Author Topic: Modifying mechanical speedometer to electronical.  (Read 2711 times)

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Offline Refrigerator

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Modifying mechanical speedometer to electronical.
« on: December 28, 2020, 05:50:22 pm »
I'm restoring restomodding a 1972 Volga 24 currently and i need to modify the original mechanical speedo to be controlled electronically.
The original engine and gearbox are shagged so i'm swapping a SAAB B204 with a different gearbox, which is why the speedo needs to be modified.
Also i want to preserve the original look, which is why i'm not going to put a screen in place of the speedo.
It's a strip speedometer, which presents it's own challenges.

I'm thinking there's two options right now.
First one is an external speed controlled motor, which would spin the cable input in the speedometer. I think this would be the least intrusive method but it might be a bit noisy depending on how it's done. Also i would be relying on the original mechanism of the speedometer to work and be accurate.

Second would be to tear the speedo apart and connect a stepper motor directly to the mechanism that moves the needle but this would be most intrusive. Of course this way the speedometer needle would be directly coupled to the stepper so i think reliability and accuracy would be better.

Also there are other gauges on the speedometer cluster, such as oil pressure, battery charge/discharge and fuel level, which complicates things, because none of the original electrical system will be left in the car and it would be nice to have these gauges work.
All of these are regular gauges and can be driven directly with PWM from a microcontroller but then i would need to interface remote sensors and i start diving down a bit of a rabbit hole full of CAN and diff pairs.

Would be nice to have some input before i blow this project too far out of proportion with feature creep.  ^-^
« Last Edit: December 28, 2020, 06:19:43 pm by Refrigerator »
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Offline pardo-bsso

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Re: Modifying mechanical speedometer to electronical.
« Reply #1 on: December 28, 2020, 06:27:17 pm »
That's a nice project.

Do you know how different are the ratios of the differential and the new gearbox to the speedometer input? If they are close enough you might be able to use it as it is and it will still be within the expected tolerance/error of the meter and law regulations.

As for the rest of the oil and water temp gauges, can you use the old sensors (or new but compatible) in the new engine and wire them directly to the cluster? If the threads are different it's a short job with plumbing parts and perhaps a bit with a lathe. Same with fuel level.
 

Offline Refrigerator

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Re: Modifying mechanical speedometer to electronical.
« Reply #2 on: December 28, 2020, 06:58:31 pm »
That's a nice project.

Do you know how different are the ratios of the differential and the new gearbox to the speedometer input? If they are close enough you might be able to use it as it is and it will still be within the expected tolerance/error of the meter and law regulations.

As for the rest of the oil and water temp gauges, can you use the old sensors (or new but compatible) in the new engine and wire them directly to the cluster? If the threads are different it's a short job with plumbing parts and perhaps a bit with a lathe. Same with fuel level.

The new gearbox does not have a cable output at all actually.
I don't think the new oil pressure sensor will be compatible with the old gauges. This is a bit of a tricky situation because even if the gauge works with some new sensor there's no guarantee that it will be accurate.

I will have an OBDII port hidden to interface to the ECU so maybe i could add some CAN interface to the gauge cluster to read oil pressure etc.
But then again i might go for a standalone ECU because the original Trionic5 might throw a fit when i try to run it standalone. Anyways that's beyond the scope of this project.
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Offline Zero999

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Re: Modifying mechanical speedometer to electronical.
« Reply #3 on: December 28, 2020, 07:19:04 pm »
The external motor spinning the cable is probably the better solution. Use a DC motor, with the appropriate gearing, if necessary. Assuming the load doesn't vary much, the speed is roughly proportional to the voltage. You could power the motor off a regulated power supply and vary the speed with PWM.
 

Offline Refrigerator

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Re: Modifying mechanical speedometer to electronical.
« Reply #4 on: December 28, 2020, 07:50:49 pm »
The external motor spinning the cable is probably the better solution. Use a DC motor, with the appropriate gearing, if necessary. Assuming the load doesn't vary much, the speed is roughly proportional to the voltage. You could power the motor off a regulated power supply and vary the speed with PWM.

