LOWER LIMB PROSTHESIS: FINAL PROTOTYPE RELEASE

AND CONTROL SETTING METHODOLOGIES

Vicentini Federico, Canina Marita, Rovetta Alberto

Robotics Laboratory, Mechanical Department - Politecnico di Milano – Itlay

Via Bonardi 9, 20133 Milano - Italy

Keywords: Lower limb prosthesis, biorobotics, human-machine interface, control, step analysis.

Abstract: The current research activity on prostheses project at the Robotics Laboratory (Mechanics Department,

Politecnico di Milano) is carried on in cooperation with Centro Protesi INAIL and STMicroelectronics. The

team is both innovative and interesting, owing to the fact that it not only involves a range of specialists but

also gives rise to interdisciplinary aspects. They are absolutely essential in project dealing with such

complex issues. This Mechanic-Leg project, called Hermes, is an original solution in the field of prosthesis.

Main aim of this research is the prototyping of a new kind of mechanical lower limb with an electronic

control. The device, resorting to innovatory mechanical and electronic solutions, allows the controller to

modify the type of step, passing from a slow to a fast walk, in an easy and intuitive way, taking care of

patient’s requirements. The Hermes M-Leg cost is comparable to the actual commercial non electronic

controlled artificial knees. The distinguishing features of Hermes M-Leg project are an higher awareness in

innovative aspects related to medical/biological/engineering research. Then, a pervasive use of cutting-edge

technology (electronics, IT, material-related technologies, etc.). The controller architecture is built upon a

low memory processing features. The hard analysis and test activity help to model the algorithm for step

control. The adaptive behaviour is mostly due to an effective experience in testing and software tuning in

cooperation with patients and clinical staff.

1 INTRODUCTION

A prosthetic system, totally replacing a lost human

body part (thereby ensuring the functionality of a

specific physiological system), acts as a true spare

part which the person is able to interact with. The

prosthesis designer takes into particular account the

man-device interface. This is done to satisfy the

patient driven requirements and to project a suitable

prosthesis. According to this statement, a innovative

design concept emerges in evaluating the prosthesis

system.

The prostheses technological evolution has begun in

sixties. The availability of advanced technologies

coming from the automotive and aerospace industry,

allowed to develop more comfortable, resistant and

light materials. The prosthesis weight limitation is

indispensable to reduce the user tiredness and to

allow a longer use during the day. In the 80s, new

materials were introduced provided of similar

mechanical resistance but lower density compared to

previous ones. The miniaturization of components is

actually fundamental in the design system; it allows

to reduce the prosthesis overall weight. So that many

global requirements have arisen from technology

development to user comfort. First, an electronic

controlled prosthesis must give stability to the

patient, support his weight and make his/her

movements easy. Therefore, it is very important to

minimize energy supplied by the patient and to fit a

natural limb behaviour. Finally, the device should be

adaptive and self learning.

In this paper it will described a methodology in

device optimization from several points of view. The

final release prototype developed mixes up design,

mechanics and software issues due to long

experience in prosthesis field. An accurate analysis

is carried on about all design process, and it will be

presented as well as a short description of produced

device.

232

Federico V., Marita C. and Alberto R. (2005).

LOWER LIMB PROSTHESIS: FINAL PROTOTYPE RELEASE AND CONTROL SETTING METHODOLOGIES.

In Proceedings of the Second International Conference on Informatics in Control, Automation and Robotics - Robotics and Automation, pages 232-241

DOI: 10.5220/0001185502320241

 SciTePress

2 DESIGN METHOD OF THE

M-LEG SYSTEM

Accurate design of a lower limb prostheses requires

analysis aimed at defining shapes, materials and way

of utilization. Analysis issues are related to general

requirements coming from both experience and

commitment outlines. One of the goals of the

research work is to design a prosthesis equipped

with some trick for storing energy. It would also

enable an amputee to perform almost the same type

of (even complex) movements as those performed

with a natural limb. Another objective of this project

is the development of the mechanical structure

strongly oriented to criteria of maximum reliability.

So, through the design process, two criteria were

identified: a functional and a structural criterion.

When the design was in progress, however, a hard

reduction in sizes was crucial. It aimed to reduce

overall prosthesis weight and get the best

compactness compared to requirements and

constraints. The prosthesis must satisfy two different

requirements relative to the de-ambulation. First is

stability. This requiring the assessment of robust

geometries in relation to steady loads. Second

requirement is related to specific leg and foot

trajectory, so it requires a variable geometry.

