Abstract:
When a switch ( 20 ) that is in series with the inductive load ( 10 ) is turned on, the rise behavior of the current (I) is first of all detected until a nominal value (Iref) is reached, and a pulse duty factor (Te/Ta) of a pulse width modulator device ( 30 ) is determined from said rise behavior. Immediately after the nominal value (Iref) of the current (I) has been reached, the switch ( 20 ) is driven in a conventional manner by the pulse width modulator device ( 30 ), using the pulse duty factor (Te/Ta) that has been determined, in order to finely adjust the pulse duty factor (Te/Ta). The invention largely avoids overshoots during control and the adjustment time is thus considerably shorter than in conventional drive circuits.

Description:
FIELD OF THE INVENTION 
   The invention relates to a method for driving an inductive load in a pulse-width-modulated manner, and to a drive circuit for this purpose. 
   BACKGROUND 
   DE 198 24 761 A1, for example, discloses such a method and such a drive circuit. 
     FIG. 1  shows the basic design of such a known drive circuit. The series circuit of an inductive load  10  and a switch  20  is arranged between supply terminals  1 ,  2 . The inductive load  10  may, for example, be a solenoid valve of an injection pump in diesel motor vehicles. Also in series with the inductive load  10  are a non-reactive resistor  12  and a further resistor  14  which is used as a current measuring resistor in order to detect the current flowing through the inductive load  10 . The anode connection of a diode  16  (whose cathode connection is connected to the supply terminal  2 ) may be connected between the junction point of the switch  20  and the measuring resistor  14 . By way of example, the positive pole of a voltage supply is connected to the supply terminal  2  and reference-ground potential of the voltage supply is connected to the supply terminal  1 . 
   So that the current I flowing through the inductive load  10  can follow a prescribed nominal current Iref as accurately as possible, the known circuit arrangement of  FIG. 1  has a pulse width modulator device  30  which turns the switch  20  on and off in a pulse-width-modulated manner. On the input side, the pulse width modulator device  30  has a PI controller  32  which has a pulse width modulator  34  connected downstream of it. A current error signal Ierr which is formed from the difference between the nominal current Iref and the actual current Iact measured across the measuring resistor  14  is supplied to the input of the PI controller  32 . Since, in the circuit arrangement shown in  FIG. 1 , the nominal current Iref is in the form of a digital value, for example in the form of an 18-bit value, it is likewise necessary to convert the analog value (ascertained using the measuring resistor  14 ) of the actual current Iact into a digital value. An amplifier  22  which is connected, on the output side, to an analog/digital converter  24  and is connected to the measuring resistor  14  is used for this purpose. The digital value for the actual current Iact is available at the output of the analog/digital converter  24 . 
   The problem with such drive circuits is the fact that they are optimized only for a single load situation. This means that an optimum transient response is ensured only in the case of a particular load situation. The actual current through the inductive load follows the prescribed nominal current only in this particular load situation. If, however, the load conditions change, the transient time may vary considerably. In particular, overshooting may result during control, which considerably extends the adjustment time in a disadvantageous manner. This relatively long overshooting is observed, in particular, in the case of large current jumps of the nominal current. 
   SUMMARY 
   The aim of the present invention is to avoid these disadvantages and to specify a method and a drive circuit in the case of which the transient response of the actual current is considerably improved in comparison with conventional methods and drive circuits. 
