Speedo Wrong by 7mph

[klarzy]

Originally Posted by HarryM1BYT

The variation in percentage, might be due to peoples interpretation of the percentage and difficulty of comparing the two whilst driving.

don't agree there... can keep a steady 40 on the digi and see 43 on the needle, similar scaling for different speeds as well so parallax is not a contributor either...

[HarryM1BYT]

Originally Posted by VMax1000

Nice thing about stepper motors is the don't require feedback

They just move between the poles........

Once it's zero'd and you know the number of poles you can drive it to the required position

That, we can agree upon.

I don't know what the speedo uses as its zero reference point though. It could possibly be a mechanical restriction on the steeper, just over drive it against an internal stop and assume it is zeroed, or some other mechanism.

[klarzy]

Originally Posted by HarryM1BYT

That, we can agree upon.

I don't know what the speedo uses as its zero reference point though. It could possibly be a mechanical restriction on the steeper, just over drive it against an internal stop and assume it is zeroed, or some other mechanism.

So accuracy in a resultant of the number of poles which is limited by space, so if there are 270 degreed between the zero and the max you would need 270 poles for a 1 MPH resolution, seems tight for such a small motor...

[klarzy]

sorry, that was wrong,, 0.7MPH resolution on a 155 mph car

[klarzy]

the reason I ask is I have been playing around with an IPK and felt the way the motor moved in test mode, I could resist it and it would move back 90 degrees but all the time it was under power it would refind zero accurately..
as if it had positional sensing...

[SD1too]

Originally Posted by VMax1000

No, I didn't say that or even implied that.

Good, that's what I thought. So it's therefore possible that the stepper motor used in the IPK isn't as accurate as those with which you work.

We're talking about gauges whose absolute accuracy doesn't really matter. We know that the temperature gauge is hopeless. Fuel gauges are an approximate indication too, and tachometers and speedometers don't need to be precise. Mechanical accuracy comes at a price so it's highly probable that the budget for the speedo and its stepper motor was not a great deal of money, and that means inaccuracy. Based on what I have learnt about the commercial world, that's the most likely explanation, in my view.

Simon

[VMax1000]

Originally Posted by klarzy

So accuracy in a resultant of the number of poles which is limited by space, so if there are 270 degreed between the zero and the max you would need 270 poles for a 1 MPH resolution, seems tight for such a small motor...

You know how many transistors they fit in a CPU the size of a stamp?

Now that is tight.

Have a look here, small and mighty http://www.linengineering.com/steppe...ize-14-09.aspx

[klarzy]

Originally Posted by VMax1000

You know how many transistors they fit in a CPU the size of a stamp?

Now that is tight.

Have a look here, small and mighty http://www.linengineering.com/steppe...ize-14-09.aspx

A motor pole winding is larger than a transistor in a cpu...

[VMax1000]

Originally Posted by SD1too

Good, that's what I thought. So it's therefore possible that the stepper motor used in the IPK isn't as accurate as those with which you work.

We're talking about gauges whose absolute accuracy doesn't really matter. We know that the temperature gauge is hopeless. Fuel gauges are an approximate indication too, and tachometers and speedometers don't need to be precise. Mechanical accuracy comes at a price so it's highly probable that the budget for the speedo and its stepper motor was not a great deal of money, and that means inaccuracy. Based on what I have learnt about the commercial world, that's the most likely explanation, in my view.

Simon

A lot of stepper motors have a build in gears. So even a cheap one with a 7.5 degree step and a 1:10 ratio can be accurate to 0.75 degrees. They can be bought for as little as £1................

Have a look here http://www.alibaba.com/product-detai...627574898.html

Step angle 5-ish degrees , coupled with internal gearing will give a nice resolution

[HarryM1BYT]

Originally Posted by klarzy

So accuracy in a resultant of the number of poles which is limited by space, so if there are 270 degreed between the zero and the max you would need 270 poles for a 1 MPH resolution, seems tight for such a small motor...

Steppers don't work like that.