Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method for Myoelectric Control of an Artificial Limb
The invention relates to a method for myoelectric
proportional control of an electric-motor-powered artificial
limb, especially a hand prosthesis, with the voltage of the
respective electrode signal being measured and conducted to
a control.
DE 18 08 934 B2 teaches:a myoelectric control circuit for
proportionally controlled electric-motor-powered, artificial
limbs especially hand prostheses, with the drive motor of
the artificial limb being connected to the power supply
pulsewise for the duration of its activity. A DC voltage
proportional to the myovoltage is superimposed on a sawtooth
voltage with a constant pulse height and pulse repetition
frequency, with the portions of this total voltage that
exceed a constant threshold each causing the drive motor to
be connected to the power supply.
DE 22 36 969 B2 discloses a motor brake circuit for a
myoelectric pulse length modulated control circuit of a
proportionally controlled electric-motor-powered artificial
limb, especially a hand prosthesis, in which the myovoltage
is amplified, rectified, integrated, and superimposed on a
sawtooth pulse voltage with a constant pulse repetition
frequency and pulse height. The superimposed voltage is fed
to the input of a square wave pulse generator to control the
pulse duty factor of the square wave generator pulses as a
function of the amplitude of,the recorded myovoltage. The
square wave generator pulses are used to control a breaker
unit by which a drive motor is connected pulsewise to a
power supply. The pulse length modulated square wave
generator pulses are fed through an amplifier to an
integrating element, whose output voltage is proportional to
the average of the pulse duty factor of the square wave
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generator pulses, which is differentiated in a downstream
differentiating element and controls an additional switch on
the breaker unit that short-circuits the armature winding of
the drive motor pulsewise. This additional switch only
switches in response to signals with a polarity of a
corresponding threshold value and can be formed by a biased
transistor.
DE 23 54 885 A teaches a myoelectric control circuit for
pulse length modulated proportionally controlled electric-
motor-powered artificial limbs, especially hand prostheses,
in which the myovoltage is amplified, rectified, and
superimposed on a sawtooth voltage with a constant
frequency. The total voltage is fed to the input of a pulse
generator designed as a form of Schmitt trigger, to vary the
pulse duty factor of the generator pulses as a function of
the strength of the recorded myovoltage. The generator
pulses are used to control a breaker unit by which the drive
motor is connected pulsewise to a power supply. In this
circuit, a voltage proportional to the drive motor current
can be supplied to the input of the pulse generator and/or a
voltage proportional to the motor voltage can be supplied to
the pulse generator as a bias voltage. A measuring
resistance is connected in series between the breaker unit
and the motor, both terminals of said resistance being
connected to the inputs of a differential amplifier, whose
output is connected to the input of the pulse generator.
The gripping force and speed controls of electromechanical
hand prostheses which are quasi-proportional to the EMG
(electromyogram) signal are known. The most frequent method
used to regulate rpm in DC motors is the gulse-width-
modulation method (PWM). A periodic DC voltage is supplied
to the motor, whose frequency is predominantly above the
hearing range, between 18 and 40 kHz. Depending on the
magnitude of the EMG signal, voltage pulses of equivalent
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length are fed to the motor and integrated by the mechanical
inertia of the motor armature into an equivalent voltage
average. It has also been proposed to short circuit the
motor during the scanning pauses in order to achieve
improved rpm regulation. However, this results in an
increase in the power consumption of the entire system.
Effectively satisfactory regulation in the opening and
closing direction as well as in the buildup of force are not
possible with known control systems because spring moments
acting from the exterior complicate linear speed regulation
by the inner hand and plastic coverings, and also because
the buildup of gripping force cannot be regulated optimally,
even if an automatic drive is incorporated.
The goal of the invention is to improve the method described
at the outset.
