Micro PLC Key to CNC Overhaul

At A Glance

Closed-loop control

Linear position transducers

Dual-axis servo control

Ethernet link

Tractor-trailers are highway behemoths, so it’s somewhat ironic that tiny rivets play a major role in the manufacturing of these vehicles. But that was the case at International Truck and Engine Corp.’s Grove City, OH, plant where the semi-cab side and back panels for tractor-trailer vehicles are produced. The plant was plagued by poor quality from its computer numerical control (CNC) machines for drilling and riveting aluminum side and rear panels to the semi-cabs’ aluminum frames.

The process of assembling a cab’s side and rear panels begins with temporary riveting of one of six pre-pressed aluminum ‘skins’ that comprise the vehicle’s body to an aluminum framework. Ten to 20 rivets give the assembled panels (which can be as large as 8 x 5 ft) just enough stability to enable workers to transport them via forklift or conveyor belt to one of five drilling and riveting tables, each of which is equipped with two dual-axis servo motors and controllers.

Until recently, PLCs equipped with CNC modules controlled servo motors that position panels beneath the drill and rivet head for permanent assembly. The drill head creates a hole in the aluminum skin and its support frame, then retracts. Mechanical fingers place a rivet in each hole and the rivet head then locks a steel cylinder in place atop the rivet. At the same time, a hydraulic ram on the underside of the panel squeezes the rivet to complete the operation, and the table shifts to the next programmed position.

This straightforward, yet complex process is subject to costly errors.

‘If the rivet is off by more than a half-millimeter in some places, it pretty much ruins the whole panel,’ says Brian Engle, a principal with Industrial Technology Solutions (ITS), a Columbus, OH-based design/build automation firm specializing in electrical systems integration. Two years ago, productivity at the Grove City plant began to suffer and waste was on the increase, prompting the company to call in ITS to make improvements.

System-wide problems

Clearly, equipment problems—which were forcing International Truck and Engine Corp. (ITEC) to scrap roughly one of every 30 panels produced—included aging PLCs and connected controls, says Engle. Inconsistent rivet patterns and table movements were hampering production. The primary problem was in the servo amplifiers, which are intended to deliver precise control of the servo motors’ torque and speed.

‘There appear to be several factors that contributed to the servos demise,’ says Engle. ‘They might have taken power strikes over time, and might have been subject to ‘dirty’ power in the plant.’ [Dirty power refers to issues such as voltage sags and surges, and harmonic distortion.]

In addition to the motion problems, operations suffered from inconsistent rivet ‘squeezes,’ he says.

‘The intended pressure for riveting was 4,000 pounds of upset force, but some rivets would wind up getting squeezed to the point that there was hardly anything left of the rivet—squeezing it completely flat. Meanwhile, other rivets would be barely touched by the hydraulic ram,’ Engle explains.

These inconsistencies appear to have been caused by the system’s lack of in-line pressure transducers. Instead, the system was equipped with standard 0-20 psi discrete pressure switches that measure a differential pressure across an orifice plate, which is very susceptible to debris in the hydraulic line and therefore rarely delivers accurate control.

‘Sometimes, the switch would trip at 5 psi, sometimes at 6 psi, and sometimes at 3 psi. Dead band on the switches was just too large and inconsistent for the quality they were trying to achieve,’ says Engle.

Further contributing to productivity problems were poor system diagnostics. By their nature, drilling and riveting units undergo a great deal of shock and vibration. Consequently, it is not unusual for them to suddenly stop working because of malfunctions or for safety reasons. However, determining what caused the shutdown at the ITEC plant was difficult because the systems rarely displayed appropriate error messages on their control screens. Consequently, machines could suffer lengthy shutdowns as operators and maintenance personnel conducted time-intensive troubleshooting.

‘It was an absolute nightmare. There was a lot of downtime,’ Engle says.

