An efficient shield room for automotive testing progresses from design to finished structure
Michael Koffink, Bob Mitchell
Intertek ETL Semko
Littleton, MA
Four years ago, we noticed a growing demand for EMC testing from the automotive industry. As cars and trucks now feature myriad on-board electronic devices, they are increasingly susceptible to stray emissions while simultaneously functioning as powerful emitters.
Clearly, it would not be possible to use our existing subcompact 3-meter chambers to meet these emerging needs. While they served our ITE and industrial customers very well, they would not accommodate the 200-V/m and 600-V/m field strengths needed for automotive radiated immunity tests nor would they provide the silent noise floor necessary for CISPR 25 radiated emissions work.
Our solution was to build a state-of-the-art 5-meter semi-anechoic chamber with a dynamic range (10 kHz up to 40 GHz) and capacity that could accommodate an entire vehicle, as well as military equipment. In use since March 2004, the chamber continues to handle all ITE, medical, and industrial work as well.
The creation of this 5-meter chamber was certainly not an off-the-shelf solution. With so much at stake for current and prospective clients, high performance and cost-effectiveness were paramount concerns. Striving for meticulous planning, our goal was to maintain control over the entire process of customized design, development, and installation.
This article traces the progress from earliest concept to final acceptance of the 5-meter chamber. We believe the process is instructive for EMC test engineers, and we’ll share what we learned along the way.
Mapping Out the Physical Specs
From the start, we focused on the efficiency and utility of the chamber over its projected lifecycle of roughly a decade. The final determination on constructing a 5-meter chamber coalesced quickly. This goal was approved after comparing the performance/capabilities of a 5-meter room, a conventional 3-meter chamber, a subcompact 3-meter, and a 3-meter/CISPR 25 combination chamber. It became evident that a 5-meter room—five feet wider, 12 feet longer, and 12 feet higher than a standard 3-meter room—could easily accommodate the FCC 3-meter scans we required. At the same time, it would be possible to test a wide range of complete vehicles, up to the dimensions of a standard full-size pickup truck.
This decision was validated by the relative cost. Accounting for the customization that typically adds features and functions to the 3-meter chambers, the 5-meter choice would offer far greater capability for an increased expenditure of approximately $80,000.
Our specifications for floors and walls were not unusual. The exterior wall was formed by two outer steel layers with a particle board material sandwiched between, and the interior surfaces featured ferrite panels with absorbers glued to the ferrite tile surfaces. Our primary concern was that the “box”, when assembled, would meet our requirements for shielding effectiveness of greater than 100 dB from 1 kHz to 40 GHz.
Next, we had to specify the performance we wanted so that the manufacturer could match the type of pyramidal absorbers that would best suit our requirements. Given the need for performance at all frequencies, with specific detail to the higher frequencies, and for as much interior space as possible, we selected polyurethane absorbers, which are shorter than the polystyrene alternative. Also, we wanted interior space of more than 30 feet in length, 18 feet in width, and 18 feet high.
The specs for joints and connections were fairly conventional as well. Specifically, we opted for hasp-type connectors to join walls and top and both surfaces with each other and high-end double knife-edge seals on all openings. This was a critical point because with a door measuring 10 feet square that would admit vehicles, there would be plenty of potential for RF leakage.
It was also necessary to specify the floor loading. Ratings of 1500 lbs/square ft. are commonplace, but we knew our room would have to withstand high point loads from vehicle tire contact points and from, for example, the nylon casters on cabinets supporting heavy aeronautics equipment. As part of our early investigation, we had spent several months researching the product families of every automobile company that sold industrial pickups and SUVs. The benchmark—the largest vehicle on offer—was Ford’s F-350 Super Duty crew cab pickup with an eight-foot truck bed. That information helped to determine that we needed a floor that would withstand 8500 lbs./square ft.
We also needed piping for testing equipment that required water cooling—e.g., HVAC and certain industrial equipment. Obviously, the floor needed to resist standing water in case of a pipe burst and would require drainage facilities. (We had heard horror stories of chambers needing costly shutdown and rebuilding after being flooded.) Also, for pneumatic equipment, we needed an air supply at up to 300 lbs/in2. Customized ventilation was also a necessity. We specified that cold air would be pumped in through the floor vents and that warm air would escape through ceiling apertures.
Even seemingly insignificant features called for careful attention—particularly the fire suppression system. Since the chamber would host emissions tests at high frequencies, we could not afford the performance degradation that would occur if fixed sprinkler systems protruded from the ceiling so, at increased cost, we chose telescopic sprinklers with ferrite rings.
The higher frequency tests we planned to run also made it critical to decide on the positioning of the shielded amplifier, the control room, and the separate shielded room for clients’ support equipment. Essentially, we had to use the shortest cable runs possible to reduce losses. Still, since the building height restrictions dictated that we had to sink the chamber below ground, the ancillary rooms would remain at ground level. Our optimal design was to use expensive, low-loss coax cables running 11 meters.
The other critical physical specs fell into place easily. They were a 10,000-lb. hydraulic lift to bring heavy equipment below ground level to meet the entry door; a 2-meter turntable rated for 8000 lbs.; and electrical power ranging from 100 A at 208/480 VAC and 60-Hz, three-phase to 30 A at variable DC power.
