Subversive Guide to Engineering

I've started a new blog called the Subversive Guide to Engineering. Its a sarcastic look at what to expect in engineering school, how to succeed (or more likely, survive), life in eng, and tips on picking majors and careers. The address again is http://subversiveguidetoeng.blogspot.com.

If you have any tips or stories to share, I'd love to hear from you in the comments of the site. Likewise, if you know someone that is in engineering or is looking to go into engineering, to give them a heads up about the site. I definitely would have loved to have a source like this back when I was starting out.

Thanks again!

Fiber Optic Gyroscopes

Introduction

Gyroscopes are key components of the navigation systems that keep vehicles on course. In fact, gyroscopes can be found in anti-skid systems in cars, satellites, the Space Shuttle, airplanes, ships, and missiles. The first practical gyroscope instruments were used to create “artificial horizons” for airplanes. The first gyroscope was invented in 1852 by Leon Foucault, however it was Elmer Sperry who made the first practical instruments in 1910 [1]. The first gyroscopes were mechanical, but they’re now made with fiber-optics, ring lasers, and even with solid state MEMS devices [1], [4].

Navigation Systems

Gyroscopes are fundamental building blocks of the navigation systems used to guide airplanes, ships, and spacecraft [1]. A gyroscope is essentially an instrument that measures rotation [1]. By itself, gyroscopes are useful in applications such as measuring the wing angle in aircraft in flight and the roll of ships in heavy seas. However, combined with accelerometers, they are used to create inertial guidance navigation systems [4].

An inertial guidance navigation system is essentially a form of “dead-reckoning”. If the starting position is known, keeping track of the acceleration and rotation that a vehicle undergoes can be used to determine its current position [4]. Such a guidance system is constructed with 3 accelerometers and 3 gyroscopes, each measuring acceleration and rotation in a single axis. Such a system can measure an aircraft’s position, velocity, acceleration, attitude, and heading to a high degree of accuracy [4].

Gyroscopes: Principles of Rotation Measuremens

Before considering photonic solutions to the problem of rotation measurement, it is important to consider previous methods. An important form of gyroscope, still in use today, is the mechanical gyro. The invention of the first mechanical gyroscope is credited to the French experimental physicist Leon Foucault in 1852, who planned to use it to measure the rotation of the earth [2]. As shown in figure 1, a mechanical gyroscope is created by suspending a rapidly spinning rotor inside three frictionless rings, called gimbals.



Figure 1 – [Left] A mechanical gyroscope contains a massive spinning rotor with an angular momentum pointed along the spin axis. If the gyroscope is tilted, [right], the rotor maintains its spin due to conservation of momentum. The tilt of the gimbals gives an indication of the tilt of the instrument [3]

Due to conservation of angular momentum, the spinning rotor maintains its direction in space even if the gyroscope is tilted. Therefore, tilting the gyroscope will cause the gimbals to reorient themselves to maintain the rotating mass in its original spin direction. The angle of rotation of the outer ring about its axis is proportional to the rotation of the gyroscope about its spin axis [3].

Principles of Operation of Fiber-Optic Gyroscopes

Fiber-optic gyroscopes (FOGs) are based on the Sagnac effect. Sagnac interferometers are based on the principle that if the interferometer is rotating, light waves traveling in opposite directions in a loop will acquire a phase difference, resulting in interference. The phase difference is due to the fact that light travels with a constant speed, c, as shown in figure 2. If the interferometer is rotating counter-clockwise (CCW) with an angular velocity, by the time the clockwise (CW) traveling wave (in red) reflects from mirror M1, it will have moved slightly closer to the wave. Likewise, mirror M1 will have moved slightly away from the CCW traveling wave (in blue).


Figure 2 – An illustration of the Sagnac effect. In an interferometer rotating counter-clockwise , the counter-clockwise propagating beam (red) will experience a shorter path length while the clock-wise beam (blue) will experience a longer path length. The difference in path length leads to a phase shift between the two beams and hence interference.

Therefore, the CW traveling wave will have a shorter path length while the CCW wave will have a longer path length. The result will be a net phase difference between the two waves, causing interference.

