METHOD FOR CONTROLLING AN ARTIFICIAL ORTHOTIC OR PROSTHETIC KNEE JOINT

Abstract

The invention relates to a method for controlling an artificial orthotic or prosthetic knee joint, on which a lower leg component is arranged and which is assigned a resistance device having at least one actuator, by means of which the bending resistance is modified depending on sensor data that is determined during use of the orthotic or prosthetic knee joint by means of a sensor, wherein the absolute angle of the lower leg component is determined exclusively by means of at least one inertial sensor, the angle determined is compared with at least one threshold value, and the bending resistance is modified when the threshold value is reached.

Claims

1. A method for controlling an artificial orthotic or prosthetic knee joint (2), on which a lower leg component (4, 5) is arranged and which is provided with a resistance device (3) having at least one actuator (31), by means of which the bending resistance is modified as a function of sensor data which are determined by means of a sensor (7) during use of the orthotic or prosthetic joint, characterized in that the absolute angle of the lower leg component (4, 5) is determined exclusively by means of at least one inertial sensor (7), the angle determined is compared with at least one threshold value, and the bending resistance is modified when the threshold value is reached.

2. The method as claimed in claim 1, characterized in that an angular velocity of the lower leg component (4, 5) is calculated from the sensor data of the at least one inertial sensor (7), and the bending resistance is reduced only when the angular velocity is not equal to zero. 25

3. The method as claimed in claim 1 or 2, characterized in that the absolute angle is determined exclusively by means of one or more inertial sensors (7), which is or are fastened on the lower leg component (4, 5) or on an orthotic or prosthetic component fastened distally thereon.

4. The method as claimed in one of the preceding claims, characterized in that the absolute angle is determined by means of 2D or 3D magnetic field sensors, 2D or 3D acceleration sensors and/or 1D, 2D or 3D gyroscopes.

5. The method as claimed in one of the preceding claims, characterized in that the sensor data of a plurality of inertial sensors (7) are merged together.

6. The method as claimed in one of the preceding claims, characterized in that the threshold value of the absolute angle of the lower leg component (4, 5) is adjusted to the value which the lower leg component (4, 5) adopts at the end of the standing phase.

7. The method as claimed in one of the preceding claims, characterized in that the bending resistance is switched between two fixed values.

8. The method as claimed in one of the preceding claims, characterized in that the bending resistance is modified when the angular velocity has reached a zero point and reversal of the movement direction of the lower leg component (4, 5) is determined.

9. The method as claimed in one of the preceding claims, characterized in that the angular acceleration of the lower leg component (4, 5) is determined, and the bending resistance is increased, or not reduced, when a threshold value is exceeded.

Description

[0016] An exemplary embodiment of the invention will be explained in more detail below with the aid of the figures, in which:

[0017] FIG. 1shows a schematic representation of a prosthetic device;

[0018] FIG. 2shows a flow chart of the control; and

[0019] FIG. 3shows a lower leg angle diagram.

[0020] FIG. 1 provides a schematic representation of a prosthetic leg with a prosthesis shaft 1 for receiving an upper leg stump and for fixing the prosthetic leg to a patient. Arranged at the distal end of the prosthesis shaft 1, there is a prosthetic knee joint 2 which is equipped with a resistance device 3, for example in the form of a hydraulic damper or a wrap spring brake. At the distal end of the prosthetic knee joint 2, a lower leg tube 4 and a prosthetic foot 5 are provided as further distal components. The functional elements: prosthesis shaft 1, prosthetic knee joint 2, lower leg tube 4 and prosthetic foot 5 are enclosed by a cosmetic covering 6 in order to impart a natural overall impression as far as possible.

[0021] In the exemplary embodiment represented, an inertial sensor 7 as an angle pickup is arranged on the lower leg component, which consists of the distal part of the prosthetic knee joint 2, the lower leg tube 4 and the prosthetic foot 5. The inertial sensor 7 may be formed as a magnetic field sensor, acceleration sensor or gyroscope. It is also possible for a plurality of inertial sensors 7 to be arranged on the lower leg component, for example in addition to the fitting at the distal part of the prosthetic knee joint 2 on the lower leg tube 4 or the prosthetic foot 5. The acceleration sensors and magnetic field sensors may be formed as 2D or 3D sensors, and in order to determine gyroscope data a gyroscope may be formed as a 1D, 2D or 3D gyroscope. A plurality of inertial sensors of the same type may be arranged on the lower leg component, likewise inertial sensors 7 of different types, for example an acceleration sensor and a gyroscope, may be fixed on the lower leg component.

