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Файл:The design of the exoskeleton. Monograph
.pdf
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The formula (2.28) contains the expression for the torque of the i-th
link:
, (2.28)
where the inductance is a function of the electric angle of rotation
of the motor rotor.
The Lagrange–Maxwell equations cannot be directly used for
calculations. Obtaining computational models for such objects is not an
easy task, since it is almost impossible to do it manually, and most methods
of computer-aided design of models is limited to the framework of classical
mechanics. In this paper, a special technology, described in detail in the
monograph [10], is proposed to be used to build a computer model of the
robot exoskeleton movement.
Summary
A model of the dynamics of a five-stage exoskeleton was built and the
expediency of taking into account the interconnectedness of the dynamics
of the links, as well as the features of gearless high-torque drives was
shown.
For drives on low-speed motors with permanent magnets shows the
presence of the dependence of the inductance of the angular position of
the rotor, which due to the lack of gear is manifested in the movement of
exoskeletons.
It is shown that the construction of mathematical models
incorporating these features effectively to build with the help of equations
of Lagrangeville, using modern computer-oriented techniques build
complex models.
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3. THE FORMATION AND DEFINING OF REQUIREMENTS
3.1. Determination of the required operating parameters and
limitations of the exoskeleton
The first priority – with the necessary power parameters to perform
actions on a certain range of movements (reproduction of the walking
cycle, different types of squats and just taking common positions, in
particular sitting on a chair), to ensure the lowest possible weight limbs
with the maximum possible reproduction of the normal functioning of the
limbs.
It is necessary to determine the range of movements that the
exoskeleton will allow the exoskeleton to perform, which, in turn, will limit
the angles of rotation for which the hinges should be designed, and the
loads with which the system is able to work, and thus set limits that must
be adhered to, and boundaries that can not be violated in any case.
The normal values of the rotation angles of joints of a healthy person
are given, which first of all set the limits of the working positions for which
it is impossible to go under any conditions, as well as to understand how
much the created system will be able to return normal functionality to the
user. First, you need to determine the range of movements that the
exoskeleton will be able to perform and what angles of rotation the hinges
should be calculated, as well as what loads the system is able to work with.
Relatively cheap metals were used in the development, steel 30 is a
priority. It is obvious that with the use of composite materials the
construction will become even easier and relieve the hinges, which will
increase the efficiency of the system, but one of the tasks is to make the
design as cheap as possible.
In further work, based on the average parameters of human feet,
obtained as a result of information research, we assume that:
• the height of the hip of the potential user 1000 mm,
• stop 220 mm,
• caviar 500 mm, • thigh 500 mm.
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• user weight up to 100 kg.
• the weight of the exoskeleton is 35-40 kg.
On the basis of these data sketches are constructed (Fig. 2.2 a, b, C, d),
which depict possible to implement econogene action. Pre-take into
account the physiology and mass of human body parts, and the placement
of the center of mass [8].
It is obvious that the greatest load on the legs occur in a person squat,
so in the future the work will be carried out mainly with the loads that arise
during the squat.
Of course, there are no less heavy exercises on the legs, for example,
different variations of the same sit down, but it should be noted that this
is all solved, in fact, the transfer of the center of mass by the inclination of
the body, where the position of the load vector is schematically depicted
depending on the change in the angle of the body relative to the vertical.
3.2. Formation of requirements for the operating parameters of
the joints
In order to form the requirements for the operating parameters of the
joints and determine the permissible limits of their variation, we will use
the data of statistical studies and expert assessments available in
information sources and make certain assumptions that will be taken into
account in the design process.
3.2.1. The angles of rotation
Based on a pre-defined functional and necessary for the action of the
rotation, decide on the angles of rotation in the range which should work
the joints of the exoskeleton. Take the corners with the maximum values
of these actions at extreme positions. Next, specify the degree measures
relative to the vertical at the General vertical position of the previous links:
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extreme anterior angle, neutral position, extreme posterior angle.
* Pelvic joint, connector: 130; 0; 23.
* Knee hinge, and: 0; 0, 115.
* Hinge rotation of the leg relative to its axis, and: -45; 0; 45.
• Stop (relative to the horizontal) : 15; 0; 30.
3.2.2. Torque
We focus on the user weighing up to 100 kg.
The desired exoskeleton weight is 35-40 kg.
Obviously, Fmax=1300 H (since this does not include part of the mass
of the legs).
We assume that the force vector will pass through the middle of all
links (a=b), and take the position of the thigh parallel to the horizontal.
It is obvious that the initial phase of the lift is the most loaded, since
the levers will be the maximum value, so the obtained torque values will
more than satisfy all other schemes.