If i did that i would most definitely add an optical speed sensor for closed loop control because i don't trust the DC motor speed to be stable especially in an automotive environment (temperature, vibration etc.). Also i think the cable rotates at a pretty low RPM so if i needed a gearbox for the motor that would also add more noise.
Cars are inherently very noisy but i find noise made by DC motors inside a car to be very noticeable and very annoying. Not to mention the Volga 24 is a massive rattle can, so if any vibration from the drive motor got coupled to the stamped steel panels it would amplify.
Steppers sound pretty cool when they make "cnc" noises so i kinda lean the more intrusive way of modifying the speedo. Also i do have a reasonably small 1.8 deg/step stepper, which is pretty much silent with micro stepping.
And let's not forget that this speedo was made in 1972 and it probably needs a good servicing by now, not to mention the bug that found it's way inside and died on the front glass  >:(
I haven't yet looked inside the speedometer mechanism so i can't say whether it will be easy or hard to modify it but i'll look inside, because right now it seems like the biggest factor is my preference, rather than the feasibility of such mod (second option).
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Offline Benta

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Re: Modifying mechanical speedometer to electronical.
« Reply #5 on: December 28, 2020, 07:54:08 pm »
Staying with the speedometer:

As you don't have an output from the gear box, my suggestion would be an ABS ring on the drive shaft with associated sensor. Rugged and proven over decades. Now at least you have a speed signal :)
Square it up (analog amplication/limiting) and you have a nice frequency proportional to your speed.

For the speedometer itself:
Forget the stepper motor, the sound will drive you nuts (OK, I know a Volga isn't a Roll-Royce, but still)
I like your idea of driving the old mechanism with a controlled DC motor. BLDC would be preferable for noise reasons.

Now you just need to translate frequency (ABS sensor) to voltage and BLDC control. A job for an Arduino or whatever.

Sounds fun.

Let us know how you proceed.

 

Offline Refrigerator

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Re: Modifying mechanical speedometer to electronical.
« Reply #6 on: December 28, 2020, 07:57:47 pm »
I think it would be possible to also make an external drive with a stepper motor but i don't know it will be able to spin fast enough.

And now that i think of it i would also need limit switches for the speedo needle if i went to modify it with the direct drive to avoid crashing the speedo needle and subsequently ruining the entire assembly.
So yeah the external drive seems like the better choice currently.
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Offline Benta

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Re: Modifying mechanical speedometer to electronical.
« Reply #7 on: December 28, 2020, 08:00:38 pm »
If i did that i would most definitely add an optical speed sensor for closed loop control because i don't trust the DC motor speed to be stable especially in an automotive environment (temperature, vibration etc.). Also i think the cable rotates at a pretty low RPM so if i needed a gearbox for the motor that would also add more noise.

Nothing forces you to place the speedo motor next to the meter. It could also be in the engine compartment using the original (shortened) speedo cable.
 

Offline Refrigerator

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Re: Modifying mechanical speedometer to electronical.
« Reply #8 on: December 28, 2020, 08:04:57 pm »
Staying with the speedometer:

As you don't have an output from the gear box, my suggestion would be an ABS ring on the drive shaft with associated sensor. Rugged and proven over decades. Now at least you have a speed signal :)
Square it up (analog amplication/limiting) and you have a nice frequency proportional to your speed.

For the speedometer itself:
Forget the stepper motor, the sound will drive you nuts (OK, I know a Volga isn't a Roll-Royce, but still)
I like your idea of driving the old mechanism with a controlled DC motor. BLDC would be preferable for noise reasons.

Now you just need to translate frequency (ABS sensor) to voltage and BLDC control. A job for an Arduino or whatever.

Sounds fun.

Let us know how you proceed.

I think it would be best to add a ring to the driveshaft instead since it would be simpler.

I will have to experiment with steppers and DC motors to see which is quieter but first i will need to find out what RPM the speedo needs to work and, most importantly, if it works at all.

