Download Neural Impulse Control Design

GEORGIA INSTITUTE OF TECHNOLOGY
College of Engineering
School of Electrical and Computer Engineering
ECE 4006
Senior Design
Neural Electrical Impulse Control
Design report
Spring 2002
Jonathan James
Matthew Morgan
Da’Janel Roberts
Date Submitted: Feb. 12, 2002
Neural Impulse Control Design
Objective
This section of our report is to describe the electronics we plan to build so as to actualize the
goals of this project. Our main goal is to build an amplifier that will magnify neural impulses enough
for them to be measured and interpreted. We have used the information from our background
research to see what frequency range we should operate in and the gain we require. Our plan to
actualize our goal is as such:
1. Build at least one copy of the board made by last semester’s group 1 (Fall 2001) using the
brainwave schematic (option 1).
2. Formulate tests to see if we are actually getting brainwaves from it. More than likely we will
be using different filters (bandpass, notch) so as to see if we are getting readings in the right
frequency ranges.
3. If the board does not function as expected then we plan to use the amplifier design of second
group of last semester and test it to see if it functions as required (option 2).
4. If neither of the two designs works satisfactorily then they will be tweaked so as to improve
performance if possible. If this is not possible then a new design will be created and
implemented (option 3).
5. Once a board is created that is able to read brainwaves and output a somewhat clean signal,
the output will be sent to an A/D converter and then the device will be handed over to the
Digital group working on this same project. This will allow them to be able to have a signal
to work with, make sense out of, and bring us closer to our final goal of Neural Impulse
control. Extra boards may be built to provide more channels for the device for at the moment,
option and option 2 only have one channel per board.
Using previous tested designs we hope have less setbacks and errors during this semester (Spring
2002) and thus speed up the time it would take to have a working prototype of the device reading the
neural impulses. We have also considered buying a prefabricated device but have decided against it
due to the high prices of such units.
Electrodes
Amplifier
Filters and A/D The Digital Group
converter
Figure 1. Overall System and plan for the semester (1st draft).
We believe this is a more manageable goal for this semester and we are able to carry this plan out
before the end of the semester, new goals will be set and worked towards.
Option 1 (Brainmaster, Group 1 Fall 2001)
This amplifier uses the plans for an old version of the brainmaster, http://www.brainm.com,
developed by Thomas Collura. The circuit is a two stage amplifier as can be seen below:
Figure 2. Schematic of the two stage amplifier courtesy of the previous group and
Brainmaster.
The AD620 component is a high quality instrumentation amplifier which is relatively
inexpensive that can deliver up to a 1000 fold gain. This high level of gain is required for a device
such as this due to the fact that the neural impulses will be very weak when being measure from the
skin. The OP-90 component is a general purpose op amp.
Quoting from the brainmaster plans, “The input amplifier IC-1 is an Analog Devices AD620
instrumentation amplifier, set up with a gain of 50. This gets the signal "out of the noise," and
provides high input impedance and high common-mode rejection. The "AC" coupling due to R4 and
C4, or R5 and C5, occurs with a long time-constant, and does not limit the low-frequency response.
It also does not affect the CMRR, since it is not in the passband. It does, however, allow the inputs
to IC-1 to be biased into the middle of their common-mode range. The amplifier IC-2 is used to
provide an integrator, used as a low-pass filter developing the reference for IC-1. This results in a
baseline-correction that produces a low-frequency cutoff at 1.6 Hz. It also allows the output of IC-1
to operate near its center, providing good linearity. The second stage provides a gain of 390 and a
frequency response to 34 Hz. The first stage is fed to the non-inverting input of an Analog Devices
OP-90 operational amplifier. The gain of the amplifier is set to 390, in the non-inverting mode. R10
is where to select the gain you want, or to put a potentiometer for adjustable gain.”
Table 1. Brainmaster circuit specifications.
