Ask the Search Engineer 28005

Capacitive switches . . . Digital measurements . . . Schematic file conversion . . .

Dear Search Engineer: I have two flanges that I need to bolt together, but they must be separated electrically as not to conduct current. I'm trying to prevent galvanic corrosion. I can find gasket material but not connection hardware. Any suggestions?—P.D., Waukesha, WI

Dear P.D.: Standard metal bolts can be used if one end uses a nonconductive bushing to separate the metal parts. This will avoid galvanic problems.

One engineer suggests using commercially available Teflon washers under the head and nut of the connecting bolt. To insulate the barrel of the bolt, he cut segments from soft plastic tubing (Tygon, etc.) to the length of the expected closed and tightened assembly. Note this tube can be a shade long as it gives. Then, making sure there are no burrs or chips to cut the plastic parts on the seating surfaces for the bolt/nut assembly, he put it together normally. If the joint has to be opened, the plastic parts probably have to be replaced but they are typically inexpensive.

Another engineer suggests assembling the flanges by providing a Bakelite or nylon sleeve in the bolt holes of the flanges, bakelite/nylon washers, at each flange end, followed by a standard carbon steel plain washer and standard bolt/nut arrangement. Ensure that a nonconducting gasket material is provided between the flanges.

Dear Search Engineer: I have a bumper paint system that paints plastic bumpers in an electrostatic fashion. The fascias themselves are not conductive, but the primer is. Due to the fact that the bumpers are moving on a conveyor system and present themselves to robots for painting, it is difficult to come up with a system to ground the bumpers to dissipate the 60 KV that we apply the paint with. My thoughts are limited to:

1. An air cylinder mechanism with a flexible arm mounted to it, and a spherical magnet with ground wire attached to it. The air cyclinder would raise up as the bumper approaches, and the strong magnet would be attracted to a ferrous bar on the backside of the bumper, thus completing the circuit.

2. I could mount a small robot under the fascia path to come out and touch each bumper and ground it, but that would be very expensive.

(We've tried many types of ground cables and clips, but the paint builds up too fast on them to make them effective). Any thoughts on these solutions or where to turn for a custom or canned solution? —R.G., DN reader

Dear R.G.: One reader votes for solution No. 1, but would use an electromagnet to control the onset of pull and release. The release could need its own electromagnet to prevent "stickiness." Or, you might try adding an antistatic component to the plastic bumper material, much like electronic component packaging. This would allow the part to be grounded by the existing support method.

Dear Search Engineer: I am working on a project in which I need to use a capacitive or field effect switch. Our circuit is a screened silver circuit on polyester, which will be placed behind a plastic panel. The circuit is supposed to generate an electric field on the surface of the panel, which needs to be monitored for changes if a finger touches the surface to activate the switch. The question is, what kind of circuitry do I use on my board to sense the change in the electric field? —J.S., Lisle, IL

Dear J.S.: QTOUCH and QMATRIX products from Quantum Research Group let you create different zones behind your plastic panel so that depending on where the finger touches the panel you can execute different instructions. The QPROX products create a capacitive field that can be projected through any material. Check them out at www.qprox.com.

Dear Search Engineer: I would like to measure the digital strength of each finger in grams and have it displayed and recorded in a computer. Any idea how to go about doing this?—M.N., DN reader

Dear M.N.: One engineer relays his experience: "In one experiment set up for IBM way back in the mid-90s, we used small plastic bladders glued and covered with silicon gel cement to the keys of a keyboard. These water-filled bladders were connected by small flexible tubes to manifold containing an array of 4-20 mA pressure transducers that gave us analog readouts. These could today be easily digitized and used effectively to achieve your end."

Dear Searh Engineer: I am looking for a linear actuator with an adjustable stroke of up to 10 inches. It has to be smooth operating from -60 to +200F and repeatable to 0.032 inch. We use it to cycle test parts in an environmental chamber. We have tried pneumatic actuators without any luck. Any suggestions? —C.M., DN reader

Dear C.M.: Among other vendors, Kerk Motion Products offers a leadscrew-driven linear actuator, which provides for adjustable stroke lengths and variable motion control. The materials allow for cycling in wide temperature ranges and the Kerk anti-backlash nut offers repeatability and positional accuracy. Check it out at www.kerkmotion.com.

