Electricity: motive power systems – Alternating current commutating motors – Universal or a.c.-d.c. motors
Reexamination Certificate
2000-02-28
2001-07-17
Ip, Paul (Department: 2837)
Electricity: motive power systems
Alternating current commutating motors
Universal or a.c.-d.c. motors
C318S132000, C318S254100, C318S434000
Reexamination Certificate
active
06262544
ABSTRACT:
TECHNICAL FIELD
The present invention relates to four quadrant motor operation, and more particularly, to a circuit and associated method for recreating actual motor current from sensed bus current information.
BACKGROUND OF THE INVENTION
Prior art of brush motor control circuits can be divided into three general categories: one quadrant operation, two quadrant operation, and four quadrant operation.
One quadrant motor operation basically describes unidirectional motor operation. This operation uses a single power device as a switch which may be operated in an on/off mode or in a pulse-width-modulated (PWM) mode. When this switch is used in a PWM mode, a free wheeling diode is used to recirculate the motor current when the power device is in the off mode. This mode of operation usually does not provide a motor braking function. Current sensing for this operation is generally done using a sensor in the DC bus which provides power device current but not actual motor current.
Two quadrant motor operation is characterized by motoring or plugging in a forward direction, motoring or plugging in a reverse direction, and braking in either direction. This operation uses four power switching devices and four free wheeling diodes in parallel with these switching devices in what is commonly referred to as an “H” bridge configuration. This mode of operation also provides bi-directional operation of the motor to provide torque to a load or actuator, and provides the ability to “plug” the motor. Plugging involves rapidly reversing the applied motor voltage while the motor continues to rotate in the opposite direction due to a previously applied motor voltage. This action results in the potential occurrence of uncontrolled motor currents and power device currents equal to twice the stall current of the motor. Finally, this mode of operation provides the ability to “brake” the motor. Braking involves turning on both upper or both lower power devices in the “H” bridge simultaneously while the motor is rotating in either direction. This action results in the potential occurrence of uncontrolled motor currents and power device currents equal to the stall current of the motor. Current sensing for this operation is generally done using a sensor in the DC bus which provides power device current but not actual motor current. This method of current sensing does not provide information to indicate actual motor current information.
Four quadrant motor operation is characterized by motoring in a forward direction, regenerating in a forward direction, motoring in a reverse direction, regenerating in a reverse direction, and braking without uncontrolled motor currents. This operation uses four power switching devices and four free wheeling diodes in parallel with these switching devices in the “H” bridge configuration. This mode of operation provides bi-directional operation of the motor to provide torque to a load or actuator and absorb torque from a load or actuator. In providing torque to the load, the motor operates in a motoring mode in either direction with controlled motor currents. In absorbing torque from the load, the motor operates in a regenerating mode in either direction with controlled motor currents. Four quadrant motor operation generally does not provide a “plug” capability in order to avoid the uncontrolled motor currents. Finally, this mode of operation provides the ability to “brake” the motor, but it monitors motor current to avoid the uncontrolled braking currents. Current sensing for this operation has previously been done using a sensor in series with the motor leads to provide actual motor current. To provide four quadrant operation, actual motor current is required so that current direction information is retained.
Two quadrant motor operation as described above has the advantage of easy and low cost current sensing but has the disadvantage of uncontrolled motor currents and power device currents in several modes of operation. The uncontrolled currents may result in catastrophic damage to the controlling power devices. The typical solution to protecting the power devices is to oversize the power devices to handle the uncontrolled currents. However, an uncontrolled overcurrent condition can also cause demagnetization of the motor magnets. Four quadrant motor operation as described above has the advantage that there are no modes of operation in which the motor currents are uncontrolled but has the disadvantage that motor current sensing is more expensive and more cumbersome. The advantage of controlled currents is that the motor torque is always at a known level and the power devices remain within their safe operating area (SOA).
Accordingly, it would be desirable and advantageous to provide four quadrant motor operation without requiring the use of a current sensor in series with the motor leads.
SUMMARY OF THE INVENTION
In one aspect of the present invention, a method of operating a four quadrant motor involves (a) monitoring a motor voltage magnitude request signal which varies between zero and a maximum value and (b) monitoring a motor direction request input. In step (c), based upon the request signal and input monitored in steps (a) and (b), a voltage request reference signal is established. In a step (d) a current magnitude request signal is monitored and in a step (e) a first current request reference signal and a second current request reference signal are established based upon the signal monitored in step (d). In a step (f) a voltage across a current sense resistor of a bus connected for powering the motor is monitored and in a step (g) a sense resistor reference signal is established based upon the signal monitored in step (f). In a step (h) the first current request reference signal is compared with a feedback current signal and in a step (i) the second current request reference signal is also compared with the feedback current signal. In a step (j) the sense resistor reference signal is provided to a controllable amplifier and in a step (k) a positive
egative gain of the controllable amplifier is controlled based upon the voltage request reference signal, the comparison made in step (h) and the comparison made in step (i). In a step (l) a signal output of the controllable amplifier is sampled to generate the current feedback signal which represents an actual current of the four quadrant motor.
This invention allows the motor to operate in a controlled torque manner at all times and allows the power devices to be properly sized. Since a single bus current sensor is used instead of a motor current sensor, the improved operation is provided at nearly the cost of two quadrant operation. Actual motor current is recreated using bus current information. Actual motor current is required for four quadrant motor operation so that motor current direction is preserved. Recreating this information using a bus current sensor provides the required information at a cost similar to the methods used which do not retain the motor current direction information. Elimination of uncontrolled currents also eliminates periods of uncontrolled torque in the motor operation. This is more important as systems become more “servo quality” oriented. Also, elimination of uncontrolled currents allows power devices to be properly sized which may also lead to additional cost savings. In systems with higher voltages than present automotive voltages, this may be the difference between a functional system and inability to properly provide a required function.
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Disser Robert John
Heaston Bruce Allen
Delphi Technologies Inc.
Ip Paul
Sigler Robert M.
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