Missile Instruments

Tactical Missile Guidance and Control Notes

Contents

1 Missile Instruments

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1.1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.2

Gyroscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.3

Types of Gyroscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.4

Mechanical Gyroscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.4.1

Free or Position gyros . . . . . . . . . . . . . . . . . . . . . . . .

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1.4.2

Rate or Constrained Gyros . . . . . . . . . . . . . . . . . . . . . .

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1.5

Accelerometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.6

Resolvers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.7

Altimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.8

Current Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.9

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 1 Missile Instruments 1.1

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.

1.2

Gyroscopes

1.3

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

1.4

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

1.4.1

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 rotations 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 3

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 compensated 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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1.4.2

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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1.5

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 accelerometers. 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.

1.6

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 V1 cosφ and the rudder servos receive −V1 sinφ ; where φ is the angle by which the missile has rolled.

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1.7

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. Radiation 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.

1.8

Current Trends

1.9

Conclusion

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