LTI 20/20 thwarted again.
Discussion
smeggy said:
Sure.
0.3 = radius (m) of the wheel (not the tyre. On reflection, this was a big overestimate; 0.2 would have been better)
x 2 = to get the diameter (width). From the LTI's POV, this is the greatest change of distance that it could detect for the spokes in this scenario
/ 0.39 = LTI measurement period, in seconds (although I did accidentally recalled the details for the Ultralyte because I was rushed. A TSM is 0.34)
x 2.237 = conversion from m/s to mph (3600/1609.3)
Ah ok - I dont think thats relevant - thats just the normal error in reading.0.3 = radius (m) of the wheel (not the tyre. On reflection, this was a big overestimate; 0.2 would have been better)
x 2 = to get the diameter (width). From the LTI's POV, this is the greatest change of distance that it could detect for the spokes in this scenario
/ 0.39 = LTI measurement period, in seconds (although I did accidentally recalled the details for the Ultralyte because I was rushed. A TSM is 0.34)
x 2.237 = conversion from m/s to mph (3600/1609.3)
My first calc that for a 50mph car the bottom of the wheel travels at 0mph (slippage excluded) and the top at 100mph.
If theres enough space to ping the large spokes now on wheels, its up to the filtering mechanism in the LTI whether or not it accepts it.
I'm interested in your tests on a large rotating wheel though
Mr2Mike said:
smeggy said:
The LTI2020 doesn't measure speed. The closest it gets is deducing the rate of change of distance
Rate of change of distance was always referred to as speed (or more correctly velocity) when I was at school..."The closest it gets is deducing the rate of change of distance to the surface the laser bounced back from over the individual samples"
This accounts for the strong possibility of different target surfaces having been struck by the laser, for each sample during a measurement.
The gun deducing a rate of change of distance by pinging different surfaces during a measurement doesn't mean the gun determined the speed of any of those surfaces.
This is how you can sweep the gun along a road surface and still get a speed reading - slip error. The road wasn't moving yet the gun gives a speed reading; well it couldn't have been measuring the speed (or velocity) of the road - right? (my personal hand-held best for that is over 100mph).
saaby93 said:
smeggy said:
Sure.
0.3 = radius (m) of the wheel (not the tyre. On reflection, this was a big overestimate; 0.2 would have been better)
x 2 = to get the diameter (width). From the LTI's POV, this is the greatest change of distance that it could detect for the spokes in this scenario
/ 0.39 = LTI measurement period, in seconds (although I did accidentally recalled the details for the Ultralyte because I was rushed. A TSM is 0.34)
x 2.237 = conversion from m/s to mph (3600/1609.3)
Ah ok - I dont think thats relevant - thats just the normal error in reading.0.3 = radius (m) of the wheel (not the tyre. On reflection, this was a big overestimate; 0.2 would have been better)
x 2 = to get the diameter (width). From the LTI's POV, this is the greatest change of distance that it could detect for the spokes in this scenario
/ 0.39 = LTI measurement period, in seconds (although I did accidentally recalled the details for the Ultralyte because I was rushed. A TSM is 0.34)
x 2.237 = conversion from m/s to mph (3600/1609.3)
My first calc that for a 50mph car the bottom of the wheel travels at 0mph (slippage excluded) and the top at 100mph.
If theres enough space to ping the large spokes now on wheels, its up to the filtering mechanism in the LTI whether or not it accepts it.
I'm interested in your tests on a large rotating wheel though
14-7 said:
I'd be more interested in peoples tests with an LTI rather than just stating mathmatics based on a guess.
If an LTI20.20 was receiving jittered returns that were +/- 0.3m or 0.2m from the mean of the expected return position they would be rejected anyway.If the laser was illuminating spokes in a rotating wheel many of the pulses of light are likely to be reflected away as well as towards the receiver so an incomplete set of data would result.
Because of the difficulty in getting a reliable return the wheel spokes are unlikely to allow acquisition so no measurement would result or take place.
The radar/wheel anomaly doesn't occur in a laser based system for many reasons so the argument is not worth consideration.
saaby93 said:
Ah ok - I dont think thats relevant - thats just the normal error in reading.
