Method and apparatus for refrigeration case control

Abstract: A method of controlling a refrigeration case of a refrigeration system including the steps of locating an expansion valve of a refrigeration case, moving the expansion valve of the refrigeration case out of a predetermined position, sampling temperature readings from an evaporator coil of a refrigeration case, calculating an adaptive proportional-integral-differential (PID) of the temperature of the evaporator coil for the refrigeration case, and moving the expansion valve to a second position closer to the predetermined position based on the calculated proportional-integral-differential until the temperature of the evaporator coil for the refrigeration case is within a predetermined temperature range.

Description: 1. Field of the Invention The present invention relates generally to refrigeration systems and, more specifically, to a method and an apparatus for controlling the temperature differential across the evaporator coil of a refrigeration case of a refrigeration system. 2. Description of the Related Art A conventional refrigeration system includes a compressor for compressing refrigerant vapor and discharging it into a condenser. The condenser liquifies the refrigerant which flows into a receiver. From the receiver, the liquid refrigerant flows through a heat exchanger and through a thermostatic expansion valve. The expansion valve expands the liquid refrigerant into a vapor which flows into and through an evaporator. Passing through the evaporator, the expanded refrigerant absorbs heat from a refrigeration case, aided by a circulating fan, and then returns to the compressor. Typically, the refrigeration system includes a plurality of refrigeration cases and compressors. The compressors are commonly piped together to form a compressor rack and pressure detection sensors are used for establishing and detecting a compressor suction pressure range in the refrigeration system for determining when upper (cut-in) and lower (cut-out) limits of the compressor suction pressure range have been exceeded. The refrigeration system uses a logic circuit for turning or cycling the compressors ON and OFF in succession or stages when the limits are exceeded to bring the compressor suction pressure within the compressor suction pressure range. One disadvantage of the above refrigeration system is that the cut-in and cut-out limits provide only a coarse control of the compressor rack in the compressor suction pressure range. As a result, the compressors of the compressor rack may be cycled frequently, resulting in a shorter life for the compressors. Another disadvantage is that the cycling of the compressors may cause the compressor auction pressure to rise or fall too quickly, resulting in excessive condenser cycling. Therefore, there is a need in the art to control each refrigeration case load to regulate the compressor rack only when the refrigeration cases cannot maintain control. Traditionally, control of the refrigeration case load employed maintaining a temperature differential such as ten degrees (10°) across the evaporator coil of the refrigeration case. The method of control used a thermostatic expansion valve to control the amount of refrigerant through the evaporator coil. Although this method of control worked well for large temperature differentials across the evaporator coil, it suffered from the disadvantage that the method could not be used to maintain small temperature differentials such as less than ten degrees (10°) across the evaporator coil. Also, the method could not be used on analog expansion valves. Further, the method produced poor results if there was liquid refrigerant in the evaporator coil. Therefore, there is a need in the art to control each refrigeration case load within a relatively small temperature range. US Patent 4745767 discloses a system for controlling flow rate of refrigerant. A controller controls an opening-closing movement of an electronic expansion valve. The controller comprises a first comparator for comparing a set signal corresponding to a preset degree of superheat as a target value with a feedback signal corresponding to a measured degree of superheat, an internal algorithmic unit serving as means for adjusting the operation of the valve, a valve drive unit serving as a means for operating the valve, an evaporator temperature measuring unit for measuring the temperature of the evaporator, a unit for measuring the temperature of the space to be cooled, a second comparator for comparing a set signal corresponding to the temperature preset for the space with a feedback signal corresponding to the measured temperature of the space, and a valve full-closing signal generator. Japanese Publication No. 02197776 discloses an electronic expansion valve control method for refrigerating a show case. A temperature in a chamber is measured by a temperature measuring means by means of a temperature-in-chamber sensor and is written in a computing and memory means. When a temperature in a chamber is lower than a set temperature, an electronic expansion valve is fully closed. When it exceeds the set temperature, if the preceding electronic expansion valve is closed, the inlet and outlet temperatures of a vaporizer, detected by inlet and outlet temperature sensors are measured by a temperature measuring means. From a difference between a superheat temperature and five degree, a valve control AV, by means of a computing and memory means, and PID control is made on an electronic expansion valve through a drive means. By adding AV to a preceding valve opening, a present valve opening is determined and an average valve value V containing the valve opening in a post time I is computed to store it. When the preceding electronic expansion valve is closed, the electronic expansion valve is opened by the