[0022] The inertial sensor 7, which is formed as an angle sensor, determines the angle value of the lower leg component relative to a center of mass line 8, which extends through a center of gravity 9. The center of gravity 9 corresponds to the center of mass of the body of the patient, and the angle is determined between the center of mass line 8 and the longitudinal extent of the lower leg component through the center of mass 9 of the body in the extended position of the prosthetic knee joint 2 at the end of the standing phase. The orientation of the lower leg component in FIG. 1 is defined by the connecting straight line 10 along the longitudinal extent of the lower leg tube 4 through the tilt axis of the prosthetic knee joint 2. In the position represented, the prosthetic knee joint 2 is in the extension position at the end of the standing phase. The existing angle of the lower leg component relative to the center of mass line 8 is stored as a threshold value. At this value, there is extension of the prosthetic knee joint 2 and of the hip joint, and therefore a step rear position of the leg, and it is safe for the user to initiate the swinging phase. If a rolling process of the prosthetic foot 5 in the anterior direction is simultaneously detectable, which is manifested by an angular velocity >0 and the angle increase, there is likewise a step in the anterior direction. On the basis of these data, an actuator 31 of the resistance device 3 of the prosthetic knee joint 2 is activated in such a way that existing standing phase damping is reduced and is kept low until the angular velocity has reached a zero point in the central swinging phase. An angle range for the position of the lower leg component at the end of the swinging phase of a normal step may be stored on the basis of empirical values. If a full step is not executed, i.e. the angle range is not reached, or a non-smooth variation in the angular velocity is established, the resistance device 3 is influenced by means of the actuator 31 in such a way that there is an increased bending resistance. If the interrupted step is continued, a reduction of the flexion resistance may be carried out again in order to end the step.

[0023] FIG. 2 represents the functionality of the control as a diagram. After the start of the control, the respective sensor value is determined in a first step 20. A plurality of sensor values 7.sub.1, 7.sub.2, 7.sub.3 may be measured by means of the inertial sensors 7. Besides the possibility of determining three sensor values 7.sub.1, 7.sub.2, 7.sub.3 of the lower leg component, for example by using three different inertial angle sensors 7 such as a magnetic field sensor, an acceleration sensor and a gyroscope, it is also possible to determine a plurality of sensor values by using a plurality of inertial sensors 7 of the same type. In principle, it is also possible to detect the sensor value with only one inertial sensor 7.

[0024] The data detected by the inertial sensors 7 are merged in a further working step 21, in order to compensate for inaccuracies and have a data situation which is as complete as possible for the calculation of the absolute angle . If only one inertial sensor 7 is provided, the data do not need to be merged.

[0025] In a subsequent evaluation step 22, the sensor data 7.sub.1, 7.sub.2, 7.sub.3 of the angles of the lower leg component with respect to the line of gravity 8 are calculated. It is likewise possible to calculate the angular velocity of the lower leg component in parallel therewith in a further working step 23.

[0026] The angle , calculated for example by a Kalman filter, with respect to the line of gravity 8, is then compared in a further step 24 with a threshold value X which was established beforehand. As soon as the absolute angle is greater than the preset threshold value X, in a control situation in which the angular velocity is not taken into account, the actuator 31 may be activated in a further step 26, in such a way that the damping device 3 adopts a reduced resistance for initiation of the swinging phase. If the threshold value is not reached, in the alternative step 27 the actuator 31 is not correspondingly actuated and the resistance of the resistance device 3 remains unchanged.

[0027] If the angular velocity is also calculated together with the angle in step 23, and the angular velocity is greater than zero, then a walking movement is detected in step 28. In a combining step 25, the angle and the angular velocity are coupled with one another, and if both threshold values are present or are exceeded, the actuator is activated according to step 26, while if one of the two threshold values for the angle or the angular velocity is absent, the actuator 31 is not activated according to step 27.