Naturally in the M-Leg prosthesis both requirements

are satisfied within the same system. A prosthetic

device for a thigh amputee must allow general de-

ambulation conditions. Each movement situation,

i.e. walking, climbing stairs, sitting, running, shows

different kinematical and dynamic characteristics.

The prosthetic mechanism must be designed for

flexible efficiency in all of them. In the prostheses

design over the last few years two phenomena have

come to light:

• The exponential development of electronics

applied to prostheses;

• The increase of models differing widely from those

currently available - from those equipped with only

a spring to those with hydraulic circuits-.

These two elements are closely connected; in fact,

both are linked to the fast progress in electronics, to

the consequent cost reduction, to increased

processing and storage performance of chips and,

most of all, to the increasing convergence of

mechanics and electronics. Listed requirements are

all related to behavioural or mechatronic issues.

They must be managed in an innovative way,

encoding a methodology starting, for instance, from

the design approach.

The knowledge in creation processes, analysis

methods and design procedures allows a team to use

not only the most suitable technology but also a

methodological approach for complex problems

solving. This process involves different stages each

of which needs subsequently to be validated.

Researchers and designers fundamental task lies in

the ideas “materialization” through effective

methods, pursuing fast and competitive product for

market. Even in research field, effective tool must be

developed in order to gain large yield in innovative

applications. The decision to begin a new system in

the high tech devices sector involves the undertaking

of a process that is generally not only long and

expensive in procedural terms but also in cognitive

terms. The first step is the design phase. It includes

an initial project plan involving:

• analysis of the available technologies as well as

those required;

• selection of technological tools to be used;

• choice of materials and validation protocol;

• realization of prototypes;

• exploration of available clinical data, either inside

the team skills or likely to be found in literature on

similar products;

• assessment and reformulation of specifications;

Figure 1: general design requirements

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233

As a result, a structure of relationships among

overall requirements leads towards project

statements. This procedure may be codified and

shared among the project team. Such procedure has

been applied in last activity and final prototype

production. In the next section it will be described

the analysis methodologies transfer to project details

and components.

3 ACTUAL RELEASE OF

ARTIFICIAL LIMB

PROSTHESIS. FROM DESIGN

METHOD TO COMPONENT

DESIGN

Long trained prototypes and maturity of skills in

limb prosthesis development have drawn into final

release of the artificial knee. This is due to a

complete review of previous releases and to the

fulfilling of special requirements in every detail.

Conceptual scheme in figure 2 depicts the

relationships between project requirements,

functional features of the device and related

hardware components. The scheme helps the team to

manage the design effort. The design action is

particularly devoted to clinical and safety aspects.

Moreover, from a technical point of view, the

motion control involves the largest part of hardware

components. Many efforts are given in

accomplishing an artificial device behaviour as

natural as possible. Both these high level

requirements are interconnected in the software

features for control. For this reason, the control

represents the final largest activity. Many tests and

control concepts are developed on the basis of

measurements and sensors acquisition. In section 4 it

will be described the general architecture and the

tests carried for that. Finally, another large efforts in

developing the final release is due to sensor

equipment. It has been enriched from the last release

and many components have been re-engineered.

Both safety and motion regulation are due to a brake

system. Figure 3 depicts the global requirements,

most of which are related to a robust and reliable

braking system. The requirements analysis in the

Figure 2: requirements and components

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scheme makes the general requirements of figure 2

deeper. It shows the features of core components to

develop or re-design. Design choices in the whole

mechanical equipment are taken upon this analysis.

The produced prototype works on combined active

and resistive principles. The device is not designed

to autonomously provide energy for walking. For

this reason a constant force spring is used to

accumulate energy, during hip backward flexion.

This energy, coming from user stump, is given back

to motion of the artificial limb by the spring itself.

The step regulation, during both the walking and

other conditions, is due to a shoe brake. It is

mounted inside the artificial knee and guarantee

reactive force and safety. It’s clear why the brake

system represents the largest design effort from a

mechanical point of view. In analysis scheme, the

choices about the braking technology, the auxiliary

mechanisms and the mounting structure are taken on

the basis of a well planned design process.

The mechanical system is designed to be functional

to control action. The control itself, however, is

submitted to the same general requirements of figure

1. In particular low power consumption, simplicity

and low costs are the main features to achieve in

software development. The electronic control must

be reliable and effective using a limited amount of

memory and processing performances. The design

challenge is related to optimization of control action

by hardware available. For this reason the device is

equipped with several sensors which supply

information to recognize artificial limb dynamic.

