LI Hansen(李瀚森), LI Xinrong(李新榮)*, LIU Hanbang(劉漢邦), LI Xingxing(李興興), LI Yuzhuo(李玉卓)

1 School of Mechanical Engineering, Tiangong University, Tianjin 300387, China

2 Tianjin Key Laboratory of Modern Mechanical and Electrical Equipment Technology, Tianjin 300387, China

Abstract: To increase the gripping area of noncontact end grippers(NCEGs), an array-type NCEG based on the Coanda mechanism is proposed, and its performance in gripping different garment fabrics(GFs) is studied. Firstly, the structure and the working mechanism of a single Coanda-based NCEG were analyzed. Secondly, four such grippers were arranged in array to form a minimum gripping unit. Then, the structure of the connecting plate(CP) to the gripper was optimized by simulation analysis to exclude airflow interference, and the adsorption performance of GFs with different fabric parameters was measured. Finally, the experimental results were analyzed to verify the scientific validity and the feasibility of the array-type arrangement. The results show that compared with other NCEGs, the array-type ones based on the Coanda mechanism are better at gripping various large-area GFs and offer better adsorption performance. This innovation provides a new solution to the problem of insufficient gripping area in GF gripping and is very important for improving the production efficiency of garment processing.

Key words: noncontact end gripping; array-type arrangement; garment fabric (GF); adsorption performance; Coanda mechanism

Introduction

As people’s standard of living increases, the demand for clothing is also increasing. Processing and production of clothing involve the important process of automatic clamping and transfer of garment fabrics (GFs). An urgent problem to be solved is how to improve the efficiency of gripping and ensure its success. Because GFs have physical characteristics such as flexibility, breathability, and various sizes, shapes, and dimensions, it is challenging to select GF grippers. Based on the various aforementioned characteristics of GFs, it is difficult to grip them using conventional grippers, and suitable grippers must be selected instead. Currently, the main methods for automatic fabric gripping and transferring include robotic gripping[1], negative-pressure adsorption[2], electrostatic adsorption[3-4], low-temperature adhesion[5], and noncontact adsorption[6-7], which have different working principles for gripping GFs. In particular, Leeetal.[8]introduced a variable-stiffness soft manipulator based on shape-memory alloy. Lien and Davis[9]designed a vacuum chuck for garment items with a porous adsorption structure. Schaleretal.[10]designed a flexible electrostatic gripper based on electrostatic adsorption and capable of gripping flexible objects. O’Shaughnessyetal.[11]designed a Peltier-driven cold adhesion gripper that cooled water vapor through Peltier elements to complete the gripping of fabrics. However, these studies have their own shortcomings: (i) robots can easy scratch the surfaces of GFs; (ii) negative-pressure adsorption is poor for highly breathable fabrics; (iii) electrostatic adsorption ionizes dust in the air, which affects the gripping accuracy; (iv) the structure of the gripper with low-temperature adhesion is complicated, and additional devices are needed to complete the release of GFs. Therefore, to make up for the shortcomings of the current GF gripping methods, Ozcelik and Erzincanli[12]fabricated a noncontact gripper that could adsorb a GF and leave no trace on its surface after gripping; however, the energy consumption was high and the adsorption force was insufficient. Also, a noncontact gripper and a Coanda nozzle were combined by Liu’s team to form a Coanda-based noncontact end gripper (NCEG) with stronger adsorption capability[13-14]. Traditional negative pressure adsorption makes airflow upward to generate suction, and at the same time, a negative pressure area is generated above the cloth. The NCEG blows out the airflow and uses the Bernoulli principle to generate a negative pressure area. In textile production, the yarn in the air is easily sucked in by inhaling air, and the gripper is blocked. Therefore, the NCEG is used for gripping. However, the adsorption area of a single Coanda-based NCEG is small, so there is a need to combine multiple NCEGs in order to increase the adsorption area and thus apply non-contact adsorption as an inexpensive, efficient and non-invasive gripping method for garment production[15].

