Experimental Comparison of Bernoulli Gripper and Vortex ...

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 16, No. 10, pp. 2081-2090 SEPTEMBER 2015 / 2081

© KSPE and Springer 2015

Experimental Comparison of Bernoulli Gripper and

Vortex Gripper

Xin Li1,#, Ning Li1, Guoliang Tao1, Hao Liu1, and Toshiharu Kagawa2

1 State Key Lab of Fluid Power Transmission and Control, Zhejiang University, 38 Zheda-road, Hangzhou, Zhejiang, 310027, China2 Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, 226-8503, Japan

# Corresponding Author / E-mail: [email protected], TEL: +86-571-87951271-6403, FAX: +86-571-87951271

KEYWORDS: Bernoulli gripper, Vortex gripper, Experimental comparison, Suction, Pressure distribution, Stress

A Bernoulli gripper and a vortex gripper are types of pneumatic grippers that have similar functions but different working principles.

They are widely used for performing gripping, transferring, and placing operations in automated production lines. This study

systematically compares the two grippers and provides an objective reference to enable users to choose an appropriate gripper. First,

we present the working principles of both grippers. Then, we measure the curves of suction force, air consumption, and upstream

pressure; furthermore, we analyze and compare the energy consumption from the viewpoint of the entire pneumatic system. The results

reveal the relationships with the air consumption and total energy consumption. Next, we measure the pressure distribution formed

on the upper surface of the workpiece. From the experimental results, the principles of the two grippers are stated and compared.

Furthermore, we use the pressure distribution data to estimate the deformation and inner stress of the workpiece. The influence of

the grippers on the workpiece are analyzed and compared from a material mechanics viewpoint. Finally, we use three types of rough

surfaces to qualitatively evaluate and compare the performance of the grippers when gripping a workpiece with a rough surface.

Manuscript received: July 4, 2014 / Accepted: July 5, 2015

1. Introduction

In many production lines, a workpiece is subjected to gripping,

transferring, and placing operations. These operations, although simple,

are a technical bottleneck in realizing a fully automated production line

owing to defects in traditional mechanical gripping tools and rubber

vacuum suckers, as described below.

(1) A mechanical gripping tool always has two or more splints that

can open and close. The gripping operation is realized by gripping the

splints and exerting force on them to make them clench. However,

fragile sheet workpieces (such as solar cell piece,1 silicon wafer, glass

NOMENCLATURE

D = bending stiffness

E = Young's modulus

gripper = gripper's energy consumption

system = energy dissipation of the air supply system

total = total energy

g = acceleration of gravity

hmax = optimal gap height

hw = thickness of workpiece

Mr = radial bending moment

Ma = circumferential bending moment

Pa = atmospheric pressure

Pc = outlet pressure of air compressor

Pu = upstream pressure of gripper

q = resultant load

Q = flow rate in the standard state

r = radius

E·

E·

E·

R = radius of circular glass workpiece

τ = shear stress

δ = deformation

ν = Poisson's ratio

ρ = density of workpiece

DOI: 10.1007/s12541-015-0270-3 ISSN 2234-7593 (Print) / ISSN 2005-4602 (Online)

2082 / SEPTEMBER 2015 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 16, No. 10

panel2) cannot be gripped using the mechanical gripping tool. Moreover,

mechanical wear occurs in the components, leading to a heavy daily

maintenance load.

(2) The rubber vacuum sucker is an effective tool for gripping a

sheet workpiece.3,4 However, it is limited to gripping flat and smooth

workpieces. With a coarse workpiece, the pressure difference between

the negative pressure inside the rubber sucker and the external

atmospheric pressure causes reverse flow of air into the sucker through

the gaps on the workpiece surface, causing the negative pressure inside

the sucker to be lost and the vacuum absorption to become ineffective.

Furthermore, dust, fragments, residues, and food juices can easily be

absorbed into the rubber sucker, causing the blocking of the vacuum

tube and the failure of the vacuum source.

In many food and leather production lines, the transferring, sorting,

and boxing of workpieces can only be accomplished through manual

operations. To change this situation, two types of grippers - Bernoulli

gripper and vortex gripper - were developed by researchers. In these

grippers, negative pressure is generated using the inertial effect of air

flow. These grippers can be used to pick up and grip a workpiece with

a coarse surface. Both grippers show similar performance, and thus far,

no study has systematically compared them. This has resulted in the

lack of a frame of reference when deciding which gripper to choose. To

address this issue, in this study, the suction of a Bernoulli gripper and

a vortex gripper was compared in terms of the energy consumption,

pressure distribution, deformation, and inner stress of the workpiece,

and the effect of a coarse workpiece surface was evaluated. From the

comparison results, the differences between the two grippers were

summarized to provide an objective frame of reference for users when

establishing an automatic production line.