Finally, the design aspects are very important to

compactness of the device. It must be stand alone,

reliable and safe. The design contribution proves

itself in hardware components definition, the

structure and shape of the device in order to let the

user feel comfortable. The user must be supported

and facilitate during all motion situations, allowing

flexibility of movements and stability.

As a result a very compact device is developed and

tested. In order to fulfil many requirements, the

electro-mechanical brake is used as regulation

system. Moreover, it’s mounted in a way that let the

housing of rotating mechanical parts. Functional

kinematical components are all mounted around the

Figure 3: brake system requirements analysis

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knee knuckle, even the elastic part in knee joint. M-

Leg is a semi-passive prosthesis because of partial

potential energy accumulation. The particular shape

of the spring makes it very easy to control. The

overall compactness is evident from the electronic

equipment above anything else. The micro-

controller by STMicroelectronics allows data

acquisition, signal conditioning and output

generation; it is a very miniaturized equipment and

provides the correct execution control algorithm. It

must be pointed out that, under many aspects, the

innovatory criteria applied along all the phases of

the development are original in solutions, ever used

before in none of the existing prosthesis.

4 CONTROL STATEMENTS

METHODOLOGY

The control target lies in device adaptation to

different dynamic conditions of user motion. The

device must follow the behaviour of a natural leg

and give a good mechanical response to user needs

in equilibrium and mobility. It must do this in real

time mode. Information coming from sensors input

is very important to outline the current situation. The

update of signal reading and output elaboration

allow the artificial knee to supply the right action.

The input channels are knee joint rotation, stress on

lower leg structure and upper segment acceleration.

The signals are provided by common strain gauges

for compression and bending, and micromachined

sensors for inertial parameters. The knee rotational

speed is calculated by the rotation angle derivative.

A calibration session has been done before using

such signal. The kinematics of knee joint is single-

centre, i.e. it has only one centre of rotation, and an

external reference is used to check the linearity of

sensor response. This procedure is required because

the potentiometer is not mounted directly on rotation

axes, but its connected to displaced integral shaft.

The software design comes after the acquisition

session of the whole sensor equipment. This is

necessary to find out the recursive patterns in step

evolution and, in parallel, in signal records. The

pattern recognition phase is especially done for

walking conditions. This is the case of major content

in regulation statements. The walking shows

periodicity of profile during the step cycle. But it’s

marked by a large variability. This is added to noise

and variability of input signals. As a result it

Figure 4: final release. Design, compactness and test.

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happens to be very useful to have several sensor

available for step condition clustering. It’s only by

all signals comparison that the walking behaviour

can be recognized. The preliminary phase for control

algorithm design is to recognize periodical pattern at

a reference conditions and to use the brake action

without any regulation. (The fundamental feature of

an electronic controlled device is the real time

adaptation to different conditions).

The walking pattern recognition is the result of an

accurate analysis on the acquisition of final release

device. The analysis is based upon the long trained

experience in step recognition during the many years

limb prosthesis development. That experience

proves to be very useful now that the input signals

for control are related to final device and very

reliable.

In this section the preliminary study of control logic

is discussed. First the signal acquisition from sensor

is described. The records are used to define the

signal recurrent patterns. Pattern are the basis on

which a real time recognition and regulation

algorithm is able to work. Then a number of states

are defined in order to cluster the recorded patterns.

This phase is very important because it is the

modelling approach for software architecture. The

states definition allows to set the transitions between

states and the pattern related to transitions. Then the

control unit is turned on, but only for simple

constant impulse. This working mode is used during

explorative tests in order to map the relationship

between step velocity, sensors information and

braking effect.

The motion analysis starts from walking. The

framework for signal interpretation is the natural

walking. the topics is well known and several studies

have been done in Biomechanics in last decades.

Many techniques let experts to measure biometric

parameters, such as rotations, angles, segment

position and so on. Literature data are very

important in finding out related pattern in records

from an artificial device. Such data set the walking

cycle or step as the complete motion of both lower

limbs between two following resting upon ground by

the same foot. The step can be split into two main

phases: the stance phase, when a foot is touching the

ground and the body weight is diversely leaning on

that foot, and the swing phase, when the same foot is

lifted from the ground and flies straightforward. The

stance starts from the heel rest. Then the foot sole

rolls as long as the toe leaves the ground. In that

moment the swing starts till the next heel ground

touch.

The whole cycle is made up of 60% of stance and

40% of swing. The symmetry and periodicity of

walking may induce to give the same duration to

both the phases, but for a small amount of cycle both

the feet are resting on the ground. This is counted in

stance. There are two short interval of simultaneous

foot resting within the cycle, each counting the 10%.

