Time may wait for no one, and neither do the needs of manufacturing plants or remote field sites when a piece of equipment goes down. But troubleshooting equipment must not only stand up to the rigors of harsh environments. It must also allow operators to get to the heart of a problem without worrying about the integrity of their diagnostic tools.
So just how do designers build ruggedness, and thus peace of mind, into their products?
When anything electrical or containing electronics gets squirrelly, whether it's in the design test lab, the plant, or even at home (an engineer's that is), most people bring out the multimeter first. Modern versions of the digital multimeter (DMM) range from simple current/voltage/resistance units to devices that can display waveforms on large LCD display screens.
Frequently used in confined spaces, a DMM can be easily dropped on or banged against unfriendly surfaces. Displays, switches, and connections have to be able to take such poundings, and the meter itself may be subjected to inadvertent electrical overloads.
No fluke. Chuck Newcombe, product planner for DMMs for Fluke Corp. (Everett, WA)--long known for the durability of its products--says there are several keys to designing robustness into test equipment.
An understanding of the user's environment, including the physical conditions, such as dust and water, and the electrical loads and electromagnetic interference, is critical, he emphasizes. "The conditions a design-test engineer on a workbench encounters are quite different than those an electrician on a catwalk in a steel mill (heat), pulp mill (humidity), or cement plant (dust) runs into," he notes. "For some time, we have put designers into the customers' shoes, getting them into the field, whether it's a steel mill or a repair truck."
When it comes to high electrical energy concerns, not only ruggedness but also safety come into play (see DN, 4/20/98, p. 96). Ideally, the test instrument should be able to continue to take measurements, or at minimum survive and be reset, after an overloading. Sturdy switches and proper fusing help ensure electrical safety and avoid failures. In any case, Newcombe explains, "You don't want the instrument contributing to a fault, and if it should fail, you want it to fail safe."
The European IP standards and tests are part of the world-wide environmental specifications being adopted, according to Newcombe, for device seal quality (dust and moisture integrity, per IEC 529). These standards specify, for instance, the contaminants that can enter a device's case and at what levels. Corrosion and oxidation concerns have lead to use of gold-plated contacts to prevent open circuits.
After designers account for the environment, they need to consider accidental dropping and other abuses. The IEC standard for hand-held devices specifies that they must survive a 1m drop onto a 50-mm layer of oak on concrete. Fluke sees the standard as a minimum and guarantees that some products can be dropped twice that distance and still maintain calibration and usefulness.
To improve the robustness of its products, the company does six different axes orientations at each drop test distance. Newcombe adds that the products are dropped from progressively higher heights onto a concrete surface until they fail, which helps to improve future designs.
Analysis software provides insight into the packaging of test instruments so they withstand high loads. Commercial design packages used by Fluke for geometry configuration and finite element analysis include Pro/ENGINEERŽ from Parametric Technology Corp. (Waltham, MA) and ANSYS/Pro from ANSYS Inc. (Canonsburg, PA). With the software, designers can try out different case shapes and explore the resultant stress development under loading.
Yellow jacket. Years ago, a removable rubber holster protected Fluke's multimeters. That has been supplanted by a protective jacket or boot, in familiar yellow. Integral to the case, this bonded layer is stronger than the rubber holster, according to Newcombe, and more effective in preventing impact damage. The design also allows for better stress distribution and eliminates velocity differences between the boot and the hard case underneath.
Protecting the instrument display, such as a DMM LCD, under shock is a prime concern. Depending on the specific device, "You may not want to 'float' the display, and maybe 'tying it down' would be better," notes Newcombe. Designers make radius and clearance changes to the case in order to produce the lowest g forces on the glass. Elastomeric strips connect the LCD glass to the circuit boards, avoiding contact breakage. Analysis results are proven out by dropping ball-bearings onto the LCDs.
Tektronix (Beaverton, OR) was also concerned with screen ruggedness in designing its new TX-DMM. Engineers stabilized the LCD by supporting it across the entire back surface using both the chassis and light diffuser. By effectively locking the display into place, the design minimizes bending and twisting motions. A plastic bezel surrounding the front of the display adds protection.
Fluke's Newcombe says the low mass of these small, hand-held devices means less impact energy, resulting in lower stress levels. In fact, the spring pressure on internal connector pins produces friction much greater than any inertial forces tending to pull them apart.
Larger devices, on the other hand, can contain more massive independent internal components, such as batteries. Likewise, big, desktop instruments require internal components, such as transformers, that under some loads (typically in transit) may even bend the chassis. To prevent damage, engineers design transport packaging to directly support these bulk masses. For example, removable bolts attached to the instrument case, housed in a padded shipping box, prevent inertial loads from entering the chassis.
Recessed switch handles that prevent direct shock loading of the switch also contribute to ruggedness, adds Newcombe. The use of surface mount components eliminate the problem of g loads bending or destroying fragile leads. Probe connectors now have integral rubberized shrouds to allow leads to flex without breaking.
Locking LEDs. When coming up with the design for their DMM, Tektronix engineers were also concerned about the LEDs for display lighting. The diffuser panel conforms to fit around each LED, which together provide even backlighting. Designers were worried that if the meter was dropped repeatedly, these light sources would begin to fatigue, ultimately shearing right off their mounting board. The amount of light available would then start to decrease. The solution: allow the diffuser and board to move together, by beefing up the pins holding them together. Eliminating the motion reduced the stress between them.
The Tektronix DMM window is a single part molded from LexanŽ polycarbonate film from GE Plastics (Pittsfield, MA). Featuring greater strength and stiffness than the three separate components it replaced, it functions not only as the window for the display but as a covering for the infrared port.
Finally, the blue rubber boot encasing the DMM has a built-in, swing-out stand. Tektronix views even this as an enhancement, as not only can the instrument be set up on a bench, it also can be hung from a shelf or hook. Either setup minimizes the actual chance of dropping the instrument.
In the end, it's a combination of all three--design software, packaging experience, and materials--that put ruggedized testing into go-anywhere packages.
TEST AND MEASUREMENT
Some standards to meet
Safety tests to specs:
IEC 1010-1, ANSI/ISA S82.01, CSA 22.2
RF Emissions - EN 50081-1
RF Susceptibility - EN 50082-1
Environmental (dust and moisture):