fuzzy logic control of modern aircraft actuators

1 downloads 0 Views 354KB Size Report
In this paper, modern civil aircraft electrical actuators are modeled, analyzed and ... voltages and frequency of the main generator and the voltage of the actuator DC bus .... Membership functions of the output variable (modulation index). 3.
3rd International Conference on Energy Systems and Technologies 16 – 19 Feb. 2015, Cairo, Egypt

FUZZY LOGIC CONTROL OF MODERN AIRCRAFT ACTUATORS Reyad Abdel-Fadil1, Ahmad Eid1, and Mazen Abdel-Salam2 1

Electrical Engineering Department, Aswan University, Aswan, Egypt Electrical Engineering Department, Assiut University, Assiut, Egypt

2

In this paper, modern civil aircraft electrical actuators are modeled, analyzed and controlled using Fuzzy Logic Control (FLC). The aircraft actuators aremodeled in PSIM and the obtained results verify that FLC is superior compared to classic controls. The FLC generates the required gate signals which drive the DC motor to follow the reference angle of the EMA actuator surface position.The aircraft system is analyzed with different number of actuators (5 kW each). In the meantime, the voltages and frequency of the main generator and the voltage of the actuator DC bus are kept within standard limits. Keywords: Fuzzy logic control, DC-DC converters, MEA, EHA, EMA

1. INTRODUCTION There is a general move in the aerospace industry toincrease the amount of electrically powered equipment on future aircrafts. This is generally referred to as the “More Electric Aircraft” (MEA) [1, 2] and brings with it a number of technical challenges that need to be addressed and overcome. High power, electric actuation systems are being proposed on many new aircrafts with ratings up to 50kW [3]. The actuators are used tomove flight control surfaces such as the rudder, aileron, spoiler etc. in order to control the speed and direction of the aircraft during flight/taxi/landing/takeoff. The flying surfaces of civil aircraft are conventionally powered through three independent and segregated hydraulic systems. In general, these systems are complex to install and costly to maintain. The concept of replacing the hydraulic system with electric actuationcoupled with changes to the electric generation technology and flight control systems, is commonly tanned the all-electric aircraft earlier studies have shown that the all-electric aircraft can give the aircraft's manufactures and operators considerable cost benefits, particularly due to reductions in system complexity and overall weight [4]. The MEA concept can be subdivided into fly-by-wire, where the hydraulically powered flight actuators are electrically signaled, and power-by-wire, where the actuators are directly powered from the aircraft's electrical system. The key to the implementation ofthe, power-by-wire aircraft,is the development of compact, reliable, electrically powered actuators to replace the conventional hydraulic system. With -149-

recent developments in high performance motors and power electronics research isbeing undertaken to develop suitable electrical actuators. Electrically powered flight actuators can take one of two principal configurations, the electromechanical actuator (EMA) with mechanical gearing, and the electro hydrostatic actuator (EHA) with fluidic gearing between the motor and the actuated surface [5]. 1.1 Electro-Hydrostatic Actuator (EHA) The control surfaces of today's large aero planes are hydraulically actuated withrecent aircraft developments (Airbus (A320/A330/A340), Boeing (B777)) most of these actuators are electrically signaled Fly-by-Wire technology superseding the mechanical cable control due to the specified high reliability there are to redundant hydraulic networks to supply the actuators installation and maintenance of such aircraft wide hydraulic systems is expensive and their weight contributes to most of the system related fuel consumption several research and development activities consider system and component solutions to replace hydraulic by electric power transmission Power-by-Wire aiming at reducing the overall systemweight increasing power efficiency which can also save component weight and reducing complexity.The hydraulic actuation system is composed by series of control valves [6], such as the servo valve, mode select valve,check valve, pressure relief valve and solenoid operated valves, all of them are joined together in order to allow a safe and proper function of the actuator. The EHA driven by electric motor, which control of hydraulic pump as shown in Fig.1. [4]. The EHA uses the standard hydraulic bypass valves to assure that the conventional active-standby, or active-active actuator architectures can be easily utilized. Thus the EHA can resemble closely to the traditional centralized hydraulic actuators in running. Therefore, the EHA is much more appropriate for the primary flight control application instead of the EMA. The EHA techniques make the quiescent power consumption lower during the standby operation. Also the EHA can achieve a rapid start-up response time by using the highly efficient electrical system due to the PBW feature of itself. The conventional hydraulic actuators have a lower efficiency (typically max 50%) than that of EHA (typically 50%-70%). The single mode failure vulnerability of the EHA is lower compared with the hydraulic actuators Moreover, it is easy to integrate several sub-systems into one single system for the EHA, which contributes to a higher modularity and easier modification, and then reduces the maintenance costs [7].