2. Working Principles

2.1 Bernoulli gripper

The Bernoulli gripper’s original structural design is very simple (see

Fig. 1(a)). Its key components are a circular plate with an air hole at its

center. There is a rubber pad on the underside of this plate, and a radial

gap flow channel is formed between the workpiece and the plate.

Driven by upstream pressure, air flows into the Bernoulli gripper along

the central air hole and then turns into the radial gap flow channel.

Because the cross-sectional area of the radial gap channel increases, the

air flow velocity in the channel decreases. According to Bernoulli’s

principle, a flow with decreasing velocity is accompanied by increasing

pressure. In other words, the internal pressure becomes smaller than the

external pressure. Moreover, because the external pressure on the plate

is actually the atmospheric pressure, there exists a negative pressure

that is lower than the atmospheric pressure inside the plate, and

therefore, upward suction is applied by the negative pressure to absorb

the workpiece. Dini and Li’s result shows that this suction is dependent

on the height of the gap flow channel (i.e., thickness of rubber pad)

between the plate and the workpiece.5,6 The relationship between the

suction and the gap height can be described by a non-monotonic curve

(see Fig. 2). Li’s study reveals that this is because of the alternation

between the viscosity effect and the inertial effect of the air flow. When

the gap height is very small, the viscosity of air flow plays the

dominant role. However, as the gap height increases, the inertial effect

of air flow dominates. Therefore, the curve of the suction force shows

two different variation trends.6 In the region with small gap height, the

suction increases sharply as the gap height increases. However, when

the gap height increases further, the suction force decreases gradually.

The maximum suction force appears at the transition phase of the two

variation trends. The corresponding gap height is the optimal design

value of the thickness of the rubber pad, which is denoted as hmax. In

general, hmax is a value of the magnitude of several hundreds of

microns. Based on the original design, many researchers have proposed

an optimized design for a Bernoulli gripper.5,7,8 Fig. 1(b) shows the

most classical and widely used design. The deflector at the center of the

plate serves to block the dynamic pressure of air flow acting on the

workpiece with changing flow direction. Air is blown out with a high

radial velocity component through a narrow circular outlet between the

deflector and the plate. This design can increase the Bernoulli effect

and the resulting suction force.

The main difference between the Bernoulli gripper and a traditional

rubber vacuum sucker is that the exhaust air flows between the

Bernoulli gripper and the workpiece. As a result, external air cannot

Fig. 1 Schematic diagrams of Bernoulli gripper

Fig. 2 Force and gap height of Bernoulli gripper

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 16, No. 10 SEPTEMBER 2015 / 2083

flow reversely into the negative pressure area at the center of the

gripper. Therefore, though the workpiece surface is very coarse, no

vacuum leaks occur. Fig. 3 shows the handling of a solar cell piece

using a Bernoulli gripper (Festo).

2.2 Vortex gripper

Fig. 4 shows a schematic diagram of the vortex gripper developed

by Li.9-11 From top to bottom, the gripper consists of a cover board, a

hollow cylinder, gaskets, an annular baffle, and a rubber sheet. The

cover board seals the upper end of the hollow cylinder, forming an

upside-down cup-like cavity. Tangential nozzles are arranged on the

inner surface of the hollow cylinder, and the upstream ends of the

nozzles are connected to the compressed air source. The annular baffle

is fixed to the hollow cylinder by using bolts. By placing gaskets

between the baffle and the cylinder, an annular gap passage is formed.

The gap passage connects the cavity to the outside atmosphere. To

increase friction between the gripper and the workpiece, a soft rubber

sheet layer is stuck to the lower end surface of the baffle. High-pressure

air is injected into the cavity through tangential nozzles, and it flows

along the circular surface of the cavity to form a rotating air flow. The

centrifugal force of the rotating flow reduces the pressure in the central

area below that in the periphery. In addition, the pressure in the

periphery is lowered to near atmospheric pressure by connecting the

periphery to the atmosphere via the gap passage. Therefore, negative

pressure is generated in the central area of the cavity. The suction force

produced by the negative pressure picks up a workpiece placed under

the gripper. Air is exhausted through the gap passage after rotating in

the cavity. The outward exhaust of air prevents outside air from flowing

into the gripper. As a result, the negative pressure in the gripper is

maintained. Therefore, the vortex gripper can pick up workpieces with

a coarse surface. Fig. 5 shows the use of a vortex gripper to pick up a

circuit board with electronic components of varying size welded on its

surface. Our previous research revealed that the gasket thickness is a

key design parameter that affects the suction force. In Fig. 6, the curve

of the gasket thickness and suction force reveals that an optimal height

value hmax exists; furthermore, this value remains nearly constant

when the supplied flow rate exceeds a certain level.