The rotation in the hinge relative to the axis of the leg will occur in the
air, and therefore, in fact, without resistance to turning.
Given the pre-said, the required torque in the pelvis for each of the
hinges:
1300N∙0.25 m/2=325N∙m/2=162.5 Nm
Obviously, a similar torque is required in the knee: 162.5 Nm.
For comparison, we indicate the necessary torques in the joints during
normal walking. From the analysis of the mechanics of the step, it is
obvious that in the power plan we are only interested in carrying the leg
forward in the air. This movement is the most loaded relative to others.
Then, provided that the length of the leg is 1000 mm, while the knee does
not experience a bend (this would give an additional load to the hinge in
the pelvis), the length of the leg removal is 300 mm, also assume that the
total weight of the leg along with the exoskeleton leg design will be equal
to 20 kg, and the center of mass of the leg will be located in the center of
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the leg link (in reality, the center of mass will be shifted closer to the hinge
in the pelvis, which will only reduce the torque on it). It follows from these
conditions that torque is required
=200 N∙0.15 m=30 Nm.
In this work we will focus on these torques for each of the hinges (162.5
N∙m when sitting down; 30 N∙m when walking), although in the future the
torques or operating conditions of the exoskeleton can be adjusted.
The stop will be considered in two versions, namely with passive
control (balancing on springs) and active (equipped with a drive).
3.2.3. The performance of movements
First of all, it is necessary to understand what parameters has walking.
Focusing on these data, set the approximate parameters of the system
performance when walking at an arbitrary pace.
Decide the goal, to create a hinge that will work in the hinge, 90
degree/sec. under appropriate power loads.
3.2.4. Supply system
In the context of this work, the development of the power supply
systems themselves will be shown superficially (in terms of requirements
and implementation possibilities). Also not considered the electrical
system as a whole and a detailed wiring diagram. However, in order to
show the reality of the implementation of this project, it is necessary to
present ways to power the system, which would satisfy the project in the
first place for the weight and battery life.
For batteries, for constructive reasons, 20-25 kg is given. At the same
time, taking into account the market of power supply technologies within
this weight, we can produce special lithium-ion batteries with a capacity of
up to 3 kW per hour., which, theoretically, will provide up to three hours of
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battery life. For a better understanding of the capabilities of this type of
battery, below is some information about this power carrier.
Modern lithium-ion batteries have high performance:
100-180 W∙h/kg and 250-400 W∙h/dm3,operating voltage ‒ 3.5-3.7 V.
If a few years ago, the developers considered the achievable capacity
of lithium-ion batteries is not more than a few ampere-hours, now most of
the reasons limiting the increase in capacity, overcome and many
manufacturers began to produce batteries with a capacity of hundreds of
ampere-hours.
* Energy capacity: 110 ... 200 W/kg
• Internal resistance: 150 ... 250 mW (for 7.2 V battery)
• Number of charge/discharge cycles to 20% capacity loss: 500-1000
• Fast charge time: 2-4 hours
• Allowable recharge: very low
• Self-discharge at room temperature: 7 % per year
• Voltage max. in element: 4.18...4.20 V (fully charged)
• Minimum voltage: 2.5..2.75 V (fully discharged) • load Current relative
to capacitance (S):
• peak: more 2S
• most acceptable: up to 1C
• Operating temperature range: -20 ‒ +60 °C
• service: not regulated.
Modern small batteries are capable of working at discharge currents
up to 2 S, powerful ‒ up to 10−20S. Operating temperature range: -20 to
+60 ° C. But many manufacturers have already developed batteries that
work at -40 °C.
It is possible to expand the temperature range and towards higher
temperatures.
Basically Li-ion batteries function best at room temperature. Working
at elevated temperatures shortens the period of their use. Increased
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temperature temporarily counteracts the internal resistance of the battery,
the increase of which leads to its wear.
Self-discharge is 4-6 % for the first month, then ‒ significantly less: for
12 months, the batteries lose 10-20 % of the stored capacity. Capacity loss
is several times less than that of Nickel-cadmium batteries, both at 20 ° C
and at 40 ° C.
Resource ‒ 500-1000 cycles.
All lithium batteries have enough parameters acceptable for storage.
Loss of capacity due to self-discharge 5-10% per year.
These indicators should be considered as some nominal benchmarks.
For each specific battery, for example, the discharge voltage depends on
the discharge current, the level of discharge, temperature; resource
depends on the modes (currents) of discharge and charge, temperature,
depth of discharge; operating temperature range ‒ on the level of life and
permissible operating voltages.
