Abstract:
An inverter control method for a hybrid vehicle includes: monitoring a torque command and a motor speed of the vehicle in real-time; determining whether the motor speed is less than a first speed; determining whether an absolute value of the torque command is less than a first torque when the motor speed is less than the first speed; changing a switching frequency to a predetermined frequency when the absolute value of the torque command is less than the first torque; and controlling an inverter operation by generating a pulse-width modulation (PWM) signal with the switching frequency changed to the predetermined frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2015-0086700 filed in the Korean Intellectual Property Office on Jun. 18, 2015, the entire contents of which being incorporated herein by reference. 
     BACKGROUND OF THE DISCLOSURE 
     (a) Technical Field 
     The present disclosure relates generally to an inverter control method for a hybrid vehicle, and more particularly, to an inverter control method for a hybrid vehicle to reduce noise and to improve fuel efficiency thereof. 
     (b) Description of the Related Art 
     Nowadays, vehicles operating with an internal combustion engine using a fossil fuel such as a gasoline, a diesel, or the like can cause problems such as environmental pollution by exhaust gases, global warming by carbon dioxide, and respiratory ailments by ozone formation. Moreover, the amount of fossil fuel on the earth is limited, so it eventually can be depleted. 
     To solve the above-mentioned problems, environmentally-friendly vehicles, such as an electric vehicle (EV) using an electric motor, a hybrid electric vehicle (HEV) using an engine and an electric motor, a fuel cell electric vehicle (FCEV) using an electric motor by electricity generated by a fuel cell, or the like has been in development. Such environmentally-friendly vehicles drive a motor by inverting a DC power charged in a main battery of a vehicle to a 3-phase AC power using an inverter, and delivers a driving torque of the motor to a driving wheel to drive a vehicle. 
     However, in a motor system including a motor and an inverter, which serves as a driving source of an environmentally-friendly vehicle, various problems such as a noise generated in a driving/regenerating operation, an efficiency deterioration caused by switching loss, an electromagnetic performance deterioration, or the like can arise. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure, and therefore, it may contain information that does not form the related art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE DISCLOSURE 
     An object of the present disclosure is to solve the above-mentioned problems as well as other problems. The present disclosure has been made in an effort to provide an inverter control method for a hybrid vehicle having advantages of reducing noise. Also, the present disclosure has been made in an effort to provide an inverter control method for a hybrid vehicle having advantages of improving fuel efficiency. 
     Embodiments of the present disclosure provide an inverter control method for a hybrid vehicle including: monitoring a torque command and a motor speed of the vehicle in real-time; determining whether the motor speed is less than a first speed; determining whether an absolute value of the torque command is less than a first torque when the motor speed is less than the first speed; changing a switching frequency to a predetermined frequency when the absolute value of the torque command is less than the first torque; and controlling an inverter operation by generating a pulse-width modulation (PWM) signal with the switching frequency changed to the predetermined frequency. 
     The changing of the switching frequency to the predetermined frequency may include changing a mode between a double sampling mode using a frequency which is twice the switching frequency as a sampling frequency, and a single sampling mode using an identical frequency to the switching frequency as the sampling frequency. 
     The inverter control method may further include controlling the inverter operation by generating a PWM signal with a fixed switching frequency when the motor speed is greater than or equal to the first speed. 
     The inverter control method may further include controlling the inverter operation by generating the PWM signal with the fixed switching frequency when the absolute value of the torque command is greater than or equal to the first torque. 
     Certain effects of the inverter control method for a hybrid vehicle according to embodiments of the present disclosure are as follows. 
     For instance, there may be an advantage of improving fuel efficiency. Further, there may be an advantage of reducing motor noise. 
     Additional possibilities for implementing the present disclosure will be apparent based on the following detailed descriptions. However, as those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit and scope of the present disclosure, so it will be understood that embodiments are provided as a mere example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram illustrating a motor system of a hybrid vehicle according to embodiments of the present disclosure. 
         FIG. 2  is a flowchart illustrating an inverter control method for a hybrid vehicle according to embodiments of the present disclosure. 
         FIG. 3  is a graph illustrating a sampling frequency and a switching frequency according to embodiments of the present disclosure. 
         FIG. 4  is a graph illustrating a simulation result of a phase current depending on a switching frequency. 
         FIG. 5  and  FIG. 6  are graphs illustrating a simulation result of a magnitude and a frequency of noise depending on a switching frequency. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, the present disclosure will be described more fully with reference to the accompanying drawings, while same or similar constituent elements are designated same or similar reference numeral and duplicating descriptions thereof will be omitted. A suffix of a component using terms such as “module”, “unit”, “part”, “member”, “element”, “portion”, and the like hereinafter are given or mixed as being taken into consideration only the ease of creating the specification, are not to be distinguished from each other having a meaning or role by themselves. Further, in describing the related art in the following descriptions, if it is determined that the subject matter of the present disclosure may be cloudy by the description, it will be omitted. Further, annexed drawings are only for ease of understanding disclosed exemplary embodiments and the spirit and scope of the present disclosure is not limited by annexed drawings. Furthermore, it will be understood that any modification, equivalents, and substitutions are included in the spirit and scope of the present disclosure. 
