CHINESE JOURNAL OF MECHANICAL ENGINEERING ·862· Vol. 22,aNo. 6,a2009 DOI: 10.3901/CJME.2009.06.862, available online at www.cjmenet.com; www.cjmenet.com.cn Series-parallel Hybrid Vehicle Control Strategy Design and Optimization Using Real-valued Genetic Algorithm XIONG Weiwei, YIN Chengliang, ZHANG Yong, and ZHANG Jianlong* Institute of Automotive Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Received June 12, 2008; revised August 24, 2009; accepted September 7, 2009; published electronically December 20, 2009 Abstract: Despite the series-parallel hybrid electric vehicle inherits the performance advantages from both series and parallel hybrid electric vehicle, few researches about the series-parallel hybrid electric vehicle have been revealed because of its complex construction and control strategy. In this paper, a series-parallel hybrid electric bus as well as its control strategy is revealed, and a control parameter optimization approach using the real-valued genetic algorithm is proposed. The optimization objective is to minimize the fuel consumption while sustain the battery state of charge, a tangent penalty function of state of charge(SOC) is embodied in the objective function to recast this multi-objective nonlinear optimization problem as a single linear optimization problem. For this strategy, the vehicle operating mode is switched based on the vehicle speed, and an “optimal line” typed strategy is designed for the parallel control. The optimization parameters include the speed threshold for mode switching, the highest state of charge allowed, the lowest state of charge allowed and the scale factor of the engine optimal torque to the engine maximum torque at a rotational speed. They are optimized through numerical experiments based on real-value genes, arithmetic crossover and mutation operators. The hybrid bus has been evaluated at the Chinese Transit Bus City Driving Cycle via road test, in which a control area network-based monitor system was used to trace the driving schedule. The test result shows that this approach is feasible for the control parameter optimization. This approach can be applied to not only the novel construction presented in this paper, but also other types of hybrid electric vehicles. Key words: series-parallel hybrid electric vehicle, control strategy, design, optimization, real-valued genetic algorithm 1 Introduction∗ Because of the energy and environment issues, the hybrid electric vehicles(HEV) are considered as one of the most viable alternatives to the conventional vehicles. HEV is a complex system combining internal combustion engine(ICE), electric motor(EM) and energy storage system(ESS). Energy management is a critical issue for the implementation of HEVs. Based on a proper strategy, the control system governs the power flow among the ICE, the EMs and the ESS to minimize the fuel consumption and emissions of pollution while meet the driver’s intention. There are many works focused on this issue. Dynamic programming(DP), fuzzy logic control(FLC) and artificial neural networks (ANN), etc are investigated in Refs. [1–8] for the hybrid control strategies. DP is a global solution which has the disadvantage of high computational cost, and it can not be applied in a real vehicle. The instantaneous optimization algorithm, which is suitable for engineering application, is often a rule-based type like the FLC. For such a rule-based control strategy, the parameters derived from the engineering intuition can not be directly used since they often result in the failure of achieving * Corresponding author. E-mail: [email protected] This project is supported by National Hi-tech Research and Development Program of China (863 Program, Grant No. 2006AA11A127) satisfactory overall system efficiency for a high non-linear system. Therefore, both the control parameters and the physical parameters for a hybrid vehicle require to be optimized to satisfy the different driving cycle conditions. Genetic algorithms(GA) are adaptive heuristic search technique used in computing to find exact or approximate solutions to optimization and search problems. Most of the previous studies depend on the binary coded genetic algorithm(BGA). POURSAMAD, et al[9] and MONTAZERI, et al[10], described an optimization approach based on BGA for the parallel hybrid electric vehicles(PHEV); ZHU, et al[11], also used BGA to optimize EQ7200’s physical parameters. Hybrid electric vehicle falls into three categories: series, parallel and series-parallel. Seen in Fig. 1, a typical series hybrid electric vehicle(SHEV) incorporates an engine, a generator, an electric motor and the batteries. The engine is