The disadvantages of Li-ion batteries include sensitivity to
overcharging and strong discharge, because of this they must have charge
and discharge limiters.
It is also possible to use a gasoline generator as a power supply, which
by itself will make the system relatively noisy and unacceptable for work in
most rooms, but will have advantages, namely the recharge time will
actually be equal to the time of filling the fuel, which can make it possible
to operate the system without long stops and give the power system up to
2 kW within a given weight. It should be noted that the presented
generators are too large for their direct application in the standard form
and need some modernization, which would allow to place such a
generator in the volume of the required shape, it is not necessarily in a
single and/or common housing.
Noise is also a very important factor that must be sustained to prevent
additional discomfort to the user and others. When selecting components
for noise, you can use the following information.
A decibel is a relative logarithmic unit used to measure a variety of
physical quantities that vary over a very wide range. In particular, the
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decibel is used to measure the sound pressure level, which is directly
related to the sound volume [7].
Approximate sound pressure levels of normal ambient sounds:
• 10 dB - whisper;
• 20 dB - norm of noise in residential premises;
• 40 dB - quiet conversation;
• 50 dB - normal conversation;
* 70 dB noise of a typewriter;
• 80 dB is the noise from the engine of the truck;
• 100 dB - loud car signal at a distance of 5-7 m;
• 110 dB is the noise of a tractor working at a distance of 1 m;
• 120 to 140 dB - pain threshold;
• 150 dB - takeoff;
Pressure above 140 dB can cause rupture of the eardrum or
barotrauma.
Summary
Thus, the parameters to be implemented in this design are selected.
Based on the information provided, it is easy to understand that providing
the design with the necessary power, with the weight and noise delivered
in the TK − is not a problem. But, of course, the priority will be in the future
to develop its own battery (generator/battery), which would be much more
satisfied with the project in size, noise, total weight and, perhaps, even the
aesthetics of the design as a whole.
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4. SELECTION DESIGN OF IMPLEMENTATION OF THE GIVEN
FUNCTIONS
Work search. It contains a lot of information and analytical studies. The
calculations are approximate. The design part is considered at the level of
3D models with certain simplifications. The principal possibility to carry out
the planned development and to realize it by available means is
considered.
4.1. Methods of transmission of torques in swivel joints
For this project were designed and developed original methods of
transmission of torques. As it became apparent that they themselves
aksonova need to make the most easy and most difficult element in classic
designs is the system with the drive gear, which is mounted directly to the
hinge. Sometimes belt and/or chain drive motor and gearbox shift for
unloading the previous hinge.
The result of my work was a new way to transmit torque, namely with
the help of cables, which can be seen in the following figures. The
advantages of the developed systems in this design is that it is actually
possible to unload the limbs from the weight of the drives, reducers,
sensors and controllers, which will ensure the operation of the hinge with
a much higher efficiency.
Construction 1
The motor transmits torque to the shaft of a ball screw pair or a sliding
screw pair, whereby the movable platform (nut) either moving in one
direction or the other pulls the cable, which passes through the corrugated
tube (is a flexible connection) and thus transmits torque to the pulley. The
direction of rotation of the motor and the pulley to which the torque is
transmitted are strictly consistent.
A very interesting and promising option, but given that the project is
not only on a theoretical basis, but also for the purpose of implementation,
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the final choice of the method of torque transmission will depend on
technical and economic capabilities. It should be noted that at least until
recently transmission ballscrews were very expensive and their use in
construction entailed significant costs. However, at the time of writing, it
could be said that the ballscrew steel is relatively much cheaper and more
affordable.
Design 2
Job description: the Motor transmits torque to the screw shaft, which
in turn winds/unwinds the cable in the appropriate direction. This, in turn,
in one direction or another pulls the cable, which passes through the
corrugated tube (is a flexible connection) and thus transmits torque to the
pulley. The direction of rotation of the motor and the pulley to which the
torque is transmitted are strictly consistent.
Simplistically we can assume that the conversion factor of the torque
can be described by the ratio of the diameter of the shaft pulley to the shaft
diameter of the winding, which will be multiplied by the resulting loss ratio.
In the equipment of the foot to use the drive, it makes no sense in the
case where it is not expected that the exoskeleton should seek equilibrium,
and, even, on the contrary, the user has to use crutches to ensure a stable
equilibrium. Therefore, the task is to make the foot such that its mechanics
while walking partially or completely unloaded the user's foot.
It was decided to develop a spring-based design that would replace
muscle function.
4.2. Selection of components
4.2.1. Electromotors
First, it is necessary to understand what reasons are generally
available today for the development and their advantages and
disadvantages. If classified by engine power, we get the following:
- direct current motors with gearbox;
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