you gotta love picaxe etc Strain gauge

[rigidigital]

Hi,
I began electronics c ouple of years ago(2007) really havent done a lot but always keep an eye on the forum.
Its been a great outlet for when Iget a bittired of farming. I remember when I was worried about calculating RPMon my mountain bike,like if the tires are a lttle flat then how does that effect wheel revs/distance per minute. To which I recieved excellent replies.

Now I want to incorporate a strain gauge to calc horsepower. I was going to source a display from 4D systems but I think that is slightoverkill. I think a standard 16 X 2 display is more than adequate.

So any help with this litle project would be very appreciated.

Mike.

 

[manuka]

Welcome back! Is this need to "-incorporate a strain gauge to calc horsepower-" on your mountain bike ? Stan

 

[MartinM57]

I think you can map out the subsystems involved in the overall solution easily:

strain gauge --> PICAXE --> display

...but I would put the selection of the PICAXE and display way down the list of priorities.

Tell us more about the gauge - what sort of gauge, where would it be fitted, can you do the the pencil and paper maths (or math if you're in USA, I believe ) to calculate horsepower from a strain measurement? Then we can move onto the PICAXE and the display...

 

[eclectic]

Just a few of several commercial units;
http://www.quarq.com/store#sram_standard

OUCH! the prices!

e

 

[MartinM57]

Well found E - but just doing your job I suppose

http://www.quarq.com/cinqo

Yeah - USD2000 retail should mean that it's possible to replicate as a DIY project for less than GBP10 after all...

(irony alert)

 

[hippy]

Tell us more about the gauge - what sort of gauge, where would it be fitted

Horse power is from what I remember 'torque x speed' and presumably the strain gauges on those expensive cranks measure and transmit the torque of the rotating crank, which equals torque on the crank axle.

I would guess you could measure force being applied to the pedal and determine torque from that which could mean a very simple force sensor fitted between foot and pedal, wire up the leg rather than fancy RF transmission of sensor data.

I would also agree it's more about the means and the maths rather than a PICAXE and display.

 

[fernando_g]

You could probably salvage a strain gage from a bathroom weight scale.
These strain gages will be in the load range you require.

 

[rigidigital]

wow, as always I am grateful for the help/replies, I don't know a lot about it but I figured I could connet the gauge to the crank. I will use my own bicycle to getit working then when I have it working I'll fit it to my brothers bicycle. He's a bit serious aout triathlons etc. He;s a funny guy.once he sat out a whole circuit of the bike leg, then joined back in and one the event. Sounds pretty bad but I laughed so hard especially when he went up and accepted the award trophy or whatever it was. don't tell anyone, this is for the picaxe forum

 

[hippy]

once he sat out a whole circuit of the bike leg, then joined back in and one the event.

You could wire up an analogue meter marked "0-100" BHP, drive it using PWM and a random value. If there's any complaint about accuracy you can say it's as honest as his bike racing is

 

[rigidigital]

wow, as always I am grateful for the help/replies, I don't know a lot about it but I figured I could connet the gauge to the crank. I will use my own bicycle to getit working then when I have it working I'll fit it to my brothers bicycle. He's a bit serious aout triathlons etc. He;s a funny guy.once he sat out a whole circuit of the bike leg, then joined back in and one the event. Sounds pretty bad but I laughed so hard especially when he went up and accepted the award trophy or whatever it was. don't tell anyone, this is for the picaxe forum

 

[premelec]

I've thought about this many times... and am a long time cyclist... the only "easy" answer I've considered is to measure chain tension by a varying force deflecting the chain a fixed amount [top side of chain]. You'd have feedback of how much current it takes to keep the chain deflected using a good quality permanent magnet motor as the defection force generator. Naturally subtract the resting chain tension. Various wireless rotating torque devices have shown up over the year for serious users - at serious prices! Hope you find a good answer....

 

[rigidigital]

Well so far aquicklook at youtube makes this little project seem not easysoI'll upload myownsolution

 

[jtcurneal]

The output of strain gauges is very low ( milivolts ) so an opamp of some type has to be used to get a level that can used to determine the force.

There are devices that vary the internal resistance with force that might be easier to use with a PicAxe.

http://www.sparkfun.com/products/9375

for one. there are others with different configs.

Joel

 

[SAborn]

I use the Texas instruments chip ADS-1231 with load cells, which are effectively a strain gauge, the chip is designed for this application and works very well the a picaxe.

The chip is 24 bit data, but the lower 8 bits are very difficult to get stable without lots of fancy circuit design, so i just use the upper 16 bits of data, this still allows a 15kg load cell to read down to 1 gram accuracy.........good enough for me.

 

[boriz]

Get an old/spare rear derailleur. Remove the tensioner (two small freewheeling sprockets mounted together on a swinging arm). Mount them on the top chain so that the chain has to go through the tensioner in sort of a stretched S shape. Spring the tentioner so that it pulls the top chain into a tighter S when torque is absent, but is forced to rotate and stretch the S when torque is present. Measure the rotation with a simple potentiometer arrangement, or if you are worried about mud+water, an inductive or magnetic position sensor.

Simpler still. Just use one of the tentioner sprockets, sprung to push down on the top chain deforming it into a shallow V shape. The more torque, the shallwer the V, the more the tentioner sprocket is forced upwards.

Hope that makes sense.

 

[erco]

Chain bounce from road bumps & shifting gears will introduce a lot of errors in a top-mounted idler system, as well as increase the chances of a chain coming off. It would also have to float sideways with the chain as it shifts.

 

[premelec]

Track bike...

It gets much easier though not trivial on a track bike - running in a velodrome...