Circuit Specifications:
Type:
differential
Inputs:
(+), (-), and "ground" return
Gain:
20,000
Bandwidth:
1.7 - 34 Hz
Input Impedance:
10 Mohms
Input Range:
200 uV full-scale
Output Range:
0-4 volts
Resolution:
0.80 uV/quantum
Input Noise:
< 1.0 uV p-p
CMRR:
> 100dB
The amplifier requires a clean 5V source. To provide this, a simple circuit can be used as can be seen
below.
Figure 3. Circuit for a clean voltage supply.
The 7805 component is a readily available regulator and B1 refers to a battery or an array of
batteries to provide the voltage.
This circuit has three electrodes, to get one channel. Two of them are inputs (active and
indifferent). These would be connected to one side of the head. The amplifier is creates a differential
reading between them (gain of 10,000). The electrode is the AGND and can be placed anywhere on
the body. For reduced noise, a 0.015 uF capacitor will be placed in parallel with R10 (see Figure 2).
Also, R10 can be replaced by a potentiometer so as to be able to vary the gain.
Option 2 (previous semester’s original design, Group 2 Fall 2001)
The previous groups answer to the high gain device needed was an instrumentation amplifier
whose gain was 86.02 dB.
Figure 4. Instrumentation amplifier used to amplify neural impulses.
The CMRR of the instrumentation amp was found to be 116dB, which is pretty high and thus well
suited for this sort of application. The output of the amp was somewhat distorted due to higher
frequency noise so a solution they found was to use a low pass Butterworth filter to clean up the
output.
Figure 5. LPF Butterworth filter used to clean up the output signal.
From the report given by the group, this cleaned up the signal quite well and allowed for a clean
amplified signal. For component parts and prices for Option 1 and Option 2 see Appendix 1.
Testing
After building the amplifier we will verify that it functions properly. In order to do this we
will use the following procedures:
1.
Test the board using different signals that approximate brainwaves (perhaps only in
frequency content) and observing the output if any.
2.
Test the board with a sine wave to see if the output is amplified as it should be and whether
any distortion is present.
3. Test the oscilloscope we will be using with a sine wave and seeing if the frequency content it
picks up is correct at the low frequencies we will be dealing with.
4.
Applying electrodes to areas of the body where a signal should be obtained (e.g. the heart,
the eyebrows, etc.) and observing the output.
5.
Applying electrodes to different parts of the head and doing an FFT and seeing the frequency
content. Will they match up with our background research finding?
6.
Taking measurements under different conditions (sleepy or asleep, under caffeine, etc.). Do
we get results that correlate to our background findings for the different type of brainwaves and their
frequency content?
When we are convinced the amplifier works properly, we will use more specific bandpass
filters to extract the various brainwaves from the amplifier signal. Once we see which features we
can extract from the signal, we will be able to figure out a viable control scheme.
One possibility could be to use the ratio of the energies between high frequencies (Beta
waves) and medium frequencies (Alpha waves). Beta wave activity increases when someone
concentrates, and so the high frequency energy should increase. Alpha wave activity would decline,
and so the ratio of Beta to Alpha should rise significantly. Such an increase could be used to issue a
'move forward' command. The only other command needed for full control would be 'rotate', and so
during testing we would look for another feature independent of Beta to Alpha ratio. The signal's
peak-to-peak amplitude change during an eyebrow movement is potentially one such feature.
After extracting our features, we could use a simple comparator circuit to generate two
discrete control outputs. These outputs could be interfaced to the remote control of a standard offthe-shelf RC car.
Appendix 1: Parts List
Part Number
Number of
parts
Supplier
Cost per
($)
C
4
http://www.digikey.com/
.70
R
8
http://www.digikey.com/
.27
Op-90
2
http://www.pios.com/
2.50
Ad620
1
http://www.pios.com/
5.90
7805
1
http://www.pios.com/
11.30
C
2
http://www.digikey.com/
.70
Option 1
Option 2
High quality
op amps (e.g.
op-90)
R
4
http://www.pios.com/
2.5
11
http://www.digikey.com/
.27
C
4
http://www.digikey.com/
.70
Proto board
3
http://www.mpja.com/
5.95
Electrodes and
wires
3
http://www.mpja.com/
5.00
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