Herein we introduce the working principle of the NCEG based on the Coanda mechanism and then the individual grippers will be arranged in array. To eliminate airflow interference, an experiment to measure the pressure distribution is conduced, and a connecting plate (CP) with airflow guiding holes is designed for optimization. Then three experiments measuring the gripping effect, the adsorption gap, and the lifting force are conduced to study the performance of the GF gripper on different GFs. The research reported herein might reduce production cost and improve production efficiency for the textile industry in its automated production applications.

1 Working Principle and Unit Optimization Design

1.1 Working principle of gripper

Figure 1 shows the physical diagram of the NCEG device and the working principle of the NCEG. The energy-saving mechanism of the device is mainly based on the Coanda mechanism. The primary compressed air enters the NCEG unit from air inlet 1, and then it enters the narrowest throat and continues to flow along the inner wall of the fluid-passage tube. The primary compressed air interacts with the surrounding air and drags secondary airflow into the NCEG body. At this point, the gas volumes into the fluid-passage tube is about 10 times that of the primary compressed air. The gas flow rate is proportional to the gas volumes, so it will increase rapidly. The high-speed airflow passes through the blocking plate and then diffuses radially outward in the gap between the bottom of the NCEG body and the GF. The high-speed airflow decelerates in this gap, creating a negative pressure at the bottom of the NCEG body according to Bernoulli’s principle, generating an upward force to grip the fabric. Because of the application of the Coanda mechanism in the NCEG unit, its lifting force is substantially higher than that of a conventional NCEG with the same input primary supply flow, thereby serving the dual purpose of reducing energy consumption and increasing adsorption capacity.

1.2 Unit optimization design

To solve the problem of insufficient adsorption area, multiple NCEGs are recommended to be used in combination to form a larger gripper. However, irregular stacking leads to low efficiency, so we propose combining an array-type of four grippers into a minimum unit and then combining multiple units to form an end gripper that meets industrial requirements. Here, we report the design optimization of the composition of the individual units. A single unit has two parts: the CP and the four NCEG units based on the Coanda mechanism. A perspective schematic of the CP with the airflow guiding hole is shown in Fig. 2. Compared with a typical CP, it has a circular hole with a diameter ofd1in its center, through which the disturbing airflow formed by the array-type NCEG modules on the adsorption bottom surface can be removed upward; there are also four connection holes with a diameter ofd2around the CP for firmly mounting the NCEG units. Table 1 shows the specific parameters of the CP with an airflow guiding hole.

1— air inlet; 2— flared housing; 3— fluid-passage tube chamber; 4— NCEG body.

Fig. 2 CP for array-type NCEG module

Table 1 Specific parameters of CP with an airflow guiding hole

The CP and four Coanda-based NCEG units are connected in the proposed array-type NCEG module, as shown in Fig. 3. This is the smallest array-type module, and an airflow guiding hole is added in the center of the CP, which is effective for reducing the airflow interference and improving the stability of the adsorption force of the array-type NCEG module. Hereinafter, for convenience, we refer to the Coanda-based array-type NCEG module as the array-type NCEG.

Fig. 3 Photograph of Coanda-based array-type NCEG module

The simulation of the whole unit revealed that the original CP caused uneven pressure distribution on the adsorption bottom surface. As the simulation results shown in Fig.4(a), the central air pressure exceeded 25 kPa, and the airflow interference in the central region was very obvious, which was not conducive to gripping objects. To exclude the airflow interference, we designed a new CP with an airflow guiding hole to reduce the airflow interference. The simulation of the array-type NCEG with the airflow guiding hole is shown in Fig.4(b), from which it can be seen that the maximum negative pressure exceeds -11 kPa and there is no airflow interference in the center of the adsorption bottom surface. The circular airflow guiding hole has been added in the center of the device, thus effectively improves the stability of the adsorption force, and facilitates the gripping of the object. From the perspective of optimal design, it could have caused serious airflow interference to the gripping process and affect the adsorption effect by using a CP without airflow guiding hole connecting the array-type NCEG. However, it could eliminate airflow interference and ensure the gripping quality by using a CP with an airflow guiding hole.

