FISITA2010-SC-O-03

POWER CONTROL ALGORITHM FOR A HIGH MOBILITY HYBRID ELECTRIC VEHICLE

Ko, Youngkwan*, 1Kim, Hyunsup, 1Jung, Yuseuk, 2Jeong, Soonkyu, 3Lee, Hyengcheol

1 2

Department of Electrical Engineering, Hanyang University, South Korea Agency for Defense Development, South Korea 3 Division of Electrical and Biomedical Engineering, Hanyang University, South Korea KEYWORDS Series HEV, Military HEV, Control strategy, HEV control algorithm structure, AVL CRUISE ABSTRACT This paper proposes a new supervisory power control algorithm structure for a high mobility hybrid electric military vehicle and new source power distribution strategy that consider the ultra-capacitor. The target vehicle has three different electric energy components, engine/generator, battery, and ultra capacitor. Therefore, the main control strategy of the target vehicle is to optimally distribute demanded power from the power sources in order to deliver appropriate traction power to the traction motors. The proposed algorithm structure is classified into three parts - driver intent determination, mode determination, and power distribution. In the driver intent part, drivers demand power and state flags, which are necessary information for mode determination, are generated by using drivers input. In mode determination part, proper hybrid system operating modes are determined based on the state flags. The power distribution part consists of two sub-parts, source power distribution and traction power distribution. In the source power distribution, the supplying power command of each power source is calculated based on the demand power and available power of each power source. In the traction power distribution part, the traction power distribution between the front and rear traction motors is determined based on the vehicle traction and stability requirement. The simulation model of the target vehicle is developed by CRUISE and the proposed algorithm is realized by using MATLAB/Simulink. Simulation results show the feasibility and effectiveness of the model and the control algorithm. INTRODUCTION More stringent emission regulation and fuel economy are forcing the development of hybrid electric vehicles (HEV) and also the study of hybrid military vehicles is actively progressing.[1,2,3] The hybrid electric vehicles (HEVs) are classified as several different architectures, such as series, parallel, and power split HEVs. Among them, the series HEV has several advantages - flexible layout, simple constitution, high availability of electric energy and so on. That is why many heavy duty HEVs adopted the series HEV configuration. [1] In HEV, a number of objectives such as the performance of vehicle, fuel economy, and reduction of emission are very much dependent on the supervisory control unit.[4] Therefore, the design of control algorithm is the most important part in developing HEV systems. So far, there are many research on control strategies of Series HEV.[4-8] In these literatures, however, very few of these can be found regarding architectures with ultra-capacitor to generate high frequency elements of required power. Also, most of them did not show how to implement this in supervisory control algorithm structure.

Therefore, this paper proposes a new supervisory control algorithm structure for a Series HEV and new source power distribution strategy that consider the ultra-capacitor. The proposed control strategy distributes the electrical power to the ultra-capacitor considering not only the high frequency elements of required powers but also the velocity of vehicle. SERIES HEV CONFIGURATION AND COMPONENTS PARAMETERS The target vehicle has three energy components, engine/generator, battery, and ultracapacitor and has two traction motors at front and rear axles as like as four wheel drive(4WD) vehicle in order to drive on rough road.(see Figure.1)
Electrical Connection Mechanical Connection Front Hub Gear Front Differential Gear

Component Vehicle

Parameter Mass Type

Value 5000 Kg Diesel 171 kW 110 kW 700 V 100 kW 150 kW 20 Ah

Component

Parameter Voltage

Value 700 V 180 kW 1.125 Ah 96.18 kW 47.20 kW 145.31 kW

Ultra Capacitor

Peak power Capacity Peak power

Engine
RMG1
Generator Battery

Rated power Peak power Voltage

MG1

Generator

MG1

Rated power Peak power

Ultra Capacitor

DC/DC Converter

MG2

Engine

Battery

Peak power Rated power Capacity

RMG2

MG2

Rated power

71.31 kW

Rear Differential Gear Rear Hub Gear

Figure 1. Series HEV configuration

Table 1. Components parameters

The battery is connected to system voltage bus without using the DC/DC converter. Hence, supervisory control unit does not control the battery power directly. On the contrary, because ultra-capacitor is connected to system voltage bus through the DC/DC converter, power control of ultra-capacitor can be possible. Therefore, it is possible to control the battery power indirectly by controlling the power of ultra-capacitor and engine/generator. Each size of components is evaluated according to systems requirements[9], and this components parameters are summarized in Table1. SERIES HEV ALGORITHM STRUCTURE The supervisory control algorithm structure of target vehicle is classified into three main parts, Driver intent determination, Mode determination, Power distribution.(see Figure 2)
Vehicle States Front/Rear Motor torque Energy mode