   The invention is essentially based on the fact that, after the prescribed nominal current has been reached, the pulse width modulator device turns the switch on and off using a pulse duty factor that was determined from the current rise behavior when the current through the inductive load was turned on or from the current decay behavior when the current through the inductive load was turned off. 
   A fundamental part of the present invention is an assessment circuit which is preferably digital and determines the pulse duty factor of the pulse width modulator device in the drive circuit in accordance with the current gradient of the actual current through the inductive load. 
   In this case, the drive circuit according to the invention operates in two different operating modes, namely in a conventional control mode in which the switch is controlled using a PI controller and a downstream pulse width modulator, as explained in connection with  FIG. 1 . In the second operating mode, the switch is turned on and off directly by the digital assessment circuit. 
   The method according to the invention functions as follows. 
   If a rise or fall in the nominal current by a prescribed value is detected, the switch that is in series with the inductive load is turned on or off by the assessment circuit. This means that the assessment circuit provides a pulse duty factor of 100% or 0%. In this case, the assessment circuit can influence the switch  20  directly or can transmit a corresponding control signal to the PI controller. If the nominal current rises or falls by the prescribed level, the switch that is in series with the inductive load is turned on or off. As a result, the current through the inductive load rises or falls at the maximum rate. When the measured current through the inductive load reaches the nominal value, the pulse duty factor of the pulse-width-modulated drive signal for the switch is established on the basis of the current rise or fall behavior. When the period of the pulse-width-modulated drive signal for the switch next starts, this established value is written to the PI controller, and the switch that is in series with the inductive load is then driven in a conventional manner using the control loop (explained in connection with  FIG. 1 ). In this conventional method of operation, the control loop (explained in connection with  FIG. 1  and also provided in the invention) ensures that the pulse duty factor is finely adjusted and finely readjusted should there be small discrepancies between the actual current through the inductive load and the nominal current on account of calculation inaccuracies. 
   The algorithm for determining the pulse duty factor that is needed to keep the average current at a particular value uses the gradient of the current through the inductive load, if the pulse duty factor was set to 100% for the first time, and that gradient which the current through the inductive load has when the nominal current is reached. 
   In this case, the pulse duty factor can be determined from the quotient (k 0 −k 1 )/k 0 , where k 0  is the gradient of the current at time t=0 and k 1  is the gradient of the current when the prescribed nominal value is reached. The pulse duty factor can also be determined from a quotient that is proportional thereto. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail below in connection with an exemplary embodiment and a plurality of figures, in which: 
       FIG. 1  shows a known drive circuit for driving inductive loads in a pulse-width-modulated manner, 
       FIG. 2  shows an example of a drive circuit for driving pulse-width-modulated inductive loads in accordance with the invention, 
       FIG. 3  shows timing diagrams for the drive circuit of  FIG. 2 , 
       FIG. 4  shows an enlarged detail from the timing diagram of  FIG. 3  in the time range when the actual current reaches the nominal current, 
       FIG. 5  shows the transient response of the actual current through the inductive load, in a circuit arrangement according to the invention (shown in  FIG. 2 ) in comparison with a known drive circuit (shown in  FIG. 1 ), in the case of a current jump from 0 to 2 amperes, if the inductive load has an inductance of 16 mH, and 
       FIG. 6  shows a similar illustration to  FIG. 5 , provision being made of a current jump from 0 to 600 mA, and the inductive load having an inductance of 9 mH. 
   

