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SIMUALATION OF ELECTRO-HYDRAULIC SERVO ACTUATOR
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ABSTRACT
Hydraulic actuators are used in many applications like aircraft flight control,
machinery and automobiles etc. This actuator when coupled with a feedback system is
called a Servo Actuator. The response of the hydraulic actuator with time is significant
particularly when the actuator is used for flight control operations. So finding the time
response of the particular hydraulic actuator much before its actual operation will be very
helpful for the designer for analyzing the performance of the system. This also helps the
designer to arrive at optimum design parameters of the hydraulic actuator. In this thesis a
position control electro-hydraulic linear actuator is selected. This actuator is used for
controlling the movements of the helicopter. Mathematical modelling of the hydraulic
actuator and its components is done and based on the mathematical equations
Matlab/Simulink models of the actuator and its components were made and the time
response of the linear actuator is obtained by using Matlab/Simulink Software. The time
response graphs which are obtained in this simulation are found to be in good
compromise with the time response graphs of Moog experimental time response graphs.
INTRODUCTION TO HELICOPTER MOTION CONTROL
A Helicopter is a Rotorcraft that derives its lift from one or more power driven rotors.
(A rotor is a system of rotating aerofoil.). Helicopters offer a facility to move from
one place to other, which are remotely located, and not having well laid out runway
which are otherwise required for fixed wing planes. Helicopters serve various
purposes from to civil transport, ambulance, police, and forest fire prevention to
sophisticated military application
ROLE OF HYDRAULICS IN HELICOPTER FLIGHT CONTROL
Nowadays Helicopter flight control in the most common configurations is
realized by collective and cyclic variation of the angle of attack of each rotor blade. The
Collective blade control pitches the rotor blades to equal angles of attack around their
longitudinal axis, changing the rotor thrust at constant rotor speed. Yaw and roll control
is realized via cycle blade motion by changing the angle of attack of every rotor blade
locally and periodically during one revolution.
The control of the rotor blade angles for small helicopters is done manually by
the pilot with the help of push-pull rods .But for a bigger helicopter the aerodynamic
forces acting on the rotor blade are so high that it becomes impossible to control the rotor
blade angles manually for the pilot. So for medium and large helicopters the control of
the rotor blade angles is done with the help of hydraulic Actuators. These hydraulic
actuators help in controlling the roll, pitch and collective Movements of the helicopter.
INTRODUCTION
Many mobile, airborne and stationary applications employ hydraulic control components
and servo systems. Hydraulic servo systems can generate very high forces, exhibit rapid
responses, and have a high power-to -Weight ratio compared to other technologies. On
the other hand, they exhibit a significant nonlinear behavior due to the nonlinear
flow/pressure characteristics, oil compressibility, time varying behavior, nonlinear
transmission effects, flow forces acting on spool and friction, which is not only largely
uncertain but is greatly influenced by external load disturbances.
The range of applications for electro-hydraulic servo systems is diverse, and
includes Manufacturing systems, materials test machines, active suspension systems,
mining machinery, fatigue testing, flight Simulation, paper machines, ships and
electromagnetic marine engineering, injection molding machines, robotics, and steel and
aluminum mill equipment. Hydraulic systems are also common in aircraft, where their
high power-to-weight ratio and precise control makes them an ideal choice for actuation
of flight surfaces.
Flow Control Valve
The electro-hydraulic flow control valve acts as a high gain electrical to
hydraulic transducer, the input to which is an electrical voltage or current, and the output
a variable flow of oil. The valve consists of a spool with lands machined into it, moving
within a cylindrical sleeve. The lands are aligned with apertures cut in the sleeve such
that movement of the spool progressively changes the exposed aperture size and alters
differential oil flow between two control ports.
The vast majority of flow control servo valves in existence employ a
double flapper nozzle pilot stage and a single spool boost stage. A stiff feedback spring is
generally used to provide feedback from the boost stage to the pilot. These types of servo
valves tend to be difficult to manufacture and expensive. A less conventional, less costly
Linear Hydraulic Actuator
Linear actuators are the devices for converting fluid power into linear motion.
They may be used to exert a force, to hold or clamp, and to initiate or stop motion. All
linear actuators are some modification of an air or hydraulic cylinder and may be either
single or double acting .the single acting cylinder receives power at one end only and is
returned to its original position by gravity or by spring action, while double acting
cylinder is powered in both directions. Double-acting cylinders permit more complete
control of movement. Ram is a form of single acting cylinder in which the piston rods are
of the same diameter.
The double rod cylinder has a rod attached to both sides of the piston. This type
of cylinder is center-mounted and is normally used when the same task is performed at
the either end on staggered cycles. Obviously the force and speed will be the same at
either end.
