A Space Simulation Chamber with Thermal Ramping

A Space Simulation Chamber with Thermal Ramping

Environmental Stress Systems Inc. (ESS) Sonora, CA

For many years there has been a requirement for microwave components used in aircraft, missiles and satellites to be screened environmentally for mission-critical functionality and latent defects affecting long-term reliability. Typically, these screens subject the device under test (DUT) to a combined thermal/vacuum environment. The thermal portion of the test is achieved using a heated or cooled platen. The vacuum is applied by mounting the platen and DUT in a sealed chamber and removing the air inside the chamber with a vacuum pump. The DUT is heated or cooled by direct contact with the platen and simultaneous exposure to a high vacuum. Spurred by the recent growth in satellite production, the need to perform thermal/vacuum testing is increasing, creating a demand for increased quantities and lower costs, and substantially changing requirements for thermal vacuum chambers. In the past, components and subsystems were tested in large, expensive and immobile thermal vacuum chambers. Due to the explosive growth in information systems, the market has shifted to a need for small, relatively inexpensive satellites produced in higher volumes with more economical, production-oriented manufacturing techniques. Likewise, functional and reliability testing must be performed in a quicker, more cost-effective manner.

New Objectives

To meet these changing demands in satellite production, a new approach to thermal/vacuum testing is required. In discussions with environmental test engineers, four objectives have been identified as design goals for a new generation of thermal/vacuum chambers: portability, economy, increased throughput and full automation. Traditional thermal/vacuum chambers are large, stationary systems that require significant floor space and facility resources. Due to its size and complexity, the system must be mounted permanently in a single location, and test instrumentation must be moved from the test lab to the system. Thus, a small, portable system that can be moved easily to the test site is required. In the past, economic realities dictated that a costly single system must be shared by numerous users and/or programs. Test engineers were required to schedule a time to gain access to the system. When their allotted time had elapsed, the entire test setup was torn down to allow the next scheduled user to set up and perform his or her testing. The solution to this problem is to give each test site its own thermal/vacuum system, although a significant system cost breakthrough is needed to realize this possibility. Due to the high cost of building and launching satellites and the inaccessibility for service and repair, component reliability is extremely important. To achieve high levels of reliability, rigorous testing must be performed. Unlike traditional component testing, some satellite components must endure days or months of testing before they are deemed reliable. The extended test duration creates throughput nightmares for test personnel. With build volumes increasing, the need for increased throughput is becoming an even more critical problem. The solution is to set up multiple, flexible test lines using a portable, economical thermal/vacuum system. The duration, cost and complexity of satellite component testing are forcing complete automation of the test process. With continuous test times measured in days and months, unattended automated operation is indispensable. In addition, automating the test eliminates failures caused by inexperience or human error. Finally, a system is needed with all operational and data-acquisition control functions addressable remotely via the IEEE-488 general-purpose interface bus (GPIB), the most common laboratory computer interface.

The SSTC Solution

Currently, a space simulation thermal chamber (SSTC) is being produced that meets all of these goals. The SSTC is a portable, cart-based, fully automated system that can be rolled anywhere in the facility by one person. The cost is as little as one-quarter of a traditional thermal/vacuum system, allowing four systems to be procured within the same capital equipment budget, which helps to meet increased throughput goals. The SSTC, shown in Figure 1 , is controllable from the front instrument panel or remotely via an IEEE-488 GPIB computer interface. A single press of a button or a remote command initiates the entire vacuum pump down sequence with all of the pumping, valving and instrumentation management occurring automatically. The use of oilless components eliminates the risk of oil contamination due to facility power loss, component failure or human error. Automatic turn on of the high vacuum gauge prevents damage to the gauge due to improper operation. The SSTC requires very little floor space. With a footprint as small as 2' X 2', the unit fits almost anywhere in the test lab. The power cord is simply plugged into an AC power source and the coolant delivery hose is connected to a liquid nitrogen (LN₂ ) or liquid carbon dioxide (LCO₂ ) supply. An optional chlorofluorocarbon-free, mechanically refrigerated recirculating chiller can be supplied if a source of LN₂ or LCO₂ is not available or if the use of expendable refrigerants is undesirable. The SSTC utilizes an oilless diaphragm roughing or backing pump and an oilless turbomolecular high vacuum pump. The chamber comprises a circular alloy base plate and a glass or stainless-steel cylindrical bell chamber. When glass chambers are used they are protected by an external metal implosion guard. User-specified feedthroughs can be installed in the base plate for connection to the DUT. Virtually any cable or connector type can be accommodated, including waveguide and coaxial connectors. A motorized lift raises the chamber to yield access to the thermal platen. The lift is safety interlocked so that the chamber cannot be raised while under a vacuum. The vacuum chamber is monitored by a microprocessor-based digital indicating instrument. Two gauges are used to cover the entire pressure span. A thermocouple gauge covers pressures from one atmosphere to 1 x 10^-3 mm/Hg. An ion gauge reads pressures from 9 x 10^-4 to 1 x 10^-7 . Programmable set points based on chamber pressure can be set by the user to activate contact closures for additional control functions. Figure 2 shows the typical pressure reduction for a space simulation test. A fully redundant alarm limit controller guards against system or DUT damage due to out-of-limits temperature conditions. In addition to the latching electrical disconnect, the limit system includes an audible 90 dB sonic alarm and visual panel indication.

Table 1 lists the SSTC system components and specifications. The thermal platen is constructed of a brazed aluminum alloy. Resistance heating elements are embedded in the platen and a serpentine internal passageway creates a path for expectorant-based refrigerants or a heat transfer fluid. Thermocouples buried in the platen monitor temperature. A microprocessor-based digital indicating temperature controller utilizing pulse-width modulation output and proportional integral derivative control techniques is used to manage the platen temperature. The chamber features locally or remotely programmable thermal ramp and soak profiles. A typical thermal rate of change for a space simulation test is shown in Figure 3 .

Conclusion

The SSTC is designed to satisfy today's need for increased throughput and lower cost for thermal/vacuum testing of high reliability satellite components and subsystems. The fully automated, portable thermal chamber makes thermal/vacuum testing a true production line function at an affordable cost.

Environmental Stress Systems Inc. (ESS), Sonora, CA (209) 588-1993.