Adaptive Control of Foot Orthosis - DiVA portal237830/FULLTEXT01.pdfAdaptive Control of Foot Orthosis 1 Introduction There are millions of individuals with gait disabilities, requiring

Technical report, IDE0704, January 2007

Adaptive Control of Foot Orthosis

Master’s Thesis in Electrical Engineering

Ali Manzoor, Hesham Elkhbai, Ziad Ekwaneen

Supervisor: Ulf Holmberg

School of Information Science, Computer and Electrical

Engineering

Halmstad University


Master’s thesis in Electrical Engineering

School of Information Science, Computer and Electrical Engineering

Halmstad University

Box 823, S-301 18 Halmstad, Sweden

January 2007

Preface

First and foremost we wish to thank our parents for their utmost moral support and freedom we

needed to move on and to achieve this milestone. We would also like to thank our advisors Ulf

Holmberg and Wolfgang Svensson, who were always supportive since the days we begin

working on this project. They have supported us not only by providing a research assistantship,

but also academically and emotionally through the rough road to finish this thesis.

Ali Manzoor, Hesham Elkhbai & Ziad Ekwaneen

Halmstad University, January 2007

Abstract

Major problems of the Foot Drop treatment are expensive and complex solutions. This work

presents the performance of a new inexpensive method named as Semi-Active Ankle Foot

Orthosis (SAAFO). The concept of this approach is to use inexpensive sensors to detect foot step

movement. The signals from the sensors afterwards will be fed to a control system of SAAFO in

runtime for a smooth foot movement of a drop foot patient while walking. Different sensors have

been studied in detail along with comparison to the proposed sensor system and mechanical

design. The signals from the sensors are used to detect different phases of human walking. These

sensors are placed at different positions on an orthosis and their signals are studied in detail.

Experiments have been done in different conditions to get a realistic picture either this assembly

can be implemented commercially. Signals are plotted and discussed yielding that the human

walking phases can be easily and accurately detected using inexpensive sensor assembly.

Tables of Contents

1 Introduction.................................................................................................................................1

1.1 Foot Drop...............................................................................................................................1

1.2 Aim of the Project.................................................................................................................3

1.2.1 Motivation.....................................................................................................................3

1.3 Background...........................................................................................................................4

1.3.1 Human Gait..................................................................................................................4

1.3.2 Passive Ankle-Foot Orthosis (P-AFO).......................................................................5

1.3.3 The Active Ankle Foot Orthosis (AAFO)..................................................................6

1.3.4 Semi-Active AFO (DACS-AFO).................................................................................7

2 System Overview.........................................................................................................................9

2.1 Sensors....................................................................................................................................9

2.1.1 Sensor Selection.............................................................................................................9

2.1.2 Sensors used in AFO.....................................................................................................9

2.1.3 Strain guage sensors...................................................................................................10

2.2 Sensor placement.................................................................................................................13

2.2.1 How strain guage used in project..............................................................................14

2.3 SSAFO Design......................................................................................................................15

2.3.1 Physical componant of SSAFO..................................................................................15

2.3.2 SSAFO principle of work...........................................................................................18

3 Experimental Methods..............................................................................................................20

3.1 Walking horizontal..............................................................................................................20

3.2 Walking up-hill....................................................................................................................21

3.3 Walking down-hill...............................................................................................................21

4 Results and Discussion...............................................................................................................23

5 Conclusion..................................................................................................................................32

References


1

Introduction

There are millions of individuals with gait disabilities, requiring either rehabilitation or

permanent assistance. In the US alone, approximately 866,000 people use an orthosis on a lower

extremity. Current assistive methods are usually labor intensive, highly variable, or require

constant tuning. Applying robotic technology to gait could provide a means to quantify lower

body kinematics and kinetics, improve and monitor locomotion during normal and clinic uses and

helps reduce the high-energy cost of disabled locomotion. Further, a robotic assistive device

could adapt to human gait, which is consistently changing in both speed and form. In initial work,

adaptive hip and knee orthosis have been able to provide more natural gait for the disabled [1].

1.1 FOOT DROP

Definition

Foot drop is a weakness of muscles that are involved in flexing the ankle and toes. It

describes an abnormal neuromuscular (nerve and muscle) disorder that affects the patient's

ability to raise their foot at the ankle. Drop foot is further characterized by an inability to

point the toes toward the body (dorsiflexion) or move the foot at the ankle inward or

outward. Pain, weakness, and numbness may accompany loss of function [2]. Walking

becomes a challenge due to the patient's inability to control the foot at the ankle. The foot

may appear floppy and the patient may drag the foot and toes while walking.

Figure 1-1: Drop Foot

Treatment

The type of treatment is dependent on the underlying cause of drop foot. Foot drop that

cannot be treated by surgery is often treated using the stimulation of the affected nerves.

The stimulation is applied as the foot is raised during a stride and is stopped when the foot

touches down on the ground. Some patients may be fitted with an Ankle Foot Orthosis

(AFO), brace, or splint that fits into the shoe to stabilize the ankle/foot.

Introduction

2

Recovery and rehabilitation

Depending on the nature of the cause of foot drop, recovery can be partial or complete.

Physical therapy and an ankle foot orthosis device worn in the shoe are important aspects of

rehabilitation. [3]


3

1.2 Aim of the project

This project is aimed to use a special orthosis device. This device will help to provide normal

range of motion to the foot and ankle during walking.

