The successful commercial testing laboratory must have the resources and capabilities to cope with a diverse market and relate to a wide range of regulatory agencies. Continuous advancements in technology and the ever-present broad spectrum of clients keep personnel on their toes.
Customers may request testing for compliance approval, to internal design standards, or in response to product problems in the field. U.S. Technologies, a 16-year-old test lab, regularly addresses these diverse requirements. We must be able to test products ranging from PCs and related equipment to military hardware and complex communications equipment.
Familiar products such as the PC and its related equipment don’t have a severe impact on the flow of work at a commercial test laboratory. They require less investigative research before quoting and less engineering involvement in the testing stages. Reports usually are presented in a simple template format.
Unique products need a bit more handholding throughout the test phase. Cost quotes are more involved to avoid committing to a project before fully knowing the requirements. Testing takes more time and requires the involvement of specialists. Finally, reports usually are customized.
Every product presents its own challenges. However, some of our recent projects are good examples of the lengths to which a laboratory goes to test an uncommon product.
Firearms Training Systems
Some of our most interesting EMC tests are for products built by Firearms Training Systems (FATS) of Suwanee, GA. These weapon-based products are used to train domestic and foreign police and military personnel.
Laser-based interactive simulation in the weapon system gives the user an uncanny feeling of realism. The weapon recoils as if the user were firing an actual round. Compressed air, smoke, and a bright orange light shoot out the back of a shoulder-fired rocket launcher. An audio system plays sounds that relate to the situation.
Conventional live-fire training with a shoulder-fired Predator anti-armor rocket costs more than $6,000 per round. During a training program, each person must fire several rounds. For training a large number of soldiers, the cost can be staggering.
The FATS system provides a lower-cost alternative. It uses real weapons modified for virtual-reality training. Room-sized screens display film footage or computer-generated imagery of the target scenario. A soldier fires at images of real tanks or infantry using a real weapon modified to fire a laser at the interactive screen. The targets react when hit. Intricate mapping on the target screen calls for a large number of possible scenarios in filming and requires complex electronics to run the system.
U.S. Technologies performs the compliance testing necessary to qualify the FATS systems for sale in both domestic and foreign markets. EMC testing ensures compliance with FCC Part 15. We also test to EN 55022 emissions standards, EN 50082-1 immunity requirements, and EN 60950 Low-Voltage Directive requirements for the CE Marking.
The first dilemma in testing a FATS system is determining an acceptable test configuration. The systems use a variety of weapons ranging from police handguns to military heavy artillery (Figure 1, see January 2001 issue of Evaluation Engineering). Deciding which weapons to use in testing can be an ordeal in itself. The active electronics added to modified weapons also must be factored into the test procedure.
When a type of weapon is introduced into the FATS repertoire, it must be tested separately with a host control unit. Some systems also have several options, including the accommodation of multiple users. In some scenarios, the bad guys shoot foam projectiles and infrared beams at trainees wearing target vests.
Some of the systems are linked into a sequence. In this way, a group being trained as antiterrorists can enter a building and receive a separate but ongoing scenario as it moves from room to room.
On FATS test days, you’d think an army was fighting its way through the test area with helicopters, tanks, artillery, and lots of machine-gun fire. We get some strange looks in our industrial park as we unload a van of machine guns and mortars. An action-packed movie on the screen can distract even the most focused test engineer. And to make the battle as realistic as possible, a powerful PA system is included.
In measuring radiated emissions to FCC Part 15 and EN 55022 on one system in the FATS family, we noted unusually high emissions at a harmonic of the clock frequency of the system control unit. Troubleshooting revealed that removing a weapon lowered these troublesome emissions. This led to the discovery that the clock frequency of each weapon was on a harmonic of the controller. A change in the oscillator of each of the weapons brought emissions closer to the limits.
However, the addition of more weapons still increased emissions. Our test procedures required us to add weapons to the system until the emissions ceased to increase. We added five weapons to identical ports on the controller, and finally the emissions leveled off. Then we were able to bring the system into compliance, using gasketing and filtering on the main controller.
