Lecture 13 and 14

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Lecture 13 and 14

• Dimitar Stefanov

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Feedback in the prosthesis control

• Visual observation
• Transmission of force and pressure through the
  socket
• Indirect feedback from the prosthesis to the body
  (force - , slip- and pressure-transducers mounted
  to the terminal device plus interface devices
  attached to the residual limb)

Force sensors, slip sensors, pressure sensors
Residual limb or other intact area of the body
Interface devices
bladders, plungers or vibrotactile actuators
Terminal device
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Prostheses controlled by myoelectric signal (MES)
The MES, captured on the surface of the skin, has
been exploited as a control source of powered
prostheses.

• In the 1950's the first powered prosthetic limb
  was built.
• By 1964, Robert Scott of the University of New
  Brunswick developed the first control system for
  prosthetic limbs.

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The myoelectric signal (MES)
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The myoelectric signal (MES)

• A single motor unit
• A motoneuron activates a group of muscle fibres
  of the motor unit
• A linear model
• A series of excitatory impulses from a motor
  neuron innervate a group of muscle fibres.
• The excitatory impulses are separated by the
  intervals x1, x2,.., xn.
• The motor unit action potential (MUAP) is the
  electrical response of each impulse.
• Potentials between the electrodes, placed in
  proximity to the motor unit, depends on the MUAP.
• The speed of the pulse wave from 2 to 6 m/s, in
  inverse relation to the fibre diameter
  (Buchtal54)
• In order to sustain a voluntary muscle
  contraction, the motor units must be repeatedly
  activated.
• Series of MUAPs motor train.

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MES during sustained contractions

• Control of a skeletal muscle is achieved by
• Recruitment varying the number and composition
  of activated motor units
• Firing rate varying the rate of activation of
  the individual motor units.

Size principle of recruitment the smallest
motor units are being recruited first
(Henneman65).

• At a contraction, which is up to 30 from the
  maximal contraction, the recruitment process in
  dominated.
• At a contraction, which is between 30 and 75
  from the maximal contraction, the firing rate in
  dominated in the force production.
• At a contraction, which is over 75 from the
  maximal contraction, the force increasing is a
  result solely of the the firing rate increasing.

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• Variants of prosthetic devices
• One MES and one single device
• One MES and n single devices (at each moment only
  one single device of the prostheses is
  controlled)
• n MES and n single devices
• n MES and m single devices (mgtn) . At each moment
  p single devices are operated. ( )

single device motor or lock belonging to the
hand, elbow, or wrist of the prosthesis In
case of lock, only one channel of MES is
required. In case of motor control, two channels
of MES are required. At each moment, one or
more single devices can be operated.
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• Control information from the MES, which relates
  to variant 2
• Selection of the device, which will be controlled
  from the MES (choice of a motor or a lock)
• Information about the speed of the selected
  single device.

Example The length of the signals from one and
the same MES source can determine the device and
its level of activation. Short MES for selection
of the single device and long lasting MES for
activation of the same device. The amplitude of
the long signal determines the level of
activation of the device.

• Simple algorithms of prosthetic control
• Control signal which relates to the estimated
  amplitude of the MES (Dorcas, 1966)
• Control signal which relates to the rate of
  change of the MES (Childrese, 1969)

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• Speed control of the motor of the prosthesis
• Analogue control (speed, proportional to the
  amplitude of the control signal)
• Binary control (The motor is either not activated
  or moves on a constant speed when the amplitude
  of the control signal exceeds certain threshold).

The relationship between a users input signals
and the movement of the prosthesis motorized
components is called a control strategy.
Analogue control of a simple device
Electrodes on agonist/antagonist muscle The
result signal determines the direction and the
force in the controlled joint.
In the most commonly used myoelectric control
strategy a flexor muscle signal is used to close
a hand and an extensor muscle signal is used to
open the hand. We refer to this as classic
myoelectric control.
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Block diagram of analogue proportional control
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Nonlinear integrator
Example
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Binary control
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One-channel control of a terminal device
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• In the past, the control strategy of a powered
  prosthesis was a built-in feature of the
  electronic circuit that was sold with the
  prosthesis.
• Changing the control strategy meant changing the
  electronics.

