Okay, so if you look at the Arduino code in the image you’ll see a commented out delay(3); I had put this in the wrong location. Because I’m not properly reading the RTS line I put in the delay, and it cleared up ANT+ communications. However it’s in the wrong location and now it’s a delay after every read! Stupid I know!
More after the break. Also I’m starting to use breaks on the blog so the amount of posting will be short. Just click the read more if you visit the main page.
So a little while ago I mentioned that it should be a very simple affair to connect the AP2 module to a Bluetooth SPP (Serial Port Protocol) transceiver. With everything working on 2.4Ghz this might be a wireless mess, but there should be lots of wireless space in the 2.4Ghz spectrum, suffice to say as long as several people are not using ANT+ and Bluetooth in the same area.
Why bridge things?
Well, it’s simple. Most phones don’t have ANT+ and it’s the #1 question I’m asked about the power meter combined with BTLE on Android, the dominate phone OS, does not have BTLE sorted out in the slightest. I don’t claim to know much about BTLE and it’s problems on Android but the group on Linkedin makes it clear that it’s not prime time ready.
So I just ordered up a HC-06 Module for about 10 Dollars CAD and I’m going to try and wire it into my Sparkfun ANT+ stick. The stick is set to 9600 Baud already and is very simple to wire up so I figure I’ll give it a try. I’ll test it with the AP2 module after.
I don’t believe the HC-06 is FCC certified so it’s not like I could sell this. An FCC certified board is a bit more expensive at around $30 – 40 dollars. That means this translator would be near $100 minimum once packaged with a regulator and a battery.
So what could you do? Well, using the basic SPP protocol an Android, iPhone, tablet, Windows, etc device could talk directly to the ANT+ protocol. Meaning that someone could easily get fitness apps talking to the holy grail… I mean the power meter! Android’s been out in the cold for a while now in the ANT+ arena, so it needs a little help.
The second letter in the name of my power meter is C, so that’s a-c-X-X-X-X-X.
Happy holidays.
I’ve been reading up on the SAR (successive approximation, eg the ADS8321) versus Delta-Sigma (eg the ADS 1247). From what I can find out the Delta-Sigma’s are slower BUT they have much better noise rejection. I’m using an averaging technique with the SAR where the Delta-Sigma may not need an averaging technique or very little.
So if anyone knows of a TSSOP 20 breakout to DIP or DIL that’s available in Canada without a fortune in shipping let me know.
This could also reduce cost. The ADS1247 is approximately $13.50 CAD which eliminates two ADS8321 and AD623 and high precision resistors. I believe that is about $45 dollars. Major electronics could be < 40 dollars. Obviously, a power meter won’t be that – labour + other parts + casing + testing + overhead and equipment.
Still looking for an appropriate gyro on breakout board. I think I’ll eventually have to pick up a hot air rework station since I’m not ready to commit to a reflow oven – Gyros are only available in LGA style. If anyone in the Kitchener-Waterloo-Cambridge or Guelph area in Ontario that has one and willing to help out, let me know!
| Speed | Time (Absolute) | ||
| Start | Zero L | Zero R | |
| 4mhz | 8017 | 8914 | 9699 |
| 2mhz | 8019 | 9037 | 9943 |
| 1mhz | 8017 | 9277 | 10424 |
| Speed | Time (relative) | Time Reading | Time Calculating | |||
| Zero L | Zero R | Zero L | Zero R | |||
| 4mhz | 897 | 785 | 121 | 776 | 664 | |
| 2mhz | 1018 | 906 | 242 | 776 | 664 | |
| 1mhz | 1260 | 1147 | 484 | 776 | 663 | |
short zeroadcL() { long adcl = 0; //initialize a long variable for (int i=0; i < 20000; i++){adcl += long(readadcL());} // Serial.print(adcl); // Serial.print(","); // Serial.println(i);} //Read 1000 times return (short((adcl/20000))); //Output the average }
I’d like to apologize for the lack of updates lately. I’ve been exceedingly busy at my day job. Long days testing has slowed the development of V3.