Incidentally, there are more test considerations than just EMC. The gasketing modification had to be respecified when our product safety engineers discovered that the original sample was not properly flame-rated to meet the requirements of EN 60950. It is important for the different disciplines of a testing lab to share knowledge of each other’s activities on a product.
Dry-Ice Maker
A machine for making dry ice caused our EMC test personnel particular concern. The machine produces shoebox-sized blocks of ice instead of the more common pellets. Most of the testing was completed at the manufacturer’s plant because the EUT is production-line equipment with large pressure tanks and conveyors. The total assembly is about 15 feet long and weighs several thousand pounds.
During a meeting with the client before testing began, we were warned that if the timing, temperature, and mixture were compromised, the large blocks of ice pumped out of the machine at a rate of about one every 15 seconds could explode like grenades. This meant that an air pocket had formed, prompting the blocks of ice to blow up when brought into the ambient temperature.
Certainly we wished to avoid this excitement, so we asked how to prevent it during testing. The response: “Avoid anything that could interfere with the proper operation of the electronic controller.” As EMC engineers, we know that interfering with the electronic controller is what EMC immunity testing is all about. How could we do our job and still protect ourselves?
We took some interesting precautions. The first step was to fabricate some makeshift safety guards. Then it was necessary to reposition the conveyor for our protection. In the end, our test engineers were particularly happy that the product passed all tests without any degradation of performance and especially without any grenade-like explosions.
High-Voltage Switch Testing System
Another EMC and product safety testing project that required special attention was an intricate system used to test sensitive switch devices. The client was Sandia National Laboratories of New Mexico.
The EUT, a tester for high-voltage current-switching devices, is comprised of off-the-shelf components plus equipment manufactured by Sandia and packaged into three subsystems (Figure 2, see the January 2001 issue of Evaluation Engineering.). One is a PC control center, the second a fully loaded seven-foot rack, and the third a temperature/humidity chamber.
An indication of the complexity of this testing project is illustrated in the test plan for electrostatic discharge (ESD). More than 18,000 static discharges are generated at 79 discharge points. This includes several voltage levels with multiple discharges to each point. Air, contact, and vertical coupling plane discharges were required.
When a few of the system components failed the immunity tests, both our staff and the Sandia engineers were concerned. In testing to IEC 1000-4-2 (ESD), we blew an LCD in an off-the-shelf component. To our surprise, this component had the CE Marking and supposedly was compliant to the ESD tests. As a quick solution, Sandia chose a different design with no LCD.
We had another problem with off-the-shelf components. The main power supply module cut out when an ESD was applied to the screw terminals. The thermal protector reset automatically within a few minutes, and normally this would not be considered a failure. However, due to the nature of the product and its unique function, the client deemed this behavior unacceptable, and we concluded that it was a failure. Together we designed a guard to cover the terminals and prevent direct discharges. The problem was solved.
All of these failures were traced to off-the-shelf components that Sandia had purchased as products with the CE Marking. These components, presumably good when they were certified, had the potential to cause the entire system to fail.
Our modifications were external to the failing components, and we solved the problems on-site. In doing so, we avoided having to go back to the device vendors. This was the most desirable route for Sandia, since the company was not concerned with mass production of this unusual assembly and very anxious to meet delivery deadlines.
Conclusion
If you have a relatively simple product with low emissions and low susceptibility, be thankful. If your product is complex and requires a great deal of operator involvement—especially if it explodes or shoots back, choose your compliance testing partner carefully.
Test laboratory engineers must know what questions to ask. They need to be flexible in operation, scheduling, pricing, and thinking. They must have vast knowledge of the EMC standards and experience in applying those standards correctly. Most importantly, they must be responsive to your requirements.
About the Author
Scott Proffitt is director of marketing and sales at U.S. Technologies. He received a degree in communications from the University of Georgia. U.S. Technologies, 3505 Francis Circle, Alpharetta, GA 30004, 770-740-0717, e-mail: [email protected].
Published by EE-Evaluation Engineering
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January 2001