I am currently involved in a project that requires me to know the offset of one vehicle's centerline from that of another, leading vehicle. My solution to this problem is to try to employ something vaguely (very, evidently) to the Instrument Landing Sytem used by aircraft during their descent and landing phase.
My testbed would involve two (lightweight, short-range) AM transmitters operating on a carrier frequency of 1MHz, separated by approximately four feet or so. I would then modulate the signal of each transmitter to 90Hz and 150Hz (or similar frequencies), respectively. As the reciever deviates from the centerline bisecting the distance between the two transmitters, I'm hoping to be able to detect whether it's to the left or the right based on depth of modulation, or some similar effect.
I should probably mention that my specialty is in aerospace engineering, not electrical engineering, so I'm hoping someone can tell me if my solution is completely off base or perhaps give me a pointer or two.
I'm not worried about wiring the transmitters or modulating the signals, but I'm unsure as to whether my solution would work how I think it will. Is it possible for me to make a reciever (optimally less than one or two pounds) to detect this offset?
Make sure that the source of 1MHz carrier is the same for both transmitters, otherwise you will have a beat note problem between the two.
Then your receiver would just compare the amplitudes of the two modulation frequencies.
...Jim Thompson
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| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
That could be a challenge with metal cars. You'd really need plastic vehicles. From recent years I only know one but it was discontinued when communism was discontinued in former East Germany:
In the UK we had the Reliant Robin ("plastic pig") 3 wheeler but that ceased production some short while ago, as I understand it the French have introduced a plastic car which I am told is to replace the plastic pig in the UK market and can be driven by people with only a motorcycle license - the law must have been changed because it used to specify no more than 3 wheels and 425kg unladen.
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Why not just use sound, or ultrasound? Who needs the RF.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Naw, just use a CLASS IIA or lower laser. Search eyesafe erbium laser or eyesafe diode laser if you want to be expensive Keep driver exposure to 330 microwatts per cm^2 or less which is OSHA Standard or look up data for max power vs pulse rate for CDRH in US or whatever handles laser safety in Europe/Asia for class I exposure. All agencies around the world use pretty much the same exposure chart, and you should be able to do this at energy levels far below the eye safety limit.
place a IR led strobe on the lead car. Also on the lead car place a diode laser with a galvanometer scanner. General Scanning model G734 or G124 low cost open loop scanner for example, similar parts using feedback are made by Cambridge technologies, Galvoline, GSI-Lumonics etc, and cloned fairly well by the Chinese. On the chase car mount three detectors. one on center, one on left side of bumper, one on right side. IR strobe is different wavelength then laser, use dichroic filters to pass each wavelength, ie 1100 nm led, 750 or 808 nm laser, Strobe signals start of scan, send a ramp waveform to the galvo scanner, scan the beam outwards towards the other car. Measure time till center of laser beam passes over each detector using microcontroller. Expand the laser beam in its vertical axis using a cylinder lens to compensate for differences in road and bumper height.
overly complex, perhaps, but much easier then a RF solution. Since the laser beam will be 1-2 mm in diameter , accuracy is good. With work you'll have both offset and distance from car to car. Based on Omni and Consolan navigation techniques, its easy to compute using say 16 bit micro.
A modulation scheme measuring amplitudes is going to be a fairly complex tuning and adjustment job, maybe even nightmare. A laser-based scheme is even worse. An ultrasound scheme will be sensitive to everything in the area.
Just build a 2Mhz transmitter, divide a 1Mhz signal from it, put one out one side and the other out the other, and measure the phase (i.e., time between crossover) between the two signals. You use a single receiving antenna, a preselector filter for each frequency, and divide the 2Mhz copy by two. Put each into one port of a synchronous detector. (It's not strictly necessary to divide the 2Mhz signal, with some logic).
Simple, easy to build.
Stable, resistant to weather, dirt, dust, etc.
Simple one-time tuning to get the transmitted phases matched.
Easy calculation of angle based on easy-to-measure timing.
didnt the Luftwaffe have something similar when they started bombing london? ISTR the poms chief scientific advisor was a kiwi, who figured out what they were doing and organised to fool, rather than jam, the system - resulting in a great deal of countryside being bombed.
Hmm. Discriminating at most 3ns from 1000ns will make things somewhat touchy. Better to go with (Joerg's?) higher frequency, though Ghz would make things tougher, given phase ambiguities.
A 10Mhz system (3ns out of 100) would still be easy to build; a 30Mhz (3ns out of 33) wouldn't be hard. Discriminating would get easier up to 50'ish Mhz. Then you start getting into serious nonlinearity issues (the crest of the sine wave), unless you do it with fast digital logic. Then the limit would be 150Mhz. What kind of accuracy would you want at angles far from center? Is this for indication or for measurement?
