Abstract:
A controller and a control method for modifying battery discharge and charge power limits in a vehicle powertrain that includes an electric battery as a power source. The modification compensates for inaccurate estimates of battery discharge and charge power limits by using a closed loop feedback control based on error between a battery voltage set point and commanded battery voltage.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The invention relates to hybrid electric vehicle powertrains having an internal combustion engine and an electric drive system with an electric motor, a generator and a battery. It relates also to pure electric vehicle powertrains having an electric motor, a generator and a battery defining an electromechanical power flow path. 
   2. Background Art 
   Hybrid electric vehicle powertrains and pure electric vehicle powertrains use a battery and an electric motor to establish a power flow path to vehicle traction wheels. In the case of a hybrid electric vehicle, an internal combustion engine complements the electric motor and the battery by establishing an independent power flow path to the vehicle traction wheels. 
   One class of hybrid electric vehicles, commonly referred to as a parallel hybrid electric vehicles, includes a powertrain with two power source configurations. The first configuration comprises an engine, a motor, a generator with a battery, and a planetary gearset for coordinating power distribution to traction wheels. A second power source configuration in a parallel hybrid electric powertrain includes an electric drive system with only a motor, a generator and a battery. The battery acts as an energy storing device for the generator and the motor. 
   The two power source configurations work together seamlessly with the planetary gearset to meet the driver&#39;s demand for power as much as possible without exceeding power limits of the powertrain and power limits of the battery subsystem. Under normal operating conditions, a vehicle system controller interprets the driver&#39;s demand for power as a function of acceleration or deceleration. The controller will determine how much torque each power source needs to contribute to total power demanded by the driver and to achieve specified vehicle performance (i.e., engine fuel economy, emission quality, driveability, etc.). 
   The power supplied by the two power sources is coordinated by the vehicle system controller as it meets the driver&#39;s demand for power without exceeding the limits of the system and without exceeding the battery voltage limits during either charging or discharging. 
   The powertrain will determine the driver&#39;s demand for torque and achieve optimum division of power between the two power sources without exceeding battery power limits. If the battery limits are exceeded and the battery voltage is outside of a specified precalibrated range, the controller will shut down the vehicle. This condition can be avoided using a closed loop power control as described in co-pending patent application Ser. No. 10/248,886, filed Feb. 27, 2003 entitled “Closed Loop Power Control System for Hybrid Electric Vehicles.” This co-pending application is assigned to the assignee of the present invention. Reference is made to it to complement the present disclosure. It is incorporated in this disclosure by reference. 
   In a conventional vehicle powertrain with a gasoline engine, combustion energy availability is the same under all operating conditions regardless of the amount of gasoline in the vehicle gas tank. In contrast, the energy availability in a hybrid electric vehicle powertrain or in a pure electric vehicle powertrain depends upon battery conditions (e.g., battery state of charge and battery temperature). A power command to the electric motor in a hybrid electric vehicle or in a pure electric vehicle therefore is limited by the battery power availability. 
   Accuracy in establishing the battery power limits is needed to maintain the battery voltage within a certain range to ensure that the powertrain system will function properly. Accuracy of the battery limits is needed also to avoid shutdown of the electric motor and its controller due to a voltage that is under or over the battery voltage limits. It is possible, however, for the battery controller to inaccurately estimate the battery&#39;s discharge and charge power limits, especially in view of the complexity of electro chemistry of the battery. An inaccurate estimation of the battery power limits could cause the battery voltage to be out of a precalibrated proper range. 
   SUMMARY OF AN EMBODIMENT OF THE INVENTION 
   The disclosed embodiment comprises a closed loop control system and method for modifying precalibrated battery limits when necessary for any given powertrain power demand. The control system compensates for inaccuracy in the estimation of the battery power limits by the battery controller so that a potential vehicle shut down can be avoided. 
   Battery voltage set points, which define upper and lower voltage boundaries, are established by calibration. The control system uses actual battery voltages as a feedback signal to create a modification to the battery power limits. The modification is determined by a voltage closed loop control system if the actual battery voltage is higher or lower than the boundaries established by the battery voltage set points. If the battery voltage is within the boundary, the control system will not change the battery power limits. 