Driver desired torque

Driver Input

Driver Intent Determination

Mode Determination
states flags

Power Distribution

Battery Power Engine/Generator Power U-Cap Power

Driving mode

Figure 2. Series HEV supervisory control algorithm structure

The part of driver intent determination generates some of information such as state flags and drivers required power by considering drivers input. In the part of mode determination,

hybrid operating mode is determined by using state flags which is generated in driver intent determination part. Finally, the power distribution part distributes the drivers required power to energy components according to hybrid operating mode and determines the torque of front and rear motors to propel the vehicle. Driver Intent Determination In the driver intent determination part, the drivers required torque and power is determined by using acceleration pedal signal(APS), brake pedal signal(BPS) and reference traction/braking torque which is maximum motor torque according to present motor speed. (see figure 3). The equation 3 shows how to calculate the desired torque and power.

Reference traction torque (Refdrv) Present motor speed Motor Sped (rpm)

Ref Derised torque = Ref

drv

APS (%) when, BPS = 0% (1)

Figure 3. Reference traction/braking torque

Also, in this part, the state flags are generated according to drivers switch input such as EV switch and Charge switch. The purpose of those switches is to operate the target vehicle in military tactical driving such as silent drive. The generated EV flag and charge flag is used in mode determination part. Mode Determination
Mode Determination Energy mode
[ Charge flag && ! EV flag] [ !Charge flag && ! EV flag]

The hybrid operation mode consists of two operation mode, energy mode and vehicle mode. By a combination of two operation modes, the hybrid operating mode is finally determined. The energy mode represents that which energy component to supply electrical power for propelling the vehicle is selected. The vehicle mode represents the vehicle driving condition such as driving, braking. The great advantage of series HEV is that engine operating condition does not depends on vehicle driving condition. Therefore, it is efficient to separate the hybrid operating mode into two parts, energy mode and vehicle mode which are independent to each other. The mode determination part uses a state-machine method [10] and this part is developed by using state-flow in MATLAB/Simulink. (see Figure 3)

Torque (Nm)

brk BPS (%) when, BPS 0%

Desired Power = Derised torque Motor speed

Vehicle mode EV mode Drive mode

[EV flag] [ !Charge flag && ! EV flag] [ Charge flag && ! EV flag]

[EV flag]

Charge mode

Normal mode

Brake mode

Figure 3. Mode determination block diagram

The vehicle mode consists of two parts - Drive mode and Brake mode. The drive mode distributes the traction power to the traction motors to propel the vehicle, and in case of brake mode braking power is distributed to traction motors and hydraulic brake system. The Energy mode consists of three parts, EV mode, Charge mode, and Normal mode. In EV mode, even if SOC goes down, the vehicle is propelled by not the engine/generator but the battery and the ultra-capacitor. Therefore, it is possible to silence driving because of removing to noise from engine. In charge mode, the engine/generator generates the maximum electrical power in order to supply vehicles required traction power and charge the battery and ultracapacitor. Because purpose of this mode is to charge the battery promptly, the engine/generator is operated to generate maximum power of battery and even if the battery SOC is higher than upper limit of battery SOC, engine/generator continuously generates the power to charge the battery. When switch inputs dont exist, energy mode enters to the normal mode to drive the vehicle without tactical purposes. In this mode, the drivers required power is distributed to the engine/generator, the battery and the ultra-capacitor. The proposed control strategy, modified power-follower control strategy, is adopted in this mode. Power Distirbution Source Power Distribution In source power distribution part, the drivers required power is distributed to each energy components according to energy mode and vehicle mode which are generated in mode determination part. For energy mode distribution, the first power distribution is performed according to drivers required power and energy mode and this distributed power is modified according to vehicle mode in vehicle mode distribution. (see figure4)
Vehicle States Energy mode Available Torque (Tavail)

Vehicle mode Modified Available Torque (Tavail_final)

EV distribution
Battery Power (Pbat) Desired Power (Pdesired)

