Abstract:
An improved method and apparatus for diagnosing the condition of an automotive HVAC refrigerant compressor based on information contained within the signal produced by a high-side pressure sensor provided for system control purposes. The output signal of the pressure sensor is divided into its DC and AC components, with the DC component being used for system control purposes, and the AC component being used for diagnosing the condition of the compressor. Pulsations of the AC component (which conventionally are removed by filtering for control purposes) correspond to individual piston stroke cycles. The pulsations indicate compressor operation and are counted for purposes of determining the actual compressor speed. Since existing pressure sensor information is utilized to verify compressor operation and to determine compressor speed, the system cost impact due to the diagnostic evaluation is minimal. Moreover, the pulsations provide failure information that could not be detected by a speed sensor since the pulsations verify that work is actually being performed by the compressor.

Description:
TECHNICAL FIELD 
     This invention relates to on-board diagnostics for the refrigerant compressor of an automotive HVAC system, and more particularly to a diagnostic method that utilizes existing sensor information. 
     BACKGROUND OF THE INVENTION 
     A significant aspect of state-of-the-art automotive engine control pertains to so-called on-board-diagnosis of various engine components or sensors, particularly when improper operation of such components or sensors can adversely influence the engine emission controls. In the case of a vehicle heating, ventilation and air-conditioning (HVAC) system that includes an clutch-driven refrigerant compressor and an electrically activated clutch mechanism, the diagnosis involves determining whether the compressor is on-normal, on-abnormal, off-normal, or off-abnormal. The on-normal condition indicates that the compressor is actually on (running) when the clutch is commanded on, and the off-normal condition indicates that the compressor is actually off when the clutch is commanded off. The off-abnormal condition indicates that the compressor is not running properly when the clutch is commanded on, and the on-abnormal condition indicates that the compressor is running when the clutch is commanded off. In addition to catastrophic failures such as a seized compressor, the abnormal conditions may be due to clutch failure or slippage, or a control unit failure. As with any diagnostic evaluation, the abnormal diagnostic indications may be used to trigger a check engine lamp or other driver alert so that the detected faulty operation can be corrected. 
     While the above-described conditions may be logically diagnosed by comparing the compressor speed with the clutch command, a measure of the compressor speed is not ordinarily available. Adding a compressor speed sensor for this purpose would significantly increase system cost, and is therefore undesirable. Accordingly, what is needed is a method of diagnosing the compressor condition without adding a special-purpose sensor. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved method for diagnosing the condition of an automotive HVAC refrigerant compressor based on information contained within the signal produced by a high-side pressure sensor provided for system control purposes. According to this invention, the output signal of the pressure sensor is divided into its DC and AC components, with the DC component being used for system control purposes, and the AC component being used for diagnosing the condition of the compressor. Pulsations of the AC component (which conventionally are removed by filtering for control purposes) correspond to individual piston stroke cycles. The pulsations indicate compressor operation and are counted for purposes of determining the actual compressor speed. 
     Since existing pressure sensor information is utilized to verify compressor operation and to determine compressor speed, the system cost impact due to the diagnostic evaluation is minimal. Moreover, the pulsations provide failure information that could not be detected by a speed sensor since the pulsations verify that work is actually being performed by the compressor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a system diagram of an automotive HVAC system according to this invention, including a pressure sensor, a signal conditioning circuit, and a microprocessor-based control unit. 
     FIG. 2 is a diagram of the signal conditioning circuit of FIG.  1 . 
     FIG. 3, Graphs A-F, graphically depict representative signals occurring in the signal conditioning circuit of FIG.  2 . 
     FIG. 4 is a flow diagram representative of computer program instructions executed by the control unit of FIG. 1 in carrying out the compressor diagnosis according to this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the reference numeral  10  generally designates an automotive HVAC system, including a refrigerant compressor  12  coupled to a drive pulley  14  via an electrically activated clutch  16 . The compressor  12  may have a fixed displacement or a variable displacement with pneumatic or electronic displacement control. The pulley  14  is coupled to a rotary shaft of the vehicle engine (not shown) via drive belt  18 , and the clutch  16  is selectively engaged or disengaged to turn the compressor  12  on or off, respectively. The HVAC system  10  further includes a condenser  20 , an orifice tube  22 , an evaporator  24 , and an accumulator/dehydrator  26  arranged in order between the compressor discharge port  28  and suction port  30 . A cooling fan  32 , operated by an electric drive motor  34 , is controlled to provide supplemental air flow through the condenser  20  for removing heat from the high pressure refrigerant in line  36 . The orifice tube  22  allows the cooled high pressure refrigerant in line  38  to expand before passing through the evaporator  24 . An air intake duct  40  housing an electric ventilation fan  42  directs outside (and/or recirculated) air through the evaporator  24 , and a heating duct  44  distributes the conditioned air in the vehicle passenger compartment. The accumulator/ dehydrator  26  separates low pressure gaseous and liquid refrigerant, and directs the gaseous portion to the compressor suction port  30 . 
