First I want to thank all of you who commented on my last week update; DAB, good suggestion - I researched some commercial head sensors and what they are used for, and it looks like a lot of research groups are looking into processing brain waves or various applications. Paul, thank you for your comment – yes, I can measure the movement of eyes and I am showing some signals below. Thank you vsluiter for your comments – yes, facial muscles are driven by electrical signals that are not EEG signals, but I think they are still generated by the brain for the purpose of moving muscles. I learned that other signals measured in EEG are related to various other functions of the human body, some are related to movements, others to thinking activities, others to controlling various organs, and others to communicating information throughout the human body. For this product I want to first use a simple to measure and reliable signal to develop the algorithm that decodes the measured signals and sends the control information to an appliance (still thinking what to use at first). After this step I will expand to other types of appliances, and I will investigate other types of brainwave signals that I can use - I want to look into building capacitive electrodes that can be applied anywhere o the scalp even above hair, and if I can get them to work I will then try to expand and diversify the types of brainwave signals used to control appliances. So most probably for now I will use some signals that control muscle and eyes movements, which I think I will be able to measure with dry electrodes without gel and skin preparation. This first approach makes the product easy to use and thus increases the chances to success. Also, thank you cy.gul for your comment about the features implemented in PsoC 5LP; I was thinking and planning to investigate this when I read in the user guide for the PsoC 4 pioneer kit about the additional PsoC 5 on this board and how this PsoC 5 can be programmed. Thank you for pointing out the additional capabilities of PSoC 5 – yes, the digital filter and also the sigma-delta converter (that I read about in the datasheet) are important advantages that I should look into.
So this weekend I focused on three things: #1 I improving the active electrodes, #2 deciding on the location of electrodes on the head, and #3 evaluating what signals I can measure and use for appliance control.
For #1, most of this work was analog circuit design and experiments trying to filter out noise, to compensate for common mode drift, and to reduce the dc offsets. This was a tedious work building a few circuits and performing various measurements. Overall I am happy to see that the analog signals are now stable over time and are not so sensitive to body movement. The current circuit that I am using now is still in “experimental” form but it is close to the final version, so I am still planning to investigate some ways of improvement. One of the things I have in plan is to replace the gel Ag-AgCl electrodes with dry electrodes. To do this I need to use a very high input impedance amplifier, which I ordered and received now, so next step will be to create a signal amplifier for dry electrodes. This will probably take most of my next weekend.
For #2, I looked at many possibilities: EEG sensors placed throughout the scalp need gel and need special preparation of the skin (they are like little needles that are pushed into the skin). I think these electrodes may work if applied correctly but I don’t see them so practical from an appliance control product perspective. Another alternative would be electrodes that can be pressed against the scalp using an elastic cap but they also need some gel to insure low impedance to the skin. These types of electrodes are easier to implement in this product but I still think that they are somehow inconvenient to use. After I investigated a few other types of electrodes I decided to use dry electrodes applied on the forehead. The main advantage of this choice is that there is no hair so I can use metallic electrodes applied directly to the skin. There will be 6 electrodes placed in a row just above the eyebrows. In the picture below I am wearing three electrodes above the left eye; this is an experiment setup – the following version will have another three electrodes placed above the right eye.
For #3, I have measured various signals with these three electrodes, and so far I see a few possibilities of signals that I can use for controlling appliances:
Selective blinking of one eye – each group of three electrodes can pick up the electric signals associated to eye blinking. Below I am showing the right-eye blinking compared to the left-eye blinking. It can be noticed that the three electrodes that I am wearing above the left eye can differentiate which eye I am blinking: left-eye signal has an amplitude of about 14mV while right-eye blinking signals have an amplitude of about 4mV. An identical group of electrodes that I will place above the right eye will measure higher amplitude on the right-eye blinking than on the left-eye blinking.
Eye movement – I was able to measure the horizontal movement of both eyes, and I could differentiate the direction of moving from the polarity of the measured pulse. Here are some examples; when I move the eyes towards the left side I measure a negative pulse and when I move the eyes towards the right side I measure a positive pulse.
Eye rotating – if I rotate the eyes slowly a full circumference I measure a signal that looks like a sinusoid. The direction of rotation, clockwise or counter-clock wise, determines the polarity of the “sinusoidal” signal. Here are two pictures showing the signals I measured for both rotation directions:
Clockwise rotation of both eyes:
Counter clockwise rotation of both eyes:
So what’s next? I want to finalize the electrodes and amplifier and then I will need to implement a low pass filter and configure the analog to digital converter. After that I will start working at the algorithm of controlling appliances using these signals. I hope to complete the analog path in the next two weeks, which will leave me about one month and a half for the algorithm and the controlling transmitter.
So until next week update I wish all of contestants good luck with your projects,
Cosmin
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