US ARMY TANK COMPONENT PUT TO THE TEST WITH ALGOR

The Laser Rangefinder

A critical component in the rangefinder of a US Army M1A1 tank was modeled and analyzed recently with the Algor Finite Element Analysis (FEA) System. The component, a compressor/heatsink subassembly, is part of the detector cooling system in the rangefinder - a device that provides accurate ranging on battlefield targets.

Ed Champion, a consulting engineer for GEC Avionics, Inc., Atlanta, Georgia, used Algor's modal, random vibration, and steady-state heat transfer analysis programs to simulate the rigorous battlefield-like conditions that the Laser Rangefinder (LRF) would encounter in actual use.

"The design of the subassembly is the key to the rangefinder's performance," says Champion. "The analyses were performed to insure that no external conditions reduced the performance of the subassembly or impaired its cooling ability."

Given the fixed design of the cryogenic compressor, the heatsink had to be designed to hold the unit in place and to provide an adequate thermal contact area for removing heat from the compressor.

Champion constructed the FEA model of the subassembly from 500 3-D Isotropic Brick elements. The model contained 800 nodes. All major features of the compressor/heatsink unit were incorporated into the model except for small fins opposite the mounting plane.

This FEA model of a compressor/heatsink assembly used in the Laser Rangefinder was constructed and analyzed on an IBM PS/2 Model 60. Some of the analyses were performed on an 80386 at 16 MHz. The overall space envelope is 4.25"w x 13.25"l x 11.50"h.

Modal analysis was performed to determine the first three fundamental frequencies. These frequencies were approximately 55 MHz above the first resonance of the LRF and the other frequencies did not coincide with the frequencies obtained from modal testing and analysis. Modal analysis results were used to determine the response of the unit to other types of excitation, for example, the effects of random vibration on the compressor and heat sink.

"Random Vibration is motion which displays no apparent pattern when recorded over time," explains Champion. "However, the frequency content of the motion is repeatable. In random vibration analysis, the excitations are assumed to follow a Gaussian distribution with zero mean value. Since this vibration is a random phenomena and the method is based on probability theory, predications can be made concerning the probability of exceeding a given displacement of stress."

Next, Champion examined the heat transfer performance of the system to see how the temperature values in the structure were distributed. This was not difficult to do since the model had already been constructed for the structural analysis. The only adjustments that had to be made to the heat transfer model were that 3-D, 8-node Thermal Brick elements were used, the heat source was specified, and the proper convection coefficients were assigned to the appropriate surfaces. "It was easy to port the thermal results to the structural model so that thermally induced stresses could be calculated," states Champion.

"The most outstanding feature of the Algor FEA system is the graphical interface for modeling and the CAD import/export abilities," Champion continues. "Algor simply has the best performance/price ratio on the market."