Development and application of fuzzy logic based full electrical braking for electric road vehicles

The bi-directional conversion between electrical energy and kinetic energy is the fundamental natural phenomenon that separates electrical and conventional internal combustion engine (ICE) vehicles. With this conversion, which occurs thanks to electrical machines, kinetic energy; that is converted to heat in the drums or discs of the conventional vehicles and dissipated to environment, can be stored in the batteries and ultra-capacitors and re-used in the electrical vehicles. This is called regenerative braking. Regenerative braking is a method which increases the total efficiency and range of an electrified vehicle, but it is only possible on certain circumstances where the electrical energy can be stored. Almost all commercial or prototype full electric vehicles and mild and more hybridized electric vehicles utilize regenerative braking. Vehicles with very large kinetic energy, especially rail vehicles like locomotives and some equipment that require very large braking power like cranes are also utilize dynamic electrical braking. In this method, it is seen that the kinetic energy is converted to electrical energy, but instead of storing, it is converted to heat on some resistor units. The main reason to use dynamic braking in such systems is to avoid the mechanical brakes to run off quickly. Similar application is seen on retarder systems that use Foucault currents to help slowing down of heavy-duty vehicles such as buses and trucks. Application of a retarder mechanism or dynamic braking was not seen on vehicles like automobiles, which have relatively less mass and kinetic energy. Another application of electric braking is to apply a field to rotate an electrical machine in reverse, while it is already rotating on a positive reference. In the literature, this method is called braking with reverse current-voltage or plugging-plug braking. When this method is applied, voltage-current levels will increase fast, due to the reverse field. In addition, energy is consumed for braking. That would reduce the application fields of this method. Generally, this method is applied on production lines with high inertia parts (i.e. metal mills), where instantaneous braking is needed to increase overall line efficiency. Today, all vehicles including the electrically driven ones, utilize mechanical braking systems. These systems are generally very complicated hydraulic systems, driven by solenoid valves and they are capable of applying chassis applications such as anti-blockage systems (ABS) or electronic stability programs (ESP) which are also safety requirements to be met. Modern electric vehicle braking systems are designed as combinations of regenerative and mechanical braking systems, which have been described above. Since electrical braking is only possible in the electrically driven wheels, algorithms to share the braking force between electrical and mechanical brakes, without risking the vehicle dynamics are being developed. Since the on-board electrical energy is limited by the capacity of the battery pack and it is very precious because of that, it is extremely important to have maximum amount of regenerative braking. Nevertheless, when the battery is fully charged or in case the braking energy cannot be absorbed, electrical braking is replaced by more mechanical braking. Additionally, to be able to charge a battery with regenerative energy, generated voltage must be above the battery voltage and the difference must produce enough current to have desired braking force. This is especially not possible in low speeds, where adequate the inverse electromotive force cannot be produced. The main purpose of this thesis is having the novel braking algorithm to stop an electric vehicle with electrical braking effect thus minimizing or eliminating mechanical braking effect. For full electrical braking, three main electrical braking methods will be used. Regenerative braking will be designed to regenerate maximum available kinetic energy. When the desired regeneration or braking effect is not obtained, dynamic braking will be available. Especially, when the vehicle is very slow to induce enough current-voltage, plugging method is planned to be used. Components in electrical vehicles are combination of electrical-mechanical-chemical systems. Therefore, environmental effects easily affect them. In addition, a vehicle may have different dynamics due to loading, ambiance, type and slope of the road, etc., even in the same running cycle. Due to numerous such reasons, instead of using a definitive control method, a fuzzy based control system is used in this thesis work. Thus, it will be possible to switch through desired braking methods to see desired effect. To be able to define how the electrical braking methods should be taken into braking process, first of all, dynamics and limits of these braking methods are investigated. A small scale test platform, consisting of a BLDC machine driving a large metal disk, which is used as an inertia, is built. Using this platform, limitations of the braking methods are investigated and the dynamics of these methods are defined. The proposed fuzzy controller uses vehicle speed, braking rate and state of charge as inputs. Purpose of this controller is to use these inputs to calculate duty ratios for three different electrical braking methods; regenerative, dynamic and plugging, with respect to their limitations. The calculated ratios, which are going to be 100% in total, represents that the whole braking effect is realized by these electrical braking methods, thus defining a Full Electrical Braking (FEB) as a new method. The control system is also applied on a controller ECU, coded by means of model based control system development methods, using Matlab®, Simulink® and relevant toolboxes and software. This controller also realized on an electric scooter. Braking ratios calculated by the controller is used to control a synchronous DC/DC converter. This DC/DC converter in buck mode, is used to control the motor mode of the scooter by adjusting the proper speed. Boost mode in opposite direction is used to boost the voltage on the terminals of the motor driver, to charge up the batteries. Also in the boost mode, a controlled chopper is added to direct braking power to be dissipated on a resistor bank. In very low speeds, again the buck mode is used in very low ratios but the driver is used to apply a field, opposite to the electric motors turning direction. To realize all this control actions, the ECU is also used as an upper level controller for the scooter. So the gas, brake, mechanical brake or SOC and tachometer of the scooter is connected to the ECU, as well. Standard gas-brake potentiometers, original motor and the batteries of the vehicle are kept. Only the motor driver is changed to have more control on the unit. Overall prototype unit is later used to make tests of the developed control method. With the tests, also some small calibrations on the controller is possible. There will be two main outcomes of the thesis. First outcome will define a structure where all mentioned electrical braking methods would be applied on a vehicle. As the cost of on board electrical energy would reduce, this structure could be used widely. In the ideal case, vehicles can be stopped by only electrical braking, which will be defined as Full Electrical Braking (FEB), in this thesis. Scientifically, a fuzzy controlled such system is not applied before, which will also be the novelty of the thesis. The other outcome will be application of regenerative braking with fuzzy controller. It is possible to use this output on a present electrical vehicle. Due to the commercial potential of this outcome, a project is defined with partnership of Istanbul Technical University and TOFAŞ Türk Otomobil Fabrikası A.Ş.. This project is supported by the Republic of Turkey Ministry of Science, Technology and Industry, Industrial Theses Program (SANTEZ), under the same name with thesis "Development and Application of a New Method for Optimal Braking of Electric Vehicles" numbered "1591.STZ.2012-2" and successfully ended in May 2014. Patent applications regarding novel control algorithm have been done to Turkish Patent Institute under topic name "Bir Fren Kontrol Sistemi ve Yöntemi" with filing number TR20120009307 and to German Patent Office under topic name "Bremssteuersystem und Bremssteuerverfahren" with filing number DE102013215763 and are in pending state. In addition, during thesis research, a solution to brake pedal feedback problem during electrical braking was patented with another patent application under topic name "A Brake System for Vehicles with Electric Drive" to European Patent Office (EPO) and granted the patent rights with the grant number EP2463163(B1). There were also an international conference paper and a national refereed journal paper published. An international cited journal paper is in the evaluation phase, as the thesis is being prepared.