Abstract:
A Proportional Integral Derivative (PID) control system controls a component by adjusting the control input and the execution of the PID calculation whenever a change in a state of a component exceeds a certain value.

Description:
BACKGROUND 
       [0001]    The present disclosure is directed to control systems, and more specifically to a proportional integral derivative (PID) evaporator temperature control scheme. 
         [0002]    In the automotive field, as well as other fields, compressors are used to control an evaporator temperature and thereby allow for heating and cooling. The evaporator temperature is typically adjusted by changing the compressor speed. In order to ensure that the compressor is operated at the proper speed for a desired evaporator temperature, electrical control systems are used. It is known in the art to use a proportional integral derivative (PID) control scheme on a micro-controller to control these systems. Typically the PID controller will have an input of the current temperature of the evaporator and the current speed of the compressor. The PID controller then attempts to drive the evaporator temperature to a desired temperature by making corresponding adjustments to the compressor speed. 
         [0003]    Current control systems determine adjustments to the compressor speed at a set frequency. By way of example, some control algorithms recalculate the needed compressor speed every 8 seconds, or at some desired time interval. Adjusting the compressor speed at a set frequency entails operating the control algorithm at the specific time interval regardless of any change in the actual temperature of the evaporator. Once the evaporator temperature has reached approximately the desired temperature minor fluctuations in temperature can occur with the evaporator temperature remaining within acceptable tolerances. Running the control scheme, and adjusting the compressor speed, consistently at the desired frequency can therefore result in unnecessary adjustments to the compressor speed, and unneeded use of electrical power. 
       SUMMARY 
       [0004]    Disclosed is a control system for operating a compressor that establishes an initial condition, detects changes in the initial condition, and operates a controller when the changes in the initial condition exceed a predetermined maximum value. The controller then establishes a new initial condition and continues to detect changes from the new initial condition. 
         [0005]    Additionally disclosed is a control scheme for controlling a compressor speed which establishes a target evaporator temperature and an initial evaporator temperature. The method detects the actual temperature of the evaporator and compares it to a previous sensed evaporator temperature to determine a change in evaporator temperature since the last iteration of the control signal. The method also detects the actual evaporator temperature and compares the actual temperature with the target temperature to determine a difference between the actual temperature and the target temperature. The difference between the actual temperature and the target temperature is used to initiate operation of a control algorithm whenever the change in temperature exceeds the predetermined value. Initiating operation when the temperature change exceeds a predetermined value provides for actuation of the control algorithm to adjust the speed of the compressor only when required to obtain a desired temperature. 
         [0006]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  schematically illustrates a vehicle which has a compressor, evaporator, and a compressor controller. 
           [0008]      FIG. 2  illustrates a block flowchart of a compressor control system using a proportional integral derivative controller (PID controller). 
           [0009]      FIG. 3  illustrates a block flowchart of a compressor control system with an additional Δt check block. 
           [0010]      FIG. 4  illustrates a sample graph of an evaporator temperature over time. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  schematically illustrates a vehicle  10  which has a compressor  20  and an evaporator  30  located in the front engine compartment. The evaporator  30  and the compressor  20  are controlled by an on-board electronic controller  40  which is capable of adjusting the speed of the compressor  20  and thereby adjusting the temperature of the evaporator  30 . The controller  40  can be a micro-processor located within the standard control system of the vehicle, or any other type of controller. The example compressor  20  is controlled by a proportional integral derivative (PID) control scheme. 
         [0012]      FIG. 2  illustrates a flowchart of a control scheme  100  for controlling an evaporator temperature  114  by adjusting the compressor speed  118 . Initially a target evaporator temperature  110  is either input into the system  100 , or manufactured into the controller  40  operating the control scheme  110 . A summation block  112  subtracts an actual evaporator temperature  114  from the target temperature  110 , and transfers the resultant value into a PID controller  116 . The PID controller  116  also accepts an input of an actual compressor speed  118  which is determined by a compressor speed sensor  120 . When the PID controller  116  performs a control cycle, it outputs a command  122  which pushes the compressor  124  toward the desired compressor speed  118 . The compressor speed  118  affects the evaporator temperature  114  in a predictable manner. For example, an increase in compressor speed causes a change in evaporator temperature in one direction, and a decrease in evaporator speed will cause a temperature movement in the other direction. 
