BACKGROUND OF THE INVENTION It is desirable to apply accelerated stress conditions to products being life-tested in order to shorten the time to failure should a flaw exist in either the design or the fabrication processes. Heretofore highly-accelerated life tests (HALT) and highly- accelerated stress screens (HASS) have used temperature, high rate-of-change of temperature, all-axis vibration and other stresses but have generally excluded high pressure, humidity and corrosive atmospheres. Highly-accelerated stress tests (HAST) have generally applied high temperature, high pressure and sometimes corrosive atmospheres but without vibration.

HALT and HAST each allow the precipitation and detection of different, but somewhat overlapping, sets of flaws. Some companies have applied HALT and then HAST and alternately applied HAST and then HALT in that order. While it would be beneficial to apply both sets of stresses simultaneously, this has not been done previously because of equipment limitations.

When using HAST and then HALT, or HALT and then HAST, it is sometimes difficult to determine when something fails because the failure may only appear during the subsequent tests. Combining the attributes of HALT and HAST not only compresses the time to failure, due to the interaction of the stresses, but also makes it possible to determine more precisely when the failure occurred. This can be important in assessing the field lifetime of the product as built and tested.

There are many types of devices for measuring and testing products in simulated environments. For example, U. S. Patent No. 4,602,503 (Hile et al.) discloses an environmental chamber for testing objects under climatic conditions of high temperature and high humidity. U. S. Patent No. 6,005,404 (Cochran et al.) discloses a test apparatus having a thermal chamber for testing products and partition-isolated auxiliary chamber in which ambient conditions prevail. A flexible partition is interposed between the thermal and auxiliary chambers. The products under test are in the thermal chamber and the monitoring equipment is in the auxiliary chamber. Also, U. S. Patent No. 5,969,256 (Hobbs) discloses a modular vibration system which uses interchangeable modules to apply various multi-axis and multi-modal vibrations to products under test.

While there are many types of environmental chambers and vibration testing devices available, none combines temperature, humidity, pressure, corrosion and vibration testing in one system.

It is therefore an object of this invention to provide a system which can apply simultaneously all the stresses of constant or time-varying temperature, humidity and pressure in combination with multi-axis vibration and corrosive atmospheres.

BRIEF SUMMARY OF THE INVENTION The invention is an apparatus for applying simultaneously to a product under test various combinations of constant or time-varying test parameters including temperature, pressure, humidity, vibration and corrosive atmosphere. The apparatus also provides for applying rapidly changing test parameters to the product.

The apparatus comprises an environmental chamber having functional devices comprising heating and cooling heat exchangers, a moisture source, an air pressure source, a corrosive materials source and a vibration system (commonly called a shaker) with provisions for mounting the product under test.

In a first embodiment, the chamber comprises two compartments, or spaces, separated by a compliant seal. The first space is a controlled-environment space where the product under test is located. The second space is an equalized-pressure space containing the vibration system comprising a vibration table, its actuators, compliant

supports and associated hardware. The compliant seal protects the vibration system from the severe environmental conditions imposed on the product under test. In a second embodiment, the chamber comprises a single compartment, without a compliant seal and without an equalized-pressure space, having instead a corrosion-resistant vibration system.

The apparatus includes a control system for controlling the temperature, humidity, pressure and corrosive level of the controlled-environment space as well as the vibration conditions applied to the product. The control system is coupled to the controlled-environment space and to all the functional devices to sense and maintain the desired conditions in the controlled-environment space. All the test conditions can be applied to the product simultaneously or individually or in any combination. Also, as well known in the art, products can be tested either passively (unpowered or inoperative) or dynamically (powered or in operation).

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of a preferred embodiment of the invention.

Figure 2 is a diagram of product mounting devices.

Figure 3 is a block diagram of a control system.

DETAILED DESCRIPTION OF THE INVENTION In the drawings, like reference numerals indicate like features; and, a reference numeral appearing in more than one figure refers to the same element. The drawings and the following detailed descriptions show specific embodiments of the invention.

Numerous specific details including materials, dimensions, and products are provided to illustrate the invention and to provide a more thorough understanding of the invention. However, it will be obvious to one skilled in the art that the present invention may be practiced without these specific details.

A preferred embodiment of the invention is shown in the schematic diagram of Fig.

