Common Refrigerator Diagnostic Test-Bed Design, Part II

The various feedback control mechanisms in many devices contribute to failures. Engineered devices that use bad choice of control algorithm (for example hysteresis control instead of Proportional Integral Derivative control) may not satisfy the design constraints. A proportional coefficient in a PID controller may need recalibration and the device it controls may start, for example, oscillating in an undesired way.

The refrigerator in the test-bed we are designing uses very basic hysteresis control (also known as on-off or bang-bang control). The thermostat, when warmed-up to the set-point temperature mechanically closes the circuit which starts the compressor. When the temperature drops below the desired set-point the thermostat opens and the compressor stops.

We will modify this basic electro-mechanical subsystem to provide computerized control. The new schematic is shown in figure 1.

modified control
Figure 1: The original refrigerator schematic is modified minimally when it becomes part of a diagnostic test-bed

Modifying the electrical circuit of the refrigerator is very simple and requires cutting the wire from the thermostat to the power connector and rewiring the compressor terminals. The process and the end-result are shown in the following photos:

fridge_modificiation_smallfridge_modificiation_completed_small

The relay-board which is off-the shelf and is put in an aluminum box for electrical protection as shown below:

relay_board_small

In the relay-board as well as everywhere else it is extremely important to do proper earthing. The reason for this is beyond safety: lack of good earthing can cause Electromagnetic Interference (EMI) problems. Finally, this is the Arduino shield which has all Arduino connectors for the refrigerator test-bed:

thermo_shield_small

In the photo of the Arduino shield we can also see the current limiting resistors (for the thermostat and temperature sensors) and Light Emitting Diodes (LEDs) that provide basic indication for the health and operation of the test-bed. In our more modern Arduino shields we use Liquid Crystal Displays (LCDs) as they are also very cheap and easy to program.

By applying the modification described above, we can (1) change the type of control for this refrigerator and (2) inject failures. One of the possible changes in the control is to use different sensor instead of the existing thermostat control (for example a digital temperature sensor). More relevant to our goal is the ability to inject and retract thermostat open-circuits and short-circuits. Another failure is modifying the hysteresis time. We will have detailed discussion on the type of failure injections once we discuss the design of the benchmark scenarios.

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