average valve opening V set as an initial valve opening. Further, control is executed at intervals of a specified sampling time. SUMMRRY OF THE INVENTION It is an object of the present invention to provide a method of refrigeration case control. It is another object of the present invention to provide a method of refrigeration case control which controls the refrigeration case load within a small temperature differential across the evaporator coil of the refrigeration case. It is further object of the present invention to provide a method of refrigeration case control which is self-turning or adaptive. It is yet a further object of the present invention to provide a method of refrigeration case control which controls an analog servo expansion valve. In accordance with the present invention, there is provided a method of controlling the temperature differential across the evaporator coil of a refrigeration case of a refrigeration system (10), the method comprising the steps of: a) sampling temperature readings from a temperature sensor of the evaporator coil of the refrigeration case at predetermined periodic sampling times; b) determining the inlet temperature and the outlet temperature of the evaporator coil, the difference between them being the temperature differential across the evaporator coil (ΔT); c) determining whether or not it is time to sample temperature readings from said sensor of said evaporator coil; d) if it is time to sample temperature readings, making adjustments to proportional-integral-difference constants based on rising/falling inside/outside window of the temperature differential determined in step b), calculating an adaptive PID of the temperature differential across the evaporator coil to produce a control signal u(t) for positioning said expansion valve, determining if ΔT is falling rapidly and, if so, adjusting u(t) accordingly, and applying the control signal u(t) to position the expansion valve; and e) if it is not time to sample temperature readings, determining if ΔT is falling rapidly and, if so, adjusting the control signal u(t) for positioning the expansion valve accordingly, and applying the control signal u(t) to position the expansion valve. Also in accordance with the present invention, there is provided apparatus for controlling the temperature differential across the evaporator coil of a refrigeration case of a refrigeration system, the apparatus comprising: a) temperature sensing means for providing temperature readings from an evaporator coil of said refrigeration case; b) sampling means for sampling said temperature readings at predetermined periodic sampling times; c) means for determining the inlet temperature and the outlet temperature of the evaporator coil, the difference between them being the temperature differential (ΔT) across the evaporator coil; d) means for determining whether or not it is time to sample temperature readings from said temperature sensor of said evaporator coil; e) means for making adjustments to proportional-integral-difference constants based on rising/falling inside/outside window of the temperature differential determined by the means recited in c), calculating an adaptive PID of the temperature differential across the evaporator coil to produce a control signal u(t) for positioning said expansion valve, determining if ΔT is falling rapidly and, if so, adjusting u(t) accordingly, in the event that it is determined by the means recited in d) that it is time to sample temperature readings; and f) means for determining if ΔT is falling rapidly and, if so, adjusting a control signal u(t) for positioning said expansion valve accordingly, in the event that it is determined by the means recited in d) that it is not time to sample temperature readings; and g) control means for applying said control signal u(t) to position the expansion valve. An advantage of the present invention is that a method is provided of refrigeration case control that controls the refrigeration case load by maintaining a small differential such as ± 0,56°C (one degree (1°)F) across an evaporator coil of the refrigeration case. Yet a further advantage of the present invention is that the method of refrigeration case control is used to control an analog servo expansion valve as opposed to a pulse or thermostatic expansion valve. A still further advantage of the present invention is that the method of refrigeration case control uses two fast Fourier transforms to position the expansion valve, thereby making the method adaptive. Other advantages of the present invention will be readily appreciated as the same becomes better understood after reading the subsequent description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a refrigeration system. FIG. 2 is a block diagram of a case controller of FIG. 1. FIGS. 3A through 3E are flowcharts of a method, not covered by the present invention, of controlling the refrigeration system of FIG. 1. FIGS. 4A through 4D are flowcharts of a method, according to the present invention, of controlling the refrigeration system of FIG. 1. DESCRIPTION OF THE PREFERRED EMBODIMENT(S) Referring to FIG. 1, a refrigeration system 10 is shown. The refrigeration system 10 includes a plurality of refrigeration cases (not shown) whose capacity or load is controlled by a rack of compressors 12 which are parallel-staged and commonly piped to produce a common compressor suction pressure and temperature. The refrigeration system 10 also includes a plurality of condenser fans 14 for condensers (not shown) of the refrigeration system. The refrigeration system 10 includes a refrigeration controller 16, a communications bus (RS-485) 18 connected to the refrigeration controller 16, a condenser input/output (I/O) module 20 interconnecting the communications bus 18 and condenser fans 14, and a compressor output module 22 