METHOD FOR DETECTING SLIPPAGE WHEN GRIPPING AN OBJECT WITH A GRIPPER

Abstract

The present invention relates to a method for detecting slippage when gripping an object with a gripper, in which use is made of at least one acceleration sensor (5) on a gripping surface (3) of the gripper (1), a sensor signal from the acceleration sensor (5) is captured during the gripping process and filtered in order to obtain a filtered sensor signal in a first frequency range, and the filtered sensor signal or a value derived therefrom is compared with a threshold value (SW.sub.1, SW.sub.2) which, when exceeded, constitutes an indication of slippage of the object. In the method, the acceleration sensor (5) is integrated into a rigid main body (2) and the main body (2) is integrated into the gripper (1) using an elastic material (4) so as to be capable of oscillation on the gripping surface (3) such that, in the event of slippage of the object, said main body can be set in oscillation on the gripper (1) only in a plane parallel to the gripping surface (3). The first frequency range is selected such that the oscillation frequencies of the main body (2) that occur in the event of slippage are within the first frequency range. The proposed method allows simple and reliable detection of slippage.

Claims

1. A method for detecting slippage when gripping an object with a gripper, in which the gripper (1) is provided with at least one acceleration sensor (5) on a gripping surface (3) of the gripper (1), wherein the acceleration sensor (5) is integrated in a rigid main body (2) and the main body (2), with the aid of an elastic material (4) which supports the main body (2) laterally, is integrated in the gripper (1) so as to be capable of oscillation on the gripping surface (3) such that, in the event of slippage of the object, said main body can be set in oscillation on the gripper (1) only in a plane parallel to the gripping surface (3), at least one sensor signal of the acceleration sensor (5) is captured during a gripping process and filtered with one or more frequency filters in order to obtain a filtered sensor signal in a first frequency range, and the filtered sensor signal or a value derived therefrom is compared with a threshold value (SW1, SW2) which, when exceeded, constitutes an indication of slippage of the object, wherein the first frequency range is selected such that oscillation frequencies of oscillations of the main body (2) which occur during slippage of the object, lie within the first frequency range.

2. The method according to claim 1, characterized in that filtering the sensor signal is carried out with a bandpass filter, the passband of which corresponds to the first frequency range.

3. The method according to claim 1, characterized in that a lower limit for the first frequency range is selected at ≥50 Hz.

4. The method according to claim 3, characterized in that the lower and an upper limits for the first frequency range are selected such that a resonant frequency of the main body (2) lies within the first frequency range.

5. The method according to claim 3, characterized in that the first frequency range is selected such that it has a width of ≤200 Hz.

6. The method according to claim 3, characterized in that an upper limit for the first frequency range is selected at ≤400 Hz.

7. The method according to claim 1, characterized in that the threshold value (SW2) is determined from a portion of the sensor signal of the acceleration sensor (5) which lies in a frequency range above the first frequency range.

8. The method according to claim 7, characterized in that from the portion of the sensor signal of the acceleration sensor (5) which lies in the frequency range above the first frequency range, a moving average value is calculated and at least twice this average value is selected as the threshold value (SW2).

9. The method according to claim 1, characterized in that a moving average value of the filtered sensor signal is compared with the threshold value (SW1, SW2).

10. The method according to claim 1, characterized in, that the gripper (1) is additionally provided with at least one force sensor (7) which is integrated in the gripper (1) and with which gripping of an object with the gripper (1) can be detected via a normal force on the gripping surface (3).

11. The method according to claim 10, characterized in that the force sensor (7) is mechanically coupled to the main body (2) in such a manner that the normal force is transmitted to the force sensor (7) via the main body (2).

12. The method according to claim 1, characterized in that the gripper (1) is provided with a three-dimensionally measuring acceleration sensor as an acceleration sensor (5) from the sensor signals of which a normal force acting on the gripping surface (3) is also calculated.

13. The method according to claim 10, characterized in that slippage is assumed only if both the threshold value (SW1, SW2) for the filtered signal of the acceleration sensor (5) is exceeded and the normal force measured with the force sensor (7) or calculated from sensor signals of the acceleration sensor (5) is above a predeterminable threshold value (SWFN) for the normal force.

