Sealed Terra computer eliminates failures due to contamination

Figure 1: Azonix's Terra computer is designed to survive harsh environments.

Azonix, a division of Crane Co., introduces the new Terra computer, which is sealed to eliminate failures due to contamination and designed to operate at temperatures up to 60°C. The design provides a tough thermal management challenge of keeping components that dissipate approximately 60 Watts of power inside the case from exceeding a 90°C surface temperature limit.

“Our thermal analysis workload is too low to keep a dedicated resource on staff,” says James Young, Design Engineer for Azonix.

“In the past our choice would have been to either build up to a dozen thermal prototypes to evaluate various design alternatives or hire a consultant to simulate the design at a cost of perhaps $5,000. But recently computational fluid dynamics (CFD) software has become available that enables design engineers without a fluid analysis background to perform thermal simulation. We used this software to optimize the design from a thermal standpoint by evaluating many different iterations in a short period of time.”

Azonix provides products used for automation, as a user interface and as a command and control center at the optimum point of attack. In addition the company provides electronic design solutions for Ethernet and wireless communication and data acquisition I/O for use in hazardous area environments.

Embedded Computer Presents Thermal Challenge

The Terra is a new embedded computer designed for use in the transportation industry. Like other products, it is sealed from the elements and designed for use in very hot environments. It was designed using the SolidWorks computer-aided design (CAD) system by a group of engineers that joined the company as part of an acquisition.

“As with most of our products, we were limited to conduction and natural convection cooling,” Young says.

“This presents a difficult challenge for modern electronics equipment and our expectation that the design would require substantial work in order to meet the allowable internal temperature requirements. As a result, we addressed thermal management early in the design process prior to building a prototype.”

Young said that the company previously addressed thermal management either by using the build-and-test method or hiring consultants to perform thermal simulation. This was because most CFD software requires the user to have a deep understanding of the computational aspects of fluid dynamics in order to be certain of obtaining accurate results.

Users need to know how to translate their computer-aided design model into the CFD environment then reverse the model so that empty flow space rather than the solid product is modeled. They also need to create a mesh with the right properties, determine boundary conditions, select the right physical models, and tweak solver settings to ensure convergence, as well as other tasks.

Engineers Perform Fluid Flow Analysis

Figure 2: Base Terra unit – Horizontal orientation detailed internal temperature cutplot.

In the past few years, a new generation of CFD software has appeared designed to be used by engineers rather than specialists. Its use of native 3D CAD data, automatic gridding of the flow space, and managing the flow parameters as object-based features eliminates the need for engineers to understand the computational part of CFD and enables them to focus on the fluid dynamics of the product.  

The newest generation of CFD software contains sophisticated automatic control functions that ensure convergence in almost every application without the need for manual tuning, and the quality of the mesh to avoid one of the biggest reasons for run divergence.

“After reviewing the advancements that have been made recently in CFD software, we selected FloEFD from the Mentor Graphics Corporation Mechanical Analysis Division (formerly Flomerics),” Young says.

“FloEFD is tightly integrated with the two CAD packages that we use at Azonix, SolidWorks and Pro/ENGINEER. The skills required to operate the CFD software are simply knowledge of the CAD system and the physics of the product. So we can focus our energy on optimizing the thermal performance of our products.”

Young opened the SolidWorks model in FloEFD and defined the heat dissipation sources, material properties, and the ambient temperature outside the enclosure at the product’s design limit of 60°C. Then he defined the goals and performed a thermal simulation.

The software analyzed the CAD model, automatically identified fluid and solid regions, and allowed the entire flow space to be defined and gridded without user interaction and without adding extra objects to the CAD model. It took about five hours to generate simulation results that showed temperatures on the surfaces of key components exceeding the allowable limit of 90°C.

Optimizing Key Design Parameters

Figure 5: Modified Terra Unit – horizontal orientation detailed internal temperature cutplot.

Young looked closely at the design parameters that were available to improve thermal performance. One key parameter was the conduction path from the heat dissipating components to the external heat sink. The other was the geometric configuration of the external heat sink.

First he improved the conduction path. A heat spreader was used in the original design to move heat to the heat sink. A larger cross-sectional area and higher thermal conductivity material improved the performance of the heat spreader.

“In this case the length of the heat spreader could not be changed but we increased its cross-section and changed it from aluminum to copper,” Young says.

“The design changes are made directly to the native CAD data because the solid CAD model is used for simulation without the need for translation or copies. We inserted a gap pad which is a thermal interface material that improves the heat conduction between the hot components and the heat spreader. Gap pads are designed to fill the air gaps that exist between the heat spreader and the devices.”

Young updated the model and re-ran the simulation several times to evaluate the effect of the design changes and optimize the conduction path. The changes substantially reduced the surface temperatures on the dissipating components, but not enough to meet the thermal requirements. He then optimized the heat sink.

“We increased the number and size of the fins in order to increase the convection heat transfer,” he says.

“But there is a point at which increasing the number of fins becomes self-defeating because it reduces the convection heat transfer coefficient. Half dozen different iterations were tried, and in each case we changed the spacing and height of the fins. The heat sink was optimized and the internal component temperatures were minimized.”

Reducing The Number Of Prototypes Required

“The changes to the heat sink reduced the surface temperatures below the maximum allowable levels,” Young says.

“The result was that we were able to complete the thermal design prior to building the first prototype. When the prototype was built and tested, the measurements were within 5 percent of the simulation predictions. As a result, this was the only thermal prototype that needed to be built. In the past, when we used the build and test method, we often required six prototypes to achieve acceptable surface temperatures and even then we were not able to achieve temperatures as low as we obtained on the Terra with thermal simulation.”

“The new generation of embedded CAD tools can save money and time by enabling design engineers to optimize the design from a thermal standpoint early in the design process,” Young concludes.

“Previous generations of CFD software only made sense for companies that produced enough new products to engage a full-time specialist to handle thermal simulation. The new generation of CFD, on the other hand, enables simulation results to be incorporated into the design process by the right person in the right place at the right time. The result is that we get the design right the first time, only have to make one prototype, and avoid expensive design changes in the late stages of the development process.”