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Issue 12 Nov. - December 2008 |
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Also at www.zupt.com |
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INERTIAL NEWS |
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In this issue of
Zest:
Inertial, Seismic, Survey and Other NEWS
A short History of the maiden flight of Ariane 5
Underground Mines
Underwater Metrology
Happy HOLIDAYS!
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INERTIAL NEWS
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A SHORT HISTORY OF the maiden flight of Ariane 5
. The weather at the launch site at Kourou in French Guyana on the morning of 4 June 1996 was acceptable for a launch that day, and presented no obstacle to the transfer of the launcher to the launch pad. In particular, there was no risk of lightning since the strength of the electric field measured at the launch site was negligible. The only uncertainty concerned fulfillment of the visibility criteria. The countdown, which also comprises the filling of the core stage, went smoothly until H0-7 minutes when the launch was put on hold since the visibility criteria were not met at the opening of the launch window (08h35 local time). Within an hour, visibility conditions improved as forecast and the launch was initiated at H0 = 09h 33mn 59s local time (=12h 33mn 59s UT). Ignition of the Vulcain engine and the two solid boosters was nominal, as was lift-off. The vehicle performed a nominal flight until approximately H0 + 37 seconds. Shortly after that time, it suddenly veered off its flight path, broke up, and exploded.
This would have been the maiden flight of the new Ariane 5 launcher of the European Space Agency, following the many successful launches of the Ariane 4, which it was meant to replace. . The information available on the launch included: - telemetry data received on the ground until H0 + 42 seconds - trajectory data from radar stations - optical observations (IR camera, films) - inspection of recovered material. The origin of the failure was thus rapidly narrowed down to the flight control system and more particularly to the Inertial Reference Systems (IRS also known as SRI or INS) All the launcher debris fell back onto the ground, scattered over an area of approximately 12 km2 east of the launch pad. Recovery of material proved difficult, however, since this area is nearly all mangrove swamp or savanna. Nevertheless, it was possible to retrieve from the debris the two Inertial Reference Systems. . In general terms, the Flight Control System of the Ariane 5 is of a standard design. The attitude of the launcher and its movements in space are measured by an Inertial Reference System (INS). It has its own internal computer, in which angles and velocities are calculated on the basis of information from a "strap-down" inertial platform, with laser gyros and accelerometers. The data from the INS are transmitted through the databus to the On-Board Computer (OBC), which executes the flight program and controls the nozzles of the solid boosters and the Vulcain cryogenic engine, via servovalves and hydraulic actuators. In order to improve reliability there is considerable redundancy at equipment level. There are two INS operating in parallel, with identical hardware and software. One INS is active and one is in "hot" stand-by, and if the OBC detects that the active INS has failed it immediately switches to the other one, provided that this unit is functioning properly. Likewise there are two OBCs, and a number of other units in the Flight Control System are also duplicated. . The design of the Ariane 5 INS was practically the same as that of an INS which was used successfully on Ariane 4, particularly as regards the software. But Ariane 5 (by design) had a high initial acceleration and a trajectory which led to a build-up of horizontal velocity, (much greater than on Ariane 4 launchers). . An internal INS software exception was caused during execution of a data conversion from 64-bit floating point to 16-bit signed integer value. The floating point number which was converted had a value greater than what could be represented by a 16-bit signed integer. This resulted in an Operand Error. The Operand Error occurred due to the unexpected high value of an internal alignment function result called BH, Horizontal Bias, related to the horizontal velocity sensed by the platform. . An angle of attack of more than 20 degrees (causing the destruction of Ariane 5) was caused by full nozzle deflections of the solid boosters and the Vulcain main engine. These nozzle deflections were commanded by the On-Board Computer (OBC) software on the basis of data transmitted by the active Inertial Reference System (INS 2). Part of these data at that time did not contain proper flight data, but showed a diagnostic bit pattern of the computer of the INS 2, which was interpreted as flight data. The reason why the active SRI 2 did not send correct attitude data was that the unit had declared a failure due to the software exception. . The OBC could not switch to the back-up INS 1 because that unit had already ceased to function during the previous data cycle (72 milliseconds period) for the same reason as INS 2, (having the same software). Full Report: Ariane 5 accident report Ariane 5 now had over 34 successful launches. OTHER NEWS
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. Graphically Cool Site of the Month |
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INERTIAL TIPS: What are the three controlling parts of a vehicle that can be used by the driver to create acceleration? Acceleration is a change in the velocity vector, so the most obvious way to create an acceleration in a car is by pressing the accelerator or gas pedal: it will increase the velocity and make you feel an acceleration. If after a few seconds you let go the accelerator, and if there was no friction between the road and the wheels or between the air and the body of the vehicle (like in deep space), the car would keep moving at a constant velocity. Most of us will have guessed the second one, nearby: the brake pedal. This second moving part allows you to decrease the speed of the vehicle, and therefore to create an acceleration (another change in the velocity vector). No so obvious is the role of the steering wheel. Speed is a scalar quantity (it only has a magnitude), but velocity is a vector quantity (with a magnitude and a direction). Since acceleration is a change in velocity and not in speed, an accelerometer does output information in a vehicle changing direction at a constant speed. The simple fact of turning creates acceleration, even though the speed doesn t change. So if you drive in a circle at constant speed, you are submitted to an acceleration. Now what is the direction of that acceleration? It is the same direction as the change of direction of the velocity vector, and unsurprisingly it is towards the center of the circle. By simply operating the steering wheel you have created an acceleration towards the center of the circle that you are following.
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GLOSSARY: Strapdown SystemsThe first Inertial Navigation Systems were gimbaled. They were oriented in a given direction in all three axes, independently of the motions of the vehicle. At turn on time they would follow a precise alignment procedure using complex mechanical motors, in order to orient themselves towards the North direction (perpendicular to the sensed earth rotation) and the Horizontal plane (perpendicular to the sensed gravity vector). From that moment on, the motors would keep the system aligned in those given directions independently of the motions of the vehicle. Strapdown inertial navigation systems on the other hand, are rigidly fixed to the moving body of the vehicle. Therefore their gyros experience and measure the same changes in angular rate as the body in motion. The strapdown INS accelerometers measure changes in linear rate in terms of the body's fixed axes. The body's fixed axes constitute a moving frame of reference as opposed to the constant inertial frame of reference. The navigation computer uses the gyros angular information and the accelerometers linear information to calculate the body's 3D motion with respect to an inertial frame of reference. MEMS & Nano.
DARPA wants microscopic atom clocks on chips: the register
Graphically cool site of the month: www.easyweb.fr
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.
From:
To: joelgi@aol.com
Sent: Fri, 21 Nov 2008 4:24 pm
Subject: Suggest For Future New Topics
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