Abstract:
An electronic drive circuit for an impulse-controlled actor ( 10 ) comprises a first capacitor and a first thyristor ( 24 ). The first thyristor ( 24 ), after its ignition, permits a discharging of the first capacitor ( 18 ) via the actor ( 10 ).

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to an electronic drive circuit for an impulse-controlled actor.  
       BACKGROUND OF THE INVENTION  
       [0002]     In many devices relating to safety for vehicle occupant restraint systems, controlling- and/or switching processes take place which necessitate a particular energy requirement. To reduce this energy requirement and to shorten the switching times, impulse-controlled actors can be provided. Such actors are controlled with short current impulses; a continuous current to maintain a state is not necessary. The required switching power is thereby distinctly reduced.  
         [0003]     In an electronic drive circuit for an impulse-controlled actor, care is to be taken that the generated impulses are matched to the design of the actor in order to ensure a correct operation of the actor and hence of the device relating to safety. For example, in a bistable lifting magnet, a continuous current or current impulses which are too long could lead to damage owing to thermal overload.  
         [0004]     The invention provides a favourably priced electronic drive circuit for an impulse-controlled actor, with which a faulty operation of the actor can be largely ruled out.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     According to the invention, an electronic drive circuit for an impulse-controlled actor comprises a first capacitor and a first thyristor. The first thyristor, after its ignition, permits a discharging of the first capacitor via the actor. The discharging of the capacitor provides for a suitable current impulse, without a time-controlled application of a voltage to the actor being necessary. With a given voltage source, the capacitor can be coordinated precisely to the requirements of the actor. Furthermore, the invention advantageously utilizes the particular characteristics of a thyristor. In the circuit according to the invention, the thyristor is used such that it causes the discharging of the capacitor via the actor through its ignition. Thereby, a timed rapid emission of a current impulse is made possible. After the discharging of the capacitor, no more current flows through the thyristor, such that the latter blocks automatically.  
         [0006]     Preferably, the electronic drive circuit according to the invention further comprises a second capacitor and a second thyristor. The second thyristor, after its ignition, permits a discharging of the second capacitor via the actor. The flow of current upon discharge of the second capacitor is opposed to the flow of current upon discharge of the first capacitor. With such a circuit, positive and negative impulses can be emitted to the actor. This is necessary in bistable lifting magnets, in order to alternate between the two stable states.  
         [0007]     According to a particular further development of the invention, the electronic drive circuit comprises a third capacitor. The discharging of the third capacitor causes an ignition of the second thyristor. The third capacitor is connected such that a discharging of the first capacitor causes a charging of the third capacitor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIGS. 1   a ,  1   b  show a bistable lifting magnet in a first position and in a second position, respectively;  
         [0009]      FIG. 2  shows a circuit diagram of the electronic drive circuit according to the invention; and  
         [0010]      FIG. 3  shows the voltage curve on two components of the circuit of  FIG. 2 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0011]     In  FIGS. 1   a  and  1   b  a bistable lifting magnet  10  is illustrated, which is used as an actor in a safety device of a vehicle occupant restraint system. The bistable lifting magnet  10  has a housing  42  and a plunger  44  which is guided so as to be linearly displaceable. A permanent magnet  46  and a coil  48  are arranged in the housing  42  which is approximately 1 cm in size. The coil  48  can be provided with current via connection lines  50 . A compression spring  54  is arranged on the plunger  44  between a support surface  52  of the outer end of the plunger  44  and the housing  42 .  
         [0012]     The bistable lifting magnet  10  has two stable end positions, namely with the plunger  44  retracted ( FIG. 1   a ) and with the plunger  44  extended ( FIG. 1   b ), which are designated below as the first position and the second position, respectively. The first position is stable owing to the force of attraction between the plunger  44  and the permanent magnet  46 ; the second position is supported by the compression spring  54 . The stroke H of the plunger  44  amounts to approximately 2 mm.  
         [0013]     The bistable lifting magnet  10  is controlled via short current impulses which are generated by the electronic drive circuit according to the invention, which is explained later in more detail. In order to move the bistable lifting magnet  10  from the first position to the second, a current must be applied briefly to the coil  48  in a direction inducing a magnetic field which neutralizes the magnetic field of the permanent magnet  46  and exerts a repelling effect on the plunger  44 . The force of the compression spring  54  in the second position serves as a holding force for the plunger  44 , which is greater than the permanent magnet force of attraction occurring again after the current impulse. A current impulse in the opposite direction induces a magnetic field which is equidirectional to the magnetic field of the permanent magnet  46 . In this case, the force of attraction is sufficient to overcome the holding force of the compression spring  54 , such that the plunger  44  is retracted into the first position again.  
         [0014]     In  FIG. 2 a  circuit diagram is shown of the electronic drive circuit according to the invention for the bistable lifting magnet  10 . The circuit comprises a voltage source  12  and two control inputs  14 ,  16  (Port  1  and Port  2 ), via which control signals are fed in. A logic part, which is not illustrated, makes provision that a high signal is emitted at Port  2  when the bistable lifting magnet  10  is to be transferred from the first position into the second position. A high signal is emitted at Port  1  for a transfer of the bistable lifting magnet from the second position into the first position.  
         [0015]     The mode of operation of the electronic drive circuit is described below.  
         [0016]     The voltage source  12 , providing a supply voltage V cc , charges a first capacitor  18  and a second capacitor  20 . At a moment t=0, a high signal is emitted via the Port  2  to the gate of the first thyristor  24  via the voltage divider  22 . At the same time, the control signal of Port  1  is low; the transistor  26  is blocking and the voltage at the gate of the second thyristor  28  is 0. The first thyristor  24  ignites and allows the discharging of the first capacitor  18  via the actor (load) to the anode of the first thyristor  24 . The first thyristor  24  remains conducting until the first capacitor  18  has discharged. After this, the first thyristor  24  is blocking again.  
         [0017]     In the load circuit, an exponentially fading current flows accordingly in a positive direction, which is sufficient to move the plunger  44  of the bistable lifting magnet  10  from the first position into the second position. The switching time amounts to approximately 16 ms.  
         [0018]     The igniting of the first thyristor  24  additionally causes a shift of the electric potentials at the cathode of the second thyristor  28  and of the third capacitor  30  to −12 V (negative supply voltage V cc  of the voltage source). This leads automatically to a charging of the third capacitor  30  via the charging resistance  32 , the charging time amounting to approximately 150 ms.  
         [0019]     Subsequently, if required, a signal change can take place at the control inputs  14 ,  16 , in order to retract the plunger  44  of the bistable lifting magnet  10 . In this case, the logic part of the circuit makes provision that a high signal is emitted at Port  1 , whilst the control signal at Port  2  is low. The transistor  26  is driven through, such that the third capacitor  30  discharges via the resistance  34  onto the gate of the second thyristor  28  and ignites the latter. In the meantime, the first thyristor  24  is blocking again owing to the previous discharge of the first capacitor  18 . The igniting of the second thyristor  28  therefore leads to a discharge of the second capacitor  20  via the actor.  
         [0020]     An exponentially fading current now flows in the load circuit in a negative direction, which is sufficient to move the plunger  44  of the bistable lifting magnet  10  from the second position into the first position.  
         [0021]      FIG. 3  shows the voltage curve at the second thyristor  28  and at the third capacitor  30 .  
         [0022]     A particular advantage of the electronic drive circuit according to the invention lies in the intrinsic security of the switching arrangement. Even if an error occurs in the logic part (software errors or the like), it is ensured that the actor is only operated with current impulses but never with a continuous current.