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This paper proposes a hybrid control structure for high pressure refrigeration systems. The methodology was applied for controlling pressure in a calorimeter for carbon dioxide compressors, replacing a valve that was operated manually. The calorimeter works with a pressure range from (20 to 50) bar for suction and (75 to 105) bar for discharge. The refrigeration cycle works with three pressure levels and two automatic valves with internally coupled stepper motors. A hybrid control comprising a linear control followed by a three level switching control was developed in order to get the pressures to the desired values. A command driver was designed and implemented to interpret the control signals and drive the valves. With the proposed control structure it was possible to obtain a satisfactory compressor model with only automatic tests, in less than half the time, when compared to the manual operation, making the calorimeter faster and stable for a wide range of working conditions. In addition to providing quicker tests, the proposed control structure showed also robustness in disturbance rejection tests and reliability in reference tracking tests, meeting the control structure project requirements.

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