Noble Development of Array Type Gripper for Robot Arm

Abstract

For grippers of industrial robots, parallel grippers and multi-function finger grippers are used. The former has the advantage of low cost but has the disadvantage of low precision, and the latter has the advantage of excellent precision but has the disadvantage of being expensive. In this paper, we developed a grip that can detects the various shapes of the object to be gripped on the gripper surface by using mesured pressure information frome Veloset Sheet sensor so that the gripper can be gripped without deforming the surface of the gripper. Also we did not only developed the array type gripper and 4 array type grippers (ATG), but also confirmed the usefulness of array type gripper developed in this study according to the 4 predefined evaluation criteria

1. Introduction

Starting with the smart factory, the core technology of the 4th industrial revolution, one of the essential devices in the overall robot industry is a robot gripper, and as shown in Fig. 1 and 2, a parallel jaw type gripper and a multi-function hand gripper are mainly used.

Fig. 1 Robot hand

Fig. 2 Dexterous robot hand

There exists an advantage from cheap price but demerit in a unstable gripping incase of irregular gripped objets in case of the jow gripper in Fig. 1 and there exist a demerit of expensive price even though multifunction and high precise as dexterous gripper in Fig. 2.

2. Array Type Gripper

Fig. 3(a), (b) show a motor controller and Fig. 3(c) a geared micro motor for FAC developed in this research, respectively.

Fig. 3 Micro geraed DC motor and it controller

The Finfer Array Cell [FAC] shown in Fig. 4 proposed in this study is designed so as to have the similar characteristics to the human finger joint in functional point of view as shown in Fig. 2.

Fig. 4 FAC including pressure sensing function

The Array Type Grippers (ATG) shown in Fig. 4 shows similar gripping function of Gripper in Fig. 2 while developed considerig a low-cost. The ATG is developped base on ths FAC, while the gripping precision as well as the stability as shown in Fig. 2 are fulfilled by combining FAC and ATG.

Unlike general robot grippers, the FAC proposed in this study uses micro geared DC motor and the rotary motion from this DC motor is converted into linear motion by the Pinion in Fig. 5 connected to the motor end, finaly the linear motion of the FAC is executed by this DC motor.

Fig. 5 Role of Pinion in FAC transforming the rotary motion to linear motion

The shaft of the reducer attached to the motor in Fig. 3(c) is connected to the pinion of FAC as in Fig. 5, The HAC is connected to the rotating part of the wrist as shown in Fig. 5.

The pinion, which receiving the rotational force from the motor, converts this rotational motion into linear motion, and engages with the rack to move the FAC in up-and-down linear motion according to the moter reotation.

In Fig. 6, the power is supplied to the top electrode attached to the top, and supplied voltage is measured by connecting the ADC (Analog to Digital Converter) of Arduino board used as controller in this study.

Fig.6 DC Geared moter and pressure measuring

When the pressure is increasing and the transfer is decreasing, the increased pressure situation is deteced in real time, the situation management logic is determind such as stopping the motor operation.

The Veloset Sheet shown in Fig. 7 below is placed in the middle of the FAC shown in Figs. 4, and the resistance of the Veloset Sheet is lowered by the twisting of the FAC. By sensing the resistance change according to the increased pressure of FAC unit, the torque between FAC and gripped object is adjusted by the micro geared motor shown in Fig. 6.

Fig. 7 Pressure sensitive conductive sheet

Fig. 7 shows pressure sensing material, Veloset Sheet, where the resistivity is changed according to the pressure change between top electrod and bottom electrod shown in of Fig. 4, and resiatence change is converted to contact pressure to the propping object. Pressure sensor is important role in falecxible operation of FAC and ATG withot any damage to the gripping objets.

Table 1 below is data on the characteristics of Veloset Sheet.

Table 1. Pressure sensitive conductive sheet spec

Fig. 8 below is a diagram showing the assambled.