Drive distribution

Modified Battery Power (Pbat_final)

Charge distribution

Modified Engine/Generator Power (Peng_final) Engine/Generator Power (Peng)

Normal distribution

Ultra-Capacitor Power (Pucap)

Brake distribution

Modified Ultra-Capacitor Power (Pucap_final)

Hydraulic brake pressure (Phydr_brk)

Energy Mode Distribution

Vehicle Mode Distribution

Figure 4. Source power distribution block diagram

Before the power distribute to each energy components in all parts of mode distribution, the available torque which can be generated in front and rear motors is calculated by considering the maximum power of motors, energy and the drivers required power.(eq. 2) The energy sources is referred to the energy components which generate the power at this moment.
Pavail = Min ( Pmax_mot , Pmax_source , Pdesired ) Pavail Tavail = mot

(2)

Where Pavail is available power and Pmax_mot and Pmax_source are the maximum power of two motors and energy sources, respectively. Min( ) is the function to get the minimum value of the functions variable. Therefore Pavail is selected to minimum value over the Pmax_mot ,Pmax_source and Pdesired . Basic strategy of power distribution at each mode is described in table2.
Energy mode Pbat
EV = LPF(Pavail) = Pavail - LPF(Pavail) Not working = LPF(Pmax_batchg) = Pmax_batchg - LPF(Pmax_batchg) = Pavail + Pmax_batchg Depends on Control strategy and vehicle state (see. NORMAL DISTRIBUTION STRATEGY session) Brake Drive Battery U-Cap Eng/Gen Hydraulic brake Battery U-Cap Eng/Gen Hydraulic brake Not working = LPF(Pregen + Plim_gen) = (Pregen + Plim_gen) LPF(Pregen + Plim_gen) = Plim_gen = Pavail - Pregen

Vehicle mode
Same as the distributed power at energy mode

Pucap Peng Pbat

Charge

Pucap Peng Pbat

Hybrid

Pucap Peng

Table2. basic control strategy

LPF() in table2 is the low pass filter function and Pregen is the regeneration power of front and real motors. Plim_gen is limited generation power of engine/generator by battery max charge power, Pmax_batdis, , and Pregen.(eq. 3)
Peng = Pmax_batdis Pregen 0 when, Peng < Pmax_batdis Pregen when, Pmin_OOL < Pmax_batdis Pregen < Peng when, Pmin_OOL > Pmax_batdis Pregen (3)

Plim_gen

Even if engine is operated in Optimal Operating Line(OOL), fuel efficiency can make a big difference. Therefore it is good to operate the engine/generator in certain range which neighbourhood of Optimal Operating Point(OOP). The minimum power and the maximum power of this range is referred to Pmin_OOL and Pmax_OOL, respectively. The engine/generator should be operated in this rage. Traction Power Distribution In traction power distribution part, the available torque which is calculated in source power distribution part is distributed to front and rear motors. Because this papers purpose is to propose the supervisory control algorithm structure and control strategy, this part is designed simply to just propel the vehicle. Basically the distributed torque of front and rear motors is constant ratio and when the wheel slip is occurred, it is redistributed to prevent the wheel slip. NORMAL DISTRIBUTION STRATEGY In this session, control strategy of normal distribution which is in energy mode is described in detail. The thermostat control(TS) and the power-follower control(PFC) is researched about control strategy of series HEV.[5,7,8] In TC, the engine/generator has only two state, On or Off. If engine/generator is turned on, it could be operated in just one point that has the highest fuel efficiency. Its condition is determined basically base on battery SOC. Because whole power needed to propelling the vehicle is supplied by battery and ultra-capacitor when engine turns off, it has a shortage that battery capacity could be large. In the PFC, against the