interconnecting the communications bus 18 and the rack of compressors 12. The refrigeration system 10 also includes a pressure sensor 24 and temperature sensor 26 attached to the condensers of the refrigeration system 10 and connected to the condenser I/O module 20 for sensing or measuring the pressure and temperature, respectively, of the refrigerant in the condensers of the refrigeration system 10. It should be appreciated that the refrigeration controller 16, communications bus 18, condenser I/O module 20 and compressor output module 22 are conventional and known in the art. The refrigeration system 10 further includes a rack input module 28 connected to the communications bus 18 and a pressure sensor 30 and a temperature sensor 32 attached to the compressor suction line (not shown) and connected to the rack input module 28 for sensing or measuring the pressure and temperature, respectively, for the rack of compressors 12. The refrigeration system 10 includes a rack output module 34 connected to the communications bus 18 and a plurality of shut-off valves and defrost coils 36 for the refrigeration cases connected to the rack output module 34. The refrigeration system 10 also includes a plurality of case controllers 38 connected to the communications bus 18 and a plurality of expansion valves 40 for the refrigeration cases connected to the case controllers 38. Preferably, the expansion valve 40 is of an analog servo type as is known in the art. It should be appreciated that each refrigeration case has an evaporator, shut-off valve, defrost coil, expansion valve, etc. as is known in the art. Referring to FIG. 2, each case controller 38 includes a central processing unit (CPU) 42 and memory such as electronically programmable read only memory (EPROM) 44, flash memory 46 and random access memory 48. The case controller 38 also includes a power supply 50 which is connected to a source of power (not shown) and provides a plurality of voltage outputs to the case controller 38. The case controller 38 further includes a liquid crystal display (LCD) 52 for visually indicating output from the case controller 38 to an operator. Each case controller 38 also includes a plurality of sensors for inputting data or information into the case controller 38. Specifically, the case controller 38 includes temperature sensors 54 for sensing the temperature of the refrigerant for the evaporator coil in, evaporator coil out, discharge air and return air of each refrigeration case. The case controller 38 also includes an air flow sensor 56 for measuring the air flow of the discharge and return air of each refrigeration case. The case controller 38 further includes door and drain sensors 58 for sensing whether the case door is open and whether fluid is draining from the refrigeration case. The case controller 38 also includes an analog to digital (A/D) converter 60 interconnecting the sensors 54,56,58 and the case controller 38. The case controller 38 further includes output drivers 62 connected to the CPU 42 and the expansion valves 40. It should be appreciated that each case controller 38 controls the opening and closing of one expansion valve 40. The case controller 38 is connected to the communications bus 18 and has a reset watchdog timer 64 connected to the CPU 42 for resetting the CPU 42. It should be appreciated that the components of the case controller 38 are conventional and known in the art. Referring to FIGS. 3A through 3C, a method of refrigeration case synchronization for compressor optimization, not covered by the present invention, is shown. It should be appreciated that the upper (cut-in) and lower (cut-out) limits of the compressor suction pressure are programmed in the refrigeration controller 16 and that the upper and lower limits of the case temperature for each refrigeration case is programmed into the case controller 38 of each refrigeration case. It should also be appreciated that other predetermined values such as a predetermined compressor suction pressure are programmed into the refrigeration controller 16 and/or case controller 38. As illustrated in FIGS. 3A through 3C, the methodology starts in bubble 100 and advances to block 102. In block 102, the methodology clears all capacity flags on all refrigeration cases. The capacity flags are used to indicate that a particular refrigeration case needs more or less cooling or capacity to regulate the load on that refrigeration case. The methodology advances from block 102 to block 104 and clears a synchronization timer or flag (not shown) of the refrigeration controller 16 and a last case flag. These flags are used to indicate that the synchronization is occurring and the number of refrigeration cases in the refrigeration system 10. The methodology then advances to block 106 and transmits the information from the refrigeration controller 16 via the communications bus 18 to the case controllers 38. From block 106, the methodology advances to diamond 108 and determines whether a compressor from the rack of compressors 12 has just staged (e.g., turned ON or OFF), for example, by looking for a flag. If so, the methodology advances to block 109 and resets the synchronization timer to a cycle time such as two (2) minutes. The methodology then advances to diamond 108 previously described. If a compressor has not just staged, the methodology advances to block 110 and sets a predetermined variable X equal to the compressor suction pressure measured by the pressure sensor 30. After block 110, the methodology advances to diamond 112 and determines whether the predetermined variable X is greater than a predetermined synchronization cut in pressure such as around 2.5 x 10 5 Pascals (thirty-six (36) PSI) stored in memory of the refrigeration controller 16. If not, the methodology advances to diamond 114 and determines whether the predetermined variable