disconnected with the wheels and it can not directly drive the wheels for a SHEV. A PHEV often uses a torque coupler (transmission) to couple the torques from the engine and the electric motor to drive the wheels together. Since the development of series and parallel hybrid system has accelerated over the past decade, the focus of R&D efforts has recently shifted to series-parallel hybrid electric vehicle(SPHEV)[12–17]. As is shown in Fig. 2, a SPHEV combines a series hybrid system with a parallel hybrid system to extract the benefits of both systems. It involves CHINESE JOURNAL OF MECHANICAL ENGINEERING an additional mechanical link compared with the series hybrid, an additional generator compared with the parallel hybrid[18]. This system often switches between series and parallel mode (configuration), which means that the control strategy is complex and the parameter is hard to be optimized for such a system. Conclusions are given in section 8. 2 Series-parallel Bus Configuration The optimization method has been applied to the case of an experimental series-parallel hybrid electric city transit bus. This vehicle is schematically shown in Fig. 2. The ICE and the generator are located upstream of the clutch, the crankshaft of the engine is directly linked with the rotor of the generator. The traction motor is placed downstream of the transmission; it is linked to the output shaft of the transmission via a torque converter, then to the propeller shaft, differential, half shafts and wheels. The specifications of the vehicle and the major components are listed in Table 1. The generator also acts as a starter which is used to start the engine as well as to combine with the ICE as an on-board generator set when the battery needs to be charged. Table 1 Specifications of the vehicle and the components Vehicle and component Vehicle ICE Fig. 1. Typical scheme of the series and parallel hybrid electric vehicles Traction motor Generator Battery 3 Fig. 2. Scheme of the series-parallel hybrid electric vehicle In this paper, the real-valued genetic algorithm(RGA) is applied for the control parameter optimization of a series-parallel hybrid vehicle which adopts a rule-based energy management strategy. The aim of the application of this algorithm is to find proper parameters to minimize an objective function after certain driving cycle. This paper is built up as follows. The vehicle configuration and the specifications will be described in section 2. The energy management strategy is then briefly introduced in section 3. The problem to be solved is formulated in section 4. The application of RGA for the parameter optimization is presented in section 5. The numerical experiment is given in section 6. The road test results are described in section 7. ·863· Parameter Curb weight M/kg Frontal area AF /m2 Aerodynamic coefficient CD Wheel radius r /m Peak power P/kW Peak torque T/(N•m) Speed range n/(r•min–1) Peak power P/kW Continuous power Pc/kW Peak torque T/(N•m) Peak power P/kW Continuous power Pc/kW Peak torque T/(N•m) Type Power capability C/Ah Rated voltage U/V Value 12 000 7.5 0.62 0.502 135 550 0–2 500 80 40 450 30 20 290 NiMH 60 336 Hybrid Control Strategy For this system, the series typed configuration is enforced when the vehicle is moving below certain a speed vCLU. This feature will help to reduce the fuel consumption as well as the emission of pollutions. When the vehicle operates as a SHEV, the vehicle is propelled by the traction motor alone, the engine is shut down unless the battery requests charging to prevent the depletion. When the speed is higher than vCLU, the vehicle runs as a PHEV, and then the requested torque should be distributed to two energy sources: the ICE and the traction motor. Of course, the generator can also act as a motor to drive the vehicle. But in this operation, it is not employed in order to simplify the strategy. CHAU, et al[18], presented several types of control strategy for the parallel hybrid electric vehicles. In this paper, a “optimal line” typed strategy to operate the engine according to an optimal speed-torque line is employed. This strategy can be illustrated in Fig. 3. The optimal ·864· YXIONG Weiwei, et al: Series-parallel Hybrid Vehicle Control Strategy Design and Optimization YUsingYReal-valued in an Isothermal Genetic Tank Algorithm Y Y engine torque at this line is scaled to the engine maximum torque at a rotational speed: TOPT = ϕ i TMAX , (1) where TOPT —Optimal torque, TMAX —Engine maximum torque, φ —Scale factor. END END where TENG is the engine torque, TCH is the charge torque, TMOTOR is the traction motor torque, TREQ is the required torque. Regenerative braking is another feature exploited by HEVs to reduce the fuel consumption. If the brake pedal is depressed (the driver requests a torque below zero), the traction motor will act as a generator to capture the kinetic power. A linear braking control algorithm is used for this vehicle as follows: TGEN = max(λ iTREQ , TGenMax ), (2) where TGenMax —Maximum generative torque of the traction motor at a rotational speed, TGEN —Regenerative torque commanded to the motor, λ —Regenerative factor. 