My first calc that for a 50mph car the bottom of the wheel travels at 0mph (slippage excluded) and the top at 100mph.
If theres enough space to ping the large spokes now on wheels, its up to the filtering mechanism in the LTI whether or not it accepts it.
I'm not sure what you are saying here.My first calc that for a 50mph car the bottom of the wheel travels at 0mph (slippage excluded) and the top at 100mph.
If theres enough space to ping the large spokes now on wheels, its up to the filtering mechanism in the LTI whether or not it accepts it.
To be clear: for your example (and applying my earlier workings), there is no way a lidar gun will deduce a speed of any greater than 53mph, or less than 47mph, when aimed at any part of a wheel of a vehicle doing 50mph - that includes any potential contribution from the slip effect.
Radar: possibly (but extremely unlikely given the wide beam spread and analysis of the returned spectrum)
Lidar: no, even with reliably detected reflections and no jitter.
Still not convinced
If you get a decent return off a curved reflective surface eg a large spoke - why wouldnt it give a speed reading?
http://mr2.com/TEXT/FAQonLidar.html#TargetName1
If you get a decent return off a curved reflective surface eg a large spoke - why wouldnt it give a speed reading?
http://mr2.com/TEXT/FAQonLidar.html#TargetName1
saaby93 said:
Still not convinced
If you get a decent return off a curved reflective surface eg a large spoke - why wouldnt it give a speed reading?
http://mr2.com/TEXT/FAQonLidar.html#TargetName1
Interesting link but not entirely accurate.If you get a decent return off a curved reflective surface eg a large spoke - why wouldnt it give a speed reading?
http://mr2.com/TEXT/FAQonLidar.html#TargetName1
You also need to consider this effect. Much simplified but you get the idea.
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Not all of your returns will be directed back to the receiver, and certainly any that are directed back from a rotating spoke will cause a variation in signal strength and position so will be rejected.
I think you would be better accepting that the wheel is simply a non-starter as a source of error in a laser speed meter.
grumpy geezer said:
Interesting link but not entirely accurate.
You also need to consider this effect. Much simplified but you get the idea.
yep You also need to consider this effect. Much simplified but you get the idea.
I'm not trying to say this would happen everytime otherwise the device would be useless, just trying to figure out what might happen sometimes.
If your spoke was curved so that as it moved it reflected the train of pulses back to the detector that might work.
If the wheel had more than one spoke, the next pulse could reflect back off the next spoke - although that would need more maths to work out spoke interval to see if it was the same as pulse interval.
Really though we could do with the other timed photos to do some coroboration
Edited by saaby93 on Thursday 5th May 12:54
saaby93 said:
Still not convinced
If you get a decent return off a curved reflective surface eg a large spoke - why wouldnt it give a speed reading?
http://mr2.com/TEXT/FAQonLidar.html#TargetName1
Please ignore GG's prior post - never accept anything because someone says so.If you get a decent return off a curved reflective surface eg a large spoke - why wouldnt it give a speed reading?
http://mr2.com/TEXT/FAQonLidar.html#TargetName1
Also, his fancy explanation doesn’t account for reflections from diffuse and dirty surfaces (top marks for his effort though).
For example: firing an LTI at the side of the car will still yield a seemingly valid speed/distance reading, even though you would think the light is being reflected away from the gun. This is fact and has been repeatedly proven.
GG's explanation is highly misleading and should be discarded.
I will endeavour to explain this to your satisfaction.
An LTI cannot get a (significant) speed from say a rotating surface, because the LTI does not measure speed; all it does is measures distances (at best) many times.
The following is a simplified (purely for ease of understanding) walkthrough:
We all know that speed is a rate of change of distance of an object.
The LTI determines speed based on the duration of flight times of many individual (time separated) laser pulses, transmitted from the LTI, reflecting off the target surface, back to the LTI.
For each of these flight time measurements it infers a distance to the target (d (m) = c (m/s) * t (s) / 2).
So for each speed measurement, there are a sequence of ~40 distance measurements (samples), all taken at known time intervals. Many samples are taken to help confirm that the final speed deduction is a valid one (partly by testing for stability).
The LTI will then (again simplified) determine that change of distance over the known time interval, to compute the speed reading.
