Abstract:
A method for controlling a throttle of an electronic throttle control-equipped engine. The method includes the steps of providing a desired throttle position derived from the driver demand and vehicle system requests. The method generates first and second throttle positions by interpolating the desired throttle position within the resolution of the throttle position controller. A duty cycle is also generated as a function of the desired throttle position and system resolution. The resulting conditioned throttle position command having the first throttle position for a first time period and the second throttle position for a second time period is communicated to the throttle controller. The ratio of the time periods corresponds to the duty cycle such that the average throttle position command is approximately equal to the desired throttle position. In this way, the control method can achieve a desired throttle position of higher resolution than the throttle position sensing system.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to a control system and method for internal combustion engines, and more particularly, concerns a throttle position control scheme for electronic throttle control-equipped vehicles. 
     Electronic airflow control systems such as electronic throttle control systems, replace traditional mechanical throttle cable systems with an “electronic linkage” provided by sensors and actuators in communication with an electronic controller. This increases the control authority of the electronic controller and allows the airflow and/or fuel flow to be controlled independently of the accelerator pedal position. Electronic throttle control systems include mechanisms for positioning the throttle plate in response to the driver demand and other vehicle system constraints such as a traction control system. 
     The most common positioning mechanism is a positioning motor. A closed-loop feedback position controller typically responds to a discrete throttle position value and commanded throttle position. Because the feedback signal is an analog signal that has been discretized by an analog-to-digital converter, its resolution is quantized and may not precisely correlate to a commanded steady-state throttle position. Thus, there is a need for an improved throttle position control system and method. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an improved throttle position control scheme. According to the present invention, the foregoing and other objects are obtained by a method for controlling a throttle of an electronic throttle control-equipped engine. The method comprises the steps of providing a desired throttle position derived from the driver demand and vehicle system requests. The method generates first and second throttle positions by straddling the desired throttle position within the resolution of the throttle position controller. A duty cycle is also generated as a function of the desired throttle position and system resolution. The resulting conditioned throttle position command comprising said first throttle position for a first time period and said second throttle position for a second time period is communicated to the throttle controller. The ratio of the time periods corresponds to the duty cycle such that the average throttle position command is approximately equal to the desired throttle position. In this way, the control method can achieve a desired throttle position which is, on average, of higher resolution than the throttle position sensing system. 
     An advantage of the present invention is that it provides higher resolution of the throttle position control. Another advantage is that the present method more accurately corresponds to the commanded steady-state throttle position as determined from the driver demand. Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of this invention, reference should be made to the embodiments illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention. 
     In the drawings: 
     FIG. 1 is a schematic diagram of an internal combustion engine and associated electronic throttle control and operator input systems in accordance with one embodiment of the present invention. 
     FIG. 2 is a logic flow diagram of a method of controlling the throttle position in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, there is shown a schematic diagram of an internal combustion engine  40  and associated Powertrain Control Module (PCM)  42  as well as an operator interface  68  in accordance with one embodiment of the present invention. The engine  40  includes a plurality of combustion chambers  41  each having an associated intake  43  and exhaust  44  operated by a respective intake and exhaust valves  45 ,  46 . Combustion occurs as a result of the intake of air and fuel from the intake manifold  47  and fuel injector  48  respectively, compressioned by the piston  49 , and ignitioned by the spark plug  50 . Combustion gases travel through the exhaust manifold  44  to the downstream catalytic converter (not shown) and are emitted out of the tailpipe. A portion of the exhaust gases may also be recirculated back through the intake manifold  47  to the engine cylinders  41 . 
     The airflow through the intake manifold  47  is controlled by a throttle comprising a throttle plate  51  and throttle actuator  52 . The throttle actuator is preferably an electronic servo motor. A throttle position sensor  53  measures the actual throttle position. The throttle position sensor is typically an analog sensor. Its output is discretized when it passes through an analog-to-digital converter such that the controller receives discrete positional values for the detected throttle position. Thus, the quandization of the positioning mechanism is typically a function of the resolution of the A to D converter. However, higher resolution typically is associated with higher cost A to D converters. 
     Other sensors include a mass airflow sensor  54  which measures the amount of air flowing into the engine  40 . An engine speed sensor  55 , provides a value indicative of the rotational speed of the engine  40 . 