DEFINITIONS
Servomechanism - A continuously acting, bidirectional closed-loop control system.
Servo valve - A device used to produce hydraulic control in a servomechanism.
Electro hydraulic Servo valve - A servo valve which produces hydraulic control in
response to electrical signal inputs; sometimes called a transfer valve.
Electro hydraulic Flow Control Servo valve - A servo valve designed to produce
hydraulic flow output proportional to electrical current input.
VALVE NOMENCLATURE
1. Hydraulic Amplifier -A fluid valving device which acts as a power amplifier, such as a
sliding spool, or a nozzle flapper, or a jet pipe with receivers.
2. Stage - The portion of a servo valve which includes a hydraulic amplifier. Servo valves
may be single stage, two stage, three stage, etc.
3. Output Stage -The final stage of hydraulic amplification used in a servo valve, usually
a sliding spool.
4. Port -A fluid connection to the servo valve; for example, a supply port, a return port, or
control port
5. Three-Way Valve - A multi-orifice fluid control element with supply, return and one
control port arranged so that valve action in one direction opens the control port to supply
and reversed valve action opens the control port to return.
6. Four-Way Valve - A multi-orifice fluid control element with supply, return and two
control ports arranged so that valve action in one direction simultaneously opens control
port #1 to supply and control port #2 to return. Reversed valve action opens control port
#1 to return and control port #2 to supply
. LITERATURE SURVEY
INTRODUCTION
Until now a lot of work has been done on control, operation and testing of
hydraulic systems. With the evolution of computer simulation techniques, this process
has become much simpler. There are many reports describing field experience related to
analyzing hydraulic actuators using Simulink software. In this chapter works published in
a wide spectrum of journals have analyzed and the goals for the present study have been
given a firm foundation with the information derived from the survey. They are presented
in the subsequent sections
MATHEMATICAL MODELING OF THE HYDRAULIC SYSTEM
Mathematical models are developed for various components of the hydraulic system in
this chapter. Mathematical modeling involves in representing the hydraulic system
components in the form of equations. These mathematical models help in representing
the hydraulics system components in Simulink Software. This mathematical modeling is
done by considering the component properties such as flow properties, functional
properties, characteristics of the component (like electrical characteristics etc).
4.1. MATHEMATICAL MODELING OF FLOW CONTROL SERVO VALVE:
The flow control valve considered in this case study is a two-stage nozzle flapper servo
valve. It consists of the following elements
1. Electrical torque motor
2. Hydraulic amplifier
3. Valve spool assembly
SIMULINK MODELS
Simulink models have been made by utilizing the mathematical models of
the subsystems. The Figure 6.1 represents the simulink model of top level diagram of the
hydraulic system. A scope block is connected to monitor the time response of the
hydraulic actuator. the connections to the various blocks in the model have been made by
considering the equations obtained in chapter 4 mathematical modelling.
Figure 6.2 represents the simulink model of hydraulic actuator system. A
scope block is connected to monitor the time response of the hydraulic actuator. The
Connections to the various blocks in the model have been made by considering the
Equations 3, 4, 5, 6,7and 8 which are obtained in chapter 4 mathematical modelling.
Figure 6.3 represents the simulink model of servo valve system. A scope
block is connected to monitor the time response of the servo valve. The connections to
the various blocks in the model have been made by considering the Equations 1 and 2
which are obtained in chapter 4 mathematical modelling.
CONCLUSIONS
Mathematical models have been developed for the hydraulic system components like
hydraulic actuator, servo valve, piston chambers, and servo controllers by considering the
system requirements, system characteristics, fluid flow properties. By using these
mathematical models
MATLAB/SIMULINK models have been made for the hydraulic system components
.This time response is a very significant factor when the system considered is a critical
system like a flight control actuator.
1. These Simulink models of hydraulic actuator function like a virtual hydraulic
actuator where in we can obtain the behavior of the system with respect to time
without physically testing the component.
2. The time response of hydraulic actuator, servo valve is obtained from the
MATLAB/SIMULINK software.
3. With the help of these MATLAB/SIMULINK models the performance of the
hydraulic system components, sub systems like servo controller; servo valve etc
can be monitored.
4. By varying the subsystem parameters like pressure, active annulus area, stroke
length, control current etc the designer can arrive at optimum parameters of the
hydraulic actuator.
5. With the help of these MATLAB/SIMULINK models of electro hydraulic servo
actuator the time response of the hydraulic actuator can be obtained without
physically testing the actuator.
6. The time responses of the hydraulic actuator, servo valve and piston chamber are
compared with the MOOG hydraulic actuator data (courtesy Flight Test Centre,
RWRDC). The time response graphs which are obtained by this simulation