Method and circuit arrangement for monitoring the operating state of a load

A invention relates to a method for monitoring the operating state of a load which is driven cyclically by a control signal, especially for an electric motor in a motor vehicle, the cyclic drive being provided by a pulse-width-modulated drive signal. In a method which permits the detection of the operating state of the load as well as driving the load in accordance with the situation on the basis of the operating state, the drive signal, whose pulse width is varied in order to limit the current flow through the load, is evaluated in order to determine the operating state of the load.

FIELD AND BACKGROUND OF THE INVENTION
 The invention relates to a method for monitoring the operating state of a
 load which is driven cyclically by a control signal, especially for an
 electric motor in a motor vehicle, the cyclic drive being provided by a
 pulse-width-modulated drive signal, and to a circuit arrangement for
 implementing the method.
 During the dynamic operation of loads, such as an electric motor in a
 control loop, the load is driven with pulse-width-modulated signals.
 During this dynamic operation it is necessary to accelerate the electric
 motor, to brake it or else to change the direction of rotation.
 In the event of an abrupt change of direction on the electric motor, a
 reversing current is produced which, for a short time, may be several
 times the operating current of the electric motor. In order to prevent
 overloading of the power source and the electronics for this case, it is
 usual to provide electronic current limiters.
 In order to protect the electronics from a short circuit, electric
 short-circuit protection is provided, whose threshold must lie above the
 high reversing current.
 However, because of the real internal resistances of the electronics, at
 low operating voltages and in the case of a non-ideal short circuit
 (R.sub.1 &gt;30 m.OMEGA.) or a stalled motor, a current flow which leads to
 the short-circuit protection responding is not reached. In this case
 because of the finite current rise, the current limiter responds only with
 a delay. Since the current limiter is reset again during each PWM period,
 a high, pulsed current flows through the output stage over the whole time,
 which leads to the electronics being destroyed or produces the risk of a
 cable fire.
 SUMMARY OF THE INVENTION
 The invention is thus based on the object of providing a method and a
 circuit arrangement which permit the detection of the operating state of
 the load as well as driving the load in accordance with the situation on
 the basis of the operating state.
 According to the invention, the object is achieved by the drive signal,
 whose pulse duration is varied in order to limit the current flow through
 the load, being evaluated in order to determine the operating state of the
 load.
 The advantage of the invention is that no additional monitoring devices are
 needed, but a signal generated for another purpose is evaluated.
 In a development, the frequency of response of the current limiter with in
 a predefined time period is counted.
 A conclusion about the operating state of the load is drawn from the
 frequency of the current limitation.
 As a result of the evaluation of the signal, there is the possibility of
 reacting to the fault. By changing the original PWM signal, the dynamics
 of the motor control system, and hence the loading on the output stages,
 can be reduced. At the same time, changes in the operating voltage and in
 the temperature are corrected by such a dynamic control system.
 In order to detect the response of the current limiter, the pulse duration
 of the varied drive signal is evaluated. By means of a desired/actual
 value comparison of the pulse duration, the effectiveness of the current
 limitation is established.
 In order to limit the current flow through the load, a current flowing
 through the load is advantageously measured and, while a control pulse of
 the drive signal is present on the load, the measured current is compared
 with a limiting value, the pulse duration of the control signal being
 varied on the basis of this comparison.
 The advantage is that, using the current flowing through the load, the
 operating state of the inductive load can be determined reliably, and
 that, on the basis of the current, the pulse duration and therefore the
 current present on the output stage can be set in such a way that critical
 situations are reliably prevented.
 Current limitation is advantageously achieved if the pulse duration is
 varied when the limiting value is reached.
 A further configuration of the invention relates to a circuit arrangement
 for driving a load cyclically, especially an electric motor in a motor
 vehicle, in which, in order to implement the method according to the
 invention, the load can be driven, via an output stage, with a
 pulse-width-modulated drive signal generated by a control device. This
 circuit arrangement is defined in that the current flowing through the
 load is determined by a current-measuring device, which is connected to a
 threshold device which, when the threshold value is reached, drives a
 switching device which terminates the control pulse while there is a
 control signal present on the output stage, it being possible for the
 control pulse from the threshold device to be fed to an evaluation device
 in order to determine the operating state of the load.
