Abstract:
A microprocessor control (10, 24) has one or more temperature responsive thermistor sensors (20, 26, 28) used to provide temperature inputs to control operation of a temperature control system. The microprocessor control subjects the thermistor sensors to a series of diagnostic tests prior to reading the thermistor sensors, the tests including short and open circuits, shorts to ground or power and leakage paths to ground. The thermistors, along with a reference resistor (R11, R26) form an RC charge circuit with a capacitor (C6, C6&#39;). The capacitor is charged through each thermistor and reference resistor for selected open and short tests while a watch dog timer is used to determine the time taken to charge the capacitor via a timer capture port coupled to the capacitor. The temperature control system has a power supply having a logic ground and a chassis ground of different potentials which are utilized in the leakage test by turning off the power source to the thermistors and setting a watch dog period to determine if leakage through a respective thermistor to chassis ground has caused the capacitor (C6, C6&#39;) to charge sufficiently to cause a timer capture.

Description:
BACKGROUND OF THE INVENTION 
     This inventions relates generally to temperature control systems and more particularly to a method and apparatus for sensing failed temperature sensors in such systems. 
     Temperature responsive thermistor sensors are commonly used in temperature control apparatus. In defrost applications, for example, one thermistor may be used to sense the temperature of ambient air and another thermistor may be used to sense the temperature of a liquid line in order to provide input signals to a control unit for controlling a defrost cycle. In another application a thermistor is disposed in an air supply duct to sense the temperature of supply air and provide an input to a control to determine when an indoor blower should be turned on or off and to vary the speed of the blower as required. It would be advantageous to use such thermistor sensors also as a limit control. For example, in the air duct connected to a fan for circulating heated air, the thermistor could be used in order to sense over-temperature conditions and turn off the heat source before fuse line breaks are actuated. This arrangement would result in a system which could not be bypassed by shorting or opening the thermistor circuit since the control&#39;s microprocessor would detect such conditions and shut down the system. The air duct thermistor can also be used to turn off the fan following de-energization of a heat source based on the actual temperature of the air rather than some arbitrary time delay. 
     However, a problem occurs when a sensor fails. A failed sensor results in erroneous readings and interferes with normal operation of the system. With regard to outdoor units, a typical mode of failure is for the insulative layer encasing the thermistor to develop cracks thereby allowing moisture to permeate and create a leakage path to ground causing faulty readings. As a result, the control unit fails to initiate a defrost cycle when needed due to the erroneous readings thereby causing the unit to ice up. By the time the thermistors are tested they often have dried out and appear to function properly so that finding the reason for the malfunction is often very difficult. In dealing with operational problems in the field it turns out that thermistor sensors are the least reliable component in the control unit. With regard to indoor air duct sensors, the limited reliability of the sensors is also a reason against using thermistor sensors as limit devices. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method and apparatus for sensing failed temperature responsive thermistors. Another object is to provide a method and apparatus for providing an indication of a failed sensor in a temperature control system and to prevent disruption of operation of the system. Yet another object is the provision of a method and apparatus which will revert to a selected default operation upon the occurrence of a thermistor failure. 
     Briefly, in accordance with the invention, a microprocessor control circuit having a thermistor sensor input subjects the sensor to a series of tests prior to taking a reading of the sensor. The tests sense for different fault conditions including short and open circuits, shorts to ground or power and leakage paths to ground. According to a preferred embodiment shorts are sensed by establishing a minimum charge time for a capacitor charged through the thermistor while opens are sensed by establishing a maximum charge time for charging the capacitor through the thermistor. Shorts to ground or power are sensed by charging the capacitor through a reference resistor and comparing the charge time to upper and lower limits for this reading. According to a feature of the invention, leakage paths to ground are detected by utilizing a potential difference between logic and chassis ground. All outputs are turned off and the timer capture pin of the microprocessor is enabled for a selected period of time so that if a leakage path significant enough to affect the circuit exists the capacitor will be charged causing a capture. In one embodiment the control uses inputs from a room thermostat and a supply air duct thermistor sensor to determine when an indoor blower should be turned on and off as well as to power resistive electric heating elements. In a modified embodiment, thermistor sensors include an ambient air thermistor and a liquid line thermistor provided to initiate defrost cycles as required in a defrost control on an outdoor unit. In another modified embodiment an A/D circuit is used for testing the thermistor sensor for fault conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of an indoor unit having an electric heat and fan control system and an outdoor unit having a defrost control system; 
     FIGS. 2a and 2b are a schematic of the control system for the indoor unit of FIG. 1; 
     FIG. 3a is a schematic of the IRQ circuit used with the FIG. 2 schematic and FIG. 3b is a schematic of the RESET circuit used with the FIG. 2 schematic; 
     FIG. 4 is a schematic of the power supply used with the FIG. 2 schematic; 
     FIG. 5 is a thermistor read routine for detecting shorts and opens in accordance with the invention; 
     FIG. 6 is a thermistor leakage check routine in accordance with the invention; 
     FIG. 7 is a thermistor read routine for detecting shorts to chassis ground or to 24 VAC in accordance with the invention; 
     FIG. 8 is a schematic of a portion of the control system of the outdoor unit depicted in FIG. 1 including the sensor portion; and 
     FIG. 9 is a schematic of a modified embodiment of a portion of a control system made in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With regard to FIG. 1 an indoor temperature control system 10 is shown comprising an electric heat and fan control 12, heaters 14, a blower fan 16 in a conditioned air supply duct 18. A temperature responsive sensor in the form of a duct air thermistor 20 is mounted in duct 18 and is electrically connected to control 12. 
