Journal of Food Engineering 98 (2010) 332338Contents lists available at ScienceDirect

Journal of Food Engineering

journal homepage: www.elsevier .com/locate / j foodengDesign of a magnetorheological robot gripper for handling of delicate foodproducts with varying shapes

A. Pettersson a,*, S. Davis b, J.O. Gray b, T.J. Dodd c, T. Ohlsson a

a SIK The Swedish Institute for Food and Biotechnology, P.O. Box 5401, SE-402 29 Gothenburg, Swedenb Italian Institute of Technology (Fondazione Instituto Italiano di Tecnologia), Via Morego, 30-16163 Genoa, ItalycDepartment of Automatic Control & Systems Engineering, University of Sheffield, Sheffield S1 3JD, UK

a r t i c l e i n f o a b s t r a c tArticle history:Received 25 August 2009Received in revised form 4 November 2009Accepted 5 November 2009Available online 18 January 2010

Keywords:GripperRobotFoodFruitFlexibleMR fluidMagneto-rheological fluidDelicate0260-8774/$ - see front matter 2010 Elsevier Ltd. Adoi:10.1016/j.jfoodeng.2009.11.020

* Corresponding author. Tel.: +46 10 516 66 42; faxE-mail addresses: apn@sik.se, anders.pettersson@sNatural food products are often variable in shape and easily bruised, making them difficult to handle for arobot. In this paper, a novel robot gripper that utilizes the effects of a magnetorheological (MR) fluid isdescribed and evaluated. In the gripping process pouches filled with MR fluid are molded around theproducts contours. Through the activation of an electromagnet in the gripper arm, a large increase inthe MR fluids yield stress confines the product in the mould produced by the gripper surfaces, allowingit to be lifted. Mounted on a six axis KUKA robot products such as apples, carrots, strawberries, broccolisand grapes have been handled without bruising or denting and without the tool changes that would typ-ically be needed. The paper presents data regarding the forces exerted on products during gripping aswell as data on maximum payloads and graspable product shapes.

2010 Elsevier Ltd. All rights reserved.1. Introduction

Many manufacturing industries today use robots for a widerange of tasks. The driving forces for such automation are increasedefficiency, consistency of quality, increased hygiene and reducedlabour costs (Gray, 2001). However, food manufacturers have beenslow to fully utilize the benefits of robot automation. Most robotsin the food industry today are used for handling products packed inprimary or secondary packing and palletizing, few are used to han-dle unpacked products in the process (Wallin, 1997). Some reasonsfor this are that food products are very diverse and display a widerange in size, texture, weight, susceptibility to damage, colour andshape, making them difficult or impossible to grip with traditionalon/off or vacuum grippers. New grippers are costly to develop andit is a significant challenge to develop a single gripper that can han-dle multiple food types. The cost and time implications of usingmultiple grippers for different food types mean this is not a viableoption in the food industry. Still, as Chua et al. (2003) concludes intheir review of robotic manipulation of food products, many foodcompanies see the use of robots and automation as vital for theirfuture survival.ll rights reserved.

: +46 46 18 87 65.ik.se (A. Pettersson).It is very difficult for a robot gripper to compete with humanworkers in terms of flexibility. A human worker can easily handlethe most delicate products, quickly change from one product to an-other and to pick objects laying on top of each other presents noproblem. These are difficult tasks for a robot (Chua et al., 2003).The robot does, however, have other advantages in food produc-tion. Hygiene is especially critical when handling unpacked foodproducts. Today more and more ready-to-eat meals are consumed.Some of these are not cooked but only heated by the consumer,increasing the risk for contaminants in the product to survive(Erzincanli and Sharp, 1997a). Human contact with food productsis a contamination risk. Humans carry microorganisms, hair, salivaand potentially dirt which can contaminate the food whilst robotscan be built to be very clean (Brumson, 2008). Other useful featuresof robots are that they are not affected by chilled or heated envi-ronments and they are not damaged by repetitive motions, ormodified atmospheres. Furthermore are they able to work 24 h aday with constant quality, speed and efficiency. To utilize all thosebenefits new grippers are needed. Patri (1991) presented an earlyconcept of a programmable kitchen system based on a Puma 760robot. A robot dished up plates of food according to received reci-pes. This work was however mostly theoretical but one conclusionwas that the dispensation and presentation (gripping/handling) ofmaterial was a major problem with no apparent simple solution.Today, however, robots are slowly entering into food production.

http://dx.doi.org/10.1016/j.jfoodeng.2009.11.020mailto:apn@sik.semailto:anders.pettersson@sik.sehttp://www.sciencedirect.com/science/journal/02608774http://www.elsevier.com/locate/jfoodeng

Fig. 1. Visualization of the concept of a flexible robot station for food production. Amixed and disordered inflow of products is identified by a vision system. Thecoloured field on the conveyers left side indicates the vision system exposure area.

A. Pettersson et al. / Journal of Food Engineering 98 (2010) 332338 333Dahlquist (2007) describes a muffin production line in a bakery.The production line is completely automated and human contactis completely avoided at all steps. Frozen hamburgers are also han-dled with robots and vacuum-cup pickup. FlexPicker robots areused to handle up to 150 burgers a minute and placing them intocartons (Rice, 2008). Many similar applications like these can befound e.g. picking croissants, confectionary, sausages, pancakesand slices of meat. However, most applications use grippers thatcan only handle minor variations in product shape and texture.

The inflexibility of grippers available today is limiting the use ofrobots in the food industry. Much research has therefore been doneto develop new and more universal grippers for food products.Erzincanli and Sharp (1997b) classification system for food prod-ucts suggested six main categories: shape, dimension, surface,compliance, temperature and weight. For each of these, 10 or moregrades are used. It can easily be seen that it is difficult to develop auniversal gripper for such a wide range of product characteristics.Furthermore, food product behaviour is not static but is affected bye.g. temperature and pressure; they may be fragile, can be easilybruised and are susceptible to bacterial contamination (Chuaet al., 2003).

