Abstract:
A circuit system and a method for operating an electric motor having variable speed from a direct-voltage source is proposed, in particular for operating a cooling air fan motor of a motor vehicle from the vehicle&#39;s electrical system. The circuit system uses pulse-width-modulated input signals that are provided by a control unit, and enables, through integration and threshold value comparison of the signals, an operation of the electric motor with different speeds based on differentiated series resistor connection, corresponding to the pulse-duty ratio of the pulse-width-modulated input signals.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a circuit system and to a method for operating an electric motor having variable speed from a direct-voltage source, in particular for operating a cooling air fan motor of a motor vehicle from its vehicle electrical system.  
       BACKGROUND INFORMATION  
       [0002]     Operating systems for electric motors, in particular for brush motors of cooling air fans for motor vehicles, are known in principle, the setting of the speed of the motor taking place either via pulse-width-modulated (PWM) signals and clocked regulators or via switched motor current circuits having different series resistors. The standard design for motor controlling by clocked regulators is to supply the motor voltage using a microcontroller, arbitrarily many operating states of the motor being capable of being realized in this way. In supplying the motor with voltage via different series resistors, switches or relays are used that are controlled directly with a voltage signal and that enable motor operation via separate current circuits having different series resistors, with respectively predetermined speeds. Here, as a rule the desired motor speed is predetermined by thermal switches.  
         [0003]     The design using separate motor switching circuits has the advantage of low cost, as long as only approximately two different speed levels are required. In contrast, the clocked controlling of the motor using pulse-width-modulated signals offers many possibilities in the allocation of an advantageous motor speed; however, the technical and economical outlay for such circuit systems is considerable, because due to the complexity of the evaluation of the PWM signals it is generally necessary to use microcontrollers here.  
       SUMMARY OF THE INVENTION  
       [0004]     The exemplary embodiments and/or exemplary methods of the present invention is directed to providing a technically advantageous and economically favorable possibility for operating an electric motor with different speeds from a direct-voltage source, enabling the selection of a larger number of speed levels using an economical circuit system. This is achieved by the features of the systems described herein, which enable a circuit arrangement and a motor operation using a few discrete constructive elements without a microcontroller.  
         [0005]     For the conversion of the pulse-width-modulated input signals of the converter, a simple integration circuit having few components can advantageously be used that is on the one hand connected to a voltage source and on the other hand is connected to the output of the control unit for the electric motor that supplies the pulse-width-modulated signals. Such a device enables the economical provision of a sawtooth-shaped integration signal, corresponding to the pulse and pause times of the pulse-width-modulated output signal of the control unit. A particularly simple circuit system is obtained if the terminal away from ground of a capacitor of the integration device of the converter is connected via an ohmic resistance to its reference voltage source, and is additionally connected via a decoupling diode and a decoupling resistor to the output of the control unit, the output of the control unit being capable of being connected directly to ground via the collector-emitter path of a transistor. Here, the discharge resistor of the capacitor can simultaneously be a part of its charge circuit.  
         [0006]     In addition, the circuit system according to the exemplary embodiments and/or exemplary methods of the present invention can advantageously be fashioned in such a way that the terminal away from ground of the capacitor of the integration device of the converter is connected directly to an input of each of a plurality of comparators connected in parallel, whose reference inputs are charged with different threshold value voltages. The outputs of the comparators can then each control a relay via a respective amplifier, for example in the form of a transistor, the switching contacts of each relay being on the one hand connected in parallel to a terminal of the electric motor and on the other hand separated via series resistors, or connected directly to ground. Thus, corresponding to the pulse-duty ratio of the PWM signals at the input of the converter, a current circuit of the electric motor having more or less strong resistance is connected to ground, while the other terminal of the electric motor is connected permanently to the positive pole of the voltage source. Corresponding to the pulse-duty ratio of the PWM signal at the input of the converter, the switching thresholds of a plurality of comparators are exceeded, so that the resulting resistance in the motor current circuit results from the connection in parallel of different series resistors.  
