I'm going to develop an Intelligent Lie detection system for my final year project.Please help me with these questions?

[Irshu]

I'm going to develop an Intelligent Lie detection system in labview.but which sensors should i use to acquire blood pressure, heart beat and respiratory signals?since i'm new to Labview i dont have an idea about what DAQ hardware to use.i preffer to have a USB interface for the DAQ hardware which i think is very cheaper.does it cost a lot of money to develop this system? i cannot afford more than $500.please help me to initiate my Project.also can i use one DAQ hardware to connect 4 sensors to monitor the signals?please help me.

[Steve.J]

Hi Irshu,

Usually lie detector systems rely on skin conductance, heart rate, and respiration signals. Getting blood pressure in real-time is difficult and invasive (usually only done in a hospital setting).  Please check which signals you really need and understand how you will analyze these biosignals to determine truth/lie in a response.

LabVIEW is an excellent tool to make such a system and our Biomedical Starter Kit (available free here: http://decibel.ni.com/content/docs/DOC-12646 ) will help you get started quickly with an example program for acquiring signals with the datalogger program.

For hardware you will be able to use one of NI's low-cost USB devices like the USB-6009.  The biosignals that you are acquiring and measuring are relatively slow signals that you can successively digitize at 500 samples/sec or even slower in the case of skin conductance or respiration.

The biggest challenge will be the sensors and amplifiers or signal-conditioners that you will need to interface the sensors to the USB DAQ device.  NI resells some biomedical sensors from Vernier that could work for you.  The key is to find a sensor or sensor/amplifier combination that outputs approximately 1-5V signal that can be directly connected to the USB DAQ device.  You can also find circuits on-line that will provide the sensor interface circuit if you are willing to build some simple electronic circuits yourself.

[Irshu]

Dear Steve,

first of all, thanks for giving me the link for your Biomedical starter toolkit.

why is that measuring blood pressure is difficult?actually blood pressure analysis is important to determine the maximum accuracy of the proposed system.i found blood pressure sensor in vernier but it costs $200,its kind of costly because i need to purchase three sensors.sweat detection sensor is just a volt meter.do you have any link where i can purchase the BP sensor for more affordable rate?

thanks for helping me out, i really glad to join this group. i hope i can get help if i needed during my project progress.

thank you.

[Steve.J]

A non-invasive blood pressure measurement (using the inflatable cuff that goes around your arm) can only make one measurement every few minutes at best.  This is not practical for use in a lie detector system where you are looking for an immediate psycho-physiological response provoked by a question.  A real-time continuous blood pressure measurement typically requires direct intra-arterial access (a catheter inserted into an artery) connected to a pressure transducer.  Obviously, this is not something that is done except in a clinical setting with very qualified personnel and the proper hospital-grade equipment.

[LuI]

A non-invasive oszillometric BP detector shall not measure more frequently than every 15 minutes over day and not more than every 30 minutes at night in order to avoid damages to the tissue at measurement side.

One can do relative measurements using pressure transducers tightly connected to the surface of ones extremity near a position where an artery is routed. But the absolute value from such measurements depends on the pressure of application of the sensor to the body.

And than there is the pulse-wave transit time measurement method, relying on the fact that pulse-wave velocity in the vessels depends, amoung others, from actual BP value (Moens-Korteweg formula).

But AFAIK (an I did an intensive research in ths area a few years ago) no-one was able to get reliable results in the fields, as there are so much influence factors that can not all be controlled for such measurements.

HTH!

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Uwe