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 Post subject: Power metering
PostPosted: Thu Mar 28, 2013 2:35 pm 
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bad kitty!
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Now that people start talking about HRM, I came to think that measuring output could be interesting.

A back o' napkin later, I came to the conclusion that making a spider with load cells, measuring the output would be fairly simple.
What also dawned on me, is that I am not capable of turning that into anything useful, as I have no clue how to handle the data.
This leads to my question:

Are any of you geniuses able to program a Rasberry Pi to sort that kind of thing?
I'm thinking Bluetooth wireless data transfer, if that's making a difference....

If so, we could have a nice HK DIY project, that would allow us to get a power meter for cheap, like sub $200.


Magura :)

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 4:14 pm 
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:thinking: go on....

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 6:38 pm 
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bad kitty!
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Well, the problem is to find somebody, who can sort the data from the strain gauge.
If so, it would be fairly simple to mount a strain gauge on the spider, calibrate it, and have a power meter.

The raspberry Pi seems like a good candidate for that, and is dirt cheap and available.

Magura :)

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 6:43 pm 
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:thinking: :think: go on....

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 6:48 pm 
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pussy
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You will need some sort of an A/D converter before you can process the signal in Pi. Since Pi can process audio inputs, perhaps that might be a cheap way to go. Edit - Pi needs an external A/D extension module to be able to process analog signals but there are plenty examples on the net available. Long time ago I built a camera shutter speed tester from a bunch of photo transistors connected to a PC via audio input. I suspect that for the needs of a reliable power meter, the A/D converter will have to be pretty good in the resolution, distortion and noise departments. When you have that, processing the signal in Pi should be a piece of cake.


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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 6:55 pm 
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bad kitty!
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The chain will have to look like this:

Strain gauge>A/D converter>Bluetooth
Bluetooth>Raspberry

So no chance of using the audio input. It would have been easy though.
The reason for the Bluetooth, is that you have to keep in mind that we need the data to get off a rotating part.


Magura :)

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 7:13 pm 
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pussy
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You could go a different route like using this Arduino signal logger, store the samples on a SD card and then process it on your PC. It should be small enough to be a part of the unit attached directly to the crank.

http://ladyada.net/make/gpsshield/logging.html

http://arduino.cc/forum/index.php?topic=11123.0


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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 7:18 pm 
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yappin' kitty
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I would have very little interest in a power meter that didn't provide some realtime display of the data. Without this, it would prove as useful as an infrared sensor counting the number of times you turned over the cranks on the ride.


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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 7:18 pm 
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stealth kitty
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Magura wrote:
The chain will have to look like this:

Strain gauge>A/D converter>Bluetooth
Bluetooth>Raspberry

So no chance of using the audio input. It would have been easy though.
The reason for the Bluetooth, is that you have to keep in mind that we need the data to get off a rotating part.


Magura :)


lol slip rings? haha

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 7:23 pm 
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bad kitty!
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...but wouldn't it be nice to be able to see the power output while riding?
That's what the one from Sram does AFAIK?

Maybe I'm getting this wrong, but to me the most interesting part, would be to compare HRM and power output in realtime.
Naturally it could be interesting to just see it on the computer as well?

Being a pretty laid back rider, I don't care much for all the efficiency babble, but just found it interesting as a gadget, and thought others might find it fun as well.

If we just log the power output, and store it on a SD card, I'd say this is about to become real cheap and fairly simple.


Magura :)

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 7:24 pm 
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bad kitty!
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Heyyall wrote:
I would have very little interest in a power meter that didn't provide some realtime display of the data. Without this, it would prove as useful as an infrared sensor counting the number of times you turned over the cranks on the ride.


Ok, there I got the answer to that question ;)


Magura :)

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 7:25 pm 
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bad kitty!
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Shibiwan wrote:
Magura wrote:
The chain will have to look like this:

Strain gauge>A/D converter>Bluetooth
Bluetooth>Raspberry

So no chance of using the audio input. It would have been easy though.
The reason for the Bluetooth, is that you have to keep in mind that we need the data to get off a rotating part.


Magura :)


lol slip rings? haha


Exactly my point :D
Or a very long wire, and just unwind it from time to time?


Magura :)

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 7:26 pm 
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yappin' kitty
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Magura wrote:
Maybe I'm getting this wrong, but to me the most interesting part, would be to compare HRM and power output in realtime.
Naturally it could be interesting to just see it on the computer as well?

Magura :)



Here is some data.

Attachment:
Overall Profile.png
Overall Profile.png [ 26.46 KiB | Viewed 9314 times ]


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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 7:30 pm 
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bad kitty!
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Heyyall wrote:
Magura wrote:
Maybe I'm getting this wrong, but to me the most interesting part, would be to compare HRM and power output in realtime.
Naturally it could be interesting to just see it on the computer as well?

