Reverse engineering a blood pressure meter

[Apothus]

I am trying to reverse engineer a blood pressure cuff. One of the automatic units that self inflates then takes a reading.
http://www.cooking-hacks.com/blood-pressure-sensor-sphygmomanometer-v2-0]This unit to be exact[/url]

The system runs off a 6V battery pack with a selection of discrete components and a processor under a blob. The only connection to the pressure cuff aside from a pump and a pressure relief valve is a pressure sensor. Removing the sensor and searching the product number turns up nothing, considering the die under the blob production method I expect they are manufacturing these devices in a high enough quantity that they have possibly had a manufacturer produce a custom sensor for them. I dont know if this unit is simply an analogue device or if it has some kind of serial interface.

Frustratingly no combination of probing the six pins on this package produces a readout on me DS1052 that I can see as being a useful signal. None of the pins are connected to the cathode of the battery [only tone tested, will check higher resistances]. The only image I see on my scope is a stable ~2v signal on some combination of pins. I cannot see any digital traffic of any kind nor can i see an appreciable ammount of analogue data that would indicate a pure analogue output, any noise is barely above the scopes internal noise and I could not see it being anywhere near clear enough for detecting the minor pressure ticks of a systolic pulse.

Is there some other testing technique I am missing? The device still operates perfectly as I run my analysis (albeit on a bottle of water and not my arm). How else can I attach my probes and why would the device not be connected to ground?

Please help I am at a complete loss here 

[joshhunsaker]

Looks like that's a 50mhz scope, could very well be that the signals are at a much higher frequency and potentially at a much lower logic level than you'd be able to view reliably.

An epoxied IC means they didn't want someone to look under the hood and copy a design.  There are ways to remove it if you like dealing with dangerous chemicals but even then, good luck.

[max_torque]

it's extremely unlikely to be a high frequency signal what ever the output type of the sensor.  It could also be a piezo sensor output.  What other ICs are on the pcb, and where so the tracks from the sensor go?  (direct to the controller, or to another IC etc??)

[Andy Watson]

It's probably silicon-strain gauge pressure sensor. It may have a built-in amplifier, if not, the output will probably be differential. Do a search for Freescale's range of MPX sensors to see similar devices.
Here is blog that explains what signals are being monitored. Do you know how to take a blood pressure reading with a manual system (i.e. pressure meter and stethoscope), it would help your understanding if you did.

 http://www.circuitlab.org/2011/09/schematics-digital-blood-pressure-meter.html

[Apothus]

It's probably silicon-strain gauge pressure sensor. It may have a built-in amplifier, if not, the output will probably be differential. Do a search for Freescale's range of MPX sensors to see similar devices.
Here is blog that explains what signals are being monitored. Do you know how to take a blood pressure reading with a manual system (i.e. pressure meter and stethoscope), it would help your understanding if you did.

 http://www.circuitlab.org/2011/09/schematics-digital-blood-pressure-meter.html

I have a pretty decent understanding of the processes. Block artery with pressure cuf then slowly deflate, listen for a pulse to return, when it does that is systolic pressure. keep deflating, when the heavy thump is no longer herd and the mercury head isnt twitching that is diastolic.

When I get home this evening I'll go through the datasheets and have another look at the output with those in mind. I dont think it is a high frequency issue considering I am not seeing anything at all on the scope, I would at least expect some garbage.

[SeanB]

Probably a millivolt signal on the steady DC level. You likely will need to use a precision power supply ( or a differential probe with Ac coupling and high gain) to get it out of the DC level it is riding on. Limit the scope bandwidth to as low as it can go as well.

[saelig]

Here are some useful sites: 

http://www.ti.com/solution/blood_pressure_monitor 

http://www.maximintegrated.com/en/app-notes/index.mvp/id/4689

http://www.freescale.com/files/32bit/doc/app_note/AN4328.pdf

Good luck!  Alan Lowne  CEO  Saelig Co. Inc. www.saelig.com

[mikeselectricstuff]

Some BP monitors use a capacitive sensor with a diaphragm - not sure what sort fo frequency they use.

[Apothus]

Well it certainly appears as though it is not going to be a nice simple 0-5v analogue signal 

I dont have the hardware to provide a precise offset, nor do I have anything other than a multimeter cap probe.
I may need to look at working from scratch with one of those ti type chips. Shame it would have been nice to see a waveform to begin with