Basic Principles Of Inertial Navigation

2y ago
11 Views
3 Downloads
626.22 KB
22 Pages
Last View : 22d ago
Last Download : 2m ago
Upload by : Tripp Mcmullen
Transcription

Basic Principles of InertialNavigationSeminar on inertial navigationsystemsTampere University of Technology1

The five basic forms of navigation Pilotage, which essentially relies on recognizing landmarks to knowwhere you are. It is older than human kind. Dead reckoning, which relies on knowing where you started fromplus some form of heading information and some estimate of speed. Celestial navigation, using time and the angles between local verticaland known celestial objects (e.g., sun, moon, or stars). Radio navigation, which relies on radio‐frequency sources withknown locations (including GNSS satellites, LORAN‐C, Omega, Tacan,US Army Position Location and Reporting System ) Inertial navigation, which relies on knowing your initial position,velocity, and attitude and thereafter measuring your attitude ratesand accelerations. The operation of inertial navigation systems (INS)depends upon Newton’s laws of classical mechanics. It is the onlyform of navigation that does not rely on external references. These forms of navigation can be used in combination as well. Thesubject of our seminar is the fifth form of navigation – inertialnavigation.2

A few definitions Inertia is the property of bodies to maintain constant translational and rotationalvelocity, unless disturbed by forces or torques, respectively (Newton’s first law ofmotion).An inertial reference frame is a coordinate frame in which Newton’s laws of motionare valid. Inertial reference frames are neither rotating nor accelerating.Inertial sensors measure rotation rate and acceleration, both of which are vector‐valued variables.Gyroscopes are sensors for measuring rotation: rate gyroscopes measure rotationrate, and integrating gyroscopes (also called whole‐angle gyroscopes) measurerotation angle.Accelerometers are sensors for measuring acceleration. However, accelerometerscannot measure gravitational acceleration. That is, an accelerometer in free fall (orin orbit) has no detectable input.The input axis of an inertial sensor defines which vector component it measures.Multi‐axis sensors measure more than one component.An inertial measurement unit (IMU) or inertial reference unit (IRU) contains acluster of sensors: accelerometers (three or more, but usually three) andgyroscopes (three or more, but usually three). These sensors are rigidly mountedto a common base to maintain the same relative orientation.3

Basic principle of inertial navigation Given the ability to measure the acceleration of vehicle itwould be possible to calculate the change in velocity andposition by performing successive mathematicalintegrations of the acceleration with respect to time. In order to navigate with respect to our inertial referenceframe, it is necessary to keep track of the direction in whichthe accelerometers are pointing. Rotational motion of the body with respect to inertialreference frame may be sensed using gyroscopic sensorsthat are used to determine the orientation of theaccelerometers at all times. Given this information it ispossible to resolve the accelerations into the referenceframe before the integration process takes place.4

What does an INS consist of? An inertial navigation uses gyroscopes and accelerometers tomaintain an estimate of the position, velocity, and attitude rates ofthe vehicle in or on which the INS is carried, which could be a landvehicle, aircraft, spacecraft, missile, surface ship, or submarine. An INS consists of the following:– An IMU– Instrument support electronics– Navigation computers (one or more) calculate the gravitationalacceleration (not measured by accelerometers) and doubly integratethe net acceleration to maintain an estimate of the position of thehost vehicle.5

Stabilized Platform and StrapdownTechnologies There are many different designs of INS with different performancecharacteristics, but they fall generally into two categories:– gimbaled or stabilized platform techniques, and– strapdown The original applications of INS technology used stable platformtechniques. In such systems, the inertial sensors are mounted on astable platform and mechanically isolated from the rotationalmotion of the vehicle. Platform systems are still in use, particularlyfor those applications requiring very accurate estimates ofnavigation data, such as ships and submarines. Modern systems have removed most of the mechanical complexityof platform systems by having the sensors attached rigidly, or“strapped down”, to the body of the host vehicle. The potentialbenefits of this approach are lower cost, reduced size, and greaterreliability compared with equivalent platform systems. The majordisadvantage is a substantial increase in computing complexity.6

Gimbaled inertial platform

Gimbaled systems A gimbal is a rigid with rotation bearings forisolating the inside of the frame from externalrotations about the bearing axes. At least threegimbals are required to isolate a subsystem fromhost vehicle rotations about three axes, typicallylabeled roll, pitch, and yaw axes. The gimbals in an INS are mounted inside oneanother. Gimbals and torque servos are used tonull out the rotation of stable platform on whichthe inertial sensors are mounted.8

How does gimbaled INS work? The gyros of a type known as “integrating gyros” give anoutput proportional to the angle through which they havebeen rotated Output of each gyro connected to a servo‐motor driving theappropriate gimbal, thus keeping the gimbal in a constantorientation in inertial space The gyros also contain electrical torque generators which canbe used to create a fictitious input rate to the gyros Applications of electrical input to the gyro torque generatorscause the gimbal torque motors/servos to null the differencebetween the true gyro input rate and the electrically appliedbias rate. This forms a convenient means of cancelling out anydrift errors in the gyro.

