Thermal Switch

Background

Rovers are designed to operate in a wide range of environments and temperatures. The intense environmental conditions demand specialized tools in order to protect equipment and other features on the rover. Thermal switches are used to ensure that equipment is kept within ideal operating temperatures by insulating or dissipating heat when necessary. This regulation of temperature works to prevent burnout and failure.

Objective

The primary objective of this project was to develop a thermal switch with two different effective thermal conductivities. These two different conductivities are at work during the main two phases of operation for the switch: cooling and insulation.

The cooling phase will be active when the electronics the switch is connected to is heated to a specific temperature due to operation. This heat activates the expansion mechanism which will lead to contact between the electronics and rest of the switch. The heat from the electronics is then consequentially dissipated through the heat sink into the environment.

The insulation phase is active, mainly at night, in order to prevent heat from leaving the component. This guarantees the component will remain within ideal operating temperatures even when the environment is extremely cold.

Figure 1 is an example of a representation of two different effective thermal conductivities at different temperatures. It is important to note that these are not the exact values at which the thermal switch operates. In Figure 1, the insulating phase would be active in the range of approximately -20 °C to 40 °C. In this portion, the thermal conductance is 0 W/K as the switch is insulating the component. In the range of 40 °C to 100 °C, there is a thermal conductance of 1 W/K. This is the cooling phase. Heat is being conducted from the component, through the switch, and into the environment.

Figure 1: Total Thermal Conductance (W/K) as a Function of Camera Temperature (°C)

Final Design

With respect to the objective mentioned above, the final design is an argon-filled bellows sealed at both of its ends and directly soldered to an action camera encasement, all of which is encased in an aluminum and polyethylene protective enclosure as shown in Figures 2a and 2b. The aluminum and polyethylene serve to protect the camera from radiation damage, as incident radiation is much greater on the Moon and Mars than on Earth. The polyethylene also serves to insulate the camera during the insulating phase and direct heat transfer during the cooling phase. Additionally, thermal interface material (TIM), shown in green in Figures 2a and 2b facilitates heat transfer during the cooling phase.

Figure 2a: Thermal Switch in Cooling Phase

Main Operating Mechanism: Heat from the camera battery causes the expansion of the bellows and the eventual contact of the bellows with TIM and thus the excess heat is dissipated.

Figure 2b: Thermal Switch in Insulating Phase

Main Operating Mechanism: The polyethylene inner encasement insulates the camera from the cold environment.

Performance Results

Below is a table containing the results of the cold and hot tests respectively.

The data suggests that in the insulating phase, which corresponds to the data from the cold test, the bellows were effective at regulating the temperature of the battery compartment. In the control condition, the condition without the bellows and thus open to the environment, the temperature dropped dramatically. However, with the bellows, the temperature remained relatively similar to the initial one differing only by 0.7 °C.

Table 2a suggests that in the cooling phase, which corresponds to the data from the hot test, the bellows effectively dissipated the increased heat of the battery compartment. In the control condition, the battery compartment temperature increased by 12.2 °C. With the bellows, however, the temperature only increased by 4.1 °C.

Overall, the results of the testing does in fact suggest that the bellows were able to operate with two, different effective thermal conductivities! In the insulating phase, the heat was prevented from leaving the battery compartment resulting in the first effective thermal conductivity. In the cooling phase, heat was able to be dissipated thus lowering the battery compartment temperature relative to the control condition with no bellows. This is evidence of the second effective thermal conductivity.

Link to Executive Summary