Vacuum-gripper for robotic manipulation of tomatoes

DCSR Project

Sólyom csaba

Gr. 1542 R.I.e.

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Contents

1. PROJECT SPECIFICATIONS ............................................................................. 2

2. SELECTING THE APPROPRIATE CONCEPT .................................................... 3

3. QUALITY ASSURANCE ...................................................................................... 6

4. DESIGNING THE PROTOTYPE ........................................................................ 10

5. CALCULATING THE NECESSARY NEGATIVE PRESSURE AND THE

REQUIRED FORCE ................................................................................................. 14

6. FINITE ELEMENT ANALYSIS ........................................................................... 15

7. BIBLIOGRAPHY ................................................................................................ 15

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1. PROJECT SPECIFICATIONS

The goal of this project was to develop a gripper prototype for the manipulation of

tomatoes of the following dimension- and weight-range:

Weight 50 - 500 [g]

Volume: 110 - 550 [cm3]

To accomplish this, the following are to be followed:

The prototype’s concept: developing three variants up to the level of kinematic chains

Ranking the requirements of the client using the AHP method

Selecting the appropriate variant based on a systematic algorithm, called PUGH

Designing the resulting prototype’s kinematic chain to the level of individual components

Planning the product’s performance

Formulating technical and economic objectives

Qualitative analysis of the prototype

Identifying vectors of innovation using the TRIZ method

Functional analysis

Planning of required functions

Detailed design of the concept

Analysis of modules

Designing and elaborating the prototype to the level of individual components

Necessary calculations for the gripping force and the required negative pressure

Analysis of modules for defection possibilities

Detailed assembly draft

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2. SELECTING THE APPROPRIATE CONCEPT

The three preliminary concepts:

1. 3-finger electric gripper 2. 3-finger hydraulic gripper 3. Gripper using vacuum

The first step is to rank the requirements of the client using the AHP method.

Said method will show us which of the 16 requirements are the most important, so it

can be decided which of these require more attention.

After this ranking is done, the PUGH method is used to deretmine which

concept will fulfill these criteria best.

AHP:

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PUGH:

Fig. 2-1. 3-finger gripper using DC servomotors and ball screws as linear actuators

Fig. 2-2. 3-finger hydraulic gripper

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After applying the PUGH method it is clear that the 3rd concept – the one using

vacuum pumps – is the optimal one for the current application. This concept uses a

single suction cup of sufficient diameter and applicable negative pressure to pick and

move all products within the given range. The electric and hydraulic concepts cannot

provide an effective approach of the problem at hand, due to the limited ability to pick

objects from dense clusters. In such cases these have no room to enter with the

fingers and grab the product without damaging the others, while also having to deal

with the possibility of trying to pick several objects at once.

The transport equipment that would be necessary to ensure a safe and

adequate contact would actually be more complex and expensive that the grippers

themselves. The suction caps eliminate this problem by approaching the object from

above, not having to deal with the problem of dense clusters, only with the assurance

of adequate contact for the necessary pressure to be created.

Functions that are needed:

Basic functions:

o Picking the product o Manipulating the product o Releasing the product

Auxiliary functions:

o Protection of the manipulated product o Holding current position in case of emergency stop o Assuring stability in motion

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3. QUALITY ASSURANCE

At this step, first QFT – phase I is applied to plan the client’s needs together

with the technical performance characteristics. The main objective is to find the

optimization areas and potential conflicts regarding optimization.

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To solve the negative correlations resulting from the above optimization

approaches, we apply the TRIZ method to find vectors of innovation to relieve the

conflicts.

Nr. Characteristics in conflict

Parameters conform TRIZ

Invention vectors

Specific solutions

1 Payload

Mass

Weight of the moving object

Stability of the object

1 19 35 39 19:

replacing continuous motion with intermittent

motion

Using intermittent

motion instead of

continuous.

2 Payload

Moment of inertia

Force(intensity )

Stability of the object

35 10 21 35:

changing physical

properties of the system

Using high-strength,

low-weight materials

3 Payload

Size

Volume of moving object

Stability of the object

28 10 1 39 10:

preemptive planning

Contact with the product

must be made in the

most convenient

location, using vision

guidance and

conveyor tracking

technologies

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As a second step QFD – phase II is applied to realize the planning of gripper

functions. This method will show the importance of every function relative to the

technical characteristics, in order to know which functions are most important in

designing the prototype.

