Measuring and testing – Fluid pressure gauge – Combined
Reexamination Certificate
1999-04-14
2001-05-29
Oen, William (Department: 2855)
Measuring and testing
Fluid pressure gauge
Combined
Reexamination Certificate
active
06237420
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to compressors used, for example, in air conditioning and refrigeration systems, and more particularly to a control for detecting unsatisfactory oil pressure conditions in a compressor and for protecting the compressor from damage from such conditions.
2. Brief Description of the Prior Art
The use of electronics to control and detect oil pressure for compressors is known. Conventional controls typically use time and duty cycle of good versus bad oil conditions to stop compressor operations. The prior art oil sensing controls utilized discrete electronics and a normally open oil pressure switch. Whenever a bad oil condition occurred the pressure switch would open and the discrete electronics would begin to time. If the time reached two minutes, a magnetic latching relay would be energized (i.e., the relay would latch) which in turn switched power off to the compressor. This system has the feature that if the pressure switch is disconnected, the control will time out in two minutes. The function of the control is to protect the compressor from the loss of lubrication. This is a time weighted average measurement to avoid nuisance trips (from conditions like defrost where liquid refrigerant may flood back into the compressor momentarily). To implement a time weighted average, the control must maintain the accumulated bad oil time during brief power outages (up to 2 minutes). This implies that voltage must be maintained in the control during these interruptions. When the power is interrupted, the pressure switch opens when the compressor stops (oil pressure is lost) if the pressure switch is a normally open device. This open switch limits power dissipation which in turn prolongs voltage on the control during the power interruption. This prior art system is adequate for the task but has many short comings. First, the oil pressure switch utilizes a magnetized shuttle which moves as the pressure changes and a reed switch. The magnetic shuttle moves toward the reed switch due to increasing oil pressure, which in turn causes the reed switch to close. The oil pressure switch could be made much smaller and less expensive if the pressure moved the magnetic shuttle away from the reed switch but this implies that the oil pressure switch is normally closed. Utilizing the prior art control, the maintenance of voltage during power interruptions would be impossible with a normally closed pressure switch (without expensive signal buffering). Furthermore, the prior art control does not detect if the wires to the sensor are shorted together (in fact the control will continue to allow the compressor to operate forever in this condition). The use of discrete electronics also has the disadvantage that the time to trip cannot be easily changed (i.e., the circuit board must be changed). Fault indicator lights cannot be used with the prior art control, an LED would constitute a continuous current drain on the power supply. This would lead to a loss of time during a power interruption as well. Finally, discrete electronics cannot easily filter noise signals from the sensor signals and require the use of expensive shielded cables in noisy environments.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a control which overcomes the above noted prior art limitations.
Briefly stated, the present invention relates to a simplified, less costly control with many added features. Features of the control include a separate power supply for the oil sensor, fault indication, compressor “jog” feature, sensor wiring fault detection, and improved noise immunity. A control made in accordance with the invention is shown in the schematic of FIG.
1
. The control includes a microcontroller in which all of its timing is derived from the 60 Hz line frequency. The microcontroller is interrupted 60 times every second. One of the features of the invention is that the microcontroller is normally in the sleep mode. In the sleep mode, power dissipation is greatly reduced (cut by two orders of magnitude), the microcontroller is “awakened” by every 60 Hz interrupt that occurs. While awake, the microcontroller checks the condition of the pressure switch, the time that bad oil has been detected (if any), and whether the relay needs to be energized. Upon completion, the microcontroller goes back to sleep. This reduces the power required to operate the control to less than 50 milliwatts. During power outages the 60 Hz interrupts stop and the microcontroller remains in the sleep mode, this insures that the bad oil time that has accumulated will remain as long as possible. Another feature of the invention is the use of a diode isolated power supply to source both the oil pressure sensor and the indicator light. By isolating the power supply of the microcontroller from the sensor, if the sensor closes (draws current) the power drain from the power supply will not affect the retention of the bad oil time in the microcontroller during power outages. In addition, the fault indicator is powered from the isolated supply. Thus the fault indicating LED will not affect time retention during power outages either. Therefore, by employing an isolated power supply for the sensor and LED, a normally closed pressure switch may be employed and a fault indicator added. This fault LED indicator chosen is a dual color device. When a bad oil condition exists the LED is illuminated RED but when a good oil condition exists the color is changed to green. If the microcontroller stops operation (such as a reset that occurs when power is first applied) both colors are illuminated and an orange color is seen. This enhances the diagnostic capability of the control to indicate a faulty control or a microcontroller failure.
The jog feature is added to the control to solve a system level problem. When a semi-hermetic compressor is shipped to a customer, it is generally pre-charged with refrigerant and much of the refrigerant is in liquid form. If the compressor attempts to operate very long with the liquid refrigerant present, damage may be done to the compressor. Once the compressor is installed, the service technician turns the compressor on for a very short duration (1 to 5 seconds) and then quickly turns off the unit. This forces the refrigerant out of the compressor and into the system. This process is called “jogging” the compressor. Notably this process is very uncontrolled (subjective energization times based on the perception of the service technician). A well known characteristic of an oil pump is that it will not produce pressure until spinning at a desired speed. Thus each time the compressor starts, a bad oil condition occurs but only lasts for approximately 8 seconds. If a normally closed oil pressure switch is employed, the switch would be closed until the motor in the compressor reaches 60% of its final rotational speed. This physical characteristic and the need for a JOG of the compressor have been combined to make a feature in this invention. The control first checks the status of the pressure switch as the compressor is starting up. The switch should be closed for 4 to 8 seconds, if the control detects an open switch (good oil) when the compressor starts the unit will shut off the system since the only ways that this could occur are as follows: the pressure switch has failed, the sensor cables are shorted together, or someone has attempted to bypass the pressure switch. This function has been combined with the JOG feature. If the oil pressure switch is open at compressor start up, the compressor is allowed to operate for 3 to 4 seconds and the unit is shut down by energizing the relay. Thus a service technician may start the jog function by simply shorting two wires in the sensor cable. At the same time cable faults, oil pressure sensor faults, and intentional bypasses of the switch will result in the compressor being turned off in at least four seconds.
The use of the microcontroller allows for software techniques to be employed to filte
Eifler Mark A.
Rowlette Mitchell R.
Baumann Russell E.
Oen William
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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