     Terms including ordinal numbers such as first, second, and the like may be used to describe various constituent elements, however the constituent elements are not limited by the terms. The terms are used only to distinguish one constituent element from other constituent elements. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     It will be understood that when a constituent element is referred to as being “connected” or “contacted” to another constituent element, it can be directly connected or contacted to the other constituent element or intervening elements may also be present. In contrast, when a constituent element is referred to as being “directly connected”, or “directly contacted” to another constituent element, there are no intervening elements present. A singular expression includes a plural expression, unless clearly distinguished therefrom in a context. 
     In the present disclosure, it will be understood that the term, “comprise”, “include”, “have”, and the like are to designate an existence of a characteristic, a number, a step, a motion, a constituent element, a component, or a combination thereof, and not to exclude other existences of a characteristic, a number, a step, a motion, a constituent element, a component, or a combination thereof, or an additional possibility thereof. 
     It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g., fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles. 
     Additionally, it is understood that one or more of the below methods, or aspects thereof, may be executed by at least one controller. The term “controller” may refer to a hardware device that includes a memory and a processor. The memory is configured to store program instructions, and the processor is specifically programmed to execute the program instructions to perform one or more processes which are described further below. Moreover, it is understood that the below methods may be executed by an apparatus comprising the controller in conjunction with one or more other components, as would be appreciated by a person of ordinary skill in the art. 
     Generally, in the case of increasing a switching frequency of an inverter included in a motor system, noise may be decreased and, in the case of decreasing the switching frequency, inverter efficiency and fuel efficiency may be improved. That is, when the inverter switching frequency is set to a low fixed frequency (e.g., a base switching frequency is fixed to 4 KHz), while electromagnetic performance may be fine, however, in a noise issue, it may be adverse. 
     When the base switching frequency is set to a high fixed frequency (e.g., the base switching frequency is fixed to 8 KHz) through an entire operational area to reduce such inverter noise, NVH performance may be improved (i.e. a PWM current ripple is reduced), while electromagnetic performance may be deteriorated and switching loss may be increased (i.e., heel hold performance is deteriorated in a vehicle constraint condition), so inverter efficiency and fuel efficiency may be degraded. In further describing electromagnetic performance, as the switching frequency increases, radiated electromagnetic noise increases (e.g., an AM radio reception becomes poor as a result) and as the switching frequency decreases, radiated noise decreases, so electromagnetic performance becomes improved. 
     In a conventional environmentally-friendly vehicle, the inverter switching frequency is set to be high (e.g., 8 KHz) and fix it to reduce inverter noise, and a sampling frequency to obtain information such as sensing current information, motor angular information, and the like for controlling the inverter is set to an identical frequency to the switching frequency (e.g., 8 KHz). 
     Herein, the switching frequency (i.e., a switching period) may be defined as a period that ON/OFF of an individual switch in the inverter is repeated each once, and the sampling frequency is a frequency corresponding to a control period in an inverter current control. Herein, the control period may be defined as a period of a repeating cycle of a current/angle sampling, a current control operating, a duty calculating, and a duty updating. 
     However, when one switching frequency is fixed and used through the entire operational area without considering a motor driving situation or the like, an environmentally-friendly vehicle has characteristics of high switching loss caused by a heat emission of, e.g., a switching element and weak electromagnetic performance. Further, in the sampling frequency, in the case that the sampling frequency is high, inverter control stability is improved, while control factors such as sensing current information, motor angular information, and the like are obtained in a shorter period, so more calculations thereof are required in a micro-computer. Accordingly, a load ratio of the micro-computer is problematically increased. 
     Hereupon, with considering noise, vibration, and harshness (NVH) performance, electromagnetic performance, a switching loss issue, control stability, the micro-computer load ratio, and the like, an appropriate control of frequencies of the switching and the sampling depending on a driving situation is required. 
     Hereinafter, an inverter control method for a hybrid vehicle improving fuel efficiency and reducing noise by appropriately controlling frequencies of the switching and the sampling will be described with reference to  FIG. 1  and  FIG. 2 . 
       FIG. 1  is a schematic block diagram illustrating the motor system of a hybrid vehicle according to embodiments of the present disclosure and  FIG. 2  is a flowchart illustrating an inverter control method for a hybrid vehicle according to embodiments of the present disclosure. 
