Abstract:
A circuit arrangement and a method are described with which a current circuit can be monitored for correct function. A first and a second current sensor are provided in the current circuit. In a test phase, the second current sensor is switched to the current circuit. Next, a current flow is brought about through the first and the second current sensors, and the current intensity is measured via the first and second current sensors. From a comparison, an error function of the current sensor can be detected. The circuit arrangement according to the invention and the method according to the invention are particularly suitable for use in safety-oriented equipment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 USC 371 application of PCT/DE 03/00188 filed on Jan. 24, 2003. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a circuit arrangement with a current circuit and to a method for monitoring a current circuit. 
   2. Description of the Prior Art 
   Current circuits are used in the most various areas of technology for supplying consumers. In supplying a consumer, in particular a consumer in a motor vehicle, in which the consumer performs safety-oriented functions, monitoring the current supply is an essential aspect of safety. The current supply is necessary if the safety-oriented consumer is to be capable of functioning correctly, and this current supply must therefore be monitored with regard to a failure or an error mode. 
   It is already known in such safety-critical systems as an electromechanical brake in a motor vehicle to provide a current sensor in the current circuit and to detect the current flowing in the current circuit via the current sensor, compare it with a comparison value, and depending on the outcome of comparison, to detect an error function or correct function of the current circuit. For instance, if an electronically commutated motor is driven via the current circuit, then via the current sensor the phase current is measured, and the torque that the motor is to output is adjusted with the aid of a phase current regulator. At the same time, the capability of correct function is monitored via the current measurement by the current sensor. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   The object of the invention is to furnish a circuit arrangement and a method for supplying a consumer that enable reliable monitoring of the function of the current supply. 
   One essential advantage of the circuit arrangement of the invention is that a second current sensor is provided in the current circuit, and that via the second current sensor, the current flow in the current circuit can be detected redundantly. It is thus possible on the basis of the second current sensor to monitor correct functioning of the first current sensor. For instance, if in the embodiments of the prior art the first current sensor produces a faulty signal, this is not detected, at least immediately, as an error signal. In contrast, the invention offers the advantage of monitoring the current circuit independently of the first current sensor. This provides double safety for correct monitoring of the current circuit. 
   Preferably, the second current sensor can be switched into the current circuit, so that the second current sensor electrically affects the current circuit only when the current circuit is being monitored. Reliable monitoring is thus furnished without adversely affecting the electrical properties of the current circuit. 
   In a simple embodiment, a switch is provided by way of which the second current sensor can be coupled to the current circuit. Preferably, a further switch is provided, with which the consumer can be decoupled from the current circuit. In this way, it is assured that monitoring of the current circuit is possible without the consumer having any electrical influence on the current circuit. 
   Preferably, the second current sensor is embodied in the form of a resistor. A voltage drop across the resistor can be detected as a measurement variable, for assessing the current flow in the current circuit. Using a resistor as a current sensor is economical and makes a reliable method possible. 
   Preferably, an electric drive mechanism, especially a winding of the electric drive mechanism, is monitored with the circuit arrangement of the invention. The circuit arrangement of the invention is especially well suited to monitoring a safety-oriented consumer in a motor vehicle, such as an electromechanical brake. 
   In a preferred application, the second current sensor is placed between two high-side switches and the corresponding supply lines to windings of an electronically commutated motor. In this embodiment, a switch for turning the current sensor on or off can be omitted. By means of the connection selected, the current sensor has no perceptible effects on the operation of the motor. An economical embodiment of the circuit arrangement of the invention is thus possible, since the current sensor, particularly in the form of a resistor, can also be integrated as part of an integrated circuit. 
   Preferably, in monitoring the current flow, the second current sensor is connected in series with a winding of an electronically commutated motor. Thus besides monitoring the current flow, it is also possible to monitor the function of the winding of the motor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in further detail below in conjunction with the drawings, in which: 
       FIG. 1 , a schematic illustration of the circuit arrangement of the invention; and 
       FIG. 2 , a detail of a trigger unit for an electromechanical brake, in which the circuit arrangement of the invention is employed. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The fundamental principle of the invention can be employed in the most various fields of technology. For instance, safety-oriented circuits can be used in the fields of civil engineering, aeronautics, control technology, nuclear power plants, or automotive engineering. The circuit arrangement and the method of the invention, regardless of the field in which they are employed, offer enhanced safety in monitoring a current circuit. The invention will be described in further detail below taking as an example an electromechanical brake for a motor vehicle. 
