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英文原文
Control strategy of the hybrid drive for vehicle mounted aerial work platform
Janusz Krasucki a, Andrzej Rostkowski a, ?ukasz Gozdek b, Micha? Barty? b,
a Construction Equipment Research Institute, Napoleona 2, 05-230 Koby?ka, Poland
b Warsaw University of Technology, Institute of Automatic Control and Robotics, Boboli 8, 02-525 Warsaw, Poland
The development process i.e. assumptions, construction, simulations and analysis of a control strategy for thehybrid drive of the vehicle mounted aerial work platform is presented in the paper. Particular attention ispaid to the development of the control system strategy ensuring appropriate energy recuperation by makinguse of energy stored in the electrochemical form. The control strategy is built up around the concept of bilevelhierarchic control system. The elevation control of the aerial work platform is assumed as the primarygoal of the control system. The secondary goal of the control system is formulated in terms of tracking andkeeping the charging level of the rechargeable electrochemical accumulator in predefined limits. A control system simulation model is developed in Matlab-Simulink environment. Exemplary results of control system simulations are shown on the example of a hydraulic power unit driving aerial work platform mounted on
special vehicle MONTRAKS.
1. Introduction
The reduction of vehicle emission has been an objective of research for many years; partly it is forced by increasingly stringent environmental legislation. The Kyoto protocol, whichwas adopted at the COP3in December 1997, is aimed to decrease the green house gas emissions(GHG) by an average of 5% referring to 1990 levels. It came into force on February 16, 2005 following its ratification by Russia.
Hybrid systems are now gaining attention as a means for reducing GHG emissions by improving fuel economy and energy eficiency.
Market for hybrid driven vehicles is growing up dynamically sincemany years. Contemporary, eleven large car manufacturers use to deliver or to intensively develop hybrid driven vehicles. Even that is mainly focusing on passenger cars segment, it should be stressed that the remarkable effort is undertaken to implement hybrid drives in the trucks, delivery vans and buses [1,2].
WestStart-CALSTART [3], an advanced transportation technologies consortium, supported by U.S. Army National Automotive Center(NAC), organized the pilot program as part of its Hybrid Truck Users Forum (HTUF?) program, to speed up and to assist hybrid commercialization. According to the forecasts elaborated by CALSTART,the hybrid driven trucks market share will grow reaching ca 9% in 2010 and near 18.5% in 2020.
Still heavy duty machines and special purpose vehicles are the object of possible implementation for hybrid drive solution. However there are some doubts, if that application is economically feasible.Considering passenger cars,in respect of environmental regulations,important role plays the “effect of the scale”. In case of heavy duty machines, aerial work platforms, pick and carry mobile cranes or special vehicles with lift equipment, the application of hybrid solution is driven with operating constrains and application.
For many cases, working conditions for that class of machinerystrongly limits or even eliminates the application of combustion engines. In particular that is case of closed space areas such as factory shops, warehouses, intrinsically safe zones, etc. Here the implementation of diesel-electric drives could considerably extend possible use of that kind of equipment. Very unique and on the other side common area of services is municipal services and works used to be processed during night in the highly populated zones (street sprayer-sweepers,garbage trucks, tramway traction networks service vehicles, etc.). It is often reported by municipalities, that the issue to be solved for that services is the level of noise generated by diesel engine.
An example on how to meet the ever-increasing regulations controlling environmental conditions during indoor lifting operations is the battery powered cranes line designed by Valla Corporation [4],which recently extended the offer for hybrid solution. Another example is a hybrid system investigated by Eaton Corporation [5,6]for medium trucks with optional aerial work platform equipment.Eaton began commercializing its medium-duty hybrid system in August 2007 in a wide variety of applications such a: telecommunications and municipality, city delivery, refuse, city transit bus, pick andcarry and so on.
A hybrid vehicle is defined as one that has more than one source of power. Several different types of hybrid solutions have beenconsidered in the past and are still undergoing extensive research,
Fig. 1. Special purpose vehicle MONTRAKS 3PS.
such as Hybrid Electric Vehicles (HEVs) [1], which use a motor/generator and battery packs (or other electrical storage devices) and mechanical hybrids which use flywheels to store energy. Hybrid Hydraulic Vehicles (HHVs) [2], which store kinetic energy captured during braking events and store it in hydro-pneumatic accumulators and return energy to driveline during vehicle acceleration. Various different structures of hybrid drives (serial and parallel) have been developed. [7,8]
The hybrid electric system maintains conventional drive train architecture while adding the ability to enhance engine power withelectrical one.