         [0007]     Furthermore, it has turned out to be particularly advantageous for the integration device and the various comparators to be connected to the same voltage source. There nonetheless results, for example given voltage fluctuations in the electrical system of a motor vehicle, an unfalsified control signal for the electric motor, because a change in the common supply voltage has the same effect on the level of the integration signal and on the switching threshold of the comparators, so that the motor speed predetermined by the pulse-duty ratio of the PWM input signal is maintained.  
         [0008]     Further details and constructions of the exemplary embodiments and/or exemplary methods of the present invention are described herein as to the exemplary embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]     The FIGURE depicts a circuit system for operating a cooling air fan motor of a motor vehicle from the vehicle&#39;s electrical system. 
     
    
     DETAILED DESCRIPTION  
       [0010]     In the FIGURE,  10  designates an electric motor that is mechanically coupled to a cooling air fan  12  and is supplied via brushes  14 ,  16  from the electrical system of a motor vehicle. The positive pole of the voltage source is designated  18  and the ground terminal is designated  20 . While brush  14  of electric motor  10  is connected permanently to positive pole  18  of the voltage source, brush  16  is connected to ground terminal  20  via a plurality of current circuits that are connected in parallel via relay contacts  22   a ,  24   a , and  26   a . Relay contacts  22   a ,  24   a , and  26   a  are actuated by relays  22 ,  24 , and  26 , which are each switched via a respective amplifier  22   b ,  24   b ,  26   b , each in the form of a transistor. Here, relay contact  22   a  is connected directly to ground  20 , relay contact  24   a  is connected to ground via a resistor  30  and relay contact  26   a  is connected to ground via a resistor  32 . The resistance values of resistors  30  and  32  can be the same or different. If standard cooling air fan motors connected to the 12-volt electrical system of a motor vehicle are used, these values are in the range from approximately 100 mΩ to approximately 400 mΩ. Given the use of equal resistors  30  and  32 , the different speeds of electric motor  10  result from the fact that when the higher switching threshold is achieved at comparator  66 , a lower overall resistance results due to the parallel connection of resistor  30  to resistor  32 .  
         [0011]     The dashed interruption of the current circuit from brush  16  to ground pole  20  is intended to indicate that the drawing merely represents an exemplary embodiment, and that instead of three relays it is also possible to use a larger number of relays, with a larger number of armature current circuits having different resistances.  
         [0012]     The target values of the respective motor speed are predetermined by the pulse-duty ratio of the PWM signals at the input of the circuit system according to the exemplary embodiments and/or exemplary methods of the present invention. This circuit system supplies a control unit  34  (shown only schematically in the drawing) having a transistor  36  connected to ground at its output  39 , to whose collector the pulse-width-modulated signals (PWM signals)  38  are adjacent for controlling the speed of electric motor  10 . Such a transistor functioning as a switch to ground is designated an “open-collector transistor.” 
         [0013]     PWM signals  38  then travel via a line  40  to input  42  of integration device  44  of a converter  46 . Integration device  44  is adjacent to positive pole  18  of the voltage source, which is connected to input  42  of the integration device via a resistor  49  and a connection point  41 . Connection point  41  is simultaneously adjacent to the cathode of a decoupling diode  50 , whose anode is connected to a tap  52  between two resistors  54  and  56  in the charge or discharge circuit of capacitor  58 . One electrode of capacitor  58  is connected to ground  20 , and its other electrode is connected via the two resistors  54  and  56  to the positive pole  18  of the voltage source, or to the anode cf diode  50 . At terminal  60  of capacitor  58  oriented away from ground  20  of the voltage source, there then appears a sawtooth signal  62  which is dependent, with respect to its direct voltage portion and its shape, on the pulse-duty ratio of PWM signal  38 .  
         [0014]     The evaluation of sawtooth signal  62  takes place through a plurality of comparators  64 ,  66 , and  68  that are connected in parallel at the input side, and that at the output side individually control relays  22 ,  24 , and  26  via amplifiers  22   b ,  24   b , and  26   b , here in the form of transistors. In principle, comparators  64 ,  66 , and  68  have the same arrangement, each having an operational amplifier  70 ,  72 , and  74 , and each having a voltage divider, adjacent on the one hand to positive pole  18  and on the other hand to ground pole  20  of the voltage source, having different divider resistors. If, for example, divider resistors  82 ,  84 , and  86 , each adjacent to positive potential  18 , each measure 20 kΩ, then divider resistors  76 ,  78 , and  80 , each connected to ground  20 , could respectively measure 10 kΩ, 5 kΩ, and 3 kΩ, in order to define suitable different switching thresholds at the respective inverting input of operational amplifiers  70 ,  72 , and  74 .  