     The PCM  42  receives as inputs the discretized throttle position signal, the mass airflow signal, the engine speed signal, and any driver demand inputs, among other things. In response, the PCM  42  controls the spark timing of the spark plugs  50 , the pulse width and timing of the fuel injectors  48 , and the position of throttle  51  by way of the throttle actuator  52 . These inputs and outputs are controlled by the main micro-controller  60 . The main micro-controller  60  controls the throttle position by outputting a throttle position command to the Throttle Plate Position Controller (TPPC)  62  to drive the throttle actuator  52  to the desired position, as will be described in more detail below. 
     The TPPC  62  is preferably a PID controller which closed-loop controls the throttle position based primarily on an error term representing the difference between the desired and actual throttle position values. The desired throttle position can be generated by any known methods of interpreting driver demand and arbitrating it with the various vehicle system constraints such as speed control and traction control. The resulting desired intake airflow value is then factored into a formula to yield a desired throttle position command. 
     With regard to throttle control, the PCM  42  generates a throttle position command. The desired throttle position command is communicated to the TPPC  62 . The TPPC  62  preferably conditions the throttle position command as described below with reference to FIG. 2, and communicates this signal to the closed-loop controller which is part of the TPPC  62 . The closed-loop controller outputs a drive signal to the throttle actuator  52  to drive the throttle  51  to the desired position. 
     The PCM  42  preferably includes an Electronic Throttle Control (ETC) monitor  64  that communicates with the main micro-controller  60  and TPPC  62 . The ETC monitor  64  includes a microprocessor  65  and associated memory separate from the microprocessor and the main micro-controller  60 . The ETC monitor  64  receives as input the engine speed signal from the engine speed sensor  55  and throttle position signal from the throttle position sensor  53 . The ETC monitor  64  then functions to monitor the throttle actuation. Although the ETC monitor  64  and TPPC  62  are shown as separate from the PCM main microprocessor, they could be partially or wholly integrated into the main microprocessor as well. Alternatively, the ETC monitor  64  and TPPC  62  can be integrated into a single controller separate from the main micro-controller  60 . 
     The PCM  42  also receives as inputs driver demand signals  66 . The driver demand signals can include such things as accelerator pedal position  70 , ignition switch position  72 , steering input  74 , brake sensor input  76 , transmission position input  78 , as well as inputs from the vehicle speed control and transmission. 
     Referring now to FIG. 2, there is a shown a logic flow diagram of a method of controlling the throttle position in accordance with one embodiment of the present invention. The method begins at step  100  by determining the desired throttle position. The desired throttle position command is preferably derived by the PCM and communicated to the TPPC. A desired or commanded throttle position can be generated by any known method but typically is a function of the accelerator pedal position input by the operator, the engine speed, the engine coolant temperature, barometric pressure, and air charged temperature. Given the driver demand, and any inputs from the speed control system and traction control system, if active, as well as any constraints imposed by engine, vehicle, or transmission speed limits, the PCM generates a desired airflow value resulting in a desired throttle position to achieve that airflow. The throttle position command can be expressed in unites of A to D counts or degrees. In a preferred embodiment, the throttle position command is expressed as opening angle degrees. Thus, in step  100 , it may be necessary to convert the throttle position command (in this case, encoded as a duty cycle) duty cycle or count into degrees of throttle opening. 
     Because the actual throttle position signal is discretized by an A to D converter, it necessarily discretizes the position information provided to the TPPC  62 . Thus, even though the commanded throttle position may effectively be continuous within the controller, the achievable steady position is discretized. For example, the actual throttle position signal may only have a resolution of {fraction (1/16)} degrees of throttle opening angle. If the desired throttle opening angle is 14{fraction (5/32)} degrees, a steady-state condition may result when the actual throttle position sensor value reads 14{fraction (3/16)} degrees due to the discrepancy and resolution between the position controller, and the position sensor. The present invention overcomes this discrepancy and provides near-continuous resolution by generating a conditioned throttle position command comprising a duty cycle schedule between two achievable discrete positions as measured by the throttle position sensor. 
     In step  102 , the throttle position command is quantized for easier handling by the TPPC controller. For example, the resulting throttle position command is expressed in a resolution of {fraction (1/256)} degrees. 
     In step  104 , the throttle position command is compared to the natural resolution of the A to D converter associated with the throttle position sensor. For example, the natural resolution may be {fraction (1/16)} degrees. Therefore, if the commanded throttle position was 7{fraction (11/32)} degrees, the rounded down throttle position would be 7{fraction (10/32)} or 7{fraction (5/16)} degrees, and the rounded up throttle position would be 7{fraction (6/16)} degrees. 
     In step  106 , the TPPC interplates between the two achievable throttle position values (tp_command_rd and tp_command_ru) to weight the rounded up and rounded down throttle position values relative to the commanded quantized throttle position value. This is expressed as follows: 
     
