Common Refrigerator Test-Bed Temperature Plots

We have discussed in previous posts how to take a refrigerator and add actuators and sensors so the refrigerator becomes a diagnostic test-bed. Today we can start running diagnostic experiments and develop diagnostic algorithms and methods. An ultimate goal of ours is to build end-to-end diagnostic solutions in the context of repair, maintenance, or decision making.

Another goal is to create a diagnostic benchmark for continuous thermodynamic refrigeration systems. Of course in the process of experimentation we shall calibrate and validate our platform (convert sensors readings to Si units, inspect signal ranges, etc.). We want to have experimental controls, to be able to reproduce experiments, and to construct representative and valid benchmark. The approach of achieving this is by careful manual validation of the intermediate results.

Let us first define a diagnostic scenario. A diagnostic scenario is a set of time-series that contain information about:

  1. all sensor data
  2. the fault injection and the positions of all actuators
  3. the conditions in the environment (these are preset constant values such as the thermostat position or the mains voltage)

A diagnostic benchmark contains many diagnostic scenarios. The length of most of our diagnostic scenarios will be 24 hours. The reason for that is there is a natural 24 hour cycle in the environmental temperature and that the slowest process of interest in our refrigerator (warming-up to ambient temperature) is several hours in duration.

Before anything else we collect data about the nominal working of the refrigerator. We will use this data for building nominal models, calibration, and validation. This is how the refrigerator temperature looks for one nominal scenario:


The plot above shows the readings from three DS18B20 temperature sensors and the thermostat position for the duration of one nominal experiment. The temperature in the freezer and in the refrigerator is oscillating around the set temperature as prescribed. The room temperature depends on the office HVAC and the outside temperature. There is a small bump in the room temperature probably due to closing the door for a noisy conversation.

The next plot zooms in the time and shows all temperature sensors and the thermostat position:


The plot above shows half an hour of the temperature and thermostat signals during the day. We can see that due to the fluid dynamics of the air in the refrigerator (the fridge is mostly empty except for the pipes that hold the sensors) some of the temperatures fluctuate more than others. The effect is pronounced for temperature sensors close to the evaporator.

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