"YOU CAN HAVE THE BEST MOTION, THE BEST PLCS, THE BEST OF EVERYTHING ELSE ON YOUR MACHINES, BUT NOTHING WILL TAKE PLACE UNTIL SOMETHING GETS SENSED," says Ron Pogats, area manager for presence sensing products for Rockwell Automation (Detroit, MI). According to Pogats, "Sensors of all types play an increasing role to enable flexible manufacturing tools adapt to product variations while guaranteeing the high level of quality required in today's automotive market. A flexible welding machine today may have double the number of sensors compared to a dedicated machine of a few years past." And not only welding machines. The same holds true for machines involved in crimping, clamping, bending, pressing, stamping, machining, gluing, materials handling, and more.
Here's a look at some recent developments in this burgeoning arena....
Capacitive noncontact displacement and thin-gap sensors can be found in a variety of automotive assembly processes, including windshield fabrication, axle runout, disc and drum break wear and analysis, and engine piston and valve manufacturing. General Motors uses these sensors to manage the gap and flushness between the mounting brackets and glass in automotive sunroofs to reduce noise, and improve aerodynamics for fuel economy. During assembly, the glass is first positioned in a fixture with several sensors around the sunroof. The sensors, accounting for the curvature of the sunroof, then dispense varying amounts of epoxy between the bracket and the glass to ensure consistent geometry and minimum gap.
An LVDT (linear variable differential transformer) device is basically an analog differential position sensor (an electromechanical transducer) that outputs a signal proportional to the displacement of a separate, movable core. That signal is easily collected by any data acquisition device. Because of that, explains Mike Weinstein, senior application engineer for Schaevitz Sensors Div., Measurement Specialties (Hampton, VA), LVDTs are replacing that handheld (read "manual") measurement devices, such as micrometers, dial indicators, and dial calipers, used by machinists all over.
LDVTs are also used in machining centers for positional feedback on the tool and tool holder. The LVDTs make measurements directly inside the machine during the machining process. These measurements go directly to a feedback system (PLC or numeric controller). When measurements get to nominal value, you just stop the machining process, back the tool out, and make the appropriate adjustments. "You don't have to predict tool wear and similar things because you're measuring as you're machining," explains Weinstein. This approach is typically used in manufacturing camshafts, crankshafts, valves, pistons—just about any machined part that goes into an automobile engine these days.
LVDTs are also used on assembly lines for measuring the body panel apertures for window glass, taillight assemblies, and headlight assemblies. Such measurements help ensure that these parts fit properly into the openings of an assembled body panel. Another application for LVDTs: automated front-end alignment equipment use LVDTs to measure toe-in, caster, and camber on the front wheels of vehicles.
Ten years ago machine vision started at over $10,000. Now, entry-level systems go for $3,200, including hardware, software, and even user training. Because of this, vision systems are now everywhere—from loading dock to shipping dock. Some applications demand a vision system. Look at parts tracking, points out Phillip Heil, chief technology officer for DVT Corp. (Norcross, GA). "Every single part that goes into a car has some kind of mark on it." This even includes those parts where you can't paste, nail, or hang a label. For those parts, such as camshafts, there's direct part marking (DPM). DPM is that tiny checkerboard symbol made up of multiple squares or dots. DPM can only be read by a vision system.
Automotive suppliers are using DVT's color vision systems, to check exterior paint colors as well as the color of plastics and fabrics in the passenger compartment. This leads to one of the more surprising automotive vision system applications Heil has seen: One final check to ensure that the proper doors are on cars. It seems that the production line at one automotive plant was running two different car models, the automaker was afraid of producing cars with the wrong doors on them. It happens. And because of that, DVT installed a couple of cameras on the production line for a final inspection of these very large features.
Laser-based sensors are used in all sorts of applications, such as detecting the presence/absence of holes, verifying die cuts/patches, and detecting weld nuts/weld studs. The latter is a problem because weld splatter is excessive in a predictable path. For this problem, Bob Arger, Corporate Business Manager for Banner Engineering Corp. (Minneapolis, MN), suggests using his company's LT3 laser sensor, which measures the time for laser light to bounce off a surface three to five meters away. "We can go screaming across a weld cell, for example, detect a weld stud, and yet have the sensor far enough away so it's safe from weld splatter and other abuses in that environment."