X is less than a predetermined synchronization cut out pressure such as around 2.2 x 10 5 Pascals (thirty-two (32) PSI) stored in memory of the refrigeration controller 16. If not, the methodology advances to the diamond 108 previously described. In diamond 112, if the predetermined variable X is greater than the synchronization cut in pressure, the methodology advances to diamond 116 and determines whether the time on the synchronization timer is equal to a predetermined value such as zero (0) stored in memory 16. If not, the methodology advances to block 118 and decrements the synchronization timer to the predetermined value. The methodology advances to diamond 108 previously described. If the time on the synchronization timer is equal to the predetermined value, the methodology advances to diamond 120 and determines whether a more capacity needed flag is set. If not, the methodology advances to block 122 and sets a more capacity needed flag and clears a less capacity needed flag for the refrigeration system 10. The methodology then advances to block 124 and sets up to transmit this information from the refrigeration controller 16 via the communications bus 18 to the case controllers 38. After block 124 or if the more capacity needed flag is set in diamond 120, the methodology advances to block 126. In block 126, the methodology calls the next case subroutine to be described in conjunction with FIGS. 2D and 2E and gets the next refrigeration case which is equal to N. It should be appreciated that if N equals 0, there are no more refrigeration cases. From block 126, the methodology then advances to diamond 128 and determines whether N is equal to a predetermined value such as zero (0) (e.g., no more cases). If so, the methodology advances to diamond 108 previously described. If not, the methodology advances to diamond 130 and determines whether the refrigeration case N is in its deadband range. It should be appreciated that each refrigeration case has a pre-programmed deadband range representing a cut in temperature such as 3.33°C (thirty-eight degrees fahrenheit (38°F)) and a cut out temperature such as 1.11°C (thirty-four degrees fahrenheit (34°F)). If the refrigeration case N is in its deadband range, the methodology advances to block 132 and sets the synchronization timer to the maximum time. If the refrigeration case N is not in its deadband range or after block 132, the methodology advances to block 134 and sets a more capacity needed flag for the refrigeration case N. The methodology then advances to block 136 and sets a flag to inform the case controller 38 for the refrigeration case N. The methodology then advances to block 126 previously described. In diamond 114, if the predetermined variable X is less than the synchronization cut out pressure, the methodology advances to diamond 138 and determines whether the time on the synchronization timer is equal to a predetermined value such as zero (0). If not, the methodology advances to block 140 and decrements the synchronization timer to the predetermined value. The methodology then advances to diamond 108 previously described. If the time on the synchronization timer is equal to the predetermined value, the methodology advances to diamond 142 and determines whether a less capacity needed flag is set. If not, the methodology advances to block 144 and sets a less capacity needed flag and clears a more capacity needed flag for the refrigeration system 10. The methodology then advances to block 145 and sets up to transmit this information from the refrigeration controller 16 via the communications bus 18 to the case controllers 38. After block 145 or if the less capacity needed flag is set in diamond 142, the methodology advances to block 146. In block 146, the methodology calls the next case subroutine to be described and gets the next refrigeration case which is equal to N. It should be appreciated that if N equals 0, there are no more refrigeration cases. From block 146, the methodology then advances to diamond 148 and determines whether N is equal to a predetermined value such as zero (0). If so, the methodology advances to diamond 108 previously described. If not, the methodology advances to diamond 150 and determines whether the refrigeration case N is in its deadband range. If so, the methodology advances to block 152 and sets the synchronization timer to the maximum time. If the refrigeration case N is not in its deadband range or after block 152, the methodology advances to block 154 and sets a more capacity needed flag for the refrigeration case N. The methodology then advances to block 156 and sets a flag to inform the case controller 38 for the refrigeration case N. The methodology then advances to block 146 previously described. Referring to FIGS. 3D and 3E, the methodology for the next case subroutine of blocks 126 and 146 is shown. From blocks 126 and 146, the methodology advances to diamond 162 and determines whether the less capacity needed flag is set for the refrigeration system 10. If not, the methodology advances to diamond 164 and determines whether the more capacity flag needed is set for the refrigeration system 10. If the more capacity needed flag is not set, the methodology advances to block 166 and sets N equal to a predetermined value such as zero (0), meaning no refrigeration case is available. The methodology then advances to bubble 168 and returns. In diamond 162 if the less capacity needed flag is set, or in diamond 164 if the more capacity needed flag is set, the methodology advances to block 170 and sets a predetermined variable Z equal to the maximum number of cases and the predetermined variable N equal to the last case. The methodology then advances to block 172 and sets the predetermined variable Z equal to Z minus 1 and the predetermined variable N equal to N plus 1. The methodology then