Dear Search Engineer: I have been sent old schematic files (formats: .$$$, .B01, .CFG, .DBF, .NET, .NPL, .P01, .S01, .SDF, .TXT, .WKS) from a company that used to use Omation SCHEMA schematic program. Unfortunately the dongle that was required to unlock these files croaked, and I have learned that Omation may have been sold to Accel or Tango. Do you know of a way that we can convert these over to Gerber files, or otherwise import them into the Camtastic program we use? The only schematic we have looks like a copy of a fax of something drawn on a napkin. Any suggestions? —M.S., High Point, NC

Dear M.S.: Router Solutions Inc. makes a suite of software that will import and convert a large number of formats. They might even be able to import a faxed napkin! Check them out at www.rsi-inc.com.

Dear Search Engineer: I am looking for an information source for fluid and air controls design to apply in automating a loudspeaker assembly line. Any suggestions for a tutorial in that discipline? —A.V., Mission Viejo, CA

Dear A.V.: Parker Hannifin used to publish tutorials for hydraulics and pneumatics and hold seminars using them as the textbooks. It would be worth a try (www.parker.com).

Dear Search Engineer: I am developing an instrument with a peltier cooler for diagnosing teeth requiring root canal therapy (so the dentist gets the right tooth). I need to get rid of the heat quickly and want 99.99 percent pure aluminum, as its thermal conductivity is much higher than 6061-grade alloy. Anyone know where I could find this material? Also, I need thermally conductive rubber to apply the heat to the tooth. I am mixing aluminum powder with polyvinyl siloxane 50/50. Anyone have any better ideas? —C.S., DN reader

Dear C.S.: Depending on the mechanical constraints of your product, you might be able to use a heat pipe, which is particularly good for small diameter long paths as might be used in instruments. Many times that container is stainless steel, which will hold up better than aluminum. The heat transfer is substantially higher than aluminum. Log on to http://rbi.ims.ca/4391-526 for more info.

Dear Search Engineer: I am working on a project where I have to operate my circuit at the maximum external (surrounding) temperature of 140C. I need to find a material that would withstand this temperature and would not allow the internal temperature of the circuit to rise more than 40 degrees. The size of the circuit, including the outer insulating covering, should not exceed more than 20 mm. This would mean the maximum thickness of the insulating material would have to be 1.5 to 2 mm. Any suggestions on off-the-shelf insulating material and/or a company that would mold such material in different shapes like a hollow sphere or a cube? —P.W., DN reader

Dear P.W.: If your circuit is surrounded by an ambient temperature of 140C for a long time, then it will eventually be heated to 140C regardless of how good the insulation is. This will be true even if the circuit remains unpowered. Therefore the amount of time that it would take for the maximum 40C temperature rise mentioned in the request for help to occur would depend on the quality of the insulation, how much heat energy the circuit itself dissipates into the enclosure, the starting temperature of the insulated circuit, and the thermal mass of the whole assembly. A greater temperature differential across the insulation will result in greater heat energy flow through the insulation. There is essentially only one way to use insulation alone to limit the temperature rise of an insulated device (the circuit) and that is to limit the amount of time that the insulated device is subjected to a high temperature coupled with allowing sufficient cool down time between sequential exposures to the high temperature.

If the insulated circuitry will be exposed to the 140C temperature for long periods of time, in addition to a suitable (possibly insulated) enclosure, you may want to use a method of heat removal such as flowing cooling air through the enclosure or possibly using some sort of heat sink or other cooling device to draw the heat from inside the insulated enclosure to a cooler location.

Dear Search Engineer: How high are the orbits for GPS satellites? —E.H., DN reader

Dear E.H.: From the U.S. Coast Guard Navigation Center (http://rbi.ims.ca/4391-527): "GPS consists of three segments: space, control, and user. The space segment consists of a minimum of 24 operational satellites in six circular orbits 20,200 km (10,900 NM) above the earth at an inclination angle of 55 degrees with a 12 hr period. The satellites are spaced in orbit so that at any time a minimum of six satellites will be in view to users anywhere in the world. The satellites continuously broadcast position and time data to users throughout the world."

Check out this NASA program for tracking current position for any satellite in orbit. The GPS satellites are the NAVSTAR satellites. Check them out: http://rbi.ims.ca/4391-528.