Electricity: motive power systems – Limitation of motor load – current – torque or force
Reexamination Certificate
1999-02-23
2001-01-23
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C318S722000, C388S903000
Reexamination Certificate
active
06177774
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a motor controller for performing drive control based on the PWM (pulse width modulation) system, for the purpose of protecting amplifiers and motors against anomalous currents and achieving high operating efficiency therein.
2. Description of the Related Art
PWM is a known system for controlling AC motors such as induced-current three-phase synchronous motors and permanent magnet synchronous motors. When control is performed with the PWM system, the voltage applied to the motor is actually varied by altering the on-off time ratio of such switching elements as a drive transistors in a power conversion circuit.
When controlling a motor using such the PWM system, there are cases where the actual current being supplied to the motor becomes abnormally high for an instant relative to a current command. There are a number of primary causes of these anomalous currents, including (a) the current exhibiting intermittent oscillation due to the current control gain being set high, resulting in spikes in the actual current, (b) noise being added to the feedback current fed back to the control circuit from the motor, resulting in spikes in the actual current, and (c) inadequate voltage for controlling motor current when the motor is turning at high speed, making it impossible to obtain the designed control characteristics in the current control loop, resulting in control instability and spikes in the actual current.
When the actual current increases abnormally as described above, anomalous currents flow to the amplifier and/or motor, creating a danger of failures in the power devices configuring the amplifier, the amplifier or motor wiring, or, in the case of permanent magnet synchronous motors, motor magnet demagnetization.
For these reasons, protective circuits have been provided in conventional motor control apparatuses to cope with such anomalous currents. These protective circuits compare the actual current with a fixed threshold value which was set for an anomalous currents and, when the actual current exceeds the threshold value, interrupt, in all phases, the PWM commands being sent to the power conversion circuit. Thus all current supply to the motor is cut off and the apparatus is put into an alarm status.
With such a protective circuit as this, it is possible to protect the amplifier power devices and the motor magnets, but, the current supplied to the motor is cut off. For this reason, the overall motor control apparatus is placed in an alarm generation status, and the apparatus being driven by the motor ceases to be under control.
A motor is ordinarily provided with a dynamic brake that runs current from the motor through a resistance and performs a stopping action. Therefore, when the protective circuit is activated and the supply of power to the motor is interrupted, an alarm status is entered wherein the entire system inclusive of the motor is stopped.
Usually, in an alarm status, not only is control of a motor stopped, but also entire system is stopped and kept uncontrolled. Accordingly, once this alarm status is in effect, it becomes impossible either to continue processing or movement, or to perform control. After this alarm status has been entered, moreover, it becomes necessary to restart the system and/or program, which requires operator intervention and results in a decline in the overall system operating rate.
Also, when the braking force of the dynamic brake is smaller than the torque required for deceleration, this can be hazardous, resulting even in work or stroke-end collisions.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a motor controller based on the PWM system, wherewith, when anomalous currents develop, the apparatus can be protected without the controller being placed in a non-controlling state.
The present invention performs control wherein, when the actual current flowing to a motor reaches a preset abnormal level, supply to the PWM command is interrupted and restarted, and the current flowing to the motor is returned from the abnormal level to a normal level by interrupting the supply of the PWM commands, thereby protecting the apparatus. The motor controller according to the present invention is a motor controller for using a PWM system to control currents flowing to motors, comprising interruption and restart control means wherewith, when the actual current flowing in at least one phase of the motor exceeds a prescribed value, the PWM command supply in at least one phase is interrupted, after which PWM command supply is restarted.
By implementing the present invention, it is possible both to improve operating rates in controllers for feed shafts and main spindles in industrial machinery and in controllers for robot arms, etc., and to obtain the maximum performance available from amplifiers and motors, etc.
The motor controller according to the present invention is now described with reference to the simplified block diagram given in FIG.
1
. In
FIG. 1
, the components involved in the control mechanism of the present invention are diagrammed in a simplified manner and other configurational elements are omitted.
A motor (not shown) uses a power converter unit
2
to convert a DC voltage supplied from a DC power supply
1
to a U-phase, V-phase, and W-phase, whereupon current is supplied to all phases of the motor. The power converter unit
2
comprises a set of switching devices for each phase, and supplies U, V, and W-phase current to the motor by complementarily turning the devices on and off according to PWM commands sent from a PWM controller
10
.
In the PWM controller
10
, a PWM command formation unit
11
receives current commands from a current control loop (not shown), forms PWM commands, and sends these PWM commands to the switching devices of the power converter unit
2
from a gate unit
12
via drivers (not shown). The PWM controller
10
also comprises interruption-restart control means. These interruption-restart control means interrupt the PWM command supply in at least one phase when the actual current flowing in at least one phase (of the U, V, and W-phases) of the motor exceeds the prescribed value, after which they restart the PWM command supply. These interruption-restart control means comprise an anomalous current detector
13
and a gate controller
14
.
The anomalous current detector
13
detects the actual current supplied to the motor, compares that detected actual current value with a predetermined value, makes anomalous level decisions, and sends the results of such decisions to the gate controller
14
. The U-phase, V-phase, and W-phase actual currents can be detected by providing a current detector
3
for each phase, or current detectors
3
may be provided for any two phases so that the current of the remaining phase can be calculated from the current values of the two phases detected by the current detector
3
.
The gate controller
14
receives detection signals from the anomalous current detector
13
, controls the gate unit
12
, and interrupts and restarts the PWM commands supplied to the power converter unit
2
from the PWM command formation unit
11
. When PWM command supply is interrupted, the switching devices of the power converter unit
2
enter an open state, and the current supply to the motor is interrupted. By interrupting this supply of PWM commands, the actual current that has reached an anomalous level drops and returns to a normal current level. When PWM command supply is restarted, the switching devices in the power converter unit
2
enter a control state, control of the current supply to the motor is restarted, and normal control can be performed.
The operation of the PWM controller
10
in the motor controller of the present invention is described now with reference to the flowchart given in FIG.
2
.
The anomalous current detector
13
compares the actual currents of each phase with an anomalous level, using detected values from the current detector(s)
3
(step S
1
), and, when an anom
Iwashita Yasusuke
Matsukane Tokuhisa
Fanuc Limited
Ro Bentsu
Staas & Halsey , LLP
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