   DETAILED DESCRIPTION 
   In the following figures, unless specified otherwise, identical reference symbols designate identical parts with the same meaning. 
     FIG. 2  shows a circuit arrangement for driving an inductive load  10 . The circuit arrangement largely corresponds to the drive circuit of  FIG. 1  but has been extended by two decisive circuit components. The reference symbols which have already been disclosed continue to represent the elements which have already been explained. In addition, the circuit arrangement of  FIG. 2  has an assessment circuit  36  which is connected, on the input side, to the output of the adder  26 . On the output side, the assessment circuit  36  is connected to a switching device  40  and controls the latter. In addition, the assessment circuit  36  has a control line  38  that couples the assessment circuit  36  to the pulse width modulator device  30 . In the exemplary embodiment shown in  FIG. 2 , the control line  38  is connected to the PI controller  32  of the pulse width modulator device. 
   The switching device  40  can be used to apply three different signals to the switch  20 . The first changeover terminal of the switching device  40  is connected to the supply terminal  1  and is accordingly at reference-ground potential. The second input terminal of the switching device  40  is connected to the output of the pulse width modulator  34 , and a positive supply voltage is applied to the third changeover terminal of the switching device  40 . Changing over the switching device  40  between these three signals is controlled by the assessment circuit  36 . Depending on which control signal is available at the output of the assessment circuit  36 , the switching device  40  is connected to the supply terminal  1  that is at reference-ground potential, with the result that the switch  20  is permanently open. If the assessment circuit  36  is used to connect the switching device  40  to the supply terminal  2 , this ensures that the switch  20  is permanently closed. If, however, the assessment circuit  36  changes the switching device  40  in such a manner that the output of the pulse width modulator  34  is connected to the switch  20 , the switch  20  is turned on and off in a pulse-width-modulated manner using the pulse duty factor determined by the pulse width modulator  34 . 
   The entire drive circuit shown in  FIG. 2  is preferably of digital design. 
   The method of operation of the drive circuit shown in  FIG. 2  becomes clear in connection with the timing diagrams in  FIG. 3  and  FIG. 4 . 
   In the timing diagram shown, various curve profiles are shown as a function of time. Successive intervals of time c 0 , c 1 , c 2  . . . c 6  are indicated on the horizontal time axis. Each interval of time lasts one clock period T of the pulse width modulator  34 . An abrupt rise in the nominal current Iref is shown, by way of example, at the top of the timing diagram shown. As shown in  FIG. 3 , the nominal current jumps from a value of 0 to a value of X. 
   The circuit arrangement of  FIG. 2  results in the current I (which flows through the inductive load  10 ) following the profile of the nominal current Iref in a relatively rapid manner and largely without overshoots. At time t 0 , at which the current jump of the nominal current Iref rises from 0 to the value X, a fault current Ierr which, in terms of magnitude, corresponds to the value X but which has the opposite mathematical sign is applied to the output of the adder  26 . The assessment circuit  36  ensures that the switching device  40  connects the supply terminal  2  to the switch  20 , so that the latter is closed. Under the control of the assessment circuit  36 , the switch  20  remains closed until time t 1 . The current I through the inductive load  10  can rise exponentially in accordance with the charging curve. After time t 1 , the assessment circuit  36  allows the current I to rise further until time t 2 . Time t 2  is determined by the exponential rise in the current I that intersects the output signal of the pulse width modulator, said output signal running in the background. At time t 2 , the assessment circuit  36  ensures that the switching device  40  connects the output signal of the pulse width modulator  34  to the switch  20 . As of this time t 2 , the pulse width modulator  34  thus controls the switch  20 . In this conventional method of operation, the control loop (already explained in connection with  FIG. 1  and also provided in the invention) ensures that the pulse duty factor is finely adjusted and finely readjusted should there be small discrepancies between the actual current through the inductive load and the nominal current on account of calculation inaccuracies. 
   In addition to the signal profiles of the nominal current Iref, the current I through the inductive load  10  and the fault current Ierr, the switching states of the switching device  40  and of the switch  20  are also shown graphically in  FIG. 3 . 
     FIG. 4  shows an enlarged illustration of the profile of the current I through the inductive load  10  at times t 1  and t 2 . 
   So that the current through the inductive load  10  keeps, on average, the nominal value Iref to be achieved, when it is being controlled by the pulse width modulator  34 , the following algorithm is used in the assessment circuit. At time t 0 , the gradient k 0  of the rise in the current I is determined. In addition, at time t 1 , the gradient k 1  of the current rise is established. These two values k 0  and k 1  are used to determine the pulse duty factor Te/Ta of the pulse width modulator  34  in accordance with the following formula: 
   
