Design and Operational Considerations of a Noncontact

7/28/2019 Design and Operational Considerations of a Noncontact

1/9

Int. J. Mach. Tools Manufact. Vol. 38, No. 4, pp. 353361, 1998 1998 Elsevier Science Ltd. All rights reserved

Pergamon Printed in Great Britain08906955/98$19.00 + .00

PII: S08906955(97)00037-0

DESIGN AND OPERATIONAL CONSIDERATIONS OF A NON-

CONTACT ROBOTIC HANDLING SYSTEM FOR NON-RIGIDMATERIALS

F. ERZINCANLI, J. M. SHARP and S. ERHAL

(Received 23 April 1996; in final form 7 May 1997)

AbstractThe application of automation for handling non-rigid products is limited due to the lack of suitableend effectors. The majority of end effectors used for handling electromechanical parts in industry are not easilyapplicable, because non-rigid products (particularly food products) have a somewhat unpredictable and unstablebehaviour which arises from their individual physical properties. End effectors based on multiple fingers aredifficult to control in real time when dealing with non-rigid materials, whose behaviour under dynamic and

gravitational forces needs to be accommodated.This paper introduces a new material handling system for use in industry. This system consists of a novelnon-contact end effector, which has been developed and applied at the University of Salford. The end effectoroperates on the principle of generating a high-speed fluid flow between the nozzle(s) and product surface, therebycreating a vacuum which levitates the product. Guidelines will be provided for the design of a non-contact endeffector that is suitable for the handling of a specific material in a practical handling application. A design anddevelopment procedure and the concept of a non-contact end effector handling system will be explained.Conclusions drawn from the experimental results will be discussed. 1998 Elsevier Science Ltd. All rightsreserved

1. INTRODUCTION

Most commercially available robot end effectors (grippers) are intended for handling rigid

three-dimensional products such as electromechanical parts. However, materials requiringhandling may be rigid, semi-rigid or exhibit characteristics more closely associated withnon-rigid materials, such as food products. Some materials to be handled are simple, flat,two-dimensional, and can be adhesive or slippery in nature. Moreover, the handling ofnon-rigid materials such as polymer sheet, jelly block and sliced meat presents additionalproblems that are not easily addressed using conventional end effectors. During the hand-ling process of some products (particularly food products) the risk of product contami-nation is high.

Although some materials are rigid, they may be flat (two-dimensional in nature), per-meable to air or non-ferrous, making the use of vacuum suction and magnetic techniquesdifficult, if not impossible. Magnetic end effectors cannot be used to handle food products,

unless used to grip a metallic device which holds the food product.Clamping grippers that have been developed in industry and successfully applied for

handling mechanical parts for machining or assembly purposes, as discussed by Tedford[1], are modified by covering the fingers of the gripper with soft materials so as not todamage the handled product. Problems exist when these grippers are applied to food pro-ducts, as they are slow, clumsy and have difficulties obtaining access during handlingon conveyors.

The needle type of gripper is typically not suitable for handling food products wherethe products appearance is important. The needles may mark and damage the productssurface and it cannot be applied to relatively thin and flat products, as it will pierce thesurface texture. There is also the risk of a needle breaking in a food product. An adhesive

end effector is not really suitable for food products because of hygiene problems and it

Research Institute for Design, Manufacture and Marketing, University of Salford, Salford M5 4WT, U.K.Author to whom correspondence should be addressed.

353


2/9

354 F. Erzincanli et al.

also may scratch the surface of products during the process of dropping the products intoa container. Bladder type end effectors [2] may be used for the handling of some three-dimensional (3D) food products. Due to its gripping principles it cannot handle flat andthin products. The accuracy of a bladder type end effector is limited.

One way of handling delicate products is to use a vacuum end effector (this is also

suitable for handling flat objects). Although it has advantages, it has disadvantages sinceit is a contact type of end effector. This functionality can lead to an accumulation of stickysubstances from the objects surface to the suction cup. Accumulation of such substancesis a vehicle to transport contamination from one product to another. Sticky substancesalso cause a product to adhere to the suction cup, which necessitates removal of equipment.Food products are usually air permeable, and due to the suction force that is applied andthe porosity of some products, they can be marked easily by air passing through them.

