Abstract:
A method of controlling the loading and unloading of a compressor includes selectively loading and unloading a compressor by engaging and disengaging, respectively, compressor members with the controller in response to system load data, monitoring at least one of the discharge pressure and the suction pressure at a predetermined time interval for a continuous time period, storing values based on the at least one of the discharge pressure and the suction pressure during the continuous time period, and determining a predetermined value indicative of compressor operation in which the compressor members are engaged. The method further includes comparing at least one of the stored values with the predetermined value and providing a signal to cease operation of the compressor when the comparison fails to indicate compressor operation in which the compressor members are engaged.

Description:
RELATED APPLICATION DATA 
       [0001]    The present application claims priority under 35 U.S.C. §119 to Provisional Patent Application No. 61/558,750, filed Nov. 11, 2011, the disclosure of which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to an algorithm for a compressor controller that initiates a shutdown of the compressor in the event of a digital control valve or other failure. 
         [0003]    In a conventional digital scroll compressor, a solenoid valve in communication with both the compressor discharge line and the compressor suction line is energized to modulate the compressor capacity for load control. The solenoid directs compressed discharge gas to separate the orbiting scroll from the fixed scroll while the compressor prime mover remains energized. In some applications, the bearings and other components are lubricated by virtue of the pressure differential between the low and high sides of the compressor in lieu of an oil pump. During separation of the scroll set this pressure differential will typically be insufficient to provide adequate oil to the bearings and other components, thus limiting the duration that the compressor can safely operate with the scrolls separated. 
       SUMMARY 
       [0004]    A compressor controller, among other things, monitors and records suction and discharge pressure data of a digital scroll compressor over time, and specifically over a series of duty cycles. Based on this data, an algorithm determines if the digital solenoid valve is stuck or if the scroll set otherwise remains disengaged. If so, the controller initiates a shutdown of the prime mover of the compressor. 
         [0005]    In one embodiment of a method of controlling the loading and unloading of a compressor, the method includes selectively loading and unloading a compressor by engaging and disengaging, respectively, compressor members with the controller in response to system load data, loading the compressor to increase a fluid pressure from a suction pressure to a discharge pressure when the compressor members are engaged, and unloading the compressor when the compressor members are disengaged. The method also includes monitoring at least one of the discharge pressure and the suction pressure at a predetermined time interval for a continuous time period, storing values based on the at least one of the discharge pressure and the suction pressure during the continuous time period, and determining a predetermined value indicative of compressor operation in which the compressor members are engaged. The method further includes comparing at least one of the stored values with the predetermined value and providing a signal to cease operation of the compressor when the comparison fails to indicate compressor operation in which the compressor members are engaged. 
         [0006]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a sectional view of a scroll compressor with the scrolls engaged and having a controller for use with an embodiment of the invention. 
           [0008]      FIG. 2  is another sectional view of the scroll compressor of  FIG. 1 , with the scrolls disengaged. 
           [0009]      FIG. 3  is a plot of the discharge and suction pressures of the scroll compressor of  FIGS. 1 and 2 , and of the control valve applied voltage, vs. time. 
           [0010]      FIG. 4  is a flow chart of a control algorithm embodying the invention. 
           [0011]      FIG. 5  is a flow chart of another control algorithm embodying the invention. 
           [0012]      FIG. 6  is a flow chart of another control algorithm embodying the invention. 
           [0013]      FIG. 7  is a flow chart of another control algorithm embodying the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. As used herein and in the appended claims, the terms “upper”, “lower”, “top”, “bottom”, “front”, “back”, and other directional terms are not intended to require any particular orientation, but are instead used for purposes of description only. 
         [0015]    Referring to  FIG. 1 , a portion of a compressor  10  is shown and comprises a generally cylindrical shell  100  having secured at one end thereof a cap  104 . A transverse partition  108  extends to the periphery of the shell  100  and separates the compressor into a high pressure side  112  and a low pressure side  114 . 
