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Integrated Inertial Positioning Systems Some Facts, Some Editorial and Some Biased Opinions

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Integrated Inertial Positioning Systems Some Facts, Some Editorial and Some Biased Opinions Integrated Inertial Positioning Systems Some facts, some editorial and some biased opinions Zupt, LLC Inertial Tools - What instruments are currently in daily use for Survey? In our business Inertial navigation systems (INS) for land seismic, Vertical Reference Units (VRU’s) for DP, USBL/SBL and Swath sonar attitude/heave corrections – Surface heading sensors – Spinning mass gyros as well as strap down Attitude Heading Reference Systems (AHRS) In other applications Inertial sensors - anti-lock brakes, anti skid, virtual reality headsets – Analog Devices, Crossbow, Systron, Bosch, BAe and many others Inertial Navigation systems as (Tactical) short term positioning sensors - Northrop Grumman, Honeywell, Kearfott, BAe, Boeing, etc. High precision (Strategic) Inertial Navigation systems for long term positioning outages - Northrop Grumman, Honeywell, Thales navigation, etc. Zupt, LLC Available How Long? In discussing this exciting new technology we must also understand that these tools have been around for a while, even in the seismic and survey business: Western Geophysical’s W-INS (“we invariably needed SHORAN”) late 1970’s Shelltech/Itech land seismic use of INS for control (helicopter based Zupt’s) Mid/late 1970’s Exxon/Honeywell’s DP reference systems – Riser and INS – 1979 British Oceanics/Intersub INS for manned submersible construction positioning (used in place of “the unreliable acoustic systems”) – early 1980’s Zupt, LLC Session Agenda A few definitions What is an inertial measurement unit (IMU)? Overview of inertial sensors Price versus performance Where is inertial technology going? Integrated Inertial Positioning Systems Loosely, tightly and deeply coupled Aiding Observations – current and future Applications for Integrated Inertial Positioning Systems Current and near term products Commercial benefits of these systems Zupt, LLC A few definitions An Inertial Sensor is a position, attitude or motion sensor whose reference are completely internal draft revision to IEEE Std 528 A Gyroscope is a sensor designed to illustrate the dynamics of a rotating body In Strapdown operations the inertial frame of reference is stored in the computer as opposed to being maintained mechanically by gimbals. Coordinate transformations and sensor compensation have to be completed within the strapdown computer. Bias - no input, but some level of output Angle random walk - white spectrum rate detection noise leads to an angle random walk (optical and coriolis gyros) Aiding - using external non inertial observations to minimize bias Scale Factor- an error in the assumed scale factor in the instrument output Schuler Oscillation/Period - 84 minutes – just think about a pendulum centered at the earth’s core and the IMU at the earth’s surface Zupt, LLC What does an IMU consist of? Zupt, LLC How does this fit together? Accelerometers Navigation Computer Integrate Speed Measure Compensate for Compensate accelerometer for Once-Velocity acceleration Twice-Distance bias and SF gravity Distance Gyros Compensate for Compensate Measure Gyro Bias, ARW, for Heading rotation rates SF and acceleration sensitivities Earth’s rotation Zupt, LLC Overview of inertial sensors Inertial sensors come in many forms and are an excuse for infinite acronym generation. The two types of sensors within an IMU are: Gyroscopes - rate of rotation Accelerometers - linear acceleration Some examples are: Gyros Dynamically tuned, (DTG), Fiber Optic (FOG), Ring laser (RLG) Accels Vibrating Beam (VBA), Quartz Resonating (QRA), Pendulous Mass (PMA) Zupt, LLC Gyro Technology Angular Rate Sensing technology principles: Spinning Mass - angular momentum Vibratory/Resonator - Coriolis Optical - Sagnac Micro Electro Mechanical Sensor(MEMS) primarily vibratory, some spinning mass, some optical Micro Optical Electro Mechanical Sensor (MOEMS) another variant Zupt, LLC Current Gyro technology Zupt, LLC More Gyro details Spinning Mass Honeywell, Northrop Grumman, Rockwell Pros Wide performance range 0.0001 to >100°/hr Very low noise - (specifically gas bearing) Cons Relatively high cost Long warm up Not well suited to strapdown applications Some types very fragile Vibratory/Resonant Watson, Systron Donner, Murata, BAe Pros Relatively small Minimum moving parts Cons Small scale factor Output noisy Rate gyro open loop Limited performance range (getting better though) Zupt, LLC Pros and Cons Optical Honeywell, Northrop Grumman, (Fibresense), Ixsea, Sagem, etc. Pros Rapid reaction and turn on(<1s) Ideally suited for strapdown operation No moving parts - very rugged Cons Performance increases with size RLG is a high voltage device FOG very temperature sensitive Micro Electro Mech.Sensors (MEMS) Draper/Honeywell, JPL, BAe, AD, Bosch, etc. (only vibratory discussed) Pros Very small No moving parts Very low cost Cons Higher precision still under development Limited performance range (only for a while) Bias stability Zupt, LLC FOG block Diag. Gyro Bias (°/hr) is usually proportional to length of fiber The