   The method of the disclosed embodiment of the invention includes computing an estimated battery power limit during battery discharge and battery charge. Commanded electric battery power is compared to an estimated battery power limit. A modified battery power limit is computed if actual battery voltage exceeds the battery charge or discharge voltage limit. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows an electric vehicle powertrain in which an electric motor, such as a high voltage induction motor, is the sole power source; 
       FIG. 2  is a schematic representation of a hybrid electric vehicle powertrain in which an internal combustion engine and an electric motor establish separate power flow paths to vehicle traction wheels; 
       FIG. 3  is a schematic illustration of a closed loop control for battery limits as applied to a hybrid electric vehicle powertrain; 
       FIG. 4   a  is a PI controller subsystem for the control system illustrated in  FIG. 3  during battery discharge; 
       FIG. 4   b  is a PI controller subsystem for the control system illustrated in  FIG. 3  during battery charge; 
       FIG. 5  is a flowchart of a control algorithm for calculating a modified battery discharge power limit; and 
       FIG. 6  is a flowchart of a control algorithm for calculating a modified battery charge power limit. 
   

   DETAILED DESCRIPTION 
   The powertrain of  FIG. 1  has a vehicle system controller  10  that receives driver commands for power at  12 . The controller will issue commands at  14  to the motor and transmission  16 , which in turn delivers power to the vehicle traction wheels through a geared differential and half shaft assembly  21 . The commands at  14  are tested by a comparator  18 . Before the commands are issued to the motor and transmission assembly  16 , the comparator determines whether limits established by battery controller  20  are either above or below the command at  14 . 
     FIG. 2  shows a hybrid electric vehicle powertrain which includes an internal combustion engine  22  under the control of an engine controller  24 . Vehicle system controller  26  responds to driver commands at  28  to issue commands at  30  to the engine controller  24 . It issues commands also to the motor, as shown at  32 . As in the case of the pure electric powertrain of  FIG. 1 , the commands to the motor are tested at comparator  34  to determine whether the command at  32  is within the battery charge and discharge limits established by battery controller  36 . 
   The power flow path established by the motor and transmission assembly  38  and by the engine  22  is delivered to the traction wheels  40  through a differential half shaft assembly  42 . 
     FIG. 3  shows an embodiment of the invention wherein a closed loop controller  44  receives battery charge and discharge limits at  46 , which are established by a battery and battery controller  48 . The closed loop control  44  includes a PI (proportional-integral) controller  50 . Input variables for the controller  50  are battery voltage set points at  52 , battery voltage at  54  and battery current at  56 . As will be described subsequently, the controller  50  establishes an increment or a decrement in the battery power limits for a given set of operating conditions. This is shown at  58 . The changes are added to or subtracted from the battery charge or discharge limits at  46  to produce modified battery limits at  60 . Those modified battery limits are transferred to the comparator  62 , which corresponds to the comparator  34  of  FIG. 2  and the comparator  18  of  FIG. 1 . 
   The vehicle system controller  64  of  FIG. 3  corresponds to the vehicle system controller  26  of  FIG. 2 and 10  of  FIG. 1 . It receives driver commands at  66  and issues engine commands to engine  68  as shown at  70 . As in the case of the configuration of  FIG. 2 , controller  64  issues commands to the motor as shown at  72 , the motor being a part of the motor and transmission assembly  74 . The motor of the assembly  74  and engine  76  are power sources in power flow paths to a differential and axle half shaft assembly  78 . 
   Although  FIG. 3  shows a hybrid electric vehicle powertrain of the type shown in  FIG. 2 , the closed loop control  44  of  FIG. 3  could be used as well with a pure electric vehicle powertrain of the type shown in  FIG. 1 . 
   The closed loop control for the battery power limits shown at  44  in  FIG. 3  is illustrated in detail in  FIG. 4   a  for a battery discharge power limit control.  FIG. 4   b  is a schematic view, similar to  FIG. 4   a , which shows a closed loop control for regulating battery charge power limits. 
   In  FIGS. 4   a  and  4   b , the battery limits are the battery discharge and charge power limits, respectively, and the sign convention for the battery power load (discharge or charge) is as follows: discharging is positive and charging is negative. Battery discharge power limit, therefore, is a positive value, while the battery charge power limit is a negative value. Positive battery current means discharging, and negative battery current means charging. 
   In  FIG. 4   a , a battery discharge set point, which is established during calibration, is indicated at  80 . The actual battery voltage at any given instant is indicated at  82 . The actual battery voltage is subtracted from the set point value at summing junction  84 . A PI (proportional-integral) controller includes a proportional term (P term ) calculation at  86 . An integral term (I term)  calculation occurs at  88 . The PI controller may be of any type generally known in the art. 
   The P term  is added to the I term  at junction  90 . The sum of the P term  and the I term  is multiplied by a filtered battery current at  92 . The product of the voltage value at  90  and the value of the current at  92  is the power limit modification determined at  94 , which is tested at  62  to determine whether it is above or below the discharge power limit at  46 . The power limit modification, which will be described with respect to the flowcharts of  FIGS. 5 and 6 , is added to or subtracted from the battery power discharge power limit at  46 . This occurs at summing junction  96 , thereby producing a modified discharge power limit  60 . 
