Author Topic: Modifying mechanical speedometer to electronical.  (Read 2851 times)

0 Members and 1 Guest are viewing this topic.

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #25 on: January 25, 2021, 10:18:09 am »
Ok let's get back to this project.
As it turns out my uni only gives enough time for boring bachelors degree final projects so i had to say bye-bye to my quadruped robot.
Anyways i've decided to use this project for my finals, might as well get two birds with one stone.
But that means i also have to get more technical about it.
Also i've since decided to go for direct drive without the external motor.
I've read that these speedometers were a little unreliable so i'll have to see how i can improve it and i've also found a guy who sells these speedometers (for parts) so reversibility of the mod is no longer important.
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
I want to avoid any wear items like potentiomenets as much as possible. Also i've seen a couple videos where servos were used to modify old mechanical speedometers and the servos were quite noisy.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #26 on: March 29, 2021, 05:14:04 pm »
So my internship is nearing its end and it looks like it would be about time to get back to this.
If i remember right the roller on the strip is 42mm in diameter, so with a 3.75 deg/step stepper i would get about ~1.4mm of movement per step.
On top of that i could also microstep but these stamped steppers don't like microstepping, at least the 7.5 deg/step ones don't.
But if i can get at least 1/2 or maybe 1/4 microstepping then the movement would drop below 1mm, which i think is acceptable for a speedometer that has about ~200mm of total travel.
Are there any standards for speedometers defining the acceptable step size for the indicator/needle?
The odometer will also need a stepper to drive it since it's driven by the throttle cable, which will no longer be there. I have a bunch of stepper motors from DVD drives and i think they'll be allright.
There isn't much space for the odometer drive so the small stepper should fit just right.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #27 on: March 29, 2021, 05:34:23 pm »
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #28 on: March 30, 2021, 06:23:04 pm »
So the key parameters for the project have been decided (a long time ago actually).
*Supply voltage 5-48V.
*Speed error less than +/- 4% (SAE J1226)
*Odometer error less than +/- 4% (also SAE J1226)
*Control signal is PPS (regular tach signal basically)
*Speed resolution no less than 1 km/h. IIRC there's about 20mm per 20km/h so that makes for around ~1mm/ km/h.
*There has to be a way to calibrate the speedometer controller. Basically two buttons and saving some value to EEPROM.

The above are practically set in stone, i can't change them but that doesn't mean i can't work around them if it so happens that i've painted myself into a corner.

I think it will be best to add a single DC-DC converter to meet the supply voltageChanged my mind. It's kind of a wide range but i think it's only because of the transients (see attached pic).
But the low voltage operation would be problematic since i don't think any DC-DC converter has a dropout voltage low enough for the stepper driver to still work on 5V supply voltage on the input of the DC-DC.
If the high supply voltage range is necessary because of transients then i can just pop in a linear regulator and call it a day right?
Anyways, 5V seems a bit low for a stepper driver i hope i don't have to finagle another DC-DC converter to boost the voltage. The drive current for the stepper motors will be low since they only need to move a rather lightweight movement.
Efficiency is also not critically important considering the application, but power dissipation is because the components will need to dissipate it obviously.
Also any stepper driver recommendations?

I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #29 on: March 30, 2021, 07:12:29 pm »
Found the A4990 "Automotive Dual Full-Bridge Driver" but it's not much of a stepper driver. What i ideally want is something that takes STEP and DIR signals but i can make this work also if need be.

Ps: Man the search on Digikey is so bad i can't select my supply voltage range but have to click through preset ranges that aren't even in any sensible order, what a pain.  |O
« Last Edit: March 30, 2021, 07:18:37 pm by Refrigerator »
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #30 on: March 30, 2021, 08:46:01 pm »
Ok so i have my list of stepper drivers with STEP/DIR controls and from what i see Trinamic stepper drivers look the most attractive because of their sensing capabilities.
This would allow me to detect lost steps and do other cool stuff especially the silent stepping but i'm not sure all these features are strictly necessary and i don't want to feature-creep my design.
Also i don't know how well those drivers cope with stamped steel stepper motors instead of the regular machined ones.

As for the brains of the operation i still have some SAMD10 MCU's and some ATmega's, at this point i just want something i'd be comfortable working with.

