Patent Application
20110231040
The present invention relates, generally, to a vehicle having an electric motor, and more specifically, to a method for controlling pulse width modulation for an inverter used with the electric motor.
Recent advances in technology have led to the use of electric motor/generators to provide all or a portion of the power used to drive a vehicle in order to improve the vehicle fuel efficiency and/or driving range. An inverter is typically used with the electric motor to convert DC power to an AC power input in order to operate the electric motor. A discontinuous pulse width modulation signal (PWM) for the inverter may be used to decrease switching losses when the electric motor has zero speed and zero torque. However, changing the control signal for the inverter to a discontinuous PWM signal every time the electric motor is at zero speed and zero torque decreases the response time of the motor.
A method for controlling an inverter coupled to an electric motor for a vehicle is provided. The method includes generating a discontinuous PWM signal for the inverter when either the torque of the electric motor is greater than a predetermined torque value or the speed of the electric motor is above a predetermined speed value. A discontinuous PWM signal is also generated when the torque of the electric motor and the speed of the electric motor are substantially zero and at least one predetermined vehicle condition is met.
The at least one predetermined vehicle condition includes at least one of a transmission is in a highest fixed gear, and an engine is off and the vehicle speed is substantially zero. Additionally, substantially zero torque is less than one percent of a maximum torque for the motor and substantially zero speed is less than one percent of a maximum speed for the motor.
The method may further include generating a continuous PWM signal for the inverter when one of the torque of the electric motor is less than the predetermined torque value and is greater than substantially zero and the speed of the electric motor is less than the predetermined speed value and is greater than substantially zero.
The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings.
Referring to the Figures, wherein like reference numbers refer to the same or similar components throughout the several views,
The transmission 14 is preferably a hybrid transmission 14 having one or more modes of operation. A transmission 14 having multiple operating moves may operate in standard, electric or hybrid modes. In standard operation mode, the transmission 14 is driven by the engine 12. Under certain vehicle 10 conditions, typically when the power demand for the vehicle 10 is low, the engine 12 may be turned off and the power required to drive the transmission 14 may be provided by the motor 16, this is known as the electric operating mode of operation. In hybrid operating mode the engine 12 provides power, and the motor 16 is controlled to function as a motor or a generator. In hybrid operating mode the transmission 14 may respond similar to a continuously variable transmission to provide smooth operation of the vehicle 10 over a wide range of speeds. However, once the vehicle 10 has reached a cruising speed, where little or no acceleration is required, the transmission 14 operates in a fixed gear. The fixed gear is selected based upon the cruising speed of the vehicle 10 and the particular transmission 14 and gear ratios provided for a particular vehicle 10. Typically, while cruising at high speeds the highest gear available for a particular transmission 14 is selected.
An electronic control unit (ECU) 20 is connected to the engine 12, the motor 16 and the transmission 14 for controlling various vehicle functions, including the operating mode for the transmission 14. The ECU 20 may also be connected to various other components, such as, but not limited to, sensors and control modules useful for controlling the vehicle 10. An inverter 22 and a controller 24 are also connected to the ECU 20 for controlling operation of the motor 16. The controller 24 receives data regarding the inverter 22 and the motor 16. For example, sensors (not shown) within the motor 16 report the operating speed and torque of the motor 16 to the controller 24. Additionally, the controller 24 may receive vehicle 10 data from the ECU 20.
Referring to
The inverter 22 includes a three-phase circuit 26, where three-phase outputs, ia, ib, ic from the inverter 22 are connected to the motor 16. The battery 18 provides a voltage source Vdc to the inverter 22. The inverter 22 includes a plurality of switches 28A, 28B, 28C, 30A, 30B and 30C to convert the DC power input from the battery 18 into a three-phase AC power output ia, ib, ic which can be utilized by the motor 16. Three of the switches 28A, 28B, 28C, are connected to the positive output of the battery 18 and three of the switches 30A, 30B and 30C are connected to the negative output of the battery 18. Additionally, the plurality of switches 28A, 28B, 28C, 30A, 30B and 30C are connected to form three pairs having three outputs ia, ib, ic from the inverter 22. That is, the output of switch 28A is connected to the output of switch 30A to form the output ia from the inverter 22. The output of switch 28B is connected to the output of switch 30B to form the output ib from the inverter 22. Finally, the output of 28C is connected to the output of switch 30C to form the output ic from the inverter 22. DC power from the battery 18 is converted into a three-phase output ia, ib, ic by repeatedly opening and closing the plurality of switches 28A, 28B, 28C, 30A, 30B and 30C based upon the signals from the controller 24 (shown in
Referring to
When the motor 16 is at near zero torque and speed, torque ripple is not a problem and a discontinuous PWM signal may be used for the inverter 22 at this point as well, as indicated at 52 on the graph 54. However, when the vehicle 10 is operating the motor 16 may frequently reach zero torque and speed momentarily. Every time the control signal for the inverter 22 is changed there may be a delay in operation of the motor 16. In most circumstances the delay in operation of the motor 16 is not noticeable. However, if the control signal for the inverter 22 is frequently changing between a continuous PWM and a discontinuous PWM (i.e. when the motor torque and speed are momentarily substantially zero) the performance of the motor 16 may be affected.
