rpm to mph HELP!

Support for creating custom user applications on the VESC.
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evoheyax
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Joined: 15 Sep 2016, 16:20
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rpm to mph HELP!

Postby evoheyax » 11 Oct 2016, 16:12

Hi, I've been writing an iphone app to pair with the adafruit UART BT module. And I am trying to give users a live time view of their speed (this app is design for electric vehicles like skateboards), but I'm not sure if my formula is correct. It seems like it could be correct, but it also seems a little low for the numbers my motors are able to hit according to speed calculators.

Currently, I use:

mph = (wheelDiameter*3.14*(rpm/7)*60)/5280;

Can someone confirm if this is correct? And if not, what I need to change? Thank you so much in advanced!

david
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Joined: 15 Dec 2015, 13:19
Location: Hamburg, Germany
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Re: rpm to mph HELP!

Postby david » 11 Oct 2016, 16:53

Assuming you mean ERPM for the rpm variable and you are calculating based on 14 pole pairs. But what's 5280?

Edit: mph, right? ;)
VESCs made in Germany

evoheyax
Posts: 4
Joined: 15 Sep 2016, 16:20
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Re: rpm to mph HELP!

Postby evoheyax » 11 Oct 2016, 19:34

I'm assuming its the erpm. There's only one rpm variable that the RollingGecko code returns from the vesc, and they call it rpm. I'm guessing this is the erpm, because if I don't dived it by 7, I get insanely high numbers.

(wheelDiameter*3.14*(rpm/7)*60) gives us feet per hour, but we want mph, so we divide by 5280 (the number of feet in a mile).

erpm/7 gives rpm from what I've been told. And the original equation I found said:

mph = (wheelDiameterInFeet*pi*rpm*60)/5280

If I use the infamous speed calculator, it says the no load speed at 12s of hummies hubs is 37 mph. But when I spin them on the bench, it maxes out at 26 mph, which is the loaded speed. I just want to confirm I'm doing this calculation correctly.

rew
Posts: 943
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.
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Re: rpm to mph HELP!

Postby rew » 12 Oct 2016, 13:24

If you mention the units, things might be verifyable.
I tried verifying your formula, but got a number in the order of 63 thousand. That's because I incorrectly guessed you'd be using inches for the wheel diameter. (even here in The Netherlands we have 28" wheels on our bike!). Apparently you'd count that as a 2.3333 feet diameter wheel. Fine.

The number of pole-pairs on a motor is something that the designer of the motor can decide. I have a bunch of 7-pole-pair motors like yours. (DT750, C5055, C8085 (?not sure)) but also some motors that have a different number of poles (Sparta ION: 22, gazelle: 10).

So your app needs to be "general" in that the "ERPM/RPM" number can be filled out by the user. A 7 default would be fine, but sometimes it is different. Also the gear ratio comes into play. It seems you're using a direct-drive setup....


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