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This idea is relatively simple -- suppose that we add an digital encoder to each of an automobile's two rear wheels (or rather, to the half-axles, where they attach to the differential), and send the data from these two encoders to a CPU.
By counting how many times each wheel has turned, subtracting
one from the other, and performing a modulo operation, it's possible to measure the vehicle's direction relative to it's starting orientation. That is, we have the rotational analog of an odometer.
This can rotational information be used to assist navigation, so that if the vehicle has a magnetic compass, but travels somewhere that has little magnetic field (inside a steel tunnel, for example), the "mechanical compass"'s direction can be used instead.
Because it accumulates errors (just like an odometer) it can't be used by itself, and must be regularly corrected... however, under many circumstances, it will be at least as accurate as a magnetic compass.
Directional Gyro
http://en.wikipedia...ki/Directional_gyro
Does the same job - whether or not it's simpler is debatable. [DIYMatt, Jun 16 2010]
South Pointing Chariot
http://en.wikipedia...th_Pointing_Chariot
Similar [goldbb, Jun 16 2010]
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Wouldn't even the most minor difference in air pressure throw the thing off completely after just a short distance? |
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That would make it even more fun. Imagine the cross country competitions allowing only navigation by this device. |
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I had assumed that that's how my car did things when out of range of the magic space clock radios - such as when in a tunnel. Though it might just look at the speed and assume things from there... |
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If turns were banked, it'd underestimate how far the car had
turned, no? (Banking ruins everything, doesn't it?) |
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This is sort of related, aircraft have a directional gyro. The pilot sets the DG to the heading of the aircraft while on the ramp, and it will stay in its original position as long as the vacuum system is working. It is used in conjunction with the magnetic compass. |
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Better to use an electric or pneumatic gyro. |
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Traveller the SF role-playing game from the 70s, which was played with two dice, pen, paper and books, had inertial compasses. This is presumably what they were doing as your character drove around under-explored planets in their ATV. |
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This might work until you spun your wheels, skidded to a stop, picked up your car with a trailering dolley... |
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[RayfordSteele] Isn't it true that modern transmissions keep
track of which wheels are skidding, so they can deliver power
to the wheels that aren't? So, couldn't this device "believe"
only the odometry from wheels currently enjoying good
traction? |
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ldischler, a minor difference in air pressure in the tires will throw it off, but I'm fairly sure that it will take many miles to cause significant error. |
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saedi, AFAIK, your navigation system (assuming it's built into the car, and not an add-on) will assume distance traveled based on the car's speed or odometer, but generally (I think) assumes that the vehicle stays pointed in the same direction it was when it last knew for sure. |
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DIYMatt, a directional gyro will suffer similar errors to an odometer-based direction-ometer. From the DG article on Wikipedia that you cited: |
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//As such any configuration of the aircraft horizontal which does not match the local earth horizontal results in gimbal error (essentially leading to a variation in the predictable 'apparent' wander known in this instance as drift.)// ... //Because the earth rotates (ω, 15° per hour), and because of small accumulated errors caused by friction and imperfect balancing of the gyro, the HI will drift over time, and must be reset from the compass periodically.// |
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Thus, banked turns will produce errors in a DG, just as they do for an odometric compass, though for different reasons. Furthermore, a DG will drift due to the earth's rotation, even if the vehicle it's in stays still, while an odometer-compass won't. |
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RayfordSteele, if your car has a GPS, but no magnetic compass, and you drive along the street, stop, and then use a trailering dolly to turn the vehicle around in-place, what direction will the nav system think that the car is facing? |
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In other words, no matter what kind of system you use, it will inevitably need to be calibrated on a regular basis. |
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Besides, the most common situations where the device will be activated will be parking garages and tunnels -- how often do you spin your wills, skid to a stop, move the car onto a trailering dolly, or drive on a banked turn whilst underground? |
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//if your car has a GPS, but no magnetic compass, and you
drive along the street, stop, and then use a trailering dolly
to turn the vehicle around in-place, what direction will the
nav system think that the car is facing?// Depends. Am I
allowed to have two GPS receivers at opposite ends of the
car? And over what time interval am I allowed to average the
difference in their estimated locations? |
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All vehicle tyres operate at a slip rate all the time. Even when you're travelling dead straight at constant speed on the flat. The slip rate depends on a whole bunch of factors; vehicle geometry, surface macrotexture (bumpiness) and microtexture (roughness) etc. After just a few rotations, you will see significant errors.
The errors suffered by a directional gyro will be insignifcant by comparison. The accuracy required to define to location of an aircraft at the end of a 3000 mile flight is entirely different to that requied to define whether your car is in your street or the next. |
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Learn to read a map and compass. |
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it would be great set into the steering wheel. |
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When driving on dirt roads your wheels skid a little
quite often. |
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The transmission doesn't keep track. The ABS
sensors and pcm do that, with some input from the
chassis control mod. |
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The simple crowning of a road is going to throw it off
pretty fast. |
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even on a perfect surface differential action isn't perfect. it wouldn't work. |
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Deciding whether to performing jumps, handbrake-turns, and 4-wheel powerslides would have to be weighed up against the possibility of knocking the device out of callibration - mind you, if it did go off, you might decide to perform another manouvre to get it back in line. |
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a) the chinese did it, as you pointed out yourself. |
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b) //we add an digital encoder // and // Mechanical Car Compass // don't really seat well together. |
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c) mechanical range calculation schemes on youtube (8 part series I think) |
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Accelerometers and a CPU would make for a more
accurate Initial Navigation System. Probably cheaper,
too. |
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//a minor difference in air pressure in the tires
will throw it off, but I'm fairly sure that it will take
many miles to cause significant error.// |
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Really? A little elementary maths perhaps? |
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Assume that one wheel has a circumference of
180cm, and the other has a circumference of
180.1cm (a tiny difference - 1 millimetre). |
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After driving ~3km (~1500 wheel revolutions), the
smaller wheel will have rotated once more than
the larger wheel. If the wheels are 2m apart, this
will give an error of around 45 degrees. |
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Realistically, that difference would easily be measured, and compensated for, minute-by-minute when the regular compass or GPS was available. |
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Otherwise, I thinks its a lousy idea, though. |
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Since the rotation is only in one plane, a basic fibre optic
gyroscope that reorientates whenever an external r3ference is
available would be the best solution. |
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you're not lapsing into leet, are you? |
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Who has been bunning this? |
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Seem to be a good idea for a magnetic compass complement, but I can't see when is really needed. You can't loose driving in a tunnel; I can't see a situation where a magnetic compass it's not enough.
Other examples could help to understand the need. |
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