I'm thrilled that so many ideas have been put forward! I suppose I should have mentioned that this design will be mounted on a small UAV, so weight is definitely a consideration at my end--I can't just pick up a box and throw it in the cargo hold!
I've modified my design somewhat from my original posting, it's as followed:
The transmitting craft has two transmitters operating at two different FM-band frequencies (say 88MHz and 100MHz). The wing of the aircraft has a directional, rearward-facing antenna on one wingtip transmitting one signal, with the other antenna on the other wingtip broadcasting the other signal.
The receiving aircraft has two forward-facing antennas in a mirror configuration. When the two signals are of equal strength, the aircraft knows it's lined up.
I have two main concerns of this design, however. For example, how will the transmitting craft know to which side it's drifting? Also, I realize I will need to take great care in designing a (light, small) Yagi antenna of some 1/2^n size to the wavelengths I'm operating on.
If this is intended for cars on public streets, no system that relies on resources on a different vehicle will do. Within a month, you would see law suits of the kind "My car hit that tree because your's told it so.." Remember that it's dangerous to sell hot coffee and Audi lost a case where the customer mistook the gas for the brake.
ILS is something different. It is calibrated officially in regular intervals and it's OK by default.
And, don't forget that your system must be immune to reflections. Imagine a bridge or tunnel made from steel and the leading car aproaches one side or the other. That could easily swap right and left as your receiver sees it.
I think that centering a leading car would be an easy add-on to the
90 GHz intelligent cruise control radars that start being deployed now.
Hi I dont think your design is going to work. There is no way to descern left from right with your current configuration, and theres no modulation trick (i can think of) that will get you a left or right. Im also worried about the distance your aircraft are following each other.... 4ft or 1.29m (for the frequency your talking about) The radiation characteristics of an antenna are much different in the near field, (less power transfered to load among other effects), vs. the far field. I dont know what the radiation pattern of the antenna is that close. but at the frequency your talking about, (for a wavelength of
3m) its usual practice to have (2*pi*r)/wavelength >>1, r being the distance between the leading aircraft and the one you want to control, to insure your in the far field, (frauhnhoffer) region of the radiator.... for your frequency(s) that would be r>>0.478m or 1.567ft. As well the paracitic elements, (director and reflector's) of yagi-uda have to be placed at significant fractions of a wavelength to work right. i still think your frequency is to low.
furthermore the radiation pattern of a yagi uda antenna is symetric about the axis of maximum power transmited (or recieved,) so you wont get a left or right signal with your current design.
in summary i think you need to go up in frequnecy, and drift farther away from the AM thing, and more twords relying on the radiation pattern of the antenna to determine your error signal. here's an idea...
plane 1 TX (at centerline of aircraft one, directional) ( with lobe maximum pointed directly back)
4ft or so ? RX RX on each wingtip, both RX antennas are directional (with no sidelobes) directed straight in front. you could then get a left or right error signal as follows.
RX (left) RX (right) are calibrated to match their recived signal strength's to the centerline of airplane one. then you should have decent dropoff in the recieved signal strength of say RX (left) while RX (right) goes up. then your following aircraft is drifting right. etc...
if you work at some big evil military industrial complex type place, you should be able to get some big evil RF/Antenna guy to help you... if no one there is evil enough to assist you in RF stuff where you work.....im looking for a job ;)
p.s. concerning radiators... you might want to take a look at a MPA array, patch antennas are probably really good for you as they are low profile (some are even conformable) so theres not the aerodynamics problems, light weight etc.. the directivity of one isnt good enough for the solution proposed above, so you will need an array. there are other microwave antennas you might consider as well.
There is no courteous way to tell you that you are hopelessly out of your league on this one. Stick to the aerospace aspect of the project, whatever that may be, and leave the radionavigation to an expert. The ILS may be old but it is highly sophisticated in concept and depth of antenna propagation application. The so-called "localizer" transmitter, which broadcasts the amplitude modulated VHF tones for determination of error off runway center in the horizontal plane, is a complex antenna array with resultant beam pattern such that the "depth of modulation" of the VHF tone is a precise function of angle off boresight, with 20% on center and varying by +/-7.55% at the specified fine angle error limits, with the two tones modulation depth varying in opposite sense so as to maintain a constant total depth of modulation over angle off boresight.
I realized from the outset that I'm hopelessly out of my league, but that isn't really going to stop me from finding a solution to my problem. Luckily for me, I work right above a group working in high-energy EM radiation, so I have a depth of knowledge to tap. Again, this is just a project for which I'm searching for a rough solution (a valid first step in any endeavor)--It's not something I'm expecting to sell to DARPA in two months or something like that.
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