         [0016]    A drive shaft or crankshaft  120  having an eccentric crank pin  124  is rotatably journaled in a bearing  128  in a main bearing housing  132 . The crankshaft  120  is driven by a prime mover (not shown) external to the shell  100 . The prime mover may be, for example, a diesel engine, an electric motor, or any other machine capable of driving the crankshaft  120 . The main bearing housing  132  includes a generally cylindrical portion  150  that defines a flat thrust bearing surface  152  on which is supported an orbiting scroll member  156 . The orbiting scroll member  156  includes an end plate  160  and a spiral vane or wrap  164  extending therefrom. Projecting from the opposing face of the end plate  160  is a cylindrical hub  170  having a journal bearing  174  therein and in which is rotatively disposed a drive bushing  180  having an inner bore  184  in which the crank pin  124  is drivingly disposed. The crank pin  124  has a flat on one surface which drivingly engages a flat surface (not shown) formed in a portion of the bore  184  to provide a radially compliant driving arrangement therebetween such that the crank pin  124  and the drive bushing  180  do not substantially rotate relative to one another. The orbiting scroll member  156  further includes an inlet  190  in fluid communication with a suction port  194  adjoining the shell  100  at the low pressure side  114 . 
         [0017]    A non-orbiting scroll member  200  includes an end plate  204  and a wrap  208  projecting therefrom which is positioned in meshing engagement with the wrap  164  of the orbiting scroll member  156 . The non-orbiting scroll member  200  has a centrally disposed discharge passage  212  that communicates with a recess  216 , which in turn is in fluid communication with an oil separator  220  positioned in the high pressure side  112 . The oil separator  220  is in fluid communication with a discharge port  224  adjoining the shell  100 . Due to the orientation of the compressor  10 , an oil sump  228  is located at a lower portion of the high pressure side  112  and receives oil separated by the oil separator  220 . An oil tube  230  extends through the partition  108  and provides oil to the bearings and other components (not shown) via the pressure difference between the high pressure side  112  and the low pressure side  114 . The non-orbiting scroll member  200  is secured to the main bearing housing  132  through a plurality of circumferentially spaced bolts (not shown) extending through associated sleeve members and configured to allow limited axial movement of the non-orbiting scroll member  200  with respect to the orbiting scroll member  156 . 
         [0018]    In order to allow orbiting motion of the orbiting scroll member  156  and prevent relative rotation between the orbiting scroll member  156  and the non-orbiting scroll member  200 , an Oldham coupling  232  is disposed between the cylindrical portion  150  of the main bearing housing  132  and the end plate  160  of the orbiting scroll member  156 . 
         [0019]    A solenoid valve  240  is connected by a control line  252  to a fitting  256  extending through the shell  100 . The solenoid valve  240  is configured to receive a pulse width modulation signal from a control module or controller  244  based in part on data supplied from a load sensor  248 , such as a temperature or pressure sensor. An internal fluid line  260  connects the fitting  256  to a passage  264  in communication with a chamber  268 . The solenoid valve  240  includes a discharge connecting tube  270  and a suction connecting tube  274  affixed to the discharge tee  278  and suction tee  282 , respectively. 
         [0020]    A recess  234  is formed in the non-orbiting scroll member  200  in communication with a compression pocket at an intermediate pressure through a bleed hole  236 . The intermediate pressure within recess  234 , along with the discharge pressure within recess  216 , will exert an axial biasing force on the non-orbiting scroll member  200  to thereby urge the tips of the respective wraps  164 ,  208  into sealing engagement with the opposed end plates  160 ,  204 . The solenoid valve  240  is closed such that the chamber  268  is in fluid communication with the suction tee  282 . 
         [0021]    Although the compressor illustrated and described with regard to  FIGS. 1 and 2  is a scroll compressor, the compressor can be any type of compressor (using refrigerant or another fluid, such as air) with a digital or other unloading device. In particular, the compressor can be of the type in which the bearings and other components are lubricated by a high/low pressure differential rather than by an oil pump. 
         [0022]    In operation, the orbiting scroll  156  orbits relative to the non-orbiting scroll  200 , drawing system refrigerant through the suction tee  282  and the suction port  194  and into the inlet  190 . The intermeshing wraps  164 ,  208 , as known by those of ordinary skill in the art, progressively decrease the size of a refrigerant containing pocket formed therein as the refrigerant is moved radially inward. This action compresses the refrigerant, which is discharged sequentially through the centrally disposed passage  212 , the recess  216 , the oil separator  220 , the discharge port  224 , and the discharge tee  278  for use in the refrigerant system. 
         [0023]    To unload the compressor, the solenoid valve  240  energizes in response to a signal from the controller  244 . Referring to  FIG. 2 , this signal opens the solenoid valve  240 , allowing high pressure refrigerant discharge to flow through the control line  252 , the internal fluid line  260 , the passage  264 , and into the chamber  268 . The pressure within the chamber  268  is increased such that the resultant applied force from the gas will overcome the previously described axial biasing force on the non-orbiting scroll member  200 . The non-orbiting scroll member  200  will therefore move axially, disengaging the non-orbiting scroll  200  from the orbiting scroll  156 . The leakage path formed between the two scrolls  156 ,  200  effectively eliminates compression of the refrigerant. 