While a pressure sensor typically measures air or hydraulic pressure on some kind of fluid media, a force transducer measures mechanical force. "Every tool used to install a fastener on an automobile has a torque monitoring device of some sort to prevent over or under tightening," says Weinstein of Schaevitz Sensors. For example, the load cells in torque wrenches help in attaching wheel lug nuts, cylinder head bolts, and similar fasteners. Torque, continues Weinstein, "is one of the most measured parameters during the assembly of an automobile."
Quartz, explains Robert Metz, Regional Sales Manager for Kistler Instrument Corp. (Amherst, NY), "is a piezoelectric material that yields an electrical charge when mechanically loaded [force]. In contrast to sensors using strain gages, no flexing spring element is required. This mechanical stress can be tensile or compressive, with only a minimum amount of mechanical deflection."
With that in mind, the uses of quartz sensors are legion; crimping, riveting, stamping, and torque assembly are just some of the applications. Successful spot welding, for example, is a form of resistance welding and is dependent on electrode clamping force, weld current level, and weld current timing. Interestingly, says Metz, current timing is best controlled as a function of electrode clamping force (turning on current must await clamping force buildup). This clamping force can be measured in two ways with quartz-based force transducers: a direct measurement can be made from a load washer mounted above the upper electrode, or an indirect measurement can be made using a quartz strain transducer mounted on some structural element of the welding machine that is stressed due to the clamping force at the electrodes. Continuously monitoring the electrode clamping force leads to detecting worn electrodes, faulty electrode drive components, out-of-tolerance parts thickness/flatness, and improper machine settings.
Historically, ultrasonic sensors weren't reliable, what with all the air valves and other ambient noise around an assembly plant. Nowadays, these sensors operate at higher frequencies that are above common plant noise. Plus, with digitized filtering, the sensor can ignore spikes in the ambient noise.
Ultrasonic sensors, according to Banner Engineering's Arger, are "color blind." "We're dealing with sound bouncing off a surface." This is especially useful where conventional sensors might be adversely affected by random surface reflections, surface color, poor ambient lighting, and monitoring distance.
Banner's upcoming Q45UR Remote Head Ultrasonics sensor has an 18 mm diameter ultrasonic head with a sensing window from 5 mm to 200 mm in a usable range of 50 mm to 250 mm with resolution as small as 0.1 mm. Need more of a range? There's a Banner ultrasonic sensor with a sensing window of 300 mm to 5.6 mm wide and a range of 0.5 m to 6 m.
Arger is happy to explain the use of photoelectric sensors to verify the presence of valve seats in an automaker's engine assembly operation. "The application was accurately perceived as ‘difficult and challenging' because of the high reflectivity of the machined surfaces and the relatively small surface of the valve seat. The automaker had already approved $100,000 for a high-end vision system."
Using photoelectric sensors turned out to be far less expensive. The system uses twelve discrete convergent-mode, mini-beam "expert" photoelectric sensors from Banner. The optics focus the visible light as a small spot on the valve seat. The "expert" reference, continues Arger, is to the sensor's push-button—teach—set-up and the sensor's ability to resolve low-contrast applications. List price for these mini-beam sensors: about $80 each.
SELECTIVE FERROUS SENSORS
The materials used in automobiles are changing: more aluminum, more plastics, more glass, more carbon fibers. Rockwell Automation, for example, has"equal-sensing" inductive proximity sensors that can sense whether a metal is ferrous or non-ferrous. This is handy for differentiating sheet metal and steel bracings when detecting studs in auto bodies.
Guy Hughes, director of operations for Controlink Systems, LLC (Lawrenceburg, IN), is working on a system to monitor and control the quality of cutting fluids—10,000 to 50,000 gallons of cutting fluids distributed throughout an assembly plant from a centralized location. The system will monitor concentration, conductivity, pH, temperature, pressure, and other parameters. As required, and based on these measurements, the system will adjust the cutting fluids chemically to keep those fluids at target levels. Moreover, the system posts pertinent information on an Internet site so that the client can see know how the system is running 24/7.
If these cutting fluids are not in optimal condition, they can affect tool wear and tool life, surface finish, and everything else regarding production and the quality of finished automobiles. Not only that, explains Hughes, if the cutting fluids go sour—bugs and other things contaminate the cutting fluid—then the plant has to dump that fluid, clean out the entire system, and recharge the system with fresh fluid. "You're talking hundreds of thousands of dollars, plus downtime," says Hughes.