Hello

I am glad that finally I get to post a thread here. Basically, I need a little clarification about some things related to my college project. It is PIC based blood pressure monitor, which will connect to Android phone via bluetooth and send SMS/email with the readings to a preset number/email address. I did a lot of research, but there are some things that I cannot fully understand. Any help will be greatly appreciated.

First, I am not sure how to inflate the arm cuff - what motor and valves should I use? Where do I get these from? (The only place I could find 5V DC air pump is a Chinese website selling tens of thousands from these.) I have never used any DC motors before, and I do not know much about air pumps either. This, along with the pressure sensor, is where I am stuck now. I have looked into other people's implementation, but still cannot explain this to myself - what type of pressure sensor do I need to use and how do I connect it? Does it have to be connected / attached to the hose inflating the cuff? And what is the difference between absolute, differential and gauge pressure sensors, which one should I use?

I am OK with all the other parts of the circuit (or at least I think I am at this stage) - amplification, processing, etc. I do not want to ask my teachers about this, because any questions lead to lower grade - 'cannot work independently, and so on'. I am very excited about the project, and want to do everything right.

So, that's a long post and I realize that I am asking for a lot here. If you have any information/links or whatever you think will help me with my project, I will be very grateful.

Thanks

 

Do you actually have to build a working device, or just come up with a design?

 

I have to build the device. I will be using PIC24FJ128GC006.

 

Absolute pressure sensor measures the pressure between the sensor and vacuum.

Gauge measures pressure between the sensor pressure and atmospheric pressure (100k Pa)

And differential sensor measures pressure between both sides of the sensor.

 

Not sure whether it will fit the bill, What about the small 12v mini compressors from an automotive wrecker, used to pressurize the automatic leveling system, they can usually fit in your hand, just.
You would need 12v but they most likely would be cheap.
Max.

 

Maybe it's useful: https://www.sparkfun.com/products/10398

I think the problem is not to find the motor or turbine, it's everything else.

 

@Mark: I need a motor to pump the air into the arm cuff, like this one :http://qdahead.en.alibaba.com/product/827359699-214417879/3v_5v_dc_micro_laboratory_air_pump.html

And it has to be not as noisy as the vacuum pump... I still do not know what to tear apart to get me such a motor. I'll find out, eventually.
About the pressure sensors - it looks that the gauge is the one I need. However, I still have not purchased the arm cuff, which is this one: http://www.amazon.co.uk/Omron-32-La...-1&keywords=omron+blood+pressure+monitor+cuff
So, I am thinking: If the arm cuff has one hose, I must connect it to the air pump, obviously. I need the pressure sensor attached to the hose as well - it needs to send information to the PIC when the arm cuff is inflated enough, lets say about 160 - 180 mmHg. This means one more hole in the hose. And on top of this I need a valve connected to the same hose for the purpose of deflating (provided that inflating and deflating use the same hose). I am not so sure about this. On conventional blood monitors (manual I mean) one hose is used for inflation/deflation, and another is used for measurement. I hope this would be the case with this arm cuff I want to use, but I'll have to try to obtain this information from somewhere.

So let's say I use the gauge sensor - does this mean I connect it to the hose and it will give me output voltage for the pressure in the hose?

 

The difference between the hose and the other connector, normaly empty (atmospheric pressure).

 

You can buy something like the uA-767 blood pressure monitor for as little as £35.00. This would give you a cuff, pump, valve and pressure transducer, may be the best way to go?

I work in medical field service and may be able to find you something useable. Drop me a PM if I can help.

 

A good teardown should be useful. We'll wait the photos.

 

Just found out that the cuff that I intend to purchase has only one tube, since it is intended to use in electronic monitor. I did some searching and I think I am starting to put the pieces together. This is a wee diagram I think will do the job:

I will need two T-tubes for that. At initiation, the valve will be closed and the MCU will start the air pump, inflating the cuff. When the pressure reaches the upper boundary (as measured from the sensor voltage) the motor will stop and the valve will open slightly to start releasing the air, while the MCU will record the pressure at the specific points (these will come from some analog conditioning). After recording the diastolic value, the valve will open 100% and release all the air.

In theory, that might work. However, I still have to find a suitable motor, valve, cuff, tubes and choose sensor.

- About the motor - I don't want to buy and dismember a new device, first because as a student I am on a budget, and second - these devices usually use surface mount packages (even available for some valves, as I just found out) and will be an ordeal to unsolder and solder them again. Actually, Microchip have a development board for blood pressure monitors using the same chip I want to use, and their motor is pretty small (they don't reveal the made, though). Motors used in commercial devices are also pretty small and noiseless, so I am aiming at something similar. I am setting high expectations here, so I am always aware that maybe I won't be able to find out exactly what I am looking for and must settle for something inferior. But it's worth a try.

-The solenoid valve - I need a valve at 5 V operating voltage, and with controllable speed of deflation through PWM. I found the perfect one here:
http://www.parker.com/literature/Li...on Fluidics Division/UpdatedFiles/X-Valve.pdf
It is three-way, which will allow me to attach the valve and the air pump to it and thus eliminate one of the T-tubes. However, I cannot find its price, which means it will be expensive, and cannot find a supplier in the UK. So, that's that with this option.
It is very important that the solenoid valve will release the air linearly. I have never used solenoids before, so I do not know what types there are, but I can't find a fit.

I don't expect ready-made solutions, but if you have any ideas, throw them in

 

I think Microchip's AN1556 is useful.

 

Hi Mark
That's what I am using to get an idea of the overall design, however, I will be using different approach when calculating the blood pressure (I don't think I will be able to get a hold of all the documents that these guys used to implement the algorithm). See the board that they have? That's a good-looking air pump there. The other thing is that they are using on/off solenoid (not sure if that's the right thing to say, but I still don't know a lot about these things). They are releasing the air immediately after inflating, since they are measuring the blood pressure in the process of inflating the cuff. I will need to measure the pressure in the process of controlled deflating, and that's why I need a controlled solenoid valve.

 

You don't need a solenoid valve with a controllable flow rate (other than on/off). You can set the deflation rate by using a flow restrictor in the tube.

 

couple of circuits here may give you a start.

 

@Alec_t: I was thinking of this, but I want the air to be released immediately after taking the readings, and also the application should have an emergency stop button to release the air if the circuitry fails. This means I would actually need two valves, and one of them will be adjustable, which is what I am looking at now. It adds a little complexity, but I can't think of a way around it.

If anyone can point me to a place to buy such solenoid valves in the UK, it would be great. I found a micro dc air pump on ebay

@Dodgydave: Thanks!

 

Just to make sure - I need a gauge pressure sensor for this circuit, right? If someone has done something similar before, please share some experience. I am going to purchase the opamps and the pressure sensor tomorrow, that's why I am asking.

I have trouble finding gauge sensor, can I use differential with one input not connected?

 

Yes, you can!