Fig. 3 Handling of a solar cell piece using a Bernoulli gripper

Fig. 4 Schematic diagrams of vortex gripper

Fig. 5 Vortex gripper picking up a printed circuit board

Fig. 6 Suction force and gasket height (air flow rate: 21 L/min)


3. Experimental Methods

3.1 Tested grippers

In this study, we use a Bernoulli gripper manufactured by Festo,

which has a strong research and development background in the

pneumatic area. This gripper has the improved structure shown in Fig.

1(b). Fig. 7(a) shows this Bernoulli gripper and its dimensions. Fig. 7(b)

shows the vortex gripper proposed and developed by the authors and its

dimensions.

3.2 Air supply circuit

Fig. 8 shows the arrangement of the air supply circuit. Compressed

air is supplied and regulated to a selected pressure. A thermal flow

meter (FD-V40, 0~250 L/min, Keyence Co., Ltd) and a pressure gauge

(A serial, 0~0.6 MPa (g), Nagano Keiki Co., Ltd) are installed ahead of

the test gripper to indicate the supply flow rate and upstream pressure.

3.3 Method for measuring suction force

The magnitude of the suction force generated by the gripper is

measured as follows. First, with large air supply, a workpiece with mass

M is picked up by the gripper. The air pressure and flow rate are then

lowered gradually by adjusting the pressure regulator; as a result, the

suction force decreases gradually. When the supply pressure and flow

rate decrease to a certain level, the workpiece drops because the suction

force becomes equal to or less than the workpiece weight. The suction

force can be assumed to equal the weight of the workpiece multiplied

by the gravitational constant at the present air pressure and flow rate.

By repeating these steps for workpieces with different weights, suction

force curves under different supply pressures and flow rates can be

plotted.

3.4 Method for measuring pressure distribution

We used a small table for measuring the pressure distribution

produced on the table by the grippers (see Fig. 9). A sliding bar with

a trapezoid-like cross section is installed in the middle of this table. To

ensure that the sliding part and the surface of the measuring table are

flat and smooth, when fabricating the table, we precisely cut its entire

upper surface with the bar installed. At the same time, the bar is moved

very slowly to ensure that its sliding speed does not influence the air

flow inside the tested gripper and its resulting pressure distribution. The

sliding bar contains a small tap hole that is connected to a pressure

sensor (KL17 series of Nagano Keiki, range: -1.0~1.0 kPa (g), precision:

0.25%F.S) via an internal connecting perforation. A wire displacement

sensor is used to record the location of the bar. When we place the

tested gripper on the measuring table, the pressure distribution on the

upper surface of the measuring table can be measured by continuously

recording the pressure and position while slowly moving the sliding bar.

4. Results and Discussion

4.1 Suction force and energy consumption

First, the air consumption (i.e., air flow rate), upstream pressure, and

energy consumption of the two grippers were measured and compared.

Fig. 10 shows that for the same suction force, the Bernoulli gripper shows

more than two times the air consumption and much lower upstream

Fig. 7 Grippers used in the experiments

Fig. 8 Air supply circuit

Fig. 9 Apparatus for pressure distribution measurement


pressure than the vortex gripper. We believe that this is because a large

air flow is necessary for the Bernoulli gripper to achieve a significant

deceleration of radial velocity. Hence, the cross-sectional area of the

circular outlet is designed to be large in order to blow out a large

quantity of air at a relatively low upstream pressure. In contrast, the

vortex gripper has only two tangential nozzles with 0.8-mm diameter.

The high upstream pressure is used to inject air from the two tangential

holes. Then, a high-speed spinning air flow is generated in the cavity.