1. A chamber 1 encloses a controlled-environment space 2 and an equalized-pressure space 21. A compliant seal 5, preferably in the form of a flexible diaphragm, separates the spaces 2 and 21. The product 3 being tested is mounted on a fixture or mounting

plate 4 which is attached to a vibration table 6. Vibration actuators 8 are attached to vibration table 6 which is supported by flexible supports 7 attached to chamber 1. A partition 9 enclosed within chamber 1 provides an air duct 45 for conditioning the air in space 2. Enclosed within air duct 45 are a fan 10, a cooling heat exchanger 11, a heating heat exchanger 12 and a moisture injector 13. A drip pan 24 is located under the cooling exchanger 11 to capture condensation from the exchanger 11. A moisture source 14 is attached to injector 13. An air pressure source 15 is coupled to spaces 2 and 21 via pressure port 17 and equalizing port 20, respectively. A moisture and corrosive barrier 16 separates space 2 from source 15 and port 20. A corrosive material source 18 is coupled to space 2 via corrosive port 19. A condensate drain 22 is located in a low point in space 2 and preferably is coupled to a condensate receptacle 23. Optionally, inlet door 24 and outlet door 25 are connected to partition 9. Fan 10 is coupled to external motor 44.

Figure 2 is an enlarged view of a preferred attachment of compliant seal 5. Seal 5 is clamped to each wall of chamber 1 and to vibration table 6 by preferably corrosion- resistant metal strips 26 held in place by corrosion resistant bolts 27 and nuts 28.

Preferably seal 5 covers the entire surface of vibration table 6. Inserts 29 are screwed into table 6 to provide mounting points or fixtures for supporting product 3 while preventing leakage between seal 5 and table 6. Inserts 29, along with sealing washers 46, can also be used to clamp mounting plate 4 to seal 5.

Preferably, mounting plate 4 or seal 5, or both, are made of thermally insulating material to minimize heat transfer from product 3 and space 2 into table 6 and space 21.

Figure 3 is a block diagram of a control system used to operate the testing system of the invention. The control system comprises a control unit 30 which is coupled to a temperature sensor 32, a humidity sensor 33, a vibration sensor 34, a pressure sensor 35 and a corrosive sensor 36. Preferably all the sensors are physically located within environment space 2 although it will be obvious to those skilled in the art of control systems that other locations can be used. Control unit 30 is also coupled to a heat source 37, a heat sink 38, a moisture source 39, a vibration system 40, a pressure

source 41, a corrosive source 42 and a mechanical controller 43. A control panel 31 is coupled to control unit 30 to provide an operator interface to the control system.

The elements 37-43 comprise the functional devices shown in Fig. 1 along with the mechanisms and controls necessary to operate those devices. That is, heat source 37 comprises heating heat exchanger 12; heat sink 38 comprises cooling heat exchanger 11; moisture source 39 comprises moisture injector 13 and moisture supply 14; vibration system 40 comprises table 6, supports 7 and actuators 8; pressure source 41 comprises pressure supply 15, barrier 16 and ports 17 and 20; corrosive source 42 comprises corrosive supply 18 and port 19; and mechanical controller 43 comprises fan motor 44, actuators for doors 24 and 25, and other miscellaneous hardware.

The control unit 30 can be programmed to operate elements 37-43 in the time sequence and duration necessary to perform a desired test. Also, control system 30 can control the output level of the various elements 37-43 as appropriate in response to feedback from sensors 32-36. Typically, control systems such as this are designed and built to perform tests for particular applications. The design of such control systems, along with their associated sensors, controllers and electromechanical devices, is well known to those skilled in the art of control systems and related software.

In operation, a product 3 is placed in the controlled-environment space 2 and mounted on the vibration table 6 by means of mounting plate 4 and inserts 29. The chamber 1 is then closed and the environment in space 2 is thereafter controlled by various combinations of the following processes.

The temperature is controlled by activating fan 10 to circulate air in the chamber past the cooling and heating heat exchangers 11 and 12. The heat exchangers can use any of several processes well known to those of ordinary skill in the art. For example, cooling can be accomplished by circulating a refrigerant from a conventional refrigeration system through the cooling exchanger or by passing liquid nitrogen through the exchanger. Also, for example, heating can be accomplished by electrical heating elements in the exchanger, or by circulating fluids from a boiler or heat pump through the exchanger, as well as by combustion devices within or thermally coupled to the exchanger.

Humidity can be increased by injecting moisture from injector 13 into the circulating air downstream from heating exchanger 12. Humidity can be decreased by cooling the circulating air with cooling exchanger 11 and condensing the vapor into drip pan 24.

The air can then be reheated downstream by heat exchanger 12 to maintain the desired temperature.

The pressure in the chamber is controlled by pressure source 15 via ports 17 and 20. Pressure source 15 provides approximately equal pressure in both space 2 and space 21 to minimize the force acting on seal 5 and table 6. Pressure source 15 can be bi-directional ; that is, it can produce positive or negative pressure relative to the pressure outside the chamber. Barrier 16 is a device preferably comprising a filter or desiccant, or both, for preventing moisture or corrosive atmospheres from being drawn from space 2 into source 15 or into equalizing space 21. Alternatively, if barrier 16 does not adequately prevent moisture or corrosive atmospheres from entering space 21, port 20 can be separated from source 15 and barrier 16. Then a separate pressure source can be added and coupled to port 20 for providing clean air, or an inert gas such as nitrogen, to space 21. A differential pressure regulator can then be used to control the separate source or source 15, or both, to limit the pressure difference between spaces 2 and 21.