Therefore, use of discontinuous PWM signal to control the inverter 22 should be limited to vehicle 10 situations where the speed and torque of the motor 16 are substantially zero for a sufficient time to take advantage of the reduction in switching losses. Several situations may exist in which the motor 16 is at substantially zero actual or commanded torque and speed for a sufficient amount of time that the reduction in switching losses is a greater advantage than the small delay in motor 16 operation. For example, when the vehicle 10 is in electric mode, i.e. the engine 12 is off, and the vehicle 10 has substantially zero speed. In this instance there is no demand on the motor 16. The vehicle 10 may be in electric mode, with no speed in a number of occurrences including when the vehicle 10 is in park, neutral, or at a stop. Another instance when utilizing discontinuous PWM is advantageous is when the vehicle 10 is at cruising speed and the transmission 14 is operating in fixed gear. For example, when the transmission 14 is in the highest fixed gear available, such as cruising at highway speeds.
Substantially zero torque and speed of the electric motor 16 would be a torque or speed that is less than one percent of the maximum operating torque and speed for a particular motor 16. Additionally, speed of the vehicle 10 is substantially zero when the speed is less than one mile per hour.
Alternatively, the electric motor 16 may also be commanded using a discontinuous PWM when the torque is substantially zero and the speed of the electric motor 16 is below a predetermined speed threshold n2. When the electric motor 16 operates at discontinuous PWM when the speed of the motor 16 is near zero than the predetermined speed threshold n2 would equal zero.
References to substantially zero torque of the electric motor 16 refer to both substantially zero actual torque and command torque. Substantially zero actual torque occurs when the estimated torque value of the electric motor 16 is less than one percent of the maximum operating torque. Substantially zero command torque occurs when the electric motor 16 is below a predetermined torque value that is sufficiently low enough that the electric motor 16 may operated as if there was zero torque. One skilled in the art would know the appropriate predetermined torque value for a particular electric motor 16 to operate at substantially zero command torque.
If the motor 16 torque is less than the predetermined torque value T1 the controller 24 compares the actual speed of the motor 16 to the predetermined motor speed n1, step 40. If the motor 16 speed in greater than or equal to the predetermined motor speed n1 than the controller 24 utilizes a discontinuous PWM to control the motor 16, step 38. If the motor speed is less than the predetermined motor speed n1 the controller 24 checks to if the actual torque of the motor 16 is substantially zero and the actual speed of the motor 16 is below the predetermined speed threshold n2, step 42. If either of the motor torque is not near zero or the speed is above the predetermined speed threshold n2, then the controller 24 sends a continuous PWM signal to the inverter 22, step 44.
If both the motor torque is near zero and the motor speed is below the predetermined speed threshold n2, then the controller 24 assesses the data from the ECU 20 to find out if the transmission 14 is in the highest fixed gear available, step 46. If the transmission 14 is in the highest fixed gear available, the controller 24 instructs the inverter 22 with a discontinuous PWM, step 38. If the transmission 14 is not in the highest fixed gear the controller 24 assesses the data from the ECU 20 again to find out if the engine 12 is in the off position, i.e. checks if the vehicle 10 is operating in electric mode, step 48. If the engine 12 is on (i.e. the vehicle 10 is not in electric mode) the controller 24 instructs the inverter 22 with a continuous PWM signal, step 44. If the engine 12 is off, the controller 24 then assesses the speed of the vehicle 10 to confirm the vehicle 10 has zero speed, step 50. If the vehicle 10 is moving, a continuous PWM signal is sent to the inverter 22, step 44, and if the vehicle is not moving, then a discontinuous PWM is sent to the inverter 22, step 38.
The above embodiment discloses using a discontinuous PWM when a motor 16 torque and speed are above predetermined levels, or when the motor 16 speed and torque are zero and other predetermined vehicle 10 conditions are met. Although the predetermined vehicle 10 conditions disclosed include operating in the highest transmission gear, or when the engine is off and the vehicle is at a stop other vehicle 10 conditions may be selected for controlling the inverter 22 with a discontinuous PWM signal. One skilled in the art would be able to select the appropriate vehicle conditions in which to control the inverter 22 with a discontinuous PWM.
While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.
