Electricity: motive power systems – Switched reluctance motor commutation control
Patent
1998-07-09
2000-09-26
Masih, Karen
Electricity: motive power systems
Switched reluctance motor commutation control
318439, 318138, 318811, H02P 506
Patent
active
061246883
DESCRIPTION:
BRIEF SUMMARY
This is a United States national application corresponding to copending international application PCT/GB97/00052, filed Jan. 9, 1997, which designates the United States, the benefit of the filing date of which is hereby claimed under 35 U.S.C. .sctn. 120, which in turn claims the benefit of British application No. 9600549.1, filed Jan. 11, 1996, the benefit of the filing date of which is hereby claimed under 35 U.S.C. .sctn.119.
This invention relates to improvements in electrical motor control strategies, especially but not exclusively for use in an electrical power steering system.
It is known in the art to provide an electrical power steering system for vehicles in which an electrical motor replaces the traditional hydraulic assistance, so as to produce a compact, efficient steering system. Steering assistance is provided by driving the motor in response to a measure of the torque in the steering column. In such an application, it is essential to have good control of the output characteristics of the motor, i.e. the motor torque.
A typical motor and drive circuitry of the kind to which this invention relates, and which may be used in a power steering system, comprises three elements; a multiple phase permanent magnet brushless motor in which the phases are connected in a star fashion, a multiple phase bridge, each arm of the bridge comprising a top switching means and a bottom switching means; and sensing means for detecting rotor position and/or motor current. The top and bottom switching means typically each comprise a single transistor or two or more transistors connected in parallel.
The control strategy controls the modulation technique used to drive the transistors, thereby controlling the current in the motor windings together with knowledge of rotor position and hence controlling the motor torque.
Traditionally, such a motor system is driven by a dedicated integrated circuit which performs the function of the control strategy, for example the Motorola MC33035 integrated circuit (IC). This provides all the features necessary to implement a motor control strategy for a 3 or 4 phase motor. The IC provides 3 outputs for driving the top transistors, and 3 outputs for driving the bottom transistors in each arm. For any given rotor position, the IC is used to enable only one top and one bottom transistor of the drive circuitry. An additional feature of this type of motor control is that the enabled top and bottom transistors are on different arms of the bridge.
In the enabled state, the bottom transistor only is pulse width modulated, with the enabled top transistor being in the ON state for the whole modulation period. Varying the pulse width modulation duty of the bottom transistor determines the motor voltage and can be used to control motor current level and speed. The selection of the desired top and bottom transistor is performed under the IC's simple control strategy which interprets the data from a position sensor in conjunction with measurements of the current flowing in the motor windings. This type of modulation will be referred to as bottom only modulation, because only the bottom transistors are pulse width modulated.
A disadvantage of the conventional commutation including the bottom control as described above is that in open loop voltage control mode, a transient drop in overall resultant motor current level (and hence torque) occurs upon commutation from one phase to the next. In the power steering application set forth, these transients are felt by the driver through the steering wheel. Other problems arise when the motor is changing direction, due to poor current control using bottom-only modulation when the motor decelerates.
An aim of the present invention is to provide an improved motor control strategy which provides accurate control of motor torque irrespective of rotor speed and position, to minimise motor torque ripple and to minimise acoustic noise from the motor.
According to a first aspect of the invention, in a control strategy for controlling the pulse width modulation of
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Coles Jeffrey Ronald
Jones Russell Wilson
Williams Andrew James Stephen
Leykin Rita
Lucas Industries Public Limited Company
Masih Karen
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