The external drive comes with the benefit that it does not devalue the speedometer assembly. I say this because the Volga 24 is a rare car here and a strip speedometer is even more rare so i would prefer to not ruin it.
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Offline Refrigerator

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Re: Modifying mechanical speedometer to electronical.
« Reply #9 on: December 28, 2020, 08:11:47 pm »
If i did that i would most definitely add an optical speed sensor for closed loop control because i don't trust the DC motor speed to be stable especially in an automotive environment (temperature, vibration etc.). Also i think the cable rotates at a pretty low RPM so if i needed a gearbox for the motor that would also add more noise.

Nothing forces you to place the speedo motor next to the meter. It could also be in the engine compartment using the original (shortened) speedo cable.

There won't be much space in that area in the engine bay because of the brake booster, which was originally not meant to be there.
Previously the brake cylinder would actuate an external brake booster cylinder, which was in the opposite side of the engine bay, which would then boost the brakes but this meant there were two hydraulic systems that were somehow meant to share one reservoir. Also the entire system was broken so i threw it away in favor of something more modern.
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Offline Refrigerator

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Re: Modifying mechanical speedometer to electronical.
« Reply #10 on: December 28, 2020, 09:06:43 pm »
There was a back door in the speedo assembly so i scraped off the white "warranty" paint off the screws took a look inside.
The entire magnet/speed cup (yep that's what it's called) assembly was attached to this cover and it did not come out very gracefully so i don't think i was meant to pull it out from the back.
Anyways it looks like it would be very easy to modify this assembly for direct drive but i noticed another problem and that is that the odometer would need a separate stepper, which adds complexity.
So the odds are against direct drive.
I can go from external drive to direct drive but not the other way around, so i will first try making an external motor to drive the cable input first.
Also the speedo works and it does require quite a low RPM to work and there's also noticeable resistance when spinning the input.
To make the strip go up the input is spun counter clockwise when looking at the back of the speedo assembly.
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Offline pardo-bsso

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Re: Modifying mechanical speedometer to electronical.
« Reply #11 on: December 28, 2020, 09:42:52 pm »
Well, mine is from 1981 but works fine. The odometer even rolled over at least once.

Perhaps I explained myself wrong but can't you make/buy an adapter if needed and use the old sensors for water and oil in the new engine?
 

Offline Refrigerator

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Re: Modifying mechanical speedometer to electronical.
« Reply #12 on: December 28, 2020, 09:59:12 pm »
Well, mine is from 1981 but works fine. The odometer even rolled over at least once.

Perhaps I explained myself wrong but can't you make/buy an adapter if needed and use the old sensors for water and oil in the new engine?

Of course you can, the threads are standard. When i was still trying to start the engine i even replaced the original oil pressure sensor with a regular pressure gauge i bought at the hardware store and it threaded right in.
But the problem is that the original pressure sensors i have don't work anymore and i don't think finding a new one will be easy. Also i don't trust old sensors in general.
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Re: Modifying mechanical speedometer to electronical.
« Reply #13 on: December 28, 2020, 10:16:33 pm »
I gave the speedo a spin and my drill in reverse can only get me to about ~150kph. The strip is a bit bouncy but if i went from 0 to 150 kph in less than a second i would probably have other things to worry about. I don't have a way to measure the rpm yet but i think the top range on the speedo is no more than 2k RPM.
I think a regular (360-ish sized) DC motor will have problems spinning the input especially at low RPM. I do have a couple 540(?, they're big) sized low RPM DC motors from some printers that i could maybe use but this entire thing is getting a little sketchy, bulky and inefficient.
A gearbox would help but it would add noise. I do have a belt assembly from a printer with a short belt and matching toothed pulleys but the input is not meant to take radial load so i can't hook it up directly so i would need to make an external assembly, which is pretty messy as well.