Programmable myoelectric control
The electronics can be easily programmed to
implement the selected strategy.
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MyoMicro TM
Software product of Variety Ability Systems Inc.,
Ontario, Canada
http//www.vasi.on.ca/prosthetic/myomicro/myomicro
.html
Software product for easily programming of the
electronic control systems from Variety Ability
Systems Inc. or Liberty Technology.
SPM from VASI Inc.
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MyoMicro TM
VARIGRIP modules from Liberty Technology.
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MyoMicro system
Hardware module software package

• The MyoMicro hardware module is connected both to
  the computers parallel port and computers
  serial port.
• The programmable prosthesis controller is
  connected also to the same hardware module.

The MyoMicro software allows selection of a
control strategy and programming the circuit
board with the selected control strategy using
"strategy wizard" .
Flexible programming - One strategy can be
initially used and later the control strategy can
be changed.
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• In MyoMicro, control strategies are described
  graphically as a collection of primitive building
  blocks.
• Three basic types of building blocks
• Input devices - correspond to electrodes,
  touchpads, and switches
• Signal processors - represent the required
  modification of the input signals (comparator,
  differencer, etc.).
• Motor Controls power amplifiers which operate
  the physical effectors (hand, elbow or wrist).

• Supported control strategies
• Proportional Control strategy
• Digital Control strategy
• "doing more with less strategy.

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Some "doing more with less control strategies
A./ Level-sensitive control
Digital control only

• The input signal is compared to two preset
  thresholds.
• Typically, if the input signal is in the middle
  region the hand will close.
• If the input signal is in the high region, i.e.,
  above the upper threshold, the hand will open.
• When the input signal is in the middle region the
  output is slightly delayed to avoid inadvertent
  hand closing while the signal is in transition
  between the high and low regions.

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B./ Rate-sensitive control
Proportional or digital control

• When the input signal crosses the lower
  threshold, the system waits for a preset period
  of time (tens of milliseconds) and then compares
  the signal to the upper threshold.
• If the signal is still below the upper threshold
  after the preset time delay, it is considered to
  be a slowly rising signal and the hand closes.
• If the signal exceeds the upper threshold before
  the end of the delay, it is considered to be a
  quickly rising signal and the hand opens.

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C./ Mode switching control
Control a hand and a wrist with two input signals
only.

• At first the control signals operate the hand
  (flexors to close, extensors to open).
• Cocontraction of the flexors and extensors
  switches control to the powered wrist. A flexor
  signal will then supinate the wrist and an
  extensor signal will pronate the wrist.
• To switch control back to the hand, the user can
  cocontract again.
• If no signal is received for a preset period of
  time, control automatically reverts to the hand.

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• The prosthesis controller is adjusted and
  preprogrammed via the computer.
• MyoMicro possesses option for monitoring of the
  users input signals from electrodes during the
  adjustment.
• User can try a variety of different strategies
  and the best suitable strategy can be found
  easily.

Menu examples
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(No Transcript)
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Other attempts to increase the number of states
from the surface MES

• Usage of many sites (or channels) of amplitude
  coded information (Schmiedl77, Wirta78,
  Almstrom81). A vector of features is subject of
  some pattern recognition. Disadvantage
  Requirement of many electrode sites, problems in
  locating and maintenance of the integrity of
  patterns.
• Usage of time-series model (Graupe82,
  Doershuk83). Disadvantage sensitivity to the
  signal amplitude.

