Transcript
Page 1: Missile Guidance Control

Tactical Missile Guidance

and Control

Notes

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Contents

1 Introduction 1

1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Autopilot Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Subsystem Interrelationships . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.4 Aerodynamic Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.5 Autopilot Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.6 Integrated Guidance and Control . . . . . . . . . . . . . . . . . . . . . . 4

1.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Missile Instruments 6

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Gyroscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Types of Gyroscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.4 Mechanical Gyroscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.4.1 Free or Position gyros . . . . . . . . . . . . . . . . . . . . . . . . 8

2.4.2 Rate or Constrained Gyros . . . . . . . . . . . . . . . . . . . . . . 10

2.5 Accelerometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.6 Resolvers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

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2.7 Altimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.8 Current Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.9 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3 Missile Servos or Actuators 14

3.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2 Types of actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.3 Pneumatic Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.3.1 Stored Cold Gas Servos . . . . . . . . . . . . . . . . . . . . . . . 16

3.3.2 Hot Gas Servos . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.4 Ram Air Servos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.5 Hydraulic Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.6 Electro-Mechanical Servos . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.7 Recent Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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Chapter 1

Introduction

1.1 Overview

Aerodynamics is part of the missile’s airframe subsystem, the other major parts

being propulsion and structure. In a broader sense, it is closely related to the autopilot

and controls that, in turn, form a part of the overall guidance loop.Thus in discussing

aerodynamic considerations for autopilot design we follow the steps given below: -

(a) First examine the interrelationships between the subsystems closely connected with

both aerodynamics and the autopilot.

(b) Then delineate the design requirements for autopilots as they are affected by aero-

dynamic input. These requirements are, in turn, affected by the choice of steering

policy chosen for the particular missile and its mission. The steering policy may

be dictated by the type of airframe configuration and propulsion system chosen

for the mission.

(c) Having developed an appreciation of the functions of aerodynamics and autopilot

in the overall system, then proceed to describe how the autopilot designer and

aerodynamicist work together in developing their subsystems to meet the design

goals of the missile system. Both start with simplified equations of motion and gen-

erally add additional terms until the final design is checked out with the standard

Euler equations of motion.

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Recently, ballistic missiles, in particular, Intercontinental ballistic missile (ICBM), with

their high speed and high maneuverability, have posed great challenges for traditional

intercepting techniques and methods. Despite the promising prospect of a new method,

head-pursuit (HP) guidance method, there still exist problems that need to be solved.

1.2 Autopilot Design

A typical approach to autopilot design is as given below:-

(a) Starting with a preliminary autopilot design with the three rotational channels

(pitch, yaw and roll) uncoupled. The aerodynamics also neglects coupling at this

stage.

(b) The development then progresses to an investigation of the coupled channels, which

requires aerodynamic data describing both stability and control couplings.

(c) At a later stage in the design process, the aerodynamic input to the autopilot may

be modified to include special requirements associated with the particular missile

design being developed, such as aeroelastic and off-design propulsion effects on

aerodynamics.

(d) The final tune-up of the autopilot can best be made with a six-degree-of-freedom

missile system simulation that requires a three-dimensional description of the aero-

dynamics of the configuration.

1.3 Subsystem Interrelationships

The aerodynamicist should be aware of the ways in which other subsystems are af-

fected by the aerodynamics of the proposed configuration. The most closely related

subsystems are as listed below: -

(a) Guidance Subsystem (Instructions to autopilot : Sensors, Computer, Filters). This

measures the error between the missile’s actual and desired courses, computes the

corrections necessary to reduce or null the error according to a chosen guidance

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law and gives commands to the autopilot to activate the controls to achieve the

corrections. The commands may be for lateral accelerations, angular rates etc.

(b) Depending on the mission, the guidance subsystem may make some demands on

the aerodynamic characteristics of the missile. For example,

(i) If the mission calls for a long duration cruise phase, it may be desirable to

have a statically stable aerodynamic configuration to minimize demands on

the control system.

(ii) On the other hand, if the mission is short range requiring large acceleration

to attack a maneuverable target, neutral stability is preferred.

(c) Autopilot Subsystem (Instructions to Controls : Accelerometers, Gyroscopes, Fil-

ters, Amplifiers). The AP receives the guidance commands and processes them

into commands to the controls such as deflections or rates of deflection of control

surfaces or jet controls through action of servomechanisms.