TC, the operating points of engine/generator are changed according to drivers required power and battery SOC state. This control strategy has a characteristic to not only satisfy the drivers required power but also maintain the battery SOC at a certain point. The average of upper and low bound of battery is used to the certain point, which has the highest efficiency of battery. This characteristic can reduce the usage of battery power and the battery capacity can be smaller than the TCs battery. However, as a result of change of engine/generators operating points, its fuel efficiency is lower than the TC. But battery loss is reduced compare to the TC because battery is operated in good efficiency points. These control strategy, the TC and the PFC, do not treat the ultra-capacitor or simply use ultra-capacitor to just generate the high frequency elements of required power. So, this paper proposes the control strategy which modifies the PFC to consider the power distribution of ultra-capacitor. This control strategy controls the ultra-capacitor power by considering not only frequency elements but also vehicle velocity. Modified Power-follower Control In this paper, a modified power-follower control(MPFC) is proposed to control strategy which adding the power distribution of ultra-capacitor to basic PFC. The purposes of this control strategy is that change of engine operating points is reduced and the battery power moves slowly by supplying the high frequency element of demanded battery power through ultra-capacitor. The original control strategy, the PFC, only battery compensation power according to battery SOC is calculated and by using this compensation power, demanded power of engine/generator is modified. But the proposed control strategy, MPFC, compensation power is calculated considering not only battery compensation power but also ultra-capacitor compensation power. The compensation power of ultra-capacitor is determined by using ultra-capacitor SOC as like the battery compensation power. However, while the target SOC of battery is a constant which is the average of upper and lower bound of the battery, the target SOC of ultra-capacitor is a variable in terms of the vehicle velocity. The target SOC of ultra-capacitor depends on vehicle velocity. When vehicle velocity is high, it should be low value to prepare the regeneration braking for receiving more regeneration power. On the contrary to this, when vehicle velocity is low, it should be high to supply more power for vehicle acceleration. By using this compensation powers of the battery and the ultra-capacitor, modified engine/generator power is determined by
Peng1 = Pavail + Pbat_SOC + Pucap_SOC (to be limited between Pmin_OOL and Pmax_OOL ) (4)

Where, Pbat_SOC and Pucap_SOC are the compensation power of battery and ultra-capacitor, respectively. Through use the ultra-capacitor like this, it can perform role of power buffer between battery and engine/generator. So, when acceleration or deceleration situations, without changing the engine operating points, it can distribute the power among energy components appropriately. Peng1 in equation (4) is the first modified demanded engine/generator power and it should be limited between Pmin_OOL and Pmax_OOL to maintain the high fuel efficiency. After the modified of that is calculated, engine operation condition is determined. Table 3 is show about summarized decision conditions of engine on/off. Pengoff in table3 is battery charge constant. The decision condition which is using the battery charge constant prevents that battery is charged excessively. After the engine operating condition is determined by using decision conditions, demanded engine/generator power is modified again to reduce the change of engine operating points. The first step of modification is calculating the average power of Peng1 during a certain time, Ta. And the second step is the

engine operating point moves to the average power when the difference between the average power and the previous engine/generator power is greater than a certain power, Pconst. If difference between those two values is smaller than Pconst, the engine operating is not changed. The certain time, Ta, depends on kbat_SOC that is the difference value between the target SOC and the present SOC of ultra-capacitor. When the absolute value of kbat_SOC is high, which means that the present battery SOC is far from SOC average point, the Ta should be small to reflect the battery compensation power, Pbat_SOC, quickly. By quickly reflect the Pbat_SOC, battery SOC can approach to the SOC average point fast. While, when absolute value of kbat_SOC is low, the Ta should be large. The low absolute value of kbat_SOC means that present battery SOC exists in neighbourhood of average point, therefore it is not need to reflect the Pbat_SOC quickly. Because engine operating points is changed frequently when Ta is small, it is good to set the Ta small if battery SOC is in neighbourhood of average point. The second modification of demanded engine/generation power is finished, drivers required power is distributed to each energy components with considering the compensation power of batter and ultra-capacitor.
Engine Operation On Previous Engine operation On Off On Off decision condition SOCbat_lower <SOCbat_pre<SOCbat_upper SOCbat_pre < SOCbat_lower or Pavail >Pmax_dischg SOCbat_pre > SOCbat_upper or Pavail + Pbat_soc<Pengoff SOCbat_pre > SOCbat_lower

Off

Table3. Modified Power-follower engine on/off condition

SIMULATION RESULTS To verity the proposed supervisory control algorithm structure and MPFC in this paper, simulation is performed. The control algorithm is developed by using MATLAB/Simulink and vehicle model is constructed by using AVL CRUISE. FTP72 drive cycle is modified to appropriate drive cycle for military vehicle by adding the inclination to original FTP72. And this modified FTP72 is used to driving cycle to perform the simulation. (see figure 5,6)
Vehicle date

