Electricity: motive power systems – Limitation of motor load – current – torque or force
Reexamination Certificate
1998-01-02
2001-05-01
Sircus, Brian (Department: 2837)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C318S458000, C318S469000, C361S031000
Reexamination Certificate
active
06225769
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a device for operating an electric motor of the type having means for detecting a current flowing through the electric motor, and having an evaluation arrangement which evaluates at least one characteristic of the current, compares it with a threshold value and outputs an overload signal in the event of overshooting of the threshold value. German Patent 28 20 330 discloses a circuit arrangement for an electric window lift drive which has an anti-jamming device. An evaluation arrangement detects a current flowing through an electric motor, and a temporal current change referred to time. The two characteristics determined for the current flowing through the electric motor are added and compared with a prescribed threshold value. After an overshoot of the threshold value, the electric motor is, for example, reversed, so that an object possibly jammed by a pane is released again. The evaluation arrangement contains a plurality of capacitors which implement low-pass filters with the aid of further components in each case. In addition to suppressing interference signals, the suppression of higher-frequency signal components in the determination of the characteristics also effects suppression of the detection of the starting operation, in the case of which a high motor current occurs which can be a multiple above the value of the steady-state motor current.
EP-A 0 116 904 discloses a drive device for a flap in air conditioning systems of motor vehicles. The flap is actuated by a stepping motor. It is ensured that the flap actuated by the stepping motor reliably reaches mechanical end stops due to the fact that, on the one hand, independently of the position of the flap which is to be prescribed the control device applies to the stepping motor at the beginning of an operating phase a maximum number of steps which suffices for one of the end stops of the flap to be reached. On the other hand, in the case of each further setting of the flap in each of the mechanical end stops, in addition to the steps required for this purpose the stepping motor has further steps applied to it in the direction of movement of the flap, in order reliably to reach the respective end stop of the flap owing to this number of extra steps, and to compensate for possible lost steps of the stepping motor.
It is the object of the invention to provide a device for operating an electric motor, which uses simple means to detect an overload or a blocked state of the electric motor, in particular a stepping motor.
SUMMARY AND ADVANTAGES OF THE INVENTION
The above object is achieved according to a first object of the present invention by a device for operating an electric motor having means for detecting a current flowing through the electric motor, and having an evaluation arrangement which evaluates at least one characteristic of the current, compares it with a threshold value and outputs an overload signal in the event of overshooting of the threshold value, and wherein the determination of the characteristic is provided within a time interval (T
I
) which starts at a first prescribed time (T
1
) after a starting instant (T
O
), the electric motor being started at the starting instant (T
O
) starting from the at least approximately de-energized state, and which ends at a second prescribed time (T
2
) before the steady-state motor current is reached.
The above object is further achieved according to a second aspect of the invention by a device for operating an electric motor, having means for detecting a current flowing through the electric motor, and having an evaluation arrangement which evaluates at least one characteristic of the current, compares it with a threshold value and outputs an overload signal in the event of overshooting of the threshold value, and wherein the determination of the characteristic is provided at a prescribed sampling instant (T
A
) which occurs after a prescribed delay time (T
V
) after a starting instant (T
O
) at which the electric motor is started starting from the at least approximately de-energized state and which occurs before the steady-state motor current is reached.
The device according to the invention has the advantage that an overload state, for example a blocked state, can be detected by determining at least one characteristic of the current flowing through a motor, in particular a stepping motor.
It is provided in accordance with a first embodiment that the determination of the at least one characteristic is provided within an interval which, starting from the at least approximately de-energized state, begins at a prescribed time after starting the electric motor and ends before the steady-state motor current is reached. It is possible with the aid of this measure to detect an overload state or blocked state as early as the starting phase of the electric motor. The device according to the invention is therefore particularly suitable in the case of stepping motors to which individual starting pulses are applied to prescribe the number of steps, the flow of current through the motor starting at least approximately from the de-energized state in each case. A comparable operating state can occur in the case of a DC motor which has a clocked operation. It is provided within the framework of such a clocked operation that a prescribed operating voltage is connected to the DC motor for a prescribed interpulse period and is subsequently switched off again for a prescribed interpulse period. Both the ratio of pulse duration to interpulse period, and the frequency can be variable. A precondition for the proper functioning of the device according to the invention in the case of clocked operation of the DC motor is that the current flowing through the DC motor decreases within the interpulse periods at least approximately to the value zero.
In accordance with another embodiment of the device according to the invention, it is provided that the determination of the at least one characteristic is provided at a prescribed instant which occurs after starting the electric motor, starting from at least approximately de-energized state, and before the steady-state motor current is reached.
In accordance with still another embodiment of the device according to the invention, the determination of the at least one characteristic of the current is provided at that instant of time within the pulse duration at which the current flowing through the electric motor reaches a maximum, and an overload signal is emitted if the instant of time falls into a predetermined time interval or duration.
The devices according to the invention are particularly suitable for detecting mechanical end stops which can be reached by an actuator driven by an electric motor.
Advantageous developments and refinements of the devices according to the basic embodiments of the invention likewise are disclosed.
The amplitude of the current occurring in the interval or the amplitude occurring at the specific instant, for example, is provided as a first characteristic of the current to be evaluated. Another advantageous refinement provides that the current change or the maximum current difference which has occurred in the prescribed interval is determined as the characteristic. A development of this measure provides that the current change is referred to time. The relationship with time corresponds to a time derivative of the current which can be performed both within the interval (difference quotient) and at the prescribed specific instant (differential quotient).
Another advantageous refinement of the first embodiment provides that the interval starts simultaneously with the starting of the electric motor. This measure ensures that the characteristic can be detected without a gap at the same time as the starting of the electric motor.
An advantageous development provides that the characteristic detected is compared with a prescribed threshold value which can be designed to be adjustable, for example, and that in the event of the threshold value being exceeded, an overload stat
Brenner Veit-Michael
Thull Lukas
Weible Reinhold
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