Vehicle reversing sensors

[roy entwistleParticipant @royentwistle24699]

I still have a tennis ball on a string hanging in my garage. Stop when it touches rear screen. Iv'e always reversed into said garage.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Nigel Graham 2 on 20/07/2021 00:50:54:

Dave –

Interesting idea, an acoustic pyrometer. Unfortunately:

"The speed of sound depends on temperature…"

To a point. It actually depends on density. Whilst the density of a gas is affected by its temperature, it also depends on pressure, so the confining conditions and gas flow. I think the variability of everything in a miniature boiler would be nighmarishly difficult to analyse to the depth necessary.

I don't presume to deny it would not work but could be far trickier than it seems.

…

Yes, I'm you're right. The more it's thought about, the more difficult it gets. Pretty sure it would work aimed into a full-size firebox, and maybe even down a full-sized firetube, but needs a certain path length and not too many false echos, Tricky in a small boiler. If I can find a pair of ultrasonic detectors I might have a play. Can they detect a blowlamp; can one receiver detect the other's transmitter and over what range, etc etc.

My lock story: my mate lent me his 850cc mini whilst on holiday. Late for work I rushed out and opened it with a key of about the right size, which turned out to be a cheap briefcase key. Am I misremembering this car as being extremely primitive? I think the doors were opened by pulling a horizontal string strung across the door cavity: factory fitting, not a bodge. Great fun to drive, but terrifying drum brakes all round, and a few years later my mate had a minor bump and about 4 foot of welded seam popped open: completely rusted inside.

Reversing, last week, at the pumps it was my delight to see a caravan driver make a complete Horlicks reversing up to the air pump. It's on a curve… Is it just me, or are caravans the most irritating vehicles on the road?

Dave

[pgk pgkParticipant @pgkpgk17461]

No it's cyclists on country roads; two abreast gently plodding along looking at the scenery with no appreciation that with all the bends I'm stuck at 10mph for the next 4 miles. But it's their right!
Tractors know that i they have 5+ vehicles tailing then they pull over. Well local Welsh tractors do – over the border the English one's seem less accommodating

pgk

[modeng2000Participant @modeng2000]

I was in a queue following a tractor type vehicle at around 15mph. When we reached a roundabout the tractor went round and round 'till the queue had passed.

John

Edited By modeng2000 on 20/07/2021 15:21:08

[Nigel Graham 2Participant @nigelgraham2]

Roy –

A very useful tip, a soft ball on string. I have seen that tip elsewhere a long time ago though if I recall correctly that motorist drove forwards into his garage.

Dave –

I would expect an ultrasonic transducer to be reciprocal as you suggest, i.e. capable of both transmitting and receiving. They can be calibrated effectively by making three of them ping at each other in turn, and apply the transmitting and receiving electrical signal levels, and the transducers' electrical characteristics, to complicated simultaneous equations. (It was easy for me. The sums lived in a computer programme written by them as know such things.) The method, "reciprocity calibrating", is roughly the sonar equivalent of making three surface-plates from each other without recourse to a reference-plate.

However, even if using a pair at their same resonant frequency, you may need know their receive and transmit sensitivities with their associated amplifier gains, in both modes, for the readings they give to make sense. It may be possible to calculate the results from purely relative signal levels, but you'd still need an optical or thermocouple pyrometer to calibrate the acoustic system, especially as it might not be linear with temperature.

The sound pressure-level emitted from a small transducer is fairly low and falls by inverse-square distance law plus absorption that rises with frequency; so although it is valuable for the sort of contact transmission and reception Pgk describes, in medicine and NDT of metal components, I would not to like to guess its likely efficiency though a turbulent gas cloud.

The sound might travel easily along a fire-tube well though. A metal tubes can be an effective acoustic wave-guide provided you can put enough of the total emitted energy into the tube's entrance. (Think of the speaking-tubes on old ships.) A single-piece sensor may be nearly omnidirectional so much of the radiated sound would miss the tube. If it is directional though, both transducers would need careful aiming for beam and tube concentricity. You could test for this by putting one sensor on a rotary table at one end of the milling-machine table, and the other on a fixed mount at the other: somewhat similar rigs are used in acoustics laboratories.

Also, although the electronics is safely away from the fire, the sensors would not be very happy too close to a high temperature heat source, whether facing through an open firebox door or in the smokebox.

Testing with a blowlamp flame would be interesting. I've sneaking suspicion the flame boundary would reflect and scatter much of the applied pulse back, due to the sharp density difference from the surrounding air. (Incidentally, whales call over long distances not only because they shout loudly and at high power, but also thanks to a natural wave-guide formed by the sea surface and a density boundary at some depth below the animals.)