(i) To provide the deflection at a desired rate, the servomechanism motors must

contend with the inertia of the control device and the torque about its shaft.

In any case the maximum value of resulting hinge moments determine the

size of the servo actuators so that reducing this maximum value is important.

(d) Controls Subsystem (Action to maneuver airframe : Aero surfaces, jet controls,

Control Servos)

(e) Airframe Subsystem (Response to control action : Aerodynamics, Propulsion,

Structure)

1.4 Aerodynamic Control

Three basic types of aerodynamic control are in use namely: -

(a) Canard Control (small surfaces forward on the body),

(b) Wing Control (main lifting surfaces near the body center of gravity), and

(c) Tail Control (small surfaces far aft on the body).

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The tail steering controls initially give an acceleration in a direction opposite to

the intended direction. For example if an upward maneuver is desired, the aft control

produces a downward force in order to turn the missile body to a positive angle of

attack that will develop a resultant upward force. This is an important characteristic of

non-minimum phase systems as will be discussed later.

The canard control has a force in the direction as does the wing control, which

generally produces most of the resultant force, with the body usually at a relatively

low angle of attack.

1.5 Autopilot Requirements

Three of the principal requirements of a good autopilot are quick response with min-

imum acceleration error, stability, and robustness.

(a) When the autopilot calls for a control deflection to achieve a lateral maneuver,

it takes time to move the surface into the called-for position against the possible

resisting aerodynamic torque and the inertia of the surface.

(b) It also takes time for the control moment to move the missile to the required angle

of attack for the maneuver.

(c) These are the factors that affect the capability of the autopilot to achieve a quick

response.

(d) The time that it takes for the airframe to achieve 63 percentage of its called-for

maneuver is generally referred to as the effective first-order time constant of the

autopilot-airframe subsystems. This response represents the transient response to

a step input in control

1.6 Integrated Guidance and Control

Integrated guidance-control systems have the potential to improve missile system

performance by taking advantage of the synergism existing between subsystems. These

systems allow the designer to impose unusual performance requirements on the missile.

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Such requirements may arise out of the new sensor and warhead technologies that may

require complex maneuvers at target interception.

1.7 Conclusion

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Chapter 2

Missile Instruments

2.1 Introduction

While the missile is moving in space, forces and moments produce accelerations and

hence velocities and displacements, with respect to the earth or any other reference

frames. Hence the missile control system needs to measure accelerations, velocities and

displacements in space. Conventional potentiometers and tacho generators cannot do

these measurements. Gyroscopes or gyros, accelerometers are generally used as sensors in

short range and medium range missiles. Long-range missiles use GPS, INS or GPS/INS

as sensors or navigational aids.

2.2 Gyroscopes

2.3 Types of Gyroscopes

Gyroscopes can be of three types which are as follows :

(a) Mechanical

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(b) Fibre optic

(c) Piezo electric

2.4 Mechanical Gyroscopes

Mechanical gyroscopes exhibit the property of rigidity and precession. Rigidity is

its ability to maintain its spin axis in the same direction in space, in the presence of a

disturbing force or torque. A gyro is said to precess, when the reaction to a disturbing

force on any one gimbal gets reflected in the movement of the other gimbal. Mechanical

gyroscopes consist of a heavy rotor spinning at a very high speed (say greater than 24,000

rev/min). This rotor is held by its spin axis by a framework called gimbal as shown

in figure. The ’inner gimbal’ holds the rotor by its spin axis, which is perpendicular

to the motion of the rotor. The ’inner gimbal’ is held by an ’outer gimbal’ which is

perpendicular to both the spin axis and axis of the ’inner gimbal’.

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Based on the degrees of freedom, mechanical gyros can be of two types namely:-

(a) Free gyros

(b) Rate gyros

2.4.1 Free or Position gyros

Free gyros have three degrees of freedom. If one angular position transducer detects

the relative movement between the missile frame and outer gimbal, another relative

movement between the inner and outer gimbal, it is possible to measure two angular ro-

tations of the missile.If the three angular rotations have to be measured, then two such

gyros are required. Gyro ”toppling” is said to have taken place, if the orthogonolity

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between the three axes is lost. ”Distortion” in the measurement takes place when the

indicated angle is not same as the actual angle. Gyro toppling and distortion are com-

pensated by means of torque motors, which gives correct moment to the outer gimbal so

that the orthogonolity between the gyro axis and missile fore and aft axis is maintained.