Fig.4 Comparison of pressure distributions on adsorption bottom surface of array-type gripper: (a) without airflow guiding hole; (b) with an airflow guiding hole

Fig. 5 Photograph of experimental setup

2 Experimental Procedure

The adsorption performance of the array-type NCEG was assessed by experiments to measure its gripping effect, adsorption gap, and lifting force.

2.1 Experimental equipment and materials

Figure 5 shows the equipment used for experimental verification, which involves a robot, the array-type NCEG, an air compressor (95 L/min, Jiangsu Dongcheng Electromechanical Tools Co., Ltd., China), a class-I electronic balance (Shanghai Youke Instruments Co., Ltd., China), a YG141 fabric-thickness measuring instrument(Laizhou City Electronic Instruments Co., China), and a 461D-II digital fabric-permeability tester (Wenzhou Dayong Textile Instrument Co., Ltd., China). Detailed parameters for the array-type NCEG used in these experiments are listed in Table 2.

Table 2 Selected parameters for experiments

In the experiments, 12 square GF samples with different fabric parameters were selected according to quality, thickness, and air permeability. The samples were all 120 mm × 120 mm in size, and their textures were relatively soft. Woven fabrics and knitted fabrics were studied. Other specific parameters for the GF samples are listed in Table 3.

Table 3 Specific parameters of GFs

2.2 Gripping effect measurement

The setup of this experiment is shown in Fig. 5. The array-type NCEG was installed under the robot and was connected to the air compressor by a rubber hose. The air compressor was turned on, the supplied gas flow rate (SGFR)Qwas adjusted to the required value (50, 70, 90, 100, and 110 L/min) using the flow-rate regulator (SMC PFM711S, 2-100 L/min, Shanghai Dasic Automation Equipment Co., Ltd., China), and then the robot was commanded to move downward so that it could just grip the GF.

The purpose of this experiment was to simulate the process of the array-type NCEG gripping, transferring, and releasing a GF. The specific measurement method is shown in Fig. 6. After the array-type NCEG on the manipulator gripped the GF, it moved along the trajectory shown in Fig. 6. The speed of the manipulator was set to 0.1 m/s during the whole process, moving 100 mm upward, then 1 000 mm horizontally, and finally 100 mm downward. After completing the above operation, the gripping effect was evaluated as follows. In Fig. 6, the three moving distances are marked asA,B, andC, respectively, and the gripping effect of the array-type NCEG is explained by observing where (if at all) the GF falls off. The more backward the drop position, the better the gripping effect. To ensure that the speed and the moving distance of each gripping, transferring, and releasing of GFs samples 1-12 in Table 2 remained consistent, the experiment was performed with software programming for the above operations.

1— fabric; 2—test platform.

2.3 Adsorption gap measurement

In this experiment (shown in Fig.7), the adsorption gaphwas the gap across which the array-type NCEG could lift the GF up from the test platform. Firstly, the array-type NCEG was installed on the robot, and then the GF sample 1 to be measured was placed on the working platform below the robot, and finnally the SGFR of 50, 70, 90, 100, and 110 L/min was supplied to complete the gripping of the GF. The SGFR was adjusted by the flow-rate regulator to measure the adsorption gaphbetween the GF and the gripper at different SGFRs, andhwas measured and recorded by a distance-measuring instrument. The remaining GF samples 2-12 in Table 2 followed the testing process of the GF sample 1 above in turn, and finally all the experimental results were measured and finished for comparison and collation.hwas measured to obtain the optimal SGFR for each GF in the steady state.

1— noncontact gripper; 2—fabric; 3—test platform.

2.4 Lifting force measurement

The setup of this experiment is shown in Fig. 8. The lifting force of the array-type NCEG gripping GFs at different SGFRs was measured. The GF to be measured was attached to a fabric-fixing device, and the lifting force of the array-type NCEG was measured by an S-type high-precision tensile force sensor (NTJL-1, Nanjing Tianguang Electric Technology Co., Ltd., China) that was connected to the fabric-fixing device through the guide post. The display instrument of the tension sensor had a zeroing function, so the lifting force of the array-type NCEG could be measured directly. After several measurements and calculations, the lifting forces of the array-type NCEG gripping GFs at different SGFRs were obtained.