Interference Fiber-Optic Gyroscopes

The interference fiber-optic gyroscope (IFOG) is based on detecting the phase shift difference that occurs in an interferometer due to the Sagnac effect. Unfortunately, the Sagnac effect is relatively weak, so to overcome this problem kilometers of fiber optic cable are used to increase the path length of the instrument.

An example of the extreme sensitivity needed by gyroscopic instruments is the fact that an inertial guidance system must be capable of detection rotations of 0.01 degrees/hour. Using light with a wavelength of 1 micron, 1 kilometer of fiber and a coil diameter of 30 cm, the resulting phase shift is only 10^-7 rad, which is at the detection limit of current instruments [5].

In order to increase accuracy, a practical IFOG system would measure interference effects at the same port as the input light, such that both lightwaves experience two reflections. This eliminates the extra phase shift from reflection that would otherwise be introduced into the measurement. A polarizer is also placed at the input to eliminate the polarization that is introduced to the light in the fiber. This extra polarization is introduced when light travels through the fibers since optical fibers are birefringent to some degree. Birefringence refers to the fact that light of different polarizations is refracted differently. This occurs in optical fibers most commonly because of small defects in the fibers themselves.

Commercial Examples of FOGs

FOGs are commercially sold by a number of manufacturers, for civilian, military, and space applications. An example of an inertial guidance system is the IMU 200, developed by Northrop Grumman [6]. The IMU 200 contains three gyroscopes and three accelerometers and is designed for high-performance applications, especially for weapons guidance systems. The system can withstand accelerations of 12 g’s, has a long-term stability of 0.5 degrees/hr and misalignment error of 0.1 mRad [6]. A version of this guidance system is currently used in the National Missile Defence interceptor missile [6]. An important point is that in many countries, including the United States, it is illegal to export highly accurate gyroscopes, because of their potential uses in weapons [1].

Future Trends: Photonic-Bandgap Fibers

The prime limitation on the accuracy of FOGs are parasitic effects that occur inside the silica fibers. These include Rayleigh backscattering, the Kerr effect, the Faraday effect, and thermal effects [7]. As mentioned previously, Rayleigh backscattering is caused by impurities in the fiber and leads to large random errors in measurement due to spurious signals. The Faraday effect causes a change in the birefringence of the fiber on the application of a uniform magnetic field. The Kerr effect, meanwhile, is a non-linear optical process that changes the index of refraction of the fiber with small variations of input power of the two beams. This causes a “drift” in the measured rotation rate [5]. Lastly, uneven thermal effects in the fiber can cause unwanted phase change. Air-core photonic-bandgap fibers reduce the thermal effects by a factor of 3-10 and the other effects by a factor of 100-500! [7].

A photonic bandgap material is one in which certain wavelengths of light are unable to propagate. This is very similar to an electronic bandgap, where certain electron energies are not allowed. The photonic bandgap is crated by a periodic microstructure. This effect is also seen in nature, for example in butterfly wings. Butterfly wings contain a fine structure, forming a photonic bandgap. Light with wavelengths in the bandgap region is strongly reflected, forming the bright colours that pattern the wings [8].

A dielectric periodic lattice will exhibit the photonic bandgap effect. For example, as shown in figure 3, this effect is exhibited by a diamond lattice [9]. To create a photonic bandgap fiber, a periodic lattice of airholes is formed in the fiber, creating the photonic bandgap material. A defect center, the central air-core is then introduced. Light propagating in the air core will not be able to leave the fiber because of the photonic bandgap material surrounding it [8]. The structure of a photonic crystal fiber is shown in figure 4.

Figure 3 – [Left] An illustration of a diamond lattice structure. [Right] The resulting bandgap structure [9].


Figure 4 – An illustration of a commercially available photonic bandgap fiber manufactured by Crystal Fibre [8].

The accuracy improvements found in air core fibers are mainly due to the advantageous properties of air over silica. The Kerr constant of air is about 800 times smaller than in silica. Likewise, the Faraday effect is about 500 times weaker. Rayleigh scattering is theoretically lower in air. Unfortunately, in current air-core fibers, Rayleigh scattering effects are actually higher than in silica fibers. This is mainly due to small dimensional fluctuations in the wall of the fiber introduced in the manufacturing process [7].