advances to diamond 174 and determines whether the predetermined variable Z is equal to a predetermined value such as zero (0). If so, the methodology advances to block 166 previously described. If not, the methodology advances to diamond 176 and determines whether the predetermined variable N is greater than the maximum number of cases (predetermined variable Z). If so, the methodology advances to block 178 and sets the predetermined variable N equal to a predetermined value such as one (1) to indicate the first case controller 38. In diamond 176 if the predetermined variable N is not greater than the maximum number of cases, or after block 178, the methodology advances to diamond 180 and determines whether the case controller 38 for the refrigeration case N has been told its new capacity (e.g., less or more needed), for example, by looking for a flag. If so, the methodology advances to block 172 previously described. If not, the methodology advances to block 182 and sets the last refrigeration case equal to the predetermined variable N. The methodology then advances to block 184 and returns to blocks 128 or 148. An example of the operation of the methodology for the refrigeration system 10 is as follows: In the refrigeration system 10, a rack of four compressors 12 may have a cut out (turn off) pressure of around 2.2 x 10 5 Pa (32 PSI) and a cut in (turn on) pressure of around 2.75 x 10 5 Pa (40 PSI) of compressor suction pressure. Six individual refrigeration cases (not shown) of the refrigeration system 10 may have cut out and cut in temperature values of the following: cases cut in (T) °C (°F) cut out (T) °C (°F) case 1 3.33 (38) 1.11 (34) case 2 0 (32) -2.22 (28) case 3 3.88 (39) 1.66 (35) case 4 2.77 (37) 0.55 (33) case 5 -6.66 (20) -8.88 (16) case 6 -5.55 (22) -7.77 (18) During normal operation, i.e., full pressure deadband control, the methodology simply turns ON another rack compressor 12 via the refrigeration controller 16 if the compressor suction pressure goes above the cut in pressure of 2,76.10 5 Pa (40 PSI). The methodology waits or delays a predetermined time interval, and if the compressor suction pressure is still above 2.75 x 10 5 Pa (40 PSI), the methodology turns ON another rack compressor 12. This staging continues until either all rack compressors 12 are ON, or the compressor suction pressure drops below the cut out pressure of 2.2 x 10 5 Pa (32 PSI). Similarly, when the compressor suction pressure goes below the cut out pressure of 2.2 x 10 5 Pa (32 PSI), and the predetermined time interval has timed out, the methodology turns OFF another rack compressor 12 until all rack compressors 12 have staged down or been turned OFF. Without synchronization in the methodology, the case controllers 38 would similarly turn ON and OFF at only the predetermined temperatures of each case. The case 1 controller 38 turns ON if the temperature of its refrigeration case gets above its cut in temperature of 3.33°C (thirty-eight (38) degrees F) and stays ON until the temperature of that refrigeration case goes below its cut out temperature of 1.11°C (thirty-four (34) degrees F). It should be appreciated that the temperature area between 3.33°C and 1.11°C (thirty-eight (38) and thirty-four (34) degrees F) is the deadband area or range. When using the full pressure deadband control described in the above example, with synchronization, the methodology determines that the refrigeration system 10 needs more capacity when the compressor suction pressure goes above the predetermined synchronization cut-in pressure which is, in this example, a predetermined value such as 2,48.10 5 Pa (thirty-six (36) PSI); that is, the predetermined value may be the fifty percent (50%) point of the deadband range of 5.5 x 10 4 Pa (2.75 - 2.2 x 10 5 Pa) (eight (8) PSI (40-32 PSI)) for the compressor suction pressure. The methodology causes the refrigeration controller 16 to scan the information from the case controllers 38 to find a refrigeration case controller 38 that has its output ON (e.g., requires more cooling), but is within its temperature deadband range. The methodology causes the refrigeration controller 16 to inform that case controller to shut OFF its load via its expansion valve 40, thus increasing the available capacity to the rack of compressors 12. If after one sixth of the staging time for the rack of compressors 12, the compressor suction pressure is still not below 2.5 x 10 5 Pa (thirty-six (36) PSI), the methodology informs another case controller 38 that is in its temperature deadband range and has its output ON via its case controller 38, to terminate its load via its expansion valve 40 earlier than its normal cut out temperature. The methodology rotates through its refrigeration cases informing all the case controllers 38 in rotation of the need for more capacity while delaying only after terminating a refrigeration case load early. This continues until all refrigeration cases have been informed or until the compressor suction pressure goes below 2.5 x 10 5 Pa (thirty-six (36) PSI). If the reduction in load of the refrigeration cases is not sufficient to go below 2.5 x 10 5 Pa (thirty-six (36) PSI), and the compressor suction pressure gets above 2.75 x 10 5 Pa (forty (40) PSI), the methodology will turn ON another rack compressor 12 via the refrigeration controller 16. If the synchronization is successful such that the compressor suction pressure goes below 2.5 x 10 5 Pa (thirty-six (36) PSI), the methodology informs all the case controllers via the refrigeration controller 16 that no more capacity is needed and goes back to full'pressure deadband control (e.g., without synchronization). Similarly if the compressor suction pressure gets below 2.5 x 10 5 Pa (thirty-six (36) PSI), the methodology informs the case controllers 38 of the need for less capacity and turns ON the case