14. The method according to claim 10, characterized in that in the case of a normal force which lies below a predeterminable value, a different threshold value (SW1, SW2) is used for the filtered signal of the acceleration sensor (5) than in the case of a normal force which lies above the predeterminable value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In the following, the proposed method is explained again in more detail by means of exemplary embodiments in connection with the drawings. In the figures

[0013] FIG. 1 shows an example of the configuration and integration of a main body with integrated acceleration sensor on the gripping surface of a gripper;

[0014] FIG. 2 shows a result of a measurement example during slipping, wherein [0015] a) corresponds to the sensor signal of the acceleration sensor, [0016] b) corresponds to the amplitude response of the sensor signal as a function of frequency, [0017] c) corresponds to the sensor signal filtered with a bandpass filter, [0018] d) corresponds to the amplitude response of the filtered sensor signal, and [0019] e) corresponds to the moving average value determined from the filtered sensor signal compared with a predetermined threshold value;

[0020] FIG. 3 shows an example of the application of a threshold value determined from the upper frequency range in the measurement example of FIG. 2; and

[0021] FIG. 4 shows a flow chart for an exemplary configuration of the proposed method.

WAYS TO CARRY OUT THE INVENTION

[0022] In the proposed method, a rigid main body with an acceleration sensor arranged therein is integrated into the gripper at the gripping surface in such a manner that the main body can only perform oscillations relative to the gripper that are parallel to the plane of the gripping surface. FIG. 1 shows an example of the integration of the main body 2 into the gripper 1, which is only partially shown in this example. The upper side of the main body 2 lies in the plane of the gripping surface 3 of the gripper 1. An elastic mass 4 is arranged between the main body 2 and the material of the gripper 1 and supports the main body 2 laterally and allows oscillation of the main body 2 relative to the gripper 1 in a plane parallel to the gripping surface 3. This elastic mass 4, for example made of silicone rubber, can enclose the main body 2 laterally in a form-fitting manner—as shown in FIG. 1—or only support it in the upper region. On the underside of the main body 2, the main body rests directly against the gripper 1, so that no oscillation of the main body 2 relative to the gripper 1 is possible in this direction (z-direction in FIG. 1). An acceleration sensor 5 with which oscillations of the main body 2 parallel to the gripping surface 3 can be captured is integrated into the main body 2.

[0023] When an object is gripped by this gripper 1, a normal force F.sub.N acts in the z-direction on the main body 2, the upper side or contact surface 6 of which can either come into direct contact with the object or can also be covered by a protective layer.

[0024] In a preferred configuration of the proposed method, a force sensor 7 is additionally arranged on the underside of the main body 2 between the main body 2 and the gripper 1, as indicated only schematically in FIG. 1. This optional force sensor 7 then captures the forces occurring in the z-direction during gripping. Such a force sensor can of course also be integrated elsewhere in the gripper 1. Due to the conical configuration of the main body 2 in this example, increased pressure is exerted on the force sensor 7 during a gripping process with respect to the contact surface 6 of the main body 2, so that the force sensor 7 responds very sensitively to corresponding gripping forces. The force sensor 7 can be used to determine whether an object is being gripped. If the grip is insufficient, slippage of the object may occur, causing tangential forces F.sub.T,x or F.sub.T,y to act on the main body 2 in the x- or y-direction. In this case, the main body 2 is set into corresponding oscillations in x- and/or y-direction relative to the gripper 1, which oscillations are captured by the 2D acceleration sensor 5 for x- and y-direction in this example. It can be seen from FIG. 2 that slippage in this example leads to an oscillation (acceleration) in the range of approx. 120 Hz, wherein this can be detected by the acceleration sensor 5 due to the increased amplitude.

[0025] FIG. 2 shows the result of an exemplary measurement during slippage of a gripped object. Partial figure a) shows the sensor signal of the acceleration sensor for the oscillation in x-direction as a function of time. Partial figure b) shows the amplitude response of this sensor signal as a function of frequency. In the proposed method, this sensor signal is filtered to obtain only signal portions in a specific frequency range, in the present example between 50 and 200 Hz, in which the oscillations of the main body 2 in the gripper 1 caused by slippage occur. Partial figure c) shows a sensor signal filtered in such a manner with a bandpass filter. Partial figure d) shows the amplitude response of this filtered sensor signal as a function of frequency.

[0026] In the proposed method, a threshold value is now specified or determined with which the filtered sensor signal or a value derived from is compared. This threshold value SW.sub.1 can be derived from empirical values or determined by preliminary measurements. In an advantageous configuration still to be described below, the threshold value, then referred to as SW.sub.2, is determined from the sensor signal itself.