     An outdoor temperature control unit 22 is also shown including a defrost control 24 with temperature responsive sensors in the form of a liquid line thermistor 26 thermally coupled to the liquid line of an evaporator unit and ambient thermistor 28 exposed to ambient air electrically connected to defrost control 24. 
     With particular reference to FIGS. 2-7, the invention will be described as applied to indoor unit 12. Referring first to FIG. 2, control 12 comprises microprocessor U1 with inputs at pins 15-19 respectively from terminals W2 (second stage heat), G (fan), O (reversing valve), E (emergency heat), and Y (compressor) through respective zener diodes ZR8, ZR7, ZR6, ZR5, ZR4, resistors R12-R16 and pull down resistors connected to chassis ground including parallel connected resistors R17, R5, R18, R29, R45, and R35 for W2, R24 for G, R19 for O, R20 for E, and R28 for Y. The above input portion of the circuit is conventional and will not be described in detail. A description of a similar input can be obtained in coassigned U.S. Pat. No. 5,272,427, the subject matter of which is incorporated herein by this reference. 
     Pins 20 and 22 of microprocessor U1 are connected to quick connects QC4 and QC3 respectively for connection to a sensing thermistor. Quick connect QC4 is connected between the cathodes of diodes CR8 and CR15 and to logic ground with capacitor C13 coupled around diode CR15. Quick connect QC3 is connected between a reference resistor R9 and capacitor C6 in turn connected to logic ground. QC3 is also connected through resistor R30 to the time capture T CAP pin 25 of the microprocessor. Capacitors C13, C11, diode CR15, resistors R30, R44 provide circuit protection functions. 
     Input pins 11, 12 and 13 are coupled to a jumper block P3 which provides a means for selecting the minimum air supply temperature for the heating mode, i.e., 85° F., 90° F., 95° F. and 100° F. 
     Off board relays for turning on or off banks of heaters, HTR1-HTR6 respectively, are controlled through relay driver integrated circuit U2 coupled to output pins 9-3 respectively. A standard oscillator OSC1 is connected to pins 26, 27 to provide a selected frequency of 2 mHz, although other frequencies can be employed, if desired. 
     With respect to FIG. 3a, the IRQ circuit connected to pin 2 of microprocessor U1 provides synchronization to the AC line to read the several inputs in the same manner as in U.S. Pat. No. 5,272,427, referenced supra, and illustrates the use of two different potentials, chassis ground and logic ground. Common or chassis ground is shown as an input to the IRQ circuit. The reset circuit of FIG. 3b tied to pin 1 of microprocessor U1 is used for brown out power interruption to ensure that the microprocessor powers up as intended. That is, it ensures that the 5 volt supply VDD comes up before the reset circuit comes up in a conventional manner. 
     The power supply network connected to VDD pin 28 of microprocessor U1 is shown in FIG. 4. Incoming power supply 24 VAC shown at R and common are connected to a full wave bridge, diodes CR1-CR4 from which logic ground is derived at the anodes of diodes CR3, CR4. A 5 volt power supply is provided by capacitor Cl, current limiting resistor R1 and a 5 volt zener diode ZR1. The remaining components, resistors R2, R32, capacitor C2 etc., provide pull down, noise decoupling and the like. Capacitors C4, C3 and associated components provide power for off board relays. 
     The control utilizes an algorithm including logic that determines the mode of operation based upon room thermostat input and supply air thermistor input. When a need for electric heat has been determined the control will energize the minimum amount of heat needed to raise the supply air temperature to a minimum set point and operates a variable speed motor by a pulse width modulated (PWM) signal based on the desired rate of air flow (cubic feet per minute or CFM) and mode of operation. 