controllers 38 that are within their deadband areas but presently not already ON. It should be appreciated that the methodology maintains or stores the first case controller 38 that went into capacity shed and the first case controller 38 that went into capacity storage so that it can rotate to another load to start the synchronization process at different case controllers 38 on the next need in that direction. The affect on the case controller 38 would be a modification of its deadband area to affect the capacity of the refrigeration controller 16. When 'told by the refrigeration controller 16 that more capacity is needed by the rack of compressors 12, the case 1 controller 38 would disregard the cut out temperature and control to a single cut in setpoint. When told by the refrigeration controller 16 that the condition is gone, the case 1 controller 38 would go back to full hysterisis deadband control. When told by the refrigeration controller 16 that less capacity is needed by the rack of compressors 12, the case 1 controller 38 would disregard the cut in temperature and control to a single cut out setpoint. This will increase on/off cycling on the expansion valves 40 to reduce cycling of the rack of compressors 35. Referring to FIGS. 4A through 4C, a method, according to the present invention, for refrigeration case control is shown for refrigeration cases of the refrigeration system 10. The method controls the temperature across the evaporator coil of a refrigeration case. As illustrated in FIG. 4A, the methodology starts in bubble 200 and advances to block 202. In block 202, the methodology initializes predetermined variables, which are restored from the EEPROM 44, and start timers in the CPU 44 and initialize the communications bus 18 for all refrigeration cases. It should be appreciated that block 202 is not performed again when the methodology is called from a main control loop of the case controller 38. The methodology advances from block 202 to diamond 204 and determines whether this is a restart or recovery from antiflood for a refrigeration case, for example, by looking for a flag set in block 224 to be described. If so, the methodology advances to block 206 and updates readings from the sensors 54,56 and stores these readings to memory 48 of the case controllers 38. From block 206, the methodology advances to diamond 208 and determines whether the temperature of the evaporator coil from the sensor 54 for a particular refrigeration case is both above the cut-in temperature and not in defrost. The CPU 44 compares the temperature readings from the sensor 54 of the evaporator coil to the cut-in temperature for that particular refrigeration case. If not, the methodology advances to block 206 previously described. If so, the methodology advances to diamond 210 and determines whether the recovery of the refrigeration case is from antiflood or watchdog, for example, by looking for a flag set in block 224 to be described. If so, the methodology advances to block 212 and sets the position of the expansion valve 40 for the refrigeration case to a predetermined average position. If not, the methodology advances to block 214 and initiates a pulldown sequence to be described. After blocks 212 and 214, the methodology advances to diamond 216 to be described. In diamond 216, the methodology determines whether it is time to sample temperature readings from the sensor 54 of the evaporator coil of the refrigeration case, for example, by looking for a flag. If so, the methodology advances to diamond 218 and determines whether the inlet or coil in temperature of the evaporator coil is cold enough. The CPU 42 determines whether the inlet temperature of the evaporator coil is below a predetermined temperature such as the inlet pulldown temperature. If not, the methodology advances to block 220 and sets a mode of the refrigeration case equal to a cold start, for example, by setting a flag. After block 220 or if the evaporator coil inlet temperature is cold enough in diamond 218, the methodology advances to diamond 222 and determines whether the outlet or coil out temperature of the evaporator coil is greater than the inlet temperature of the evaporator coil for the refrigeration case. The CPU 42 compares the outlet temperature of the evaporator coil to the inlet temperature of the evaporator coil for the refrigeration case. If not, the methodology advances to block 224 and sets the mode of the refrigeration case equal to antiflood, for example, by setting a flag. After block 224 or if the evaporator coil outlet temperature is greater than the evaporator coil inlet temperature in diamond 222, the methodology advances to block 226 and makes adjustments to proportional integral differential (PID) constants based on rising/falling and inside/outside window of the temperature difference between the outlet temperature of the evaporator coil and the inlet temperature of the evaporator coil. The methodology then advances to block 228 and calculates an adaptive PID according to the following formula: u(t) = K p e(t) + K i ∫ edt + K d de dt where: u(t) is the control signal to the valve relative to current position; e(t) is an error signal in the ΔT reading from its setpoint; and K p , K i and K d are PID gain multipliers which are adjusted by adaptive tuning. The methodology then advances to diamond 230 and determines, if a peak occurred, whether the new valve position for the expansion valve 40 is less than or equal to the old valve position for the expansion valve 40 of the refrigeration case. If not, the methodology advances to block 232 and recalculates K d so that the new valve position equals the previous. The methodology then advances to block 228 previously described. In diamond 230, if a peak occurred and the new valve position was less than or equal to the old valve position for the expansion