A corrosive atmosphere is produced by injecting corrosive materials from corrosive source 18 into space 2 via port 19. A common corrosive is salt water but other corrosives such as acids or ozone can be used. A condensate drain 22 and a sealed receptacle 23 can be used to collect and contain corrosive liquids which accumulate at low points in space 2.

The vibration system, comprising vibration table 6, compliant supports 7 and actuators 8, along with power sources for the actuators (not shown), applies multi-axis vibration to the product 3 under test.

Compliant seal 5 protects table 6, its compliant supports 7 and its actuators 8 from the humidity and corrosive atmospheres in space 2. Preferably, compliant seal 5 covers the entire surface of table 6 and provides an air-tight, or hermetic, seal to the walls of chamber 1. Strips 26 are bolted or screwed to the chamber walls and to table 6 to ensure that seal is tightly fastened in place. Sealants may also be used to ensure

against leakage. Also preferably, a corrosion resistant mounting plate 4 covers seal 5 in the area where products are mounted to protect the seal from damage when products are moved into and out of the chamber. Inserts 29 are inserted into table 6 to provide rigid mounting points for attaching products or mounting fixtures to the table. Such inserts may also be used to attach mounting plate 4 to the table as shown in Fig. 2.

The walls of chamber 1 are preferably insulated to minimize heat transfer between the chamber and its surroundings. Also, mounting plate 4 or compliant seal 5, or both, are preferably made of thermally-insulating material to minimize heat transfer between the product, along with the controlled-environment space, and the vibration system.

Fan motor 44 is preferably mounted outside of chamber 1 to protect it from extreme temperatures and corrosive atmospheres within the chamber. Motor 44 can be coupled to fan 10 via various well-known drive means capable of containing the environment in the chamber.

Liquid nitrogen, if used for rapid cooling, is preferably not directly injected into the chamber. Instead, liquid nitrogen or other cold fluid is forced through cooling heat exchangers without introducing any material into the chamber which could affect the humidity, pressure or corrosive atmosphere.

As described above, humidity control is maintained via moisture injection to increase the humidity and via moisture removal, by cooling heat exchangers, to decrease humidity. The cooling exchangers may form ice which can be removed from the chamber either as a solid or as a liquid. Reducing the moisture content of the controlled space while maintaining temperature is accomplished by collecting moisture on the cooling heat exchangers and simultaneously reheating the air downstream to maintain temperature. Increasing the humidity while maintaining temperature is accomplished by injecting moisture in the form of steam or atomized liquid preferably after preheating the air as necessary to maintain temperature.

In applications where ice forms on the cooling exchanger, defrost heaters can be provided in the cooling exchanger and the following procedure can be used. As ice forms on the cooling exchanger, periodically doors 24 and 25 can be closed and the heaters turned on to melt the ice. The water is then collected in the drip pan and removed via a drain or pump. If necessary, parallel air ducts and additional heat

exchangers can be added and used to permit the cooling exchangers in one duct to be defrosted while those in the other are working.

Vibration can be accomplished by utilizing a vibration system of any type. A vibration system using pneumatic actuators is illustrated in Fig. 1 although other types well known to those skilled in the art can be used. The air to drive the pneumatic actuators is provided by an external source (not shown) and exhausted outside the chamber by an exhaust line (not shown) so that no pressure change is introduced by operation of the vibration system. The compliant seal used to separate the area beneath the table from the controlled-environment-space can serve two purposes : first, to protect the vibration system from corrosive atmospheres, and second, to insulate the vibration system from temperature extremes. Pressure balancing is accomplished by the pressure source which injects or removes clean, dry air or nitrogen from the space below the table as required to maintain the balance.

In a second embodiment of the invention, the compliant seal 5 in Fig. 1 is not used and spaces 2 and 21 become a single space. Then, to eliminate the need for the compliant seal, all the critical components of the vibration system (such as the table, its actuators and supports) having exposed surfaces can be made of corrosion-resistant materials such as stainless steel, fiberglass or various kinds of plastics. Or, as an alternative, all such exposed surfaces can be coated with various corrosion-resistant materials known to those skilled in the art of protective coatings. in addition, the coating materials can provide thermal insulation, either by the inherent properties of the protective coatings or by laminating layers of insulating coatings or other materials between the exposed surfaces and the protective coatings.

Products under test can be monitored or powered by systems outside the controlled environment space via cables passed through the chamber walls or through the compliant seal or the vibration table if those systems are located in the equalized pressure space.

While the invention has been described above with respect to specific embodiments, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

All references mentioned herein are hereby incorporated by reference to the extent that they are not inconsistent with the present disclosure.