Both stance and swing phases can be divided into

sub-phases. This is due to better understanding the

step dynamic and recognizing it in signals records.

The stance phase is formed by five sequences.

1. initial contact: this is very critical in the

step dynamic. The safety of standing on the

artificial limb depends on this moment for

the largest part. The firmness of the

artificial limb must be comparable to

natural one, both for safety and for self

confidence in motion.

2. first double touch (10% of walking cycle):

the body weight is pushed forward lifting

the rear foot heel and lowering the front

foot toe. In this phase the weight is passing

from a leg to the other and it can be easily

detected by the stress on the device

structure.

3. half touch (20%): from the lifting of the

rear foot toe to the lifting of the front foot

heel. During this phase the rear foot passes

the front one. The weight rests on a single

limb.

4. final touch (20%): starts from the resting

foot heel lifting and goes up to the finish of

the other limb swing phase. This is a very

complex and slight movement to detect.

The touching limb shows a flexion and a

waving pattern affected by large variability.

5. second double touch (10%): inverted

compared to the first.

The swing phase is formed by three sequences:

1. swing start (10%): starts from the lifting of

foot toe. The limb gets a backward

acceleration.

2. half swing (15%): the knee flexion

reaches the maximum extension. The sub-

phase ends at the touching heel lifting. It’s

hard to recognize because of the large

variability of simultaneous values of data

related to maximum extension and heel

lifting.

3. end of swing (15%): starts from the

touching heel lifting till the flying heel

touching. The limb decelerates in order to

prepare the following ground touch.

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The sensors signal records are analyzed in

comparison with the natural walking cycle

definition. This is useful for modelling the step

behaviour and building the software architecture. In

real time control is not so important to recognize and

define the whole set of sub-phases.

But there are some critical transitions that must be

detected and complied. In particular, it’s necessary

to react to the swing end before the ground contact,

the flexion extension of flying limb and the whole

weight resting upon only one limb for stability. The

signal used to detect the limb behaviour are the

rotation angle, its derivative, a mixed signal coming

from an arrangement of compression and bending

signals. These signals are given by a double

Wheatstone bridge.

First, such signal are acquired in a test session with

the brake system turned off. The black lines in figure

5 mark the limits of each walking cycle. The red

ones mark the stance and swing phases. The rotation

angle is reported in indirect degrees size. This is due

to the potentiometer return shaft. The relationship

between the knee rotation and the potentiometer

shaft angle has an amplifying factor due to the

different diameters of return mechanism. The

calibration set up guarantee the linear ratio between

knee rotation and potentiometer angle. The mixed

signal size is reported in percentage of weight. The

signal gets the contribution of two types of stress

signal, so the size is not directly related to an

absolute value. The signal used for first analysis are

collected from a standard step succession. The user

is invited to walk as naturally as usual trying to keep

the speed constant. The resulting signals are

averaged among several walking acquisition on the

basis of a predetermined reference point. The output

pattern are very regular and predictable of standard

pattern.

The rotation signal shows the typical pattern, no

hyperextension is supposed to be detected and the

knee flexion happens to be short compared to step

cycle. This is due to the reduction of swing

percentage in prosthesis users. The flying phase of

the artificial limb is faster than the natural one.

One of the objective in device control is to allow the

user walk as much naturally as possible. This means

to arrange the symmetry of walking cycle between

both the limbs. The zero axes crossing in velocity,

Figure 5: acquired signals from sensors equipment

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related to changing versus of rotation belongs to a

narrow distribution, and the average value of

maximum knee angle recorded is 45°.It’s smaller

than natural value because of the shorter duration of

knee flexion. The lower leg has not enough time to

accelerate and reach larger values. The region of

rotation related to maximum extension is one of the

most interesting in regulation of rotation range. The

reason lies in complete absence of direct control by

the user. The user has non chance to control the

artificial limb backward flexion. This is over the

impulsive energy give to the hip at the start of swing

phase.

The mixed signal is very useful to evaluate the

swing-stance transition and the stance sub-phases.

The mean non-scaled value is related to absence of

weight. It corresponds to swing phase when the

rotation is active and for a while after the complete

knee extension just before touching the ground.

After the heel touches the ground an increase in

signal due to compression stress is gathered. The

peak is short and not marked because of the step

velocity. The body weight, in fact, is thrown

straightforward passing the vertical axis of the

device. In this way the torque due to bending

changes sign and decreasing the value below the

mean. The absolute value is larger the compression

phase one because of the longer leverage for torque

and the duration of rolling on the foot sole. In the

final release the two contribution are separately

evaluated.

