Speedometer motor

[Graham RounceParticipant @grahamrounce14492]

Hi all – those old simple reliable mechanical speedometers, with the aluminium cup and rotating magnet, have been replaced, at least in the BMW, with insanely complex electronic ones. They have dozens of parts, including a high speed processor fgs, and Gb of flash memory with god knows how many millions of transistors etc in them!

It’s an incredible waste of technology, imho, but the miniature pcb-mounted motor that drives the pointer (!! grrr) is cute.   There’s one in the enclosed pic in various stages of undress.  Would anyone like to explain exactly how it works? They’re about 20mm across, and have only four connections, two for each electromagnet. The black wheel is very lightly magnetised. I imagine the coils are powered alternately, but what stops the black wheel from just switching back and forth in the same position? And how does it reliably forward and reverse?

Interesting, albeit infuriating!

Thanks,

[duncan webster 1Participant @duncanwebster1]

It appears to be some kind of stepper motor

[OldironParticipant @oldiron]

All controlled by our little friend the computer I suppose.

[Graham RounceParticipant @grahamrounce14492]

Yes.

I’ve searched for an inet article on them, but people seem to believe the mechanical ones are still currently used…  🙁

[Graham RounceParticipant @grahamrounce14492]

PS It seems a shame not to use one of them for something sensible!

[John HaineParticipant @johnhaine32865]

<p style=”text-align: left;”>They are much cheaper than the mechanical type, easy to drive and would take a very small proportion of the available processor power.  Look on the Arduino support forum, for example, to see how they can be driven.</p>

[Graham RounceParticipant @grahamrounce14492]

I’m unconvinced! But anyway, if you’ve found it already, could you post a link? Thanks

[Graham Rounce]

On Graham Rounce Said:

I’m unconvinced! But anyway, if you’ve found it already, could you post a link? Thanks

Traditional cable driven speedos are terrible things; bulky, unreliable, inaccurate, slow, expensive and you have to route the cable from the gearbox through the car to the instrument. Driving them with an electric motor removes most, if not all, of these flaws.

[Michael GilliganParticipant @michaelgilligan61133]

See Post_4 here : https://forum.arduino.cc/t/driving-mechanical-car-speedometer-using-arduino-controlled-motor/1171960/4

… then follow your nose

MichaelG.

[duncan webster 1Participant @duncanwebster1]

You’ve already got the wheel sensor as part of the ABS brakes, so it makes a lot of sense to have an electronic speedo

[Graham RounceParticipant @grahamrounce14492]

Michael G : Thanks, but that link goes to “Driving mechanical car speedometer using Arduino controlled motor“.

I thought John Haine had a link to how to operate the micro-motor from an electronic speedo (say using an Arduino).

[duncan webster 1]

On duncan webster 1 Said:

You’ve already got the wheel sensor as part of the ABS brakes, so it makes a lot of sense to have an electronic speedo

There used to be lots of gearboxes that had the speedo sensor installed where the drive cable used to fit – Ford’s Type 9 as used in Granadas for example.

[John HaineParticipant @johnhaine32865]

I have certainly seen stuff on Arduino using these but I don’t have a link to hand.  Google is your friend.

[Graham Rounce]

On Graham Rounce Said:

Michael G : Thanks, but that link goes to “Driving mechanical car speedometer using Arduino controlled motor“.

I thought John Haine had a link to how to operate the micro-motor from an electronic speedo (say using an Arduino).

Did you actually look at Post_4 and then follow the PiHut link ???

MichaelG.

 

[SillyOldDufferModerator @sillyoldduffer]

If there was no advantage in electronic speedos, they wouldn’t exist.   Actually they provide many opportunities, starting with being cheaper to make, and more accurate and reliable than the electromechanical type.

But the first important advantage is the electronic type doesn’t need a long spinning bowden cable threaded between gearbox and  dashboard.   Admittedly not difficult to fit speedo cables to private cars, but long cables are a serious nuisance in large vehicles.

The advantages above mostly benefit manufacturers, but there are customer goodies too.   The electronic type measures speed, distance, calculate acceleration and understand the information.  As hard braking and fast acceleration on public roads both indicate a bad driver – boy-racer, or drunk – it’s helpful to highlight them on the display.   More.  The speedo can be integrated with the Engine Management Unit,  tyre pressure and anti-skid systems.   My speedo reminds me of service intervals.  Sad to say we live in a somewhat dishonest world, so it’s good they’re difficult to clock, or replace in order to mislead second-hand purchasers.