Tokisue et al. [3] have worked to develop a non-contact handling system to handlerigid semiconductor wafers by using vacuum and pressurised air supply simultaneously.This handling system has successfully been applied to real components made from rigidmaterials. Such an end effector is suitable for handling non-rigid material [4].

The need for a new range of end effectors suitable for compliant products has beendiscussed in detail elsewhere by Erzincanli et al. [5] and Erzincanli and Sharp [6], andaccording to the handling requirements of non-rigid products, novel non-contact end effec-tors have been developed and applied for lifting various compliant food materials at theUniversity of Salford.

In this paper, the experimental results are discussed, including examples of productssuccessfully lifted with this end effector, and recommendations are given for robotic hand-ling in real manufacturing applications. From the experimental results, guidelines will beprovided for the design of a non-contact end effector that is suitable for the handling ofa specific material in a practical handling application. The operational sequence of theend effector differs from the other conventional counterparts, therefore an operational

sequence is presented. A design and development procedure and the concept of a non-contact end effector handling system will be explained along with auxiliary equipment.

2. THE NON-CONTACT END EFFECTOR

The non-contact end effector consists of radial air outflow nozzle(s). The nozzle operateson the principle of generating a high-speed fluid flow between the nozzle head and thematerial surface, thereby creating a vacuum which lifts the product. A radial outflow nozzlesystem consists of two narrowly spaced circular disks placed parallel to each other, andperpendicular to a central tube.

A schematic diagram of the radial outflow configuration is shown in Fig. 1, wherecompressed air from a source enters through a tube at the centre of the upper disk (referredto as a nozzle) and after striking the lower disk flows radially outwards between the twodisks. The radial outflow of air between the parallel disks causes either an attracting or

Fig. 1. The phenomenon of radial air outflow.


3/9

355Non-contact robotic handling system

a repelling force to exist between the disks. In order to create an attracting force, theclearance gap of a nozzle must be very small compared with the diameter of the centraltube and the radial distance through which the fluid flows. The gap between the endeffector and the non-rigid product is to some extent self-regulating, and imposes localrigidity which is useful during manipulation.

The review of previous research on radial flow reveals a lack of information for thecase of lifting a material that is floating freely under radial flow nozzles, other than twoseparate U.S. patents [7, 8]. These patents were certified on lifting a rigid circular semicon-ductor device. Other than patent information, there are no experimental or practical appli-cations. In previous research on the radial air outflow, both disks were fixed to the experi-mental rig and the clearance gap was adjusted when different gap settings were required[9], in other words the front disk was not floating freely. In the case of lifting a materialusing forces caused by radial outflow, the front disk (in this research it is a lifted material)must be free from any mechanical fixing. This makes considerable differences to the resultsthat are taken during experimentation. In this research the lifted material makes the clear-ance gap self-adjusting. This self-adjusting must be investigated in order to observe the

behaviour of the lifted material. This observation will help to establish an appropriateapplication of the end effector.The other lack of information in the literature is for the case of lifting a rigid or non-

rigid material with a non-circular physical structure and the size of the material largerthan the nozzle head diameter. In this research, square shaped specimens of non-rigidmaterials are used.

3. APPLICATION OF THE END EFFECTOR

A series of experiments was carried out during the development program and the non-contact end effector was applied to handling a variety of materials. The effects of sizeand configuration of the nozzles were investigated on the lifting conditions. Various nozzle

configurations and handling conditions were obtained using different sizes and numbersof nozzles [4]. It was revealed that the variations of the number and size of the radialflow nozzles have significant effects on lifting conditions when the end effector is usedto lift rigid and non-rigid materials with particular properties (size and weight) [4]. Theresults of the experimentation are summarized in Fig. 2. It should be emphasized thatsome of the results were visually observed and could not readily be translated into dataor figures.

