Electronics Gurus - Stages Power Crank

Ackland

Ackland

chats d'élevage
Is anyone here some sort of tech wizard?

The pod sensor on my stages crank became dislodged from the arm and pulled the wiring off of the circuit board.
Stages denied the warranty back when it happened claiming that a rock impact wasn't part of normal use for a MTB component...

Anyway...
I found it in the cupboard recently and wanted to see if I could get it firing again.
Any ideas on how to set up a rig to test the possible pin location combinations?
Tried a straight point to point configuration but that doesn't seem to be the winner.
Apparently Engineers don't lay out the strain gauge wires in a simple manner


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Comic Book Guy

Comic Book Guy

Likes Bikes and Dirt
Not sure if I can help as I only dabble in electronics.

A quick google search and it could be a 4 wire accelerometer sensor. You would have power (red wire), ground (black wire) and two signal wires (X and Y signal). What would be good is a photo of the component side of the board. There may be some numbers on the chips that could help identify the circuit components/design.
 
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JTmofo

JTmofo

XC Enthusiast
I'd put my Money on the "sensor" being a load cell of some descriotion. 4 wire would indicate excitation voltage (+ and -) and signal (+ and -).

Colour codes are all different for various makes of loadcells , making it tough to rewire.




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D

Daniel Hale

She fid, he fid, I fidn't
stages suck -most warranty denied -u let water get into it, mine is wrecked, if it got water in it its due to their faulty battery cap
 
cokeonspecialtwodollars

cokeonspecialtwodollars

Fartes of Portingale
It will definitely be a strain gauge connected in a wheatstone bridge arrangement. Unfortunately the Stages representative is somewhat correct in that even inherent resistance of the solder joints can impact the accuracy of the reading so they probably calibrate each sensor after assembly, doesn't mean that he shouldn't be able to provide a wiring diagram or colour codes. It will be difficult to determine the layout of the bridge even if you had access to a decent multimeter as it will depend on whether Stages use quarter, half or full bridge rectification. The second lot of photos didn't come through very clearly, can you post them again? We need to be able to trace the positive and negative tracks from the battery to the load cell though realistically I think that your best bet would be to get hold of somebody that would be willing to pull theirs apart and check the colour codes.

EDIT: My bet would be the red will go to battery positive and black to battery negative, this would leave green and white as the signal wires going to the remaining terminals.
 
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Nambra

Nambra

Definitely should have gone to specsavers
Think cos2d is on to it, and I’d be leaning toward it being a full bridge as you can see the four foil sensing elements, two either side of the blue tape or whatever it is on the sensor. A bit like the examples below, except that the four solder connections are orientated differently on the Stages sensor. A full bridge setup will provide more accurate sensing too, which is another reason it is likely to be this way.

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If you have a multimeter, you could measure the resistance between any two wires coming from the sensor. Assuming each sensing element is about the same resistance, you’ll get one of two readings, either one sensing element in parallel with the other three in series, or two pairs of series elements in parallel. Either way, the important thing is the measured resistance - if it’s in the order of 50 Ohms or more, then I wouldn’t thing that the resistance of the solder connections on the circuit board would affect the accuracy of the sensing that much.

However, which sensing element wire is connected where on the PCB will still be important as the unit would have been factory calibrated for the sensing elements in a particular arrangement, as they won’t all be exactly the same resistance - there will be some manufacturing tolerance. As cos2d says, this is where you take a punt on wire colours and look closely at the tracks on the circuit board to reverse engineer what each solder pad connection is likely to be. Generally the supply +/- should be fairly evident.

Just looking at the Stages website too - there appears to be a zero offset calibration function in the App and you need to have the crank in the 6 o’clock position to do the calibration. This is an accelerometer on the circuit board that measures cadence and I’d be guessing that the strain gauge sensor elements will be expected to be in a certain configuration in order for the system to work properly.

The health of the circuit board is the other thing - it doesn’t look water affected. Can you connect it via Bluetooth at all? If it does still seem functional, I reckon you’ve got better than even odds that it will work again, but unless the sensor elements are reconnected as they originally were, the resulting power readings might be out by maybe 5% worst case.

I see you’re SA based too @Ackland (I was there just last week too!) - I know a pretty good electronics engineer in Adelaide who might be able to help if you’re interested.
 
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Ackland

Ackland

chats d'élevage
Thanks @Nambra

The unit turns on with a battery connected.
I can connect to the stages app and have updated firmware.
Both the app and GPS unit can "zero" it so the board seems fine.

When it comes to transmitting cadence or power data, nothing.

No dramas with absolute figures being out by 5% or so as I'd only look at my own data as a training tool.
 
Nambra

Nambra

Definitely should have gone to specsavers
Well that’s a promising start then. I wonder if the programming in the electronics has some sort of dependency between cadence sensing and the strain gauge sensor... that might explain why you haven’t got cadence. Maybe it’s looking for changes in power as well. Or maybe there is no accelerometer and it actually determines cadence from the strain gauge itself.

So if the electronics appear ok, and there’s little reason to think that the strain gauge is broken too, it’s now all about the wiring. Four wires to four connections means 1 of 24 permutations to choose from, although there might be more than one combination that might work.

Any chance of getting some really nice high res pics of the strain gauge and both sides of the circuit board near the connections? That would help to reverse engineer the circuit, as it’s going to follow a standard design (techy info here if you’re interested). We know how you need to connect and excite a strain gauge, so if we better understand which wire goes to where on the sensor (a multimeter would help here too, but good pics might be enough) and have a better understanding of what each solder pad on the circuit board does (by seeing where it goes), we can eliminate a few of those 24 permutations and hopefully get it fixed sooner.

By the way, I don’t think attaching the wires to the circuit board incorrectly is likely to cause any permanent damage, just don’t short them out on the circuit board, or accidentally create a solder bridge between two pads!
 
Ackland

Ackland

chats d'élevage
Thanks again @Nambra
I've had to downsize the images for rotorburn to accept them.
Apparently 10mb is too large for the forum software.
Can email?
If your Adelaide based mate is keen to help for some beers then that would be greatly appreciated.

It just annoys me that the response from stages has been "not warranty, not our problem".

Even back when this all started, I was only offered a 10% discount on a refurbished replacement which is a joke.

Nowadays they don't do an alloy SRAM MTN crank arm at all.
 
cokeonspecialtwodollars

cokeonspecialtwodollars

Fartes of Portingale
Nambra said:
I see you’re SA based too @Ackland (I was there just last week too!) - I know a pretty good electronics engineer in Adelaide who might be able to help if you’re interested.
Click to expand...
This will by far deliver the best outcome, much easier to decipher when you've got it physically in front of you. Make sure you take some photos once it's working again and post them up here for prosperity and because fuck Stages, maybe someone else will be able to find them in their time of need.
 
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