【正文】 icles with less environmental impact. One promising method that is currently being implemented is the hybrid electric vehicle. Hybrid vehicles are defined as vehicles that have two or more power sources [25]. There are a large number of possible variations, but the most mon layout of hybrid vehicles today bines the power of an internal bustion engine (ICE) with the power of an electric motor and energy storage system (ESS). These vehicles are often referred to as hybrid electric vehicles (HEV’s) [25]. These two power sources are used in conjunction to optimize the efficiency and performance of the vehicle, which in turn will increase fuel economy and reduce vehicle emissions, all while delivering the performance the consumer requires. In 1997, the Toyota Prius became the first hybrid vehicle introduced into mass production in Japan. It took another three years for the first mass produced hybrid vehicle, the Honda Insight, to be introduced into the North American market. The release of the Honda Insight was closely followed by the release of the Toyota Prius in North America a couple of months later [35]. Hybrid electric vehicles have the distinct advantage of regenerative braking. The electric motor, normally used for propulsion, can be used as a generator to convert kiic energy of the vehicle back into electrical energy during braking, rather than wasting energy as heat. This electrical energy can then be stored in an ESS (. batteries or ultracapacitors) and later released to propel the vehicle using the electric motor. This process bees even more important when considering the energy density of batteries pared to gasoline or diesel fuel. Energy density is defined as the amount of energy stored in a system per unit volume or mass [44]. To illustrate this point, 4 kilograms ( litres) of gasoline will typically give a motor vehicle a range of 3 50 kilometres. To store the same amount of useful electric energy it requires a lead acid battery with a mass of about 270 kilograms [25]. This demonstrates the need for efficient regenerative braking to store electrical energy during driving, which in turn will keep the mass of the energy storage system down and improve the performance and efficiency of the HEV. Research Scope Regenerative Braking Systems The scope of the research presented is to create a low cost regenerative braking system to be used on future economical hybrid vehicles to study the interaction between regenerative and mechanical braking of the system. This system should be able to control the bination of both regenerative and mechanical braking torque depending on driver demand and should be able to do so smoothly and safely. Controlling the regenerative braking torque can be done using control algorithms and vector control for induction motors. However, controlling the mechanical braking torque independently of the driver pedal force, while maintaining proper safety backups, proved to be more of a challenge. To overe this problem, a system was developed that would attenuate the pressure in the brake booster in order to control the amount of mechanical torque developed by the braking system． Hybrid Electric Vehicle Overview Hybrid vehicles have emerged as one of the short term solutions for reducing vehicle emissions and improving fuel economy. Over the past 10 years almost all of the major automotive panies have developed and released for sale their own hybrid electric vehicles to the public. The popularity of hybrid electric vehicles has grown considerably since the turn of the century. With enormous pressure to bee more environmentally friendly and with unpredictable gas prices, the sales of hybrid electric vehicles have increased dramatically in recent years. Hybrid Configurations For the past 100 years the objective of the hybrid has been to extend the range of electric vehicles and to overe the problem of long recharging times [35]. There are three predominant hybrid electric vehicle configurations currently on the market today. These configurations are known as series hybrids, parallel hybrids and 4 series/parallel hybrids. Each configuration has its advantages and disadvantages which will be discussed in the following sections. Series Hybrids In series hybrids the mechanical output from the internal bustion engine is used to drive a generator which produces electrical power that can be stored in the batteries or used to power an electric motor and drive the wheels. There is no direct mechanical connection between the engine and the