As can be seen in Fig. 2, semi-rigid and non-rigid materials require multiple nozzles.

Fig. 2. Evaluation of the results of the experimentation.


4/9


Each nozzle should cover a large surface area of the product to be handled, and a highair flow rate should be used. Consequently a large clearance gap can be obtained. For arigid material, a single nozzle could be used, applying a low air flow rate and having asmall coverage area [10]. As a result, a small clearance gap can be obtained. A semi-rigidmaterial falls between the requirements of rigid and non-rigid materials. The modulus of

elasticity of a particular semi-rigid material will indicate the configuration and conditionsto apply. For example, a semi-rigid material with a high modulus of elasticity will havesimilar requirements to a rigid material.

Both the inner structure of the material to be handled and its surface stiffness affectthe handling conditions when a non-contact end effector is used. It may be very difficultto categorize a material as being either rigid or semi-rigid. More research is necessary toinvestigate and establish the limits exactly.

The clearance gap between the nozzles and the non-rigid specimen increased withincreasing air flow rate. When air flow is increased with a non-rigid specimen, the airdownstream can flow out to atmosphere easily due to the bending of the non-rigid speci-men. Lifting forces between the nozzles and specimen cannot be created across the whole

of the specimen surface. Some of the experimental results have been reported by Erzincanliand Sharp [6] and in more depth by Erzincanli [4].At a low air flow rate, lifting could be obtained but the specimen tended to slide horizon-

tally. This is because the low air flow rate could not create the conditions necessary forequilibrium. For secure lifting without any tendency to slide, the air flow should be highenough to create a depression on the specimens top surface. The surface hardness andthe qualities of the material internally affect the lifting conditions. Although the internalstructure of the jelly is compliant, the surface layers may be relatively more rigid and alifting process can then be successful. The surface layers tend to harden with exposure tothe atmosphere, and newly produced material often has insufficient rigidity for the liftingprocess to be maintained.

The novel non-contact end effector has several advantages over the other conventionaltypes of end effector. The following are the specific advantages for the non-contact endeffector.

1. The nozzles can be manufactured not only from stainless steel but also from rigidplastics and aluminium, depending on the hygiene requirements and availability of thematerials. These materials are of low specific mass and therefore a non-contact endeffector that is made from these materials will be light in weight. One of the primaryconsiderations in end effector design, i.e. to reduce the payload on the robot arm, willtherefore be met.

2. Running costs for the non-contact end effector are relatively low, since only compressedair is used. Compressed air systems can be found in most industrial environments.

3. During the releasing process of a lifted product at the end of a handling cycle, the non-contact end effector requires no additional equipment. When the end effector reachesthe required position, the air will be shut off and the lifted material will drop.

Although the non-contact end effector has several advantages over the conventional endeffectors, it does have some disadvantages and limitations. In some cases the end effectoris incapable of handling products due to some of the specific limitations which are outlinedas follows.

1. If a product has a relatively compliant structure, the radial air outflow cannot beobtained since there are indentations and local bending on the surface of the product.

2. If the fibres of the product are loose (for example beefsteak) the behaviour of the

surface of the product is inhomogenous. The air flow may diffuse between fibres.

4. DESIGN CONSIDERATIONS FOR THE NON-CONTACT END EFFECTOR

During the design and selection process for a non-contact end effector, several para-meters should be taken into consideration, such as material compliance and porosity, and


5/9


the required clearance gap. Other influencing factors may be considered after these mainparameters and requirements have been decided upon. A flow chart is given in Fig. 3 toguide the designer during the design stage of a non-contact end effector to handle eitherrigid or non-rigid materials. According to this flow chart, the mechanical properties (suchas compliance) of the product are the main parameters to be considered initially. Firstly, the

structural characteristics of the product should be considered with respect to the handlingrequirements. This could be carried out using the materials modulus of elasticity. Inaddition to the compliance of the material, the stiffness of the materials surface isimportant. A stiff surface structure makes the handling easier, because the material hasless local bending and behaves more like a rigid material.