The flow and upstream pressure requirements are different owing to the

different working principles of the two types of grippers. The energy

consumption of the grippers is an important evaluation index. In this

article, Cai’s algorithm is used to calculate the energy of compressible

air. The gripper’s energy consumption ( gripper) is calculated as follows:

(1)

where Pa is the atmospheric pressure; P

u, the upstream pressure; and Q,

the flow rate in the standard state. This equation thus converts the

upstream pressure and flow rate into energy (unit: Watt). The vortex

gripper has lower air consumption yet much higher upstream pressure

than the Bernoulli gripper; therefore, the vortex gripper has relatively

lower energy consumption (see Fig. 10). However, this is not our final

conclusion. Next, the energy consumption problem is considered from

the viewpoint of the overall pneumatic system. The two types of

grippers are pneumatic end effectors that need to be connected with a

compressed air supply system (see Fig. 11). In general, compressed air

reaches the gripper after passing through many pneumatic components

such as the cooler, dehumidifier, filter, piping system, regulators, and

switching valve. In this process, a large amount of energy is

unavoidably dissipated. More importantly, the larger the air flow, the

greater is the energy dissipation of the air supply system. Then, when

the gripper is used, the total energy consumed ( total) is the sum of the

energy consumption gripper of the gripper itself and the energy

dissipation of the air supply system ( system). The total energy total

can be calculated by substituting Pu in Eq. (1) with the outlet pressure

of the air compressor (Pc). Considering that on a production line, the air

for the gripper is supplied by decreasing the pressure using regulator(s)

instead of configuring a low-pressure air source for the gripper,

irrespective of the upstream pressure of the gripper, the comparison of

the total energy total is only dependent on the air flow rate. The outlet

E·

E·gripper P

aQ P

u/P

a( )ln=

E·

E·

E·

E·

E·

Fig. 10 Comparison of flow rate, upstream pressure, and energy

consumption

Fig. 11 Compressed air supply system


of the compressor configured in our laboratory is 700 kPa (g), and the

total energy of the two types of grippers is shown in Fig. 12. Because

the air consumption of the vortex gripper is very low, its total energy

is only half that of the Bernoulli gripper.

The above discussion clearly suggests that the vortex gripper is

advantageous in terms of energy saving relative to the Bernoulli

gripper.

4.2 Pressure distribution

Fig. 13 shows the measurement results of the pressure distribution

when the same suction force (1.0 N) is generated for the two types of

grippers. To generate a suction force of 1.0 N, the Bernoulli gripper and

vortex gripper respectively require an air flow of 36.0 and 15.0 L/min.

Despite the fact that the pressure distributions of the two grippers, as

shown in Fig. 13, are very similar, the mechanisms of generating the

negative pressure are quite different.

The Bernoulli gripper injects air from the circular gap at high radial

velocity, following which air flows with decreasing velocity along the

radius. Hence, the pressure changes from negative to atmospheric

pressure. Because there is no air flow below the deflector, the pressure

distribution at the central part of the gripper is uniform without any

obvious pressure variation. Moreover, a pressure distribution is observed

slightly beyond atmospheric pressure in the periphery of the gripper (20

< r < 30 mm). This is caused by the viscous friction between the

airflow and the walls (i.e., the bottom surface of the Bernoulli gripper

and the upper surface of the workpiece).

In contrast, the vortex gripper injects high-pressure air into the cavity

through tangential nozzles, following which air flows along the circular

surface of the cavity to form a rotating air flow layer. The rotating air

flow induces a cupped pressure distribution in the cavity. No obvious

pressure variation is observed in the central area. It is suggested that the

air in the central area does not rotate with the external rotating air flow

layer. This phenomenon has already been observed in CFD simulations.

The CFD simulation results show that the flow velocity decreases sharply

from the periphery to the center, forming a flat pressure distribution

pattern in the central area. Furthermore, a slightly negative pressure is

observed at the contact area (r > 25 mm) between the annual rubber

sheet and workpiece. In the ideal case (rubber sheet and workpiece have

tight contact), there will be no negative pressure. However, in reality,

the workpiece and rubber sheet cannot be in tight contact. Therefore,

the negative pressure in the cavity and the external atmospheric pressure

create a tiny amount of air flow between the workpiece and the rubber

sheet, which produces the pressure distribution in the contact area.

Both types of grippers show low pressure distributions in the central

area, and the peripheral pressure is near to the atmospheric pressure.

Hence, the external air can be effectively prevented from flowing

reversely into the grippers.