         [0013]    The evaporator temperature  114  is sensed by a sensor  128 , which outputs the evaporator temperature  114 . The example control system shown in  FIG. 2  includes a condition check within the evaporator temperature block  114 . The condition check evaluates a specific condition, such as evaporator temperature, and determines how much the condition has changed since the last control cycle. A control cycle is a single iteration of the control scheme  100  which determines an adjustment to the compressor speed using the control scheme  100 . If the change in condition exceeds a predefined amount, a control cycle is performed. 
         [0014]    The control system  100 , utilizes a double feedback loop, in that it uses the current evaporator temperature  114  compared with the target evaporator temperature  110  as one input into the PID controller  116 . The control scheme  100  also utilizes the current compressor speed  118  as a second input into the PID controller  116 . The feedback loops ensure that as the temperature of the evaporator approaches that of the desired target temperature  110 , a progressively smaller input is sent to the PID controller  116 , thereby causing the PID controller  116  to perform a smaller adjustment to the compressor speed  118 . 
         [0015]      FIG. 3  illustrates the example control system  100  of  FIG. 1 , with a separate Δt check block  210 . Δt represents the difference between the current evaporator temperature  114  and the evaporator temperature  114  from the previous evaporator temperature data reading from the evaporator temperature sensor. The Δt check block  210  prevents the PID controller  116  from operating whenever Δt is below a predetermined value. This allows the PID controller  210  to recalculate a desired compressor speed  122  only when a speed correction is necessary. Each time the Δt check block  210  passes a value to the summation block  112 , it also stores that value as an “initial value.” The initial value is then compared to the incoming sensed evaporator temperature  114  to determine the Δt value. When the Δt value exceeds a predetermined Δt value, the Δt check block  210  passes the current evaporator temperature  114  to the summation block  112 , and the PID controller  116  operates a control iteration. 
         [0016]    Alternatively, a timing component  220  can be utilized to prompt operation of a control iteration, in addition to a change in condition prompting the control cycle, as is indicated in the Δt check block  210 . The timing component  220  determines how much time has passed since a value has been passed to the summation block  112 . If a predetermined maximum time has elapsed, the actual evaporator temperature  114  is passed to the summation block  112  regardless of the Δt value. By way of example, the maximum time could be set to three minutes, thereby ensuring that the control scheme is operated at least every three minutes. This allows the control system  100  to make minor necessary adjustments to the compressor speed  118 , without constant unnecessary adjustments to the compressor speed  118 . 
         [0017]    Illustrated in  FIG. 4  is a sample graph  300  of evaporator temperature control operations using the above described system. In the graph  300 , the line  310  represents the temperature of the evaporator over time, the axis  312  represents temperature, and the axis  314  represents time. Each of the bars  316  represent a control cycle which is run by the controller. Since the controller uses the Δt value to determine when to operate a control cycle, that is the control cycle is only run when Δt is greater than a certain number, the bars are closer together at the beginning of the time period when the temperature is changing at the fastest rate. As the time progresses and the temperature changes at a slower rate, the Δt minimum is not exceeded for longer periods, and the control cycles  316  are spaced farther apart. By the end of the time period the evaporator temperature  310  has reached the desired temperature line  318 . The example system illustrated here includes the optional maximum time element described above, and as such the latest three control cycles  316  are evenly distributed and were initiated because a maximum time had elapsed since the last control cycle  316 . 
         [0018]    An example of the above described system uses the control scheme to drive an evaporator temperature to a desired value by adjusting a compressor speed. The system initially detects an actual evaporator temperature when it is first turned on, and this temperature is set as the initial operating condition. The control system then polls the evaporator temperature and compares actual temperatures to the initial operating condition. When the difference between the two values exceeds a predefined amount, the control scheme operates one cycle of the PID controller. The PID controller accepts the evaporator temperature as a control input and determines an adjustment to the compressor speed which is necessary to drive the evaporator temperature to the desired value. The controller then resets the “initial operating condition” to be the actual operating condition at the start of the control cycle, and the system returns to polling the actual evaporator temperature. 
         [0019]    Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.