4 Fig. 3. Brake specific engine fuel consumption contours (g/(kW•h)) A SOC sustaining policy is also included in the control strategy. When the SOC drops down below the lowest state of charge allowed (Sl), the engine should be enforced to operate at the maximum torque to provide additional charging power. When the SOC rises up higher than the highest state of charge allowed (Sh), the original request power, but not TOPT, should be directly commanded to the engine to avoid deep charge of the battery when the request torque is below TOPT. The control strategy, except the regenerative braking, can thereby be structurally described as follows: IF vehicle speed<vCLU (SHEV) IF Battery Request Charging Idling Charge (TENG=TCH); Motor Drives Alone (TMOTOR= TREQ); Charge State=1; ELSE Engine Shuts Down (TENG=0); Motor Drives Alone (TMOTOR= TREQ); END ELSE (PHEV) IF S>Sh TENG=min(TOPT, TREQ); TMOTOR=TREQ–TENG; ELSE IF S<Sl Engine Propels Fully (TMOTOR= TMAX); TMOTOR= TREQ – TMAX; ELSE Optimal Line Control (TENG=TOPT); TMOTOR=TREQ – TOPT; Problem Formulation The optimization objectives are as follows: (1) to minimize fuel consumption; (2) to sustain the battery state of charge. The property of an optimization problem can be exploited by Eq. (3): ⎧⎪min J ( x), ⎨ x∈ X ⎪⎩s.t. g ( x) ≤ 0, (3) where x —Solution to the problem including a vector of the control parameters within the solution space X which defines the lower and upper bounds of the parameters, J(•) —Objective function, g(•) —Non-linear constraints. For a fuel targeted instantaneous control strategy, the objective is to minimize the fuel mass flow rate at each period for a charge-sustained hybrid electric vehicle: min m f (t ), ∀t, x (4) where m f —Fuel mass flow rate, t —Time. From a global point of view, the objectives are transferred to minimize the fuel consumption at the terminal time of a driving cycle, but not at local periods. The objective is then formulated as follows: tn min J ( x) = min ⎛⎜ ∫ m f (t )dt ⎞⎟ , x x ⎝ 0 ⎠ (5) where tn— Terminal time of a driving cycle. Note that the charge sustaining policy is not included in CHINESE JOURNAL OF MECHANICAL ENGINEERING this objective function. So as to recast this multi-objective nonlinear optimization problem as a single linear optimization problem, a tangent penalty function (Eq. (4)) is added to the objective function to constrain the final battery state of charge as well as to minimize the battery energy consumption as follows: P (tn ) = α i tan( S0 − S (tn ) ), is the same as that between Sd and Sh, and then if ∆S and Sd are determined, Sl and Sh are consequently determined. In order to operate the battery in a multitude of times, Sd are all set to 0.55 on the contrary the battery resistance is the lowest. The relationships between these variables are as follows: ⎧ Sl = Sd − ΔS , ⎨ ⎩ Sh = Sd + ΔS . (6) where P(•) —Penalty function, α —Penalty factor. Then the objective function is redefined as follows: CJ ( x) = tn ∫0 FC (t )d t + P(tn ). ·865· (8) (7) 5 Application of RGA 5.1 Overview of genetic algorithm GA is premised on the evolutionary ideas of natural selection and genetic, which relies on the use of selection, crossover and mutation operations to generate the offspring of the existing population. GAs do not require derivative information or other auxiliary knowledge. Only the objective function and corresponding fitness values influence the directions of search. The more fit individuals have, the better chance to be selected as the parents. The genetic algorithm normally works in this way: first, a population is created with a group of individuals randomly; and then two individuals selected based on their fitness reproduce one or more offspring; after that offspring are mutated randomly. This continues until a suitable solution has been found or a certain number of generations have passed. Compared with binary coded genes, the real-valued genes often offer a number of advantages over them[19]. For example, the binary-coded chromosome should be converted to a phenotype so as to evaluate the fitness. However, the real-valued type does not need the additive process, which increases the efficiency of the GA. RGA is used to represent the chromosome in this study while most of the previous studies depend on a binary-coded type. Fig. 4 shows optimization flowchart of RGA for the SPHEV. The series-parallel simulation program(SPSP) is the simulation tool including the vehicle models coded in the Simulink environment. 