So where is it? The short is that V3 contains some bugs. I last had some zeroing issues which I believe is sorted by replacing the LM4140 with a voltage divider but it becomes hard to test this in conjunction with the magnetic pickup code. This is the major slowdown.
As a result I very much dislike the magnetic pickup. The code will need to be edited and trimmed I suspect. My research into gyro’s indicate they consume about 400 microamps. This isn’t much compared to the current consumption, but still adds to the target.
I’m actively exploring the nRF51422 and sorting out getting either an evaluation kit or development kit into Canada. After some back and forth NuHorizons seems that they thought they originally collected Canadian tax… however they do not. This means that a 99 dollar development kit costs 99 + 25 dollars shipping + either 19.45 or more likely 29.00 to process the customs and 13 dollars taxes totalling 166 dollars, or pay the 66 dollars for express shipping totalling 179 dollars as customs brokerage is included.
Symmetry Electronics seem a little more reasonable as the Fedex Express options are only 44 dollars and include brokerage processing, but it still totals 157 dollars for a 100 dollar dev kit.
While I had earlier mistaken that Digikey carries Nordic things they do carry Dynastream modules using the nRF24AP2. The difference is Digikey is express shipping for 8 dollars, or free ver 200 dollars and no brokerage! That would mean if they did sell Nordic products then it’s total cost would be 121 dollars shipped next day.
In terms of development costs 166 vs 157 vs 179 vs 121 shouldn’t be significant, however considering all the costs up until this point I’m about 2000 – 3000 dollars into development costs depending on if you include certain elements that I use (FR60 watch kit, Garmin Edge 500, programmers, tariff processing, etc). Keep that in mind if you think this is the cheap way out to getting a working power meter. (I could have bought 2 Quarqs or an SRM) That being said I’ve been pushing my budget for this project and there are huge limitations to the way it’s built right now.
The nRF51 would allow me to drive down the board / electronics costs. The AP2 Module is very expensive (25 dollars in low volume) compared with the cheap nRF51 ($2.63) and the AP2 module still requires a micro controller. Thought a finalized arduino style board is expensive, using an atmel atmega 32u4 is about $6.50. So even if I was to drop the pre-made board, the nRF51 still comes in cheaper by a significant margin.
Oh, and one more think nRF51422 (ANT+) is interchangeable with nRF51822 which means BTLE is possible without a significant change over in board design – in fact no change over! It’ll require different code (if BTLE ever pulls together a power meter profile).
To do:
Get V3 working well enough to make some videos of it working and post the board + code
Get nRF51 development equipment.
I mentioned a problem with the LM4140 reference voltage as a poor design decision. Adding a capacitor didn’t solve it nor did adding a resistor. Realizing I was unhappy with this I decided to hack the board and put on a voltage divider for the reference voltage.
I explained before but the short is you need a voltage reference in between the +ve voltage and the ground in order to have a single sided amplifier. This reference raises the differential voltage from the strain gauge.
I’ve attached a picture of the old reference resistor and the hack of two resistors and a bridge wire. It’s giving much better stability.
So why is this ultra precise high grade IC failing… because it’s source is a poor linear regulator on the Arduino Pro Micro. So this means that as the supply voltage drifts up and down that means that relative the reference it’s drifting. This is now ratiometric and even if the supply drifts the reference is still at the same “relative” voltage keeping it all nice and stable.
I seriously hate magnetic reed sensors. I dislike how I have to program the microcontroller. V4 will have a MEMs gyro.
I spent yesterday figuring out how would I program the nRF51422 if I switched to that for development. Kicking myself for missing out on the free board Nordic offered, but even more so for not getting the free day of training. That could be invaluable. However, my self learning has revealed that the nRF51422 is not as scary too deal with. Segger J-Link to 9 pin “JTAG” (not true JTAG as I understand it for the nRF51422 which actually uses a two pin arrangement to program, but it does contain the normal JTAG lines) is all there is to it.
