Download Design Goals

EEG Biofeedback
Design Report
Adrian Smith, gte198f
Daniel Shinn, gte539f
Ken Grove, gte262f
ECE 4006 - Group N1
Spring 2002
March 19, 2002
Georgia Institute of Technology
College of Engineering
School of Electrical and Computer Engineering
Abstract
The objective of this project is to continue the work of previous groups and design
a system that will ultimately be able to control a remote vehicle using brainwaves. From
the digital aspect we are not necessarily concerned if the control signal is a brainwave,
rather the focus is on the identification, process, and assignment of these signals to
specific commands. In order to achieve these plans, muscle movement signals will be
used. This paper discusses our revised design that will be fine tuned as we progress into
the project.
Design Goals
The digital EEG design groups have 3 primary goals for the overall project. The
first goal is to test and understand the design produced by a previous group. This will
require our group to be familiar with how they designed and built the amplifier. During
this process we will compile a bill of materials for ordering parts to replicate the design.
This will need to be completed early in the design process so that we can have a working
test bed to base our continued work upon. The second primary goal is to become familiar
with the operation of the analog-to-digital converter that has been used in the past to
control a mobile vehicle. Part of this goal will be to relocate the board to a computer that
will be available for use after this semester. The group will also look into the feasibility
of replacing the existing card with newer PCI card technology. The final goal of the
project is to interface the amplifier with the A/D card and process the data collected.
During this portion of the project we will begin to interrupt input signals as different
commands and time permitting we will output control commands to a mobile vehicle.
The Amplifier Board
The amplifier that was built in a previous semester was designed following the
specifications furnished by the founder of Brainmaster Thomas Collura. His design
called for a 2-stage amplifier, which was supplied with power by a 7805 voltage regulator
circuit and a midpoint voltage circuit.
Stage 1 of the amplifier, shown on the left side of Figure 1, consists of a high
impedance amplifier with a gain of 50. It also has a high Common Mode Rejection
Ratio. This stage provides noise reduction and signal centering for the higher
amplification of the second stage. The OP-90 in stage 1 is used for baseline-correction.
Figure l. Amplifier Design
Stage 2 of the design uses an OP-90 to amplify the signal by 390 times. This
allows a very weak EEG signal to be digitized and analyzed on a computer. A voltage
ranging from 5 volts to 36 volts powers the amplifier. This is accomplished through the
addition of a power supply circuit for a clean and level voltage supply. The 7805 chip
accepts the varying voltage between 5 and 36 volts and dissipates the excess voltage to
always output a clean 5 volts. Capacitors are also used in the power supply design to
make the output more stable. This allows the circuit to be powered through the use of a
9-volt battery. The power supply also contains another OP-90 in combination with
resistors to obtain 4 and 2-volt supplies for use in the amplifier. Figure 2 displays the
overall specifications for the amplifier.
Type:
differential
Inputs:
(+), (-), and body ground
Gain:
19,500
Bandwidth:
1.7 - 34 Hz
Input Impedance: 10 Mohms
Signal Input Range: 200 uV full-scale
Signal Output Range: 4 volts: from 0.0 to 4.0 volts
Resolution:
0.80 uV/quantum
Input Noise: < 1.0 uV p-p
CMRR:
> 100dB
Power Requirements: 5 to 36 Volts
Figure 2. Amplifier Specifications
Figure 3 contains the bill of materials needed to make a signal amplifier of this
design. Items that are needed in addition to the components for the physical amplifiers
include a 9-volt battery connector, pre-holed circuit board and a set of 3 conductor signal
leads.
Resistors:
(1) 10K 1/4W 5%
(2) 1K 1/4W 5%
(3) 130K 1/4W 5%
(2) 200K 1/4W 5%
(2) 10M 1/4W 5%
(2) 200K 1/4W 5%
(1) 51K 1/4W 5%
Capacitors:
(1) 0.47uF 400V polypropylene (P474J)
(3) 0.1uF 400V polypropylene (P104J)
(2) 0.001uF 400V polypropylene (P103J)
(1) 10uF 6.3VDC Tantalum
Integrated Circuits:
(3) OP-90 amplifiers
(1) 620AN amplifier
(1) LM7805C voltage regulator
Other:
(1) Set of 3 conductor signal leads
(1)of
Pre-holed
Figure 3. Bill
Materialscircuit board
(1) 9 Volt Battery Holder
Although mostly 1% resistors were used on the board that is currently built, a
Spice simulation showed that 5% resistors could be substituted instead. The resistors
used in the creation of 2 and 4 volts may need to remain 1% resistors to more closely
achieve 2 and 4 volts. There are enough 1% resistors currently in stock to avoid ordering
them for our version of the amplifier. If the board does not perform like the current
board, these resistors will be used and possibly ordered for the other groups. The
components have been sourced through several distributors, but a significant portion can
be acquired through Digikey.