Researchers at Stanford University have created the first air-core photonic bandgap based fiber-optic gyroscope. The FOG was built with 235 m of commercially available fiber manufactured by Crystal Fiber. The minimum sensitivity of the gyroscope was 2.7 degrees/hour and the long term drift was 2 degrees per hour. To compare the performance with regular fiber, the crystal fiber was replaced with 200 m of silica single-mode fiber. The resulting minimum sensitivity was 7 degrees/hour and the long term drift was 3 degrees per hour [7].

The gyroscope created was essentially a proof-of-concept design. Currently, much work is left to be done in the development of photonic bandgap fibers. Current fibers have high loss (~19 dB/km for high quality fiber) and scattering mechanisms. Ultimately, research and development into improved photonic bandgap fibers will lead to FOGs with improved long-term stability, simplified design, lower cost, and higher reliability [7].

References:

[1] Fischetti, M., Gyroscope Guidance: Hidden Guides, Scientific American, Vol. 286, n. 6, June 2002, pp. 96-97
[2] Greenslade, T., Gyroscope, Instruments for Natural Philosophy, 2006
http://physics.kenyon.edu/EarlyApparatus/Mechanics/Gyroscope/Gyroscope.html
[3] Hyperphysics, Gyroscope, Hyperphysics, 2006
http://hyperphysics.phy-astr.gsu.edu/hbase/gyr.html
[4] King, A.D., Inertial Navigation – Forty Years of Evolution, GEC Review, Vol. 13, no. 3, 1998. pp. 140-149
[5] Bergh R. et al., An Overview of Fiber-Optic Gyroscopes, Journal of Lightwave Technology, Vol. LT-2, no. 2, 1984. pp. 91-107
[6] Northrop Grumman, IMU 200 Product Brochure, Northrop Grumman Navigation Systems Division, 2000
http://www.nsd.es.northropgrumman.com/Html/IMU200/BrochureIMU-200_Inertial_Measuring_Unit.pdf
[7] Kim H.K. et al., Air-Core Photonic-Bandgap Fiber-Optic Gyroscope, Journal of Lightwave Technology, Vol. 24, no. 8, 2006. pp. 3169-3174
[8] Crystal Fibre, Technology – Air Guiding Fibers, Crystal Fibre, 2005
http://www.crystal-fibre.com/technology/technology_tutorial4.shtm
[9] Johnson, S., Photonic Crystals: Periodic Surprises in Electromagnetism, MIT, 2004
http://ab-initio.mit.edu/photons/tutorial/
[10] Sherman, R., Star Wars Programs, FAS.org, 2006
http://www.fas.org/spp/starwars/program/000708-nmd_ift5_8.jpg
[11] Sensormag, Sensors, Sensormag.com, 2006
http://sensorsmag.com/sensors/data/articlestandard/sensors/142006/318761/0900_101f.gif
[12] Northrop Grumman, Navigation Systems Division Brochure, Northrop Grumman Navigation Division, 2006
http://www.nsd.es.northropgrumman.com

Milestones: Flash Memory [1984]

Note: This article is a lead-in to an excellent story about the invention of flash memory, and its inventor, Dr. Fujio Masuoka, titled “Unsung Hero” at Forbes.com

Introduction

Today, Flash memory is ubiquitous, compromising a $76 billion dollar a year market, largely cornered by one chipmaker: Intel. Flash memory is integral to a large number of consumer and industrial applications such as:

• PCs and notebook computers
• Solid state music players
  
• Cell phones and PDAs
• Security systems
• Embedded systems
• Networking products
• Medical devices

The key aspect of Flash that makes it unique is that the memory is non-volatile and re-writeable. Traditional RAM memory must be constantly refreshed every few milliseconds in order to maintain its contents, without power the memory is reset. Meanwhile, traditional ROM memory is non-volatile, but is not easily re-written with new information. Devices used to store information more permanently, such as hard disk drives, meanwhile, are not solid-state devices but instead have many moving parts. Therefore, they tend to consume a lot of power, are sensitive to impacts and vibrations, are bulky, and suffer from a higher rate of failure. Flash memory would turn out to be a proverbial silver bullet, addressing all of these concerns.