[0027] Depending on the configuration of the acceleration sensor, the sensor provides a sensor signal a.sub.x for a direction (for example x-direction) or—in the case of a two-dimensional acceleration sensor—two separate sensor signals a.sub.x, a.sub.y for x- and y-direction. These sensor signals can then either be evaluated separately and in each case compared with the threshold value or further processed to obtain a signal corresponding to the magnitude of the acceleration |a|=(a.sub.x.sup.2+a.sub.y.sup.2).sup.0.5 and then to compare it with the threshold value.

[0028] In the present example, in partial figure e), a moving average value of the filtered sensor signal formed after rectification is compared with the predetermined threshold SW.sub.1. If the moving average value exceeds this threshold value, slippage is detected.

[0029] In an advantageous configuration, the threshold value can also be taken from the sensor signal outside the frequency range specified for the detection of slippage. For this purpose, in the example of FIG. 3, a portion of the sensor signal above the frequency range used for detecting slippage (50 to 200 Hz) is filtered out with a high-pass filter and a moving average value of this filtered sensor signal is calculated. In the present example, double of this moving average value is then used as the threshold value SW.sub.2 and compared with the moving average value of the sensor signal filtered in the frequency range from 50 to 200 Hz. In the measurement of FIG. 3, the moving average value of the sensor signal filtered to detect slippage and the corresponding curve of the threshold value SW.sub.2 can be seen. Here, too, slippage is assumed to have occurred when the threshold value SW.sub.2 is exceeded. A combination of this temporally varying threshold value SW.sub.2 with a fixed threshold value SW.sub.1 as in FIG. 2 can also be used, in which case both threshold values must be exceeded in order to detect slippage. This use of two threshold values can be used for plausibility checks and further improvement of reliability.

[0030] A further increase in reliability can be achieved by additionally measuring the normal force F.sub.N acting on the gripping surface 3 or the main body 2 with a force sensor. For this purpose, a threshold value SW.sub.FN is specified, above which the gripping of an object with the gripper 1 can be assumed due to the occurring forces. When the threshold is later exceeded by the signal received from the acceleration sensor, slippage is only assumed to occur if the threshold for the signal from the force sensor was exceeded at the same time. This again only serves to check the plausibility of the detection. Instead of using a force sensor, the normal force F.sub.N can also be determined according to Newton (F.sub.N=m.Math.a.sub.z) from the acceleration signal for a.sub.z (acceleration in the z-direction) from a 3D acceleration sensor.

[0031] In a further configuration of the proposed method, the threshold values for the filtered sensor signal for detecting slippage can also be selected as a function of the holding force detected with the force sensor. When gripping larger objects and with the associated greater holding force, a fixed threshold value SW.sub.1 can be selected. When gripping lighter and/or sensitive objects, the holding force is lower and the amplitude of the filtered acceleration signal is also correspondingly lower when slipping. In this case, the changing threshold value SW.sub.2 is then preferably selected based on the moving average value of the portion of the sensor signal that contains only frequency portions above the frequency range selected for detecting slippage in order to be sure that no interferences are identified as slippage due to the smaller signal. For example, a normal force F.sub.N of 0.5 N can be selected as the value for distinguishing between the two situations. If a holding force F.sub.N of >0.5 N is detected with the force sensor, the fixed threshold value SW.sub.1 described above is used. If a smaller holding force F.sub.N is detected, the varying threshold value SW.sub.2 is used. This procedure is illustrated again as an overview in a flow chart in FIG. 4.

[0032] The further processing, in particular filtering, and evaluation of the sensor signals can be carried out sufficiently quickly in the proposed method in a simple manner with the aid of a microcontroller.

[0033] If a three-dimensional acceleration sensor is used in the main body, the sensor signal can also be used additionally to evaluate the movement of the gripper, since the frequencies of the acceleration signal occurring in this case generally lie outside the range specific to sliding.

[0034] The signals captured in the proposed method can also additionally be used to determine the minimum mass of the object to be gripped, for example by means of the holding force and the direction of the acceleration due to gravity.

REFERENCE LIST

[0035] 1 gripper [0036] 2 main body [0037] 3 gripping surface [0038] 4 elastic mass [0039] 5 acceleration sensor [0040] 6 contact surface [0041] 7 force sensor