One feature of this system is its ability to recover energy normally lost during braking and store the energy in batteries. The stored energy is used to improve fuel economy and vehicle performance for a given speed or used to operate the vehicle with electric power only.
The control of hybrid power trains is more complicated than the control of ICE only power train. First, one needs to determine the optimal operating mode among five possible modes (motor only,engine only, power assist, recharge, and regenerative). Furthermore,when the power assist mode or the recharge mode is selected, the enginepower and motor power needs to be selected to achieve optimal fuel economy, battery charge balance, and operability. With the increased power train complexity and the need to achieve multiple objectives, most often a two-level control architecture is adopted [5].
Fig. 2. Structure of the hybrid drive unit. Notion: x — piston stem displacement, v — piston stem velocity, p1 — under piston pressure, R1 — switching signal of valve (8), p2 — supply pressure, R2 — switching signal of valve (7), n1 — EM rotational speed, U — battery voltage, I — battery current, n2 — ICE rotational speed, OUT — setpoint of electric motor controller.
Fig. 3. Structure of the control system. Notion: SP_xp — Setpoint of the AWP position. PV_xp — Actual value of the AWP position. e_xp — AWP position control error. SP_vp — Setpoint of the lifting or lower velocity of the AWP. PV_vp — Actual value of the AWP velocity. SP_SOC — Setpoint of the battery SOC. PV_SOC — Actual value of battery SOC. PV_P1 — Actual value of the pressure p1. PV_P2 — Actual value of the pressure p2. OUT1, OUT2 — Outputs of PID controllers.
The analysis of power control systems optimizing: power efficiency factors, fuel consumption and emissions has been given in[3,9,10]. Investigations have been mainly focused on the possibility of kinetic energy recuperation in the phase of vehicle braking.
In this paper, the design of a power management control system isdescribed for a hybrid drive system of special purpose vehicle with hydraulic aerial work platform (AWP) equipment. For that type of vehicles (stop-and-go duty cycles) the potential energy of the load being handled with AWP should be seriously considered as recyclable [11,12].
The major advantage of the proposed hybrid drive over othersolutions is a simple drive architecture. It differs from known solutions, thosewidely used in personal cars. The classic approach (personal cars) needs full redesign of power transmission system. The innovative approach for the special purpose vehicles requires only extension of classic ICE drive with extension unit. Extension unit is composed of electricmotor coupledwith hydraulic pump/motor. That solution allows to differentiate the power flowbetween the thermal and electrical path with help of hydraulic subsystem. However, even that solution is not straightforward from the point of view of power flow, it demands for advanced control system strategies.
Two-layer hierarchical control system architecture is considered in this paper. A lower control level is built by application of local classic proportional-integral-derivative (PID) controllers. A higher control level is developed around a fuzzy logic controller (FLC) with the intention of dynamically setting out control rules for lower level local controllers
2. Characteristics of the target system
A specialized automotive vehicle MONTRAKS (Fig. 1) is intended for repairing and maintenance of tram and trolley-bus overhead wire system, assembling and disassembling of rail track sections and is exploited by the municipal communication services.
Such types of vehicles are usually designed on the bases of regular trucks undercarriage equipped with appropriate working accessories. The equipment is built up around the aerial work platform (AWP) (1) embedded at the end of the boom (2) driven by the set of two hydraulic cylinders and hydraulic swing motor (3).Besides a standard road running on the tires, the major feature of these vehicles is the possibility to move on rail run. That is achieved with additional set of rail wheels (4) which are driven with low speed hydraulic motors.As often as not, maintenance and repairing of the traction networks take place throughout the night, and these are time consuming operations. For the period of the time that repair work is carried out, the vehicle is parked; instead of the engine is continuously running and driving the hydraulic pump which is used to supply oil to the hydraulic equipment. In this phase of duty cycle, a power demand from the working equipment is low — does not exceed 3% value of engine rated power [2], due to that the diesel operation point approaches the regions of its low efficiency and significant emissions. Simultaneously, the diesel generates particularly bothersome noise.