 By terminating the control pulse on the output stage, the current flow
 through the inductive load is limited. The evaluation device observes the
 response of the current-limiting circuit over a number of PWM periods
 within a predefined time duration. Since the behavior of the load under
 various operating conditions has been determined in preceding
 measurements, the frequency of response of the current-limiting circuit is
 known for each of these operating states. The comparison of the frequency
 actually registered with the frequency corresponding to the normal state
 permits conclusions about the operating state of the load.
 The evaluation of the drive signal does not always have to be carried out
 in hardware, but is also readily possible using suitable software.
 In order to evaluate the output signal from the threshold device, this
 signal is fed to an integration element or to the control device which
 generates the original pulse-width-modulated drive signal. Using the
 response frequency detected over a number of periods of the
 pulse-width-modulated signal, the loading situation of the motor is
 assessed and necessary countermeasures are initiated.
 The switching device advantageously contains a flip-flop, whose set input
 leads to the threshold device, while the pulse-width-modulated signal
 generated by the control device is present on its reset input. Using such
 a simple circuit arrangement, not only is digital current limitation
 provided by evaluating the output signal from the flip-flop, but at the
 same time a dynamic control loop for driving the inductive load is
 established.
 In this case, the load to be switched is a constituent part of a bridge
 circuit, each half bridge of this bridge circuit being driven by the
 varied pulse-width-modulated signal.
 However, depending on the application, the load can be a constituent part
 of a half bridge or can be driven by the controller via only one switch.

Identical features are identified by identical reference symbols.
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 In FIG. 1, an electric motor 1 is connected between the battery voltage
 U.sub.B and ground. A switch 2 is driven cyclically by a driver 4 on the
 basis of a pulse-width-modulated signal generated by a control device 5,
 by which means the electric motor 1 is connected to the battery voltage
 U.sub.B at a predefined frequency. A shunt 6 is connected between the
 electric motor 1 and ground. The voltage across the shunt (current
 measuring resistor) 6 is picked off by a threshold switch (comparator) 7,
 whose output drives the set input of a flip-flop 8. The reset input of the
 flip-flop 8 leads to the control device 5. The pulse-width-modulated drive
 voltage generated by the control device 5 is present on the reset input of
 the flip-flop 8. The output of the flip-flop 8 actuates a switch 11, which
 is connected to the connecting line 12 between control device 5 and switch
 2. Furthermore, there is a connection 3 between the flip-flop 8 and the
 control device 5.
 The functioning of this circuit arrangement is to be explained with
 reference to the pulse diagrams in FIG. 2. Diagram a shows the current I
 flowing in the motor 1 over the time t, as is measured by the shunt 6. The
 time axis is subdivided into time sections t.sub.PWM, which correspond to
 the period duration of the pulse-width-modulated signal generated by the
 control device 5.
 FIG. 2b shows the PWM signal output by the control device 5 to the switch
 2. Here, it can be seen that this signal has a constant mark/space ratio.
 When the electronics are switched on, such a signal drives the motor 1.
 When a pulse is present, the switch 2 closes, as a result of which the
 motor 1 is connected to the operating voltage U.sub.B, and a current flows
 through the shunt 6. In the pulse space, the current drops and rises again
 only after the start of the application of a new pulse by means of the
 pulse-width-modulated signal generated by the controller 5.
 The voltage drop corresponding to this current flow through the shunt 6 is
 measured by the comparator 7 and amplified. In the comparator 7, it is
 determined whether the current flowing through the motor has reached a
 threshold of 35 ampere (cf. FIG. 2a). If this is the case, the set input
 of the flip-flop 8 is set. At the same instant, there is present on the
 reset input of the flip-flop 8 an item of information relating to whether
 a pulse or a space of the pulse-width-modulated signal is present on the
 switch 2. If a pulse is present, the flip-flop 8 switches the switch 11,
 which results in a pulse t.sub.SB (illustrated in FIG. 2c) being
 generated, which limits the time duration of the original drive signal.