         [0015]     The circuit system operates as follows:  
         [0016]     At the output of control unit  34 , or at the collector of transistor  36 , there appear PWM signals  38  whose level changes between the potential of voltage source  18  and ground potential. The pulse-duty ratio of PWM signals  38 , i.e., the respective pulse or pause duration, determines the speed of electric motor  10 ; in the present case, a higher speed of motor  10  is allocated to a longer pulse duration of PWM signal  38 . Corresponding to the curve of PWM signals  38 , the potential at connecting point  41  of resistor  49  to the cathode of diode  50  also fluctuates between the potential of positive pole  18  of the voltage source and ground. As long as the potential at connecting point  41  corresponds to the potential of the voltage source, capacitor  58  is charged via the series circuit of resistors  54  and  56 . The charging process ends when ground potential appears at connecting point  41 , and capacitor  58  discharges via partial resistor  56  of its charge circuit and via diode  50 , until the positive edge of PWM signal  38  again appears at connecting point  41 . At terminal  60  of capacitor  58 , there arises a sawtooth-shaped direct voltage whose mean value increases with the pulse-duty ratio of PWM signals  38 .  
         [0017]     Sawtooth voltage  62 , whose level is variable, is now simultaneously adjacent to the inputs of the three comparators  64 ,  66 , and  68 ; here, only those operational amplifiers  70 ,  72 ,  74  switch through whose reference signal at the inverting input is lower than the sawtooth signal  62  at the non-inverting input. This means that given a small pulse-duty ratio of PWM signals  38  at the input of converter  46 , or at the input of integration device  44 , only the comparator  68  having the lowest threshold voltage corresponding to the ratio of its divider resistors  80  and  86  switches through, thus closing relay contact  26   a . Because series resistor  32  in the armature current circuit of electric motor  10  represents the greatest possible series resistance, electric motor  10  now rotates with the lowest speed. As the level of sawtooth voltage  62  increases, comparator  66  also subsequently switches through, and activates, via amplifier  24   b , relay  24  in order to close relay contact  24   a . The resistance value of the parallel circuit of resistors  30  and  32  is now smaller than that of resistor  32  alone, so that a higher speed results for electric motor  10 . Correspondingly, given a further increase in the level of sawtooth signal  62 , relay  22  is activated via comparator  64  and amplifier  22   b , and relay contact  22   a  is closed, so that the motor is connected to ground without a series resistor and achieves its maximum speed, independent of whether relay contacts  24   a  and  26   a  are closed. On the other hand, relay contacts  22   a ,  24   a , and  26   a  can also be realized so as to be able to be switched separately, so that when relay  24  responds relay contact  26   a  is opened, and only series resistor  30  is effective in the motor current circuit. If necessary, intermediate stages for the motor speed can also be created through optional single or parallel switching of the series resistors. In this case, the value of resistor  30  is dimensioned smaller than the value of resistor  32 .  
         [0018]     The number of three comparators  64 ,  66 , and  68 , is selected only as an example. This number, and thus the number of predeterminable speed stages of electric motor  10 , can also be selected to be greater; an upper limit in terms of cost results from a comparison of the costs of the circuit according to the exemplary embodiments and/or exemplary methods of the present invention with a clocked regulation using a microcontroller.  
         [0019]     In sum, the circuit system according to the exemplary embodiments and/or exemplary methods of the present invention can be characterized in that with a low circuit outlay the information contained in the pulse-duty ratio of PWM signal  38  is economically used to set the speed of the motor via the control value of the motor. Here, from input-side PWM signal  38  output signals are generated directly without using a microcontroller, and in the exemplary embodiment the signals are used to control relays. Instead of relays, of course, it is also possible to use other switching devices, such as for example MOSFETs, in the motor current circuits.