Figure 1. The Electro-hydrostatic actuator (EHA).

-150-

1.2 Electro-Mechanical Actuator (EMA) An Electro-Mechanical actuator driving an inboard spoiler surface of an aircraft was chosen as an example of a flight actuator being a part of the power distribution system. The system diagram of the electromechanical actuator (EMA) is shown in Fig. 2. It consists of a DC-DCpower converter feeding a dc motor, which moves the inboard spoiler surface through a mechanical transmission consisting of a gearbox and a ball screw mechanism. The DC-DCconverter is connected to the dc power distribution bus through an input filter, whose purpose is to prevent the switching ripple of the converter from going to the bus. Finally, a feedback controller is employed to precisely control the surface movement. An electro-mechanical actuation technology is chosen for this study because it is used in many industry applications, including manufacturing, process plant, railway vehicles and aerospace systems [8].

Figure 2. The Electromechanical actuator (EMA).

2. FUZZY LOGIC CONTROL APPROACH The fuzzy logic unlike conventional logic system is able to model inaccurate or imprecise models. The fuzzy logic approach offers a simpler, quicker and more reliable solution that is clear advantages over conventional techniques [9-10]. This paper deals with speed control of separately excited DCmotor in aircraft electrical actuator, actuator DC bus voltage and main ACbus voltage and frequency using fuzzy logic controller. 2.1 Structure of Fuzzy Logic Controller The fuzzy controller has four main components: first, Fuzzification, which modifies crisp inputs (input values from real world) into linguistic variables. Enabling the input physical signal to use the rule base, the approach is using membership functions. Second the Rule-base, where fuzzy inputs are compared and based on the membership functions of each input. Third, Inference mechanism which evaluates which control rules are relevant at the current time and then decides what the input to the system should be. Lastly, DeFuzzification Interface that converts back the fuzzy outputs of the Rule-base to crisp onesselect membership functions for the different control outputs from the rule base [11]. The FLC components are as shown below in Fig.3.

Figure 3. Fuzzy logic control components. -151-

In Fuzzification process, linguistic variables are used instead of numerical variables. The basic fuzzy set operations needed for evaluation one of three rules method such as AND (∩), OR (∪) and − NOT (-) [10]. The fuzzy set part is represented by the following equations: AND-Intersection: µ A∩B = min [µ A (X), µ B (X)] OR Union: µ A∩B = max [µ A (X), µ B (X)] NOT -Complement: µ A∩B = 1- µ A (X) D

(1) (2) (3)

The Rule base stores the linguistic control rules required by rule evaluator (decision-making logic). The Inference evaluates which control rules are relevant at the current time and then decides what the input to the system. Three different methods used in inference mechanism, center of area or centroid (COA), Bisector, and Middle of maximum (MOM). The most popular method, the center of gravity or area is used, which is presented in (4) [12]. (4) In Defuzzification part, the fuzzy logic generates demanded output in a linguistic variable, according to real world requirements.The linguistic variables have to be transformed to crisp output (real number) using this process. 2.2 EMA Position Control using Fuzzy Logic Instead of using system model, FLC operation based on heuristic knowledge and linguistic description is used. The lack of knowledge ofdeveloping membership functions and rules can give wrong results. Thus with sufficient knowledge of adjusting the rules and membership functions the performance of FLC can be improved.The inputs to the FLC are normally the error (E) and change of error (CE) instead of using the system transfer function, where E is input is error between the referenceand actual theta position. Input variable membership show in following Fig. 4. NB

NM

NS

ZE

PS PM

PB

Term PB PM PS ZE NS NM NB

Definition Positive Big Positive Medium Positive Small Zero Negative Small Negative Medium Negative Big

Figure 4. Fuzzy membership functions of the inputs E/CE.

The Rule base stores the linguistic control rules required by rule evaluator (decision-making logic). One of The rules used in this paper shown in Table 1. The variables are defined in the legend.

-152-

Table 1. If-then rule base for fuzzy logic control

E CE CNB

NB

NM

NS

ZE

PS

PM

PB

VL

VL

VL

VL

CNM CNS

VL VL

VL VL

VL LO

LO UM M AM UM M AM B

Term

LO

UM

M

CZE VL LO UM M AM CPS LO UM M AM B CPM UM M AM B VB

B VB VB

VB VB VB

CPB

VB

VB

M

AM

B

VB

VB

VL LO UM M AM B VB

Definition Very Low Low Under Medium Medium Above Medium Big Very Big

The output for FLC is the value of modulation index (M) of DC-DC converter, The DC-DC converter output voltagedue to the change in duty cycle (D), and will change DC motor armature voltage and motor shaft speed, the output membership used in this work show in Fig. 5. VL

LO UM M AM

B

VB

Term Definition VL LO UM M AM B VB

Very Low Low Under Medium Medium Above Medium Big Very Big

Figure 5. Membership functions of the output variable (modulation index).