 The pulse-width-modulated drive signal resulting from this is illustrated
 in FIG. 2d, from which it can be seen that the pulse t.sub.SB set by the
 flip-flop 8 during one period shortens the original pulse-width-modulated
 signal, by which means the current rise through the motor 1 is limited.
 The measuring and control operation just described is repeated in each
 period of the drive signal. As can be seen from FIG. 2, because of the
 concrete operating conditions, a pulse-width-modulated signal is generated
 in which different pulse durations occur within the individual periods.
 How often the current limiter responds in this case depends on the
 operating conditions of the motor 1.
 The pulse-width-modulated signal varied in this way is fed to the
 controller 5 for evaluation. Information relating to the percentage to
 which the current-limiting device responds at different loading states of
 the motor 1 is stored in the form of a table in a memory device (not
 further illustrated) of the controller 5. This percentage behavior is
 determined from the frequency of the current limitation by means of which
 the control pulse generated by the controller 5 is reduced.
 This means, for example, that it is assumed that the current-limiting
 device responds by up to 50% in the normal case during a defined measuring
 time of 0.5 s. That is to say, the current is limited in 50% of the PWM
 periods during a measuring time of 0.5 s. If the drive signal has a
 frequency fpw of 10 kHz, for example, then 5000 periods will be generated
 in 0.5 s. Given a response frequency of 50%, the current limiter responds
 during 2500 periods.
 If the evaluation of the varied pulse-width-modulated signal results in the
 current-limiting circuit responding greater than 50%, it is concluded
 there is a stiff mechanism. The evaluation device must react appropriately
 to this. In order to protect the output stage, the control gain is
 reduced, for example.
 Stalling of the motor 1 is detected if the current limiter responds at
 100%. In this case, the drive of the motor is stopped by the controller.
 By means of a signaling device, the attention of the operator is drawn to
 the fault, and the controller 5 can initiate further measures.
 FIG. 3 illustrates a circuit arrangement for driving an adjusting motor
 variably, for example one which adjusts the stroke of valves in motor
 vehicles. As can be seen from FIG. 3, the motor 1 is a constituent part of
 a bridge circuit consisting of the switches 2, 17 and 16, 15. Each half
 bridge 16, 17 and 2, 15 is connected between the operating voltage U.sub.B
 and ground. Here too, a current-measuring resistor 6 is arranged between
 the bridge and ground. Each switch is driven by a driver. In this case,
 the driver 4 switches the switch 2. The driver 10 drives the switch 17
 cyclically, while the driver 14 controls the switch 16. The switch 15 is
 controlled by the driver 13. An OR circuit 9 is used to determine the
 output of the controller 5 on which the pulse-width-modulated signal is
 present, the OR circuit being connected to the reset input R of the
 flip-flop 8.
 In order to simplify the explanation, only one half bridge will be
 considered, the switch 2 being at low, while the PWM signal is present on
 the second switch 17. The switch 16 is open and the switch 15 is
 continuously closed.
 The motor 1 is regularly switched on and off by the control device 5 with
 high currents (for example between 10 and 100 ampere). In the initial
 situation, current is applied to the motor 1, in the present example the
 switch 17 is driven cyclically by the PWM signal, while the switch 15 is
 permanently closed. As already explained in connection with FIG. 1, the
 current flowing through the motor 1 is measured by means of the shunt 6
 and evaluated in the threshold circuit 7. If a limiting value of 35 ampere
 is exceeded, and if a pulse is simultaneously present on the driver 10,
 the flip-flop 8 switches the switch 18, as a result of which the
 pulse-width-modulated drive voltage is varied in accordance with FIG. 2.
 The varied pulse-width-modulated signal is fed, via the line 3, to the
 controller 5 for evaluation. The evaluation of the varied pulse duration
 of the drive signal is carried out simply, as already described, in the
 control device 5, which is a microcontroller.