All methods for incensement the number of
signals, commented until now, use steady-state
analysis to the MES (signals that are produced
during constant muscle effort).
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Recent tendency reduction of the number of
electrodes and reduction of the sensitivity to
the electrode placement.
Multifunction Myoelectric Control Systems
(Control of Artificial Limbs using Myoelectric
Pattern Recognition)
Electrodes placed over a set of muscles
Multiple control signals
Control system's ability to control more than one
device, such as an elbow and a wrist by one MES.
Control of prostheses for high-level amputations
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Structure of the transient MES
MES which coincide with the onset of rapid
contractions Hudgins (1991) placed a single pair
of surface electrodes over the biceps and triceps
and investigated the signals between the
electrodes during small isometric and
anisometric contractions.
Patterns of transient MES, corresponding to
flexion/extension of the elbow and
pronation/supination of the forearm
Elbow flexion
Elbow extension
Forearm pronation
Forearm supination
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Basmajian (1985) The motor unit recruitment order
appears stable for a given task, once the task
has been learned.
Two channel transient MES patterns

• Kuruganti (1995) The performance of pattern
  recognition Hudgins myoelectric scheme can be
  enhanced by using two channels (localized MES)
  instead of one channel (global MES).
• The localized activity of the biceps and the
  triceps can be better analyzed by two sets of
  closely spaced bipolar electrode pairs.

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Patterns of transient MES using two sets of
electrode pairs
Elbow flexion
Elbow extension
Biceps
Biceps
Triceps
Triceps
Forearm pronation
Forearm supination
Biceps
Biceps
Triceps
Triceps
Localization of the signals provides a greater
distinction between the classes than one channel.
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Hudgins (1991, 1993) investigated the information
contents of the transient burst of myoelectric
activity accompanying the onset of sudden
muscular effort.
The Hudgins multifunction control scheme for
powered upper-limb prosthesis
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• Time-domain features of the MES are used
  (zero-crossing, mean absolute value, mean
  absolute value slope, and trace length)
• Simple multilayer perception (MLP) ANN is used as
  a classifier.
• The controller identified four types of muscle
  contractions using signals measured from the
  biceps and triceps.

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• Multifunction control from a single site
• Output control signals derived from natural
  muscle contractions

Results from the experiments Average
classification performance 89 for a group of
15 subjects (9 normally-limbed subjects and 9
persons with limb deficits).

• Two ways for further improvement of the
  classification accuracy
• Improvement the classifier (type and
  characteristics)
• Improvement the means of the signal
  representation (improvement of the feature set).

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• Although some classifiers possess obviously
  better characteristics, the effect of the
  improved classifier to the recognition
  characteristics is not so significant.
• The signal representation most significantly
  affects the classification performance (Bishop
  96).

• The problem of the signal representation
• for pattern classification can be broken down
  into following 3 tasks
• Feature extraction
• Dimensionality reduction
• Classification.

Procedure of signal representation
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• The transient MES patterns have their structure
  both in the time and the frequency domain.
• It is supposed that the better recognition
  methods should be based on time-frequency
  representation (time-frequency based feature set).

Feature sets based upon the short-time Fourier
transformations, the wavelet transform
Dimensionality reduction very important
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Relating Surface EMG Amplitude to Joint Torque
Ongoing project at the Worchester polytechnic
Institute, Dept. of EECS, http//www.ece.wpi.edu/
ted/research.htm
Relation between the surface EMG amplitude to the
tension (or force) produced by individual
muscles. non-linear relationship

• Difficulties
• EMG from muscles other than that which the
  experimenter intends to record may be included in
  the signal ("cross-talk).
• Relating EMG to individual muscle tension
  requires independent verification via direct
  mechanical measurement of individual muscle
  tension.  At present, there is no practical
  method for reliably making such in-vivo
  measurements.

Easier task is finding relations between the
surface EMG amplitude and the joint torque.
EMG-torque relationship can be modeled as a
polynomial relationship in case of
constant-posture, nonfatiguing contractions about
the elbow.
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Example of cable driven, body powered prostheses.