Mail Archives: geda-user/2014/01/23/13:50:25
I've been reading this thread with some interest. There have been a lot
of good suggestions - they all make the point that there is no "Best"
solution. Nevertheless, I do have another idea that should require no
special additions to the shaft. Since you already have cranks mounted on
the shaft, I would suggest taking a look at a reflective proximity
sensor. They are cheap, small, relatively immune to ambient light and
come in a lot of versions. I would mount it sideways - looking at the
passing of one of the cranks (or both and take an average). This would
give you two different types of measurement. First, you could just count
crank passings (pulses), and/or measure how long it takes the crank to
pass the sensor (pulse duration). The first could be used to measure
average crank rate (RPM) and the second to measure instantaneous crank
radial velocity (i.e, you know the width of the crank arm in degrees of
arc so this is just a simple ratio.
The electronics is very simple - almost a simple as the magnetic reed
switch solution - and almost exactly the same as the photo-interrupter.
It's much simpler than the inductive bridge needed for the variable
reluctance sensor next to a gear (and it doesn't need the gear!). Here's
Digikey PN's for a few model's I've tried:
480-1936-ND :This has a 1.27mm sensing distance.
365-1092-ND :also a 1.27mm sensing distance.
CNB13020S0LF-ND or CNB13020R-ND : 1mm sensing distance (the second is an
axial lead version of the first)
They do need to be rather close to the crank, and mounted in a stable
fashion, but this should not be a real problem on any rigid mechanical
device.
Stephen R. Besch
On 01/23/2014 09:33 AM, Dave Kerber wrote:
> I can hit 120 or slightly more with my legs and a light load, so I expect
> I could go higher with my arms. That's with a shorter radius, though;
> 170mm crank arms. I'd bet that if you allow up to 120-150rpm, you should
> be able to handle all but the most extreme cases.
>
>
>
>> -----Original Message-----
>> From: Rob Butts [mailto:r DOT butts2 AT gmail DOT com]
>> Sent: Wednesday, January 22, 2014 3:51 PM
>> To: geda-user AT delorie DOT com
>> Subject: Re: [geda-user] Best way to measure RPMs
>>
>> I was just looking at slot type photointerrupters. I'd like
>> to have the option to take advantage of resolution too. They
>> are cheap and I think will do what I need.
>>
>> The rpm range is a guess but 0 - maybe 60? How fast can
>> someone pedal a hand bike with roughly a 9" handle radius?
>>
>>
>> On Wed, Jan 22, 2014 at 3:30 PM, Ed Simmons
>> <ed AT estechnical DOT co DOT uk> wrote:
>>
>>
>> Do you care about resolution within a revolution?
>> Typically, for example on the front wheel of a motor bike,
>> you'd have a toothed wheel with one missing tooth as an index
>> pulse read by a hall effect sensor, equally simple is doing
>> the same thing with an optical setup. You can count 'normal'
>> pulses to get the resolution you need within an individual
>> turn and also look out for the longer pulse (or gap) caused
>> by the index notch.
>>
>> If you only care about single revs, stick a magnet on
>> the shaft and have a reed switch to count revs.
>>
>> HTH,
>>
>> Ed
>>
>>
>> On 22/01/14 20:23, Rob Butts wrote:
>>
>>
>> The application is an arm bicycle where the
>> spinning shaft is connected to a handle on each end so with
>> no gearing the rpms will not be too high. I don't have any
>> encoder yet and I can attach anything to the shaft.
>>
>>
>> On Wed, Jan 22, 2014 at 3:13 PM, Nathan Stewart
>> <therealnathanstewart AT gmail DOT com> wrote:
>>
>>
>> That depends a lot on whats attached to
>> the shaft, how fast it's turning, and what's driving it.
>> Obvious solutions are encoder, interrupter (photo or magnetic
>> - ic engines typically use an inductive pickup on gear
>> teeth), or even counting pulses on the drive current. If you
>> already have an encoder for other purposes, that makes a lot
>> of sense. If you don't already have anything else that might
>> work, interrupter and divide by is probably the most direct
>> method. Sensing the driving forces can be a lot of things,
>> from stepper motor pulses to spark plug firing.
>>
>>
>> What's the application?
>>
>>
>>
>>
>>
>> On Wed, Jan 22, 2014 at 3:00 PM, Rob
>> Butts <r DOT butts2 AT gmail DOT com> wrote:
>>
>>
>> I have a shaft that I want to
>> measure the rpm of. I'm looking for opinions on the best and
>> cheapest way. The design is still in development phase so I
>> have freedom.
>>
>> Thanks
>>
>>
>>
>>
>>
>>
>
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