5.2 Individuals The individuals represent the control parameters to be optimized, since the control strategy switches the mode mainly based on four parameters which are Sh, Sl, vCLU and φ. These parameters should be optimized to meet the objectives. To reduce the computational cost, an approach described below is used to cut down the individual numbers. As shown in Fig. 5, Sd represents the desired state of charge. It is assumed that the interval value (∆S) between Sd and Sl Fig. 4. Optimization flowchart of RGA for SPHEV Fig. 5. Resistance characteristics of the battery pack Then, ∆S, φ and vCLU are considered as the optimizing parameters. The aim of the optimization is thereby to find a variable vector xOPT to minimize the objective function above: OPT x OPT = (ΔS OPT ϕ OPT vCLU ). (9) Based on the desired performance and characteristics of the components, the bounds of the variables are listed in Table 2. YXIONG Weiwei, et al: Series-parallel Hybrid Vehicle Control Strategy Design and Optimization YUsingYReal-valued in an Isothermal Genetic Tank Algorithm Y Y ·866· Table 2. where X At , X Bt —Pair of old populations before crossover operation, t+1 t+1 X A , X B —Pair of new populations after crossover operation, σ —Random number controling the variance of each crossover operations. Bounds of the decision variables Parameter SOC interval ∆S/% Optimal torque ratio to maximum torque φ Series-parallel switch speed vCLU/(km•h–1) Lower bound 5.0 Upper bound 20.0 0.5 1.0 12.0 20.0 5.3 Fitness function In this case, the objective function value is transformed into a measure of relative fitness to solve a minimization problem by the non-linear ranking method. 5.4 Selection operator Stochastic universal sampling ensures a selection of offspring, which is closer to what is deserved than roulette wheel selection. The stochastic universal sampling selection method is adopted here to decide whether a chromosome can survive to the next generation. The chromosomes that survive to the next generation are placed into a matting pool for crossover and mutation operations. 5.5 Crossover operator Once a pair of chromosomes has been selected for crossover, one or more randomly selected positions are assigned to the to-be-crossed chromosomes. The newly crossed chromosomes are then combined with the rest of the chromosomes to generate a new population. An arithmetic crossover operator is used, which is a linear combination of two chromosomes. This method can be formulated as follows: 5.6 Mutation operator After the crossover operation, the mutation operation determines if a chromosome should be mutated in the next generation, which plays the roles of recovering the lost genetic materials as well as randomly disturbing genetic information. With real-valued representations, mutation is achieved by either perturbing the gene values or random selection of new values within the allowed range. In this study, uniform mutation method is applied: ⎧ X t = {x1t , x2t , ⋅⋅⋅, xkt , ⋅⋅⋅, xnt }, ⎪⎪ t+1 ⎨ xk = Lk + r (U k − Lk ), ⎪ t+1 t t t+1 t ⎪⎩ X = {x1 , x2 , ⋅⋅⋅, xk , ⋅⋅⋅, xn }, where X t —Population before mutation operation, X t+1 —Population after mutation operation, n —Number of variables, k —Position of the mutation, r —Random number between 0 and 1, Lk, Uk —Lower and upper bounds on the variables at location k. 6 ⎧⎪ X At+1 =σ + (1 − σ ⎨ t+1 t t ⎪⎩ X B = σ X A + (1 − σ ) X B , X Bt ) X At , (10) Fig. 6. (11) Numerical Experiment Case As mentioned in section 4, SPSP shown as Fig. 6 is used to perform the optimizations. Interface of SPSP CHINESE JOURNAL OF MECHANICAL ENGINEERING SPSP is forward simulation tool for series-parallel evaluation on fuel economy, emission and other performance. In this tool, the quasi-static model of the engine as well as the electric motors, and an internal resistance typed battery model are used. The vehicle is considered as a moving mass subjected to the traction force. The road load includes the aerodynamic drag force, the rolling resistance of the tires and the gravitational force. Computational simulations works were carried out on the Chinese Transit Bus City Driving Cycle which are shown in Fig. 7. The following parameters are configured to perform the GA for the application under consideration. (1) Max number of generations: 40; (2) No. of individuals in a population: 25; (3) No. of variables: 