Control apparatus and methods, heat transfer systems and apparatus and

Abstract

Control apparatus for producing a control signal indicating that a physical variable has reached a userselected level. A transducer generates for the control apparatus electrical pulses having a repetition period which is a function of the physical variable. The control apparatus repeatedly counts clock pulses in response to the transducer pulses to provide a first digital signal representative of the repetition period of the transducer pulses. The apparatus also repeatedly generates a second digital signal corresponding to the user-selected level of the physical variable by adding to a preset binary code a binary amount related to a userselected voltage. The first and second digital signals are compared to produce the control signal for a switching circuit which responds thereto. The comparison is overriden if an analysis of most significant bits of the first digital signal shows that the physical variable varies beyond a predetermined level. In control apparatus for a heat transfer system the comparison is made with a high fluid pressure level, and a low charge pressure level, as well as a userselected low fluid pressure level. Subject to predetermined time delays, the system is actuated when the high pressure is reached, and deactuated when the selected low pressure is reached. Abnormally low fluid charge is indicated when an excessive rate of change from the high pressure level to the low charge pressure level repeatedly occurs. An automotive air conditioning system and methods for controlling such a system are also described.