     As shown in  FIG. 1 , the motor system includes a monitoring unit  11 , a current command generator  12 , a current monitoring unit  13 , a d-q/3-phase coordinate converter  14 , a PWM signal generator  15 , a PWM inverter  16 , a motor  17 , a resolver R, and a speed calculator  19 . 
     The current command generator  12  supplies a current command i d * of d-axis and a current command i q * of q-axis depending on an input torque command T e * and a motor speed ω rpm  to current monitoring unit  13 . Herein, the current command generator  12  includes a current command map per a torque command and a motor speed, and the current command generator  12 , may extract current commands i d * and i q * of d and q axes respectively, corresponding to the torque command T e * and the motor speed ω rpm  from the current command map. 
     Then, the current monitoring unit  13  generates d and q axes voltage commands V d * and V q * for operating the motor  17 , depending on d and q axes current commands i d * and i q *. The current monitoring unit  13  receives a d-axis feedback current i d  applied to d-axis, and a q-axis feedback current i q  applied to q-axis from a 3-phase/d-q coordinate converter  18 , and removes a torque error by calibrating d and q axes voltage commands V d * and V q *. 
     The d-q/3-phase coordinate converter  14  obtains 3-phase voltages V a *, V b *, and V c * by 3-phase converting d and q axes voltage commands V d * and V q *. The PWM signal generator  15  generates PWM switching signals S a , S b , and S c  by using the switching frequency from the monitoring unit  11  and 3-phase voltage commands V a *, V b *, and V c *, and outputs PWM switching signals S a , S b , and S c  to the PWM inverter  16 . 
     The PWM inverter  16  includes a plurality of switching elements selectively turned on and off by inputted PWM switching signals S a , S b , and S c , and outputs 3-phase currents I a , I b , and I c  for controlling the motor  17 . 
     Further, the 3-phase/d-q coordinate converter  18  outputs the d-axis feedback current i d  and the q-axis feedback current i q , and feedbacks d and q axes feedback currents i d  and i q  to the current monitoring unit  13 . 
     The inverter control method according to embodiments of the present disclosure will be described. 
     First, in the case of driving using the motor  17 , the monitoring unit  11  monitors the driving situation, i.e., the current torque command T e * and the motor speed ω rpm  at step S 10 . Specifically, the monitoring unit  11  may monitor the torque command T e * inputted to a current command generator (e.g., generating d and q axes current commands) for a motor controlling. Further, the monitoring unit  11  may monitor the motor speed ω rpm  calculated from the speed calculator  19  based on an absolute angular position θ detected by the resolver R of the motor  17 . 
     Then, the monitoring unit  11  determines whether the current motor speed ω rpm  is less than a first reference speed ω rpm   _   cal  at step S 20 . 
     In the current switching frequency, in the case that the motor speed ω rpm  is equal to or greater than the first reference speed ω rpm   _   cal , the inverter is controlled in a double sampling mode (F samp =2×F sw ) using a frequency that is two times the switching frequency as the sampling frequency at step S 42 . 
     Next, in the case that the motor speed ω rpm  is less than the first reference speed ω rpm   _   cal , the monitoring unit  11  determines whether a current absolute value of the torque command |T e *| is less than a first reference torque T e   _   cal  at step S 30 . 
     In the current switching frequency, the absolute value of the torque command |T e *| is equal or greater than the first reference torque T e   _   cal , the inverter is controlled in the double sampling mode (F samp =2×F sw ) using the frequency that is two times the switching frequency as the sampling frequency at step S 42 . 
     In the case that the absolute value of the current torque command |T e *| is less than the first reference torque T e   _   cal , the inverter is controlled in a mixed sampling mode alternately repeating a single sampling mode (F samp =F sw ) and the double sampling mode (F samp =2×F sw ) for a predetermined period at step S 40 . 
     The mixed sampling mode will be described with reference to  FIG. 3 . 
       FIG. 3  is a graph illustrating the sampling frequency and the switching frequency according to embodiments of the present disclosure. In the case that the motor speed ω rpm  is less than the first reference speed ω rpm   _   cal  and the absolute value of the current torque command |T e *| is less than the first reference torque T e   _   cal , the monitoring unit  11  controls the inverter in the mixed sampling mode at step S 40 . 
     For example, by a signal repeating 1 or 0 per a half period 0.5T —ss  of a predetermined period T —ss , the double sampling mode (F samp =2×F sw ) for setting the switching frequency F sw  to the frequency that the sampling frequency F samp  is two times the switching frequency F sw , and the single sampling mode (F samp =F sw ) for setting the sampling frequency F samp  identical to the switching frequency F sw  are repeated. 
     In time periods of t 1  to t 2  and t 3  to t 4 , the double sampling mode (F samp =2×F sw ) is selected, and in time periods of t 2  to t 3  and t 4  to t 5 , the single sampling mode (F samp =F sw ) is selected. In this case, each time period has an identical period to the half period 0.5T —ss . 
