When you are making a cuff, the critical factor is precise sizing and positioning," says Hall. "Each cuff consists of two pieces of plastic-a top sheet and a bottom sheet. The top sheet and bottom sheets are heat-sealed together into a single piece that forms the sleeve. The sleeves are wrapped around the patient's arm (or thigh) and secured with a Velcro hook and loop system. The top sheet has the hook and the bottom sheet has the loop. Precision is everything. Our parameters are +/- 40/1000 of an inch. It is critical to stay within them. There are instructions and medical information silk-screened on each cuff that have to line up perfectly. The cuff must be aligned properly over the artery for an accurate reading. We ensure this by printing an arrow on the cuff at the exact point where cuff and artery should make contact. The machine has to automatically sense where that arrow is, position the sheets exactly on the table, and seal them correctly."

"The material that goes into the machine becomes the dielectric of a capacitor. It is part of a tuned circuit-you feed energy through the tuned circuit to create the heat for the seal. If the material doesn't feed in correctly, the seal may be bad. You can feed the material manually and tweak it to get it to go through, but you can't get the exact repetition you need to ensure that each part is correct and of the same uniform high quality." Hall's solution called for a PC-based control system with major system components supplied by Mitsubishi Electric. The system is operated by Mitsubishi's proprietary MC2 software. Major system components are supplied by Mitsubishi. "This is our first PC-based project," Hall says. "We're the only U.S. manufacturer in the RF sealer industry that makes automated equipment. Our competitors are in Japan and Germany. I felt the PC systems offered us a number of competitive advantages that will give us a significant edge over them."

"When we decided to go with a PC-based system," Hall relates, "we talked to all the players and it appeared that the Mitsubishi system was the best in quality, components and pricing. This was a new area for us." Hall worked with Mitsubishi Applications Engineer Dale Ross to develop the machine's control system. Their system has a total of 25 separate stations and includes forty-three axes of servos with approximately 800 digital I/O points. It is split into two sub-systems with a MC2 runtime controlling each sub-system. Devices are connected to the PC via Mitsubishi's high-speed BUS Network, Control and Communications Link (CC-Link). Remote devices include digital input modules, digital output modules and servo motion control modules. "We considered a variety of control system configurations," said Ross. "We looked at installing a PLC at each machine station with a communications link back to the PC at a Master PLC with remote PLC racks installed at each machine station. And a communications link from the PC to the Master PLC at a Master PLC with remote I/O and remote intelligent motion installed at each machine station and a communications link from the PC to the Master PLC. We also explored a PLC-based solution combined with PC-based motion. In the end, we went with a PC-based control solution. Twenty-five PLCs meant twenty-five PLC programs that would have to be developed and maintained. We had determined that we needed an HMI and the recipe program for that would also have to be written to interface the 25 PLCs."

Creating and troubleshooting 25 relay ladder logic programs is an awesome task. Interfacing these programs would prove to be an even bigger task. Hall also wanted a flexible system that could be easily reconfigured or expanded, a very time-consuming proposition with ladder logic. "But with the integration of software components available in a PC-based control system, we felt that we had the easiest and most flexible development platform for all the programs, particularly if we used Mitsubishi's MC2 software package. MC2 is an integrated PC-based control solution that combines real time control and Windows-format flow-chart programming with integrated diagnostics. The software allows the operator to watch the flow charts as they execute in real time and provides a step-by-step representation of the control process as it progresses." Hall is sold on it. "When you are wiring a PLC program, you need to do a lot of pre-engineering with pencil and paper. The flow-chart programming is easier to input and easier to modify. You're creating the flow. It's right off the brain as you enter the data. This was the first time I did it, so I didn't get a big break in terms of speed, but as I get better at it, it's going to go faster. I enjoy it. It pushes you to the limit with something new."

"The flexibility is also attractive. Our customer can change the size of the cuff they are running in about an hour, which is what it takes to enter a different recipe. We can expand the system, change out stations or even change the product with relative ease. If they want to run tourniquet cuffs in the future, we can do that with some reprogramming."

Hall reports that, "Probably the biggest advantage here is the diagnostic capability. When you're using flow rather than ladder, its easier to diagnose faults when something is wrong. Programming error conditions is more difficult with a PLC, and more difficult for the operator to grasp. As you're creating the sequential flow-chart programming, you are creating the diagnostics at the same time. With MC2, you can input tags on the screen for monitoring different things important to the process. If something goes wrong, a screen pops up that shows an illustration of the problem, complete with part numbers."

"The pay-off is that the machine is easier to trouble shoot. A big system like this would normally require an engineer that stands by in case of trouble. On this machine, the operator is shown a picture detailing the problem and what to do to clear the error. He can repair it himself or work through it over the phone with us. Either way, that's money saved on the cost of the engineer, and money saved on reduced downtime."

Mitsubishi was able to solve another problem frequently encountered with RF heat sealing. "This is a fairly large machine with lots of I/O points scattered around it," says Mitsubishi's Ross. "They wanted the modularity of a PLC at each station with somewhat localized per-station control, but they also wanted to be able to manage the 25 different programs from one master program. The problem is that RF sealers generate huge amounts of electrical noise that can be detrimental on a remote-network control system. The noise can radiate out and cause the PLCs to fault even when the sealers are properly shielded. MC2 offers multiple connection methods to devices in the field, including Mitsubishi's CC-Link, a10MB high-speed BUS Network that supports a variety of remote devices, such as those found on this machine. We were able to demonstrate that CC-link could give them the best of both worlds in this application, despite the noise--a remote network with control devices at each station linked back to the master on two wires."

"In our testing we took the CC-Link networking wire and wrapped it around the blood pressure cuff machine's high frequency sealer," says Hall. "We took the shields off and cycled the machine to create a hazard. Only when we had completely exposed the link to tremendous noise interference well beyond the limits of normal operating conditions did we experience problems." "You can't help but be impressed with the capabilities Hall has demonstrated in building this machine," observes Ross. "In precision manufacturing, it is very important that you never lose control of the product. Hall has taken every measure possible to build a system over which they always have 100% control." Given the initial track record, it's not surprising that Hall Dielectric is moving ahead aggressively with PC-based controls. A second blood pressure cuff system is now being built. "It's a separate system for making neonatal cuffs," says Hall. "It will do cuffs in five sizes with the same requirements for accuracy, even though the components are very small-about the size of a big band aid."

"We're also working on another, more complicated PC-based control system for another medical equipment customer. We're on the bleeding edge. Right where we want to be."