valve 40, the methodology advances to diamond 234 and determines whether the change in temperature, ΔT, for the evaporator coil is falling rapidly. If so, the methodology advances to block 236 and makes an adjustment to the position of the expansion valve 40 for the refrigeration case. If not, or after block 236, the methodology advances to block 238 and positions the expansion valve 40. It should be appreciated that the CPU 42 sends a signal to the'output drivers 62, which positions the expansion valve 40. After block 238 or if it is not time to sample in diamond 216, the methodology advances to diamond 240 and determines whether the change in temperature, ΔT, is falling rapidly as previously described for diamond 234. If so, the methodology advances to block 242 and makes an adjustment to the position of the expansion valve 40 for the refrigeration case before the sample time. After block 242 or if the change in temperature, ΔT, is not falling rapidly in diamond 240, the methodology advances to diamond 244. In diamond 244, the methodology determines whether the temperature of the evaporator coil for the refrigeration case is above the cutout temperature and not in defrost. The CPU 42 determines whether the temperature is below a predetermined temperature such as the inlet pulldown temperature. If so, the methodology advances to diamond 246 and determines whether the refrigeration case is in the antiflood mode, for example, by looking for a flag set in block 224. If so, the methodology advances to diamond 248 and determines whether a dryout time has been completed, for example, by looking at a timer in the CPU 42. The CPU 42 starts a timer at the beginning of the defrost cycle for the evaporator coil of the refrigeration case. If not, the methodology advances to block 250 and makes adjustments to the timer based on the temperature of the refrigeration case and negative change in temperature of the evaporator coil for the refrigeration case. The methodology then advances to diamond 248 previously described. In diamond 204, if this is not a restart or recovery from antiflood, the methodology advances to diamond 252 and determines whether this is a control cycle of the refrigeration case, for example, by looking for a flag. If so, the methodology advances to diamond 216 previously described. If not, the methodology advances to diamond 254 to be described. Also, if the temperature of the evaporator coil is above the cutout temperature (out of defrost) in diamond 244, the methodology advances to diamond 254. Further, if the refrigeration case is not in an antiflood mode in diamond 246, the methodology advances to diamond 254. Additionally, if the dryout time has been completed in diamond 248, the methodology advances to diamond 254. In diamond 254, the methodology determines whether the expansion valve 40 for the refrigeration case is forced to a position, for example, by looking for a flag. If so, the methodology advances to block 256 and positions the expansion valve 40 for the refrigeration case. After block 256 or if the expansion valve 40 is not forced to a position in diamond 254, the methodology advances to diamond 258 and determines whether it is time to sample temperature readings from the sensor 54 for the refrigeration case. If not, the methodology advances to diamond 268 in FIG. 4D to be described. If so, the methodology advances to block 261 and logs the temperature readings and temperature cycle times in memory 48 of the case controller 38 for the refrigeration case. The methodology then advances to diamond 262 and determines whether there has been a predetermined number of sample temperature time points, such as one hundred twenty-eight (128). If not, the methodology advances to bubble 260 previously described. If so, the methodology advances to block 264 and calculates a fast Fourier transform (FFT) of a waveform as a function of time created by sample temperature time points representing the change in temperature, ΔT, for the evaporator coil and makes parameter adjustments based on the transformed waveform which represent the quality of the data gathered in the logging step. The methodology then advances to block 266 and makes valve position adjustments of the expansion valve 40 for short temperature cycling of the refrigeration case based on the parameter adjustments made in the previous step. After block 266, the methodology advances to diamond 268 in FIG. 4D. In diamond 268, the methodology determines whether a more capacity needed flag is set for the refrigeration system 10. If so, the methodology advances to block 270 and sets the cut-in temperature limit 0,56°C (one degree F) above the cut-out temperature limit for the refrigeration case. The methodology then advances to block and returns to the main loop of control for the refrigeration system 10. In diamond 268, if the more capacity needed flag is not set, the methodology advances to diamond 274. In diamond 274, the methodology determines whether a less capacity needed flag is set for the refrigeration system 10. If so, the methodology advances to block 276 and sets the cut-out temperature limit 0,56°C (one degree F) below the cut-in temperature limit for the refrigeration case. After block 276 or if the less capacity needed flag is not set in diamond 274, the methodology advances to block 272 and returns. Accordingly, the method of refrigeration case control allows control of the refrigeration case even if the temperature sensors fail. The method of refrigeration case control provides control of an analog servo expansion valve for the refrigeration case. The method of refrigeration case control uses two fast fourier transforms to be self-tuning and therefore adaptive. The method of refrigeration case control allows closer temperature control of the refrigeration case within ± 0,56°C (one degree (1°) F).

