EEVblog Electronics Community Forum
EEVblog => EEVblog Specific => Topic started by: EEVblog on September 23, 2014, 11:36:56 pm
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Teardown Tuesday.
Inside a Polar Wearlink chest strap fitness heart rate monitor monitor transmitter.
Also a look at and some attempted decoding of the signal with the Tektronix MDO3000 oscilloscope.
Polar RMCM-01 Receiver Module Datasheet: https://www.sparkfun.com/datasheets/Wireless/General/RMCM01.pdf (https://www.sparkfun.com/datasheets/Wireless/General/RMCM01.pdf)
https://www.youtube.com/watch?v=bDxabiM-WF8 (https://www.youtube.com/watch?v=bDxabiM-WF8)
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Cool.
It would be interesting to see what the guts of an ANT+ heart rate strap has. The ANT+ ones typically only report the average heart rate once a second and you don't get any R-R beat to beat timing. It looks like you can actually pull out the beat to beat timing from the Polar strap.
The beat to beat variation is said to be a good indicator of fatigue recovery.
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It doesn't look complicated.
Watch crystal frequency / 6 = 5461.3 Hz. That's going to be rounded up to 5.5kHz by marketing.
It seems like there might be 4 potential pulse positions in each 20ms division on the scope, so about 5ms per pulse. 5ms would give 27.3 cycles of "5.5" kHz per pulse (or it could be 28). If you look at the video at about 10:30 and count the peaks to the left of where is starts to fall off, back to the noise, I count 28 peaks which would make the pulses about 5.1ms for 28 cycles (27 cycles would be 4.9ms).
The packet seems to be a start pulse (at the heart rate) followed by a second pulse after so many positions delay identifying the transmitter. The delay will be more than 4 positions, perhaps 8 (ie. 40 ms). Then the third final pulse 4 positions (20ms) after that.
The leftmost and rightmost positions of the 2nd pulse in your video seem to cover 9 positions (spanning about 110ms for the longest packet displayed). There could well be a few more positions than that but I would have thought not many.
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That chip is most likely a microcontroller with an opamp or two and a comparator or ADC built in.
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At 5 KHz the only way to transmit data between the sensor and a receiver is by magnetic fields, I wonder if Dave has a magnetic field sensor that can operate at this frequency in the lab?
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I was thinking the pulses were just indicating start and stop. Either the first being the start and second being the stop or the full 3 pulse section being the start and the rate is the lag time between pulse sections.
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I wonder if the 31 CODED mark on the battery cover is a clue? 31 hex in binary is 00110001. The ones could match the 3 pulses seen for each "packet" (LSB first). I don't know about the variations in pulse spacing between power-ups, though.
If units exist, and anyone has them, with a different CODED number, perhaps scoping one may verify this theory.
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I was thinking the pulses were just indicating start and stop. Either the first being the start and second being the stop or the full 3 pulse section being the start and the rate is the lag time between pulse sections.
I thought the same thing. The first and last posts of the three are just start and stop indicators and the middle position is the actual heart rate. It seemed to move around so that makes sense. Would allow for a certain minimum and a defined maximum rate this way.
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31 CODED indicates the name of this coding scheme: T31. There is/was also a T61.
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Dave, did you do any current measurements on it?
I am interested to know how much it uses in on mode.
Thanks
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While footage of you jumping up and down in your lab *is* entertaining, I think there's a few things you could have done to make this little project more technically interesting.
-- it would have been interesting to bodge together a resonant LC circuit to use as a receiving antenna -- could make a fundamental Friday out of that alone -- and mess about with your signal generator and all
-- I would have liked to see a more systematic approach to decoding the signal -- bodge up a very simple circuit to demodulate and suck the data into a logic analyzer and mess around with that
-- I would have been interested in the analogue design -- what is the input circuit like to read your heartbeat? where are the FETs or amplifiers? what does the signal look like going into that chip? Could you fake out the unit to perceive a heartbeat by feeding pulses from your sig-gen through a couple of static wrist-straps or something?
-- what is the design of that chip? It seems like an interesting mixed-signal job. If not fully custom, what sort of mask- or field-programmable chips would be suited to this type of application that seems to require some mixed signal goodness. At least run a scope over the pins to see what looks like what.
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The space between groups of packets is heartrate, not encoded in any way at all: one group of three pulses per heartbeat. So this device is very simple indeed: each time it detects a beat, it transmits one group of three pulses. I say this just because one of the measures group intervals (I think there was only one such measurement shown) matches a reasonable heartrate, and also because simpler is better, couldn't be simpler that this.
I suggest that one of the pulses is the marker for beat, and the distance to the others is device ID (temporary ID, it seems).
The reason the device has to be away from the heartbeat so long to start transmitting a new ID is practical: it's to avoid the device and the receiver having to find each again, in event the device loses the heartbeat. This happened to me a lot when cycling with a chestband that had lost its elasticity, and kept sliding out of position.
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This is possible one of the stupid questions, but can you use a analog scope to do the same think
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An analog scope won't be usable at such slow timeframes unless you have one of the old analog storage scopes.
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I think the time difference between 1st and 2nd pulse dependence on heart beat rate & seed value
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Are there any measurements which can be made on the chest strap alone (not the transmitter) between the two contacts to determine whether it might be defective? My unit stopped working and I wasn't sure whether the problem was in the strap or the transmitter module. I spoke with Polar customer service and they were pretty sure (correctly) that the problem was with the strap and not the transmitter.
I made some simple resistance and capacitance measurements with a multimeter to compare the defective strap with the good replacement, but I didn't see any obvious differences.
Don