If aiming at a quickly rotating tyre (the bike and gun are both static), each sample measurement will deduce an unchanging distance to the tyre for each sample because the distance from the gun to the tyre surface (where each pulse struck) will be the same each time, e.g. will always be at say 10m away. So the LTI will deduce a speed of 0mph (no change of distance), even though the tyre edge where the laser bounced off is spinning at +100mph. There is no measurement error.
The same principle applies to the spokes on a quickly rotating wheel of a bike, but with a slight addition.
For one sample it could deduce a distance of 10.0m, the next 10.1m, the next 9.9m, then 10.2m, then 9.8m, then 10.1m, then 9.8m, etc. The range of the distance measurements are bounded to within the 20cm radius of the rim i.e. they are limited to anything between 9.8 to 10.2m. For my numerical example, the gun will very likely conclude an instability of the overall speed deduction, so voiding the speed measurement and instead displaying E03; there must be a very smooth change of distance between each sample for the speed reading to be considered valid.
The worst possible case is where the pulses from the gun are in synchronisation with the rotation of the spokes, resulting with a smooth change of distance, from the near side of the radius to the far side. In that case, the distance measurements would change linearly from 9.8m to 10.2m, over the 1/3 second period when then samples are taken, thus resulting with an error of 2mph (maximum).
Does the beam from the laser stay focused in a tight point or does it diffuse to a larger contact patch the further it has to travel?
If the latter, then at this distance how big would the area be?
Would it be big enough so that it would be reflecting from an area significantly larger than a point on a spoke thus making the spoke argument irrelevant.
If the latter, then at this distance how big would the area be?
Would it be big enough so that it would be reflecting from an area significantly larger than a point on a spoke thus making the spoke argument irrelevant.
smeggy said:
The same principle applies to the spokes on a quickly rotating wheel of a bike, but with a slight addition.
For one sample it could deduce a distance of 10.0m, the next 10.1m, the next 9.9m, then 10.2m, then 9.8m, then 10.1m, then 9.8m, etc. The range of the distance measurements are bounded to within the 20cm radius of the rim i.e. they are limited to anything between 9.8 to 10.2m. For my numerical example, the gun will very likely conclude an instability of the overall speed deduction, so voiding the speed measurement and instead displaying E03; there must be a very smooth change of distance between each sample for the speed reading to be considered valid.
The worst possible case is where the pulses from the gun are in synchronisation with the rotation of the spokes, resulting with a smooth change of distance, from the near side of the radius to the far side. In that case, the distance measurements would change linearly from 9.8m to 10.2m, over the 1/3 second period when then samples are taken, thus resulting with an error of 2mph (maximum).
I think we're getting there For one sample it could deduce a distance of 10.0m, the next 10.1m, the next 9.9m, then 10.2m, then 9.8m, then 10.1m, then 9.8m, etc. The range of the distance measurements are bounded to within the 20cm radius of the rim i.e. they are limited to anything between 9.8 to 10.2m. For my numerical example, the gun will very likely conclude an instability of the overall speed deduction, so voiding the speed measurement and instead displaying E03; there must be a very smooth change of distance between each sample for the speed reading to be considered valid.
The worst possible case is where the pulses from the gun are in synchronisation with the rotation of the spokes, resulting with a smooth change of distance, from the near side of the radius to the far side. In that case, the distance measurements would change linearly from 9.8m to 10.2m, over the 1/3 second period when then samples are taken, thus resulting with an error of 2mph (maximum).
Your 20cm is only 8" I'm not familar with bike wheels but expected them to be at least twice that, however setting that aside.
Provided there were enough spokes at the right angle couldn't the pulses coming back increment the measured distance at a rate higher than the bike speed.
Edited by saaby93 on Thursday 5th May 13:53
BrianMillar said:
Does the beam from the laser stay focused in a tight point or does it diffuse to a larger contact patch the further it has to travel?
If the latter, then at this distance how big would the area be?
Would it be big enough so that it would be reflecting from an area significantly larger than a point on a spoke thus making the spoke argument irrelevant.
The laser is emitted conically. So yes, it does get wider with longer distances. However, this is further complicated by the possibility of the edge of the beam catching the wheel.If the latter, then at this distance how big would the area be?
