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Файл:The design of the exoskeleton. Monograph
.pdf
61
- servomotors;
- stepper motor;
We will examine the positive and negative sides of each of the engines.
Consider electric motors that have satisfactory characteristics in the
supply modes, that is, provide Mconst.
1. DC motors.
Advantages:
- easily available in the market;
- wide range of engines;
- powerful;
- easy to connect.
Disadvantages:
- unable to operate at low speeds (<10 rpm, gearbox required;
- large consumption;
- the most expensive among the compared;
- not able to work on their own with precise movements.
2. Servomotors:
Advantages:
- built-in reducer;
- diversity;
- relatively not expensive;
suitable power for small robots;
- easy installation;
- average energy consumption.
Disadvantages:
- pretty slow speed.
3. Stepper motor:
Advantages:
- precise control;
- diversity;
- good speed;
- not expensive.
Disadvantages:
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- large consumption;
- not much power.
Powerful and economical permanent magnet synchronous motors
operate without additional transmission elements and provide very high
torque due to a more advanced design.
The variety of configurations of electric motors allows to solve a variety
of problems. One of the special types of motors, known as high-torque
direct drive motor with permanent magnets (PM), is characterized by a high
diameter-to-length ratio and a large number of magnetic poles, which
optimizes the creation of torque. These low-speed electric motors, typically
operating at frequencies below 1000 rpm, are available on the market in
both chassis and chassis-free versions. Based on the above, for
engineering reasons, it was decided in the future in this work to work with
stepper motors.
4.2.2. Methods of control and their implementation
The desire to expand human capabilities through wearable or
implantable devices is understandable. In many fantasy stories, the
characters transform their bodies or use cybernetic devices to improve
their abilities and acquire new ones. Following the flight of imagination in
scientific laboratories and even at University departments, the first, still
timid, steps in this direction are made. Within the framework of the
presented work, a detailed overview of the control systems is not provided,
but we will present material that will give an understanding of the nature
of the control methods and ways of their implementation.
Recently, a lot of information about the prospects of
electromyographic sensors has appeared in information sources. But
when used in the development of such a given, there will be a number of
problems.
For example, electromyographical sensors used in cyberpirate where
they are mounted on the forearm in the course of muscle fibers. Signals
from them come to the processing of the microcontroller on the Arduino
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Board, mounted on the back of the glove. After analyzing the information
about the local muscle tone, the chip sends a command to the servos of
the finger.
To learn how to move your finger, which was lost, will require some
training and software improvements components Arduino, but after all,
and the child does not immediately master fine motor skills.
Today there are ready electromyographically module ‒ the device for
detection and amplification of muscle activity. For the tasks such devices
are indispensable, as the basis of the design includes anthropomorphic
mechanism, the realism and performance of motor movement which
depends on the speed of perception of the control signal sensors, than in
our case the priority will be to eat the muscle activity of the user.
Working in half with the human nervous system, we will get a
minimum delay in the signal and a number of additional benefits, which
we will return to a little later.
In 1907 by the German scientist G. Pieper (it.) Grown. (Von Piper, H, B.
1 877, Elektrophysiologie menschlicher Muskeln, von H. Piper. Berlin, J.
Springer, 1912 ) first applied the method of electromyography to humans.
The study is carried out using an electromyograph with a special input
for EMG registration.
Electromyogram (EMG) is a curve recorded on film, paper or magnetic
media.
The amplitude of the potential muscle usually does not exceed several
millivolts, and the length is 20-25 MS.
It should be understood the placement of human muscles and their
functions.
With the help of needle electrodes inserted into the muscle. They
detect potential fluctuations in individual muscle fibers or in a group of
muscle fibers innervated by one motor neuron.
With the help of cutaneous electrodes, the process of muscle
excitation is read.
Stimulation electromyography ‒ electromyography with artificial
stimulation of the nerve or sensory organs. This allows you to explore
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neuromuscular transmission, reflex activity, to determine the speed of the
excitation of the nerve.
The scope of electromyographic sensors:
• In psychophysiology to study age patterns.
• In the physiology of work and sports.
• In the study of motor function of animals and humans.
• In studies of higher nervous activity.
• In engineering psychology (for example, in the study of fatigue, motor
development experience).
• To assess the restoration of impaired motor function in orthopedics
and prosthetics.
Electromyography techniques often include the so-called "Global
EMG", recorded by surface electrodes installed on the patient's muscles.
However, its use often leads to incorrect diagnoses, due to the impact on
the results of such a study a large number of subjective factors:
- The patient's desire to strain the muscle
- The patient has other diseases (conditions)
- The distance between the electrodes
- The direction of the electrodes relative to the muscle fibers
- Resistance under the electrodes
- The accuracy of the electrodes relative to the muscle
- Effect of contraction of other muscles of this group
So, having dealt with the capabilities of this equipment, it is necessary
to allocate areas of muscles on which sensors will be installed in the future.
In other words, one of the control options will be to control the signal
received directly from the muscle area performing similar functions in the
anthropomorphic mechanism opposed to it.
Ready to solve the problem can be a special suit ‒ Athos suit ‒ a tight
compression jacket and shorts, stretch and comfortable, with a flat seam.
These things gently tighten the body, do not interfere with movement,
reduce fatigue and muscle pain. At the same time, the material absorbs
moisture so that the skin remains dry.
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Right in the suit built electromyographic sensors that monitor the
activity of certain muscles and do not require replacement over time.
Just the jacket 14 is embedded EMG sensors, in shorts ‒ 8. Even in the
jacket there are two heart rate sensors (in shorts - 4 such sensors) and two
breathing sensors.
So, sensors collect information about the state of your body, and the
"core" ‒ a light shock-resistant processor that is attached to the clothes,
collects these signals, analyzes them and transmits via Bluetooth for
example to your mobile phone or computer.
Training with Athos will become more useful and effective. You know
what muscles are tense and how many percent, and which are in a relaxed
state, how accelerated your heart rate and breathing.
Athos shows muscle strength as a percentage, muscle building or
muscle toning, the degree of muscle fatigue.
Athos suit will be useful for yoga. Often in the description of posture
indicate which muscle groups should be tense. Using Athos, you will be
able to check whether you are performing this or that asana correctly.
After training, you can throw the suit in the washing machine. He is not
afraid of automatic washing at 40 °C. and, of course, to assess your
progress. The app stores data about your past workouts so you can keep
track of your progress.
The question remains what kind of information and how accurate
information is provided by the sensors of this suit, since there is a high
probability that despite good advertising from the manufacturer, the suit
system works with a narrow flow of data, for example, one value per
second, which generally makes it possible to give information about
different kinds of activity of the user and his muscles, however, will not be
able to act as a means of control.
This is not without problems. For research purposes today
electromyographic sensors have incredible potential, but we should not
forget about the very nature of this sensor. During operation, a significant
issue will be noise filtering and complex algorithmization of actions that in
different circumstances under the operation of the sensor. Also, the use of
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these sensors will deprive of the opportunity to use the exoskeleton of
people who have lost their legs at all or in which the connection between
the legs and the spinal cord is terminated. Therefore, after analyzing the
possible options for the operation of the system and ways to control them,
it was decided to make a relatively simple but very reliable control system,
namely to provide the user with special crutches with control buttons on
the handles, which would be programmed for specific algorithms of action.
This would be a kind of game interface, where there are joysticks and
buttons, the combination of which, as in games would give the user to
perform a certain action. Besides, it would deprive the problems with
samomoderirovanie system, with crutches in a pair of mechanical legs
would have absolutely stable support.
In further development, as an option control could be applied
neurocomputer interface, but to date, all attempts to implement such
control has not yet given stable results in any such control system would
work without failure under any circumstances.
4.2.3. Development of control systems
To implement the project, it was decided to use stepper motors
because they are relatively compact, lightweight, affordable and have easy,
convenient and stable control. So now we will understand how they work
and how to control them. At the same time, the possibility of connecting a
stepper motor with a standard worm gearbox is considered, which in turn
will give at first glance a reduction in a number of non-standard parts and
elements, but will entail an increase in weight and complication of the
braking and retention system, since this system will be forced to hold a
torque greater than a multiple of the conversion factor to the reducer, and
not the torque on the motor shaft.
Of course, there may be an opinion that it would be worthwhile to
develop your own control driver for the task, but through the reasons
described in the future, it is obvious that in order to ensure stable and
accurate operation of the drive, you should not try to simplify something
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in this aspect. Therefore, the control drivers will be selected from among
the standard ones, according to the engine. For clarity, we present several
wiring diagrams provided by manufacturers of common stepper motor
control drivers
For each specific driver, accurate connection information can be found
in the public product documentation, based on experience, often even in
the official documentation of manufacturers, some parameters are often
overlooked.
When working with Arduino, the drive control is reduced to using the
standard Arduino – Stepper library.
Different drive drivers can be used for different drive capacities and
the complexity of the tasks assigned to them, but the control program
remains unchanged.
On the control contact DIR must be fed HIGH or LOW -- depending on
this - the stepper motor will rotate in one direction or the other. So, for
testing - it can simply be connected to the GND or 5V connectors of the
Board.
The control signal to the controller has a pulsating nature and is in two
logical States, mainly 5V and 0V. The simplest example of a control program
on Arduino will be the following sketch:
int pul = 13;
int dir = 12; void setup() { pinMode(pul, OUTPUT); pinMode(dir,
OUTPUT); digitalWrite(dir, HIGH);
} void loop() { digitalWrite(pul, HIGH); delay(10); digitalWrite(pul, LOW);
delay(10); }
4.2.4. Stepper motor. Features of stepper motors
Working with stepper motors, it is important to understand what these
engines are and the peculiarities of their use, so we will pay a little attention
to the analysis of this issue.
