Stepper Motor to replace magnetic mechanism on classic car speedometer.....

Good Morning

I have a 1978 1275GT Austin Mini and would like to replace the existing magnetic speedometer mechanism with a stepper motor(?) based setup as the gear ratios and wheel diameters are now non-standard and the speedo is way out- recalibration of the speedo works out around $150+ (£100++)

the use of an arduino either:

(i) taking a pulse from a driveshaft rotation (via a hall-effect transducer)

or

(ii) using an optical trigger off of a rotating disc on the end of the speedometer

and then moving the original needle around the existing gauge would enable me to use the existing kit but display an accurate speed as well as potentially allowing the odometer to function if the rotation rate of the cable or the driveshaft could be converted into a drive for it

Possibly the alternative is to control the speed of a motor that was intermediate between the speedo cable and the speedometer?????

I think probable controlling the rpm of a small motor might be easier as a multiplier could be used between the two speeds?

What are your thoughts?

Dr B

recalibration of the speedo works out around $150+ (£100++)

And, you think you can build something cheaper???

I've heard of stepper motors being used for dial indicators. But, you'll probably need to micro-step because a regular stepper motor is 1.8 degrees per step, and that's not smooth. You also need a "home" sensor because a stepper won't automatically return to zero and it has to know where it is.

A servo motor might be a better solution. Servos don't step, they don't have to be zeroed, and they come with the driver circuitry built-in.

I assume hot rod shops deal with this issue all of the time, so you might want to research the usual solutions.

Possibly the alternative is to control the speed of a motor that was intermediate between the speedo cable and the speedometer?????

I think probable controlling the rpm of a small motor might be easier as a multiplier could be used between the two speeds?

That sounds complicated and it might be tricky to keep it linear. And of course, you have to find a DC motor (maybe geared-down) with the right speed-range.

DVDdoug: And, you think you can build something cheaper??? I assume hot rod shops deal with this issue all of the time,

Pretty good advice. Once was the time when you could get a little gearbox that screwed onto the back of the speedo. This converted it from mph to km/h - and fixed the odo too. There is a fair chance it is available in other ratios.

These two links may give you some ideas - I have not tried any of them myself, I bookmarked them "just in case"

https://www.tindie.com/products/TheRengineer/analog-gauge-stepper-breakout-board/?pt=ac_prod_search

http://www.ebay.com/itm/10PCS-4-Wire-2-Phase-Mimi-stepper-motor-for-Canon-micro-stepping-motor-D6mm/370953776904

...R

If it were me I'd be printing a sticker with an accurate speed scale and sticking that on top of the existing speedometer.

Hi, What are you going to do about an Odometer? If you are going to be street legal, nice car by the way, I think you will find you will need an Odometer.

What you propose is possible, in fact they make stepper units to act as car intruments, but unless you can provide a legal odometer, as mentioned before , it might be worth getting the speedo fixed.

Check this link.... https://www.adafruit.com/products/2424

Tom..... :)