Electricity: motive power systems – Limitation of motor load – current – torque or force
Reexamination Certificate
1999-12-15
2001-09-18
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C318S801000
Reexamination Certificate
active
06291954
ABSTRACT:
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
1
>30 m&OHgr;) 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.
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Hadamik Matthias
Joch Christoph
Duda Rina I.
Farber Martin A.
Mannesmann VDO AG
Nappi Robert E.
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