A big issue with this is that if I don’t incorporate an OEM J-LINK means others can’t reprogram it. Mixed feelings on this. Let me know your thoughts? I haven’t decided if V4 will be a minor increment or more major. Feel free to let me know your thoughts via email.
Why is the Left crank arm of V2 connected to V3 via frankenwiring? I was experiencing some very peculiar ADC behaviour. It’s hard to see, but next to the LM-4140 reference is now a blue capacitor. Turns out it was drifting very very high and erratically so that as the load changed so did the output. I missed the 0.1uF capacitor that was supposed have been after the regulator. I’ll need to recalibrate it but things are now a bit more stable. I’ve been asked about releasing PCB schematics. I’m not decided on this but this is a good reason not, to at least not yet. I also hold that without training installing strain gauges is a recipe for disaster and wasted money.
Power consumption is still an issue and with the drifting reference voltage solved, I was able to rewrite my “readadcR” routine to the “wiggle” version. My naming conventions aren’t all that sane when testing (I’m “wiggling” on and off a random pin). However interrupting the transfer to switch a pin didn’t impact the overall readout. According to the datasheet it shouldn’t but until it’s tested I believe nothing.
short readadcR() {
digitalWrite(slaveSelectPinR,LOW); // take the SS pin low to select the chip:
byte one = SPI.transfer(0xFF); // send in the address and value via SPI:
byte two = SPI.transfer(0xFF);
byte three = SPI.transfer(0xFF);
digitalWrite(slaveSelectPinR,HIGH); // take the SS pin high to de-select the chip:
return (one << 14 | two << 6 | three >> 2); //output the value from the three bytes as one short
}
short readadcRwiggle() {
digitalWrite(slaveSelectPinR,LOW); // take the SS pin low to select the chip:
digitalWrite(wiggle,HIGH);
byte one = SPI.transfer(0xFF); // send in the address and value via SPI:
digitalWrite(wiggle,LOW);
byte two = SPI.transfer(0xFF);
byte three = SPI.transfer(0xFF);
digitalWrite(slaveSelectPinR,HIGH); // take the SS pin high to de-select the chip:
return (one << 14 | two << 6 | three >> 2); //output the value from the three bytes as one short
}
Running the two routines (one reads and averages 10 values then the next routine does the same)yielded the above graph. Very little difference. The difference between the two techniques is highlighted below.
It’s obvious that at times the readings are slightly different, but this is on a 65000 value scale and averaged out the difference was 0.38 bits. Pretty insignificant as this should just be the last few bits plus a little offset for reading 10 values then reading 10 other values causing some error as I dynamically load the device.
What does this mean. Earlier here, I showed that currently I’m at 34ma, 17 - 19ma is the bridge itself! That is much too high! I’m targeting a lower value and V4 might not be perfect but it needs to be in the single digits at least. So assuming that reading the ADC takes 3/4 of the time and calculations take 1/4, and there are 3 read periods and only 1 bridge needs to be active for 1/3 of the reading that means that 19ma *1 bridge/2bridges * 1/4 duty cycle = 2.37ma.
Okay, so we dropped 19 to 2.37ma but we can still do better. If I added a minor delay to reduce read cycles in half (8000/s per side totalling 16000 samples/s) we drop both the bridge and the ADC’s to 1.185 and the adc to 0.4ma. This should in turn put the microcontroller to sleep for effectively half the time, dropping it from 17 to 8. I suspect this is wrong due to the regulator drawing more power.
So current consumption would be at about 10.5ma so far for V4. that should give about 20 – 22 hours of operation. A far cry from Quarq’s 300 hours. I suspect that they must reduce read rates even further leading me to question how much more accurate these devices could be if they scarified operating time. Original Quarq Saturn is listed at 400 hours on a 600mah battery. Total consumption when in operation is just over 1 ma. This is brutally low as I’m still not dealing with the ANT+ current consumption, however that is very very low.