Control system revision

After complete machine analysis, ITS recommended a solution that would require replacement of the PLC and motion control system with newer, reliable controllers as well as replacement of the machines’ complete electrical system.

In designing the solution, ITS had to consider several of ITEC’s goals. First and foremost was quality, says Engle. ‘International stressed the importance of keeping the quality of its parts at the maximum level.’ Another consideration was reliability. If something went wrong, they wanted diagnostics in place that would quickly tell them exactly what the problem was to minimize downtime. In case replacement parts were needed, the company wanted assurances of two-day delivery at the Ohio plant, meaning that ITS would have to select readily available off-the-shelf components from a reliable provider. ITEC also wanted to increase machine speed to increase production.

Finally, the company insisted that ITS complete the overhaul quickly.

‘We were given a four-week window in which to get each of the five machines up and running. In that time, the machine had to be disassembled, cleaned, painted, have an entirely new electrical system installed and tested, and then the machine had to be released back into production,’ says Engle.

To resolve the problem of inconsistent rivet squeeze pressures, ITS determined that rebuilding the lower ram assembly would be insufficient to make rivet squeeze pressures consistent. An entire redesign was needed, says Kevin Sheedy, a senior design engineer and principal for ITS.

ITS created a closed-loop control system using an integrated linear position transducer gun drilled into the lower ram assembly plus a high accuracy, quick response in-line pressure transducer.

‘By using a pressure transducer in the main hydraulic line, we reduced the chance of erroneous pressure readings that can be associated with differential pressure readings,’ says Sheedy.

As for the control system, ITS turned to AutomationDirect, a supplier of brand name and private-labeled automation components. To control the drilling and riveting machinery, ITS selected AutomationDirect’s Direct Logic 205 PLC equipped with the company’s D2-260 processor. Configuration of the unit includes two 32-point input cards, a 32-point output card, an 8-point analog voltage input card, and an 8-point analog output card.

‘The DL-205 is actually classified as a micro-PLC. It’s a modular, slotted style unit that is powerful and compact enough to meet our space requirements,’ says Engle. ‘It was also imperative that the processor have enough memory locations to enable the controller to run five parts programs, each with different rivet locations.’

The PLCs also are equipped with a Direct Logic H2-CTRIO high-speed counter, which delivers high-speed pulse output signals for dual-axis servo control.

‘They’re capable of 25,000 pulses per second. That gives us high accuracy on the servo in a short amount of time. So response on the machines is very quick. This new system gives an accuracy within 10 micrometers on table movement, which has made the customer very happy.’

ITS also replaced the existing servo motors with a model made by Sanyo Denki—a choice made largely by availability of a DL-205 module designed to run Sanyo Denki servo amplifiers.

In addition, the refurbished systems include an Ethernet link to an office-based PC running AutomationDirect’s DataWorx data logging software. DataWorx provides plant management with production rates, speeds per machine, maintenance history, and other information.

To reduce downtime caused by lengthy troubleshooting sessions, ITS also installed AutomationDirect’s 15-in. color EZTouch displays, mounted on cantilevered arms extending from each drilling and riveting machine.

‘The new system provides us with far more detailed, easy-to-understand information than the old system,’ says Engle. ‘If an operator tries to start a machine, and it doesn’t respond, text shows up on the screen indicating exactly what’s wrong. If plant personnel need more information, a complete system diagnostics screen is only one click away. Operators and maintenance personnel can change any and all values affecting the process without need for any interface other than the touchscreen’ and appropriate security clearance. ITS undertook the project in November 2003 and completed the overhaul earlier this year. Thus far, the new drilling and riveting units are yielding excellent results, says Engle.

‘For example, we cut the average time to produce the 97-in. panel from 5 minutes, 45 seconds down to 4 minutes, 3 seconds… a 30% improvement,’ he says.

For more related products see www.controleng.com/buyersguide, and visit the following Web sites:

www.automationdirect.com

www.industrialtechnologysolutions.com
www.sanyodenki.com

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