Temperature-control system simulates space

Fig. 1  A vacuum chamber simulates conditions in space.

The company wanted to replace the unsupported VAX-based hardware that served as the backbone of the system, so its engineers worked with National Instruments to design a dual-chassis PXI-based system (Figure 2) that could control the thermal chambers and perform the necessary tests. Lockheed-Martin then contracted with Cal-Bay Systems, a developer of PC-based measurement and control systems, to make the design a reality.

Fig. 2  This thermal system overview depicts power distribution among various component areas.

The Lockheed-Martin thermal-vacuum system consists of a high-vacuum chamber lined with liquid nitrogen cold walls. Radiant solar heat is simulated using 12 resistive heaters operated through a power-distribution system. This power system has a serial interface that controls 12 stepper motor controllers. These motors in turn control a series of autotransformers, which adjust proper power output to the heaters.

Inside the control room, a PXI chassis controller executes the custom Cal-Bay software library application while serving the test data to engineering servers through proprietary datasocket technology. A local SCXI chassis monitors and provides inputs to the control room power distribution control panel by measuring the voltage and current on each heating zone. The chamber pressure reading is also captured by the software for flight hardware safety protection.

To complicate matters, a natural phenomenon called Corona arcing occurs at specific pressures where the dielectric of air ceases to prevent electrical conductivity between heaters and ground. Such a situation can cause electrical arcing capable of destroying flight hardware. By monitoring the system vacuum, the control software must automatically override the power settings to eliminate any harmful electrical potential should vacuum reach the Corona pressure.

Fig. 3  A run screen capture displays conditions across several zones.

By making use of its extensive software libraries, Cal-Bay was able to complete the projected 24 "man months" of work in just four "man months." Also, the system's cost was in line with the industry-standard $100 per channel.

With the new system, Lockheed-Martin has reduced test-commission time, improved operational procedures, and integrated test data into its engineering network. The system provides ultimate operator control with integrated safety features while maintaining subsecond control loops over a widely distributed PXI control system.