Magura :)



Here is some data.

Attachment:
Overall Profile.png


That's kinda what I had in mind.

When I think about it though, STI has a point.
Realtime is not all that interesting, as it will be all over the place. Looking at something like the graph you posted, once in front of the computer, would offer a lot better info.
From a logical standpoint, HR vs. power output, is something that's going to change slowly, so just comparing average and max/min from ride to ride, would be sufficient to make informed decisions.

Magura :)

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 7:39 pm 
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yappin' kitty
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So bike power meters transfer the data from the meter to a head unit (e.g., Garmin Edge) in real time (~1 hertz). The head unit processes the data into instantaneous power (meaningless), x-second averaging (~10 second average is good), and then longer averaging for the ride. The head units can also strip out the zeros (e.g., coasting) before computing the averages.


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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 7:41 pm 
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yappin' kitty
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ps-there is a lot of good information here:

http://www.dcrainmaker.com/2013/01/stag ... eview.html


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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 9:07 pm 
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:thinking: go on....

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 9:18 pm 
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bad kitty!
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Shibiwan wrote:
:thinking: go on....

Image


How about you throw your little yellow brain into this as well?

After giving this some thought, I'd say STI is right. Real time info, is of no real value.
So essentially what we need is a DA converter and a Arduino recorder, like Sti linked to.
That would be small enough to fit a spider.

Magura :)

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 9:20 pm 
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stealth kitty
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Why bother - hollow crank. so there!

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 9:48 pm 
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yappin' kitty
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I'm trying to decide if it is April Fool's day. To make an analogy to cars: the car speedometer is the speedometer on a bike, that's easy. The HR monitor is probably more like the tachometer on the car than anything else. The power meter might be like the transmission oil temperature. Under load, the oil temperature goes up, sometimes way up. A dashboard that displays all of this data is sort of cool, but it can also be practical.

Good grief, now I've convinced myself I need to buy a power meter.


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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 10:15 pm 
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friendly kitty

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Magura wrote:
...but wouldn't it be nice to be able to see the power output while riding?
That's what the one from Sram does AFAIK?

Maybe I'm getting this wrong, but to me the most interesting part, would be to compare HRM and power output in realtime.
Naturally it could be interesting to just see it on the computer as well?

Being a pretty laid back rider, I don't care much for all the efficiency babble, but just found it interesting as a gadget, and thought others might find it fun as well.

If we just log the power output, and store it on a SD card, I'd say this is about to become real cheap and fairly simple.


Magura :)


You're looking at $75 in sensor packages for a good quality strain gauge, arduino with power source, and a datalogger shield, or a little more cash and a little less weight/size if you want it all on a custom PCB. If you made enough of them with the custom PCB, you could end up with the price point being MUCH lower.

But yeah, nothing complex here. Especially if you calibrate it rather than calculate the theoretical power-strain outputs.

If you wanted an output while riding, get a bluetooth xbee board for the arduino and output it realtime...


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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 10:29 pm 
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bad kitty!
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axisofoil wrote:
You're looking at $75 in sensor packages for a good quality strain gauge, arduino with power source, and a datalogger shield, or a little more cash and a little less weight/size if you want it all on a custom PCB. If you made enough of them with the custom PCB, you could end up with the price point being MUCH lower.

But yeah, nothing complex here. Especially if you calibrate it rather than calculate the theoretical power-strain outputs.

If you wanted an output while riding, get a bluetooth xbee board for the arduino and output it realtime...


To throw a xbee board in the mix, is a great idea.

Regarding calibration, that's exactly why I want calibrate each gauge, rather than doing the theory behind.
I've worked a bit with strain gauges for medical device tests, and soon found out that simple hands on calibration, is the way to go. It takes 5 minutes to do, and works no matter how clumsy the mounting of the strain gauge was.

A custom PCB is maybe a bit out of the scope of this ;)
Off the shelf parts from RS components, would be the ideal parts source in my opinion.

Now Sti, are you already done writing the SW?

Magura :)

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 Post subject: Re: Power metering
PostPosted: Thu Mar 28, 2013 11:07 pm 
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friendly kitty

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Magura wrote:
axisofoil wrote:
You're looking at $75 in sensor packages for a good quality strain gauge, arduino with power source, and a datalogger shield, or a little more cash and a little less weight/size if you want it all on a custom PCB. If you made enough of them with the custom PCB, you could end up with the price point being MUCH lower.

But yeah, nothing complex here. Especially if you calibrate it rather than calculate the theoretical power-strain outputs.