Given the EEG amplifier design, we can proceed with the design of capturing the
signal and manipulating it to our needs. The proposed design for this project is illustrated
in Figure 4. This design captures the signal from the EEG amplifier and manipulates it
using a command interface to control a mechanical device.
Figure 4. Proposed design for EEG signal manipulation.
The Analog-Digital Converter
The existing board is a Keithley DAS-1701ST. However the computer that it is
installed in is borrowed and may not be available for further use. The card can be moved
to another computer but lacks the newer PCI interface. The solution to this dilemma is to
order a new card. The data acquisition board optimal for this application is the Keithley
KPCI-3107 board. The board contains all the necessary elements needed for proper data
capture.
The Keithley KPCI-3107 board has a PCI interface for fast data transfer to the
microprocessor of the computer. It has a sampling rate of 100kSamples/second. This
will enable us to not only obtain a clear sample of the desired low frequency signal, but
also manipulate the high frequencies that were not filtered out in the transfer of the signal
from the EEG amplifier to the interface which connects via cable to the PCI card. This
can be seen in Figure 5 below.
Figure 5. Photo of Keithley KPCI-3107 Data Acquisition Board.
The key features for this board are listed below:
Maximum sample rate of up to 100kS/s
16-bit inputs: 16 single-ended or 8 differential inputs
32 digital I/O lines
24 software programmable ranges
12 gain ranges (1, 2, 4, 8, 10, 20, 40, 80, 100, 200, 400, 800)
Extensive triggering options
DriverLINX™ and TestPoint™ software drivers
LabVIEW™ VIs
VisualSCOPE™ digital storage oscilloscope
ExceLINX™ (Excel add-in)
There are many advantages to this data acquisition board compared to competitors
other than its price ($680 with educational discount). The primary reason for choosing
this board is to stay with the Keithley brand and retain the ability to reuse the software
code written by the previous group. It also features an AutoZero capability that filters out
drifts in the signal acquisition. In addition the board allows assignment of a gain specific
to each channel, which can improve the flux in gain associated with the electrode or the
EEG amplifier. Data cables and breakout boards have recently been ordered.
The construction of a new lab interface box is also needed. The current box
features a metal case with the breakout board mounted inside. Cold connections are
available but instead they are wired to female banana plug connectors. This allows any
type of input to be used and changed quickly and easily. For our purpose alligator clips
are used to connect to the electrode pads. This simple and effective design can be
duplicated for the new box. The only difference is that there will be two breakout boards,
one for analog I/O and the other for digital I/O. Because of time restraints it will be
unlikely that we will be able to build such a box. This may be left for future groups to
design and develop.
Software
Drivers for DriverLINX are included in the software bundle that comes with the
KPCI-3107 board. DriverLINX is a program which can create DLLs for the data
acquisition and route the signals to other applications or send a digital pulse through the
KPCI-3107 board. The interface for DriverLINX can be programmed in C, C++, Visual
Basic, and Active X. Thus when an analog signal is seen by the A/D board, the user can
program a trigger to manipulate any interrupt or application on the computer or send a
signal of 1 (5Volts) through one of its 32 digital outputs. This is key for the transmission
of output to the mechanical device. Full documentation is included with the software
package and programming a driver under DriverLINX seems like a simple task. The
main hurdle for this design is what to do with the new redirected digital signal after the
A/D board is triggered. We must familiarize ourselves with the code written by the
previous group.
Sources
http://www.ece.gatech.edu/academic/courses/fall2001/ece4006/Mind/group1/index.html
This is the previous neural group’s website. It contains all of their work and several
useful links under the resources heading.
http://www.ece.gatech.edu/academic/courses/spring2002/ece4006c/N1/
This is our group’s website. Links can be found under the resources and websites
headings.
www.keithley.com/
Keithley’s homepage. Information about the 3107 data acquisition card as well as the
data cables and breakout boards can be found here.