Fig. 8 Array type robot gripper

The ATG Gripper is comprised of four HACs at one side and Two ATGs are combined to be ATG gripper as shown Fig. 8 and Fig. 9.

Fig. 9 Assembling operation using FAC and ATG

Fig. 9 is about the process of combining one ATG by assembling 4 FACs, and 2 ATGs are installed on both sides of the Robot End effector.

The operation of the robot gripper using ATG with FAC can be executed by proceeding of following steps:

Step 1: Operate the moter so as to rotate arm part joint and move the robot hand to the position to gripi the object.

Step 2: When the robot arm is moved to an appropriate position, the motor at the bottom of the wrist is operated to start the rack and pinion motion, by which circular motion can be converted to the linear motion of FAC.

Step3: Through the operation of Step 2, the robot hand at both ends is opened according to the size of the object to be gripped.

Step 4: When both sides ATGs are gripped and reached the gripped objects, the finger part of the FACs are doing an adpting motion in order not to damage to gripping objets.

3. Peformance Evaluation of ATG

3.1 Final Assembly

The application example of FAC-based ATG developed in this study as a robot gripper is shown in Fig. 10 below as as computer simulation.

Fig. 10 Gripping operation of FAC based robot gripper

Fig. 11 shows the results of assembling the HAC-based ATG developed in this study.

Fig. 11 Final assembling including wiring: (a) final assembling process (b) ATG based gripper motion on egg

3.2 Performance evaluation and its criteria

For the performance evaluation of HAC-based ATG developed in this study, the four evaluation criteria is determined as shown in Table 2.

Table 2. Evaluation items and test methods

(1) Sensor nose test

Table 3 and Fig. 12(a) show the Sensor Noise Measurement result and setup for noise measure, respectively.

Table 3. Sensor Noise Measured result

Fig. 12 Sensor Function Test setup (a) Sensor noise test (b) Sensor sensitivity test

Table 4 and Fig. 12(b) show the Sensor Noise Measurement result and setup for noise measure, respectively.

Table 4. Sensor Sensitivity Measured result

Fig. 13 show recorded sensitivity graph.

Fig. 13 Recorded sensitivity graph in real time

(2) Finger contraction and relaxation test

With the FAC in no-load condition, the reciprocating motion of FAC is generated betweem maximim distance and FAC is returned to the start point for five times without stop. After 30second recess, the pevious test is restarted 5 times. Table 5 shows results of contraction and relaxation of FAC.

Table 5. Finger contraction and relaxation measured result

(3) Controllability test of FAC

Table 6 and Fig. 14 shows results of contraction and relaxation of ATG and test equipments, respectibly.

Table 6. Finger sensor controllability measureed results

Fig. 14 finger controllability test setup: (a) front side on various heights (b) top side on various heights (c) finger sensor control test

Table 7 shows analyed results according to the performance evaluation criterion defined on Table 2. We can confirm the developed FAC and ATG satisfy the criterion on Table 2 as shown in Table 7.

Table 7. Analysis of test results

4. Conclusion

In the case of grippers of industrial robots, the higher the precision, the higher the price, and the lower the precision, the lower the price, however there is lot of difficulties in object gripping with lower precision gripper.

In this study, considering these points, we developed an Array Type Gripper (ATG) and Array Cell Cell (FAC) that can improve the precision of the gripper even if low price.

Through the experiment, it was confirmed that the performance was comparable to that of the existing robot hand, and in particular, the following results were derived.

First, the ATG can detect the status of gripping object surface in real time, and grip the object without causing any deformation.

Second when it is necessary to grasp an object other than a flat surface even with a protrusion, the object can be grasped without damage by using the 3D spatial object grasping function of the ATG which is comprised of a built-in contact surface pressure sensor.

Third, after making an array type gripper comprised of four ATGs and pre-defined four kinds of feasiblity test rules. we confirmed the usefulness of FAC and ATG as gripper of robot.

Appendices