Yes, it’s finally getting near the point where I can put this version on a bike and not have to fight with soldering, wires, and tape to hold it together. Anyone who has seen my videos for V2 will remember that.
| Current (ma) | ||
| Item | 3.3V | 2.5V |
| Bridge | 9.428571 | 7.142857 |
| Instrumentation Amp | 0.55 | 0.55 |
| ADC | 0.792 | 0.6 |
| Sum x2 | 21.54114 | 16.58571 |
| Measured | 35 | - |
| Goal (100 hours) | 2.25 | 2.25 |
Coming soon, current consumption measurements!
Above is the raw data from the calibration test. I used the setup below with a few weights from my dumbbell that weight approximately 10lbs. Here’s is how it breaks down. As you may realize that the sensors max out at a relatively low load – It’d probably be fine for testing or a 140lbs climber, but not an expected maximum. I have another resistor to set a lower gain, so I might be able to adjust this easily.
What should the maximum per pedal load be. Let’s take a worse case scenario of a 1800 watt sprinter. We’ll assume 172.5mm crank arms and 90rpm.
P = T * w = 1800 = T* (90/60 *2*pi) => T = 191 N-m
T = Lever arm * force => force = T/L = 191/0.1725 = 1107 N = 112kg
Surprisingly my max load with margin is approximately this! However we’d have to account for the downstroke being stronger than the upstroke, so the best way to do this is assume one leg is doing all the work. So in short, I need to reduce my gain to about 125 from 250 approximately. Although I might use different gains as swapping the differential pins for the Right should give me back some sensitivity.
| Left | Right | |
| Zero (bit) | -13273.2 | -5097.73 |
| Loaded (bit) | -17315.5 | -7141.56 |
| Unloaded (bit) | -13286.1 | -5104.38 |
| Difference (bit) | -4042.31 | -2043.82 |
| Load(N) | 44.59091 | 44.59091 |
| Torque (172.5mm) | 7.691932 | 7.691932 |
| Bits/N-m | -525.526 | -265.71 |
| Max Torque (with margin) | 83.80372 | 96.60148 |
| Load (kg) | 49.5228 | 57.08548 |
So I realized a couple of things. Switching the polarity of the supply and ground on the strain gauge also flips the direction it changes when loaded, so I need to swap the signal outputs for the right sensor as it goes negative when positively loaded. The problem is I was hoping to use more of the ADC range by using a lower ref voltage (e.g: –10000 to +32765, instead of 0 to 32765 which allows me to use a higher gain and thus add accuracy). I don’t know why I didn’t realize this in the last post. I’ll look at this fix tomorrow or Tuesday.
I’ve left it running for approximately an hour and it’s holding steady at –13273 +/-4 and –5097 +/3.
A little two sided tape holds the board on nicely two the back of the crank arm. Should nicely clear the crank ring too!
If you are good at reading between the lines you’ll understand that the title alludes to two separate tasks, but they have not been combined.
So the short is, the AP2 module, after a lot of fighting, is talking to the microcontroller. I was concerned when I couldn’t get the AP2 Module surface mount soldered (with only a soldering iron). It was very discouraging but I eventually solved it. This was already mentioned, and a dummy program was loaded up to transmit meaningless data on the basic power profile. See below.
So the next thing was soldering on all the MSOP and SOIC components which turned out to be very easy by comparison. I guess I’m getting good at soldering tiny components by hand. So I loaded up my dummy program that reads the raw sensor data from the ADS8321 16 Bit ADC. It works fine and at a gain of 250 (using 2.5 dollar 0805 resistors instead of 15 dollar through hole resistors) gives great sensitivity.