Motivation

The major motivation behind this project is to assist those individuals with gait disabilities. The

goal is to build lightweight, portable and easy controllable adaptive assistive device for Foot Drop

disability. To achieve this two main parts are suggested.

1. Detecting the human walking gait phases. It will be achieved by using simple sensor

structure, strain gauge sensor, to measure the force exerted by foot while walking.

2. Control signals depending on these sensor signals will be used to control the mechanical

structure and to keep the walking as smooth as possible.

3. Magneto-Rheological (MR) Damper technology with controllable damping

characteristics will be used in this project, to generate the braking strategy for the orthosis

structure in the suitable position, which has not been used so far for this purpose. It will

help to operate the Ankle Foot Orthosis (AFO) as a semi-active ankle foot Orthosis [SA-

AFO].

Introduction

4

1.3 Background (Related Works)

1.3.1 Human Gait

Definitions:

To explain the idea of the human gait, some technical terms are needed to be explained

first.

Stride Cycle is the time interval of occurrence of repetitive events of walking, i.e. from

initial heel lift to the following initial heel lift.

Dorsiflexion/Plantar Flexion is act of bending a joint; especially a joint between the bones

of a limb so that the angle between them is decreased. [12] In this case bending of the limb

with respect to the foot.

Gait is a particular way or manner of moving on foot. Walking and running are the two

basic human gaits [13]

Gait Cycle is defined as the time interval between two successive occurrences of one of the

repetitive events of walking. Although any event could be chosen to define the gait cycle, it

is generally convenient to use the instant at which one foot contacts the ground [14]. The

gait cycle consists of Stance and Swing. Stance is the period in which the foot is in contact

with the ground and Swing is the period in which the foot is in the air. Stance comprises

60% of the walking cycle. While 40% is the swing.

The following terms are used to identify major events during the gait cycle:

1. Initial contact

2. Opposite toe off

3. Heel rise

4. Opposite initial contact

5. Toe off

6. Feet adjacent

7. Tibia vertical

These seven events subdivide the gait cycle into seven periods, four of which occur in the

stance phase and three in the swing phase. [14]


5

Figure 1-2

The duration of a complete gait cycle is known as the cycle time, which is divided into

stance time and swing time [14].

There are four major objectives for the design of any Orthosis [1]:

• Restoration of normal function and ability to:

control of motion

correction of deformity

compensation for weakness

• Make it as comfortable to wear as possible.

• Minimize the abnormal appearance of the Orthosis.

• Lightweight, thickness of sensor, accuracy, range, and speed.

Most advanced Ankle Foot Orthosis (AFO) has not been able to improve on all four

objectives from the basic AFO.

There can be many different ways to treat Drop foot problem as mentioned before. Below

are explained main methods, which have been used so far.

Since the project is divided into two parts, sensor part and actuating part , previous projects

will be mentioned on these two criteria , the actuating part will consist of : passive , active

and semi active actuating system and the sensor part will consist of : Force sensors ,

pressure sensors , special sensors ( which depends on each research method) and the strain

gauge sensors .

1.3.2 Passive Ankle-Foot Orthosis (P-AFO)

Most orthosis devices provide passive support to the Ankle –Foot Orthosis joint, Passive

Ankle-Foot Orthosis (P-AFOs) constitute a special class of ankle braces designed to

Introduction

6

enhance gait function by providing natural support to the lower limb as it progresses over

the stance foot. However, a fundamental design characteristic of all P-AFOs is that they are

not capable of replicating all dynamic characteristics of the natural ankle complex.

Standard Ankle-Foot Orthosis (AFO)

An ankle foot Orthosis (AFO) can be defined as “a medical mechanical device to support

and align the ankle and foot, to suppress spastic and overpowering ankle and foot muscles,

to assist weak and paralyzed muscles of the ankle and foot, to prevent or correct ankle and

foot deformities, and to improve the functions of the ankle and foot.” [5]. However, the

model currently used is still a basic mold made out of polypropylene or metal-alloy made

to fit the ankle of the patient (Fig.1-3).

The major advantages of polypropylene AFOs are that they distribute pressure over a larger

surface area of the limb resulting in less discomfort. They are lighter than metal braces,

more cosmetically appealing, and can be worn with a variety of shoe types.

With this AFO design there is no ability of the ankle-foot to [plantar-flex] or [dorsi-flexion]

from heel-strike through gait cycle. The ankle-foot is held only in a rigid neutral position

during swing phase.

Figure 1-3: Standard Ankle Foot Orthosis (AFO)

1.3.3 The Active Ankle Foot Orthosis (AAFO)

The Active Ankle Foot Orthosis (AAFO) developed in [7]. It consists of a polypropylene

ankle-foot orthosis powered by a Series Elastic Actuator (SEA). They demonstrated that an

adaptive impedance controller has good potential for aiding patients with drop-foot.

The main advantage is that the SEA exhibits stable behavior while in contact with all

environments, including in parallel with a human. However, the further complication is that

the system used to control the AAFO needs to be on the AFO being worn by the subject.


7

The main disadvantage is that the minimum thickness for a load cell used to measure the

Ground Reaction Force (GRF) and centre of pressure (COP) is 9.6 mm (0.375 in), which

does not allow this system to be worn within a sneaker. A rubber pad could be placed under

the metal plate to act as a sole. This, however, would make the system cumbersome.

Force sensitive resistors (FSR) are polymer thick film devices that exhibit a decrease in

resistance with an increase in the force applied to the active surface this technology allows

for extremely thin and flexible sensors; however, they lack accuracy and repeatability.

Capacitance Force Transducers are more accurate and durable than FSR’s, but are also

larger.

Before the variable-impedance, AAFO can have broad utility for individuals suffering from

drop foot gait, system hardware and software must be advanced. The series-elastic actuator

used in this investigation is too heavy and power intensive to be practical in a commercially

available active ankle foot system, and would therefore have to be redesigned. Further,

control strategies and sensing architectures specifically suited for the ascent and descent of

stairs and ramps would be necessary.

Artificial Pneumatic Muscles active Ankle Foot orthosis:

The basic idea was to provide plantar flexion and dorsiflexion torque during walking to the

patient. It was achieved by computer software, which adjusted air pressure in each artificial

muscle while walking. It was a Prototype (Not portable), Heavy (mass 1.4 kg), Complex.

[17]

Functional Electrical Stimulation (FES)

Functional Electrical Stimulation (FES) is a methodology that uses shorts bursts of

electrical pulses to generate muscle contraction [4]. These pulses generate action potentials

in motor neurons attached to a muscle, using self-adhesive skin surface electrodes placed on

the side of the leg, which cause that muscle to contract.

The FES system must produce pulses at a minimum of 20 Hz; otherwise the muscle

twitches continuously and does not generate a steady output force. Both monophonic and

biphasic current or voltage pulses can be used to stimulate motor neurons. The common

belief, however, is that the injected charge should be removed from the body and not

allowed to accumulate. Most surface electrodes use biphasic current pulses, which changes

the positions of the anode and cathode during stimulation.

1.3.4 Semi Active Ankle Foot Orthosis

Dorsiflexion Assist Controlled by Spring AFO (DACS-AFO)

This type of AFO Generates a dorsiflexion assist moment during plantar flexion and no

moment during dorsiflexion using a spring located at the calf. The initial dorsiflexion angle

of the ankle joint is adjustable and three springs with different moments are available [6].

Introduction

8

The DACS-AFO, consists of body and sole portions, which are made from polypropylene

and are connected with two joints and a cylinder. The cylinder generates a dorsiflexion-

assist moment with an angle of plantar flexion at heel strike.

Unfortunately, no significant difference was found between the DACS-AFO and basic AFO

and most subjects of their study preferred their old AFO.


9

2. System Overview

2.1 Sensors:

As mentioned before, basic idea of this project is to develop semi active ankle foot orthosis,

which is actuated by MR damped. To achieve and build this system, accurate signals for the gait

cycle and human walking should be measured in order to use these signals to operate the

actuating system (The MR damper). And to choose the most suitable sensors, which can measure

all gait cycle cases, several options are available such as: Force sensors and Pressure sensors,

which have been used before in previous projects. Below, is the description of the procedures

used to choose sensor type in this project.

2.1.1 Sensor Selection:

Under coming text is the description of the main and general criteria to choose any type of sensor

for an application.

Criteria for Sensor Selection:

Range; the sensor should be able to measure the full-scale range of the event. Sufficient

margin should remain at the upper end of the range so the measured event should not

exceed the range of the sensor and cause damage.

Frequency response; of the sensor is the range of frequencies over which the sensor gives

an accurate response. It should be able to measure the full range of frequencies expected

from the experiment.

Sensitivity; is defined as the ratio of the change in sensor output to a change in the input to

be measured.

Accuracy; refers to how close the output of sensor is to the actual event.

Environmental conditions; such as temperature, humidity, water and other should be

considered while choosing a sensor.

Cost; is an important factor and depends on the standard of the project. [15]

2.1.2 Sensor used in AFO:

Force Sensors:

As its name implies, force sensors are used to measure the force. Depending on its

application, it can also be used to measure the weight or mass.

System Overview

10

Force sensors can be integrated into an orthosis or prosthesis to measure ankle/wrist forces,

moments and loads. It can also be used in gait analysis to measure ground reaction forces

and loads exerted by a strap or pad in an orthosis.

A Force Sensing Resistor (FSR) is a thin polymer device that exhibits decreasing electrical

resistance in response to increasing force-applied perpendicular to the surface of the device.

A force sensing resistor is made up of two parts. The first is a resistive material applied to a

film. The second is a set of digitizing contacts applied to another film [15].

The force versus resistance relationship of an FSR is almost logarithmic, meaning it is non-

linear. With no load on the FSR, there is no contact between the electrodes and the resistive

material and the FSR is seen as an open circuit.

Due to the mechanical nature, FSRs have a very slow rise time. One of the major

limitations of FSRs is their creep response. Creep is the deformation or strain of a

mechanical member over time when exposed to a constant force. It is important to make

note of the fact that FSRs are not appropriate for accurate measurements of force due to the

fact that parts might exhibit as much as 15% to 25% variation between each other. In FSRs

this creep manifests itself as a decrease in resistance over time. If the FSR is used to

measure the absolute value of a force, this creep can be a source of error, [15]. Also the size

of FSR is considered big comparing to other sensors, and due to the Foot orthosis

application which need a very thin sensor in order to be able to build in the foot structure

without any difficulties. These sensors have been used in some of previous foot orthosis

projects, and they face the same problems as mentioned above.

Pressure Sensors:

To measure pressure, an electrical signal must be generated in response to a pressure input.

Pressure is measured either by deflect or strain. This strain can then be measured in a

variety of ways, using capacitive, PR and PE as well as other techniques to measure

displacement.

Pressure is defined as the force exerted over a surface per unit area of surface. Pressure is

measured in the same units as mechanical stress N/in2 (Pascals, Pa). [15]

The use of these sensors was limited, due to there size, same as the force sensors, non

linearity signals , which need some signals analysis before it will be entered into the control

system. And the use of this sensor in foot orthosis will be limited for measuring some

points under the foot, which maybe changed from one foot to another.

Wireless In-shoe Force System:

The WIFS is a portable system, intended for ambulatory use in a variety of locations. It

developed to measure times of foot contact occurrence, the approximate weight on each foot

, and the center of pressure (COP) on each foot (as a function of time) while being reliable,

long-lived, easily mounted or installed and cosmetically unobtrusive. This was done using a


11

set of thick-film force sensors mounted in multi-layer shoe insoles fitted inside the shoe.

Information obtained by this insole is transmitted from small, battery powered transmitters

mounted on the shoe to a receiver. This system consider very expensive.

2.1.3 Strain Gauge Sensors:

Definition

In this section we will describe the sensors which used in our project, strain gauge sensors.

These sensors normally used to measure the stress or strain in materials depends on there

amount of bending due to applied force. So this bending will be proportional to the change

in sensor resistance, and voltage value applied.

Stress and strain are very closely related and it is possible to calculate one if the other is

known. Strain, is defined as the fractional change in length per unit length (delta L/L) or the

change in length divided by the original length. Strain can be tensile (positive) or

compressive (negative) and is a dimensionless quantity since length units cancel one

another.

Strain Gauge senor previously was made up of a thin wire bonded to a dimensionally stable

substrate in a zigzag pattern. Advances in the design of the device have resulted in a photo-

etched foil strain gage. This new construction technique has improved the reliability and

sensitivity of the device while reducing its size and cost.

These photo-etched strain gages consist of a very thin foil sheet (commonly Constantan

alloy or nickel-chromium alloy) bonded to a dimensionally stable substrate such as

polyimide film. The foil is then etched to produce a single conductor which zigzags across

the substrate. The gage is then attached to the component to be tested usually by bonding or

gluing the strain gage to the component with cyanoacrylate or an epoxy adhesive. As the

component is stressed, it undergoes some dimensional change or deformation. Since the

strain gage is bonded to the surface of the component, it also will undergo the same

deformation as the component.

Figure 2-1

As the thin foil wire changes dimensions, the resistance of the wire changes. This resistance

change is very small and needs to be converted to voltage and amplified before it can be

System Overview

12

measured. To convert the small changes in resistance of a strain gage into a voltage, a

Wheatstone bridge is used.

A Wheatstone bridge is a circuit consisting of four resistors and a power supply. One or all

of these resistors can be strain gages. With voltage supplied to the circuit, a small change in

resistance of any one of the four resistors will unbalance the bridge and cause a voltage

change at the output points. This voltage can then be amplified and read by a volt meter.

The sensitivity of strain gages, called the gage factor, is the ratio of the fractional change in

resistance to the fractional change in length along the axis of the gage. Foil strain gages

typically have gage factors near two.

Strain gages can be useful in measuring many properties of orthosis and prosthesis. [15]

The advantages of using strain gauge senor in this project are:

The Size of strain gauge is very small and very thin, and available in different shapes and

sizes. So it can be easily fitted inside the foot orthosis.

The signal obtained from these sensors is a voltage signal, which can be easily and directly

used in the control system. The output signals from the strain sensors are considerably very

small, but it can be easily amplified by using an amplification circuit.

Sensitivity of strain gauge sensors is very high; any small change in the materials attached

to the sensor will be directly sensed by the strain gauge sensor.

Accuracy and repeatability of strain gauge sensors are very high also. And it was seen in

the experiments done in this project.

Inexpensive; these sensors are very cheap comparing to other sensors, such as force or

pressure sensors. There cost vary and depends on the application they are used for.

The limitation of using strain gauge sensors occurs because of there sensitivity to ambient

temperature variations, but that can be compensated by using some certain arrangements in

sensor circuits design. But in this project the influence of temperature variations on the

strain gauge sensor was very small, and can be neglected without using additional

components.


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2.2 Sensor placement:

The second most important task is the sensor placement on the foot in order to reflect the

gait cycle cases in the right and accurate way.

As mentioned before, gait cycle contain three phases: Heel phase, stance phase and swing

phase. And the target was to determine these three phases by using the minimum number of

strain gauge sensors with high accuracy and repeatability.

For this purpose four main positions on the foot orthosis were chosen to measure the

bending during the gait cycle, and analyze the signal obtained from each sensor and try to

extract these phases from them. We numbered the sensors depends on there position as: 1,

2, 3 and 4. See figure 2-2.

2-2(a) 2-2(b)

2-2(c) 2-2(d)

Figure 2-2: Sensor placement

Figure 2-2(a) shows four different sensor positions on the lower side of the orthosis, out of

which only two positions i.e. 1 & 4 were used. Figure 2-2(b) shows the discarded sensor

System Overview

14

positions. Figure 2-2(c) shows the upper two sensor positions which were used in place of the

discarded ones. Figure 2-2(d) shows the electrical equipment mounted on the orthosis.

2.2.1 How to use these sensors in project:

The strain sensor will be used to measure the bending in the foot during the walking, since

the bending on foot will occur because of the force applied by the human body on the foot

ends (heel and foot front) while walking. So by applying the force on the front fingers (F1),

multiplying it with the distance from the front fingers to the heel (r1), a Torque (M1) will be

generated on the foot during walking from the stance phase( Dorsiflexion direction). And

the same happen when the heel phase happened, the force (F2) from the body and the

distance (r2) from the heel to the end of the foot generate a torque (M2) in the other

direction (Planterflesxion). See figure 2-3.

Figure 2-3

One of the aim of this project is to find a way to balance these two torques during the gait

cycle from the sensor signals.


15

2.3 Semi-Active AFO Design:

This project is about building a Semi active AFO hardware, and test it on a real Foot drop

patient. But due to the fact that most of the time was spent on the sensor part to find the

right signals and a logical strategy to control our orthosis as per project target, not much

was done along the mechanical side. But to compensate that, a sketch design along with the

proposed working principle of the orthosis is also explained. In later paragraph, orthosis

design, components, and working is mentioned. See figure 2-4.

Figure 2-4: Schematic drawing of proposed semi-AAFO [SAAFO]

The actuating system of the ankle foot Orthosis will be A Magneto-theological (MR)

Damper, with controllable viscosity, which helps in controlling the damper extension in

exact position to support the foot during the gait cycle. Figure 2-4 shows a proposal

drawing of the semi-active ankle foot Orthosis that will be build.

2.3.1 Physical component of AFO:

The Semi-Active Ankle Foot orthosis (SAAFO) consist of:

Foot orthosis structure: This is the mechanical part of the project. Due to some reasons,

the mechanical building was revised and a new structure was supposed to be build. A new

foot orthosis that will be able to hold all components as per new design. Below is defined

the proposed structure for that orthosis, which is not that much different from the normal

orthosis but with extra supports and holders.

Sensors: for detecting and navigating the gait cycle phases, as mentioned before, these are

strain gauge sensors built in the foot structure. These sensors are flexible and easy to install

so it can be used with any type of orthosis material.

System Overview

16

MR damper: used is a compact, Magneto-Rheological [LORD RD-1005-3] damper. This

(MR) fluid damper is suitable for industrial suspension Applications, with continuously

variable damping which controlled by the increase in yield strength of the MR fluid in

response to magnetic field strength. So it was fitted in AFO system in a certain design to

control the ankle joint angle as what will be described later in the principle of work of the

proposed semi active ankle foot orthosis.

This new technology has been used for its great features and benefits such as:

Fast Response Time: responds in less than 15 milliseconds to changes in the magnetic field.

Easy to Use: provides simple electronics and straightforward controls. So no need for any

complicated circuits or signals conversions.

Durable: provides excellent long-term stability.

Low power consumption: The Input Voltage to operate the MR damper is 12 Volt DC, and

Input Current if operated in continuous mode for 30 seconds is 1 Amp max. That means we

do not need for a huge size of battery to operate it, and make the foot orthosis lighter.

In the next section below, we will describe the MR technology in more details.

Electronic Components: the strain gauge sensors will be used through Wheatstone bridge

circuit. Each sensor will be connected with one bridge, known as a quarter bridge, see figure

2-5. Then the output of the bridge will be the input of the amplifying circuit figure 2-6. This

circuit has been designed and fabricated in Halmstad University Labs. And also filter circuit

has been used to avoid the noise in sensor signals.

Figure 2-5: Strain Gauge Figure 2-6: Amplifying Circuit

sensors in a wheatstone bridge circuit


17

Control Circuit: the sensors signals then will be transferred to the control kit (this kit has

been designed and fabricated in Halmstad University). Which consist of a PIC

Microcontroller, analog and digital input/output ports, power supply.

The control program will be downloaded to this controller, and it will control the SSAFO

during walking.

All these components; the Wheatstone bridge circuits, amplifying and filtering circuits,

Microcontroller kit and power supply for electronic circuits and MR damper will be

mounted inside one box on the foot orthosis. The patient will wear the foot orthosis and be

able to walk with it anywhere.

Magneto-Rheological (MR) Technology

In this part MR Technology is described in more details.

Magneto-Rheology is a branch of Rheology that deals with the flow and deformation of the

materials under an applied magnetic field. The discovery of MR fluids is credited to Jacob

Rabinow in 1949 [2]. Magneto-Rheological (MR) fluids are suspensions of non-colloidal

(~0.05-10 µm), multi-domain, and magnetically soft particles in organic or aqueous liquids.

Magneto Rheological (MR) materials constitute examples of controllable (“smart”) fluids,

whose Rheological properties vary in response to an applied magnetic field. These materials

typically consist of micron-sized, magnet sable particles dispersed in an insulating medium.

The essential characteristic of MR materials is that they may be continuously and reversibly

varied from a state of free flowing liquids in the absence of an applied magnetic field to that

of stiff semi-solids in a moderate field. This dipolar interaction is responsible for the chain

like formation of the particles in the direction of the field, see Figure 2-7.

Figure 2-7: Schematic of the formation of chain-like formation of magnetic particles in MR fluids in

the direction of an applied magnetic field

System Overview

18

MR technology offers: Real-time, continuously variable control of Damping, motion and

position control, High dissipative force independent of velocity, Greater energy density,

Simple design, Quick response time, Consistent efficacy across extreme temperature

variations, minimal power usage.

MR fluids can be operated directly from low-voltage power supplies and are significantly

less sensitive to contaminants and extremes in temperature.

Applications of MR fluids

In the marketplace today, state-of-the-art MR fluids are becoming increasingly important in

applications concerning active control of vibrations or torque transfer. Shock absorbers,

vibration dampers, seismic vibration dampers, clutches and seals are the most exciting

applications of MR fluid.

The Lord Corporation [9] has currently manufactured MR fluid devices for commercial

applications including heavy-duty vehicle seat suspensions, rotary brakes that provide a

tunable resistance for exercise equipment and vibration dampers for various industrial

applications such as dampers for washing machines. The damper that contains MR fluids

for commercial Applications such as seat suspensions is presented in Figure 2-8.

Figure 2-8: MR fluid damper 1) Plastic shaft, 2) Sponge saturated with MR fluid, 3) Coil,

4) Steel tube, 5) Wire supplying current

The biggest challenge of MR fluid is to have high turn up ratio, temperature stability and

durability. The second biggest challenge of MR fluids is the materials science oriented

studies such as surface chemistry, polymer physics, in synthesizing stable and redispersal

MR fluids.


19

2.3.2 SSAFO principle of work:

The principle of work for our SSAFO will be as follow:

During the gait cycle, there are three main phases: Heel phase, stance phase and swing

phase, as mentioned before. So in each phase the ankle angle will change in a way that is

suitable to ground level so the human body remain balanced during walking. In case of the

patient with foot drop, he cannot change this ankle angle during the walking. So the SSAFO

should do this mission for him.

In stance phase, the SSAFO will be in the zero position, when the MR damper and the bar

(a) are parallel to each other, see figure 2-9(a). So the MR damper will be max compressed

by the human body weight and the ankle angle is 90 degree (corresponding to ground level).

When the person start to walk, the ankle angle will start to decrease because of the body

weight and the force generated on the front foot fingers, and generate the momentum torque

in Dorsflexsion direction, so the position of the MR damper will be changed corresponding

to the bar 2-9(b) position, and make a triangle with it. In this moment the controller will

give the command to the MR damper to freeze, and fix the foot ankle angle in that position,

depends on the sensor signals, and this will be the end of stance phase.

After finishing the stance phase, the swing phase will start, and signals from the sensors will

define it to the controller. In this phase the controller will command the MR damper to

release and make it self free moving, so by using the body weight again and the mechanical

joints shown in the figure 2-9(c), the SSAFO will change the ankle angle to increase,

because of the force applied on the heel and the momentum torque generated in

Planterflexsion direction. The angle then returns to the zero position. After that the

controller will give the command again to the MR damper to Freeze and keep the foot in

that position.

´ 2-9(a) 2-9(b) 2-9(c)

Gait Phases using new SSAFO

By repeating these procedures, the SSAFO will be able to control the ankle angle and

change it depending on the body weight and MR damper.


19

Experimental methods

20

3. Experimental methods:

The Experiments that have been done in this project depends on the human walking, so three

different procedures were done in order to recognize different signals from the sensors that are

used, and also try to define each gait phase from these signals. Below are the descriptions for

each procedure in details:

Walking horizontal:

The Experiment have been done with respect to the human cycle different criteria such as:

speed (normal speed equal to 1.4 m/s , slow speed equal to 1 m/s and fast speed equal to 1.6

m/s ), walking uphill ( + 5 degrees ), and walking downhill ( - 5 degrees ).

The picture below shows the human gait steps for one gait cycle.

..... Step ...... ....... Step ........

........................ Stride ........................

Figure 3-1: this figure showing the one cycle.

• Step Length means the distance from one foot hit to the next (left to right or right to left) -

with reference to 0.75 m for normal adults.

• Stride Length (one gait cycle) means the two following steps (by both left and right feet -

with reference to 1.5 m for normal adults.

Swing means the period when the foot is off the ground (38% of gait cycle in normal

walking).

Then the time and distance have been measured in order to analysis the signals obtained

from the sensor for each cycle. See figure 3-2 below.


21

Figure 3-2:

Walking up hill:

Experiments on walking up hill were done using 5 degree inclination with a distance 1.5

meter each. Data was collected 100 samples per second with different walking speeds, i.e.

fast and slow walking speeds. In walking up hill, heel comes in contact with the ground first

and hard, this can be seen in the results also, which will be discussed later in chapter 4.

Figure 3-3

Walking down hill:

Experiments on walking down hill were done using 5 degree declination with a distance 1.5

meter each. Data was collected 100 samples per second with different walking speeds, i.e.

fast and slow walking speeds. In walking down hill, heel comes in contact with the ground

first but the force is applied more on toe.

Experimental methods

22

Figure 3-4

Further details will be discussed in detail in later chapter.


23

4. Results and Discussion:

This chapter enlists the experimental results done using strain gauge sensors at four different

positions. Hence to determine which sensor positions are most appropriate for this kind of

sensing equipment.

Different sensor positions have been shown in section 2.1.3.1. Sensor 1&4 are on the lower side

of the orthosis while 2&3 are on the top. Four different sensor combinations were used to

determine gait steps, these are;

1-4

2-4

1-3

2-3

Signals from these sensors are in milli-volts, hence they need to be amplified. These signals are

discrete and out of phase, but for easiness heel sensor signals are inverted while being plotted to

differentiate different phases of the gait cycle. Results from these will be discussed below in

detail.

Sensor 1&4 (Normal Walking):

The idea behind these sensor positions was that to detect heel down and front down phase

without any delay and it was also seen that sensors respond fairly good at these positions.

These were tested on walking with different speeds and angels as mentioned in chapter 3.

Figure 4-1: Sensor 1&4 (Normal Walking)

V

O

L

T

S

Number of samples

Red: Front Sensor

Blue: Back Sensor

Green: Heel Down

Navy Blue: Flat foot

Grey: Front down

Yellow: Swing

Grey line: Gait

Boarder

Red Line: Threshold

Result and Discussion

24

Above mentioned figure shows the signals from walking on a normal speed on a horizontal

plane with shoes. Red curve describes front sensor and blue curve shows heel sensor signal.

For each sensor a threshold is configured to differentiate between noise/unwanted signal

and the actual step. Red line shows this threshold. For sensor 1 it is Zero and for sensor 4 it

is -1.25. These values remain constant for these sensor positions.

One assumption taken is that, the gait cycle will always start from heel down. Analyzing the

figure, blue values below -1.25 are the actual heel down (which have been marked as light

green color). Blue square shows the flat foot phase. Red curve above 0 displays the front

down. Yellow square shows the swing phase.

In phase 1 (heel down) control circuit for the damper starts decreasing the voltage and in

effect the damper starts increasing in length from size A to size B. In phase 2 (flat foot),

damper remains on position B. In phase C (front down), the damper will decrease in length

from size B to size C, in effect lifting the heel. In phase 4 (swing phase), damper will move

from position C to position A, as discussed in section 2.2.2. This way the gait cycle

continues.

There are two ways of detecting what next phase is going to be.

1. A clipper/clamper can be used to get rid of the noise/unwanted signal below threshold. In

this case next case will be assumed, i.e. after heel down phase it will always be a flat foot

and so on. The advantage of this approach is a simpler design of the orthosis system.

2. Signal processing can be used to determine the different phases. The advantage of this

approach is that gait phases can be determined accurately. After a heel is detected, in

accordance with the figure, signal from sensor 1 is increasing while signal from sensor 2

is decreasing. It shows flat foot phase of the gait cycle. Opposite is the case for swing

phase. If the signal from sensor 1 is decreasing and from sensor 2 is increasing, it is

considered to be a swing phase. This approach is accurate but time consuming.

Sensor positions 1&4 both give pretty good signals and different gait phases can be detected

quite easily.

It is seen that while walking normal with sensor positions 1&4, the swing phase and flat

foot phase are almost have the same time interval.


25

Sensor 2&4 (Normal Walking):

Sensor position 2 lies under the toe which is the maximum bending point and relatively high

signal was expected from this position.

Same measurements were taken with these sensor positions as done on the previous ones.


It can be seen that by using these positions accurate phases in gait cycle can be determined.

The only disadvantage if compared to the sensor position 1 is that it gives more bending in

the opposite direction thus giving us more noise/unwanted signal. Due to this fact the

threshold for sensor position 2 is also changed from 0 to 1.20.

Number of

V

O

L

T

S

Red: Front Sensor

Blue: Back Sensor

Green: Heel Down


Grey: Front down

Yellow: Swing

Grey line: Gait

Boarder

Red Line: Threshold

Number of samples


26

Sensor 2&3 (Normal walking):

Position 3 is right beneath the heel and idea for this placement was that while in heel phase

a fairly good signal can be attained.


It can be seen that by using sensor position 3, the gait cycle phases cannot be determined.

As the signal from the heel decreases very slowly absorbing the next phases. Due to this

reason the threshold for sensor 3 also cannot be determined.

Sensor position 3 therefore is not a good sensor position.

Number of samples

V

O

L

T

S

Red: Front Sensor

Blue: Back Sensor

Green: Heel Down


Grey: Front down

Yellow: Swing

Grey line: Gait

Boarder

Red Line: Threshold


27

Sensor 1&4 (walking uphill):

Figure 4-4: Sensor 1&4 (walking uphill)

It can be seen that while walking uphill the signal from front sensor is quite large as

compared to the heel signal. It can also be seen that the front down phase is noticeably

larger if compared to the heel down.

Number of samples

V

O

L

T

S

Red: Front Sensor

Blue: Back Sensor

Green: Heel Down


Grey: Front down

Yellow: Swing

Grey line: Gait

Boarder

Red Line: Threshold


28

Sensor 2&4 (walking uphill):

Figure 4-5: Sensor 2&4 (walking uphill)

Each sensor is calibrated at 0.1 V while in swing phase. This calibration gives an advantage

that when front/toe sensor bends; it gives signal in positive while heel sensor operates

opposite to it. According to the measurements following table is deducted.

Number of samples

V

O

L

T

S

Red: Front Sensor

Blue: Back Sensor

Green: Heel Down


Grey: Front down

Yellow: Swing

Grey line: Gait

Boarder

Red Line: Threshold


29

Sensor 1&4 (walking downhill):

Figure 4-6: Sensor 1&4 (walking downhill)

It can be seen in the figure that we can easily differentiate all gait cycles. It is to be noted

that heel down phase is larger than front down in this case.

Number of samples

V

O

L

T

S

Red: Front Sensor

Blue: Back Sensor

Green: Heel Down


Grey: Front down

Yellow: Swing

Grey line: Gait

Boarder

Red Line: Threshold


30

Sensor 2&4 (walking downhill):

Figure 4-7: Sensor 2&4 (walking downhill)

Table 1 below shows signal strength on different sensor positions according to different walking

conditions.

Sensor No. Up Hill Signal Down Hill Signal Normal Signal

1 > 2V < 1.5V 1.5 < V < 2

2 > 2V < 1.5V 1.5 < V < 2

3 > 1.5V > 1.5V < 1.5V

4 < 1.5V > 2V 1.25 < V < 1.5

Table 1

Table 1 shows different values from sensors while experimenting with different conditions. The

table is self explanatory and this information can further be integrated with signal gait analysis to

get a more accurate state.

Number of samples

V

O

L

T

S

Red: Front Sensor

Blue: Back Sensor

Green: Heel Down


Grey: Front down

Yellow: Swing

Grey line: Gait

Boarder

Red Line: Threshold


31

As described previously that walking uphill Front Down phase is fairly larger than the heel down

phase. Integrating this analysis with table entry col. 2, row 1 a more accurate decision boundary

can be made. All these values in the table are a mean value of multiples of foot step values while

experimenting.

Conclusion

32

5. Conclusion:

This project studies the successful implementation of the proposed sensor system to detect

different phases of gait cycle. Four gauge sensors were used to determine these phases. These

sensors were placed at different orthosis points for better detection of the bending forces while

walking.

A lot of work has been done in this area using many type of sensors and orthosis equipment. Such

as pressure, force, capacitive sensors combined with rotary angel potentiometer to determine

different gait phases. Our contribution to the work is that low cost, high accuracy, simple design

sensor system consisting of four strain gauge sensors only, along with a design of a new

technology MR Damper has been proposed.

Experiments show that by using only one kind of a sensor without any rotary angel

potentiometer, we were able to successfully determine the gait phases accurately. We can also

differentiate between horizontal, uphill and downhill walking. It can clearly be seen that while

walking uphill, the front-down phase is fairly larger as compared to the heel-down phase.

Opposite is in the case of walking downhill. While walking horizontal, front-down and heel-down

phases are almost the same. Further is it also concluded that sensor positions 1, 2&4 are most

appropriate for these kinds of sensors. Sensor combination 1&4 and 2&4 give somewhat similar

results, so either one of these can be used for measurements. For more accurate readings these

three sensors can be combined along with the values from the table 1.

References:

1. Joaquin A. Blaya, “Force-Controllable Ankle Foot Orthosis (AFO) to Assist Drop

Foot Gait”. : Master thesis in Mechanical Engineering at the MASSACHUSETTS

INSTITUTE OF TECHNOLOGY - February 2003

2. Seval Genç, “SYNTHESIS AND PROPERTIES OF MAGNETORHEOLOGICAL

(MR) FLUIDS”. Master thesis at University of Pittsburgh 2002

3. Ronthal, Michael. Gait Disorders. Boston: Butterworth-Heinemann, 2002.

4. M. Goldfrab, “A control brake Orthosis for FES aided gait”. Mechanical Engineering.

Cambridge, MA: MIT Press, 1994, p. 272.

5. J. Michael Wheatley, Ph.D. “Ankle Foot Orthosis (AFO) what is it?”

http://www.mhcop.com/articles/ankle.htm . 10-09-2006.

6. Sumiko Yamamoto, PhD, Masahiko Ebina, CPO, Shigeru Kubo, Eng., Takeo Hayashi,

Yoshiyuki Akita, CPO, Yasuyuki Hayakawa, CPO, “ Development of an Ankle-Foot

Orthosis with Dorsiflexion Assist”. Part 2. Journal of Prosthetics and Orthotics 1999;

Vol 11, Num 2, p 24.

URL: http://www.oandp.org/jpo/library/1999_02_024.asp

7. Joaquin A. Blaya and Hugh Herr, “Adaptive Control of a Variable-Impedance Ankle-

Foot Orthosis to Assist Drop-Foot Gait”. IEEE TRANSACTIONS ON NEURAL

SYSTEMS AND REHABILITATION ENGINEERING, VOL. 12, NO. 1, MARCH 2004

8. Daniel Grafström, ‘“GAIT ANALYSES USING PORTABLE TACTILE SENSORS”.

Master thesis in Mechatronical Engineering , Halmstad university - 2005

9. Lord Company , http://www.lord.com/tabid/3318/Default.aspx , 10 – 09 – 2006

10. AKAZAWA Yasushi, NAKAGAWA Akio, MATSUBARA Hiroyuki, NAKAMURA

Toshiya NOMURA Tsuyoshi, TANAKA Masao, “Introducing Mechatronical

Technology into Ankle-Foot Orthosis Joint”. Experimental gait wearing an AFO with

Variable Joint Viscosity GRADATE school of Osaka university - 2005

11. D. P. Ferris, J. M. Czerniecki, and B. Hannaford, “An ankle foot Orthosis (AFO)

powered by artificial muscles” JOURNAL OF APPLIED BIOMECHANICS, 2005, 21,

189-197.

12. http://www.thefreedictionary.com/dorsiflexion , 10 – 09 – 2006

13. http://en.wikipedia.org/wiki/Gait , 10 – 09 – 2006

14. Michael W. Whittle, “Gait Analysis an introduction”.

15. Robert M. Havey, “Methodology Measurments, Part II: Instrumentation and

Apparatus”.

16. http://www.cs.princeton.edu/sound/learning/tutorials/sensors/node8.html) , 10 – 09

– 2006

17. Daniel P. Ferris, University of Washington,

1998.

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