I'm starting to lean more towards direct drive again.
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Online langwadt

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Re: Modifying mechanical speedometer to electronical.
« Reply #14 on: December 28, 2020, 10:24:47 pm »
I gave the speedo a spin and my drill in reverse can only get me to about ~150kph. The strip is a bit bouncy but if i went from 0 to 150 kph in less than a second i would probably have other things to worry about. I don't have a way to measure the rpm yet but i think the top range on the speedo is no more than 2k RPM.
I think a regular (360-ish sized) DC motor will have problems spinning the input especially at low RPM. I do have a couple 540(?, they're big) sized low RPM DC motors from some printers that i could maybe use but this entire thing is getting a little sketchy, bulky and inefficient.
A gearbox would help but it would add noise. I do have a belt assembly from a printer with a short belt and matching toothed pulleys but the input is not meant to take radial load so i can't hook it up directly so i would need to make an external assembly, which is pretty messy as well.

I'm starting to lean more towards direct drive again.

is noise really an issue at speed in such an old car?


 

Offline Zero999

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Re: Modifying mechanical speedometer to electronical.
« Reply #15 on: December 28, 2020, 10:32:26 pm »
How long is the speedo cable? Can you put the motor outside the cabin, in the engine bay?
 

Offline Refrigerator

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Re: Modifying mechanical speedometer to electronical.
« Reply #16 on: December 28, 2020, 10:40:56 pm »
I gave the speedo a spin and my drill in reverse can only get me to about ~150kph. The strip is a bit bouncy but if i went from 0 to 150 kph in less than a second i would probably have other things to worry about. I don't have a way to measure the rpm yet but i think the top range on the speedo is no more than 2k RPM.
I think a regular (360-ish sized) DC motor will have problems spinning the input especially at low RPM. I do have a couple 540(?, they're big) sized low RPM DC motors from some printers that i could maybe use but this entire thing is getting a little sketchy, bulky and inefficient.
A gearbox would help but it would add noise. I do have a belt assembly from a printer with a short belt and matching toothed pulleys but the input is not meant to take radial load so i can't hook it up directly so i would need to make an external assembly, which is pretty messy as well.

I'm starting to lean more towards direct drive again.

is noise really an issue at speed in such an old car?

I think yes, especially because this car counts as a "classic car" and i want to keep it as classy as i can with the lowest amount of noise possible. Just because it's already noisy doesn't mean that i can just pile more noise on top.
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Offline Refrigerator

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Re: Modifying mechanical speedometer to electronical.
« Reply #17 on: December 28, 2020, 10:45:26 pm »
How long is the speedo cable? Can you put the motor outside the cabin, in the engine bay?

I would have to find it to measure since it's been tucked away into storage somewhere in the attic. The exhaust of the engine will be on the driver side so i try to keep things there scarce to avoid getting cooked.
« Last Edit: December 28, 2020, 11:03:22 pm by Refrigerator »
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Offline Refrigerator

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Re: Modifying mechanical speedometer to electronical.
« Reply #18 on: December 28, 2020, 11:13:34 pm »
I dug around in my stepper box and found some small pancake steppers with 3.75 deg/step angle.
Here's a datasheet for one: https://pdf1.alldatasheet.com/datasheet-pdf/view/176408/MITSUMI/M35SP-11NK.html
I could easily fit one of these inside the speedometer enclosure to drive the strip pulley directly. Not sure if 3.75deg/step is good enough because these stamped steel steppers don't like microstepping.

Edit: assuming 40mm roller diameter for the strip (i forgot to measure), a 3.75deg/step would get me about 1.3mm strip movement per step, which is not great. With 1/4 microstepping i could get these numbers down to 0.33mm, which i think is good enough but these steppers don't like microstepping, at least the 7.5deg/step ones don't.
« Last Edit: December 28, 2020, 11:18:44 pm by Refrigerator »
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Offline Refrigerator

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Re: Modifying mechanical speedometer to electronical.
« Reply #19 on: December 29, 2020, 04:54:45 pm »
Quite fortunately all of the gauges are their own individual modules and can be removed from the back side.
I have exams coming up so i'll put this project on the back burner for a week, after that i'll start experimenting some more with the motors.
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Offline Zero999

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Re: Modifying mechanical speedometer to electronical.
« Reply #20 on: December 29, 2020, 05:41:29 pm »
How long is the speedo cable? Can you put the motor outside the cabin, in the engine bay?

