Real-time hierarchical control strategy design in electric hybrid vehicles

Master's Thesis

Electricity-power

Pollution in big cities has become an acute issue for years. Expert research shows that the main cause of pollution in cities are cars with internal combustion engines. Combustion vehicles have many disadvantages, among which we can mention things such as dependence on a specific type of energy (oil), production of greenhouse gases, production of toxic gases, noise pollution, low efficiency of the system and, as a result, energy loss. According to the above, electric cars have been introduced since the 1890s and have been popular until the 1930s. With the progress of combustion cars, electric cars were slowly forgotten until in 1960 and later, considering the problems of combustion cars, researchers thought of a solution and started various researches on electric cars. Electric hybrid cars are a generalized type of pure electric cars, in which the disadvantages of pure electric cars have been solved to some extent. In fact, these cars are intermediate between conventional cars with a combustion engine and pure electric cars. The use of a high-efficiency electric engine, the possibility of energy recovery, and the ability to move the operating point of the combustion engine to areas with optimal efficiency, has provided a reduction in pollution and an increase in the overall efficiency of these cars. Types of hybrid cars: In general, a hybrid car consists of an energy storage system, a power generation unit, and a transmission system. Spark internal combustion engines, direct injection combustion engines, gas turbines and fuel cells can play a role as a power generation unit, which can be used to provide the driving force of the vehicle by combining them and using an electric motor.

For the energy storage unit, flywheels, capacitors, and batteries can be considered. But among these choices, batteries are the most used. The transmission system consists of mechanical gear box, gears, differential, clutch, etc.

According to the control structure and the method of connecting components to each other, hybrid cars are divided into the following three categories:

1-series hybrid cars

2-parallel hybrid cars

3-combined hybrid cars (series-parallel)

In series cars, the electric motor is the main driving force. In fact, the set of batteries feed the relatively high power electric motor. In a situation where the state of charge of the battery decreases from the lowest allowed value, at this time the combustion engine starts working and by turning the generator, it charges the batteries. It is natural that this action increases the driving range of the car. In this type, the electric motor will charge the batteries in the role of a generator when the car is working in combustion mode only. Depending on the type of control strategy, the electric motor may start working at the beginning (at low speeds) and then the combustion engine will enter the system (at high speeds). Hybrid hybrid car is actually a combination of two series-parallel systems. The components of the propulsion system in hybrid hybrid cars are: 1-two power generation sources, a combustion engine or fuel cell. together with a traction motor to create driving force and energy recovery. 2- Continuously variable transmission system, CVT[1] 3- An electromagnetic clutch for the power transmission system 4- A small electric motor to generate electrical energy (charging) and start the combustion engine 5- Batteries The way the components of this system are connected in different motion modes is done by the control units. accept Two points that should be taken into consideration about hybrid electric cars, one is the issue of energy recovery in the process of deceleration and braking by the electric motor, which can somehow improve energy consumption. The second point is the lack of pollution due to the lack of fuel consumption in stopping conditions. In this case, which is caused by the problem of urban traffic, the car works in electric mode, and as a result, it will reduce pollution.

Control strategies in hybrid electric vehicles

So far, various control strategies have been presented for optimal energy management in hybrid electric vehicles. Control strategies or energy management for hybrid electric vehicles are basically used to meet several objectives at the same time. The first goal is usually to minimize fuel consumption, and trying to reduce pollution and meet the vehicle's drivability is one of the main goals. Regardless of the structure of the electric hybrid vehicle, the main goal of the control strategy is the momentary management of power transfer between energy sources and achieving the main control goals. One of the important characteristics of the control strategy is that the control objectives are mostly integral (fuel consumption and pollution per mile of track) or quasi-local in time (propulsion capability in each time interval). While control functions are local in time. In addition, the control goals are often under integral constraints, such as not keeping the charging state of the batteries in the desired range. The general nature of all objectives and constraints cannot lead to general optimization techniques, because the future is uncertain in a real motion situation. For this purpose, there are some methods that build the control strategy based on the results of general optimization on a predetermined cycle. But these methods do not directly lead to practical implementation, because the main problem with the overall optimization criterion is that the entire driving program must be predetermined, and in this case, the real-time control strategy is not easily implemented. For this purpose, in this thesis, according to the complexity of the drive system of the electric hybrid car, a hierarchical control strategy for the electric hybrid car has been investigated. For this purpose, first the dynamic modeling of the subsystems is done, then for each of the subsystems, the corresponding local controller is designed. After that, to achieve the performance goals, the switching strategy between the subsystems is designed to achieve the real-time control strategy. 

Content of next chapters

The main goal of this thesis is to achieve a real-time control strategy for electric hybrid vehicles. For this purpose, the existing control strategies have been identified in the first chapter. In the second chapter, due to the fact that smart methods have been used in this thesis, smart control strategies have been examined. In the third chapter, the hierarchical control structure of the electric hybrid vehicle as a hybrid system has been investigated with emphasis on the dynamic modeling of subsystems. In the fourth chapter, the design of the hierarchical intelligent control strategy for the electric hybrid car has been discussed, and in the fifth chapter, the real-time hierarchical control strategy for the electric hybrid car and its simulation have been explained.

Introduction

Due to the complexity of the electric hybrid car, different control methods and algorithms have been used to control it. In a general classification, the control strategies in hybrid electric vehicles can be divided into five categories:

1) Experimental control strategy

This method is based on the results obtained from experimental and laboratory information and is based on static models of the system. In this method, the functional modes of the hybrid vehicle system can be identified and this method can be easily implemented in practice.

2) Control strategy based on static optimization

In this method, static and quasi-static assumptions are used for modeling, and a control strategy is built by using combustion engine efficiency maps and other sub-systems of the car's drivetrain.

3) Control strategy based on control Optimum

This method is based on the dynamic and quasi-static nature of subsystems and is based on dynamic planning methods and optimal control theory. 4) Control strategy based on dynamic control.  It is used to analyze the stability of the system.

5) Control strategy based on intelligent methods

In this method, intelligent methods such as genetic algorithm, fuzzy control, neural network and so on. is used Smart strategies are presented in general in the second chapter.

1-1) Control strategies based on empirical laws

Many functional control strategies are based on observations and empirical laws.