Mapping Out the Performance Criteria
Analyzing what our clients’ expected testing requirements might be, we determined that the chamber must be qualified to 17 different standards—including EN55011, EN55022, CISPR 25, and U.S. FCC Class B, parts 15 and 18 radiated emission requirements. The unit also had to comply with immunity standards such as SAE J551, ISO 11452, and all North American automotive related immunity standards, as well as European Directive 2004/104/EC and MIL-STD 461E. The chamber had to be designed with the flexibility to handle fast-evolving standards far into the future—beyond the normal seven-to-10-year lifecycle of such facilities. If the unit could meet stringent military specs, we knew it should be able to handle new standards for automotive EMC testing, as well as the full range of test on ITE, medical equipment, and industrial products. Additionally, the ability to handle a range of international test standards would allow us to serve clients with products targeted toward the global market.
To assure that the proposed chamber could handle these wide-ranging demands, its shielding effectiveness had to be determined. It had to be greater than 100 dB at as much as 40 GHz—a level that had to be tested and verified before the manufacturer installed the ferrite tiles and absorbers. Good field uniformity (from 26 MHz to 18 GHz) at 16 points within the chamber was another critical necessity. Anticipating possible customer requirements, we knew that we would be dealing with very different field strengths—i.e., levels differing markedly from those found in typical EMC testing of telecom equipment. While normal field strengths for telecom gear or heavy industrial equipment range from 3 V/m to around 10 V/m, automotive equipment requires anywhere from 70 to 150 V/m, with pulses for radiated immunity tests of up to 600 V/m. Military testing runs even higher—in the continuous 50 to 200 V/m range, depending on classifications.
Selecting the Vendor
After sign-off from senior management and the determination that space was available for a chamber of this size, we began the process of short-listing and evaluating vendors. We developed a 56-page RFQ outlining all the physical and performance specs that would go into the chamber. That document was forwarded to seven different manufacturers with which we had signed confidentiality agreements. It quickly became apparent that two would be unable to meet our specs while another two failed to convince us that they could meet the tight timeline established for installation and final certification. Although it is common for construction of a chamber, even a 3-meter chamber, to require up to nine months from site preparation to certification sign-off, we required completion within five months.
The three short-listed vendors were invited for all-day meetings. Meeting face-to-face with their project managers and design engineers, we were able to grill them on their technical capabilities, project management expertise, cost controls, and contingency plans. As we moved into formal contract negotiations, we were able to apply various experiences and expertise to minimize those elements that often add significant cost overruns, including building refits and obtaining construction permits.
Managing the Installation
The key to rapid, trouble-free installation was a collaborative stance with the manufacturer. We relied on in-house experience for all site preparation and project management while they provided the chamber components and subassemblies that were erected on-site. We avoided the costs of purchasing a turnkey project; and after cutting the first concrete on November 27, 2003, the new chamber was fully accepted on February 28, 2004—less than four months later.
Our three months of prep work truly paid off. Early in the construction process, we established that we could locate the chamber in such a way that only one support column in the building would need to be reinforced. (Alternative siting could have meant tens of thousands in increased costs.) We also determined that local building ordinances would not permit us to raise the building’s roof to accommodate the tall chamber. The only other option was excavating eight feet below grade, an alternative that entailed an array of other challenges. For example, after obtaining final drawings from the three finalist vendors, we determined that we could ensure that the necessary excavation would not disrupt our underground utility lines. Simultaneously, we had to obtain an agreement from the landlord consenting to such a large excavation. In turn, we had to agree to a contingency plan for filling in the hole should we leave the building.
We found that another key to successful installation was having our staff take on the role of project manager. In-house personnel selected and managed most of the subcontract services, including dealing with the electricians who would wire power into the chamber. We also made sure that the vendor’s team (usually two engineers, with one more needed for assembly of the heavy floor panels) had adequate working space as well as 24-hour access to the facility.
Containing Total Costs
The complete process of purchasing, installing, and accepting a 5-meter chamber with the relevant specs could easily run to as much as one million dollars. Fortunately, we were able to complete the project for far less, a savings attributable to exhaustive prep work and careful project management. For example, our RFQ included a five-year warranty on parts and labor, rather than the two years that is the standard offering of most chamber manufacturers. That stipulation has already paid dividends in terms of minor fixes for which we incurred no costs. Also, the chamber’s fabrication materials themselves are guaranteed for 10 years. We broke with the standard practice of fronting fifty percent payment upon contract signing and instead negotiated for progress payments at due on agreed milestones. Finally, we put in place stiff penalty clauses for time overruns (as much as $6000 per day).
Conclusion
Since its startup early in 2004, the chamber has allowed us to verify EMC compliance for many customers. We emphasize asset efficiency and see that the chamber is staffed appropriately at all times. This emphasis helps keep operating costs in check, helps maintain an optimum scheduling of tests, and backs up our 24-hour quotes and 3- to 5-day average lead times for test programs. We believe that our experience with specifying, purchasing, and installing such a major test facility is instructive for test engineers everywhere. It has confirmed for us the value of comprehensive preparation and the importance of having a diverse group of skill sets in-house. It has also underscored the merits of disciplined project planning, budgeting, negotiation, and cost control. As product development costs come under greater scrutiny and as launch windows tighten further, efficiency and the ability to provide fast and conclusive EMC tests become even more important.
Michael Koffink is the Operations Manager of Intertek’s Littleton, Mass. lab.
Bob Mitchell is a Staff Engineer at the Littleton lab.