Gimbaled INS example

Strapdown INS

Strapdown inertial navigation concept Accelerometers mounted directly to airframe(strapdown) and measure “body” acceleration Horizontal/vertical accelerations computedanalytically using direction cosine matrix(DCM) relating body coordinated and locallevel navigation coordinates DCM computed using strapdown bodymounted gyro outputs

RLG instrument cluster (MarconiFIN3110 strapdown INS)

Two‐dimensional navigation forstabilized platform14

Two‐dimensional navigation forstrapdown system15

Strapdown inertial navigation unitblock diagram

Strapdown INS building blocks17

Accuracy vs. price18

Advantages of INS It is autonomous and does not rely on any external aidsor visibility conditions. It can operate in tunnels orunderwater as well as anywhere else. It is inherently well suited for integrated navigation,guidance, and control of the host vehicle. Its IMUmeasures the derivatives of the variables to becontrolled (e.g., position, velocity, and attitude). It is immune to jamming and inherently stealthy. Itneither receivers nor emits detectable radiation andrequires no external antenna that might be detectableby radar.19

Disadvantages of INS Mean‐squared navigation errors increase with time.Cost, including:– Acquisition cost, which can be an order of magnitude (or more) higher thanGPS receivers.– Operations cost, including the crew actions and time required for initializingposition and attitude. Time required for initializing INS attitude bygyrocompass alignment is measured in minutes. TTFF for GPS receivers ismeasured in seconds.– Maintenance cost. Electromechanical avionics systems (e.g., INS) tend to havehigher failure rates and repair cost than purely electronic avionics systems(e.g., GPS). Size and weight, which have been shrinkingPower requirements, which have been shrinking along with size andweight but are still higher than those for GPS receivers.Heat dissipation, which is proportional to and shrinking with powerrequirements.20

Synergism with GPS GPS integration has not only made inertial navigationplatform better, it made it cost less Sensor errors that were unacceptable for stand‐aloneINS operation became acceptable for integratedoperation Manufacturing and calibration costs for removing theseerrors could be eliminated New low‐cost MEMS sensor technologies could beapplied INS also benefits GPS performance by carrying thenavigation solution during loss of GPS signals andallowing rapid re‐aquisition21

Relation to guidance and control Navigation is concerned with determining where youare relative to where you want to be. Guidance is concerned with getting yourself to yourdestination. Control is concerned with staying on track. There has been quite a bit of synergism among thesedisciplines, especially in the development of missiletechnologies where all three could use a common setof sensors, computing resources, and engineeringtalent. As a consequence, the history of developmentof inertial navigation technology has a lot of overlapwith that of guidance and control.22

INS operation became acceptable for integrated operation Manufacturing and calibration costs for removing these errors could be eliminated New low‐cost MEMS sensor technologies could be applied INS also benefits GPS performance by carrying the navigation solution during loss of GPS signals and

Related Documents:

Visual Inertial Navigation Short Tutorial Stergios Roumeliotis University of Minnesota. Outline . "Visual-inertial navigation: A concise review," IRA'19. Introduction Visual Inertial Navigation Systems (VINS) combine camera and IMU . Continuous-time System Equations: Quaternion of orientation: Rotation matrix: Position: Velocity

Redundant Inertial Navigation Unit (RINU) The RINU is a redundant inertial navigation system manufactured by Honeywell International, Inc (HI). The RINU is derived from the Fault Tolerant Inertial Navigation Unit (FTINU) INS previously flown on the Atlas V launch vehicle. The RINU features a redundant set of five inertial instruments channels.

Inertial Sensors, Precision Inertial Navigation System (PINS). 1 Introduction Presently Inertial Navigation Systems are compensated for gravitational acceleration using approximate Earth gravitation models. Even with elaborate model based gravitation compensation, the navigation errors approach upto several hundred

2.2 Fundamentals of Inertial Navigation, 19 2.2.1 Basic Concepts, 19 2.2.2 Inertial Navigation Systems, 21 2.2.3 Sensor Signal Processing, 28 2.2.4 Standalone INS Performance, 32 2.3 Satellite Navigation, 34 2.3.1 Satellite Orbits, 34 2.3.2 Navigation Solution (Two-Dimensional Example), 34 2.3.3 Satellite Selection and Dilution of Precision, 39

only inertial navigation system. Objective of the proposal: The objective of the proposal is a combination of the existing inertial navigation system (INS) with global position system (GPS) for more accurate navigation of the launchers. The project's product will be navigation algorithms software package and hardware units.

correct the inertial navigation solution and also to constrain future development of navigation errors by correcting the incoming inertial measurements. While different approaches for navigation-aiding can be found in the literature, arguably the most common approach is based on various variants of the well-known extended Kalman filter (EKF).

Inertial Navigation System (AINS) consists of an inertial navigation system (INS), Doppler velocity log, depth meter and intermittent DGPS fixes. The data acquired are fused by an extended Kalman filter. After preliminary tests, this navigation system will be installed on an Autonomous Underwater Vehicle (AUV) where it will

Inertial sensors used in the mechanization of Newton's laws of motion, hereafter called the inertial navigator or inertial navigation system (INS), are gyroscopes and accelerometers. The gyroscopes sense angular orientation or motions of the vessel. The accelerometers sense the vessel's linear accelerations, which are changes in linear