Stepper driver list:
http://www.farnell.com/datasheets/2549178.pdf
http://www.farnell.com/datasheets/3119151.pdf
http://www.farnell.com/datasheets/2815804.pdf
https://www.ti.com/lit/ds/symlink/drv8834.pdf?ts=1617134868217&ref_url=https%253A%252F%252Fwww.google.com%252F
http://www.farnell.com/datasheets/3119145.pdf
http://www.farnell.com/datasheets/3119141.pdf
http://www.farnell.com/datasheets/2914769.pdf

I'll pick whichever i feel like using, max input voltage is not necessarily important as long as it's around the 12V (or more) mark since i can put a voltage regulator to knock it down.
And whatever needs knocking down will on be a transient so i'll throw a filter on the input of the regulator as well (LCL T? CLC PI? RC? i'll think about it).
Max input voltage will remain 48V just might not be continuous.  ::)

Also i don't know what sensors gearbox speed sensors use but wheel sensors use reluctance sensors so gearboxes should be the same so i'll need some circuitry to make the signal micro-friendly, which i'd have to do anyways since i don't want to feed automotive noise straight into my GPIO.

Ps: i'll be spamming this thread a lot, it's kind of my place to dump ideas as i go.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Offline james_s

  • Super Contributor
  • ***
  • Posts: 15225
  • Country: us
Re: Modifying mechanical speedometer to electronical.
« Reply #31 on: March 30, 2021, 09:53:48 pm »
I think I would have gone with a good quality DC motor driving the original speedo, a piece of vinyl tubing makes a decent low torque shaft coupler.

If you want a more direct approach, how about taking the electronic speedometer out of a newer car and transplant the mechanical bits behind the existing faceplate? With a bit of luck you might even find one that has the odometer in the same location. IIRC the speedometer in my Volvo is essentially a moving coil meter.
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #32 on: March 31, 2021, 05:49:41 pm »
I think I would have gone with a good quality DC motor driving the original speedo, a piece of vinyl tubing makes a decent low torque shaft coupler.

I read in some article that these strip speedometers were notoriously unreliable so direct drive would let me bypass most of the mechanical components that may be contributing to this reputation.
If i were to drive the speedo cable with a motor i would not only add a wear item, but it would need to be constantly running and whirring in the background. I would also be trusting a mechanism from the early 70's to be operating accurately and reliably.
And i don't think that's a good solution.


If you want a more direct approach, how about taking the electronic speedometer out of a newer car and transplant the mechanical bits behind the existing faceplate? With a bit of luck you might even find one that has the odometer in the same location. IIRC the speedometer in my Volvo is essentially a moving coil meter.
I think that's a bad idea actually. For starters what i have is a strip speedometer and i don't think there are any electronic strip speedometers around.
I would then have to shoehorn a dial speedo in, which would ruin the aesthetic (that retro strip speedo vibe) and before that i would have to find one that even fits and looks right.
After that i would have to somehow reverse engineer the interface for said speedo and chances are it wouldn't work with my gearbox/ gear ratio combo so i would need a custom interface to convert the signal to match anyways. So i would be making some kind of control board brainbox regardless.
So basically i would be doing the same thing except it would be more work, more convolution, less reliability (imo) and the result wouldn't look as good in the end also.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Offline james_s

  • Super Contributor
  • ***
  • Posts: 15225
  • Country: us
Re: Modifying mechanical speedometer to electronical.
« Reply #33 on: March 31, 2021, 09:04:08 pm »
Oops yes I missed the part about the strip speedo. Well you could use a servo to drive that. Doesn't have to be a cheap hobby servo, you can get very high quality servos that are precise and reliable. The electric motor drive does have the problem of relying on the original mechanism, but the motor itself does not have to be unreliable. I have seen some of those solar powered air vents for example that have run for decades, the motors are almost silent and last a very long time. They are designed for that low torque application and have good bearings and beefy brushes that have a light tracking force so it doesn't wear the commutator.

The rest is pretty trivial though. All of the electronic speedos I've seen use a reluctance sensor, essentially a guitar pickup that reads slots or teeth in a metal wheel. The only difference is the calibration and that can usually be adjusted or you can buy or build an adapter that reads whatever signal you have, and puts out pulses at the correct frequency.
« Last Edit: March 31, 2021, 09:05:52 pm by james_s »
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #34 on: April 01, 2021, 03:24:29 pm »
I don't feel like spending money on expensive servos, especially when they wouldn't even fit the application.
The roller for the strip rotates more than once over the entire range of the speedometer, so a servo wouldn't work. And if i tried to make it work i'd be adding gearing and it would become a huge mess.
Right now my plan is to replace the original shaft of the roller with a stepper motor, so that the roller is directly coupled to the stepper motor.
I think this way it would be the simplest and most reliable.