Flash was invented at Toshiba in 1984 by Dr. Fujio Masuoka. Dr. Masuoka, pictured on the left, got to his senior research position at Toshiba by making incremental improvements to Toshiba’s bread-and-butter product, DRAM memory. However, the idea of creating non-volatile solid state memory drove him. He worked on the idea largely working on his own, without the blessing of upper management. After introducing the first prototypes of the memory in 1984, he received a bonus, worth a few hundred dollars from Toshiba. Intel, convinced the new type of memory would be an important cornerstone of the memory market invested heavily in commercializing the new technology, while Toshiba gave Masuoka a few part-time engineers.

In the end, Toshiba was embarrassed by its failure to capitalize on Flash, and Toshiba spokesmen have even tried to claim the memory was actually invented at Intel. Toshiba, meanwhile, allegedly tried to demote Fujio Masuoka, who was regarded at the company as not a team player and insubordinate due to his independent work on Flash.

Without further ado, the rest of Fujio Masuoka’s fascinating story is available here: Unsung Hero at Forbes.com.

Interesting Links:
Unsung Hero: Forbes.com
Samsung’s Solid State Disk Drive

Milestones: The First Telephone Call [March 1876]

Introduction

On March 10, 1876, Alexander Graham Bell made the world's first telephone call, speaking the words "Mr. Watson, come here, I want to see you." to his assistant in an adjoining room. The communications landscape in 1876 was much different than it is today. Communications were carried out largely through mail or telegraph, with telegraph lines increasingly criss-crossing the developed world. However, the mail system was slow, and telegraph messages had to be laboriously tapped out in Morse code, making them necessarily short and limited to urgent communications. In other words, the world was a much larger and less connected place.


Development of the First Telephone

As often happens, the telephone was the result of research and development by a large number of innovators, however it was Alexander Graham Bell that filed the first patent, and hence is widely credited for its invention. The first telephone, with which Bell transmitted his historic message, used a liquid microphone, invented by Elisha Gray. Gray's liquid transmitter consisted of a diaphragm attached to a metal needle placed just barely into contact with a conducting liquid (any liquid with free ions will do, such as a water/acid mixture). As the diaphragm vibrated, the needle would dip into and out of the liquid, resulting in a variation of current passing to the receiver.

Bell, meanwhile, had been working on a similar concept, however using the movement of a reed in a magnetic field alone, rather than in a liquid. His first devices using this concept, in 1875, however, suffered from a large amount of interference and tended to have the reeds stick to the electromagnets. Bell developed his first bi-directional telephone, and the one used in the famous demonstration with Watson, using Gray's water transmitter idea. The water transmitter was not ideal however, and Bell's later experiments focused on perfecting the electromagnetic transmitter.

Bell soon created an improved version of the electromagnetic transmitter. The mouthpiece consisted of a stretched membrane to which was attached an iron armature. An electromagnet was placed behind the membrane. As someone speaks, the soundwaves emitted by their voice cause the membrane to vibrate. The vibrating iron armature, in turn, induces a current in the electromagnet. This current was transmitted along a conductor to the receiver, which converted the varying current to sound using the exact opposite process (i.e.: A varying current in an electromagnet causes a diaphragm to vibrate, producing sound). The patent application submitted by bell is shown on the right.


The First Commercial Instruments

The rest, as they say, is history. Bell would continue to make pubic demonstrations of his telephone through the late 1870s. Bell and Watson made the longest call yet, a distance of two miles, on October 9th, 1876. More inventions would be needed before the telephone became a commercial device. In 1877, Thomas Edison filed a patent for an improved transmitted, that used an independent power source, rather than just the energy contained in the user's voice, to power the device. This sparked a competitive race between Edison and Bell.

The spread of the telephone was slow at first. The first exchanges began in Britain, where by the end of 1878 there were 200 subscribers. Initially the telephone was the purview of the rich, costing 20 pounds a year (when the average yearly wage was 80 pounds or so).

The first telephone companies were set up, consisting of public "exchanges", where one could come to and use the telephones if a subscription fee was paid. By the 1890s, automatic exchanges started cropping up, as well as more sophisticated telephones, as pictured on the right. It would only be a matter of time before the telephone became a staple of modern life.

Perhaps one of the most interesting aspects of the invention of the telephone is the shear amount of litigation involved. Decades-long legal battles were fought by Elisha Gray and Alexander Graham Bell (who beat Gray to the patent office by several hours, although this is hotly contested) and between Bell and Edison, among others. While Bell was a visionary inventor, his legacy stands above the others partly due to the fact that he was also an articulate and astute businessman, who was able to secure funding and had well positioned business partners.