If you wanted an output while riding, get a bluetooth xbee board for the arduino and output it realtime...


To throw a xbee board in the mix, is a great idea.

Regarding calibration, that's exactly why I want calibrate each gauge, rather than doing the theory behind.
I've worked a bit with strain gauges for medical device tests, and soon found out that simple hands on calibration, is the way to go. It takes 5 minutes to do, and works no matter how clumsy the mounting of the strain gauge was.

A custom PCB is maybe a bit out of the scope of this ;)
Off the shelf parts from RS components, would be the ideal parts source in my opinion.

Now Sti, are you already done writing the SW?

Magura :)


I'd imagine there are thousands of codes for very similar applications already written... "wireless strain gauge monitor" and whatnot.
Maybe have to mix and match boxed code for arduino-xbee-strain-gauge-lcd-output, but yeah... nothing complex.

If you mount an xbee on the crank, have it transmitting the digital equivalant of the analog data... the other xbee shield hooked to an arduino on your chosen dashboard (add a datalogger shield too, if you desire), and have the arduino handle the basic calculations of "this strain = this load, this load over this time = this power... output the values you want to the LCD in whatever format you want, or output another digital signal to your bike computer if it can accept it and have it handle the data however you wish.


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 Post subject: Re: Power metering
PostPosted: Fri Mar 29, 2013 12:04 am 
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http://www.analog.com/en/analog-to-digi ... oduct.html
http://www.ti.com/lit/ds/sbas366b/sbas366b.pdf

A couple of A/D solutions worth considering.


Magura :)

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 Post subject: Re: Power metering
PostPosted: Fri Mar 29, 2013 1:57 am 
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Whoa, I start a new and get busy and miss all this fun! I always looked at a power meter sort of like an MPG gauge. The harder you press the gas, the worse your MPG, but, speed goes up, Tach goes up.


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 Post subject: Re: Power metering
PostPosted: Fri Mar 29, 2013 2:20 am 
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Magura wrote:
http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7710/products/product.html
http://www.ti.com/lit/ds/sbas366b/sbas366b.pdf

A couple of A/D solutions worth considering.


Magura :)


https://www.sparkfun.com/products/9365
referenced from
https://forum.sparkfun.com/viewtopic.php?f=14&t=29578


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 Post subject: Re: Power metering
PostPosted: Fri Mar 29, 2013 5:04 am 
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 Post subject: Re: Power metering
PostPosted: Fri Mar 29, 2013 10:26 am 
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bad kitty!
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Sti wrote:
Magura wrote:
http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7710/products/product.html
http://www.ti.com/lit/ds/sbas366b/sbas366b.pdf

A couple of A/D solutions worth considering.


Magura :)


https://www.sparkfun.com/products/9365
referenced from
https://forum.sparkfun.com/viewtopic.php?f=14&t=29578



That's a nice solution.

I've got shit to do today, but will try to make a list of components tomorrow.
From there we can decide if it's worth the effort.


Magura :)

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 Post subject: Re: Power metering
PostPosted: Fri Mar 29, 2013 11:32 am 
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Trying to wrap my head around this concept, so pardon me if I'd got it all wrong..

A graph like the one Heyyall posted isn't really interesting to me except for the max wattage part. Time to discover that my max output is like 8% of what Mario Chipollini did in his prime (think he was clocked at 2 kW during a sprint one, total monster).

As for real time wattage I don't really care. When I'm pedaling hard I have no time to watch the meter anyway. Maybe a max wattage readout since last reset, that way I could go "ooh, 317 watts on that last climb" and then reset before the next climb to see if I could beat that. Still, mainly just a "fun fact".

However the real useful bit (in my opinion) would be a summed up total after the ride. The graph Heyyall posted just shows when I'm pedaling hard and when I'm relaxing, that's not really useful to me, but a grand total of total watts put in over the entire ride would be.

Currently my bike computer gives an estimate on how many kJ I've spent during a ride, but to be honest it's highly inaccurate. It just looks at my heart rate over time and does an estimate.

A setup like the one you're talking about allows to take the guesswork out of it, measuring exactly how many watts I'd put into the pedals over a ride. That I could see as very useful.


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 Post subject: Re: Power metering
PostPosted: Fri Mar 29, 2013 12:14 pm 
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yappin' kitty
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I showed you just the profile, I also have the data on how many kJ, too. By stage and overall.

What I like about a power meter is the real time information. You convert your 20 min power into other values. You learn what you can hold for x- minutes, so on a long climb, you can pace yourself. Same thing when biking in the wind.

Real time info makes for better intervals too. Here is a nice article on that.
http://www.fascatcoaching.com/rightway_ ... rvals.html


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