But one problem with a stepper is that i don't know the position of the strip and i can't assume it to always be where it left off last time.
So i think i might glue a little piece on the strip to protrude slightly and use either a micro switch or an IR sensor.
To home the speedo i could make it run to max range and back to zero, kind of like motorbike speedos do. I think that would be neat.
Suggestions for any other kind of simple sensor for the strip position are welcome also.
Currently i'm leaning more towards a microswitch, this way there would be a hard stop for the strip and sort of act like an end stop. The servo would be adjusted to skip steps if the strip jams.
Opto sensor would not stop the strip from rolling over too far in case of a fault or error. There are teeth on the roller that fit into holes in the strip to keep it from slipping and might damage the strip if it were to get misaligned.
Also the ends of the strip are joined by springs that hold onto aluminum end pieces that are pressed on the strip and these aluminum pieces would definitely interfere with the roller if the strip were to go too far.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Offline james_s

  • Super Contributor
  • ***
  • Posts: 15225
  • Country: us
Re: Modifying mechanical speedometer to electronical.
« Reply #35 on: April 01, 2021, 08:44:45 pm »
Use a Hall effect sensor and glue a small magnet onto the roller, or use a slot type opto sensor with a small tab connected to the moving roller. Another option is to use a stepper with an absolute encoder on it.
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #36 on: April 02, 2021, 04:28:22 pm »
Use a Hall effect sensor and glue a small magnet onto the roller, or use a slot type opto sensor with a small tab connected to the moving roller. Another option is to use a stepper with an absolute encoder on it.
The stepper would make more than one revolution so the absolute encoder wouldn't work.
I think a slot sensor would work with an endstop to keep the strip from rolling too far.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #37 on: April 04, 2021, 12:54:03 pm »
In case the gearbox doesn't have a speed sensor i should add two pulse inputs for the ability to run wheel sensors.
If the gearbox does have a speed sensor the single sensor output will be fed into both inputs.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Offline james_s

  • Super Contributor
  • ***
  • Posts: 15225
  • Country: us
Re: Modifying mechanical speedometer to electronical.
« Reply #38 on: April 04, 2021, 06:08:09 pm »
The gearbox must have had some sort of output to drive a speedometer, at least every FWD gearbox I've seen does, either to drive a cable or an electronic sensor. For cable drives you can get retrofit sensors that connect to the original mechanical drive and give you pulses, or you could build something.
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #39 on: April 04, 2021, 07:26:14 pm »
The gearbox must have had some sort of output to drive a speedometer, at least every FWD gearbox I've seen does.

My 90's SAAB doesn't have a speed sensor in the gearbox, rather ir relies on the wheel speed sensors, which are necessary for the ABS also (it's there so why not use it, right?)
This shows that not all cars are built the same so to keep my design more future proof i'll add two inputs.
It should be relatively easy, since they will both be the same so i'll just copy and paste, then (A + B)/2 = my speed.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Offline james_s

  • Super Contributor
  • ***
  • Posts: 15225
  • Country: us
Re: Modifying mechanical speedometer to electronical.
« Reply #40 on: April 08, 2021, 04:38:34 am »
My 90's SAAB doesn't have a speed sensor in the gearbox, rather ir relies on the wheel speed sensors, which are necessary for the ABS also (it's there so why not use it, right?)
This shows that not all cars are built the same so to keep my design more future proof i'll add two inputs.
It should be relatively easy, since they will both be the same so i'll just copy and paste, then (A + B)/2 = my speed.

Hm that's interesting, I wonder how they average the input from the two wheels? My ex had a Saab, it was a 1988 900s, fantastic car. I could swear that had a speedometer cable drive on the gearbox but it's been quite a few years since I worked on that car. Every car I can remember working on had either a speedo drive cable from the gearbox or a reluctance sensor in the differential housing, which in the case of FWD cars is also the gearbox.
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #41 on: April 08, 2021, 02:59:09 pm »
My 90's SAAB doesn't have a speed sensor in the gearbox, rather ir relies on the wheel speed sensors, which are necessary for the ABS also (it's there so why not use it, right?)
This shows that not all cars are built the same so to keep my design more future proof i'll add two inputs.
It should be relatively easy, since they will both be the same so i'll just copy and paste, then (A + B)/2 = my speed.