[0028] FIG. 3 represents a diagram of the lower leg angle as a function of time. The individual positions during a stepping cycle are represented above the diagram. The step begins with planting of the heel, the so-called heel strike, so that the lower leg angle is initially slightly increased. In the course of continued stepping, the prosthetic foot is fully planted and after about 0.4 second the lower leg, or the lower leg component, reaches the vertical, so that the lower limb angle is 0. As a result of movement further forward, the lower leg, or the lower leg component, is tilted further so that the lower leg is increased in magnitude. The end of the standing phase is reached after about 0.8 second, and the front of the prosthetic foot leaves the ground so that swinging phase flexion occurs. At the end of the swinging phase, in the case of normal walking, the reversal point is reached at a lower leg angle of 45 . This is the case at about 1.3 seconds. A movement reversal subsequently takes place, the lower leg angle is reduced in magnitude and approaches the vertical, until at about 1.7 seconds it reaches the vertical again and is then tilted further relative to the vertical, but this time in the positive angle range, by the foot being extended forward. Over the profile of the lower leg angle , a plurality of threshold values may be established for this lower leg angle . If these threshold values are reached or exceeded, or fallen below, depending on the direction in which the threshold value is considered, the variation of the bending resistance may be carried out; in particular, after a maximum bending angle is reached, the bending resistance may be increased in order to provide security against unintended and unbraked folding of the knee joint in the event of stumbling. Simple and effective control of the damping of the resistance device can be carried out by means of the setting of the lower leg relative to the vertical.

[0029] It has been found that the simple algorithm described above can be used well in order to switch to and fro between standing phase damping and swinging phase damping. The control is particularly reliable when the angle value is greater than a previously stored threshold value, which is adjusted by operating a control knob on the prosthetic device when the patient is in backward movement and the angular velocity is greater than zero. If both conditions are satisfied, the resistance device 3 may be switched over from high standing phase damping to lower swinging phase flexion damping by switching a hydraulic valve or by releasing a lock of a brake device, for example.

[0030] The use exclusively of inertial sensors reduces the costs, since the torque sensors based on strain gauges for alternative methods are very expensive. Furthermore, inertial angle sensors are free from wear.

[0031] Besides the first derivative of the angle signal for determining the angular velocity, it is also possible to use the second derivative of the angle signal, in order to determine the angular acceleration. The acceleration signal may be used to detect falling, the angular acceleration exceeding a fixed limit value indicating that the smooth walking movement is interrupted and a different damping value would be more expedient, usually increasing the flexion resistance.

[0032] Furthermore, by observing the angle profiles, it is also possible to deduce the walking speed; in addition, not only may the resistance behavior, and therefore the damping behavior during the swinging phase, be switched to and fro between two values in a binary fashion, but the switching may be adjusted in very small time intervals for discrete angle values or for any angle value, or any sampling of the angle value. For example, hydraulic valves may be adjusted stepwise or brake devices, which may likewise be used in resistance devices, may be adjusted in an adapted fashion in order to produce a smooth gait.

[0033] Furthermore, on the basis of the angle profiles, the resistance device may be adjusted in such a way as to allow standing phase flexion in the event of a heel strike with an extended joint, by reducing the flexion resistance. After this has happened, a progressive variation of the damping may also be provided, so that, after reduction of standing phase flexion after the heel strike, strongly increasing damping occurs, which allows bending up to a particular angle value after the heel strike; in addition, a further increase in the angle is prevented by adjustment of the resistance device.

[0034] The artificial knee joints may be used both as prostheses, as described in the exemplary embodiment, and as ortheses. The resistance devices may be configured as simple locks, complex hydraulics or wrap spring brakes. By taking the angular velocity into account, it is also possible to use drives in order additionally to permit folding or extension. For security against undesired folding or reduction of the damping under load, i.e. in the case of an applied flexion moment, the actuator may be configured in terms of power so that the switching power to be applied when a flexion moment occurs lies above the output power of the actuator, so that reduction of the damping and therefore abrupt folding cannot occur.