Power meter for Hubbers

[WorcesterWheelers]

You can also add my name to the following lists:

1. Eager potential user

2. Retail outlet to sell you new "Watt-ever-you-wanna-call-it"

[Christie]

Still think you should go for a replacement spider. The offset mounting for the chaonrings is fine for the prototype, but will look less elegant.

 

Another advantage of your method is that the stucture can be made from carbon composites. The strain gauge method is very very hard to do right on composites, have not been tried so far. The composites will be good for marketing!

[bruce]

Still think you should go for a replacement spider. The offset mounting for the chaonrings is fine for the prototype' date=' but will look less elegant.

 

Another advantage of your method is that the stucture can be made from carbon composites. The strain gauge method is very very hard to do right on composites, have not been tried so far. The composites will be good for marketing!

[/quote']

 

Christie, do composites compress linearly?  I have no idea.  In order for the phase difference approach to work I the material would have to compress linearly.

[Christie]

Bruce, I have no idea how the electronics would work! I have worked with composites a fair bit. Although the epoxy & graphite have different stinnfess moduli,  laminates have linear tension & compression untill ply failure occurs.  

[Rocket Man]

I get the feeling that the existing products have many engineering compromises in them, such as non-linearities in the strain response of the mechanical parts.

 

What we should aim for is a simple construction that has as few parts as possible using materials that have almost ideal characteristics. Whilst I personally love the idea of something that has "that composite look" it would probably be better to keep that for cosmetics rather than function.

 

I'm putting together concept drawings of the mechanical parts. The aim is to resricting all "movements" to a single degree of freedom - in this case a simple rotation. There has to be minimum error at the zero load end and absolute linearity along the scale.

 

The complication is that the "pull" of the chain is radial on the chain ring, not axial. This means that there are forces trying to both "twist" and "pull backwards" on the spider at the same time. Not an easy thing to measure. I would rather "get rid of" the pull and deal only with the twist.

 

As a design philosophy we should aim to avoid the modern trend towards trying to "fix" fundamental design flaws in software. If the basic transducer is stable and linear then life becomes real easy...

[bruce]

I get the feeling that the existing products have many engineering compromises in them' date=' such as non-linearities in the strain response of the mechanical parts.

 

What we should aim for is a simple construction that has as few parts as possible using materials that have almost ideal characteristics. Whilst I personally love the idea of something that has "that composite look" it would probably be better to keep that for cosmetics rather than function.

 

I'm putting together concept drawings of the mechanical parts. The aim is to resricting all "movements" to a single degree of freedom - in this case a simple rotation. There has to be minimum error at the zero load end and absolute linearity along the scale.

 

The complication is that the "pull" of the chain is radial on the chain ring, not axial. This means that there are forces trying to both "twist" and "pull backwards" on the spider at the same time. Not an easy thing to measure. I would rather "get rid of" the pull and deal only with the twist.

 

As a design philosophy we should aim to avoid the modern trend towards trying to "fix" fundamental design flaws in software. If the basic transducer is stable and linear then life becomes real easy...

[/quote']

 

Some issues to consider:

The response of the materials around the transducer.  Different crank materials such as carbon, aluminium, titanium will react differently to the forces applied.  I know with the SRM, the transducer get's calibrated to the material of the cranks and the chainrings.

Temperature response of the material used to make the transducer, as well as the materials in the chainrings etc.  Change in temperature will cause the chainrings to compress the transducer.  Different chainring materials will have different thermal expansion coefficients.

Thermal response of the transducer itself.

The SRM also suffers from inaccuracies due to changes in the chainline in different gear combinations.

The quarq is trying to overcome the problem of the zero point shift when temperature changes by storing a set of offset vs. ambient temperature instead of just a single point like the powertap and SRM.  The layout of the strain gauges in the powertap seems to be more temperature stable than the SRM though.

[Rocket Man]

A typical design becomes fraught with peril when trying to use strain guages. They tend to measure the very things that you don't want to measure, like temperature and lateral loads.

 

Whilst purists will scream that you can't allow the chain ring to move with respect to the spider I think that the way to get a useful signal out of a mechanical device is to allow some significant and measurable motion - much larger than the effects of temperature expansion. I'm thinking of a rotation that equates to 1mm/kW at the circumference.

 

Using this relatively large motion we need to measure a movement of about 0.01mm in order to achieve 1% resolution. Using interfering magnetic discs we could have magnetic domain sizes of the order of 0.1mm and easily reach this value.

 

What makes the proposed design insensitive to the usual mechanical tolerances is that the magentic tracks will be balanced - ie there will be two sets of tracks generating interference patterns that move in opposite directions. It is only the relative movement of these patterns that needs to be detected. Also, the interference mixing takes place magnetically before the pickup so there is no need to use high speed, low sensitivity pickups. Instead, simple hall effect devices will detect the two slow beat patterns and measure their relative phase difference.

 

The electronics part of the design is almost trivial. This is a mechanical gizmo...

 

 

[Velouria]

Shouldn't you be a bit careful posting too many details on an open forum? Before you know it someone might have stolen this idea, or used it as the basis for their own new power meter. If there is a possibility of such a thing working, shouldn't some sort of protection be sought first?

 

[Rocket Man]

Shouldn't you be a bit careful posting too many details on an open forum? Before you know it someone might have stolen this idea' date=' or used it as the basis for their own new power meter. If there is a possibility of such a thing working, shouldn't some sort of protection be sought first?
[/quote']

 

I was waiting for someone to ask this question...

 

The purpose of this thread is to try and get a low cost power meter into the hands of everyday cyclists. It doesn't make any difference how this objective is achieved. If, by some some chance, there happen to be good ideas here that someone can use to produce such a device then the end justifies the means. I am not doing this to make a fast buck.

 

As long as nobody objects to me using this forum to collect and disseminate information about power meters (a cycling related subject) then I am willing to share any and all ideas that come up.

 

Also, open forum information cannot be patented, ergo, the rider is protected from monopoly and the final retail price will have to be kept low.

 

 

I am an engineering anarchist!

[TitusTi]

The engineering equivalent of open source, love it!