Gyros can be ”blast” started, which are utilised for short total reaction missiles such as

anti-tank, air to air and short range surface to air systems. Sometimes the rotors are

started with compressed air. Missiles having flight timings more than 40 seconds have

electrically driven gyros. A drift rate of about 1 deg/min is acceptable for tactical grade

missiles. However the drift rate better than 0.01 deg/hour is required for navigational

grade gyros.

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2.4.2 Rate or Constrained Gyros

Rate gyros measure angular rate about one axis. As shown in figure, a rotor is

mounted in a gimbal, whose motion about an axis at right angles to the spin axis is

constrained by a torsion bar or friction free spring system. There are no other gimbals,

so the rotor has one degree of freedom only, about its spin axis. The cylindrical gimbal

is enclosed in a hermetically sealed outer case and the gap between them is filled with

viscous fluid in which the gimbal is floated with neutral buoyancy. The fluid provides

viscous shear damping, minimal pivot friction and protection from shock. If the missile

turns, as indicated, a gyroscopic precession will occur as indicated, which in the steady

state, will be the angle of twist proportional to the input rate. The ”E” type pick off is

an a. c. pick off, which would give signals proportional to the rate of turn. Very good

resolution and linearity can be obtained with rate gyros. The rotor can be blast started

or electrically driven.

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2.5 Accelerometers

Linear accelerometers are the most commonly used accelerometers. They can be of

three types namely:-

(a) Spring - Mass accelerometers (or) Seismic Mass accelerometers

(b) Piezo - electric accelerometers.

(c) Force - Balance accelerometers.

Spring-Mass accelerometers, most often employed in tactical missiles, consists of a mass

suspended in a case, by a low hysteresis spring and fluid damping. The displacement

of spring is proportional to the linear force and hence acceleration. The displacement

is picked off using a. c. pick offs. There is only one sensitive direction for these ac-

celerometers. Hence three accelerometers, placed orthogonally, are required to measure

the accelerations in the three mutually perpendicular axes.

Piezo-electric accelerometers, exhibit an electric charge across two faces, proportional

to the impressed force and hence acceleration. They require special charge amplifier for

low frequency acceleration.

Force-balance accelerometer is a more accurate version of spring-mass accelerometer

and is used when great accuracy is required.

2.6 Resolvers

Resolvers are used to resolve the guidance commands, issued from the ground, to the

freely rolling missile axes, so that the commands are executed properly.

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The induction resolver consists of a rotor and a stator, each with two windings whose

electrical axes are at 90 degrees to each other as shown in the figure above. The secondary

voltages, which result, are proportional to the sine and cosine of the shaft angle. The

rotor is held stationary in space by means of a ’roll gyro’ and the stator is allowed to

rotate/roll with the missile. The guidance commands (up-down or left-right) are given

to each of the primary windings of the rotor. Due to the rotation/rolling of the missile

about the roll axis, induced voltages are produced in the stator winds (secondary), which

is a function of the roll angle. The output of each winding of the stator is given to the

rudders and elevators for the left-right or up-down movement. If the guidance command

of V1 is given for the up-down movement, then the elevators servos would receive a

command proportional to V1cosφ and the rudder servos receive −V1sinφ ; where φ is

the angle by which the missile has rolled.

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2.7 Altimeters

Altimeters measure the height of the missile with reference to the ground/sea level or

some selected elevation. For a missile flying above 100 m from the ground, over a distance

of 20 to 30 Km, a simple barometric capsule or piezo electric pressure transducer would

be accurate. This is not suitable for heights less than 100 m, due to local variations in

atmospheric pressure, resulting in poor accuracy. FM/CW radar altimeters are more

accurate in the range of 0 − 10 m. Pulsed radar techniques are also used in finding

heights.

Another accurate altimeter is laser altimeter. This device illuminates the target

(ground) with a short duration package of radiation derived from a laser source. Radi-

ation reflected or scattered from the target (ground) is detected by a receiver in close

proximity to the laser source. Conventional radar timing techniques are used to give

the altitude information. Since the laser altimeters have narrow beam widths (of the

order of a degree or so), can give spot measurements of altitude above the terrain. The

accuracy is 0.1 for a range up to 10 m and 1 percentage from 10m to 50m.

2.8 Current Trends

2.9 Conclusion

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Chapter 3

Missile Servos or Actuators

3.1 Requirements

The basic requirements of a servo or actuator used for moving the control surfaces

in a missile are as follows:-

(a) Minimum weight and volume.