Hm that's interesting, I wonder how they average the input from the two wheels?

Probably just average the individual speed values, maybe only from the rear wheel sensors since they're mostly tangential to the turning circle and because they're not driven (in fwd) so their readings will be more accurate.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #42 on: April 16, 2021, 06:21:19 pm »
Found this nifty buck-boost converter IC. Quite expensive though.
https://www.ti.com/lit/ds/symlink/lm5118.pdf?ts=1618575613071&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM5118
This would open up more possibilities for stepper drivers because it would make the voltage range wider.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #43 on: April 16, 2021, 06:35:38 pm »
Ok so i just had a little outside the box idea.
Basically i could use a negative voltage DC-DC converter that would take whatever voltage came from the 12V line and convert it to -12V.
And then use the -12V line as a virtual ground for my electronics and 0V as the +12V rail.
This way i would get the buck-boost functionality without actually needing a buck-boost converter, which should cut the cost down (imo).
The current should be low enough for this to work, i'll have to do more research.
Only problem would be coupling a digital output in case i wanted to add anything in the future that needs to know the speed.
The inputs should be easy enough, a capacitor in series with the sensor output to AC couple it should do the trick.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #44 on: April 16, 2021, 06:49:08 pm »
The idea is kind of like this but now that i think of it this converter would nuke my circuitry if it ever stopped working so it might not be the best choice.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #45 on: April 16, 2021, 06:53:27 pm »
farnell component search could not be any more of a pain in the ass if it tried
EDIT: AND THE STUPID WINDOW KEEPS CHANGING SIZE EVERYTIME I CLICK ON A VOLTAGE (to refresh results) SO I CAN'T EVEN MULTI-SELECT BY HAND  :rant: :rant: :rant: :rant: :rant:
« Last Edit: April 16, 2021, 06:56:29 pm by Refrigerator »
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #46 on: April 16, 2021, 07:05:50 pm »
Well it doesn't look like there are any suitable LDO's anyways. Most are low power jobbies.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #47 on: April 17, 2021, 04:13:41 pm »
Ok so i came up with a quick and dirty discrete-ish LDO that should do the trick in case i find a suitable stepper driver.
It's not complete but works as a proof of concept.
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #48 on: April 27, 2021, 11:25:31 am »
Ok so i came up with a quick and dirty discrete-ish LDO that should do the trick in case i find a suitable stepper driver.
It's not complete but works as a proof of concept.
Oh yeah and forgot to explain what it does to those who don't see it.
One opamp is used to make a linear regulator while another is used to make a charge pump.
Schmitt trigger would have been better but opamps come in dual-per package, most commonly (Eg. TL082, LM358, JRC4558 etc.)
Compared to a schmitt trigger this only increases the part count my three resistors and allows me to use 1 less IC (the schmitt trigger)
The charge pump is there to increase the drive voltage for the main pass element, which is N-type because it's easier to make the regulator stable this way.
MOSFET is used as the main pass element because a BJT is current driven and my little charge pump can only supply so much.
Efficiency is not important considering the application, as long as it's not excessively inefficient it's not a concern and the charge pump adds very little to that.
« Last Edit: April 27, 2021, 11:29:17 am by Refrigerator »
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Online Refrigerator

  • Super Contributor
  • ***
  • Posts: 1343
  • Country: lt
Re: Modifying mechanical speedometer to electronical.
« Reply #49 on: April 27, 2021, 12:12:58 pm »
Ok so since buck-boosting or staging regulators and converters is a bit of a mess i've chosen to stay away from any stepper driver that can not operate at 5V.
Drivers with 4.5 or 4.75V min input are on the edge of being opted out, such as TMC2209/8 and MP6600.
So far i have three candidates: STSPIN220, DRV8834 and TMC2300, which i really like for its feature set.
Also seems like Farnell is completely out of stock on almost everything and TMC2300 can only be found on Digikey.
Is shipping through Digikey in EU expensive? I've heard there can be gotcha's with which warehouse you ship from.
« Last Edit: April 27, 2021, 12:24:18 pm by Refrigerator »
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf