Many People Severely Affected by Disorders Such As Amyotropic Lateral Sclerosis and Cerebral

Abstract

Many people severely affected by disorders such as amyotropic lateral sclerosis and cerebral palsy retain the ability to move their eyes, even though they have lost the ability to control all other muscles. These eye movements create micropotentials, known as electrooculogram (EOG) signals, which can be detected by placing electrodes near the eyes. Once these signals are properly amplified, they can be interpreted by a microcontroller to operate a variety of devices otherwise inaccessible to the severely disabled. This project will focus on using EOG signals to control a wheelchair.

1.0 Introduction

1.1 General Background

A common characteristic of those suffering from such disorders as amyotropic lateral sclerosis (ALS) or cerebral palsy is the ability to move the eyes despite having lost most other muscle control. Fortunately, this eye movement can be detected. As a result, the eyes can be used to control devices that would otherwise require the use of the hands or arms.

It has been shown that a micropotential exists between the front and back of eye due to the concentration of negatively charged neurons making up the retina. This micropotential is shown in Figure 1 below.

Figure 1. Eye micropotential

As a result of this phenomenon, when the eye looks up, the top of the eye becomes more positively charged than the bottom. A similar potential exists when the eye moves horizontally. The micropotentials resulting from this movement are referred to as electrooculogram (EOG) signals, and can be detected by electrodes placed appropriately around the eyes. By using two sets of electrodes to monitor both the horizontal and vertical EOG signals, eye movement can be completely decoded and used to control a variety of devices.

A summer intern and two professors from the Electrical and Computer Engineering Department at Iowa State University have designed a system which uses EOG signals to control an electric wheelchair. This design uses an amplifier/filter circuit and a Mindstorm[1] microcontroller to interpret the EOG signals from two independent sets of electrodes. This circuitry is then interfaced to the wheelchair through the chair's joystick port. Essentially, the EOG signals from the electrodes take the place of the joystick in this design. The current system is battery powered and is designed to rest in the lap of the user.

The current system has had limited success and needs to be redesigned. This project will focus on debugging the old system with a final goal of designing a new system with more accurate control and improved safety features.

1.2 Technical Problem

The technical problem to be solved in this project is to detect small, high noise EOG signals and convert them into control signals for a wheelchair. The system will use a set of electrodes placed above and below the eye to detect vertical movement, and a set of electrodes placed on the temples of the user to detect horizontal movement.

Since EOG signals are in the microvolt range, they must be amplified in order to produce a usable signal for the microcontroller. These signals are subject to noise from both the user’s body and the environment, therefore, a filter must be designed to eliminate noise and present clean signals to the microcontroller.

Once the signals have been appropriately processed, a microcontroller-based system will be used to decode the eye movements. This involves interpreting left, right, up, and down movements of the eyes, as well as handling other events such as an eye blink. The algorithms to control this system will be implemented in the C programming language, and possibly in the assembly language of the processor as needed. The microcontroller will also need to handle the A/D conversions for the input signals and the D/A conversion for the output control signals sent to the wheelchair.

The system will be interfaced to the wheelchair through the chair’s joystick control port. This interface will be designed as an independent circuit that will simulate the joystick’s output to the wheelchair. Ideally, this interface could be customized for a variety of user applications.

A diagram of the system is shown below in Figure 2.

Figure 2. Overview of ocular control system.

1.3 Operating Environment

The operating environment for the system will be indoors. Adapting the system to outdoors would require a weatherproof casing for both the circuitry and electrodes in order to maintain accurate signals. As part of its use in an indoor environment, the system is expected to be subject to rough and jarring movements that are typical of a wheelchair's motion. The mounting of the circuitry and the final packaging of the system will be designed with such conditions in mind.

1.4 Intended Users and Use

The intended users of this system are those who are unable to operate a wheelchair via a joystick with their hands. The system will use eye movement to emulate the joystick of the wheelchair making this device useful to those afflicted by debilitating disorders such as ALS or cerebral palsy.

1.5 Assumptions

The design of the system is based on the following assumptions.

• Sufficient input signal strength to practically control wheelchair

The electrodes are expected to provide EOG readings in the 100μV range. The amplifier circuitry is based on this assumption in order to provide signals to the microcontroller in an expected range. Any signals considerably smaller than this will not be useable by the microcontroller as a result of the present amplifier design.

• EOG signal properties similar for different users

Although the amplifier/filter circuits can be fine-tuned for each user, a basic signal level is expected across users.

• EOG signal readings similar for electrodes

New electrodes are required each time the system is used. Therefore, the signals output from similar electrode types are expected to be the same.

1.6 Limitations

The following limitations have been placed on the design.

• Simple eye movement commands to control wheelchair

The number of movements possible with the eyes limit the number of control signals for the wheelchair. Since complex combinations of eye movements are not practical, the system must be able to provide movement of the chair in all direction, as well as support safety commands, using only a simple series of movements.

• Indoor operating environment

The system will be limited to an indoor operating environment.

2.0 Design Requirements

2.1 Design Objectives

The ultimate objective of this project is to use the eyes to control the movement of a wheelchair. This objective can be broken into the following three areas of work:

• Research electrode capabilities

In order to design an appropriate amplifier and filter for this system, the signals from the electrodes will need to be analyzed. In addition, research into the placement and type of electrode for this system will need to be conducted before any circuitry is developed.

• Debug current system

The previous design group has developed a prototype system utilizing the Mindstorm microcontroller. This device has been interfaced to a wheelchair with some success.

This system will need to be debugged to allow the previous design team the opportunity to test their Mindstorm algorithms. Also, this will provide a test platform for future design revisions.

• Design new system.

The current system is divided into three components: amplifying and filtering circuitry, a microcontroller subsystem, and the wheelchair interface. These three subsystems need to be redesigned in order to extend and improve the capabilities of the current system. A new design will include improving the existing implementation of electrodes, EOG signal quality, signal processing, and interface functionality.

2.2 Functional Requirements

The finished system is required to perform the following functions:

• Full 360-degree movement.

The system must be able to control the wheelchair in all directions. It must translate each eye movement into the appropriate wheelchair movement.

• Safety capabilities.

The system must provide emergency stop capabilities and other safety features. Also, the user is physically connected to the electronics of the system, and therefore precautions must be taken to isolate the user from the high power supplies.

2.3 Design Constraints

The design and implementation of the system is constrained by the following factors:

• Compact

The system must be small enough so as not to inhibit the movement of the wheelchair, and should fit easily on the back of the wheelchair.

• Temperature and shock resistant

The device must be able to withstand quick changes in temperature associated with indoor and outdoor transition. Since the system is mounted to a wheelchair it must be able to withstand rough and jarring movements.

• Safe to use

The safety of the person controlling the system is crucial. The electronics of the system must be isolated from the user and must contain safety features to prevent loss of control of the wheelchair.

2.4 Milestones

Project success will be measured against the following milestones:

• Completed research on placement and type of electrode to use in system. (5%)

In order to progress in any other area of the design, the electrodes utilized in the present system must be fully understood. This includes determining the placement of the electrodes around the eye where the strongest and most accurate signal is measured. Furthermore, the type of the electrodes to be used in the system must be determined. These areas will need to be decided upon early in the design process, as both the amplifier/filter design and the microcontroller program rely on this knowledge.

• Completed current system. (15%)

The overall milestone of the first semester will be finishing the current design. The goal will be to use the existing components and design, with small modifications, to provide a stable test platform for the redesign of the system. In addition, this will allow the previous design team to test their microcontroller code with a working system.

• Completed redesign of amplifier and filter electronics. (25%)

Improvements will be made to the amplifier and filter circuit to increase signal amplification and decrease noise. After the final design has been tested, it will be fabricated on a printed circuit board.

• Completed redesign of microcontroller system. (25%)

A new microcontroller, providing more functionality for the system, will need to be decided upon. A program written in C for the new microcontroller will provide full 360-degree movement, decoding of blinks, and provide emergency safety features.

• Completed redesign of interface circuitry. (10%)

A more permanent interface to the wheelchair will need to be designed. In addition, a general interface for other devices will be investigated. This milestone will be attained when the final interface design has been mounted on a permanent board and an investigation has been completed into other devices the system could control.

• Completed testing and debug of entire system. (20%)

All three components will be integrated and tested. The result will be a complete and working system.

A breakdown of these milestones with respect to their importance to the project overall is shown below in Figure 3.

Figure 3. Milestone breakdown

3.0 End-Product Description

The final deliverable system will provide a wheelchair with the ability to be controlled with the eyes. The system will utilize two independent sets of electrodes taped around the user's head to decode horizontal and vertical eye movement. This movement will replace the use of the joystick to control the direction of travel of the wheelchair. With a single look up, the wheelchair will begin to move forward at its slowest speed. With two looks up in series, the wheelchair will move forward at twice the original speed. Such movement will be allowed in all directions. In addition, a variety of safety features will be a critical part of the system. A series of blinks, for example, would allow the wheelchair to be immediately stopped. This device will allow those who are unable to control most muscles of the body, the ability to operate a wheelchair using their eyes.