(b) Good shelf life.

(c) Low cost.

(d) Reliability.

(e) Good dynamic performance.

3.2 Types of actuators

Based on the medium or method used, actuators can be of the following types:-

(a) Pneumatic

(i) Cold Gas

(ii) Hot Gas

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(b) Ram Air

(c) Hydraulic

(d) Electro-mechanical

3.3 Pneumatic Actuators

Pneumatic actuators are used when the force requirement is moderate/low and the

time of operation is short. Some of the advantages are of using pneumatic actuators in

missiles are as follows:-

(a) Low cost.

(b) Less number of components required.

(c) Freely available medium.

(d) Very good power to volume ratio as well as weight ratio.

(e) Self-cooling.

However some of the major limitations of pneumatic actuators are as follows:-

(a) Storage at high pressures.

(b) Leakage problems.

(c) Low efficiency.

(d) Low torque.

(e) Low stiffness due to compressibility of air.

(e) Slow response.

Based on the source of air, pneumatic actuators can be of two types namely:-

(a) Stored cold gas servos.

(b) Hot gas servos.

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3.3.1 Stored Cold Gas Servos

In this type of actuator, actuation power is obtained from stored gas, which is released

just before the firing of the missile by means of a solenoid operated start valve. A pressure

regulator is used to maintain a constant servo supply pressure. The movement of the

actuator arm is controlled by the signal given to the solenoid, which in turn operates

the valve, to move the actuator arm up or down. This in turn results in the movement

of the control surface to the desired degree. Helium or Nitrogen is used as the medium.

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3.3.2 Hot Gas Servos

In this type of pneumatic actuator, Cordite or Isopropyl nitrate is burnt to produce

hot gases, which in turn is used to operate the piston. A relief valve is employed when

cordite is used, since its burning rate is high. Used for short time flights only, but highly

reliable.

3.4 Ram Air Servos

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The salient features of ram air servos shown in figure above, are as follows:-

(a) They are used when the missile speed exceeds 1.5 Mach, below which the perfor-

mance is poor.

(b) The air is supplied to the actuator by a number of pitot intakes positioned around

the body and connected to a common manifold.

(c) The paddle valve is controlled by a switch motor, which controls the supply of ram

air to the vane actuator.

(d) Altitude of operation of the missile also plays a significant role, since pressure

decreases with height.

(e) The design is simple and gives advantage of size and weight.

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(e) Since ram system can be used only after certain speeds, a backup servo is required

during the boost phase.

3.5 Hydraulic Actuators

Hydraulic actuators are used when large actuation force is required. They are ide-

ally suited for long operation time, provides high stiffness and good speed of response.

Generally used for missiles having ’moving wing’ configuration. The hydraulic medium

can be ’recirculating’ or ’non-circulating’, where it is expelled into the atmosphere. The

pressure for the oil can be generated by means of a turbine operated by burning cordite

or iso-propyl nitrate. They are also used in thrust vector control (TVC), during boost

phase.

3.6 Electro-Mechanical Servos

In this system, an electric motor (DC) is used to generate the motion either through

gears or ball screw arrangement. Very low torque is available using gear, which can

control small sized control surfaces or seeker motion. Ball-screw arrangement provides

higher force and used in the ”flexible nozzles” of TVC. For flight times less than 20

seconds, the DC motor can withstand currents more than its ’rated’ value, to provide

higher torques.

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The construction and working principle of the electro-mechanical actuator shown in

figure above is as follows:-

(a) This electro-mechanical servo has two ’contra-rotating’ drums, in constant motion

from a direct drive from a motor.

(b) A spring is wrapped round each drum and is attached to the nut in one end and

one member of a clutch at the other.

(c) Actuation of one clutch provides positive torque to the final drive, the capstan

acting as the main power amplifier; actuation of the other clutch provides torque

in the opposite sense.

(d) The output end of each clutch spring is attached to the rotating nut of a screw

jack.

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(e) The translating screw jack drives the missile fin shaft through a rack and sector

gear.

(f) When the error is zero, the springs act as brakes, locking the output shaft.

(g) A mechanical interlock prevents simultaneous operation of both clutches.

3.7 Recent Developments

3.8 Conclusion

Hot or Cold gas servos provide very high performance standards. Hydraulic servos

are ideally suited for heavier medium range missiles. Missiles against stationary or slow

moving targets may use electrical servos for their autopilots.

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