4.3 Stress and deformation

The pressure distribution generated by the gripper acts directly on

the workpiece surface. As a result, shear stress (τ), circumferential and

radial bending moment (Mr and Mα, respectively), and deformation (δ)

are produced on the workpiece. The stress and deformation are harmful

for some fragile thin workpieces (such as solar cells and LCD glass

panels). Therefore, when choosing a gripper, it is necessary to consider

the potential hazards caused by its pressure distribution. There is no

method to directly measure the stress and deformation of the

workpiece. However, it is believed that these can be estimated by using

the experimental pressure distribution results.

Next, we study the situation of gripping a circular workpiece. The

workpiece is subjected to three external forces (see Fig. 14): uniformly

distributed gravity, negative pressure distribution of the gripper, and

Fig. 12 Comparison of total energy

Fig. 13 Pressure distributions formed on the upper surface of the

workpiece for generating 1-N suction force


contact force between the gripper and the workpiece. Because the

pressure distributions shown in Fig. 13 are all basically axisymmetric, it

can be considered a two-dimensional axisymmetric case. The workpiece

is subjected to the resultant load q(r), and then, the corresponding shear

force τ(r) is produced inside the workpiece. At any position with radius

r, the shear force τ(r) is given as follows:

(2)

The shear force causes the workpiece to deform. The relationship

between the shear force and the deformation is shown as follows:

(3)

where D is the bending stiffness, as defined by the following equation.

(4)

ν is Poisson’s ratio, i.e., the ratio of the lateral deformation to the

vertical deformation of the material. Poisson’s ratio is determined by

the type of material. For example, it is 0.24~0.3 for glass and ~0.3 for

metallic material. E is Young’s modulus of the material, and hw is the

thickness of the workpiece. The deformation δ(r) produces radial and

circumferential bending moments (Mr and Mα) (see Fig. 15) inside the

workpiece, respectively calculated as follows:

(5)

(6)

To solve the deformation and stress, the following boundary

conditions are required.

(1) The peripheral position is set to the reference point for the

deformation, i.e., δ = 0, where r = radius of the workpiece R.

(2) Because this is an axisymmetric problem, the deformation

variation at the center is zero, i.e., dδ /dr = 0, where r = 0.

(3) The peripheral position is a free end without any constraints, and

therefore, the radial bending moment is zero, i.e., Mr = 0, where r = R.

Thus, the deformation and internal stress can be solved by the

flowchart shown in Fig. 16. Considering that two of the boundary

conditions are at the periphery, we start the calculation circle from r =

τ r( ) 1

2π r--------- q r( )

0

r

∫– 2π rdr⋅=

τ r( )D

---------–d3δ r( )

dr3

--------------- 1

r---

d2δ r( )

dr2

---------------1

r2

----dδ r( )

dr-------------–+=

DEh

w

3

12 1 ν2

–( )-----------------------=

Mr

r( ) Dd2δ r( )

dr2

--------------- ν

r---

dδ r( )dr

-------------+⎝ ⎠⎜ ⎟⎛ ⎞

–=

Mα

r( ) Dd2δ r( )

dr2

--------------- 1

r---

dδ r( )dr

-------------+⎝ ⎠⎜ ⎟⎛ ⎞

–=

Fig. 14 A circular workpiece with a symmetrically distributed load

Fig. 15 Bending moment of a tiny volume

Fig. 16 Flowchart of calculation


R. The inverse integration from the periphery to the center is employed.

X is the predicted value of dδ /dr at the position r = R. After one circle

ends, X is revised according to whether dδ /dr at r = 0 is zero, and then,

a new circle begins. The calculation ends when the condition

( ) is met.

One typical application of these types of grippers is for the assembling

line of a mobile panel. Therefore, a thin glass disk workpiece (hw = 0.6

mm, D = 0.3, E = 48 GPa, ρ = 3000 kg/m3, and R = 50 mm) is taken

as an example to study the deformation δ(r), internal stress τ (r), and

bending moments Mr(r) and Mα(r) generated in the glass disk by the

pressure distribution of the two grippers. After being picked up and

gripped, the glass workpiece stays in a force equilibrium state under the

action of three forces: pressure distribution of gripper, its own uniform

gravity distribution, and contact pressure between the gripper and itself.

In the last section, the pressure distribution was measured experimentally.

Hence, the experimental values shown in Fig. 13 can be directly used

for the calculation. With the density and thickness of the glass workpiece

already known, its uniform gravity distribution (= ρghw) can be obtained.

Moreover, because the workpiece stays in the force equilibrium state,

the contact force between the gripper and itself can be easily calculated.

