Electricity: motive power systems – Constant motor current – load and/or torque control
Reexamination Certificate
2000-09-15
2002-03-26
Nappi, Robert E. (Department: 2858)
Electricity: motive power systems
Constant motor current, load and/or torque control
C318S700000, C318S809000, C318S811000, C318S799000
Reexamination Certificate
active
06362586
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and an electronic microcontroller unit for controlling the torque of a permanent magnet (PM), synchronous, alternating-current (AC) motor over an extended speed range, wherein the method and the microcontroller unit are both suitable for controlling the motor of an electric vehicle.
BACKGROUND OF THE INVENTION
Recent developmental advances in high-energy batteries, combined with the development of smaller and more powerful motors, have made it possible for new technological and commercial markets to open for a wide range of products, including, for example, portable electric appliances, electric entertainment equipment, and electric vehicles. With particular regard to electric vehicles, improved electric motor drives have also been made possible by the development of solid-state devices such as the MOSFET (metal-oxide-semiconductor field-effect transistor) and the IGBT (insulated-gate bipolar junction transistor), for each of such devices has the capacity for switching and delivering a significant amount of electrical power to a motor. In light of such, along with recent increases in energy costs, energy conservation concerns, environmental concerns, and strict legislation requiring improved internal combustion engine (ICE) efficiency, the motor vehicle industry is pressing for the development of improved electronic motor controls for electric vehicles.
The basic premise on which electronic motor control is based is that the speed, torque, and direction of a motor are all controlled by electronically switching or modulating phase currents and voltages which are ultimately transmitted to the motor. In a closed-loop, electronic motor drive and control system for a synchronous, three-phase, alternating-current (AC) motor, for example, the basic elements of such a system may include: (1) the AC motor, (2) a direct-current (DC) battery (or battery pack), (3) a DC-to-AC inverter, (4) a user command signal device, (5) current sensors, (6) a rotor position sensor, and (7) a microcontroller or microprocessor unit.
In such a system, the user command device is connected to the microcontroller unit and thereby enables a user (that is, the vehicle operator) to select a desired speed or torque at which the motor is to operate. The current sensors are utilized for sensing the phase currents of the motor so that the microcontroller unit can process the currents for feedback control purposes. Similarly, the rotor position sensor is utilized for sensing the position of the rotor of the motor so that the microcontroller unit can instantaneously determine the position and/or speed of the rotor for feedback control purposes as well.
Further in such a system, the DC battery defines a DC power bus which is connected to the inverter, and the inverter is connected to the AC motor. The inverter serves to convert the DC power from the battery into three sinusoidal (AC) phase current signals (i
a
, i
b
, i
c
) which are transmitted to the stator of the motor to thereby operate the motor and control the torque. The inverter includes three drivers wherein each driver is dedicated to driving one of the three AC phase currents. Each driver has two power switches, one “top” switch for driving a particular phase current high and another “bottom” switch for driving the same phase current low. Thus, the three drivers of the inverter have a combined total number of six power switches. Common designations for these six power switches are A_TOP, A_BOT, B_TOP, B_BOT, C_TOP, and C_BOT. The individual conductive states (“on” or “off”) of the six power switches dictate both the frequency and the magnitude of the three phase currents which are transmitted to the motor. The inverter receives electrical switching signals from the microcontroller unit which dictate the conductive states of the six power switches at any given point in time.
In general, to properly control the motor, the microcontroller unit must perform two primary tasks. One, the microcontroller unit must generate switching signals for helping the inverter create sinusoidal waveforms for the motor. To accomplish this, the microcontroller unit must implement a “modulation technique.” There are many different types of modulation techniques, some of which include, for example, sinusoidal pulse-width modulation (PWM), third-harmonic PWM, 60° PWM, and space vector modulation (SVM). Two, the microcontroller unit must generate electrical control signals for adjusting the frequency and magnitude of the sinusoidal waveforms. To accomplish this, the microcontroller must implement a “control algorithm.” Although there are many general types of control algorithms, such as, for example, open-loop volts-per-hertz control, volts-per-hertz with DC current sensing control, direct or indirect vector control (field orientation), and sensorless vector control, a significant number of torque control motor drives implement an indirect “vector control technique.” In such a technique, both the phase currents and the rotor position/speed of the motor are sensed to establish closed-loop, feedback control of the motor.
In a vector control technique, electrical signals representing data concerning the sensed phase currents are communicated to the microcontroller unit from the current sensors. In addition, electrical signals representing data concerning the position of the rotor are also communicated to the microcontroller unit from the rotor position sensor. Based on such communicated data, the microcontroller unit then mathematically “maps” the measured phase currents as a stator current vector (I
a
) onto a two-axis (direct axis “d,” quadrature axis “q”) coordinate system for the purpose of achieving feedback control. In such a d-q coordinate system, the stator current vector is broken down into two current components, I
d
and I
q
, which are orthogonal to each other on the coordinate system. The I
d
current component is used to represent and control the flux of the motor, and the I
q
current component is used to represent and control the torque of the motor. If the d-q coordinate system is then mathematically “rotated” synchronously with the rotor flux of the motor, both I
d
and I
q
can then be treated and controlled as DC values, and the AC motor can thus be controlled almost as if it were a DC motor. Thus, in this way, independent and decoupled control of both the flux and the torque of the motor is achieved.
In addition to sensing the phase currents and rotor position to generate values for I
d
and I
q
, the microcontroller unit must further implement the vector control technique to also generate a desired value for a first (direct-axis) command current variable (I
d
*) and a desired value for a second (quadrature-axis) command current variable (I
q
*). Generated values for the first command current variable and the second command current variable are ideal values which are most desired and preferred and are used for controlling and operating the motor. These generated values are based on and derived from, for example, the sensed rotor position/speed data, the voltage supplied by the DC battery, and a user command signal, all of which are electrically communicated to the microcontroller unit. In ultimately generating the values based on such communicated information, the microcontroller unit must typically be involved in very complex and time-consuming processing.
Once both the “measured” I
d
and I
q
values and the “preferred” I
d
* and I
q
* values are successfully determined and generated, the microcontroller unit then typically utilizes a “current controller” to compare the measured and preferred values. The current controller is basically an implementation of difference equations. Based on the comparison, the current controller then generates electrical control signals, sometimes referred to as “adjustment” or “correction signals,” which are used to help conform future “measured” I
d
and I
q
values with the “preferred” I
d
* and I
q
* values. To accomplish such, the control signals generated by the current
General Motors Corporation
Grove George A.
Leykin Rita
Nappi Robert E.
Sedlar Jeffrey A.
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