The size, weight and shape of a semi- or non-rigid product affects the extent to whichits inherent compliance determines its handling characteristics. These parameters shouldtherefore also be considered, together with the compliance of the product, when determin-ing the number of nozzles to use. The shape and size of the product also affects theconfiguration of the nozzle(s) and the volumetric air flow rate that will be used for eachnozzle individually. For example, if it is necessary to lift a triangular shaped product, the

nozzles should be configured to cover this shape. The nature of the product surface shouldbe considered in determining the number, dimensions and type of the nozzles. For a pro-duct with a rough surface, a small number of large size nozzles may be used to obtainsuccessful lifting.

To handle a semi-rigid or non-rigid material, the nozzle diameters should be as largeas possible. The dimensions are limited by the number of nozzles to be used. An optimumshould be investigated experimentally. As can be seen in Fig. 2, non-rigid materials requiremultiple nozzles that provide a large coverage area. Large diameter nozzles have provedto be more effective for this type of material.

The nozzle should be manufactured from a material (such as stainless steel or rigidplastic) that meets the requirements of hygiene, and that is otherwise suitable for the

working environment. The surface of the end effector should be well finished, in order toreduce contamination problems. Non-contact end effectors that are developed followingthese guidelines should be tested before installing them in a working environment.

Design and selection specifications of a nozzle for the non-contact end effector for aparticular product are illustrated in Table 1. This table helps the designer to determine thecriteria during the design stage of a non-contact end effector. From this table the mechan-

Fig. 3. Selection procedure for a non-contact end effector configuration.


6/9


Table 1. Design, selection and configuration specifications of a nozzle

Objectives Criteria

To design and select a non-contact end effector CompliancePorosity

Surface stiffness

Surface qualitySurface size

ThicknessTo configure the nozzles Surface size

WeightShape

Required clearance gapTo handle non-rigid material Large diameter of nozzle

High number of nozzlesNozzle materials Stainless steel

Rigid plasticAluminium

etc.

ical properties of the product are the main criteria to meet the design and selection objec-tives. In order to configure the nozzles, the size, weight, shape and required clearancegap need to be considered. The table also shows the number, size and material criteriaof nozzles.

5. OPERATIONAL SEQUENCE FOR THE NON-CONTACT END EFFECTOR

The non-contact end effector should be employed with the nozzle axes oriented verti-cally, in order to obtain adequate non-contact handling. The end effector could be usedin other orientations, providing that the products can be adequately supported and pos-itioned to ensure that the end effector has access. The end effector will operate in a

sequence of four main steps. These steps are illustrated in Fig. 4. Before starting anysequence, the properties of the material to be lifted should be known, and appropriatenozzle dimensions, nozzle configuration and volumetric air flow rate should be determined.

During the first of the steps illustrated in Fig. 4, denoted (a), the end effector will movevertically down until it reaches a specified clearance distance. Care should be taken that

Fig. 4. An operational sequence concept for the non-contact end effector.


7/9


the nozzle does not touch the material. This distance could be 0.10.3 mm, and it can becontrolled using sensory systems. During this movement the air supply should be shutoff. If the air supply is on, the material may be blown out of position by the compressedair jet flowing from the nozzles.

At the second step, denoted (b), the vertical movement of the end effector will halt,

and the air supply will be switched on. The air flow will create attraction forces and thematerial will be picked up. The end effector will move upwards with the product untilthe required height is reached. At the third step, denoted (c), the end effector and productwill move to the required horizontal and vertical position, which could be a conveyor orpackaging tray.

At the fourth and last step, denoted (d), the end effector will move downward to alocation where the material will be dropped. When the end effector reaches the requiredposition, the air will be shut off, and the lifted material will drop. The location could bea conveyor system, packaging tray, or perhaps other ancillary equipment. When the air isshut off, the material will be dropped at the desired location without any releasing system.After dropping the product, the end effector can return to the first pick-up position (a) to

repeat the cycle.The non-contact end effector could be mounted on a robot or a programmable robotarm in a process, pick and place operation, or packaging line. During this research, theend effector was mounted on the arm of a Hitachi robot in the Manufacturing Laboratoryat the University of Salford. The valves of air pressure and volumetric air flow are con-trolled by a computer and a computer program according to the handling and releasingtimes, and requirements such as weight and compliance of the product to be lifted [11].These requirements will be discussed in the following section.