Certain exemplary embodiments can provide a process for myo-
electric proportional open-loop control of an
electromotively actuated artificial limb including a hand
prosthesis, wherein a voltage of an electrode signal is
measured and passed on to an open-loop control that
controls, via a closed-loop feedback control system, a
rotary speed of a drive motor and a gripping force to be
provided by the artificial limb, the process comprising:
a) for the purpose of implementing a proportional regulation
of speed, a desired value of rotary speed that is assigned
to the electrode voltage measured in a given case is
ascertained from various desired values of rotary speed that
relate to the drive motor and are each defined by a
particular electrode voltage; b) an actual rotary speed of
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3a
the drive motor is measured, the desired value of rotary
speed ascertained is compared with the measured actual
rotary speed of the drive motor and is supplied through
negative deviation to a proportional-integral (PI)
controller; c) in the PI controller a control deviation
corresponding to set parameters is converted into a pulse-
width modulation (PWM) signal, and the drive motor, which is
thereby given a rotary speed proportional to the measured
electrode voltage, is driven with said PWM signal; d) a
respective actual current value of the drive motor is
measured and compared with a predetermined current maximum,
the exceeding of which causes the PI controller to be
disconnected; e) for the purpose of implementing a
proportional regulation of gripping force, building-up of
gripping force in stepwise manner; f) a maximum gripping
force is subdivided into stages, whereby actual electrode
voltage corresponds to a particular stage; g) a particular
pulse-width modulation (PWM) value and an associated
current-disconnection value are assigned to each stage; h)
in a counter within, counting is effected from a counter-
reading zero up to a stage predetermined by an actual
electrode voltage, the PwM value selected by the counter in
a given case being output with a current-disconnection value
that is equivalent to it; i) in parallel with this, a
current of the drive motor is measured and compared with the
current-disconnection value that is output in a given case;
j) in the event of the disconnection value being attained,
the counter is increased by one and a subsequent PWM value
is output; k) if the counter attains the stage predetermined
by the actual electrode voltage, such that the predetermined
gripping force is attained, the drive motor is disconnected;
and 1) a respective actual current value of the drive motor
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3b
is measured and compared with a predetermined current
maximum, the exceeding of which likewise causes the drive
motor to be disconnected.
This goal is achieved according to embodiments of the
invention by the following features for proportional speed
regulation:
Proportional speed regulation is performed, with the rpm
setpoint associated with each measured electrode voltage
being determined from various rpm setpoints affecting the
drive motor and each being defined by a certain electrode
voltage; this rpm setpoint is compared with the actual
measured motor rpm and fed as a system deviation to a
proportional-integral regulator (PI regulator); in the PI
regulator the system deviation is converted in accordance
with set parameters into a pulse-width-modulation signal
(PwM signal) and thus controls the drive motor, which as a
result develops an rpm that is proportional to the actual
electrode voltage; the actual current value of the drive
motor is measured and compared with a set current maximum,
with the PI regulator being switched off when this maximum
is exceeded.
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This goal is achieved according to embodiments of the
invention for proportional gripping force regulation by the
following features:
Proportional gripping force regulation is performed, with
the gripping force buildup taking place stepwise; the
maximum gripping force is divided into stages, with the
actual electrode voltage corresponding to a specific number
of stages; each stage is assigned a specific pulse-width-
modulation value (PWM value) and a corresponding current
shutoff value; the circuit counts upward from counter
position 0 up to the number of stages set by the actual
electrode voltage, with the PWM value selected by the
counter in each case being output with the current shutoff
value equivalent to it; at the same time the drive motor
current is measured and compared with the current shutoff
value being output; when this shutoff value is reached, the
counter is stepped up by 1, and the next PWM value is
output; when the counter reaches the number of stages set by
the actual electrode voltage, the preset gripping force has
been reached and the drive motor is shut off; the actual
current value of the drive motor is then measured and
compared with a set current maximum, with the drive motor
likewise being shut off when this maximum is exceeded.
According to the invention therefore, both speed regulation
proportional to the EMG signal and gripping force regulation
are possible.
According to the invention it- is advantageous to combine the
two forms of regulation into one system that operates as
follows:
Two electrode signals are measured and averaged, and fed to
an interlock that compares the signals with internally set
switching thresholds and authorizes the corresponding motor
CA 02148577 2005-09-28
direction. A mechanical gripping farce switch, whose
switching point is above the switching point of an automatic
drive, separates the system into two regulating circuits.
Below the switching point, the system operates with
proportional speed regulation. When the switching point is
exceeded, a switch is made to proportional gripping force
regulation.
Additional features of the invention are the subjects of the
subclaims and will be described in greater detail in
conjunction with additional advantages of the invention with
reference to embodiments.
The drawing shows some of the embodiments of the invention
that serve as examples.
Figure 1 is a block diagram showing the function of a
system according to the invention with
proportional speed regulation and proportional
gripping force regulation;
Figure 2 is a regulating circuit for speed-proportional
operation;
Figure 3 is a regulating circuit for gripping force
proportional operation; and
Figure 4 is a timer-interrupt process for PWM generation.
