Robotic E-field Visioning Based on Leon Theremin's Design of the "Theremin"
Invented in 1918 by Soviet scientist Leon Theremin, the instrument that bears his name is generally regarded as the first electronic musical instrument.
"By using an alternating current of suitable frequency, tones of varying pitch are easily obtainable. A small vertical rod is used as the antenna. When the instrument is in operation, electro-magnetic waves of very weak energy are generated around this rod. These waves are of a definite length and frequency. The approach of a hand, which is an electrical conductor, alters the conditions in the electro-magnetic field surrounding the antenna, changes its capacity and thus affects the frequency of the alternating current generated by the apparatus. In this manner, a kind of invisible touch is produced in the space surrounding the antenna, and, as in a cello, a finger pressing on a string produces a higher pitch as it approaches the bridge, in this case also, the pitch increases as the finger is brought nearer the antenna." Leon Theremin |
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Vast amounts of information on the "Theremin" can be found by searching on the web...
Not only can the principles of the Theremin be used to control a musical instruments, they can also be used to control robots. Just as the hand controls the pitch and volume of the Theremin, a robot equipped with an array of Theremin sensors can detect and respond to objects around it.
http://www.maxiespages.com/Articles/Theremin_tech/Theremin_patent_application/ - Theremin's patent. Note that there are many sensor patents (new and very old...) based on Theremin's original patent. Many of them "re-claim" Theremin's work :-D
http://www.thereminworld.com/ - A great site with schematics and details of various Theremin instruments.
http://web.media.mit.edu/~jrs/ - An excellent site about E-field sensing.
Basic system:
As Theremin states, the principle of the Theremin can be used in many ways. The one we are interested in is where he uses many sensors to control various functions. Each sensor covers a different area of space and causes some specific response depending on which sensor is stimulated. The theory behind the Theremin Vision Sensor is very old (87 years). Our only task is to condense it into a reliable, sensitive, and small system that will integrate into small robots of say the "Sumo" class. To that end, the use of surface mount components and limiting the "parts" to the fewest and smallest needed is very important.
The version I have come up with is called "Theremin Vision" in honor of Leon Theremin. He seemed to like to have his invention named after him, so we will continue the tradition ;-) He also did all the work. I just provided a little modern engineering to repackage his system for a modern use.
Sensors:
The sensors are the heart of the system. Each one is based off the common CD4093 CMOS Schmitt triggered QUAD NAND gate. This IC preforms four functions. The first gate is used as the main sensing oscillator. Combined with the antenna, a 20pF capacitor, and variable resistor, it forms the main oscillator whose frequency varies ever so slightly when other objects come near the antenna. The second gate forms the reference oscillator with a fixed capacitor and a fixed resistor well shielded from the outside world. The third gate takes the two frequency signals from the main and reference oscillators and mixes them into a heterodyne signal. This signal is a mixture of the oscillators frequencies but it also contains the "difference" of the two signals. The advantage of heterodyning is that if two signals are say 1000000Hz and 1000010 Hz, there is produced a separate 10 Hz signal. It is normally difficult to detect the 10Hz difference when it is only 0.001% of the main signal, but the heterodyne process separates out the subtle difference into a nice "big" signal. In our case, the capacitance, and thus frequency, changes we are tying to detect are very tiny. However, by heterodyning the signals, they become very large!! The fourth gate is simply used as an enable to the sensor. Each sensor can be turned off to prevent interference with the others and to act like a "shield" to help direct the other sensors.
Processor:
The processor is controlled by a microcontroller. I use the Basic Stamp BS24p but practically any microcontroller would work. The microcontroller turns on and reads the information from each sensor. It also does some math to provide a nice output. There is some circuitry which provides power to the sensors and makes the connections. However, it also has a "low pass" filter which filters out the high frequency from the heterodyne signal and pulse shapes it leaving only the low frequency difference pulse signal we want. There is also a binary counter that divides the signal by 64/256/1024 counts to average and smooth it for greater stability. The basic stamp then measures the time of this pulse to which is proportional to the capacitance of the antenna and thus provides the crucial data we want.
System:
The prototype is simply constructed on a piece of pine board that holds four sensors and the processor. It is powered by a 9 volt battery. A computer serial cable can be plugged into it for programming or experimenting. There are 8 ports on the basic stamp that are left unused. In the simplest mode, a laptop computer is connected to the board which reads out the levels from the four sensors. One can move one's hand around the area of the sensors an experiment with different objects place near the sensors.
Fine Details:
For detailed information on each part of the system, the following links go into more theory of operation, schematics, PC board layouts, parts, etc.
Note: The "original" content of this site's ideas, text, pictures, schematics, layouts, software, theory, etc. are freely and completely released to the Public Domain and have no patent, copyright, or trademark.
Specifically - The "Thermin Vision" described here is a public design project that actively solicits ideas from anyone who wishes to help in the design. All details of the design have published in a number of public forums or can be obtained just by asking.
This project was funded by the research and development department of Team-Titanium and managed by Team-Titanium Combat Platform Management. Technical resources were provided under project "We are a Cyborg!".
Inquiries may be directed to:
platform.management@team-titanium.com