Homemade Bicycle Power meter V3.1 update and long term power error 

Thanks, hoping to get a second one on the go as just released my other bike accepts the hollow crank and bottom bracket.

No problem, thanks for commenting. Strain gauges are so highly sensitive to temperature, the wheatstone bridge with 4 stain gauges is good a compensating. However there is always going to be some variation in the thermal expansion of the material. Can't have all the stain gauges in the same place. Even a bright light shining on one side of the crank changes the value which is incredible. I hadn't thought about the nRF51s, they are no doubt up to the job and as you say could be cheaper. Will look out for some. Thanks for pointing out the link, it is now corrected.

@@Ed.R Makes sense. Thanks for explaining. Do you know how the commercial crank based units account for this? Just more calibration or sensors?

@@avibank I believe they spend a while measuring and quantifying it which is part of the reason for the high prices. Also other factors like picking up torque from the other crank. Putting the strain gauges in a pocket inside the crank will help avoid the effect of the sun shining on one side of the crank. I assume they will measure temperature and the software will use a look up table to apply a compensation. Not really sure what goes on to achieve 1% accuracy but this is as much as I've picked up along the way.

Thanks that would be another possibility.

​@@Ed.R I was watching this Keith Wakehem video where he goes through the innards of a new Wahoo pedal power meter. At around 6:39 he talks a bit about the TI ADC they use. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-hIK4F9xcA-Q.html

@@avibank I notice that the setup is a combination of amplifier and ADC. I've no doubt there are better ADCs however my goal with my power meter is keep it cheap and simple. The HX711 is so readily available and cheap and does everything amp and ADC all in one.

@@Ed.R I replied to this last week and RU-vid seems to have deleted it and any record of it in my history :/ In essence I agree hx711 is the best choice given the goals and constraints. I did find some TI ADC's being sold in little purple modules, not too much more expensive than the HX711. The chip is ads1232/1. Also has a built in PGA like the HX711. I put a link in the comment that got deleted so I won't again. Would be interesting to experiment with if I can get my hands on some.

@@avibank That's annoying how RU-vid is deleting comments. I asked and they didn't give me any option to allow all comments. I had a look at the ADS1232 data sheets and actually rather like the look of it. Appears to be very similar in most ways to the HX711 but seems more professional. I like the smaller clock dividers meaning full 130Hz sample rate is achievable with 8MHz, an easily achievable PWM frequency. Also notice the boards on Ebay come with a position for a basic 2 pin crystal like I should have done 🤦‍♂️. Even states that capacitors aren't required. The HX711 data sheet is a bit vague about clock sources. If I find a library for the ADS1232 that works on nRF52 chips I might be giving it a go.

Thanks, maybe I need to make a video soon explaining where I'm going with my power meter and put the code up. I don't intend to use an extra module for cadence. I currently use an nrf24l01 but the future plan is ANT+ using Segger embedded studio. I think I have chosen the hardest option but in the end it will be a better learning experience even if it takes me years.

@@Ed.R Isn't ant+ being replaced by BLE? Phones seem to be dropping support. But nRF52 does both, no? I imagine the best eventual solution would be a custom PCB with nRF52 surface-mount module. Can put the hx711 and mpu6050 chips on that.

@@ewetoob1924 I guess ANT+ will go eventually as people seem to prefer BLE but at the moment my Garmin Vector pedals and computer use it. You are correct nRF52 chips can do both with the right soft device. I'm going to be using ANT+ for communication between my power meter and the automatic shifter. So can use it as long as I want. A custom PCB would be the way to go but I may not bother as I have plenty space inside my crank axle. Especially once I loose the nrf24l01 module it will just be the power latch circuit and an nRF52840 board and whatever 1S lipo fits. Code and video are up if you check out my latest video. Thanks for the motivation to finally get it up.

Sorry I get the same appears the link has been shortened by RU-vid I'll try putting it again here normandnadon.com/forum/showthread.php?tid=3083 The forum is actually a bit out of date and only shows the build of my original power meter which used an amplifier not the HX711. Best to look through my video playlist here on RU-vid for the more recent developments. ru-vid.com/group/PLGwDuFncb0Ky7Xtqh5f4zIB2kjJFK9wUi

@@Ed.R aightt ! thanks for the response ! right now, im quite optimistic about the concept of the strain gauge, but im a little worried about how the gyroscope works on a crankset. how did you specifically made the MPU6050 works on the crank? have you done the kalman digital filter on it? because ive read some of the journals that says MPU6050 have some output issues against angle measurement on something that rotate. and did you use those 3 respects (X,Y,Z) of the MPU or do just using 2 of em?

@@MrSafetyRiderI'm afraid I'm not going to be very helpful on this one because I don't use an IMU. I measure cadence off the force waveform. It has its limitations but is surprising accurate. Is worth learning how to use one though as they are interesting. I followed this playlist of video tutorials to get the basics. ru-vid.com/group/PLGs0VKk2DiYwEo-k0mjIkWXlkrJWAU4L9 I did find some code somewhere that was great at measuring rpm will see if I can find it. Never completely understood it myself so can't really explain how it's done to someone else. One thing I did remember was that its important to combine the accelerometer and gyro. Accelerometer never drifts but suffers a lot of vibration noise. The gyro drifts badly but doesn't suffer from noise due to vibrations. Getting the right blend of the 2 was the clever bit.

@@MrSafetyRider The rpm code I was thinking of is in the examples of the Arduino_LSM9DS1 library. That is for the Nano-33-ble on board IMU. Appears if you only need rpm then the gyro on it's own works ok once calibrated. If you wanted an angle that's where the accelerometer is needed. Hope this helps a little.

@@Ed.R aightt, thanks a bunch for your time and information sir!! ill let you know in a few months for the final systems.

Hi, thank you. I have never felt that the project is finished enough to be fully made public for others to copy. I'd like to get the ANT+ working first otherwise the nrf24l01 communication is of limited use to most people. Having said that now that I'm beginning to get an idea of the potential and limitations of the project I might consider it. I wouldn't provide detailed instructions for the entire build because cranks are different and I have no idea what I'm doing when it comes to strain gauge installation. Definitely could share code and wiring diagram.

@@Ed.R Thanks for your answer! I'm really interested in the code. Do you think there is a possibility to send the Power via Bluetooth to a Garmin Edge 530 instead of Ant+? Wouldn't that be easier?

@@felixp.2535 Have now put the code up on Arduino project hub find link in description of my latest video ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-2bmOP68gpBo.html I think BLE might be easier but I wanted the challenge of ANT+. Not sure if Garmin computers receive BLE, I assume they do. My old Edge 500 just says ANT+ on it.

@@Ed.R Hey, Thanks for sharing and for your explanation line by line! I'm really looking forward to your project with ant+:) Best regards, Felix