According to Figure I, two electrode signals are measured,
averaged, and fed to an interlock that compares the signals
with internally set switching thresholds and authorizes the
corresponding motor direction. A mechanical gripping force
switch, whose switching point is above the switching point
of an automatic drive, separates the system into two
regulating circuits. Below the switching point, the system
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6
operates with proportional speed regulation (Figure 2); when
the switching point is exceeded, a switch is made to
proportional gripping force regulation (Figure 3).
With proportional speed regulation, the gripping force
switch is open; according to Figure 1, the preferred
electrode voltage is converted into an rpm setpoint by a
table. The corresponding table value constitutes the
setpoint (set rpm) for the downstream proportional-integral
regulator (PI regulator). With this form of table
conversion it is possible to perform an adjustment to a
specific electrode signal-rpm characteristic
(proportionality). In this connection, evaluation using a
table has the great advantage that minor disturbances in the
input signal can simply be filtered out by conversion using
the table values (filtration) and at the same time
individual characteristics can be generated.
The setpoint that is now available is compared with the
actual measured rpm of the motor and fed as a system
deviation to a PI regulator. The motor rpm is measured in
the PWM gap as a feedback generator voltage. In the PI
regulator the regulating deviation is converted into a PWM
signal in accordance with the set parameters thus
controlling the motor shunt.
The regulating circuit is closed as a result. The motor now
turns at an rpm that is proportional to the actual electrode
voltage.
In a superimposed control circuit, the actual measured motor
current value is compared with a current maximum. When this
maximum is exceeded (the hand encounters a stop in the open
direction), the PI regulator and hence the motor shunt are
shut of f .
2~~~57~
If on the other hand the gripping force switch is actuated
as a result of the set gripping force being exceeded, a
switch is made to proportional gripping force regulation.
The gripping force buildup takes place stepwise, with the
maximum gripping force being divided into stages. Thus the
actual electrode voltage corresponds to a specific number of
stages. A program-internal counter begins to count upward
on two tables (PWM value and the corresponding current
shutoff value) from counter position 0 up to a number of
stages that is set by the actual electrode voltage. The PWM
value selected by the counter is output. At the same time,
the motor current is measured and compared with the current
shutoff value equivalent to the PWM value. When this
shutoff value is reached the counter is increased by 1 and
the next PWM value is output.
The continuous output of increasing PWM and current shutoff
values corresponds to the proportional gripping force
buildup.
When the counter reaches the number of stages preset by the
electrode, the set gripping force has been reached; the
motor is shut off. The amplitude of the electrode voltage
is stored and established as the switching threshold for
possible further gripping by the hand. For this purpose the
muscle must be tensioned correspondingly more to exceed this
switching threshold.
In order to permit a rapid grasp with maximum gripping
force, when a preset switching threshold is exceeded by the
electrode voltage, the counter position is not set to 0 but
to a higher table value. Then the maximum gripping force is
reached in a shorter time and the motor is switched off.
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When a given internally set maximum current value is
exceeded by the actual motor current during gripping force
buildup in the hand, the motor is likewise shut off.
Figures 2 to 4 will now be explained in detail:
Figure 2:
~ Two electrical signals generated by muscle activity
(myosignals) are picked up by skin electrodes,
amplified, filtered, rectified, and fed as signals E1
and E2 to the circuit.
~ An analog/digital conversion (8 bits) is performed by
AD1 and AD2.
~ The values undergo an averaging process (averaging) to
a lock (LOCK).
~ Averaging:
Goal: Generating an average value and
increasing the range, with division only
by half the number of totaled values.
Performance: 128 measured values/64 = digital value.
~ LOCK:
Goal: Choice of active signal, locking of
inactive signal.
Performance:
Rest: If neither A nor B is active, wait until
one of the two exceeds the turn-on
threshold. The turn-on thresholds for
the two directions (hand OPEN/hand
CLOSED) are different.
Hand OPEN: A fixed turn-on threshold
von-open exists .
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Hand CLOSE: A variable turn-on
threshold Uon-close exists
whose value can be
influenced by the current
(force) proportional
operating mode.
Active: If one of the two signals has exceeded
the turn-on threshold, it is deemed to
be an active signal until it drops
beneath the set shutoff threshold Uoff
and again enters the "resting" state.
