Controls on the freezer temperature

• Freezer compressor
  The computer can control the compressor to be on or off. There are limits to the compressor on/off duty cycle. To ensure the nominal life of the compressor, the manufacturer recommends no less than 10 minutes on or off.
• Heater array
  A heater array is located just above the floor of the freezer compartment. This array can be continuously controlled by the computer to keep the temperature at a constant or varying set point. A small fan placed in the freezer helps homogenize the spacial temperature variations.

To smooth the temporal temperature variations of the freezer as well as to add an effective thermal capacitance to the system, aluminum plates 3 cm thick were placed on the walls and floor of the freezer. Although this does reduce the maximum cooling rate of the freezer, it is still capable of simulating diurnal temperature variations of Mars in real time.

Minimum Temperature: A minimum temperature of -90oC can be obtained in the freezer by running the compressor 100% of the time and not activating the heater.

Temperature Stability: The temperature stability depends on the duty cycle of the compressor and on the quality of the PID (Proportional, Integral, Derivative) control algorithm. If the compressor is on 100% of the time, then the control is expected to be quite good. However, if the compressor is cycled on and off (to either lengthen the compressor life span or to raise the maximum achievable freezer temperature) then stability will depend on how well the PID algorithm deals with the compressor off time.

Maximum Temperature: Due to current and power limitations in the freezer heater array, the maximum freezer temperature will depend on this maximum heater power as well as the duty cycle of the compressor.

Sample Data

Mars Diurnal Temperature Simulation
The following figure shows that a diurnal temperature cycle of the Martian atmosphere can be achieved. The setpoint profile was obtained from Viking Lander data and is linearly interpolated between points. It can be seen that the temperature deviation from the imposed setpoint is less than 1 degree, which can be minimized further by use of a fans.