Fig. 8 Setup of lifting force measurement

3 Results and Discussion

The gripping performance of the array-type NCEG was elucidated by the experiments to measure the gripping effect, the adsorption gap, and the lifting force. In this section, the results of those experiments are analyzed.

3.1 Analysis of gripping effect

Table 4 shows the experimental results of the gripping effect with the GF samples 1-12 in Table 3, where W represents that the GF falls off during the gripping rising process, X represents that the GF falls off during the horizontal moving process, Y represents that the GF falls off during the releasing process, and Z represents that the GF does not fall off during the whole movement process as shown in Fig. 6. Table 4 shows that at the same SGFR, the less heavy the GF is, the less likely it is to fall off, indicating a better gripping effect on lighter fabrics. Regarding the air permeability, the lower the air permeability of the GF for the same mass is, the better the gripping effect is, whereas the thickness of the GF has less influence on the gripping effect. Regarding the SGFR, the gripping effect is improved with increasing SGFR.

Fig. 9 h-Q diagram

Table 4 Results of gripping effect

3.2 Analysis of adsorption gap

To obtain the best SGFR for each GF in the steady state, adsorption gap experiments for the twelve kinds of GFs were conducted. Figure 9 shows theh-Qdiagram of the GF samples 1-12 at different SGFRs. Regarding the mass of the GF, the lighter it is, the larger the adsorption gap is, which indicates that at the same SGFR, the lighter the GF is, the easier it is to grip. Regarding the air permeability of the GFs, among those GFs with almost the same mass, the smaller the air permeability is, the larger the adsorption gap is, which indicates that at the same SGFR, the smaller the air permeability is, the easier the GF is to grip. Regarding the thickness of the GF, it has less influence on the height of the adsorption gap. Regarding the SGFR, the adsorption gap of the GF samples 1-12 increases significantly with increasing SGFR.

3.3 Analysis of lifting force

The relationship between the lifting forceFand the SGFR was explored in order to study the influences of GF quality and air permeability. Only one sample was selected for experimental verification. The variation in the lifting force with SGFR is plotted in Fig. 10, which compares the results of the experiment for sample 5 at different SGFRs.

From the experimental results, the adsorption capacity of the array-type NCEG increased with increasing SGFR, but when the SGFR exceeded 90 L/min, the slope of theF-Qdiagram curve began to decrease, as the lifting capacity began to decrease.

3.4 Contrast and discussion

It can be seen from Figs.9 and 10 that the two influence factors are mainly related to the GF quality and the air permeability. The lower the GF quality and the air permeability are, the better the adsorption effect is. It can be seen that with increasing SGFR, the lifting force of the array-type NCEG increases significantly. However because of the limitation of the internal structure of the gripper, the growth rate of the lifting force slows gradually when the airflow reaches a certain value. Through comparative analysis, it can be obtained that the greatest influence on the gripping effect at the same SGFR is the GF quality, followed by the air permeability and the GF thickness.

4 Conclusions

In this study, the structural design and the adsorption principle of an NCEG based on the Coanda mechanism were analyzed systematically. Single Coanda-based NCEGs were arranged in array, and four experiments were conducted to optimize the structure of the GF gripper and analyze the adsorption performance.

Compared with the array-type NCEG without airflow guiding hole, the one with an airflow guiding hole had a more uniform and symmetrical pressure distribution on its bottom adsorption surface. The airflow interference was significantly reduced, and the adsorption force was more stable. It was conducive to gripping objects. The GF quality and the SGFR had high influence on the adsorption performance of the array-type NCEG followed by the air permeability and finally the GF thickness. The array-type NCEG based on the Coanda mechanism can grip a larger area of the GF and meets the requirements for garment automation use. The present work provides a new solution to the industrial problem of automatic GF gripping and transfer, and it offers technical support for improving the automation of the apparel industry.