       
         duty_cycle_unq=(tp_command_tp_command_rd)/NATURAL_RES  (1) 
       
     
     In step  108 , the duty cycle is quantized to a specified resolution similarly to the commanded throttle position in step  102 . This may be expressed as follows: 
     
       
         duty_cycle=quantize(duty_cycle_unq,DUTY_CYCLE_RES)  (2) 
       
     
     wherein DUTY_CYCLE_RES is a predetermined constant such as 0.5. This step serves to simplify the controller&#39;s implementation. 
     In step  110 , the conditioned throttle position command is generated that alternates between the rounded up and rounded down throttle positions according to the calculated duty_cycle value. For a constant period implementation, this can be expressed as follows: 
     
       
         tp_command=tp_command_rd for (1-duty_cycle)*PERIOD, then tp_command_ru for (duty_cycle)*PERIOD  (3) 
       
     
     Thus, in the example above, if the desired throttle position from the PCM was 7{fraction (11/32)} degrees, and the natural resolution of the system was {fraction (1/16)} degrees, the corresponding rounded down and rounded up position values would be 7{fraction (5/16)} degrees and 7{fraction (6/16)} degrees, respectively. The corresponding duty cycle would also be 50%. Thus, in the case of a constant 20 msec period, the throttle position command would be 7{fraction (5/16)} degrees for 10 msec and 7{fraction (6/16)} degrees for 10 msec for as long as the desired throttle position was 7{fraction (11/32)} degrees. 
     In step  112 , the throttle position is driven by closed-loop feedback control according to the conditioned throttle position command generated in step  110 . 
     An example of the present method as shown in FIG. 2 for a variable period follows. Assume that the resolution of the A to D converter associated with the throttle position sensor (the feedback signal) is {fraction (1/16)} degrees. If the desired throttle opening angle position command is 5{fraction (1/64)} degrees, then the conditioned throttle position command provided to the closed-loop position controller would be as follows: for 6 milliseconds, the commanded throttle position would be set to 5 degrees, and for 18 milliseconds, the commanded throttle position would be set to 5{fraction (1/16)} degrees. This conditioned throttle position command is then be repeated as long as the desired throttle position command was 5{fraction (1/64)} degrees. In this example, the total control period is 24 milliseconds. Depending upon the responsiveness of the controller, however, a minimum control time period is preferred to achieve a desired throttle position. In other words, a 50% duty cycle having a one millisecond dwell time at each of two commanded throttle positions, i.e., a commanded time period of two milliseconds, may be too fast to achieve the desired throttle position. Thus, it may be desirable to implement a variable control time period to ensure that the conditioned throttle position command does not generate a dwell time less than the responsiveness of the closed-loop controller. Thus, if the conditioned throttle position command were 10 percent at the rounded down value and 90 percent at the rounded up value and the minimum dwell time necessary to effectuate a response by the closed-loop controller was 4 msec, the rounded down value would be commanded for 4 msec and the rounded up value for 36 msec. This contrasts with the constant period example of 20 msec wherein the 10 percent rounded down value would be commanded for 2 msec (10 percent of 20 msec) which would be less than the response time of the closed-loop system. 
     From the foregoing, it can be seen that there has been brought to the art a new and improved throttle position control method which has the advantage of high resolution near, near-continuous throttle position control. While the invention has been described in connection with one or more embodiments, it should be understood that the invention is not limited to those embodiments. On the contrary, the invention covers all alternatives, modifications and equivalents as may be included within the spirit and scope of the appended claims.