I know mechanical engineers find it hard to believe, but electronics are orders of magnitude more reliable than mechanisms.   Almost always best to ditch mechanical options if a job can be done electronically.

Dave

[NealebParticipant @nealeb]

…and quite a few cars are moving to “no moving parts” TFT displays. Curiously, the one in my own car has borrowed a feature that you often find in the stepper-motor speedo/rev counter units, that it winds the needle from 0 to max when you first switch on. What’s the point of doing that with a pixel-based display?

[John HaineParticipant @johnhaine32865]

Since all the car systems now are digital and software based it would be logical to just have a digital display of a speed number calculated in a computer.  It’s just these pesky humans that still like to see needles on dials so they needed some way to make them.

[Graham RounceParticipant @grahamrounce14492]

MichaelG: “Did you actually look…???” – Kindly don’t talk to me like that. I, and most people here, make the effort to be polite, fyi with no ill-effects.

Here, for other folks’ benefit, is the useful bit:
Automotive Gauge Stepper Motor (x27.168)

I’ll have a look, especially at the datasheet, to see if there’s enough info for me to be able to make it do something 🙂

[bernard towersParticipant @bernardtowers37738]

Didnt they try the digital speedo on a super astra and everyone moaned they couldn’t see the direction it was going and really big nos as well, a bit like DVMs and analogue meters!

[duncan webster 1Participant @duncanwebster1]

I can read an analogue gauge to a ‘near enough’ level of accuracy a lot faster than a digital one. Of course a screen can display an analogue lookalike

[Nigel Graham 2Participant @nigelgraham2]

I’d have thought it a lot simpler for the electronics simply to produce a voltage or current proportional to speed, then the display is just an analogue or digital V or A meter.

[Graham Rounce]

On Graham Rounce Said:

MichaelG: “Did you actually look…???” – Kindly don’t talk to me like that.

There was nothing impolite about it

… just a simple request for information

A full quote of my question would make that clear.

MichaelG.

[Graham RounceParticipant @grahamrounce14492]

@sillyoldduffer – Ok, I take what you say, but I still have a preference for things which (a) actually do normally work very reliably, and especially (b) that I can easily fix myself if they go wrong.

The panel from a BMW that I’m playing with has speedometer, rev counter, fuel gauge, temperature gauge, kitchen sink gauge (not really, though I wouldn’t put it past them). What would you do if the speedo bit developed a fault? Study how it works and fix it in an evening? Hahaha. No, you throw a computer’s worth of electronics and micro-mechanics in tbe bin and buy another whole panel. Call me old-fashioned if you like, but that’s so against the grain!

BTW, “My speedo reminds me of service intervals” Does it indeed. For me, that feature is in the same category as when you open a window and a super-annoying message pops up informing you of the effect on your mpg.

As an aside, I think all these so-called helpful features should be easily configurable by the owner. I suspect that many people would opt to switch off 90% of them.

[Michael GilliganParticipant @michaelgilligan61133]

If sellers on Ali can offer prices like this … I wonder what they cost to make ???

https://www.aliexpress.com/item/1005005466656068.html

MichaelG.

 

[duncan webster 1Participant @duncanwebster1]

Back when you could easily fix cars yourself you got 35 mpg on a good day and you had to service them every 6000 miles. I can still do an oil and filter change every 10000 and there are no points, and I get 55 mpg on a good day. Even in the 70s instruments we’re built into the plastic dashboard, so good luck fixing them. I’m now on my 4th car with a ECU, the previous 3 all did 150000 miles plus without changing the clutch. Compare that with my Vauxhall Viva which did well to get 20000 before I discovered the van had the same assembly except the friction material was a lot thicker. OK changing the clutch was easy, but I’ll take the hit not to have to spend the weekend on my back under the car. I could go on, SWMBO has a Hillman Imp when we met, I spent a lot of time under that, potentially a nice car, but not properly developed. I had a VW Beetle, OK it was reliable, but the handling was absolutely awful, and performance wasn’t startling. Heating system was prone to leaking CO into the car as well since it relied on blowing air over an exhaust heat exchanger.

I know a good supplier of rose tinted glasses

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