Monthly Archives: November 2010

Holiday Engineering Project - Beginning an Ultrasonic Mapper

Over the Thanksgiving break I decided to tackle a small engineering project that has been on my mind for the past month or so.  The basic idea was inspired by learning about Bill Stone's underwater cave mapping system (Stone Aerospace).   It is a relatively complex system constructed by more funding that a college student has at his/her disposal.  Being a caver myself I decided to build a much simpler and cheaper version for both above and below ground mapping.

Since this project is not too large it should be easily constructed over a few weekend hacking sessions, and over the last day the first steps have taken place. The machine uses and Arduino Uno as an interface between a computer and the real world.  While it is true that it would be possible to make a self-contained unit that would store data on a flash drive and maybe even display initial results on some simple display, I opted out of that option for cost and control concerns.  It should be simple to make a simple GUI that allows a netbook to control the setup very nicely and process the data real time.  
Above the basic layout from my notebook is shown.  The ultrasonic ranger (an SRF05) is attached to a rotating mount to sweep through the space around it.  In the initial prototype stages I'm using a simple RC servo since I have it on hand.  Servos only sweep close to 180 degrees to avoid bending control links on models.  While continuous rotation servos are made, they have no position feedback which does not allow accurate positioning of the servo.  It would be possible to use multiple rangers so 180 degrees is enough rotation, but it will be more economical to switch to a slow geared motor with an encoder (maybe a quadrature rotary encoder?) to detect the position as the unit slowly sweeps the space around it.  
Also pictured is the Arduino board and the range finder mounted in a custom case built this afternoon (the window added allows the ping LED to be seen inside the case to confirm the link).  Modification of the PCB and mounting hardware was required.  CAT5e cable along with 4-pin radio mic connectors are used for the power, ground, echo, and trigger pins.  The ranger has a built in PIC controller to pulse the output with a length of the echo time.  The unit seems to range effectively up to about 17 feet.  
While I don't know the accuracy at large distances, or the spread of the 'beam' with the increasing path length, some simple tests should help sort that out.  Currently I've written the interface on the Arduino in C, which will listen for commands over the USB connection.  It this executes those commands (such as move servo, get distance, etc) and returns the appropriate values if necessary.  The communication is asynchronous and not really 'handshaking', but using startbits there is some error checking involved.  
The control from the computer is in the form of a python script that takes easy user input and converts it to the short serial commands in the home-made quick transfer format in the Arduino C code.  Currently it moves the servo and returns distances in cm.  At distances less than 30 cm results seem to be accurate to about 2-5mm.  

The next steps include mounting everything up to get 180 degree scans produced, assessing the accuracy and noise limits of the unit.  I also plan to implement a scanning algorithm that scans more quickly over plan surfaces and slows down over more interesting surfaces.  It will also throw out false returns, which could be due to a range folding issue in large spaces (I have not done anything past back of the envelope on if the signal is strong enough to cause a range folding issue).  A leveling mechanism (manual or automatic) would be nice for field applications.  If this were to be used in mapping a tunnel some kind of simple inertial navigation system with a combination of gyros and accelerometers could be used.  These are all for future development and future posts.  
Have a happy and safe Thanksgiving! 

New Nuclear? - The Traveling Wave Reactor

For a few decades the debate has raged: are you pro or anti-nuclear power?  While nuclear power is a fantastic source of energy it can be dangerous and produces waste that we are currently hard pressed to store.  Movies such as 1979's 'The China Syndrome' have aided public fear of nuclear power once the new rubbed off in the 1960's.  Since the Three Mile Island accident, there have been no new reactors built in the US.  What if there were a new kind of reactor that was safer, cleaner, and used our stockpile of waste as fuel?

Lately there has been some press on the traveling wave reactor (TWR).  A spinoff from a Bill Gates backed company called TerraPower is working with the design, but it's really nothing new.  The TWR has been studied since the late 1950's and there are several articles in the literature that describe the theory of the units.

The principle of operation is to use depleted uranium and spent fuel that would traditionally be stashed away in a repository to fuel the reaction.  The reaction starts with ~10% enriched uranium at one end of the fuel column.  As a chain reaction takes place in the critical zone it converts material downstream into fissile material which is then the new critical zone.  Basically the critical zone that is actually producing the power moves down the fuel column with time.  This means if we were to look at a movie of where the power is produced through time we would see a soliton (wave that travels maintaining its shape) pass through the fuel column over a period of ~60 years.

The advantages of such a reactor include using depleted stocks of uranium and not producing material that could proliferate into atomic weapons.  As with every process governed by the laws of physics there is no free lunch.  Currently the reactor is a 'paper reactor'; no prototype has ever been built, though many have been run as numerical simulations.  The unfortunate thing is reactors often don't live up to their numerical ideal models and that could push the TWR into the realm of being currently non-economical.  While the units are smaller and safer they do produce less power and would require a significant investment to start building in an industry that is currently set in its ways.

In reality, the reactors sound fantastic, but I don't plan on seeing them anytime soon as the money for research, even energy related is just not there.  To get a reactor from paper to common production takes decades and often includes unforeseen problems.  Full scale reactors of the fission sort built in the early days had many kinks to work out.  Some would startup and then mysteriously shutdown by themselves; it turns out this was xenon poisoning, something that was handled poorly in the reactor design.  Other reactors such as the SL-1 were thought to be safe, but engineering flaws caused an accident and ended up impaling a man to the roof of the containment building with a control rod.  It has taken decades to get the current reactor technology to where it is.  While remaining optimistic about the TWR design, it is sadly not ready to swoop in and help us in the energy crisis.

TerraPower Website

Rule #32 - Enjoy the Little Things (and why it's scientifically sound)

I'm going to deviate from my normal format of the physical sciences and take a short look into the human brain.  Anyone who has seen the recent comedy 'Zombieland' probably remembers the rules that the main character (Columbus) had.  If fact, Columbus had many rules including: cardio, double tap, beware of bathrooms, don't be a hero, etc that he used to survive Zombieland.

Today I came across an article about happiness and it made me recall the rule he learns from a redneck type called Tallahassee.  The rule (#32 in his book) is 'Enjoy the Little Things'.

The Psychology Today article (found here) examines the negative bias of our brains.  Scientists have shown time and again that negative events have more influence on us that positive events.  While this is likely a remnant of what helped us survive early on, it can sometimes get in the way of happiness today.  Just think about the last time one bad event ruined a whole day, week, month, or in extreme cases, ruined your year.

A group of researchers even examined married couples and found that those who were long-term and happy had a ratio of happy to bad events of about 5:1.  Couples that had less, especially much less, were more likely to get separated in the near future.  For years we've heard that marriage isn't 50:50, but who would have guessed it's approximately 83.3:16.7?

After the study, scientists have decided that we are better of to enjoy more small happy events than just a few great big happy events.  While doing something big like buying a new car may make you happy, it can be negated by just a few simple bad events.  If you had many small happy events in life, you would likely be a happier person.  To sum it up: enjoy the little things!