Claims: A method of controlling the temperature differential across the evaporator coil of a refrigeration case of a refrigeration system (10) , the method comprising the steps of: a) sampling temperature readings from a temperature sensor (54) of the evaporator coil of the refrigeration case at predetermined periodic sampling times; b) determining the inlet temperature and the outlet temperature of the evaporator coil, the difference between them being the temperature differential across the evaporator coil (ΔT) ; c) determining (216) whether or not it is time to sample temperature readings from said sensor (54) of said evaporator coil; d) if it is time to sample temperature readings, making adjustments (226) to proportional-integral-difference constants based on rising/falling inside/outside window of the temperature differential determined in step b), calculating (228) an adaptive PID of the temperature differential across the evaporator coil to produce a control signal u(t) for positioning said expansion valve, determining (234) if ΔT is falling rapidly and, if so, adjusting u(t) accordingly, and applying (238) the control signal u(t) to position the expansion valve (40); and e) if it is not time to sample temperature readings, determining (240) if ΔT is falling rapidly and, if so, adjusting the control signal u(t) for positioning the expansion valve accordingly, and applying (256) the control signal u(t) to position the expansion valve (40). A method according to claim 1, which includes the step (204) of determining whether the method is a restart or recovery from an antiflood of the evaporator coil. A method according to claim 2, including the step (206) of updating temperature sensor readings if the method is a restart or recovery from an antiflood of the evaporator coil. A method according to claim 3, including the step (208) of determining whether the refrigeration case is above a predetermined cut-in temperature and not in defrost, and repeating said step (206) of updating if the refrigeration case is not above the predetermined cut-in temperature or if the refrigeration case is in defrost. A method according to claim 4, which includes the step (252) of determining whether the method is a control cycle if the method is not a restart and not a recovery from an antiflood of the evaporator coil, and/or determining (210) whether the recovery was from an antiflood or watchdog if the temperature of the refrigeration case is above the predetermined cut-in temperature and not in defrost in step 208. A method according to claim 5, including the step (212) of setting the expansion valve to a predetermined average position if it has been determined in step 210 that the method is a recovery from an antiflood or watchdog, and/or the step of employing a pull down sequence (214) if it has been determined in step 210 that the method is not a recovery from an antiflood or watchdog. A method according to any preceding claim, which includes the step (218) of determining whether the inlet of the evaporator coil is cold enough prior to said step (228) of calculating. A method according to claim 7, including the step (220) of setting a mode of the refrigeration case equal to a cold start if it has been determined in step (218) that the inlet of the evaporator coil is not cold enough, or the step (222) of determining if the outlet temperature of the evaporator coil is greater than the inlet temperature of the evaporator coil if it has been determined in step (218) that the inlet of the evaporator coil is cold enough. A method according to claim 8, which further includes the step (224) of setting a mode of the refrigeration case equal to antiflood if it has been determined in step 222 that the outlet temperature of the evaporator coil is not greater than the inlet temperature of the evaporator coil. A method according to any preceding claim, which includes the step (230) of determining, if a peak occurred, whether a new valve position of the expansion valve is less than or equal to an old valve position of the expansion valve. A method according to claim 10, including the step (232) of recalculating one (K d ) of the PID constants (K p , K i , K d ) if a peak occurred and the new valve position was not less than or equal to the old valve position. A method according to any preceding claim, including the step (261) of logging sensor temperature readings and temperature cycle times if it is time to sample temperature readings. A method according to claim 12, including the step of determining (262) whether a predetermined number of temperature readings have been sampled. A method according to claim 13, wherein if it has been determined in step 262 that a predetermined number of temperature readings have been sampled, the method includes the step of transforming (264) change in temperature data and making parameter adjustments based on transformed change in temperature data. A method according to claim 14, wherein said change in temperature data is transformed using a fast Fourier transform. Apparatus for controlling the temperature differential across the evaporator coil of a refrigeration case of a refrigeration system (10), the apparatus comprising: a) temperature sensing means (54) for providing temperature readings from an evaporator coil of said refrigeration case; b) sampling means (42) for sampling said temperature readings at predetermined periodic sampling times; c) means for determining the inlet temperature and the outlet temperature of the evaporator coil, the difference between them being the temperature differential (ΔT) across the evaporator coil; d) means for determining whether or not it is time to sample temperature readings from said temperature sensor (54) of said evaporator coil; e) means for making adjustments to proportional-integral-difference constants based on rising/falling inside/outside window of the temperature differential determined by the means recited in c), calculating an adaptive PID of the temperature differential across the evaporator coil to produce a control signal u(t) for positioning said expansion valve (40), determining if ΔT is falling rapidly and, if so, adjusting u(t) accordingly, in the event that it is determined by the means recited in d) that it is time to sample temperature readings; and f) means for determining if ΔT is falling rapidly and, if so, adjusting a control signal u(t) for positioning said expansion valve (40) accordingly, in the event that it is determined by the means recited in d) that it is not time to sample temperature readings; and g) control means (38,62) for applying said control signal u(t) to position the expansion valve (40).

Inventors: ANGLIN MARK E, HENDERSON JAMES P, OTT JAMES H, ANGLIN, MARK E., HENDERSON, JAMES P., OTT, JAMES H.
Applicants: NOVAR ELECTRONICS CORP, NOVAR ELECTRONICS CORPORATION
Application year: 1995
Main classification: F25B49
IPC codes: F25B41, G05D23, F25D29