Abstract: A thermal anemometer having a large number of thermally isolated fins is placed in a fluid flow. The flow velocity is computed from the temperature of the fin tips and the fluid temperature along with knowledge of the amount of power supplied to the fins.

Abstract: A unit and system for sensing fluid velocity which is particularly well suited for use in the control system of an HVAC unit. A salient feature of the system is the combination of heat generating structure, a plurality of thermally conductive elements, and thermal insulation between the heat generating structure and a fluid flow path. Each of the conductive elements is in thermal communication with the heat generating structure at one end and the fluid flow path at a second end. As a result, heat transfer to the fluid is channelled through the conductive elements. This achieves a distributed heat flow path which results in an efficient and accurate heat flow measurement.

Abstract: A solenoid valve used as an expansion valve in a refrigeration circuit has an opening that varies as a function of the pulse width of a high frequency PWM (pulse width modulated) control signal. A transistor controls the electrical current delivered to the solenoid valve. The PWM control signal rapidly cycles the transistor on and off at a constant frequency that is sufficiently high to prevent the valve from having enough time within a cycle to move between fully open and closed. As a result, the valve balances at an intermediate position determined by the relative lengths of the on and off periods of each pulse cycle. The relative on-off pulse widths, often referred to as, "duty cycle", is varied in response to a temperature sensor to control the flow of refrigerant through the refrigeration circuit. The duty cycle is also adjusted to compensate for any variations in the supploy voltage applied across the solenoid valve.

Abstract: A microcomputer based fault detector for identifying a phase reversal, a phase loss, and a power loss in three-phase circuits. A microcomputer samples a pattern of timing signals generated in response to three current transformers associated with each of the three phases. The changing pattern of timing signals represent the phase relationship of each of the three phases. The sampling rate of the microcomputer is synchronized to the alternating current in each of the three phases by an interrupt signal indirectly generated by one of the phases. In the absence of current in this interrupt generating phase, a backup interrupt, internal to the microcomputer, is enabled which directs the microcomputer to determine if the absence of the first interrupt is due to a phase loss or a power loss. Either interrupt prompts the microcomputer to sample the pattern of signals and compare it to a predetermined pattern.

Abstract: A microprocessor-based frequency shift keying (FSK) data communications module. The microprocessor selectively modulates and demodulates data directly in terms of a square wave pulse train in which a first frequency is used to convey a binary "1" and a second frequency to convey a binary "0". An input and an output port on the microprocessor are selectively coupled to a transmission line through a transformer, by means of electronic switches. Also included are a bandpass filter for shaping the square waves into sine waves and for filtering noise, and means including a notch filter for generating a carrier detect signal input to the microprocessor, when incoming data is detected on the transmission line. Many of the components in the module are used for both transmitting and receiving data.

Abstract: A capacitive liquid level sensor for sensing the height of liquid in a liquid storage tank at a plurality of locations. The liquid level sensor includes a cylindrical housing which extends along a line perpendicular to a predetermined reference level which is approximately parallel to the expected level of liquid in the storage tank. Several capacitors are included within the housing, each employing an air/liquid dielectric, whereby the capacitance values of the capacitors are individually variable due to any variations in the liquid height at each location. Several alternative cross-sectional structures are shown whereby the cylindrical housings are employed as a common plate to the several capacitors located within each housing.

Abstract: A method and apparatus for measuring the level of a liquid in a storage tank having a predetermined reference level. A liquid level sensor is located in the tank so as to monitor the upper level of liquid over a predetermined height range at a plurality of locations within the tank. The sensor is interrogated at each location to determine the corresponding sensed level and the sensed levels are compared to see if their values are close enough to indicate that the measured level approximates a plane parallel to the predetermined reference level in the tank. Only when the sensed evels are close enough in value, is one of their level values read as a measure of the level in the tank.