         [0024]    To load the compressor, the controller  244  deenergizes the solenoid valve  240 . Referring to  FIG. 1 , this closes the solenoid valve  240 , which discharges the gas within the chamber  268  back through the passage  264 , the internal fluid line  260 , the control line  252 , and to the suction tee  282 , which moves the orbiting and non-orbiting scrolls  156 ,  200  back into engagement. 
         [0025]    The control module  244  switches the solenoid valve  240 , and thus the compressor  10 , between engaged and disengaged states while the prime mover remains energized. One or more load sensors  248 , such as temperature or pressure sensors, alone or in combination, provide system load data to the control module  244 . The control module  244  adjusts the pulse width of the control signal to modulate the compressor  10  between its full load and no-load states to meet the system demand for refrigerant. Rather than being directly responsive to the difference between set point and real time parameters (e.g., of temperature or pressure), the modulation frequency is a function of the duty cycle calculated by the controller to meet the system demand. 
         [0026]    When the scroll set is disengaged, the pressure differential required to lubricate the bearings and other components (in the absence of an oil pump) is insufficient for continuous operation. As a result, the duration of time that the compressor  10  can be safely operated in the disengaged mode is limited. If the solenoid valve  240  remains in the open position for an extended period of time, damage may occur to the compressor  10  requiring replacement of multiple components, or, for non-serviceable compressors, total compressor replacement. 
         [0027]    The controller is configured to chart the compressor discharge pressure and suction pressure over time. Referring to  FIG. 3 , plots  300 ,  304  show pressure vs. time and solenoid valve voltage vs. time for corresponding time periods and for a given ambient and conditioned space temperature. As an example, the plots  300 ,  304  are based on an operating condition of 15° F. ambient and 15° F. box conditions, with a 50% duty cycle (e.g., 5 seconds on, 5 seconds off). 
         [0028]    Referring to plot  304 , during the loading phase of a cycle  308 , the valve  240  is deenergized (V closed ) and the scroll set (scroll members  156 ,  200 ) is biased together or engaged (see, e.g.,  FIG. 1 ). During an unloading phase of the cycle  308 , a voltage (V open ) is applied to the solenoid valve  240  and the scroll set disengaged (see, e.g.,  FIG. 2 ). In normal operation, the discharge pressure trace  310  and the suction pressure trace  314  are out of phase, as shown in plot  300  of  FIG. 3 . For example, as the discharge pressure  310  decreases over the cycle  308 , the suction pressure  314  increases. Specifically, when the solenoid valve  240  closes and the scroll set moves to the engaged position, the refrigerant discharge pressure  310  increases while the suction pressure  314  decreases over the same time period. When the solenoid valve  240  opens and the scroll set moves to the disengaged position, the discharge pressure  310  decreases while the suction pressure  314  increases. Similar plots can be derived for additional operating conditions. 
         [0029]    Referring to  FIGS. 3 and 4 , in one embodiment of a control algorithm, the routine begins at step  400 , in which the discharge pressure is monitored at a predetermined time interval, for example, every 0.5 seconds, over the course of a continuous time period, for example, one minute (or any time period sufficient to include a number of duty cycles). During this continuous time period the maximum and minimum values of discharge pressure monitored are stored in the controller  244 . At the end of the continuous time period (step  404 ), the controller  244  calculates the difference between the stored maximum value and the stored minimum value (step  408 ) and determines whether the discharge pressure increased a certain amount, for example, 5 psi, at any time during the continuous time period (step  412 ). If the discharge pressure did not increase by at least 5 psi within the continuous time period, then a shutdown signal is sent from the controller  244  (step  416 ) to shut down the compressor  10  (i.e., deenergize the prime mover). The shutdown signal indicates that the solenoid valve  240  may be stuck in the open position or that the scroll set remains otherwise disengaged. 
         [0030]    To avoid nuisance shutdown cycles, the controller  244 , after waiting a certain time interval, for example, 15 minutes (step  420 ), and if fewer than three shutdowns have occurred (step  424 ) sends a signal to restart the compressor  10  and the control algorithm is reinitiated (step  428 ). If the controller  244 , after once more completing steps  400 - 412 , determines that the discharge pressure has not increased by 5 psi within the continuous time period, the controller again shuts down the compressor  10  in accordance with step  416 . As previously noted, the controller  244  is programmed to allow a certain number, for example, three, such shutdowns and restarts during a continuous one hour period, at which point an error code will be displayed on the controller  244  (step  432 ) and the compressor  10  will not restart without service. 