I would have to find it to measure since it's been tucked away into storage somewhere in the attic. The exhaust of the engine will be on the driver side so i try to keep things there scarce to avoid getting cooked.
I think it will be fine, as long as it's not too near anything hot. The starter motor, alternator, battery etc. don't cook and in this case the motor will have minimal loading. Another thing you could consider is, putting the motor and gearbox in an enclosure lined with polystyrene foam, or cotton wool for sound insulation.
 

Offline Refrigerator

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Re: Modifying mechanical speedometer to electronical.
« Reply #21 on: December 29, 2020, 06:38:39 pm »
How long is the speedo cable? Can you put the motor outside the cabin, in the engine bay?

I would have to find it to measure since it's been tucked away into storage somewhere in the attic. The exhaust of the engine will be on the driver side so i try to keep things there scarce to avoid getting cooked.
I think it will be fine, as long as it's not too near anything hot. The starter motor, alternator, battery etc. don't cook and in this case the motor will have minimal loading. Another thing you could consider is, putting the motor and gearbox in an enclosure lined with polystyrene foam, or cotton wool for sound insulation.

On the SAAB B204 the starter and alternator are both on the intake side so they don't get any heat actually. The problem with the exhaust manifold is that it radiates heat, it's not just hot air.
Oh and it's a turbocharged engine (forgot to mention) so there's only a limited amount of space on the driver side.
But anyways, the motor assembly would need a custom enclosure because i wouldn't expect a printer belt (or a GT2 belt) to hold up very long in the elements, especially in the engine bay.
I would need to make a custom enclosure and common materials like acryllic and 3D printed plastics (PLA, ABS) might deform under the heat.
Also wrapping the assembly in anything would be counter productive because even if it kept the heat from getting in there would be no way for the heat to get out.
Additionally this would only delay the heat-soak of the module, not prevent it.

I'm slowly leaning more and more towards the direct drive approach, even though it is very intrusive and likely non-reversible.
I would have to weigh the pros and cons of both approaches.
Also the will be very different both ways, because the external drive will be running constantly when i'm driving and the direct drive will only be stepping when the speed changes.
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Offline minifloat

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Re: Modifying mechanical speedometer to electronical.
« Reply #22 on: January 01, 2021, 07:07:14 pm »
For the speedometer, would it be possible to attach a rugged RC car servo instead of the eddy current bell?
For the odometer, a electromechanical counter would save the look'n'feel.
Both could be controlled by a microcontroller based circuit, which could also convert signals.
What do you think?

Happy new year,
br, mf
 

Offline Refrigerator

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Re: Modifying mechanical speedometer to electronical.
« Reply #23 on: January 01, 2021, 07:36:58 pm »
For the speedometer, would it be possible to attach a rugged RC car servo instead of the eddy current bell?
For the odometer, a electromechanical counter would save the look'n'feel.
Both could be controlled by a microcontroller based circuit, which could also convert signals.
What do you think?

Happy new year,
br, mf

Servos are horribly inaccurate, especially the cheap ones, also the bell rotates more than 360 degrees through the entire speedo range so a servo wouldn't work.
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Offline minifloat

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Re: Modifying mechanical speedometer to electronical.
« Reply #24 on: January 01, 2021, 10:14:41 pm »
Servos are horribly inaccurate, especially the cheap ones, also the bell rotates more than 360 degrees through the entire speedo range so a servo wouldn't work.
Understood, but does the pulley, which is moving the ribbon, also rotate more than 360°?
I've seen from your photos, the bell is driving the pulley via gears.
My idea was to replace the speedometer cable and bell mechanism by a servo or 270° motor potentiometer.
Of course, with custom gear ratio, so the full servo/motorpot range equals the speedometer range. The gear ratio may be e.g. 2:1 :D

It looks as if this kind of modification is reversible.
br, mf
 


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