problem with piezoelectric sensor..

as a part of my project,digital heart beat counter, in order to sense the heart pulses from the wrist, we selected a piezoelectric sensor.

the sensor is working fine, it detects the pulse in the wrist..
sensor convert pulse to mV range..

but the problem is , the sensor detects even a small movement of hand too..
so in order to, take the pulse correctly , we have to keep the hand still for 1 min..

can any one suggest anything, to avoid this?

thanks

First you need to make good hardware filtering of the signal - reject all frequencies outside of the working range. Then use some techniques to predict the appearance of the pulses and further reject the undesired signals

The PZT sensor will measure movement ; due to the blood through the wrist arteries(the wanted signal) as well as any body movement(unwanted signals). It will be very difficult to discriminate between the stated two groups of signals. Rather use and electro optic (IR) sensor.

A rough and ready way could be to use a second sensor adjacent to the first but NOT on the blood vessel. Then using a comparator the thump of the blood should be more apparent.
Frank

luben111 said:

First you need to make good hardware filtering of the signal - reject all frequencies outside of the working range. Then use some techniques to predict the appearance of the pulses and further reject the undesired signals

Click to expand...

hard ware filtering?
we had expected only high frequency noises..
we included a low pass filter to eliminate all the frequencies above 2Hz..

how ll we design filters to elimate the external movement?

thanks

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klystron said:

The PZT sensor will measure movement ; due to the blood through the wrist arteries(the wanted signal) as well as any body movement(unwanted signals). It will be very difficult to discriminate between the stated two groups of signals. Rather use and electro optic (IR) sensor.

Click to expand...

IR sensor was our first option..
As this was not available in near by stores , we decided to go for piezo..

you have to build very sharp low pass filter, if possible not a single RC but cascaded (more than 20db). To filter the high frequency noises is extremely important. Also after few measurements you need to roughly estimate the pulse rate and reject undesired signals (you need to search for some kind of correlation).

Probably your biggest problem is that the pulse tones amplitude is too small compare to piezo signals caused by mechanical movements.

BTW, there is one method for detecting the pulse (and also blood pressure) when you apply some pressure over the arthery until you hear specific tones, unfortunately this is not for long term use because of the pressure.

The best pulse monitors are like rubber clips with infrared diodes - you put it on the finger and you can monitor the pulse for extended period of time.

luben111 said:

Probably your biggest problem is that the pulse tones amplitude is too small compare to piezo signals caused by mechanical movements.

Click to expand...

ya,exactly..

pulse signals are really small..
when we try to amplify it, the voltage from the external movement also gets amplified..

---------- Post added at 08:38 ---------- Previous post was at 08:35 ----------

luben111 said:

BTW, there is one method for detecting the pulse (and also blood pressure) when you apply some pressure over the arthery until you hear specific tones, unfortunately this is not for long term use because of the pressure.

Click to expand...

we attached a small weight above the piezo,for increasing the pressure above the artery,but this dint improve the result much

No, you need not a small pressure, you need a pressure which is a little lower than the blood pressure - in this case the arthery is normally closed and opens for short time on the pressure jump creating specific noises. Doctors use this technollogy for measuring the blood pressure - when the noises appear for first time it shows that the pressure is equal to the highest level of blood pressure, when they disappear - to the lowest level. The sounds are called Korotkoff sounds - See Korotkoff sounds - Wikipedia, the free encyclopedia