3; (4) Survival selection: stochastic universal sampling; (5) Crossover (recombination) method: arithmetic crossover; (6) Mutation method: uniform mutation; (7) Mutation rate: 0.025; (8) Penalty factor α: 1 000; (9) Initial battery state of charge: Si= Sd=0.55. Fig. 7. parameters are set to the optimal values, and the vehicle is loaded 3.5 t. We use an on-board monitor system based on controller area network(CAN) to guide the tracing of the driving schedule. The vehicle parameters including the vehicle speed, the transient SOC and the fuel consumption are collected by this system. Table 3. Optimized parameters and results Parameter SOC interval ∆S/% Optimal torque ratio to maximum torque φ Series-parallel switch speed vCLU/(km•h–1) Fuel consumption C/(L•100 km–1) Value 18.12 0.62 13.15 32.29 As shown in Fig. 9, the dot line is the actual speed, and the real line is the corresponding desired speed. The transient SOC is presented in Fig. 9, and the fuel consumption with the index time is shown in Fig. 10. The data shows that the optimal fuel economy as well as the optimal final SOC can be achieved using the optimal control parameters. Because some tracing is missed during a real road test, the absolute fuel economy is incomparable; the specific fuel consumption in liters per 100 km is used to evaluate the optimal fuel economy and actual fuel economy. Table 4 shows that the actual results are similar to the optimal results, which validate the optimization is effective. Chinese Transit Bus City Driving Cycle Fig. 8 shows the optimization process history for the driving cycles. In this optimization case, the final SOC is sustained to the initial (desired) value after a driving cycle, which means that the minimum objective value equals to the optimal fuel consumption. In addition, the optimized results are shown in Table 3. Fig. 9. Simulated and actual battery state of charge Fig. 10. Table 4. Simulated and actual fuel consumptions Comparison on simulation and road test results Parameter Fig. 8. ·867· Optimization process history of RGA Distance d/km Initial SOC 7 Road Test Validation Final SOC Si/% Sf/% Fuel consumption C/mL A road test is carried out to validate the optimization parameter’s performance. During the test, the control Specific fuel consumption C1/((L•100 km–1) Simulation Road test Pessimistic 5.87 5.74 5.36 55.0 55.0 55.0 55.0 54.7 54.75 1 894.4 1 903.5 1 826.5 32.29 33.16 34.08 ·868· 8 YXIONG Weiwei, et al: Series-parallel Hybrid Vehicle Control Strategy Design and Optimization YUsingYReal-valued in an Isothermal Genetic Tank Algorithm Y Y Conclusions (1) Series-parallel electric vehicle control parameters need to be optimized to meet the requirements on fuel economy as well as to prevent the depletion while satisfying the driver’s intention. (2) HEVs’ multi-objective nonlinear optimization can be transferred to a single-objective optimization problem using penalty method. For the sake of reducing the computational cost, it is necessary to cut down the number of optimization parameters. (3) Real-valued genetic algorithm is a feasible approach to optimize the parameters for an “optimal line” typed control strategy; road test results validate its effectiveness. 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CA: Morgan Kaufmann, 1991: 205–218. Biographical notes XIONG Weiwei is currently a doctoral candidate in Institute of Automotive Engineering, Shanghai Jiao Tong University, China. He received his master’s degree from School of Mechanical Engineering, Shanghai Jiao Tong University, China, in 2005. His research interest is in advanced hybrid electric vehicle control and optimization. E-mail: [email protected] YIN Chengliang is currently a professor in Institute of Automotive Engineering, Shanghai Jiao Tong University, China. He received his PhD degree from Jilin University, China. His research interests include hybrid electric vehicle, vehicle electronics, advanced transmission system, etc. E-mail: [email protected] ZHANG Yong is currently a post-doctorate in Institute of Automotive Engineering, Shanghai Jiao Tong University, China. He received his PhD degree from School of Mechanical Engineering, Shanghai Jiao Tong University, China, in 2006. His research interests include hybrid vehicle electronics, ESP. E-mail: [email protected] ZHANG Jianlong is currently a post-doctorate in Institute of Automotive Engineering, Shanghai Jiao Tong University, China. He received his PhD degree from School of Mechanical Engineering, Shanghai Jiao Tong University, China, in 2008. His research interests include hybrid vehicle regenerative brake control, ABS.
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