INERTIAL NAVIGATION SYSTEMS

BY:

BHARATH KUMAR.A.V.

1MV07EC017

SIR MVIT

CONTENTS

Introduction

History

Description

Applications

Conclusion

WHAT IS INS? An INERTIAL NAVIGATION SYSTEM (INS) is a

navigation aid that uses a computer , motion sensors and rotation sensors to continuously calculate the position, orientation, and velocity of a moving object without the need for external references.

WHY INS?A navigation system that could operate without having to depend on any outside signals, relying instead on its own signals as a standalone unit.

AN INS CAN DETECT

Change in its geographic position (a move towards east or north).

Change in its velocity (speed and direction of movement).

Change in its orientation (rotation about an axis).

INS CONSISTS OF Accelerometers: measuring linear acceleration.

Gyroscopes: measuring angular velocity.

ACCELEROMETER

Newton’s Second Law• F = mA.

Measure force on object of known mass (proof mass) to determine acceleration

ProofMass (m)

Direction of Acceleration w.r.t. Inertial Space

Displacement Pickup

Case

a

Spring

PENDULUM ACCELEROMETERS

Floated Pendulum Flexure Pivot Pendulum

MEMS

Micro electromechanical device (MEMS) solid state silicon accelerometer

DISADVANTAGES Fixed Bias Scale Factor Errors Cross-Coupling Vibro-Pendulous Error Clock Error

ADVANTAGES

GYROSCOPES

Two primary types• Mechanical• Optical

Measure rotation w.r.t. an inertial frame which is fixed to the stars (not fixed w.r.t. the Earth).

MECHANICAL GYROSCOPES

A rotating mass generates angular momentum which is resistive to change or has angular inertia.

Angular Inertia causes precession which is rotation of the gimbal in the inertial coordinate frame.

CORIOLIS PRINCIPLE

Axis velocity caused by harmonic oscillation (piezoelectric).

Axis rotation. Axis acceleration

measurement.

Problems: High noise. Temperature drifts. Translational acceleration. Vibration.

EQUATIONS OF PRECESSION

Torque vector T Angular Momentum vector H

Torque is proportional to • Angular Rate omega cross H plus• A change in angular momentum

δH = Change in angular momentum

SPIN AXIS (At time t = t + δt)

SPIN AXIS (at time t)

DISC

Precession (rate ω)H

H

O

A

B

Problems with Mechanical Gyroscopes

Large spinning masses have long start up times.

Output dependent on environmental conditions (acceleration, vibration, sock, temperature ).

Mechanical wear degrades gyro performance.

Gimbal Lock.

GIMBAL LOCK Occurs in two or more degree

of freedom (DOF) gyros Planes of two gimbals align

and once in alignment will never come out of alignment until separated manually

Reduces DOF of gyroscope by one

Alleviated by putting mechanical limiters on travel of gimbals or using 1DOF gyroscopes in combination

OPTICAL GYROSCOPE

Y

X

Ω

Beam Splitter Position at time t = t + δt

Beam Splitter Position at time t = t

Light Input

Light Output

• Measure difference in travel time of light traveling in opposite directions around a circular path

TYPES

Ring Laser Gyroscope

Fiber Optic

RING LASER GYRO They utilize the principle of the Doppler effect.

Two laser beams travel around a closed circuit (made with three or four mirrors) in opposite directions and are sensed by a detector.

As viewed from a reference point inside the gyro (the detector), there is a shift in the frequencies of the two laser beams

The rate of rotation of the gyro determines the phase difference of the frequencies.

Each ring laser gyroscope only rotates on one axis, therefore three of them are required to register changes in pitch, roll, and yaw.

GENERAL PRINCIPLE

ADVANTAGES Few moving parts Small size and light weight Rigid construction High tolerance to shock, acceleration, and vibration High level of accuracy Low cost over the lifetime of the gyro

Base cost of Laser ring gyros is more expensive than mechanical gyros.

Laser ring gyros are susceptible to an error known as “LOCK-IN”

DISADVANTAGES

LOCK-IN When the rate of turn is very small,

the frequency difference between the two beams is small.

There is a tendency for the two frequencies to “couple” together and “lock-in” with each other.

As a result of lock-in, a zero turning rate is indicated.

By mechanically moving or twisting the system, the coupling of frequencies does not occur.

This mechanical adjustment is called DITHERING.

FIBER OPTIC GYROSCOPE

Ω

Coupling Lens

Beam Splitter

Light Source Detector

Fiber Optic Coil

Measure phase difference of light traveling through fiber optic path around axis of rotation.

Two strands of optical fibre are wound in opposite directions on a coil form.

Laser light is sent from a single source down both fibres.

The outputs of the two fibres are combined at a photodiode.

The gimbals are a set of three rings, each with a pair of bearings initially at right angles.

They let the platform twist about any rotational axis.

There are two gyroscopes (usually) on the platform.

Two gyroscopes are used to cancel gyroscopic precession, the tendency of a gyroscope to twist at right angles to an input force.

GIMBALLED GYROSTABILIZED

PLATFORMS

STRAPDOWN SYSTEMS

A strap-down system is a major hardware simplification of the old gimbaled systems.

The accelerometers and gyros are mounted in body coordinates and are not mechanically moved.

Instead, a software solution is used to keep track of the orientation of the IMU (and vehicle) and rotate the measurements from the body frame to the navigational frame.

MATHEMATICAL EQUATIONS

The six degrees of freedomA body's actual spatial behavior / movement can be described with three translatory and three rotary components.

ADVANTAGES

The ins cannot be jammed. Will not fall prey to signal degradation. Does not require costly nationwide maintenance of

hundreds of transmitters. The ins will work deep underground and underwater

where gps signals can't reach. Inertial navigation even works in space.

DISADVANTAGES

The need for complete and precise maps was another disadvantage of inertial navigation

Position/velocity information degrade with time (1-2NM/hour).

Equipment is expensive ($250,000/system) - older systems had relatively high failure rates and were expensive to maintain

newer systems are much more reliable but still expensive to repair

Initial alignment is necessary - not much of a disadvantage for commercial airline operations (12-20 minutes)

APPLICATIONS

CONCLUSION

THANK YOU