From this first analysis two main output are

available: the states definition and the transition

average values. These issues are fundamental in

setting the architecture model for control software

and in software requirements statements. These

features are related to transitions, so the braking

action coming from control regulation must fulfil the

needs of the user shown through the signal record.

From the detected pattern it can be assumed:

1. a brake action is required between swing

and stance in order to guarantee the safety

and stability in touching the ground. The

brake must be on as long as the weight is

passed across the vertical axis.

2. the velocity at the end of swing phase drops

very quickly. This is due to initial

acceleration in forward rotation due to

spring elastic force and the mechanical

block at the complete extension, 0°. This

provokes a stroke to the device transmitted

to the socket and, finally, to the hip and the

backbone of the user. The return rotation

must be decelerated before the end of its

range.

3. other requirements could be revealed by a

finer analysis of device behaviour with the

brake turned on. An important feed back is

given by the user, pointing some features

he may be consider useful or comfortable.

These requirements come from pattern first analysis

and must be added to general ones dealing with

Figure 6: acquisition tests at constant velocity.

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239

emergency management, safe standing upon the

device with the whole or partial body weight,

different motion situations. In particular sitting down

and climbing stairs are test routine run in order to

achieve typical data. The methodology is the same

about different shaped patterns.

The control model is thought to be implemented

through a Finite State Machine (FSM). It’s a

traditional tool to describe formal requirements and

relationships between defined states. It’s not the

control algorithm structure but the ideal framework

of transition management. Such tool is quite

powerful in setting states and transitions, is fit for

limited amount of memory of processor and can be

managed by several people inside the

multidisciplinary team. A first prototype of FSM is

implemented to turn on the brake system in detected

and required points. The initial rules are based only

on pattern analysis. This feature allows the tester to

run some experiments for mapping the dynamic

relationship between braking and step conditions.

Test regulation in control logic is assigned to fixed

velocity/braking position ratio. It’s obviously a

simplification because this ratio changes during the

walking. But for in lab test on leg simulator this is

very useful. It helps to check the right brake

activation due to sensor record and software

regulation. The dynamic step regulation must adapt

the braking action to the velocity and rotation angle.

The dynamics of brake achieve effective resistive

torque as a function of velocity, angle and time

delay in impulse transmission. The larger the

velocity, the larger must be the angle of activation

or, in other words, the advance in getting the speed

reduction. This set of relationships must be fitted

among an empirical data setting and collection. The

experiments take the first step towards the adaptive

control required to electronic controlled prosthesis.

They are carried by a specific tool of calibration and

tuning described below. It’s used to change

regulation parameters both for initial test and for

customization of stand alone final release. For such

reasons a Calibration and customization tool has

been developed.

Initial setting must be run before using the device.

The controller sets internal parameters on the basis

of user features. The main quantity to measure is the

user weight. As usual procedure, the user stands on

the device for a while. The device records a large

amount of values of stress on its structure. The

duration of record may last from six to twenty

seconds. This allows the user to feel free to stand in

natural way. In such way the weight distribution on

both legs, the natural one and the artificial one, has

the chance to vary in a wide range of usual

conditions.

The setting tool has been developed for fine tuning

the recorded parameters. The operator can set

thresholds or constants related to step regulation.

The fine tuning of thresholds is not so usual because

of the initial auto-setting procedure. It’s very useful

to change comparison values inside control software.

This is done to check particular features of device

behaviour. For instance, several tests were run for

Figure 7: tuning, setting and customization tool.

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understanding the right shape of velocity table curve

during the software development. The user was

forced to experience the same brake activation

response for the whole walk. In that way the step

time and step percentage profile were forced to be

tightly constant. The user was so forced to walk at

fixed velocity. The user was helped in do this by

walking on a tapis roulant so that he could slightly

feel the unnatural step regulation. The amount of

tests was collected varying walking velocity and

brake activation position.

5 CONCLUSIONS

A final prosthesis prototype is the result of a long

design process. Experience and skills are supported

now by coded methodologies and analysis tool. This

process starts from a design approach leading

towards details optimization. It’s important to

underline the methodology contribution to several

re-engineering stages. By means of final release a

large development in direct signal acquisition and

testing became possible. The proposed methodology

for step analysis was done with the constant help and

experience supply of patients and INAIL staff. The

presented methodology is basic for the further FSM

development with self-learning and adaptive

features.

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