It should be noted that the contact pressure is supposed to distribute

uniformly. This assumption is beneficial to simplify the calculation,

and it is believed that this treatment will not fundamentally affect the

comparison result. Fig. 17 shows the calculation result. Under the

action of an external load, the shear force and bending moment vary

along the radial direction. The bending moment causes the glass

workpiece to deform, and extrusion/stretching stress is generated inside

the workpiece. Excessive shear force and extrusion stress will damage

the material, and deformation of the workpiece will influence the

assembly accuracy. The result in Fig. 17 indicates that the vortex gripper

shows larger deformation and bending moment but smaller shearing

force than the Bernoulli gripper. However, because these differences

are very small, it is considered that the two types of grippers do not

differ obviously in these respects.

4.4 Influence of surface roughness

The Bernoulli gripper and vortex gripper discussed in this study can

pick up workpieces with a coarse surface, unlike traditional rubber

suckers, because they generate a negative pressure while constantly

exhausting air. This effectively prevents the external air from flowing

reversely into the grippers. In this section, the influence of the surface

roughness of the workpiece on the two types of grippers is studied. To

quantitatively investigate and evaluate the effect of surface roughness

on suction force, we used sandpaper. The roughness of sandpaper is

evaluated by the grit size, which refers to the size of the particles of

abrading materials embedded in the sandpaper. The roughness of the

sandpaper surface increases as the grit size decreases. Fig. 18 shows the

dδ r( )dr

-------------r 0=

0→

Fig. 17 Load and resulting deformation and inner stress


three types of surfaces used in the experiments: a smooth surface and

150 and 60 grit sandpaper. 150 grit sandpaper has small and fine

particles, whereas 60 grit sandpaper has protruding sand particles

distributed evenly on its surface.

Fig. 19 shows the experimental result. As the surface of the

workpiece becomes coarse, both grippers need to consume more energy

to generate the same suction force. We believe that this is because the

coarse surface blocks the air flow. When the Bernoulli gripper injects

air through the gap flow tunnel, the coarse surface will undoubtedly

increase the viscous friction. Hence, the pressure in the gap tunnel is

raised, and thus, the negative pressure and suction force are decreased.

For the vortex gripper, the coarse surface weakens the rotation of air

and decreases the effect of centrifugal force of the rotating air. Thus,

the negative pressure and suction force are weakened. Moreover, the

extents of influence on the vortex gripper are observed to be slightly

larger than on the Bernoulli gripper. In the present test sandpapers, the

vortex gripper still maintains the advantage of low energy consumption

when gripping a workpiece with a coarse surface. However, the vortex

gripper may lose this advantage when the surface becomes very rough

and rugged.

5. Conclusion

In this study, we presented the working principles of the Bernoulli

gripper and vortex gripper. Then, we experimentally analyzed and

compared the energy consumption, pressure distribution, deformation

and internal stress of workpiece, and impact due to surface roughness.

From the experimental results, the following conclusions can be derived.

(1) When the same suction force is generated, the operating condition

for the Bernoulli gripper is low upstream pressure and large flow rate,

whereas that for the vortex gripper is high upstream pressure and low

flow rate. From the perspective of total energy including energy loss of

the pipe system, the vortex gripper with low flow rate has a greater

advantage in terms of energy saving.

(2) Though the air flow forms of the two types of grippers are quite

different, the pressure distributions are similar with a low pressure being

distributed in the central area and the peripheral pressure approaching

the atmospheric pressure. Hence, external air can be effectively prevented

from flowing reversely into the gripper.

(3) The workpiece deforms under the action of its gravity, negative

pressure distribution of gripper, and contact force between the gripper

and itself. According to the material mechanics model, the experimental

results of pressure distribution are used to infer the deformation and

internal stress of the workpiece. Because the pressure distributions of

the two types of grippers are similar, the deformation and internal stress

of the workpiece are at the same level.

(4) When the workpiece surface becomes rough, the two types of

grippers need to consume more energy to generate the same suction as

when gripping a workpiece with a smooth surface. The experimental

results reveal that the surface roughness has a slightly larger influence

on the vortex gripper than on the Bernoulli gripper. It implies that the

vortex gripper may lose its advantage in terms of energy saving when

the workpiece surface becomes very rough and rugged.

ACKNOWLEDGEMENT

This work is supported by the National Natural Science Foundation

of China (No. 51375441) and the Science Fund for Creative Research

Groups of National Natural Science Foundation of China (No. 51221004).

Fig. 18 Views of surfaces used for experiment

Fig. 19 Influence of workpiece’s surface roughness


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