CA2148577C

CA2148577C - Method for myoelectric control of an artificial limb

Info

Publication number: CA2148577C
Application number: CA002148577A
Authority: CA
Inventors: Peter Gammer; Karl Susz; Janos Kalmar
Original assignee: Otto Bock Healthcare GmbH
Current assignee: Otto Bock Healthcare GmbH
Priority date: 1994-05-10
Filing date: 1995-05-03
Publication date: 2007-01-09
Anticipated expiration: 2015-05-03
Also published as: JP2005334675A; JP3923987B2; CN1116855C; CA2148577A1; JPH0898853A; DK0681818T3; RU2108768C1; RU95107479A; ES2079337T1; ES2079337T3; DE4416509C1; EP0681818A3; DE59508553D1; AU1786695A; ATE194477T1; EP0681818A2; EP0681818B1; CN1117839A; TW241202B

Abstract

For myoelectric proportional control of an electric-motor-powered artificial limb, either proportional speed regulation or proportional gripping force regulation is performed. In proportional speed regulation, the set rpm value is determined from various rpm setpoints. The rpm setpoint is compared with the actual measured rpm of the motor and fed to a proportional-integral (PI) regulator as a system deviation. In the PI regulator, the system deviation is converted into a pulse-width-modulation (PWM) signal and the drive motor is then controlled at an rpm proportional to the actual electrode voltage. The actual current value of the drive motor is measured and compared with a set current maximum which, when exceeded, causes the PI regulator to be switched off. In proportional gripping force regulation, the maximum gripping force is divided into stages, with the actual electrode voltage corresponding to a certain number of stages. Each stage is associated with a pulse-width- modulation (PWM) value and a current shutoff value. Counting takes place from 0 to the number of stages specified by the current electrode voltage. Each PWM value is output with the equivalent current shutoff value. At the same time, the drive motor current is measured and compared with the outputted current shutoff value. When this shutoff value is reached, the counter is increased by 1 and the next PWM value is output. When the counter reaches the number of stages defined by the actual electrode voltage, the set gripping force is reached and the drive motor is shutoff. The current value of the drive motor is measured and compared with a set current maximum which when exceeded, causes the drive motor to shutoff.

Description

214~~~~
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 21~~~~~

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 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 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 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.

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 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 - 2~.48~ ~~

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.

21~~~~

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 .

~~~~e~~~

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.

~ 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.

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.

2~48~7~

~ 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.

Claims

1. 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 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 l) a respective actual current value of the drive motor is measured and compared with a predetermined current maximum, the exceeding of which likewise causes the drive motor to be disconnected.

2. The process according to Claim 1, wherein in event of the predetermined gripping force being attained, amplitude of the actual electrode voltage is stored and established as a switching-threshold for a possible reflex clutching of the hand prosthesis.

3. The process according to Claim 2, wherein for the purpose of enabling a rapid grasping at maximum gripping force, in event of the predetermined switching-threshold being exceeded by the electrode voltage the counter-reading is set to a value higher than zero.

4. The process according to any one of Claims 1 to 3, wherein a pair of electrode signals is measured, averaged and supplied to a locking stage where the pair of electrode signals are compared with predetermined switching-thresholds and a corresponding direction of the drive motor is unlocked.

5. The process according to any one of Claims 1 to 4, wherein the measured and an optionally locked electrode signal is supplied to a control circuit, and a gripping-force switch subdivides the control process into proportional speed regulation and gripping-force regulation, whereby when the electrode signal does not attain a switching-point lying above the switch-over point of an automatic drive the process operates with proportional speed regulation but in event of the switching-point being exceeded the process operates with proportional gripping-force regulation.