Would it be big enough so that it would be reflecting from an area significantly larger than a point on a spoke thus making the spoke argument irrelevant.
saaby93 said:
ep
I'm not trying to say this would happen everytime otherwise the device would be useless, just trying to figure out what might happen sometimes.
If your spoke was curved so that as it moved it reflected the train of pulses back to the detector that might work.
If the wheel had more than one spoke, the next pulse could reflect back off the next spoke - although that would need more maths to work out spoke interval to see if it was the same as pulse interval.
Really though we could do with the other timed photos to do some coroboration
Without going into all of the details about this the answers to your queries are all well known and do not cause a strange or false reading.I'm not trying to say this would happen everytime otherwise the device would be useless, just trying to figure out what might happen sometimes.
If your spoke was curved so that as it moved it reflected the train of pulses back to the detector that might work.
If the wheel had more than one spoke, the next pulse could reflect back off the next spoke - although that would need more maths to work out spoke interval to see if it was the same as pulse interval.
Really though we could do with the other timed photos to do some coroboration
In summary:
The Radar finds it a problem because it is deriving speed from a Doppler shift and so the wheel effect is operative in that process. It doesn't cause a significant problem because the reflected signal back from more reflective areas of the vehicle swamp out the much lower incidental returns from a wheel. Think of it like talking in a Metallica concert next to the speaker stack. Do you hear what your friend is saying or do you hear Metallica and see your friends mouth moving but hear no words?
For the laser with a sympathetic rate of rotation and spoke angle. If the wheel had more than one spoke and each spoke in turn as it revolved reflected back the laser pulse at a similar angle (it would have to be almost the same or it wouldn't get there) then the laser would indicate how fast the wheel was moving away or towards it. When you think about it, the wheel, being attached to the car (hopefully) will be moving towards or away from the laser speed meter at the same speed as the car.
Neither cause a problem so no need to worry about it?
saaby93 said:
think we're getting there
Your 20cm is only 8" I'm not familar with bike wheels but expected them to be at least twice that, however setting that aside.
Provided there were enough spokes at the right angle the pulses coming back could increment the distance at a rate higher than the bike speed.
Yes, but the point is "by very little" i.e. the maximum effect being a lot less than the difference between a speed limit and its associated prosecution threshold.Your 20cm is only 8" I'm not familar with bike wheels but expected them to be at least twice that, however setting that aside.
Provided there were enough spokes at the right angle the pulses coming back could increment the distance at a rate higher than the bike speed.
Hence the effect, while possible, is not significant.
BrianMillar said:
Does the beam from the laser stay focused in a tight point or does it diffuse to a larger contact patch the further it has to travel?
If the latter, then at this distance how big would the area be?
Would it be big enough so that it would be reflecting from an area significantly larger than a point on a spoke thus making the spoke argument irrelevant.
YesIf the latter, then at this distance how big would the area be?
Would it be big enough so that it would be reflecting from an area significantly larger than a point on a spoke thus making the spoke argument irrelevant.
smeggy said:
Please ignore GG's prior post - never accept anything because someone says so.
Also, his fancy explanation doesn’t account for reflections from diffuse and dirty surfaces (top marks for his effort though).
For example: firing an LTI at the side of the car will still yield a seemingly valid speed/distance reading, even though you would think the light is being reflected away from the gun. This is fact and has been repeatedly proven.
GG's explanation is highly misleading and should be discarded....
We can all do complicated but I have accounted for the diffuse in my explanation and have made it simplified to suit the reader. No offence to saaby93 intended.Also, his fancy explanation doesn’t account for reflections from diffuse and dirty surfaces (top marks for his effort though).
For example: firing an LTI at the side of the car will still yield a seemingly valid speed/distance reading, even though you would think the light is being reflected away from the gun. This is fact and has been repeatedly proven.
GG's explanation is highly misleading and should be discarded....
The wheel is of no concern and the returns from it would be rejected as I said.....oh then so did you.
grumpy geezer said:
We can all do complicated but I have accounted for the diffuse in my explanation and have made it simplified to suit the reader.
Not quite.You made a grave assumption that "Not all of your returns will be directed back to the receiver," which is obviously false for a diffuse surface.