longer the fiber - the better the FOG Zupt, LLC RLG Block Diag. Gyro Bias (°/hr) is usually proportional to path length The longer the path length - the better the RLG Path length control mirror Cervit block Anode Anode Dither mechanism Optical beams Mirror Cathode Detector -Schematic of ring laser gyro. Input axis is perpendicular to the plane of page. Zupt, LLC Accelerometer Technology Linear acceleration sensing technologies: Pendulous/Translational Mass displacement/rebalance Electrical Restraint Rotational Restraint Elastic Restraint Resonant Element Frequency Vibrating String Vibrating Beam Double Ended Tuning Fork Zupt, LLC Current Accelerometer Technology Zupt, LLC More accel. details Force Rebalance Accels - Honeywell Q-Flex, Northrop Grumman A4, Kearfott Mod VII Pros Highly Reliable - relatively low cost Wide bandwidth Low bias error Cons Analog output self heating under changing acceleration Power consumption Pendulous Rebalance Accels. Pros Reliable, rugged, small Well understood error model Pendulous Integrating Gyro Accel. (PIGA) as good as it gets used for ICBM and general missile guidance Cons PIGA - Cost Resonant Element Accel. Sundstrand, Allied Signal Adkem Pros Digital output Low power Cons Not good in high shock environment Detailed calibration required Zupt, LLC Sensor Advances - MEMS Wafer thick gyros - 400µm Critical assembly process for MEMS Assembly issues being worked on to make a low cost, mass produced “instrument”. Noise is the challenge I-O have low noise, low G product – VectorSeis® Zupt, LLC MEMS DoD Development Program A low cost, high G MEMS and guidance effort is underway for a DoD joint forces program. This effort has the following goals: Phase 1 <75°/hr, >10,000G, <8 cubic inches This phase should have been delivered 3 vendors selected – 2 delivered 4 months ago Phase 2 <10 °/hr, >20,000G, <4 cubic inches This phase should be delivered this/next year 2 vendors selected – one ready to deliver Phase 3 <0.5 °/hr desired (<1 °/hr acceptable), >20,000G launch survivable, <2 cubic inches volume. DoD’s cost expectation for this IMU is <$1,200 Should have been delivered in 2006 – may not be needed due to deeply coupled Phase 2. Deeply coupled L1 and L2/Lm, WAAS, SAASM GPS receiver should be incorporated as an option to Phase 2/3 Zupt, LLC So how good is a “good” INS? Once the sensors (just discussed) have been combined to make an Inertial Measurement Unit software has to be added to turn the raw rate (incremental rate - ? ?) and the raw acceleration (incremental velocity - ? V) data into something useful as a: Attitude Heading Reference System (AHRS), or an Inertial Navigation System (INS) The performance of an INS is usually rated in terms of its position error growth rate once the INS is navigating in “free inertial” mode (no aiding). The USAF define INS in the following manner: INS Classification Position Error Growth Rate Heading Errors Low > 2nm/hr >0.2° Medium 0.5 to 2nm/hr 0.05 ° to 0.2 ° Precision <0.5nm/hr <0.05 ° Following a standard ground alignment at 50 ° or lower latitude – USAF SNU84-1 Zupt, LLC Cost versus Performance PRICE $K 160 Department of State Controlled Technology Dept. of Commerce Controlled ? Primarily for internationally sourced IMU's only IMAR .003º $155K 20cm RLG 140 120 100 Thales Totem .001º Thales $100K 30cm RLG T24 .003º $130K Kearfott 24cm RLG LN250 .005º? $90K 1200m? FOG 80 LN100 .003º $80K Northrop 18cm RLG PHINS .003º $80K Ixsea 1200m FOG CIMU .0035 $80K Honeywell 6" path RLG Sigma 10 .05º SAGEM $65K 10cm RLG 60 Octans 0.01º $60K Ixsea 700m FOG (AHRS only) 40 T16 .01º $100K Kearfott 18cm RLG T90 1º $39K Tamam 20 LN200 1º $22K Northrop 200m FOG BOEING 3º $20K MEMS 0.002 0.015 0.15 1.5 15 BIAS STAB º/hr Zupt, LLC Where is this technology going? Zupt, LLC Maturity of technology Draper Laboratory’s view on the state of current development. The suggestion is that most technologies are now mature except for MEMS gyros. Zupt, LLC Export Control The fastest way to go to jail (without passing go) will be to flaunt the export controls associated with this technology. The International Traffic in Arms Regulations (ITAR) and the Arms Export Control Act (AECA) are the governing law that is overseen through the Directorate of Defense Trade Controls (DDTC www.pmdtc.org) within the U.S. Department of State (remember Colin Powell). Simply put (to me by the DDTC) “if you screw up, it will be you, not the company, that goes to jail”. DDTC is responsible for all licensing issues if the commodity is controlled by State. To get a commodity under the more understanding Dept. of Commerce control a “Commodity Jurisdiction” has to be filed with the D.o.State. Most US manufactured and “high end” international IMU’s will fall under the control of the DDTC Do not listen to the vendors when they say don’t worry about this – talk to your own export attorney and get their advice. Zupt, LLC What type of sensor do you need to buy/integrate? • As can be seen from the previous charts IMU’s are available in many flavors. The one underlying suggestion I would make is to: Only buy the sensor with the performance you really need Do not over specify the performance requirements of your sensor or it will cost significantly more than it should.
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