Disadvantages mentioned above may be eliminated for instance by introducing an additional electric motor (EM) powered by an electrochemical battery pack. In this case, the ICE will deliver
mechanical power when the vehicle moves from/to its operation area. While parking the vehicle's power demand from the working equipment will be balanced from the EM and optionally from the ICE.
The structure of discussed hybrid drive is shown in Fig. 2
Energy for the motor is supplied from a set of electrochemicalaccumulators (5). The primary power source of the equipment drive unit is the EM. Motor traction parameters are controlled by the pulse width modulator (6). It is possible to reverse the motor's operation into generator mode. The EM runs the hydraulic pump (3) supplying the hydraulic actuation system. The ICE, running in the appropriate chosen operating point, drives the second hydraulic pump (2).Hydraulic oil flows frompumps (2) and (3) are added together in the common supply line. Hydraulic switching valves (7) and (8) redirect the oil flow in the main supply line either to the tank via overflow valve or to the under piston chamber of the hydraulic cylinder (9).The piston stemof the cylinder (9) controls the elevation angle of the boom (10) and indirectly the position of AWP (11). It is obvious that the control of the cylinder (9) influences the potential energy of load Q while the platform is lifting or lowering.
The following phases are to be distinguished in the duty cycle of the
hybrid drive unit:
? SPL phase — lifting of the AWP,
? SPD phase — lower of the AWP,
? SPP phase — parking of the AWP.
In SPL phase, as a result of movements of the cylinder's (9) piston and appropriate boom lifting movements, the addition or differentiation of oil flows from pumps (2) and (3) takes place. In case of subtraction of flows, one part of the pump flow (2) is directed to the main supply line and the reminder part of flow drives the pump (3) switched into motor mode. In SPD phase, the direction of oil flow in the main hydraulic supply line changes, oil runs the pump (3), and the mechanically coupled electric motor (4). In both phases it is possible to supply cylinder (9) by the oil delivered by the pump (3) driven by electric motor (4). Charging a battery (5) occurs in the SPP phase. In this phase, the AWP is fixed, and the pump (3) is driven by oil provided by the pump (2).
Fig. 4. Membership functions of the AWP position control error.
3. Control strategy
In general, the main objective of the power control strategy is to operate the hybrid drive with possible high energy efficiency and low emissions while maintaining specified vehicle performance [13].Maximal use of electric power is the main task of the hybrid drive control system. This corresponds with specific requirements for noise level and economic operation of MONTRAKS vehicle.This can be achieved by applying of the proposed power control strategy. This strategy is based on operation of AWP velocity closed to required trajectory and effectively capturing of the regenerative energy by controlling the state of charge (SOC) of a battery. As it is only possible,the electric drive should be used in SPL and SPD phases of duty cycle.SOC is the ratio of present charge of a battery to the maximum charge that can be possibly stored in the battery and in time instantt=T may be expressed as:
;
where:Q(t0)=Qmax maximal capacity of the battery, SOC(t0)=1,i(t) battery charging or recharging current.Meanwhile, the SOC of a battery should be controlled between a minimum SOC and a maximum SOC to obtain regenerative braking energy effectively with the least amount lost and stress on the battery.The minimum and maximum SOC levels are determined according to
the ability of a battery to absorb regenerative energy and to restart vehicle systems. In general, the larger the difference between the minimum SOC level and the maximum SOC level, the more regenerative energy a battery can effectively absorb. However, the larger span of operating SOC levels may reduce the battery's life, which is affected by the depth of discharge. Hence, the SOC levels should be appropriately determined between optimal minimum and maximum levels [SOCmin, SOCmax]. Considering the battery charging and discharging efficiency, the SOC range is set to [0.3, 0.8] in this paper. The power flow distribution between engine and electric motor may be defined through degree of hybridization (DOH) of the drive:
where: PICE — engine power, Pmot — motor power.
The combined power management/design optimization problem can be written as follows:
where:
XSP(t) 2 desired AWP trajectory
XPV(t) 2 actual AWP trajectory.