6. CONCEPTUAL APPLICATION OF THE NON-CONTACT END EFFECTOR

A conceptual handling procedure for a non-contact end effector in a real application is

shown in Fig. 5. In this concept, a host computer will be used to collect and evaluate the

Fig. 5. A conceptual handling procedure for the non-contact end effector.


8/9


incoming information from peripherals (sensor(s), check weighers and vision systems) andavailable information about the material(s) to be lifted. The computer will determine anadequate nozzle configuration (number, dimensions and positioning of the nozzle(s)), andwill instruct the robot to acquire a suitable end effector from the range available. Quickchange mechanisms may be employed. The computer will adjust the air flow rate and

pressure, isolating any nozzles that may not be required for irregularly shaped products.This host computer will then activate the robot to handle the products as necessary.In this concept, one of the information inputs concerns the physical properties of

materials to be handled. Although some classification systems exist in manufacturingindustry for mechanical parts [1214], where these systems are being used successfullyon assembly lines [15, 16], no classification systems exist that can feed information oncompliant products, especially food products, into such a computer system. Product infor-mation could be compliance (modulus of elasticity), surface quality (i.e. smoothness),weight, handling surface size, shape, and the thickness of the products. All of these para-meters affect selection considerations, from the nozzle dimensions to the configuration ofthe nozzle(s). The determination of such parameters for a comprehensive range of food

products is necessary, and this should be the subject of further research into product hand-ling characteristics.The experimental results should be given to the computer system to enable the computer

to adjust the air pressure and volumetric air flow rate. These values will create optimumhandling conditions and also an optimum clearance gap. It is very difficult, particularlyfor non-rigid materials, to calculate the volumetric air flow rate theoretically using theinformation currently available in the literature. Further research is required, both theoreti-cally and practically, to investigate and establish a satisfactory analysis of the fluid flow,using nozzles of the type described, so that attraction forces and other parameters can beaccurately predicted.

The non-contact end effector may be used in handling environments where products

having different surface dimensions and shape are handled. In such a case, if there isadequate access, a large number of nozzles (sufficient for the largest product) may bemounted on the nozzle holder, and some or all of those may be operated or isolated, asrequired. The computer will select the appropriate nozzle(s), and create a configurationsuitable for the handling size and shape of the product. During the handling cycle, thecomputer will switch on and off the air flow for only the selected nozzle(s). The air flowfor each nozzle can be controlled using individual valves. Information relating to the pro-duct shape, size, location and orientation can be obtained using a vision system in orderto configure the nozzle(s) [17].

If there is inadequate access for an end effector with a large number of nozzles on thehandling line, nozzle configurations can be mounted onto nozzle holders and stored nextto the robot. When necessary, end effectors with appropriate nozzle configurations can beselected and acquired by the robot using quick changing mechanisms.

The environmental conditions may affect the structure of compliant products, thereforethe system should be fed under controlled conditions of temperature and humidity. Thesystem may be required to operate at a specific temperature and humidity suitable for theproducts to be handled. If necessary the temperature of the air flow should be altered, anddifferent gases may be used, such as nitrogen for meat products.

According to Bonfield [17], a vision system can supply information to the computerrelating to the location, orientation, size and shape of the products to be handled. Thisinformation can then be evaluated and used by the computer to activate the robot andnozzle valves in this application concept. The robot will orientate the end effector asnecessary, before moving above the pick-up location.

Sensors will measure the gap distance between the nozzle(s) and the product surface.According to this gap information, the computer will stop the approach of the end effectortowards the product and switch on the air valve(s). These sensors will eliminate the possi-bility of nozzle contact with the product to be lifted.