In the "active" period, the inactive
signal is blocked.
~ If neither A nor B is active ("resting" state), the
Timer Interrupt (TIR) is blocked (TIR locked) by the
STOP branch, in other words PWM generation is
suppressed.
~ If one of the two signals is active, the signal value
is used as the pointer of a table (V table) which
contains a set rpm value for the motor for each signal
value.
~ This set rpm is used as the input value for the PI
regulator.
~ PI regulator:
The difference is generated from the actual (motor) rpm
converted by AD3 and the set rpm (e).
Proportional share: P a ~ v (v = proportional
amplification)
~'.~4~~77
to
a
Integral share: I ~ - (f = slope factor)
f
Starting value k:
a
k = const. - P + I = const. - a ,~ v +
f
k is a digital value that corresponds to a certain
pulse duty factor in PWM generation.
~ PWM limitation:
Depending on the system, the regulator output (k value)
is limited at the top and bottom in order to ensure
reliable regulating function.
~ TIR unlocked:
A or B is active and a PWM pulse duty factor value is
formed (k). Therefore, TIR can now be unlocked (TIR
unlocked).
~ Motor current:
...converted from analog/digital by AD4 and smoothed
during averaging in I/4. This is followed by a
sustained check of the motor current for a system-
dependent current constant I max. If this value is
exceeded by the motor current, the TIR will be locked
(TIR locked).
Figure 3:
Switching in this operating mode is performed by a switch
(gripping force switch) which, by its design, reports
gripping above a certain force threshold (s = 1).
The process in this state should create a proportionality
between the electrode signal and the gripping force.
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~ Value B:
Since a force buildup takes place only when the hand is
closed, the signal B (hand CLOSE) is used as the
reference for the entire process.
~ Description of the individual points of application and
times:
0... Gripping force switch closed (s = 1)
1... 60 ms pause (fix)
2... First value taken from PWM table and output
for 100 ms
3.1... Current I actual measured and compared with
I_set (from the I set table):
a) I actual > I set: TIR is locked.
b) I actual < I set: Measurement of I-
actual for a
max imam o f an
. additional 100 ms.
If the case
described under a)
does not occur
during this time,
TIR is locked after
these 100 ms.
3.2... Independently of I set, I actual is
continuously compared with I max. If I max
is exceeded, shutoff occurs (TIR locked) to
prevent "continuous pumping." This state is
unlocked only~by activating signal A (hand
OPEN).
4... TAB pointer is incremented. (The values in
both tables are numbered from 0 to 18). When
the TAB pointer and B show the same table
number, the desired gripping force has been
reached and TIR can be blocked.
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If this state has not yet been reached, an
advance is made to point 1.
If this state has been reached, B is stored
and used as a new turn-on threshold in LOCK.
As a result, the hand remains in this
(gripped) state and can only be moved from
this position by
Activation of A (hand OPEN) to reduce
the gripping force, or
Activation of H (hand CLOSED) with a B
value that must be higher than the last
value stored (new turn-on threshold)
resulting in an increase in the gripping
force.
If electrode signal B above 1.2 V (corresponds to a
table number > 18) is triggered by a strong muscular
contraction (corresponding to the wish to grip quickly
and powerfully), the TAB pointer, without speeding up
rampwise to the corresponding value, is set immediately
to table number 13.
This is only valid when the TAB pointer is below table
number 13 when activation takes place.
Figure 4:
If the conditions required for issuing the TIR are met, this
8-bit timer triggers an interrupt at the transition from 255
to 0, whereupon a jump is made to the Timer Interrupt
Routine that serves to generate the PWM.
~ Reg.save [1]:
Since the main program is interrupted at an undefinable
place (depends only on the timer running out), the
values that are in the work must be stored.
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~ Channel selection/invert [2]:
A determination is made as to which of the two inputs A
or B is active in order to determine the current
rotational direction of the motor. This is followed by
inversion of the motor signal output prior to that
time.
~ t on/t off [3]:
Determination as to whether one is involved in the
formation of PWM in the "motor on" or "motor off"
state. Accordingly, the value far the on time (t on)
or the off time (t off) is formed from k.
~ t~timer [4]:
The time value calculated in Point 3 is loaded in the
timer and the latter is started. This produces the
actual pulse duty factor.
~ Reg.recall [5]:
The values stored at the beginning are activated once
more and the main process is continued.