         [0031]    Referring to  FIGS. 3 and 5 , in another embodiment of a control algorithm, the routine begins at step  500 , in which the suction pressure is monitored at a predetermined time interval, for example, every 0.5 seconds, over the course of a continuous time period, for example, one minute. During this continuous time period the maximum and minimum values of suction pressure are stored in the controller  244 . At the end of the continuous time period (step  504 ), the controller calculates the difference between the stored maximum value and the stored minimum value (step  508 ) and determines whether the suction pressure decreased a certain amount, for example, 2 psi, at any time during the continuous time period (step  512 ). If the suction pressure did not decrease by at least 2 psi, the shutdown signal is sent from the controller  244  (step  516 ) to deenergize the prime mover. The absence of a pressure decrease of at least 2 psi signals that the scroll set remains in a disengaged state. The controller will energize the prime mover as previously described, steps  520 - 528  and, after three such shutdowns, generate the error code (step  532 ). 
         [0032]    Referring to  FIGS. 3 and 6 , in another embodiment of the control algorithm, the slope of the discharge pressure trace  310  and the slope of the suction pressure trace  314  are monitored and stored by the controller over the course of a continuous time period (step  600 ). At the end of the continuous time period (step  604 ), the controller  244  analyzes the discharge pressure slope and/or the suction pressure slope profiles and determines the change in slope(s) over the time period (step  608 ). As previously described, the discharge pressure of the refrigerant increases when the solenoid valve  240  is deenergized, i.e., the slope of the discharge pressure changes from negative to positive upon closing the solenoid valve. The suction pressure of the refrigerant decreases when the solenoid valve  240  is deenergized, i.e., the slope of the suction pressure changes from positive to negative upon closing the solenoid valve. If the discharge pressure slope does not change from negative to positive, or if the suction pressure slope does not change from positive to negative during the time period (step  612 ) the controller  244  initiates the shutdown sequence previously described (steps  616 - 632 ), indicating that the system remains disengaged. In this algorithm, the discharge and suction pressures can be analyzed alone or in combination, and the controller-initiated signal can be triggered by either condition or by a combination of conditions. 
         [0033]    Referring to  FIGS. 3 and 7 , in another embodiment of the control algorithm, the difference between the discharge pressure and the suction pressure is monitored at a predetermined time interval for a continuous time period (step  700 ), during which the maximum and minimum difference values are stored in the controller. At the end of the time period (step  704 ) the controller  244  calculates the difference between the stored maximum value of pressure differential and the stored minimum value of pressure differential (step  708 ) and determines whether the differential rose by at least a certain amount, for example, 10 psi, during the continuous time period (step  712 ). If not, the controller sends a shutdown signal (step  716 ) to deenergize the prime mover and continues with the shutdown sequence as necessary (steps  720 - 732 ). 
         [0034]    In another embodiment of the control algorithm, the pressure ratio, which is the ratio of the discharge pressure to the suction pressure, can be monitored continuously by the controller  244 . If this ratio drops below a predetermined ratio, for example, 1.4, at any time during compressor operation, the shutdown signal and sequence are initiated. 
         [0035]    Any of the algorithms can be activated during digital operation of the compressor  10 . In some compressor systems, the compressor is operable in both a digital and non-digital mode (e.g., to facilitate a change in a parameter setpoint or a change in system environment) and the controller  244  may direct the compressor  10  to switch from digital to non-digital mode during a period in which the scroll set is stuck in the disengaged position. The controller  244  is configured to continue to monitor the algorithm embodied in steps  700 - 732  and the pressure ratio of the compressor for an additional time period, for example, one minute, immediately after the compressor  10  is transitioned out of the digital mode, to ensure normal operation and, if necessary, initiate the shutdown signal and sequence as previously described. 
         [0036]    The above-described embodiments can be used together to monitor the compressor system. Alternatively, one or more of the embodiments can be used as a backup to another of the embodiments. In some applications, one or more of the embodiments may be preferable. The numerical values provided for pressure, temperature, length of time, or any other parameter above are exemplary only and not limiting within the scope of the invention. 
         [0037]    Various features and advantages of the invention are set forth in the following claims.