3. SIMULATION RESULTS AND DISCUSSIONS The MEA (B787) electrical power system composites from four identical channels, and hence, only one channel is simulated for simplicity and time saving in PSIM commercial software [13]. The FLC control is carried out using a C-code capability in PSIM software. Because of the lack of the engine model, a DC motor replaces it. The simulated single channel of B787 aircraft is shown in Fig. 6 [14]. The FLC ensures that at every bus of the aircraft, the magnitude of the voltage and its frequency comply with the aircraft standards. The aircraft system is studied at full load includingthe actuator dynamic effects at different frequency range of 400-800 Hz. To meet the aircraft standards of the generator terminal voltage and frequency, the field excitation voltage and shaft speed are controlled. The shaft speed of the engine (DC motor equivalent) is controlled by changing the armature voltage keeping the excitation voltage constant. The terminal voltage of the DC motor is regulated by the PD-FLC to get the required speed and hence the frequency.By the same way, another PD-FLC controller is used to regulate the excitation voltage of the main synchronous generator yielding the required terminal voltage.Applying the full load at 0.1 s and switching on the EMA load at 1 s, the generator rms voltage value during

-153-

transient and steady state conditions is shown in Fig.7(a) and its frequencyis shown in Fig.7(b) at 600 Hz frequency operation.

Figure 6. One channel of the simulated B787 aircraft model. V 125

120

115

(a) 110

105

100 0

1

2

3

4 Time (s)

5

6

7

8

HZ 602

601

(b)

600

599

598 0

1

2

3

4 Time (s)

5

6

7

Figure 7. Main generator AC busvoltage: (a) magnitude (b) frequency.

-154-

8

It can be seen that the generator terminal voltage is almost around 115 V even with a switching EMA load at 1 s. At the same time, the generator frequency is constant at 600 Hz with a deviation of less than the standard value of ±1 Hz. This ensures the effectiveness of the applied FLC method. The 270 VDC bus is considered a very important bus at MEA as about half of total load is connected at this bus [14]. Three phase PWM controlled rectifier shown in Fig. 8 is used instead of using conventional tap changing transformer. The inverter is controlled using PI-FLC type. The DC bus voltage profile is shown in Fig. 9 for the same operating conditions. The total DC bus voltage ripple ∆V= ±5 V satisfying the aircraft standards [15].

ωm ,τm

ωm ,τ m

θ

ωm_ out Vref = 270V

θ _ out −

+

+

− VOut

ωm_ ref

θ _ ref



Figure 8. Three phase PWM controlled rectifier topology and EMA dynamic model with their FLC controls. Vdc 300

250

200

150 0

1

2

3

4 Time (s)

5

6

7

8

Figure 9. 270 V DC bus voltage.

The mechanical actuator module includes a gearbox and a ball screw mechanism. The model for the mechanical transmission takes into account inertia, damping, and stiffness of the ball screw mechanism and stiffness of the bearing structure. Themodel for the surface dynamics is also considered. The model reflects the horn stiffness, surface inertia, damping, and a nonlinear relationship between the mechanical actuator movement and the surface deflection angle as shown in Fig. 8. Feedback controller provides the duty cycle of the DC/DC converter according to the actuator position command. The EMA simulation model excluding the DC motor is shown in Fig. 10 [16]. The EMA is controlled to provide a certain deflection angle profile. It is -155-

worth to mention that taking the angle as a feedback signal is better than taking the DC motor speed signal, as in this case the EMA dynamics are included and verified. 1 Kr

1 h

ωm

1 N

1 + s −

− K act

+ −

1 N

Tex

1 Mp s

− 1 1 + Ks + + Js s h θc − −

− +

1 s

1 θ s

Bs

Bp

Figure 10. Electro-mechanical actuator simulation model excluding the DC motor.

The actuator output position (deflection angle, theta) is taken as the feedback signal and compared to the reference signal. Then, the error and its change are applied to the FLC controller. The EMA performance is shown in Fig. 11 (a) for a complete period of deflection angle (from 0°-78°). The deflection angle overshoot is shown in Fig. 11 (b) with a zoom at 2 s.The corresponding DC motor speed is shown in Fig. 12. Thete_r

Theta

Thete_r

Theta

100 79

50 78

0 77

-50

76

-100

75

0

2

4 Time (s)

6

8

2

2.1

(a)

2.2 Time (s)

(b)

Figure 11. EMA performance: (a) complete operation period, (b) overshoot at 2 s. RPM 1.5K

1K

0.5K

0K

-0.5K

-1K

-1.5K 0

1

2

3

4 Time (s)

5

6

Figure 12. EMA motor speed profile.