The vision system may also provide guidance at the final location where the product


9/9


is to be dropped. The final location could be a tray for packaging, or a conveyor systemto transfer the product during a processing sequence. In other words, the handling sequencethat was comprehensively discussed in the previous section will be fulfilled. By the methoddescribed, the non-contact end effector will handle compliant products whilst reducing therisk of contamination.

7. CONCLUSIONS

This paper has introduced the requirements of a new end effector for handling non-rigid materials, and a novel non-contact end effector along with its working principles.The results of experimentation have been discussed regarding the criterion of nozzle designand configuration of the nozzle(s). A conceptual application of the non-contact end effectorin a live handling environment, with the auxiliary equipment required, has been discussed.

This non-contact end effector is capable of handling materials that are rigid, relativelynon-rigid, flat, two-dimensional and with slippery or adhesive surfaces, whilst reducingthe risk of contamination. The results of the experimentation showed that contaminationfree handling can be obtained, since there is no contact between nozzle(s) and product.

However narrow, a clearance gap was always observed during the lifting processes.A design consideration for the non-contact end effector was outlined regarding experi-mental results. An operational sequence and a conceptual application of the non-contactend effector in a practical handling environment were introduced. Such a procedure canbe used to design and develop end effectors to handle a wide range of non-rigid products.

REFERENCES

[1] Tedford, J. D., Developments in robot grippers for soft fruit packaging in New Zealand. Robotica, 1990,8, 279.

[2] Perovski, A. P., Universal grippers for industrial robots. Russian Engng J., 1980, 60/8, 11.[3] Tokisue, H., Kuse, T. and Hashimoto, T., Development of non-contact handling system for semiconductor

wafers, in Microcontamination Conference Proceedings, p. 206 (1989).[4] Erzincanli, F., A non-contact end effector for robotic handling of non-rigid materials, Ph.D. Thesis, Univer-

sity of Salford (1995).[5] Erzincanli, F., Sharp, J. M. and Dore, A. M., Grippers for handling non-rigid food products, in Proceedingof EURISCON94, Malaga, Vol. 3, pp. 798806 (1994).

[6] Erzincanli, F. and Sharp, J. M., Non-contact end effector for robotic handling of compliant products, inProceedings of the 31st International MATADOR Conference, Manchester, pp. 629634 (1995).

[7] Benjamin, J. M., Pneumatic probe for handling flat objects, U.S. Patent 3.425.736 (1969).[8] Mammel, W. K., Pickup device for supporting workpieces on a layer of fluid, U.S. Patent 3.431.009 (1969).[9] Wark, C. E. and Foss, J. F., Forces caused by the radial out-flow between parallel disks. Trans. ASME,

1984, 106, 292297.[10] Erzincanli, F. and Sharp, J. M., Robotic handling using non-contact end effectors: the effects of nozzle

configuration on rigid materials, in CESA96 IMACSIEEE/SMC Multiconference, Lille, France (1996).[11] Erhal, S., Design and manufacture of a non-contact gripper and its control system, M.Sc. Dissertation,

University of Salford (1994).[12] Gombinski, J., Classification and coding, Engng Materials and Design, September, 600605 (1964).[13] Opitz, H. and Wiendahl, H. P., Group technology and manufacturing systems for small and medium quantity

production. Int. J. Prod. Res., 1971, 9/1, 181203.[14] Boothroyd, G., Automatic handling of small parts. Annals of the CIRP, 1975, 24/1, 393395.[15] Swift, K. G. and Redford, A. H., Classification for automatic assembly of small parts. Annals of the CIRP,

1978, 27/1, 435440.[16] Boothroyd, G. and Dewhurst, P., Design for assembly, Department of Mechanical Engineering, University

of Massachusetts, Amherst, MA (1983).[17] Bonfield, T., Flexible vision-assisted handling systems for the packaging of fresh meat and other products,

M.Sc. Thesis, University of Salford (1993).

Documents

Design and Operational Considerations of a Noncontact