Command signal

Figure 5. Simulation Environment

Figure 6. Modified FTP 72 driving cycle

EV/Charge mode simulation

Charge mode
350 400 450 Time (s)

EV mode
500 550 600

Figure 7. EV/Charge mode simulation results

Figure 7 shows that the power distribution among energy components according to drivers input of EV/Charge switch. In the charge mode, even if the batter SOC is higher than upper limit, battery is continuously charged by generating power of engine/generator. For EV mode, the battery and ultra-capacitor generate the drivers required power to propelling vehicle and engine/generator is not working. Both EV mode and Charge mode, ultra-capacitor generates high frequency elements of drivers required power. Normal mode simulation Power distribution between energy sources
Thermostat control
300 200 100 0 -100 150 300 Power (kW) 200 100 0 -100 150 300 200 100 0 -100 150 Req Pw Bat Pw Gen Pw UCap Pw Req Pw Bat Pw Gen Pw 200 250 300 350 400 450 500 550 Req Pw Bat Pw Gen Pw

Power-follower control

200

250

300

350

400

450

500

550

Modified power-follower control

200

250

300

350 Time (s)

400

450

500

550

Figure 8. Comparison of power distribution

Thermostat Control

700 600 500 400 300 200 100 0 500

Power-follower Control

700 600 500 400 300 200 100

Modified power-follower Control

1000 1500 2000 2500 Engine Speed (RPM)

3000

0 500

1000

1500

2000

2500

3000

Figure 9. Comparison of Engine operating point

Figure 7 shows the power distribution among energy components for the each case of power distribution control strategy and Figure 9 represents the engine operating point during the simulation.n For TC, the engine/generator generates the constant power at fixed one point which has highest fuel efficiency. But, because of constant output power of the engine/generator, engine operating condition should be off when battery charged power greater than maximum battery charging power. Therefore, engine operating condition is often changed more than the others. For PFC and MPFC, the output power of engine/generator is varied so engine on/off condition change is not frequently occurred compare to TC. Especially, in the case of MPFC, change of engine operating point and engine on/off condition is smaller than PFC because ultra-capacitor performs the role of buffer between engine/generator and battery power distribution. Also, battery power move slowly because ultra-capacitor generates the high frequency element of demanded battery power instead

Battery SOC Figure10 shows a simulation result of battery SOC trajectory. In case of TC, we can see a wide variation and we confirm smaller variation rather than TC in case of PFC and MPFC. Also, when battery SOC goes down greatly, battery SOC of PFC and MPFC approach the average SOC faster than battery SOC of TC because engine/generator generates more power to compensate the battery SOC. If battery SOC has a wide variation, it could be a bad efficiency of the battery and reduce the battery life. So PFC and MPFC are a better performance than TC in viewpoint of battery management.
80 Battery SOC (%) 70 60 50 40 Thermostat Power-follower Modified Power-follower

200

400

600

TIme (s)

800

1000

1200

1400

Figure 10. Comparison of battery SOC

Fuel economy
Control Strategy Initial SOC (%) Final SOC (%) Fuel Consumption (L/100km) Thermostat 55 63.0952 45.809 Power-follower 55 60.9414 46.644 Modified Power-follower 55 (U-cap : 70) 61.5447 (U-cap : 72.2694) 46.535

Table 4. Comparison of fuel economy

The result of fuel economy which is corrected value by considering final SOC is summarized in Table 4.The best fuel efficiency is achieved in using TC which engine/generator is only operating in OOP and the PFC has the lowest fuel efficiency. The MPFC proposed in this paper has improvement fuel economy compare to PFC, but has lower fuel economy than TC CONCLUSION This paper proposes the overall structure of supervisory control algorithm and the modified power-follower control(MPFC) which is energy management considering the ultra-capacitor. The proposed structure of supervisory control algorithm is verified through the simulation. For validity of proposed supervisory control strategy, MPFC, simulation results of that are compared with the simulation results of thermostat control(TC) and power-follower control(PFC). With respect to fuel economy, the MPFC has higher fuel efficiency than PFC but has lower than TC which engine/generator is operated in OOP. In viewpoint of battery management, the result of MPFC and PFC show similar ability and have better performance compare to TC. Also MPFC show that the change of engine/generators operating point and operating condition is smaller than PFC. ACKNOWLEDGE This research is supported by expenditure of Dual Use Technology Center in South Korea

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