   Unlike  FIG. 4   a ,  FIG. 4   b  shows a comparison of the charge voltage boundary at  98  rather than a discharge voltage boundary at  80 . The actual battery voltage at  82 ′ is subtracted from the voltage at  98 . The difference is transferred to a proportional-integral (PI) controller corresponding to the PI controller of  FIG. 4   a.    
   The elements of the PI controller of  FIG. 4   b  corresponding to the elements of the PI controller of  FIG. 4   a  have been designated by similar reference numerals, although prime notations are added. In the case of  FIG. 4   b , the modified charge power limit is shown at  60 ′. In  FIG. 4   a , the modified charge power limit is shown at  60 . These modified power limits are tested at  62 , as described with respect to  FIG. 3 , to determine whether the discharge or charge limits are exceeded. 
     FIG. 5  shows a flowchart of an algorithm for determining a discharge power limit using closed loop control. The controller reads input variables at  96 . These variables include battery discharge power limit, battery discharge voltage set point, battery voltage and battery current. Also, the battery voltage limit modification and the integral term for the PI controller is set to zero. The index run number for the closed loop is set to one. 
   At action block  98 , a battery current is filtered using a low pass filter. The filter time constant can be varied depending upon the noise level of the signal. The battery current is clipped to a value greater than or equal to zero. 
   The next step in the routine indicated at  100  involves a calculation of the discharge voltage error. This was seen at  84  and  84 ′ in  FIGS. 4   a  and  4   b , respectively. 
   Having determined the discharge voltage error, the integral term and the proportional term for the PI controller of  FIG. 4   a  are calculated. In the calculation of the proportional term, the voltage error is clipped at action block  102 . The error signal is clipped to positive values so that the proportional term only modifies the battery discharge limit if the voltage is below the set point. The P term  then is calculated at action block  104  by multiplying the clipped error signal by the proportional gain K P . 
   In the calculation of the I term , it is first determined at decision block  106  whether the battery voltage limit modification is greater than the battery discharge power limit. If it is greater, the I term  is not updated. This will prevent further increases in the I term  by freezing the integrator and preventing the integrator from winding up (increasing the value of the I term ). If the battery limit modification is not greater than the battery discharge power limit, the I term  is updated at action block  108 . This is done by adding the I term  for the previous control loop to the product of the integration constant and the voltage error determined at  84  and  84 ′ in  FIGS. 4   a  and  4   b , respectively. If the present I term  becomes negative as shown at decision block  110 , the I term  is reset to zero at action block  112 . The previous I term  is stored at  114  and reset to the present I term . 
   Having determined the P term  and the I term , the power limit modification is calculated at action block  116 . Power limit modification is the sum of the proportional and integral terms multiplied by the clipped and filtered battery current. The battery current is low pass filtered with a calibratable filter time constant. This will allow the same PI controller with the same PI gains to be used when the internal battery resistance is higher (e.g., when the battery temperature is low). 
   If the battery power limit modification is less than the power discharge limit, as determined at decision block  118 , the modified power discharge limit is calculated at action block  124 . This is done by subtracting the power limit modification from the discharge power limit. If the battery power limit modification is greater than the battery discharge power limit, as determined at decision block  118 , the battery power limit modification is equal to the battery power discharge limit at action block  122 . The routine then proceeds to action block  124  where the modified discharge power limit is calculated, as explained previously. 
     FIG. 6  is a flowchart illustrating the controller routine for a closed loop controlled battery charge power limit. The routine is similar to the routine of  FIG. 5 . The steps in the routine of  FIG. 6  have been identified by reference numerals that correspond to the reference numerals used in  FIG. 5 , although prime notations are used. In the case of the routine of  FIG. 6 , the inputs that are read by the controller at the beginning of the routine are battery charge power limit, battery charge voltage set point, battery voltage and battery current. In step  98 ′, the battery current is low pass filtered and clipped. The absolute value of the clipped current is used. The end result of the routine of  FIG. 6  is the calculation of modified charge power limit rather than a calculation of modified discharge power limit, as in the case of the flowchart of  FIG. 5 . 
   Both during charging and discharging of the battery, the embodiment of the invention described above is capable of compensating for inaccuracy of the battery limits estimated by the battery controller. 
   Although an embodiment of the invention has been described, it will be apparent to a person skilled in the art that modifications may be made without departing from the scope of the invention. All such modifications and equivalents thereof are intended to be covered by the following claims.