-156-

7

8

2.3

According to the deflection angle profile, the speed of the DC motor should change very sharply from zero to its maximum speed of 1000 rpm, but, due to the motor inertia, it takes some time to reach the maximum speed.In this case, a large current is drawn in instants of changing the motor speed. These changes in the current are reflected in the drawn power as shown in Fig. 13 for one actuator. The number of studied actuators is seven with the same model and performance. W

100K

50K

0K

-50K 0

1

2

3

4 Time (s)

5

6

7

8

Figure 13. EMA demand power.

4. CONCLUSIONS Fuzzy logic control is applied to the MEA aircraft actuation system to follow a certain deflection angle profile. Triangular membership functions with two types of fuzzy logic (PI-FLC and PD-FLC) are applied to control the aircraft actuator theta position, DC bus voltage and main generator voltage and frequency. The voltage ripples as well as the frequency deviations are compatible with the aircraft standards. The obtained results verify the suitability and effectiveness of the proposed control method for the aircraft actuation system.

5. REFERENCES [1]

[2]

[3]

[4]

[5]

A. Eid, M. Abdel-Salam, H. El-Kishky, and T. El-Mohandes," Simulation and transient analysis of conventional and advanced aircraft electric power systems with harmonics mitigation", Electric Power Systems Research, Vol. 79, Issue 4, pp. 660-668, April 2009. Reyad Abdel-Fadil, Ahmad Eid, and Mazen Abdel-Salam, “Electrical distribution power systems of modern civil aircrafts,” 2nd International Conference on Energy Systems and Technologies, Cairo, Egypt, pp. 201-210, 18-21 February 2013. D.R. Trainer, and C.R. Whitley,"Electric actuation - power quality management of aerospace flight control systems", IET International Conference on Power Electronics, Machines and Drives, pp. 229-234, 4-7 June 2002. Richard M. Crowder, "Electrically powered actuation for civil aircraft", IET Colloquium on Actuator Technology: Current Practice and New Developments, pp. 5/1 - 5/3, London, UK, 10 May 1996. K.P. Louganski, “Modeling and analysis of a dc power distribution system in 21stcentury airlifters”, M.Sc. Thesis, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, September 1999. -157-

[6]

[7] [8]

[9]

[10]

[11]

[12] [13] [14]

[15] [16]

C.A. Constantino, L.S. Góesand, and F.J. Moreira, "High frequency modeling of a hydraulic actuation flight control system", 9th Brazilian Conference on Dynamics Control and their Applications, 7-11 June 2011. Ian Moir, and Allan Seabridge, "Aircraft systems: mechanical, electrical, and avionics subsystems integration", 3rd Ed., John Wiley & Sons, Apr. 2008. X. Du, R. Dixon, R.M. Goodall, and A.C. Zolotas, "Modeling and control of a high redundancy actuator", Mechatronics, Vol. 20, no.1, pp. 102–112, February 2010. Rahul Malhotra, and Tejbeer Kaur, “DC motor control using fuzzy logic controller”, International Journal of Advanced Engineering Sciences and Technologies, Vol. 8, no.2, pp. 291-296, 2011. A.I. Al-Odienat, and A.A. Al-Lawama, “The advantages of PID fuzzy controllers over the conventional types,” American Journal of Applied Sciences, Vol. 5 (6), 2008. R.P. Suradkar, and A.G. Thosar, "Enhancing The Performance Of DC Motor Speed Control Using Fuzzy Logic", International Journal of Engineering Research & Technology (IJERT), Vol. 1 no.8, October 2012. M. Algreer, and F. Ali,”Position control using fuzzy logic”, Al-Rafidain Engineering, Vol. 16, no.1, 2008. Powersim software,http://powersimtech.com/products/psim/ Reyad Abdel-Fadil, Ahmad Eid, and Mazen Abdel-Salam ,”Fuzzy logic control of modern aircraft electrical power system during transient and steady-state operating conditions”, The IEEE International Conference on Power Electronics, Drives and Energy Systems, Mumbai, India, 16-19 December 2014. Aircraft Electric Power Characteristics, MIL-STD-704F, 25 Oct. 2013. V.B. Blaignan and V.A. Skormin, “Stiffness enhancement of flight control actuator”, IEEE Transaction on Aerospace and Electronic Systems, Vol. 29, no. 2, April 1993.

-158-