I think it fair to say that you are not in any position to say that the change of reflection coefficient with angle, from a diffuse surface, is great enough to be reliably detected by an LTI.
grumpy geezer said:
The wheel is of no concern and the returns from it would be rejected as I said.....oh then so did you.
Yes, but I gave the reasons, instead of your ‘just accept it’.
Very interesting as i posted last year when i joined about my experiance but then was unable to reply to people for 15 days as i was a new member. by then so pissed off didnt bother.
I was alleged doing 97mph. but knew I wasnt went to great lenghth requesting evidence etc. pleaded not guilty,(court admin person phoned advised i sub evi in advance to ensure admissable) I spent ages down loading stuff from internet familiarised myself with slip angle etc.
police would not supply everything I requested would not allow adjournment date I requested.
when I got in court was told my evi. was not admissable as i was not an expert The police officer was a goody two shoes who made statements I had no way of contesting. Result 6 points and £100 fine plus £265 costs. 1.ph. member messaged got what i deserved. Another member messaged sounds like i was on a fishing expedition although i had detailed why i was contesting and not exploring every excuse or loophole possible.
!
This is just a short discription as i cant be bothered typing all day.
The internet is awash with evidence of how these machines are fallable, but if forces invest about £2000 per machine plus expence of all other resources they dont want to be found out do they the thieving b
ds
I was alleged doing 97mph. but knew I wasnt went to great lenghth requesting evidence etc. pleaded not guilty,(court admin person phoned advised i sub evi in advance to ensure admissable) I spent ages down loading stuff from internet familiarised myself with slip angle etc.
police would not supply everything I requested would not allow adjournment date I requested.
when I got in court was told my evi. was not admissable as i was not an expert The police officer was a goody two shoes who made statements I had no way of contesting. Result 6 points and £100 fine plus £265 costs. 1.ph. member messaged got what i deserved. Another member messaged sounds like i was on a fishing expedition although i had detailed why i was contesting and not exploring every excuse or loophole possible.
!
This is just a short discription as i cant be bothered typing all day.
The internet is awash with evidence of how these machines are fallable, but if forces invest about £2000 per machine plus expence of all other resources they dont want to be found out do they the thieving b
dsI cant be arsed reading all the replies, but when i use the LTI (although, we use a handheld device, which gives no photgraph) we have to conduct checks before and after usage.
We need to check the scope is aligned and do a distance check, so we then know that the laser is firing at the correct target and at the correct distance.
Its also not mentioned in the course, but i always record that there's no fresh damage on the device, and the seals over the screws are still intact, in an attempt to prove nobody has pulled the thing to pieces and been messing.
Also need to check the display, so that all numbers are showing correctly, and that a 6 doesnt show as an 8 or whatever.
Assuming all this is done, then you're pretty bulletproof im my opinion.
Im always a bit wary of trying to target bikers unless the device is on a tripod, because i find the target too small to clearly hit when it's just being held.
We can also only use them at specified risk managed sites.
Whether any of this matters, or helps, i dont know
Edit: The LTI also really doesnt like it unless the laser is hitting the number plate.
I've hit radiators, windscreens and all sorts before and it really doesnt like it. Hit that nice shiny reflective number plate and bam, you get the result in a second.
We need to check the scope is aligned and do a distance check, so we then know that the laser is firing at the correct target and at the correct distance.
Its also not mentioned in the course, but i always record that there's no fresh damage on the device, and the seals over the screws are still intact, in an attempt to prove nobody has pulled the thing to pieces and been messing.
Also need to check the display, so that all numbers are showing correctly, and that a 6 doesnt show as an 8 or whatever.
Assuming all this is done, then you're pretty bulletproof im my opinion.
Im always a bit wary of trying to target bikers unless the device is on a tripod, because i find the target too small to clearly hit when it's just being held.
We can also only use them at specified risk managed sites.
Whether any of this matters, or helps, i dont know
Edit: The LTI also really doesnt like it unless the laser is hitting the number plate.
I've hit radiators, windscreens and all sorts before and it really doesnt like it. Hit that nice shiny reflective number plate and bam, you get the result in a second.
Edited by Acheron on Thursday 5th May 16:00
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