     Referring back to FIG. 2, the thermistor 20 and capacitor C6 form a timed charge circuit using a digital input. When pin 20 (PC2) is turned on a timer is initiated. When capacitor C6 charges to a level high enough for timer capture pin 25 (T CAP/PD7) to see a high the timer is stopped. The particular routine for detecting shorts and opens is shown in FIG. 5. The read cycle is initiated at step 20, a watch dog timer is initiated at step 22 and a decision step at 24 checks to see if a reading has occurred. If not, the routine goes to decision block 26 to see if the watch dog timer has expired and if not, it cycles back to step 24. If the watch dog timer has expired an error flag is set at block 28. If a reading has occurred at decision step 24, then the routine goes on to decision step 30 to see if the reading is above a selected minimum value. The minimum reading is used for detecting a shorted thermistor. If not, an error flag is set at block 28 and if the reading is above the minimum the routine goes to decision block 32 to check if the reading is below a selected maximum. The maximum reading and watch dog timer are used to detecting opens. If the reading is not below the maximum, an error flag is set at block 28 but if the reading is below the maximum then it is considered valid, step 34, and the routine goes onto a leakage check routine. When an error flag is set at block 28 an error capture routine is run at step 36 with the routine then going back to block 20. 
     Thermistor leakage is checked by a routine shown in FIG. 6. The routine is initiated at step 38. At step 40 the power source for the thermistor readings is turned off, i.e., pins 20, 22 and a watch dog period of time is initiated at step 42. Decision block 44 checks to see if the watch dog period has expired and if so the routine goes to step 46 with the leakage test being passed and then to step 48 which returns to step 20 of the thermistor read routine. If the watch dog time period has not expired, the routine from decision block 44 goes to decision block 50 to see if a reading within a specified range has occurred. If not, the routine cycles back to block 44 and if such reading has occurred an error condition flag is set at process step 52. An error capture routine is run at process step 54 and the routine goes on to block 48. 
     The routine for detecting shorts to chassis ground or 24 VAC is shown in FIG. 7. Depending upon the severity of the short, this condition may be sensed as a leakage path, discussed supra, prior to being diagnosed as a short to chassis ground or 24 VAC. The routine involves the initiation of the readings of a known reference resistor R9 at step 60, the initiation of a watch dog timer at process step 62, the decision block 64 to determine whether the watch dog has expired. If the watch dog has expired, an error flag is set at block 66; if it has not expired, the routine goes on to decision block 68 to determine whether a reading has occurred. If no reading has occurred, the routine cycles back to step 64 but if a reading has occurred the routine goes on to decision block 70 to determine if the reading is greater than a selected maximum value and if not on to decision block 72 to determine whether the reading is less than a selected minimum value. If the decision is yes at either decision block 70, 72 an error flag is set at block 66. If the decision is negative at decision block 72 the routine goes on to block 74 and returns to the start of the thermistor read routine (block 20). If a flag is set at block 66, an error capture routine is run at process step 76 and then the routine proceeds to block 78 and returns to the start of the thermistor read cycle. It will be noted that the FIG. 7 routine is very similar to that of the FIG. 5 thermistor read routine; however, the minimum and maximum values are tightly controlled since a known value R9 is being read. Suitable decimal values for the minimum and maximum are noted below in table 1. 
     
         ______________________________________             MAX   MIN______________________________________Reference Resistor (R9)                352    32Thermistor          13928   16______________________________________ 
    
     These values represent the number of microprocessor clock cycles counted while waiting for the R-C circuit to charge. It will be understood that for an A/D circuit the appropriate values would be the minimum and maximum A/D readings taken rather than clock cycles. 
     A very short charging time reflects a short condition, i.e., a time less than the minimum value. On the other hand, a charge time exceeding the maximum reflects an open condition. With respect to thermistor leakage, by turning off inputs at pins 20 and 22, there should be no charging; however, if a leakage path through the thermistor to another voltage source exists this can be detected at time capture pin 25 (T CAP/PD7) during a selected watch dog period. That is, the leakage path through the thermistor is to chassis ground which is at a different potential than logic ground. If the thermistor sensor is determined to have failed the control can be adapted to go into a selected default operation, e.g., turn on the blower fan. 
     FIG. 8 shows a modification relating to an outdoor unit such as the one depicted in FIG. 1. As shown in FIG. 8, connections Q3, Q4 and Q5, Q6 are for connecting an ambient temperature thermistor and a liquid line temperature thermistor, respectively. The thermistors are coupled to U3 which functions as an analog switch. That is, 5 volts from VDD can be switched onto any one of lines U10, U9, U3 or U2. The microprocessor (not shown) selects the appropriate charge path through switch U3 to charge capacitor C6&#39; through either the ambient thermistor, the liquid line thermistor or a reference resistor R26. Resistor R27 is a discharge resistor in order to discharge capacitor C6&#39; prior to the commencement of a charge cycle. After discharging capacitor C6&#39; the appropriate thermistor or reference resistor is enabled and the watch dog timer initiated as in the FIG. 2 embodiment. When the time capture pin goes high the time is captured in the same manner as in the FIG. 2 embodiment. If a failed thermistor is detected the control, as in the previous embodiment, can be adapted to go into default operation. For example, if the ambient sensor fails the ambient readings can be taken using the liquid line thermostat when the unit is off. On the other hand, if the liquid line sensor fails then the control can go into a timed defrost operation thereby avoiding damage. 