4.0 Approach and Design

4.1 Technical Approaches

At the request of the previous design team, this project will be approached by first completing the implementation of the existing design. This will allow for a more accurate test environment for the final design, as well as offer the previous team the opportunity to test their microcontroller code. An alternative method would be to redesign the current system immediately. However, since the current system is close to completion, spending the necessary time to make this system work is a feasible and welcomed approach. This will be the approach taken in the first semester.

Following completion of the current design, revisions will be made to the three main components of the system: the amplifier/filter circuit, microcontroller, and the wheelchair interface. These components will be worked on concurrently. This allows the group to split into smaller teams and concentrate on areas specific to their discipline. A second strategy would be to concentrate on a single component at a time. Since the existing system has well-defined interfaces, it seems logical to split the team up and work on the components concurrently. Furthermore, this will allow potential problems to surface early in the design process.

4.2 Technical Design

The final, redesigned system can be broken into 4 main areas: the electrodes, amplifier/filter circuit, microcontroller system, and wheelchair interface. Detailed descriptions of each of these areas are given below.

Electrodes

In order to design an appropriate amplifier and filter for the system, the signals from the electrodes had to be analyzed. Research into the placement and type of electrode for this system was conducted before any circuitry was developed.

The electrodes offering the best signal were found to be the A10 and the A7 from LeadLok[2]. These electrodes offered the strongest signal with the least amount of noise from the user and environment. The placement of the electrodes giving the best signal was found to be directly above and below one eye for detecting vertical movement. Horizontal eye movement was most effectively detected from electrodes placed on the temples. This placement is illustrated below in Figure 4. Notice that a fifth electrode, attached to the mastoid bone behind the ear, is tied to the common of the amplifier circuit.

Figure 4. Electrode placement

This electrode type and placement provides a signal of approximately 100μV, considerably larger than what was expected. The expected range was estimated at 10-20μV. This information was critical to the redesign of the amplifier and filter circuits.

Amplifier/filter circuit

The purpose of the amplifier/filter circuit is to process the raw electrode waveform into a signal that can be successfully decoded by the microcontroller. Using data retrieved from electrode research, the EOG signal resultant from an eye movement is on the order of 100μV. This noisy signal must then be processed and amplified to a reasonably sized signal (several volts) to the microcontroller. The basic block diagram of this system can be seen below in Figure 5. This design has been applied to the current system and will also be used, with improvements, in the new system.

Figure 5. Amplifier/filter block diagram

In the following section, each component of the amplifier/filter circuit will be described, then possible improvements will be mentioned. A single channel of the current design can be seen below in Figures 6 and 7. Figure 6 includes all components seen above in the block diagram, except for the voltage converter, which is shown in Figure 7.

Figure 6. Schematic of amplifier/filter circuit (single channel)

The first component in both the block diagram and the schematic is the Burr-Brown instrumentation amplifier. This is a variable gain differential to single-ended amplifier with a common mode rejection ratio around 90 dB (with gain = 100). This component allows for excellent amplification, while dampening the noise associated with the human body. Using the eye signal as an input, this component has been able to amplify the EOG signal to the 20-25mV range while letting through only small amounts of 60Hz noise. This component seems to have no problems at this point, for this reason this part of the system is predicted to see no changes in the final design.

The second part of the system is a high-pass filter. This is realized as a simple resistor-capacitor filter in Figure 6. The current system places the cutoff frequency of the high-pass filter at around 0.16Hz, removing the DC component of the Burr-Brown output. This DC signal results from the offset of the instrumentation amplifier and a potential that exists on the human scalp. Without this high-pass filter, the DC signal would saturate the next amplification stage. This filter is mainly used to remove a DC component, therefore the current RC filter will continue to work in the new design.

The third and fourth components of the system are built out of a single LM385 chip. This is realized by using one opamp as an amplification stage and the second as an output DC level shifter. Both the gain of the first stage and the offset of the second stage are made adjustable through the use of potentiometers. This allows the system to be tweaked for use on different body types, which may require more or less gain at changing body potentials. Although this component seems to be working properly, it will need to be modified to interface with the HC11 microcontroller instead of the Mindstorm. Currently, to interface with the Mindstorm, this component outputs a 2V (peak-to-peak) signal around a 3V DC offset. To interface to the HC11, this signal will change to a standard 0-5V input. Other possible improvements could also be made to make the calibration process automated. By removing the potentiometers and adding voltage controlled feedback circuitry, the microcontroller could adjust the gain and offset of the amplifier/filter circuit to provide the standard 0-5V input without the user needing to adjust the circuit.