VITA 75 vs. VPX: Optimizing unmanned vehicle thermal and payload efficiencies

Military vehicles are getting smaller. Today's fighting forces are increasingly equipped with vehicles that are remotely teleoperated by soldiers, and also operate semi-autonomously. These unmanned vehicles operate in the air, on ground, at sea, and underwater (UAV, UGV, USV, UUV), and are rapidly comprising an increasing percentage of the U.S. fleet. These vehicles are tools used to perform the vital military task of protecting and extending the capabilities of the modern warfighter.

The absence of troops in-vehicle does not lessen the processing demand for unmanned vehicles. High-performance computer systems with greater processing capabilities, wider and higher-speed data buses, and more and higher-resolution sensors are required to enable the autonomous functions when a vehicle is unmanned. The higher data input and processing needs demand that these smaller vehicles provide the space necessary for cooling that allows for reliable, high-performance operation.

Thermal dissipation requires space, and space in these small unmanned vehicles is a scarce commodity. The operating environment of unmanned vehicles is no less demanding than that of manned vehicles. Temperatures can be just as extreme, but in these smaller vehicles, cooling computer systems becomes even more difficult.

3U and 6U VPX embedded systems have evolved to keep up with the latest bus speeds of modern CPUs while holding SWaP at bay. These relatively compact solutions are currently fulfilling modern requirements for high-performance computing in mobile military platforms. These systems provide connectivity, as well as the wide, high-speed I/O required to support visible spectrum and infrared cameras, radar, and other fast, high-definition sensors. At the heart of these systems is the processing power (CPUs, GPGPUs, FPGAs) required to process that data for object detection, classification, and tracking.

All these functions are required in smaller unmanned platforms, but these vehicles have tighter payload restrictions than their larger brethren. Every bit of weight and volume that can be removed from a system has the potential to improve the range, capabilities, or cost of a deployed unit, so engineers must consider the function of every cubic centimeter of space, and each gram of weight. This is where Small Form Factor (SFF) VITA 75-based systems excel; all unnecessary space can be squeezed from the system to reduce its size, and unnecessary mass can be eliminated. And while dissipating the thermal energy produced by these systems does require space, making the electronics portion of a system smaller leaves more space available for that thermal dissipation.

Efficiencies and space

One part of unmanned systems[ ] design that is often overlooked is empty space (air space), or mass. In military unmanned vehicle applications, each ounce or cubic inch of space that can be removed from a computing subsystem has the potential to improve the range, capabilities, or cost of a deployed unit. To this end, again, engineers must consider the function of every bit of cubic space and each ounce of weight. All unnecessary space should be squeezed from the system to reduce its size, and any unnecessary mass should be eliminated (see Sidebar 1). If included in the system design, the purpose of air space should be well understood. For instance, systems may include empty space because the design requires a particular internal or external surface area for convective thermal dissipation. Similarly, thermal transfer or sinking might require material mass to be designed into the unmanned vehicle system.

Consider the large and heavy thermally conductive pathways that distribute heat from critical components in VPX design. This conductive scheme distributes the energy from each board in a chassis via contact with slotted card guides, thereby facilitating the flow of internal temperatures to the external skin of the enclosure. The conductive components, as a function of their mass, also have a sinking capacity within themselves. This mass adds thermal sinking capacity, and can therefore average out peaks of the thermal demand of the processing components. Physical pressure, provided by wedge locks, maximizes the contact area between the thermal shunts on the blades and the slots. While wedge locks increase the shunt-to-slot contact area opposite the wedge lock, the wedge locks themselves, which provide pressure based on interlocking wedges, present a low (< 50%) contact area. Every other wedge on the device can only contact the slot or the blade, but not both (Figure 1). The resulting effect is that the use of bladed slots actually reduces potential contact area, conductivity, and the resulting thermal efficiency of VPX.