A structure of the proposed control system for this purpose is given in Fig. 3.
Fig. 3 shows the structure of the control system. The control system consists of two loops:
— control of the AWP position and velocity,
— control of the SOC of battery pack.
Each loop may control electric motor controller. Control signals are governed by the logic unit. It is aimed to provide smooth switching of control signal for appropriate time instants. Control system for AWP positioning and velocity control has a cascade structure. Fuzzy controller processes the velocity of the AWP. It is calculated from the real and desired platform displacement. Velocity signal from the auxiliary controller SP_vp is fed as the reference to the classic PID controller and it is compared with actual velocity of the platform PV_sp. The second control loop keeps the SOC of battery in predefined limits. This loop consists of PID controller and logic unit. PID unit controls the level of charge of the battery through continuous adjustment the hydraulic valves positioning.
Fig. 5. Membership functions of the AWP velocity.
3.1. AWP position controller
A controller of the AWP has been developed based on the cascade of classic PID controller and FLC. The FLC has been chosen because of its suitability for control of nonlinear, multiple-domain, and timevarying plant with multiple uncertainties [3]. This controller has two inputs: a control error of the AWP postion (SP_xp?PV_xp), and acurrent velocity of the AWP (PV_vp). The FLC calculates setpoint value of the AWP velocity SP_vp for the PID controller of the electric motor.
The FLC [14] consists of three basic blocks: fuzzyfication, inference and defuzzyfication. Inputs of the controller are fuzzyfied in the fuzzyfication block. In fact, fuzzification maps the space of crisp values onto the space of fuzzy ones. In this process, each crisp sample of each input signal is transformed into the set of numbers interpreted as the membership degrees of this samples to the appropriate fuzzy values (fuzzy sets). Fuzzyfied inputs are fed to an inference machine. The inference machine makes fuzzy outputs based on: fuzzy inputs, fuzzy logic rules and knowledge embedded in the rule base (Fig. 6). The rule base is created based on the appropriate knowledge or by means of learning from data or is acquired from real or simulation experiments. Fuzzy output from the inference machine is transformed into the crisp value by means of defuzzyfication procedure. Exclusively the triangle and trapezoidal membership functions have been used in the process of fuzzyfication. In fuzzy AWP velocity controller each input was fuzzyfied by means of seven membership functions (see Figs. 4 and 5).
The rule base applied for the inference process is depicted in Fig. 6. Rule base is assumed as the set of quantitative knowledge. A total of 49 rules have been formulated for the FLC. For the clarity, the rule base is displayed in the form of colored matrix. Every entry to the matrix corresponds with the appropriate fuzzy output (SP_vp); that is presented in the form of vertical bar in the right side of Fig. 6. Conventional, center of gravity [14] method has been applied for the defuzzyfication of fuzzy output. A control surface of developed FLC has been presented in Fig. 7. As mentioned above, the output from the FLC is fed to the AWP velocity PID controller. Velocity of the AWP is controlled in the follow-up control system by controlling rotational speed of the hydraulic pump (Fig. 2). Settings of the velocity controller have been carefully tuned to ensure aperiodic transition (without overshoots) even in case of stepwise excitation (see Figs. 10 and 11).
3.2. SOC controller
The linear PID controller has been applied for the control of the battery's SOC (Fig. 3). The actual value of SOC is continuously estimated fromEq. (1) making use of the measurements of the battery current. An additional control unit allows for driving the coils of electro-hydraulic valves R1 and R2. Control signals for the electro-hydraulic valves are obtained from the measurements of supply pressure p2, under piston pressure p1, and current and voltage (I, U) of the battery.
Fig. 6. The rule base of the AWP velocity FLC. Notion used is given in Table 1.
Fig. 7. Control surface of the FLC.
In the lifting phase of the AWP, the control unit delivers an appropriate excitation for the electro-hydraulic valves (7) and (8). In outcome, the under piston chamber of the cylinder is connected with the main hydraulic supply line. After a demanded position of the AWP is reached, the valve (8) will be driven towards its neutral position, which will finish the movement of the platform.
Here, the energy of the combustion motor may be used for battery charging. In the battery charging phase, the charging controller controls also the pressure in the under piston chamber of the hydraulic cyli