Slip sensing

Humans can grasp an object without information such as a coefficient of friction or weight. To implement this graspingmotion with the robot hand, sensors have been proposed that detect an incipient slip within the contact surface or stick-slip.

Method of slip detection

The sensor is constructed of electrode and pressure conductive rubber(Inaba Rubber Co., Ltd.) as shown in figure.

The voltage difference Vp is measured and the signal processing is performed. Then the initial slip can be detected. The pressure conductive rubber was a high polymer material primarily composed of silicone rubber with carbon particles uniformly distributed within.
In an unloaded condition, the electrical resistance is infinity. However, the electrical resistance changes when the normal force was added, because the mutual contact between carbon particles increases. Moreover, when added a tangential force, the electrical resistance randomly changes by changing the mutual contact between carbon particles.

As shown in figure, the object is placed on the surface of the sensor. The upper graph shows the output of the slip sensor when pulling force is applied to the object.
The lowest graph shows the pulling force and its position shifts through slippinge.
First, the pulling force is increased until about 0.15s, after which it remains roughly constant. Specifically, it can be considered that a transition from static friction to dynamic friction occurs at the place marked with a verticalline in the figure. At which point, a slip occurs between the object and the surface of the sensor. This is also clearly seen with the measurement of a laser displacement sensor.
Here, looking at the enlarged portion of the upper graph, a complex change in the voltage emerges immediately before the occurrence of slip (the time of the initial slip).
Upon performing a frequency analysis with respect to this voltage change, it was found that the sensor output Vp at the time of the initial slip includes a high frequency component of several kHz to several 10kHz.
In this regard, such high-frequency change does not occur when the change in force is in normal direction.
The slip sensor presented here extracts this high-frequency component by applying the discrete wavelet transform (DWT) and detects the initial slip of the object.
The middle graph presents the results of DWT power using Haar wavelets. It is clear that immediately before slip occurs, the DWT power increases.

• Seiichi Teshigawara, Kenjiro Tadakuma, Aiguo Ming, Masatoshi Ishikawa, and Makoto Shimojo, High Speed and High Sensitivity Slip Sensor Utilizing Characteristics of Conductive Rubber–Relationship Between Shear Deformation of Conductive Rubber and Resistance Change–, Journal of Robotics and Mechatronics Vol.21 No.2, pp. 200-208,2009.
• Teshigawara, S.; Tsutsumi, T.; Shimizu, S.; Suzuki, Y.; Ming, A.; Ishikawa, M.; Shimojo, M., Highly Sensitive Sensor for Detection of Initial Slip and Its Application in a Multi-fingered Robot Hand, 2011 IEEE International Conference on Robotics and Automation pp.1097-102, 2011

experiment

We carried out an experiment on gripping force adjustment by using a parallel hand with installed slip sensors. This parallel hand is shown in video. We installed the slip sensors in the right and left parallel fingers. The object was held by these fingers. The slip sensors output was fed back to the parallel hand, and the grip force was adjusted in proportion to the weight of the object.

In this experiment, we performed the following two operations.
M1: After an empty glass (made of PET) is gripped with minimal force, nuts are thrown into the glass and the weight is increased. The slip sensor reacts when the object seems to slip, and the grip force is strengthened.
M2: The hand decreases the gripping force, and when the slip sensor detects the initial slip of the glass, the hand strengthens the gripping force slightly. By repeating this operation, the parallel hand can hold the object while applying minimal force.
The control algorithm is simple. The gripping force is strengthened depending on the output from the slip sensor.

In video, Oscilloscope shows gripping force and lighting green lamp shows occurrence of slip.

install slip sensor on robot hand

First, we equipped the CoP sensor on the fingers to detect contact position and force. Next, the slip sensor was covered over the CoP sensor.

The ob
ject is a PET bottle of 100g. The laser displacement sensor was arranged above the bottle. The slip sensor was installed in opposite finger tips respectively.

First, holds the bottle by a certain grasping force, and after one second, begins the grasping using adjustment control.

The result of experiment was shown in figure. These graph indicate, from the above, the displacement of the bottle, the force output of the CoP sensor, and the DWT value.
The dotted line in the middle graph shows minimum force that this robot hand can hold the bottle.
As a result, the grasping force decrease when slip of the bottle is not occurred. On the other hand, the grasping force increases when the slip sensor detects initial slip.
Moreover, focusing on the laser displacement sensor's output, the slip of the bottle is hardly occured. This fact means that by using this slip sensor, the grasping force adjustment control with the multi-fingered robot hand has been achieved.

Following video shows the experiment.

• Teshigawara, S.; Tsutsumi, T.; Shimizu, S.; Suzuki, Y.; Ming, A.; Ishikawa, M.; Shimojo, M., Highly Sensitive Sensor for Detection of Initial Slip and Its Application in a Multi-fingered Robot Hand, 2011 IEEE International Conference on Robotics and Automation pp.1097-102, 2011.