     In an alternative system configuration, the orifice tube  22  is replaced with a thermostatic expansion valve (TXV). In this case, the accumulator/ dehydrator  26  is omitted, and a receiver/drier (RID) is inserted in line  38  upstream of the TXV. 
     The compressor  12  includes a number of internal reciprocating pistons (not shown) that successively and repeatedly pump refrigerant into the high pressure pipe  36  when the clutch  16  is engaged. The refrigerant pressure in pipe  36  is detected by a pressure transducer  46 , which develops a compressor outlet pressure (COP) signal on line  48 . As in conventional controls, the DC component of the COP signal is used for one or more control purposes, including cycling the cooling fan motor  34  to optimize cooling and driveability concerns, cycling the clutch  16  to account for various ambient conditions, and disengaging the clutch  16  in the event of an abnormally high compressor outlet pressure. These functions are carried out by the microprocessor-based control unit  50 , which develops a clutch control signal (CL) on line  52  and a fan control signal (FC) on line  54 . 
     According to this invention, the control unit  50  additionally utilizes the AC component of the of COP signal for purposes of diagnosing the operation of compressor  12 . To this end, the COP signal on line  48  is applied as an input to the signal conditioning circuit (SCC)  56 , described in detail below in reference to FIGS. 2-3. As indicated in FIG. 1, SCC  56  produces two output signals: a high side pressure (HSP) signal on line  58  and a compressor speed pulse (CSP) signal on line  60 . The HSP signal on line  58  is based on the DC component of the COP signal, and is used by control unit  50  for control purposes as discussed above. The CSP signal on line  60  is based on the AC component of the COP signal, and is used by control unit  50  for purposes of diagnosing the operation of compressor  12 . An engine speed signal (ES) on line  62  enables enhanced diagnostic evaluation, as described below in reference to FIG.  4 . 
     FIG. 2 depicts the signal conditioning circuit SCC  56 , and Graphs A-F of FIG. 3 show representative waveforms at various points in the circuit, on a common time base. As seen in FIG. 2, the COP signal on line  48  is applied to two circuits: a low pass filter  64  for forming the HSP signal on line  58 , and an AC-coupled differentiator and squaring circuit  66  for forming the CSP signal on line  60 . In an abbreviated period of clutch engagement represented by the ON level of trace  68  in Graph A of FIG. 3, the COP, HSP and CSP signals are depicted by the traces  70 ,  72  and  74  in Graphs B, C and F, respectively. 
     As indicated above, the compressor pumping events or cycles are directly responsible for the pulsations seen in the COP signal of Graph B, FIG.  2 . These pulsations are detrimental for control purposes, and the series resistor  76  and shunt capacitor  78  of low-pass filter  64  effectively remove the pulsations to form the HSP signal of Graph C, which represents the DC component of the COP signal. A representative time constant for the filter  64  would be approximately 100 msec. In circuit  66 , the series capacitor  80  isolates the pulsations. The signal at node  82  therefore corresponds to the AC component of the COP signal, and is depicted by the trace  84  in Graph D. The DC offset in trace  84  is determined by the resistors  92 - 94 , which divide a source voltage VCC. A differentiator comprising the operational amplifier  86  and the feedback elements  88  and  90  amplify AC portion of the signal (i.e., the pulsations), and the resistors  92  and  94  provide a reference offset voltage (REF) on line  96 , resulting in a differentiator output on line  98  as shown in the trace  100  of Graph E. The differentiator time constant, which may be on the order of 0.5 msec, is defined by the feedback elements  88 ,  90 , and the gain is defined by the relative resistance values of elements  88  and  102 . The pulse amplitude of the offset AC signal on line  98  can be used for diagnostic purposes as a measure of the pumping capacity of compressor  12 , if desired. In FIG. 2, the signal on line  98  is applied to a squaring circuit comprising the comparator  104 , forming the CSP signal on line  60 . The resistors  106  and  108  divide the source voltage VCC to provide a reference voltage (which may be the same as offset voltage REF) to the inverting input of comparator  104 , and the pull-up resistor  110  holds line  60  at VCC when the voltage on line  98  exceeds the reference voltage. When the voltage on line  98  is below the reference voltage, the comparator holds line  60  at ground potential, resulting in the square-wave, or pulsation, trace  74  depicted in Graph F. 
