Sensors make sense: clever deployment of sensors can help manufacturers reach single-digit ppms

Automotive Design & Production, June, 2004 by Lawrence S. Gould

The TRW Automotive Machine Building Div., Body Control Systems (Winona, MN), has in many situations replaced soldering the electrical cables from the car directly to the printed circuit boards (PCB). The approach now is to use "compliant pin" connection, wherein the electrical connection is through an array of needle-sized eyelets that collapse when the mating connector on the cable is press fit to the PCB. To monitor that press fit operation, TRW Automotive uses a linear voltage displacement transducer (also called linear variable differential transformer, LVDT), a load cell, and other instrumentation to measure force and displacement. If one of those needle pins is bent or closed slightly, the sensor system will detect a higher force than acceptable. Conversely, if the eyelets on the PCB side are too open, which would cause a loose and inferior connection, the sensors will detect a lower force. Either situation is cause for a rejection during assembly.

In another part of the plant, sensors help TRW Automotive keep things quiet inside automotive passenger compartments. The application involves actuators driving the louvers behind dashboards for ventilation. TRW Automotive makes "millions" of these. TRW doesn't want these actuators and associated gear trains making noise. This is hard to do out in production; the plant is "pretty loud," says Doug Hinrichs, electrical engineering manager at the TRW Automotive Machine Building Div. "It's even difficult to stand next to each other and talk." To get around that, TRW uses a piezoelectric accelerometer, a type of strain gauge, to measure the vibration caused by the actuators. This vibration has already been correlated to audible noise. By comparing the vibration measured in assembly against limits TRW developed in a sound room, the assembly operators can know what actuator units are out of tolerance.

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NEW SAVINGS

Scattering sensors around for production, mistake- and error-proofing, and quality control seems to make sense--but what does it mean quantitatively? "Without the use of sensors and other poke-yoke activities, I don't think TRW Body Control Systems would have been able to get down to the 15 ppm (defect rate) we're currently at," says Russell Rattunde, plant manager for TRW Automotive's Machine Building Div. Agreeing with Rattunde is Tom Sliwa, director, TRW Automotive Advanced Manufacturing Engineering--North America Braking and Suspension. "Right now, our anti-lock braking system facility in Fowlerville, Michigan, is well down in the single digits. So is the plant in Brighton, Michigan. They're at 5 to 7 ppm. World-class performance."

TRW uses a vision system to ensure that no chips are in the grooves of a caliper machined for a disk brake. The manual approach sometimes let defects through, which get caught further down the assembly line during test. At which point, defective brakes needed to be reworked. "Shutting down an assembly line is not a popular thing," one of the managers from TRW Automotive notes with more than a little irony.

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WELD SENSORS, PART I

One way some European vehicle manufacturers are assuring that their weld tips are up to the job after redressing is through the use of a vision system. The system consists of an expert system, a special fiber optic head, and a photoelectric sensor from Banner Engineering Corp. (Minneapolis, MN; www.baneng.com). Essentially, the sensors detect low-contrast differences in the reflections off the weld caps. The expert system then makes a pass/fail judgment.

According to Bob Arger, corporate business manager of Banner, some U.S. OEMs are using another approach. Rather than looking at reflections, they're using a vision sensor to check the shape of the cap. In this arrangement, the weld tips are backlighted and the vision sensor is capable of determining shape, profile, and misalignment of the tips.

WELD SENSORS, PART II

Weld spatter is particularly problematic when it accumulates around the face of a sensor. In some cases, the sensor ends up "thinking" that the spatter is the target, thereby resulting in false sensing--and production downtime. According to Greg Wise, product manager of proximity sensors for Rockwell Automation (Chelmsford, MA; www.ab.com/sensors/index.html), companies are taking different approaches to prevent this buildup from occurring. For example, the Allen-Bradley Copper Barrel Proximity Switch uses thermal conductive material in its copper housing to resist buildup. Wise says that they have some sensors that employ epoxy-based materials that keep the spatter from sticking.

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FAST SCANS

"This machine vision system will run the full array of FrameWork (software) tools and in some cases will be suitable for applications that formerly required a cluster of high-resolution cameras." That's Steve Gieseking, director of R & D, DVT Corp. (Duluth, GA; www.dvtsensors.com) talking about the company's Legend LS Line Scan vision system. It incorporates a Texas Instruments DSP process and has a 2K linear sensor that yields up to 18,000 lines per second. "Using an encoder feature in FrameWork 3.0," Gieseking says, "the Line Scan Legend LS will provide consistent sampling for a number of applications."