     As such, after the inverter switching frequency F sw  is changed into the switching frequency set according to the current torque command T e * and the motor speed ω rpm , the changed switching frequency is delivered to the PWM signal generator  15 . Accordingly, a triangle-wave oscillator signal is generated by using the changed switching frequency, and a PWM signal is generated. 
     Herein, a method of generating the triangle-wave oscillator signal and the PWM signal depending on the switching frequency F sw  is a conventional art, so detailed descriptions will be omitted. 
     However, in embodiments of the present disclosure, the switching frequency F sw  is varied depending on the current torque command T e * and the motor speed ω rpm , and the PWM signal generator  15  generates the PWM signal by using the changed switching frequency. Depending on the PWM signal generated in this process, operating On/OFF of the switching element in the PWM inverter  15  is controlled. 
     Since a conventional system configuration such as the current command generator  12  for generating d and q axes current commands i d * and i q *, the current monitoring unit  13  for generating d and q axes voltage commands V d * and V q *, the d-q/3-phase coordinate converter  14  for outputting 3-phase voltage commands V a *, V b *, and V c *, the 3-phase/d-q coordinate converter  18  yielding d and q axes feedback currents i d  and i q , and the like. For this, detailed descriptions thereof will be omitted in the present specification, because they are a well-known conventional art. 
     Referring to  FIG. 4  to  FIG. 6 , phase currents and measured noises in the case of using switching and sampling frequencies in the double sampling mode, and in the case of using switching and sampling frequencies in the mixed sampling mode will be described in comparison. 
       FIG. 4  is a graph illustrating a simulation result of a phase current depending on the switching frequency, and  FIG. 5  and  FIG. 6  are graphs illustrating a simulation result of an amount and a frequency of noise depending on the switching frequency. 
       FIG. 4( a )  is a graph illustrating a U-phase current I a , a V-phase current I b , and a W-phase current I c  of a measured PWM current, in the case of using switching and sampling frequencies only in the double sampling mode. Further,  FIG. 4( b )  is a graph illustrating the U-phase current I a , the V-phase current I b , and the W-phase current I c  of the measured PWM current, in the case of using switching and sampling frequencies in the mixed sampling mode. 
     In time periods of t 11  to t 12  and t 13  to t 14 , the double sampling mode (F samp =2×F sw ) is selected, and in time periods of t 12  to t 13  and t 14  to t 15 , the single sampling mode (F samp =F sw ) is selected. Accordingly, the ripple of the phase current in time periods of t 11  to t 12  and t 13  to t 14  is smaller than the ripple of the phase current in time periods of t 12  to t 13  and t 14  to t 15 . Hence, NVH performance is improved in a first case of using the mixed sampling mode, in comparison with a second case of using only the double sampling mode (F samp =2×F sw ). 
       FIG. 5  is a graph illustrating a magnitude and a frequency of measured noise in the case of using switching and sampling frequencies only in the double sampling mode. Further,  FIG. 6  is a graph illustrating the magnitude and the frequency of measured noise in the case of using switching and sampling frequencies in the mixed sampling mode. 
     In a 4 KHz region (illustrated by a dotted line) of  FIG. 5  and  FIG. 6 , the magnitude of noise when operating in the mixed sampling mode is smaller than the magnitude of noise when operating in the double sampling mode. Accordingly, the amount of noise is reduced in a first case of using the mixed sampling mode, in comparison with a second case of using only the double sampling mode (F samp =2×F sw ), 
     Since the switching frequency is appropriately varied depending on a motor driving condition, and, for the sampling frequency, an appropriate mode change is executed between the double sampling mode and the single mode, so through the entire operational area, an overall improvement in switching loss, electromagnetic performance, NVH performance, control stability, and the like may be achieved in comparison with using one switching frequency and one sampling frequency. 
     While this disclosure has been described in connection with what is presently considered to be practical embodiments, it is to be understood that the disclosure is not limited to the disclosed embodiments. Accordingly, those skilled in the art may easy select and substitute therefrom. Further, a person of an ordinary skill in the art may omit a part among aforementioned constituent elements without a degradation of performance, or add an additional constituent element to improve performance thereof. Furthermore, a person of an ordinary skill in the art may alter a sequence of steps described in the present specification depending on a process environment or equipment. Accordingly, the scope of the present disclosure shall not be determined by aforementioned exemplary embodiments, but shall be determined only according to the attached claims. 
     DESCRIPTION OF SYMBOLS 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 11: monitoring unit 
                 12: current command generator 
               
               
                   
                 13: current monitoring unit 
                 14, 18: coordinate converter 
               
               
                   
                 15: PWM signal generator 
                 16: PWM inverter 
               
               
                   
                 17: motor 
                 19: speed calculator