     
       
         
           
             Pulse 
             ⁢ 
             
                 
             
             ⁢ 
             duty 
             ⁢ 
             
                 
             
             ⁢ 
             factor 
           
           = 
           
             
               Te 
               / 
               Ta 
             
             = 
             
               
                 
                   k0 
                   - 
                   k1 
                 
                 k0 
               
               . 
             
           
         
       
     
   
   This formula is derived as follows: 
   Curve profile for current rise: 
   
     
       
         
           
             
               
                 
                   y 
                   ⁡ 
                   
                     ( 
                     t 
                     ) 
                   
                 
                 = 
                 
                   A 
                   · 
                   
                     ( 
                     
                       1 
                       - 
                       
                         ⅇ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           
                             - 
                             t 
                           
                           τ 
                         
                       
                     
                     ) 
                   
                 
               
             
           
           
             
               
                 
                   
                     y 
                     ′ 
                   
                   ⁡ 
                   
                     ( 
                     t 
                     ) 
                   
                 
                 = 
                 
                   
                     
                       A 
                       τ 
                     
                     · 
                     ⅇ 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     
                       - 
                       t 
                     
                     τ 
                   
                 
               
             
           
         
       
     
   
   Values at time t=0: 
   
     
       
         
           
             
               
                 
                   y 
                   ⁡ 
                   
                     ( 
                     0 
                     ) 
                   
                 
                 = 
                 0 
               
             
             
               
                   
               
             
             
               
                   
               
             
             
               
                 
                   
                     y 
                     ′ 
                   
                   ⁡ 
                   
                     ( 
                     0 
                     ) 
                   
                 
                 = 
                 
                   A 
                   τ 
                 
               
             
           
         
       
     
   
   Values at the present time: 
   
     
       
         
           
             
               
                 
                   y 
                   ⁡ 
                   
                     ( 
                     tpres 
                     ) 
                   
                 
                 = 
                 
                   A 
                   · 
                   
                     ( 
                     
                       1 
                       - 
                       
                         ⅇ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           
                             - 
                             tpres 
                           
                           τ 
                         
                       
                     
                     ) 
                   
                 
               
             
           
           
             
               
                 
                   
                     y 
                     ′ 
                   
                   ⁡ 
                   
                     ( 
                     tpres 
                     ) 
                   
                 
                 = 
                 
                   
                     
                       A 
                       τ 
                     
                     · 
                     ⅇ 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     
                       - 
                       tpres 
                     
                     τ 
                   
                 
               
             
           
         
       
     
   
   Convert equation for y: 
   
     
       
         
           
             
               
                 
                   y 
                   ⁡ 
                   
                     ( 
                     tpres 
                     ) 
                   
                 
                 = 
                 
                   A 
                   · 
                   
                     ( 
                     
                       1 
                       - 
                       
                         
                           τ 
                           A 
                         
                         · 
                         
                           
                             y 
                             ′ 
                           
                           ⁡ 
                           
                             ( 
                             tpres 
                             ) 
                           
                         
                       
                     
                     ) 
                   
                 
               
             
           
           
             
               
                 = 
                 
                   A 
                   - 
                   
                     τ 
                     · 
                     
                       
                         y 
                         ′ 
                       
                       ⁡ 
                       
                         ( 
                         tpres 
                         ) 
                       
                     
                   
                 
               
             
           
           
             
               
                 = 
                 
                   A 
                   - 
                   
                     
                       A 
                       
                         
                           y 
                           ′ 
                         
                         ⁡ 
                         
                           ( 
                           0 
                           ) 
                         
                       
                     
                     · 
                     
                       
                         y 
                         ′ 
                       
                       ⁡ 
                       
                         ( 
                         tpres 
                         ) 
                       
                     
                   
                 
               
             
           
           
             
               
                 = 
                 
                   A 
                   · 
                   
                     ( 
                     
                       1 
                       - 
                       
                         
                           
                             y 
                             ′ 
                           
                           ⁡ 
                           
                             ( 
                             tpres 
                             ) 
                           
                         
                         
                           
                             y 
                             ′ 
                           
                           ⁡ 
                           
                             ( 
                             0 
                             ) 
                           
                         
                       
                     
                     ) 
                   
                 
               
             
           
           
             
               
                 = 
                 
                   A 
                   · 
                   
                     ( 
                     
                       1 
                       - 
                       
                         kpres 
                         k0 
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
   
   Final value: 
   
     
       
         
           A 
           = 
           
             ypres 
             
               1 
               - 
               
                 kpres 
                 k0 
               
             
           
         
       
     
   
