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Old 20th March 2014, 23:03   #16
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Default Re: About Non-Linear Speedometers

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Originally Posted by zenren View Post
Difference of 1-2km/h on a GPS based speedo has multiple reasons:
  • GPS always computes speed based on time and straight line distance travelled between two consecutive points that it measures. If the roads are slightly curvy, you actually travelled a higher distance in that time than your displacement indicates and hence the car speedo indicates slightly higher speed.
  • In most GPS devices, readings would have a slight time lag and hence what you see now would be your speed 1-2 seconds ago. You can check the speeds on the GPS speedo when you slow down - it'll take a second or so before starting to decrease.
Sorry, this is not entirely accurate.

Nearly all GPS units measure speed based on doppler phase shift effect. Even if it would measure time/distance (and some do) it would do that several times per second, so a curve road has no bearing on speed.

What is true is the fact that in order to get an accurate GPS speed reading you need to be traveling at a constant speed. So no acceleration or deceleration. At constant speed they are very accurate, much more accurate than your car's speedometer.

Whereas the absolute error in most car speedometer increases as the absolute speed increases the absolute error on the GPS remains (by and large) the same.

I have been thinking about these non linear speedometers. I have had a few cars that had those (VWs). I'm not yet convinced that they are more accurate. Because of the scale it appears as if you read the speed with great accuracy, but that doesn't mean that the actual reading itself is more accurate in that section of the scale than the "higher up" sections. Just because you have a more granular scale doesn't make the measurement itself more accurate, only the reading of the value.

It probably depends on how these non linear speedometers work. Is it driven by a motor, or is it in essence just a voltmeter? I.e. a coil in an electric field. The needle moves because of a different voltage applied.

Believe it or not, but I used to teach instrumentation, calibration and that sort of stuff nearly 30 years ago. It might come back to me. It probably doesn't really matter for the driver. For all intents and purposed you think you get a more of an accurate reading. As long as you don't break the speed limit nobody is the wiser as to what the real speed and or accuracy is. One of the reasons that typically car speedometer reads to high anyway all the time.


Truth be told, I never noticed that the speedometer was nonlinear until somebody pointed it out to me. So it appears, well at least on me, to work quite naturally.
Jeroen

Last edited by Jeroen : 20th March 2014 at 23:04.
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Old 21st March 2014, 20:54   #17
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Default Re: About Non-Linear Speedometers

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Originally Posted by Jeroen View Post
Nearly all GPS units measure speed based on doppler phase shift effect.
Correct. Which is also why the non realtime postprocessed speed which is essentially based on ds/dt is quite bad. That is until and unless you have raw data (like .sbn files for the SiRF3 chips) files. Most recreational GPSrs dont.

Quote:
Even if it would measure time/distance (and some do) it would do that several times per second, so a curve road has no bearing on speed.
The 5Hz and 10Hz chipsets are a very recent phenomenon. Old ones are 1Hz.


Quote:
It probably depends on how these non linear speedometers work. Is it driven by a motor, or is it in essence just a voltmeter? I.e. a coil in an electric field. The needle moves because of a different voltage applied.
Unless there has been a sea change in technology, it should be an air core (Sin Cos) movement, not steppers.

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Believe it or not, but I used to teach instrumentation, calibration and that sort of stuff nearly 30 years ago.
Any other pies which have your fingerprints we should no off?
(To know that GPSrs use the doppler shift speed (and Kalman filters) requires knowledge of another pie!)

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Sutripta
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Old 22nd March 2014, 07:43   #18
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Originally Posted by Sutripta View Post
Any other pies which have your fingerprints we should no off?
(To know that GPSrs use the doppler shift speed (and Kalman filters) requires knowledge of another pie!)

Regards
Sutripta

I just like to pick things apart. See how they work.
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Old 22nd March 2014, 19:10   #19
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Default Re: About Non-Linear Speedometers

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Originally Posted by sarathlal View Post
Does this mean that there is no relation between calculating speed and the wheels of the vehicle? Reason for asking is, on most cases related to changing the profiles or sizes of tires are discussed, one of the most common things listed as affected are, the speed indicated by the speedometer alters if oem tire specs are altered.
Yes. The speed indicated will change if tyre size is changed. For each revolution of the output shaft, distance travelled depends on tyre size. Bigger tyre size means more distance travelled per revolution of the output shaft.
The speedo is programmed to co-relate output shaft rotational speed (rpm) with vehicle speed and indicate it based on OEM tyre size. Increasing tyre size would mean vehicle will actually be tarvelling faster whiles the speedo thinks it to be slower and indicates lesser speed.
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Old 22nd March 2014, 21:31   #20
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Default Re: About Non-Linear Speedometers

^^^
You found out about the doppler shift, and the Kalman filter by taking apart a GPSr?

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Sutripta
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Old 23rd March 2014, 11:38   #21
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Originally Posted by Sutripta View Post
^^^

You found out about the doppler shift, and the Kalman filter by taking apart a GPSr?



Regards

Sutripta

Having lived in Kansas City near the Garmin HQ and knowing some folks there helps. I was also involved in the TomTom Live Traffic service. So I have had a few peeks under the hood so to speak. But in general I'm still fascinated by GPS and anything to do with navigation and charts. So I've done a bit more research than googling it and ending up on Wikipedia.
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Old 23rd March 2014, 20:32   #22
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Default Re: About Non-Linear Speedometers

^^^
And now that makes sense!

Regards
Sutripta

PS: Any observation on our (ie Indian) map reading abilities?

Last edited by Sutripta : 23rd March 2014 at 20:34.
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Old 24th March 2014, 09:18   #23
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Default Re: About Non-Linear Speedometers

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Originally Posted by Sutripta View Post
^^^
Any observation on our (ie Indian) map reading abilities?
We have maps in India??? Not so I have noticed!

I can honestly say that during the 18 months I've been in India I have never seen a map being used by anybody. I have not seen a map in any car I have used so far.

I have been on several weekend trips with my Royal Enfield, usually with 10-20 other bikers and nobody brought a map ever!

I have a Garmin Etrex 30, fitted with maps from Openstreetmap.

(http://garmin.openstreetmap.nl/)

Notice the domain??

So far these maps have done me fine here in and around Delhi up to about 350 kilomter radius. Accurate enough. So I plan my rides on my PC and then upload them in the Etrex.

There is also a special openfietsmap for bicycle in the Netherlands. Incredible level of detail. There is a little alley next to our home and that is for instance visible and you can plan a route through it.

One of my favourite sites is this one: http://gpstracks.nl/

All in Dutch I'm afraid. In essence a huge collection of tracks for cars, bikes, hiking, bicycling etc. When I'm in the Netherlands I often use these when I want to go for a hike or a ride on my bicycle. I've contributed myself with some 15 odd bicycle tracks as well.

Jeroen
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Old 24th March 2014, 20:17   #24
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Default Re: About Non-Linear Speedometers

OT OT OT
Quote:
Originally Posted by Jeroen View Post
We have maps in India??? Not so I have noticed!
Neatly, diplomatically sidestepped. (Necessary for self preservation from the torch and pitchfork carrying crowd.)

Quote:
There is also a special openfietsmap for bicycle in the Netherlands. Incredible level of detail. There is a little alley next to our home and that is for instance visible and you can plan a route through it.
To be expected, given the cloggies love affair with their (pedal) bikes. And the fact that both Navteq and Teleatlas have strong Dutch DNA.
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Sutripta
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