   The invention will be described in further detail below taking as an example an electromechanical brake for a motor vehicle. 
     FIG. 1 , in a simplified illustration, shows the fundamental principle of the circuit arrangement of the invention. A direct-voltage source  1  is provided, which furnishes a fixed direct voltage. The direct-voltage source is in communication, via a current line  2  and a current sensor  5 , with an electric drive mechanism  3 . The electric drive mechanism  3  is for instance an electronically commutated motor, which has windings  4 . An electronically commutated motor is well known and is described for instance in “ Handbuch der elektrischen Anlagen und Maschinen ” [Manual of Electrical Systems and Machines] by Egbert Hering, published by Springer-Verlag, 1999, ISBN 3-540-65184-5, chapter A8.5.1.3, pages 203 ff. The drive  3  is operatively connected via a drive rod  6  with final control elements of an electromechanical brake  7 . Via the drive rod  6 , the drive  3  controls the mode of operation of the electromechanical brake  7 . The drive  3  can be connected to a ground potential via a first switch  9 , which is preferably embodied as a field effect transistor. The first switch  9  is in communication with a control unit  11  via a control line  10 . 
   Between the current sensor  5  and the drive  3 , a second current sensor  12  is connected to the current line  2 . The second current sensor  12 , in a simple embodiment, can be embodied as a defined resistor. One output of the second current sensor  12  can be connected to ground via a second switch  13 . The second switch  13  is in communication with the control unit  11  via a second control line  14 . The control unit  11  is also connected to the current sensor  5 . A memory  15  is also provided, in which control information and control fields are stored. The control unit  11  is furthermore connected to the drive  3  via a third control line  16 . 
   In a known manner, the control unit  11  controls the phase currents for the windings  4  of the electrically commutated motor  3  in accordance with the desired rpm and the desired torque that the motor  3  is to furnish to the electromechanical brake  7 . To that end, operating parameters of a motor vehicle  8  in which the circuit arrangement is disposed are taken into account. In addition, via the current sensor  5 , the control unit  11  detects the current supplied to the drive  3  via the current line  2 . The current is compared with reference values, and an error function of the current supply is detected if the detected current differs from the reference values by more than a predetermined differential value. 
   The input and output of the second current sensor  12  are connected to an A/D converter  17  via measurement lines  36 . One output  37  of the A/D converter  17  is carried to the control unit  11 . 
   For monitoring the current sensor  5  and the current supply of the drive  3 , the control unit  11 , in a monitoring phase, switches the resistor  12  to the current line  2 ; that is, the control unit  11  closes the second switch  13  and thus connects the output of the resistor  12  to ground. Preferably, the first switch  9  that is closed in normal operation is switched by the control unit  11  into an open position, so that current no longer flows via the drive  3  but solely through the second current sensor  12 . In the monitoring phase, the control unit  11  detects the voltage both at the input and at the output of the resistor  12 . From the voltage drop across the resistor  12 , the control unit  11  detects the current flowing through the current sensor  5 . 
   With the knowledge of the resistance of the resistor  12 , the control unit  11  calculates the current that is supplied to the drive  3 . The current ascertained via the second current sensor  12  is compared with the current ascertained by the current sensor  5 . If the comparison shows that the current ascertained by the current sensor  5  differs from the current that was calculated via the voltage drop of the resistor  12 , then the control unit  11  detects an error function of the current sensor  5  and outputs an error signal, for instance to a display system of the motor vehicle  8 . In addition, the control unit  11  can switch over to an emergency function, to assure safe stopping of the motor vehicle  8 . 
   The monitoring is preferably done cyclically, whenever the drive  3  is not needed to actuate the electromechanical brake  7 . 