     The flow diagram of FIG. 4 represents a software routine executed by the control unit  50  in carrying out a diagnostic evaluation of compressor  12  based on the CSP and ES signals. As indicated, the control unit  50  executes other software routines for controlling the compressor clutch  16  and cooling fan motor  34  via lines  52  and  54 . As part of such other controls, the control unit  50  controls the status of a CLUTCH ON flag to indicate whether the clutch  16  is being commanded on or off. 
     Referring to FIG. 4, the block  120  is first executed to read and process the relevant inputs, including the CSP signal on line  60  and the ES signal on line  62 . Processing of the inputs may involve some filtering, and in the case of the CSP signal, updating a counter (CSP counter) to reflect the number of pulses that have occurred since the last execution of the routine. 
     If the CLUTCH ON flag indicates that clutch  16  is being commanded to an engaged state and the COMPRESSOR RUNNING flag has not been set, as determined at blocks  122  and  124 , respectively, the blocks  126 - 128  are executed to monitor the CSP pulses to verify that compressor  12  is actually beginning to operate. If no CSP pulses have been received in the first 100 msec of clutch operation, the block  126  is answered in the affirmative, indicating that the compressor has not started operating; in such event, the block  130  is executed to set a compressor-off-abnormal diagnostic indication. However, if at least one pulse is received within the 100 msec interval, the block  128  is answered in the affirmative, and the blocks  132 - 134  are executed to set the compressor running flag and to set a compressor-on-normal diagnostic indication, completing the routine. Once the COMPRESSOR RUNNING flag has been set, the block  124  is answered in the affirmative, and the blocks  126 ,  128 ,  132 ,  134  are skipped. 
     If the clutch  16  is on and compressor operation has commenced, the blocks  136 - 140  are executed to determine if the compressor is running at a normal speed. The block  136  determines the compressor speed CS based on the number of CSP pulsations received over a given interval of time, or in other words, the pulsation frequency. In a six-cylinder compressor, for example, six pulses will be received for each revolution of the compressor, and the compressor speed will be computed as one-sixth of the number of CSP pulses per unit time. If the compressor speed CS is less than a reference such as  50  RPM, as determined at block  138 , the block  130  is executed to set the compressor-off-abnormal diagnostic indication, completing the routine. If the compressor speed CS is at least 50 RPM, block  140  is executed to determine if there is excessive clutch slippage; this is achieved by computing the clutch slip magnitude based on compressor speed CS, engine speed ES, and the compressor/engine pulley ratio PR, and comparing the computed slip to a threshold THR. As indicated at block  140 , the magnitude of slip is computed according to the expression |CS−(ES*PR)|, where the quantity (ES*PR) is the expected running speed of compressor  12  based on engine speed ES. If the slip magnitude exceeds the threshold THR, the block  142  is executed to set a compressor-on-abnormal diagnostic indication, completing the routine. 
     If the clutch has been commanded off for at least a reference interval such as 100 msec, as determined by blocks  122  and  144 , the blocks  146  and  148  are executed to compute the compressor speed (as described above) and to compare the compressor speed to a reference speed such as 50 RPM. If the compressor speed is greater than 50 RPM, the block  142  is executed to set the compressor-on-abnormal diagnostic indication, completing the routine. If not, the blocks  150  and  152  are executed to clear the COMPRESSOR RUNNING flag and to set a compressor-off-normal diagnostic indication, completing the routine. 
     In summary, the control of this invention enables reliable and cost-effective diagnosis of the compressor operation by utilizing existing but formerly un-used sensor information. The pulsations of the high side pressure sensor  46  are used not only to verify that the compressor is actually working, but also to determine the running speed of the compressor. This enables the diagnostic algorithm to verify proper starting and stopping of the compressor, and in conjunction with the engine speed information, to detect excessive clutch slippage. 
     While the present invention has been described in reference to the illustrated embodiments, it is expected that various modification in addition to those mentioned above will occur to those skilled in the art. For example, the pressure sensor  46  may alternatively be located at the outlet of condenser  20 , or elsewhere upstream of the orifice tube  22 , and the system  10  may be a heat pump as opposed to the arrangement depicted in FIG.  1 . Also, the various reference time intervals, time constants, and so on, are representative only, and may vary from application to application. Thus, it will be understood that systems and methods incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.