What is correct, Speedo or sat nav?

Lum

Not necessarily.

For example on my old Escort the speedo cable attached directly to the dash unit, where it rotated a little magnet. The speedo needle itself was attached to a little iron bowl that surrounded said magnet and a spring pushed the needle towards zero. The faster the magnet rotated, the harder the needle would push against the spring and the higher the reading it would give.

jase1

OK, on modern cars then

Lum

Fair enough, on modern cars they probably use a stepper motor or something that's fed from the speed pulse.

Even so the lag should be almost zero. Even the little digital unit I have soldered into the ECU signal updates pretty much instantly. It's certainly several orders of magnitude faster than any GPS I've compared it against.

I don't like modern cars anyway :P

jase1

Lum wrote: »

I don't like modern cars anyway :P

I agree that they've become a bit too complex for their own good.

I try to avoid cars with too many gadgets, but the problem is that I do a lot of miles so unfortunately it's sensible to have some newish bland Eurobox. Much as I'd like a Japanese luxobarge

Strider590

Hall sensor detecting pulses as the drive shaft rotates, or optical sensor doing the same........

Then a frequency to DC voltage converter feeds what is effectively an analogue voltage meter. Alternatively the input is fed to the ECU and the speed calculated in pulses per second and fed to a digital 7 segment display (like in the Yaris) or sent out to a digital to analogue converter (DAC) and fed to the above mentioned "analogue meter". You wouldn't use a stepper motor to drive a speedo needle, stepper motors are high torque devices.

The reaction speed would depend on how many pulses per revolution of the drive shaft and any smoothing capacitance used after conversion to DC voltage (frequency to voltage converters tend to create a lot of DC noise), too little smoothing and you'd see the needle flickering up/down, too much and it'd react very slowly to changes in speed.

I designed a vehicle instrument panel 10 years ago and did quite a lot of research into it.

epninety

Sorry Strider, but many modern speedos are stepper motor driven, mine included. Ford and GM at least use them in the mainstream, VDO in the commercial sector, Stack and SPA in the motorsport market, and that's just the ones I've seen.

birkee

Have we agreed yet, that the speed indictor belongs to the vehicle, and is not linked to something outside the vehicle? (satellites)

Gloomendoom

birkee wrote: »

Have we agreed yet, that the speed indictor belongs to the vehicle, and is not linked to something outside the vehicle? (satellites)

Nope. Although it seems common sense as a gps signal cannot be relied upon in all circumstances... tunnels, deep ravines etc.

Another problem is that without a vehicle linked speedo there is usually no odometer linked to the vehicle. Is that legally required?

adouglasmhor

birkee wrote: »

It's like trying to knit fog isn't it?

An eddy current speedometer IS directly connected to the vehicle. The fact that the indicator works from magnetic induction is neither here nor there. The cable is rotated according to the vehicles speed.
The electronic one also works from sensors that rotate according to the vehicles speed. The fact that there is no mechanical connection doesn't change the fact that the vehicles actual speed controls the information fed to the instrument. But it's not "linked" then

I know tunnels have been mentioned, I mentioned them myself, as being the reason why sat navs wouldn't meet requirements, because they can't indicate a vehicles speed at any given time, i.e. in difficult reception conditions, or upon satellite failure.
(Suns magnetic storms wiping them all out? Put on my tinfoil hat and sit it out, if it's that strong a magnetic storm your engine management system and electonic ignition are probably kaput tooMeteor shower must be some meteor shower? Enemy actionI won't be giving a flying one what speed I am doing and the last place i want to be is probably in my car?.... What do you all do? Park your vehicles up till they get new satellites?)

I have already said the bit about tunnels; I already said that was where using them as a speedo fell down. Because you are of limited intelligence you are like a dog with a new bone and can’t let go even though it’s been said already. I also mentioned later specifically built in sat nav systems with built in gyroscopic and pulse input.
The speedo cable is not connected to the cup and is rotated according to the engine’s rotation which is converted to a speed reading at the speedo, not linked for someone so sure of yourself you talk some nonsense. The magnet spins in an aluminium cup which pushes the speedo cable round they are not “connected” “linked” or whatever, if they were it wouldn’t work, it needs to be open.

cyclonebri1

birkee wrote: »

Have we agreed yet, that the speed indictor belongs to the vehicle, and is not linked to something outside the vehicle? (satellites)

No, absolutely no. The speedo works within the manufacturers wwiiddee tolerance band, accepted.. The satelite system is independant and based in the present, not in the technology of the past.
Only the cost of designing a rapid updating system holds it back from common useage today, as they say, watch this space;)