All food for thought anyway… (No, not the whales.)

Pgk Pgk –

You could well be right about bats using doppler effects; and that combined with interference modulation might be what distinguishes a flying insect from a fluttering leaf.

I've tried to guess what certain enviroments may "sound" like. One study I read was of a pipstrelle colony in Holland, using a canal as guidance between their roost and their forest hunting-land. Although these bats hunt at dusk so probably helped by sight, when flying along the line of a calm waterway the sound impinging on the water at an angle ahead of the bat might be reflected ahead so lost (the canal would appear "black", or "dark" bank vegetation such as rushes would probably give low-level, diffuse narrow-band noise; the hard ground each side give much more definite echoes.

Regarding your coming up behind tractors, I can say most I have followed have let traffic pass when possible.

The most irritating behaviour I find with cyclists is to play rabbits by trying to cycel as fast as they can, so are very hard to overtake safely, instead of pulling in to some refuge like a field gateway. Worst though was a big bunch, evidently a cycle-club, I once encountered on a rural road. Despite a half-mile of empty road ahead I could not pass, because they insisted on riding as a thrr-or four- abreast bunch taking the middle half of the road, even across the markings at a T-junction with a larger road where fortunately they and I were taking opposite directions.

I have no qualms about towed caravans as most I have seen, as a resident of a tourist-magnet county, are towed competently and at sensible speeds (I accept they have their own speed-limits). Some years ago the DoT studied traffic behaviour and found the largest number of slow vehicles causing long queues are not caravans but HGVs. There are far more of them, all year round and on all manner of roads.

The worst for discourtesy are a large proportion of drivers of horseboxes, both fixed-wheelbase and towed. I appreciate they have to be careful with a large, very heavy, unpredictably nervous animal forced to stand for a long time in a jolting wagon; (I feel sorry for the 'oss); but too many of the drivers seem to delight in ambling along at 40mph on all ordinary roads, and not pulling in where and when safe to let everyone else pass.

[SillyOldDufferModerator @sillyoldduffer]

Pleasant hour with an Arduino tells me more about the HC-SR04 rangefinder module:

The receive and transmit transducers appear to be identical.

The module is self-contained. Supplied with 5V, it's set off by a short pulse of about 2uS on the trig pin. This causes the module to emit 8 12uS pulses at about 41.7kHz (my module).

The unit averages the echo and replies with a pulse proportional to distance. Trigger pulse in yellow, reply blue:

The Arduino measures the length of the blue pulse, and calculates distance based on the speed of sound in air. Off the web, I used 334m/s which is correct for Standard Temperature & Pressure and 50% humidity, but not my dining room today. The distance is roughly correct; aimed at a target 270mm away, I got:

The variable results are probably due to my wobbly experimental lash-up: target is an address book leant against a fan-cooled oscilloscope while I rest my elbows on the not very stiff table and the sensor is within 20mm of the tabletop

Given that the module sends 8 pulses, and needs to be triggered, I don't think it would be easily confused by a second module, but it could happen. Can't try it because my spare module has gone awol. I remember putting it safely in a box, not which box.

Dave

Typos.

 

Edited By SillyOldDuffer on 20/07/2021 16:55:59

[duncan webster 1Participant @duncanwebster1]

Posted by SillyOldDuffer on 20/07/2021 11:31:21:

Posted by Nigel Graham 2 on 20/07/2021 00:50:54:

………..

Reversing, last week, at the pumps it was my delight to see a caravan driver make a complete Horlicks reversing up to the air pump. It's on a curve… Is it just me, or are caravans the most irritating vehicles on the road?

Dave

The weekly commute to my last paid job involved about 70 miles across single carriageway roads in North Wales. At least tractors are not usually going that far, but Shed Draggers holding queues of 20+ cars for mile after mile were not uncommon. Gaily driving past laybys and then pedal to the metal when they got to the occasional wide straight bit, no thought of letting anyone past. Tin Snails are nearly as bad. I've been told that in some states of the USA there is a legal requirement to pull over if you're holding up more than 6 cars

Edited By duncan webster on 20/07/2021 17:11:50

[MeunierParticipant @meunier]

S.O.D,
Your 850 mini recollection is correct, cable operated door-lock, rubber floor-mats and horizontal sliding windows.
In the 70's I did part-time work off-shift for a car-parking outfit at Heathrow and parked an Innocenti mini.
Proper door-cards not a big hole, normal door-release and wind-up windows plus better finished interior.
Make of it what you will, it impressed me at the time.
DaveD

[Author]

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