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Next the QFT – phase III is applied to analyze the necessary modules to see

the connections between the each module and the individual functions of the gripper.

The results are the importance values for each module.

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4. DESIGNING THE PROTOTYPE

Before starting to design the prototype, some parameters need to be

considered: the products should be fresh, with consistent compressive resistance to

be manipulated. The forms of the products can vary in large ranges as long as they

have a sufficient contact area for the suction cups – as such this method can be used

primarily for tomatoes with shapes close to spherical, or having a round profile (the

oval, elongated variant can pose a challenge, because it doesn’t have a suitable

contact surface).

All the components used in the prototype were selected from Anver Corp. to

ensure seamless integration and ease of maintenance. The only exception is the

support element which links and holds the components and is interfaced with the

robot’s flange.

The chosen concept uses a single 18.3 mm double-bellow suction cup (Part No.

B2.5-18-SIT) made of food-grade translucent silicone (material designation SIT)

capable of handling 620 grams at approx. 0.8 bar negative pressure. The double-

bellow structure allows adequate contact with non-regular surfaces, like those of

various tomato types inside the given range.

The parallel robots suitable for the application at hand, like ABB’s IRB 340 or

the Adept Quattro usually come equipped with integrated vacuum pumps capable of

creating a maximum of approx. 0.9 bar negative pressure, meaning that the payload

requirements can be easily met. If the robot chosen does not have a vacuum pump

and the associated sensory equipment, these can be acquired from Anver,

specifically the JV series mini vacuum generators (having 0.84-0.87 bar capacity)

and JVSF fitting modules (able to accommodate any M5 10/32’’-port vacuum sensor)

can provide the necessary capabilities.

The suction cup is attached to a suspension assembly (Part. No. SLSA-110NR),

via a barbed-fitting (Part No. BM5), to ensure a smooth and soft touch, consistent

pressure (through adequate contact) leading to increased precision, less

maintenance (inadequate contact can lead to the deterioration of the suction cups)

and reduced risks of damaging the product.

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The components:

Suction cup B2.5-18-SIT

Barbed-fitting BM5

SLSA-1 series vacuum cup suspension assembly – SLSA-110NR

Tube fitting for the suspension assembly – PTC1-5/32S

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Polyurethane tube – PC-T532

SIT material

SIT is excellent for high heat or food packaging. Soft and pliable, meets FDA

Title 21 and German spec. BGVV (BGA) Part XV for contaminant-free load handling.

Contains no dyes that can leach out when handling baked goods, drugs, glassware,

hot products from molds, etc. Our new formulation minimizes almost completely the

yellowing with age which used to occur with clear silicone in the past.

The support element with the interface flange:

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The above figure shows the exploded assembly of the chosen equipment, with

the only major difference being the elimination of the suspension bracket, since the

lower part of the support is designed to hold the suspension rod and provide

structural stability.

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5. CALCULATING THE NECESSARY NEGATIVE PRESSURE AND THE

REQUIRED FORCE

Where:

o F – the necessary force

o m – the mass to be manipulated

o k – safety factor, =2 as per Anver recommendation for vertical lifting

In our case:

[ ] [

]

Where:

o A – effective contact surface

o a – 0.8 (only 80% of the contact area is taken into consideration, as per Anver

recommendation)

o n – number of suction cups, =1

o d – diameter of the suction cup, =18.2 [mm]

In our case:

[ ]

[ ]

To calculate the necessary negative pressure we use the following equation:

In our case:

[ ]

[ ] [

]

This value is almost half of the maximum capacity of both the Anver mini pumps

and the built-in pumps of some parallel robots, meaning lifting and maintaining

stability should not be a problem.

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6. FINITE ELEMENT ANALYSIS

Finite element analysis of the support structure under a load of 20 [N], double

the calculated value for safety reasons. We can note that the rectangular shape of

the support pillars poses some risks – by introducing rounded corners these can be

effectively eliminated, by removing the areas of minimum resistance.

7. BIBLIOGRAPHY

1. Suport de curs la disciplina Robotizarea fabricatiei I si II de Dr. Ing. Bogdan MOCAN

2. Anver catalogs at www.anver.com