     Most defrost problems with outdoor units evolve from sensor failure which, at times, is difficult to diagnose, as noted supra. Frequently, sensors have microscopic cracks in their insulative encapsulant thereby providing a leakage path which dry up by the time a field technician has an opportunity to visit the site and, when dry, the sensor works as intended. By means of the invention such sensors are detected at the time of failure and a suitable default operation is initiated as long as the fault condition is in existence. The invention also allows the use of a thermistor as a limit device for the first time. In the past, a separate fuse or thermostatic device was required to provide protection for overheating conditions. 
     A control circuit made in accordance with the FIGS. 2-4 embodiment comprise the following components: 
     
         ______________________________________R1    1.1K    2W      R44    5.1M 1/8WR2    10K     1/4W    R45    1.5K 2KR3    2K      1/4W    U1     MC68HC05P97 (MOTOROLA)R4    10K     1/4W    U2     ULN2003R5    1.5K    2W      LED1   RadialR6    100K    1/8W    OSC1   59275-1R7    1K      1/4W    ZR10   59231-0126-12VRB    100K    1/8W    ZR1    59231-0115-5.1VR9    5.11K   1/4W    ZR2    59231-0115-5.1VR10   10K     1/8W    ZR3    59231-0144-43VR11   2K      1/8W    ZR4    59231-0126-12VR12   200K    1/8W    ZR5    59231-0126-12VR13   200K    1/8W    ZR6    59231-0126-12VR14   200K    1/8W    ZR7    59231-0126-12VR15   200K    1/8W    C1     47mF-50VR17   1.5K    2W      C2     10uFR18   1.5K    2W      C3     100uF-50VR19   1.5K    2W      C4     47uF-50VR20   1.5K    2W      C5     .1uF-100VR21   51K     1/8W    C6     .1uF-50VR22   51K     1/8W    C7     .1uF-10QVR23   1M      1/8W    C8     .1uF-100VR24   1.5K    2W      C9     .01uF-50VR25   2K      1/8W    C10    10uF-16VR26   2K      1/8W    C11    .0056uF-50VR27   100K    1/8W    C13    .0056uF-50VR28   1.5K    2W      CR1    59226-0007R29   1.5K    2W      CR2    59226-0007R30   1.1K    2W      CR3    59226-0007R31   10K     1/4W    CR4    59226-0007R32   10K     1/4W    CR5    59226-0007R33   51K     1/8W    CR6    59226-0007R34   10K     1/8W    CR8    59320-1001R35   1.5K    2W      CR9    59226-0007R36   100K    1/8W    CR15   59320-1001R38   100K    2W      CR16   59320-1001R42   10K     1/8W    CR19   59320-1001______________________________________ 
    
     As noted above, if desired, an analog to digital, A/D, network can be used in place of the timed charge circuit described above. In such a modified embodiment, referring to FIG. 9, thermistor 20 and resistor R46 form a voltage divider with their junction 30 tied to an A/D input 32 of the microprocessor. The voltage divider is connected between a reference voltage source 34, e.g., 5-7 volts, and logic ground. An npn bipolar transistor Q1 is coupled across the voltage divider with its emitter connected to logic ground and its base, through drive resistor R47, to I/O port 36 of the microprocessor. A small current limiting resistor R48, e.g., 100 ohms, is connected to the collector of transistor Q1. The reference voltage source 34 is coupled to reference voltage input 38 of the microprocessor. A blocking diode CR20 has its cathode connected to resistor R46 for a purpose to be described below. The value of reference R46 is selected based on the particular range of thermistor resistance being measured. 
     In the FIG. 9 embodiment the microprocessor converts the analog voltage level to a relative bit number, e.g., an 8 bit number. For an open circuit condition the bit number would equate to the reference voltage minus one diode drop while a short circuit condition would result in a bit number equating to ground voltage. In conducting a leakage test transistor Q1 is turned to short the reference voltage source to ground and then a reading is taken at the A/D input 32. A reading above a threshold level equivalent to ground voltage indicates that leakage has occurred from chassis ground. Diode CR 20 prevents any signal from getting back to the reference voltage source. 
     Although the FIG. 9 embodiment adds some cost to the system it offers the advantage of a simplified approach regarding timing since it provides essentially instantaneous readings. 
     Numerous variations and modifications of the invention will become readily apparent to those familiar with HVAC controls. The invention should not be considered as limited to the specific embodiments depicted but rather as defined in the claims. 
     The LST file for the thermistor diagnostics is set forth as attached herewith: ##SPC1##