Interesting Links
Library of Congress: Alexander Graham Bell
Telephone History
Connected Earth: The Telephone

Alessandro Volta [1745 - 1827]

Introduction

These days, its difficult to go for more than a few hours without using some sort of electronic device powered by a battery, whether it’s a cellphone, laptop, mp3 player, or a remote control. Batteries are without a doubt a critical part of our modern world, but whom do we have to thank for their invention? It turns out that the letters of gratitude can be sent to an Italian physicist, Alessandro Volta (that is, if you could send letters back in time, Volta passed away in 1827).

While the battery is important now, it was perhaps even more critical when Volta first introduced it in 1800. His first batteries, or ‘voltaic piles’, provided a steady, constant supply of electricity that proved to be crucial in some of that era’s most important electrical experiments that established the field. It’s important to realize that at that time, accessing electricity was not a trivial task. There were no power plants or electrical power lines criss-crossing the countryside. In fact, electricity in general was a relatively recent discovery and was still poorly understood.

Childhood

Young Volta had the advantage of a high birth, his parents were members of the Lombard aristocracy, who had close ties with the church. At an early age, Volta did not show any extraordinary ability, but quickly began to distinguish himself as he progressed in his schooling. Tragically, his father died when he was seven, and his raising was entrusted to an uncle. Alessandro Volta’s uncle and teachers attempted to persuade him to join the priesthood, however he resisted, and elected to become a physicist. At the age of 14, Volta finished grammar school and ended his formal education, instead undertaking study electrical phenomena that were sparking interest in scientific circles at the time.

Scientific Career

The beginning of Alessandro Volta’s is an interesting one. He began exchanging scientific correspondence with some of the major authorities on electricity at the time at the age of 18, without any university preparation. His first, officially published work was his 1769 dissertation, entitled “On the Forces of Attraction of Electric Fire”. He continues writing, and eventually becomes a Professor of Experimental Physics, at Como Grammar School. Finally, in 1778, he is appointed to the chair of Experimental Physics, at the University of Pavia, a position he would occupy for the next 25 years, where he would develop some of his most important scientific contributions.

The Voltaic Pile

Volta would not develop the voltaic pile, until 1800, when he was 55, and near the apex of his scientific career. The voltaic pile was born out of a heated debate between Volta and Luigi Galvani, who discovered that the muscles of dead frogs twitch when exposed to a spark. Galvani and his supporters termed this “animal electricity”, and believed that electricity came from the muscles, and was not separable from a biological organism. The debate began in 1792, when Volta argued that electricity was a physical phenomenon, and the frog legs simply acted as a detector.

Volta started his work by replacing the frog legs with brine-soaked paper, through which he was able to detect the flow of electricity using instruments he had developed earlier in his career. He would continue his experiments, ultimately discovering that electricity can be generated by separating two dissimilar metals by an electrolyte (a substance that contains free ions). After much trial and error, he discovered that the most effective pair of metals to produce electricity was zinc and silver.

Volta would go on to create the voltaic pile, or the first electric battery. He did so by alternating layers of brine-soaked cardboard sandwiched between copper and zinc electrodes, as pictured on the right. By attaching a wire to either end of the pile (the terminals), the battery created a steady source of current.

Volta’s invention would make him famous world-wide. He would demonstrate the invention to some of the most famous people of his day, including Napoleon Bonaparte. Volta would end up receiving many honours for his ground-breaking discovery, including being made a count in 1810 and a Professor of Philosophy at Padua University in 1815.

Personal Life and Legacy

Perhaps the most striking thing about Volta was his humble and unassuming personality. Indeed, the fact that he gradually accumulated his research, positions, and honours speaks of a man who was humble and methodical. Volta would not marry until he was nearly 49, but would end up having three children. By 1813, Volta had ceased his research work, because of attachment to family and increased political involvement. In 1819 he finally retired to his country home in Camnago, where he died on March 5th, 1827 at the age of 82.

In 1881, the unit of electric potential was named in Volta's honour, being designed the volt (V).

Interesting Links
Alessandro Volta Biography (many interesting pictures of his inventions)