In the ideal engine there is no friction, its torque is proportional
americam windings and a single electrical characteristic is inductance.
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Inductance L characterizes the ability of the winding to store energy in a
magnetic field. Inductance have the property of inductive resistance, that
is, resistance to alternating current, which is the greater the faster the
current changes, which means that the inductive resistance increases with
the speed of rotation of the engine. According to Ohm's law, the current is
directly proportional to the voltage and inversely proportional to the total
resistance, which means that the winding current decreases with
increasing rotational speed. Since the moment is proportional to the
Amper-turns and the current is inversely proportional to the velocity, the
moment will also be inversely proportional to the velocity. That is, at zero
speed the time tends to infinity, as the speed increases the torque (and
current) begins to approach zero.
Electrically, the real engine differs from the ideal one mainly by the
non-zero resistance of the winding, as well as by the ferromagnetic
components, which tend to be saturated with a magnetic field, which leads
to hysteresis losses and eddy current losses. Saturation limits torque, and
eddy currents and hysteresis losses cause the motor to heat up. Consider
the curve of the torque of the stepper motor speed.
As can be seen from the graph, at a speed below a certain limit, the
moment, and hence the current, grow very quickly, up to levels that lead
to damage to the motor. To avoid this, the driver must limit the current rise
to a certain value. Since the moment is proportional to the current, the
moment will be constant from the moment of holding to the speed
threshold, and at a speed above the threshold ‒ the current will be limited
by the inductance of the windings will begin with a segment where the
moment is constant, to the point when the motor stops generating and
consuming reactive power. A real step motor has losses that modify the
ideal speed-torque characteristic. A particularly large contribution of time
from Zubovich harmonics of the magnetic field (it is sometimes indicated
in documentation on the engine). There are always losses in the engine,
and the faster the shaft of the stepper motor rotates, the greater the
losses, and they also need to be counted from the ideal characteristic.
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Note how real power falls with increasing speed, including on the
segment of "constant power". The rounding at the transition point is due
to the transient in the circuit ‒ the driver is gradually converted from a
current source to a voltage source.
The stepper motor is highly susceptible to resonance, being in fact an
analogue of the pendulum "suspended on a spring load", where the load
is the rotor, and the spring is a magnetic field, and has a frequency of
natural oscillations, which depends on the current and inertia of the rotor.
At the moment when the difference between the phases of torque and
speed reaches a value of 180 degrees. there is a resonance - the change in
the magnetic field begins to coincide with the speed, and the speed of the
rotor when positioning on a new step becomes too large. At resonance, a
significant part of the magnetic field energy is used to overcome the inertia
of the rotor when oscillating near the equilibrium position, which is
expressed in a significant drop in torque on the shaft. The accumulated
kinetic energy of the rotor is consumed in the event of resonance for about
1-10 seconds., therefore, you can accelerate the engine, passing the
resonance zone without consequences, but it will not be possible to work
for any long time- the shaft will stop. To eliminate this phenomenon in the
drivers used by various anti-resonant algorithms.
The output power of the motor (speed × torque) is proportional to the
voltage divided by the square root of the inductance. If we double the PWM
voltage, we get another SMV curve, which is higher, and the power in the
area of constant power will double. With the flow of another picture. The
figure below shows what will be when the driver current is set to 2 times
the nominal for the engine. The motor starts to emit 4 times more heat,
and the torque at low rpm increases less than 2 times due to saturation of
the winding cores.
As you can see, the power is not increasing at all. It is always
recommended to set the current on the driver equal to the nominal value
for the motor. This will reduce vibration at low frequencies, improve the
characteristics of the course in microstep mode.
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The power of the stepper motor is not constant, it is large enough at
the time of retention, somewhat increases at a speed up to a certain point
("breakout point"), after which it begins to gradually decrease with
increasing ferromagnetic losses. Torque with increasing speed drops
sharply, and the graph of the fall can be considered roughly linear.
To select the drive will have to use empirical approximations.
To select a similar power drive on stepper motors, start with the
definition of the moment, which should give the motor and the desired
speed. For example, the drive needs a torque Md = 10 Nm at a speed of n
= 200 rpm. To calculate the engine power P use the approximate formula:
This approximation allows in practice to estimate the engine power
directly from the speed - torque characteristics of the motor graph. As a
rule, the power and speed of the active window and the asynchronous
motor are known, and you can perform the reverse operation - to obtain
the moment from the inverse formula:
where z - is the number of complete steps per revolution, f is the
frequency of steps in Hz, and then
the torque and speed of rotation to choose the engine based on the
documentation and the curve of the moment.
4.3. Noise and its elimination on the example of the car
4.3.1. Sound source
As you know, perceived in the car noise can be of two types:
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