     FIG. 2  shows a further advantageous embodiment of the circuit arrangement of the invention, which is preferably integrated with a trigger circuit for an electronically commutated motor  3 . Advantageously, the second current sensor  12  is also integrated into the trigger circuit  18 . An economical realization of the circuit arrangement can thus be achieved. The motor  3  is shown only schematically in the form of the three windings  22 ,  25 ,  27 . 
   A direct-voltage source  1  is provided, which can be connected to one input of the current sensor  5  via a third switch  38 . One output of the current sensor  5  is in communication with inputs of a first, second and third transistor  19 ,  20 ,  21 . One output of the first transistor  19  is connected to one input of a first winding  22 . One output of the first winding  22  is connected to one input of a fourth transistor  23 . One output of the fourth transistor  23  is in communication with a ground line  24 . 
   One output of the second transistor  20  is in communication with one input of a second winding  25 . One output of the second winding  25  is in communication with one input of a fifth transistor  26 . One output of the fifth transistor  26  is connected to the ground line  24 . One output of the third transistor  21  is connected to one input of a third winding  27 . One output of the third winding  27  is connected to one input of a sixth transistor  28 . One output of the sixth transistor  28  is connected to the ground line  24 . The outputs of the first, second and third windings  22 ,  25  and  27  are connected to the input of the current sensor  5  via a first, second and third diode  29 ,  30 , and  31 , respectively. The inputs of the first, second and third windings  22 ,  25 , and  27  are connected to the ground line  24  via a fourth, fifth and sixth diodes  32 ,  33 , and  34 , respectively. A capacitor  35  is connected between the ground line  24  and the input of the current sensor  5 . In addition, test resistors  39  are also connected parallel to the fourth, fifth and sixth diodes  32 ,  33 ,  34 . The control terminals of the transistors  19 ,  20 ,  21 ,  23 ,  26 ,  28  are connected to the control unit  11  via control lines. In a known manner, the control unit  11  controls the current supply to the windings  22 ,  25 ,  27 , in order for a desired rpm and/or a desired torque to be output to the electromechanical brake  7  via the drive rod  6 . 
   In the embodiment shown, the second current sensor  12  in the form of a resistor is disposed between the input of the first coil  22  and the input to the second coil  25 . The terminals of the second current sensor  12  are in communication with the A/D converter  17  of the control unit  11 , via measurement lines  36 . Thus the second current sensor  12  is incorporated into an existing current circuit without additional switches. Because of the disposition of the second current sensor  12  between the high-side field effect transistors  19 ,  20  of the first and second windings  22 ,  25 , respectively, the second current sensor  12  embodied as a resistor does not make itself annoyingly apparent in a normal motor commutation operation. 
   In a test of the current sensor  5 , which is preferably performed when the motor is stopped, the first transistor  19  and the fifth transistor  26  are made conducting. The other transistors  20 ,  21 ,  23 ,  28  are blocked. In this state of the circuit, a test current flows to the ground line  24  via the current sensor  5 , the first transistor  19 , the second current sensor  12 , the second winding  25 , and the fifth transistor  26 . The current flowing via the second current sensor  12  is calculated via the voltage drop detected across the second current sensor  12 . The resistance of the second current sensor  12  is known, and the current can thus be calculated from the equation
 
 U=R×I, 
 
where U stands for the voltage drop across the second current sensor  12 ; R stands for the resistance of the second current sensor  12 ; and I stands for the current intensity.
 
   Instead of the arrangement shown in  FIG. 2 , the second current sensor  12  can also be disposed between the input of the first and the input of the third winding  22 ,  27 , or between the input of the second and the input of the third winding  25 ,  27 . Depending on where the second current sensor  12  is disposed, suitable transistors should be made conducting in the monitoring process, so that a current flows across the second current sensor  12  and one winding. 
   In the test method, the current is detected both via the current sensor  5  and via the second current sensor  12 . By way of a comparison of the two currents detected, a defect of the first and/or second current sensor  5 , 12  can be recognized. 
   The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.