This is going to be a bit technical but bare with me; the ADS8321 is a differential ADC so it gives –32768 to +32765. My instrumentation amp, the AD623, is a single supply differential instrumentation amp, so a reference voltage is used to raise the voltage. So we could have +/- 4mv differential, for example, but once amplified it could be +/- 1v. Since we have a single supply this needs to be raised as we can only go from 0 to +vcc. Imagine we have a 2v supply voltage, we would want to lift the 0mv differential after amplification to 1v so that we have a full range for the +/- 4mv swing corresponding to +/- 1v after the amp. This gives a range of –1v corresponding to –32768 and +1v to 32765 all from the original input of +/-4mv.
I’m using a 3.3V regulator to provide the supply voltage, but I only really care about one direction of the force so what I was expecting was the voltage to always go in one direction. HOWEVER, a strain gauge has a tolerance so you might have an offset electrically. This is more of a problem because if you don’t use a reference voltage to raise the level sufficient and it’s hanging out at –1mv and you are only reading positive voltages, you have a dead sensor area and then a massive offset when it kicks in. So it still needs to be raised, but not necessarily by the whole range. In my case I’m raising it 1.024v with the LM4140 precision reference voltages.
If you are still there this all culminates in one of my bridges has the supply and ground swapped as it’s going in the reduced range direction when I apply a force that wouldn’t freewheel the bike.
The next steps are mounting the PCB more securely and switching to coin cells. I currently have concerns regarding coin cells as my drain might be too high in which case I would switch to my Li-ion cells I have already. Power Management will be a very important part of development from here on out. Power reduction hasn’t been at the top of the list and I still wish to change to the nRF51422 Soc when the evaluation kits become available.
Once I can get a handle on the current consumption I’ll post the results.
As you can see above the new circuit boards have arrived from Alberta Printed Circuit Boards. These guys have some of the quickest turn around I’ve seen – you do pay for it though. I’ve designed this board to not deviate too much from the smaller circuit boards I initially built – mainly to speed development of the V3.
A previous post showed a circuit board with an atmega32u4 chip and the associated circuits to support it. Considering I do not have a reflow over I opted for the simpler approach of only supporting the analog and wireless on this PCB and leaving the regulator and micro-controller on an Arduino pro micro. You can see how I broke out the ISP to make it more convenient to program.
Both of the boards mount to this main one – one via pins and the other via surface mount. I haven’t, but I should, incorporate power planes but I consciously did not include them as the AP2 module suggests not having anything metal under the F-Antenna. I probably took this to the extreme but after the difficulties with V2 maintaining a good signal I felt this was a good choice. The analog components are very very low power, on the order of micro-amps of current draw at 3.3v and I never had issues with the little circuit boards so I’m comfortable.
The voltage divider used on V2 is replaced with a precision voltage reference, the LM4140.
This will go on a bike without too much problem but having seen the size of the “Stages” power meter I’m keen to drive the size down. This would have been a great time to know some electronic engineer who works at any of the mobile phone companies as they can pack some power into a small space.
I’m keen to use the AP2 module as the nRF51422 hasn’t become available yet. I unfortunately missed my opportunity to get a free one as I had to renew my passport (Expired on the 25th of Sept, and the Boston conference was on the 24th). So I couldn’t book tickets or sign up for the tour, and as a result I’m waiting for nuhorizons to get in the evaluation kit.
I just submitted my order for parts with Digikey just then, so maybe a new prototype by the end of the week, but likely next week some time.
The most challenging part to the design and development of a power meter is certainly the packaging – or maybe it’s the programming, either way it’s actually not the instrumentation side of things. I think I’ve found a way to package the electronics out of the way and not interfere with the RF signal which was an issue with V2. I’m planning to move to the nRF51422 eventually but until then I’m using a hobbyist microcontroller board to speed development. The new board is to be mounted to the inside of the the chain ring side crank arm. The AP2 module will hang off behind the arm. This means there is no metal near the module to interfere with the signal. I still need to incorporate the battery. I might take the Rotor route and use the higher capacity cell rather than the 2032. Perhaps the 2477 cell.
The unrouted trace is a ground, there are multiple grounds and I’m avoiding ground loops.
