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Flight Simulator Vibration

Due to the fact that the technology discussed on this page is protected by a confidentiality agreement, I will not be able to discuss it in detail.  However, this page will discuss the problem solved by the new technology invented by my team for Evans & Sutherland. I would be more than happy to discuss more details of this project during an interview.

Evans & Sutherland produces hardware and software to create highly realistic visual images for simulation, training, engineering, and other applications throughout the world. E&S visual systems are used in both military and commercial systems, as well as planetariums and interactive theaters.  The company's depth in technology and experience uniquely positions E&S to provide complete, tailored visual solutions for virtually any simulation or visualization application.
Beginning in September 1999 and ending in April 2000, E&S sponsored a project through the Brigham Young University Mechanical Engineering Department to solve a vibration issue they were having with a product they called Spectra View.  Shortly after the teams formation, I was appointed by the team as the team leader.  This multidisciplinary team consisted of three mechanical engineers, one manufacturing engineer and one industrial designer.

Problem Description

Pictured to the right is an illustration of a flight simulator that offers pilots a 260 degree view (E&S refers to this as a "Mosaic Display").  In the picture to the right, the cube represents the flight simulator, the grey partial sphere represents the screen, the three multicolored boxes represent the projection equipment, and the simulated cockpit is located right in the center.  This entire unit sits on top of three hydraulic actuators  that simulate the motion of the aircraft.  The problem my team was hired to solve dealt specifically with unwanted vibration in the screen.
The picture to the right is a close-up of the screen and projection system that is used in the flight simulator pictured above.  The display works by projecting an image, using four projectors, onto the mirror screen that reflects the image back into the eyes of the pilot.  The combination of projector positions and the spherical mirror screen create images that appear to be positioned at infinity (this makes for incredibly life like 3D simulations).  To have this enormous mirror screen made in the conventional manner (with silvered glass) would have been cost prohibitive and required seams in the screen.  Therefore, E&S decided to make the mirror another way.
E&S made their mirror screen by stretching Mylar (the same stuff used to make balloons) over the spherical frame and sealing it to the edge.  A vacuum is then pulled behind the Mylar to stretch it into the shape of a sphere.  This same sort of thing happens when you place plastic wrap over a bowl of hot food and place it in the refrigerator.  As long as the plastic wrap remains sealed at the edges and doesn't touch the food in the bowl, the plastic wrap is pulled into the shape of a sphere as the food cools.  An illustration of a cross sectional view, cut top to bottom, of the mirror screen can be seen to the right.
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Because the of this design, the Mylar mirror acts like the head of a drum.  At certain times during simulation, the hydraulic actuators will excite the volume of air behind the screen causing the Mylar to vibrate like the head of a drum.  This vibration distorts the image seen by the pilots and can even cause nausea.  

The Solution

When you blow over the top of a glass soda pop bottle the air inside is excited.  This excitation causes the air inside the bottle to resonate and make a sound.  The smaller the bottle the higher the sound.  The basic principle behind the solution was to raise the frequency at which the volume of air behind the Mylar vibrates by making the volume of air smaller.  By raising the frequency of the Mylar/Frame system above the frequency of any vibration created by the flight simulator, the Mylar vibration could be completely eliminated.
This seamed like a simple enough solution.  There was a 1.5 inch space between the Mylar and the back of the frame.  By decreasing this space the frequency would be raised and the problem would be solved.  Unfortunately, the1.5 inch space was needed to install the Mylar mirror and could not be reduced.  The solution we invented, which I cannot discuss in detail here, raised the resonate frequency of the Mylar/Frame system by decreasing the air volume behind the Mylar without decreasing the 1.5 inch space behind the Mylar.  My team's solution proved to eliminating unwanted Mylar vibration.