   Pulse duty factor: 
           =       Inom   A     =     Inom     ypres     1   -     kpres   k0                   
Use for Current Controllers:
 
   Only when the current rises are k 0 , kpres and ypres measured continuously and the value for A continuously updated (or averaged): 
   
     
       
         
           A 
           = 
           
             ypres 
             
               1 
               - 
               
                 kpres 
                 k0 
               
             
           
         
       
     
   
   When the current rises and falls, the pulse duty factor is calculated using the following formula: 
   
     
       
         
           
             Pulse 
             ⁢ 
             
                 
             
             ⁢ 
             duty 
             ⁢ 
             
                 
             
             ⁢ 
             factor 
           
           = 
           
             Inom 
             A 
           
         
       
     
   
   When the current rises and the nominal current has been reached, Inom=ypres. In this case, the duty cycle can therefore also be calculated in accordance with the following formula: 
   
     
       
         
           
             Pulse 
             ⁢ 
             
                 
             
             ⁢ 
             duty 
             ⁢ 
             
                 
             
             ⁢ 
             factor 
           
           = 
           
             duty_pres 
             = 
             
               
                 Inom 
                 
                   ypres 
                   
                     1 
                     - 
                     
                       kpres 
                       k0 
                     
                   
                 
               
               = 
               
                 
                   k0 
                   - 
                   kpres 
                 
                 k0 
               
             
           
         
       
     
   
   The fundamental advantage of this method is that all information for determining the pulse duty factor Te/Ta is available at any time for the current I. This is important because delays in calculating the pulse duty factor after the nominal current Iref has been reached may lead to overshoots during control, and the adjustment time would therefore be extended considerably. 
     FIG. 5  compares the transient response of a conventional drive circuit (shown in  FIG. 1 ) with the transient response of a drive circuit according to the invention (shown in  FIG. 2 ). In this case, it is assumed that the nominal current Iref jumps from 0 A to 2 A and the inductive load has an inductance of 16 mH. Curve profile A shows the transient response of a conventional drive circuit (shown in  FIG. 1 ) and curve profile B shows the transient response in the case of the method according to the invention using a drive circuit as shown in  FIG. 2 . It can clearly be seen that curve profile B overshoots to a lesser degree than curve profile A. In addition, the nominal value is reached more quickly in the case of curve B. 
     FIG. 6  shows a similar curve profile. However, provision is made of a current jump from 0 A to 600 mA, and the inductive load has an inductance of 9 mH. It can be seen that, in accordance with curve profile B, the value of the nominal current is likewise achieved more quickly with the drive circuit according to the invention or with the use of the method according to the invention, even though this is less pronounced than in the case of larger current jumps of the nominal current. 
   
     
       
             
           
             
             
           
         
             
                 
             
             
               List of reference symbols 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               1 
               Supply terminal 
             
             
               2 
               Supply terminal 
             
             
               10 
               Inductive load 
             
             
               12 
               Resistor 
             
             
               14 
               Resistor 
             
             
               16 
               Diode 
             
             
               20 
               Switch 
             
             
               22 
               Amplification 
             
             
               24 
               A/D converter 
             
             
               26 
               Adder 
             
             
               30 
               Pulse width modulator device 
             
             
               32 
               PI controller 
             
             
               34 
               Pulse width modulator 
             
             
               36 
               Assessment circuit 
             
             
               38 
               Control line 
             
             
               40 
               Switch 
             
             
               T 
               Clock period 
             
             
               Te 
               Turned-on time 
             
             
               Ta 
               Turned-off time 
             
             
               t0 
               Time 
             
             
               t1 
               Time 
             
             
               t2 
               Time 
             
             
               Iref 
               Nominal current 
             
             
               I 
               Current 
             
             
               Ierr 
               Fault current 
             
             
               IPP 
               Current span 
             
             
               c0 . . . c6 
               Clock periods 
             
             
               k0 
               Gradient at t0 
             
             
               k1 
               Gradient at t1 
             
             
               Iact 
               Actual current 
             
             
               X 
               Nominal current value