However, some projects have presented interesting approachesto this problem and often with good results. Lien and Gjerstad(2008) have developed a gripper that uses a Peltier element tofreeze products to the gripper surface. The gripper was shown tomanage gripping forces and release times, for cod fish meat, withinrequirements for industrial use. The materials used and principle ofoperation makes a hygienic design possible. A number of non-con-tact grippers utilizing the Bernoulli effect have been presented.One example is a gripper presented by Davis et al. (2008) for han-dling tomato slices. A positive airflow is used to create a pressuredecrease in the gap between the gripper surface and the product.The gripper design can be made very simple, and together withthe non-contact feature the gripper is potentially very suitablefor the food industry. As shown by Naghdy and Esmaili (1996) itis even possible to extract information about fruits ripeness inthe gripping process. They used a soft force sensor on a paralleljaw gripper to measure firmness and correlate this to fruit matu-rity. Choi and Koc (2006) present a gripper with inflatable rubberpockets on the gripper surfaces. This gripper was able to handlevarious three dimensional shapes and still retain a low pickingand placing positioning error. The gripper could be set to use lowforces. Already in 1977 had Hirose and Umetani developed a softgripping universal gripper. The design was made up of a linkedwire and pulley system. When wrapping itself around objects eachlink exerts the same force on the product, making it a very suitablegripper for delicate and variable products (Hirose and Umetani,1977). In 1980 Perovskii presented an interesting gripper usingthe hardening effect that occurs if a vacuum is applied to small par-ticles enclosed in a rubber pouch. At normal pressure these parti-cles can move around freely and be formed around an object.When a vacuum is applied the shape is locked and the product isconfined in the form created between the pouches (Perovskii,1980). Imitations of the human hand could be ideally suited forhandling delicate products and have been researched for a longtime but have not come onto the market due to demands and lim-itations of such hands (Caldwell and Tsagarakis, 2000).

Even if much research has been done there is still a lack of suit-able grippers. Partly due to this most robots in the food industryare used as only slightly more flexible then hard automation. Hardautomation can often be seen as a production unit with a very highthroughput but with low flexibility. For these robot stations the cy-cle times are expected to be as high as for hard automation and lit-tle extra are expected, preferably a robot should be fitted at anexisting production line. A significant aim of this work is to developa gripper that is as flexible as possible. The concept was to create aone-robot robot station with a universal gripper able to assemblewhole, or part of, meal trays e.g. ready-to-eat meals, Sushi lunchboxes or fruit boxes. Not only to put one type of product in abox. Focus was therefore put on the development of a universalgripper. Fig. 1 shows a visualization of the concept with a robothandling a mixture of products arriving on a conveyor where aflexible robot gripper is able to handle the product variabilityand produce an ordered output.

In this paper, we present a novel universal robotic gripper to in-crease the functionality of robots in the food industry. The gripperutilizes the increase in yield stress of a magnetorheological (MR)fluid achieved when a magnetic field is applied. With this tech-nique various product shapes and sizes can be gripped using verylittle force. During the research the products used for testing havebeen, models of or fresh, whole fruits and vegetables, specificallycarrots, strawberries, apples, tomatoes, grapes and broccolis tocover a wide span of product properties. Integrated in a robot sta-tion, creating a flexible production unit, and using product data ex-tracted with a vision system this gripper is demonstrated to handlea wide variety of products without time consuming readjustmentsbetween products and without bruising or denting.

2. Gripper design and experimental setup

The experimental description of this paper consists of threeparts. Firstly the design of the novel MR Fluid gripper is explainedin more detail and secondly the robot station in which it is in-tended to operate is described. In Section 2.3, the methods to as-sess the gripper are described.

2.1. Designing a magnetorheological (MR) fluid gripper

To overcome the hard contact found with traditional jaw grip-pers, and thus reduce the risk of bruising and denting, a compli-ant/deformable covering for the jaws was investigated. Acompliant surface also facilitates handling of various shapes. Itwas found that MR fluids have the unique features of both anexcellent compliance and the ability to secure a solid grip.

A magnetorheological fluid is a material where the rheologicalproperties change when a magnetic field is applied. MR fluids aresuspensions of micron-sized polarisable particles. The particlesare universally iron and the carrier fluid used can be e.g. oil or

Table 1Typical properties for the MRF-140CG magneto-rheological fluid (Lord Corporation,2008).

Viscosity, Pa s (at 40 C) 0.280 0.070Density g/cm3 3.543.74Solid content by weight, % 85.44Operation temperature, C 40 to +130

334 A. Pettersson et al. / Journal of Food Engineering 98 (2010) 332338water, at ambient conditions MR fluids show a Newtonian flowbehaviour, but in a magnetic field the fluid exhibits a yieldstrength. A higher magnetic field universally generates higher yieldstrength up to a saturation level. To generate a magnetic field, per-manent magnets or electromagnets can be used. The advantagewith the electromagnet is that the magnetic field can be activatedand deactivated instantaneously and controlled from a microcon-troller. MR fluids have been used both in research and in commer-cial applications. Rong et al. (2000) presented a flexible fixturesolution to reduce the number of fixtures needed in production.The concept was to use MR fluid under high pressure to generatesatisfying fixture strength. MR fluids have also been used forsemi-active seat suspension system dynamic dampers (Jollyet al., 1998), and for clutches.

To contain the MR fluid oil resistant polyurethane pouches wereused. The pouches are only partially filled to allow for free flow ofthe fluid at forming, 22 cm3 was used for each pouch. A pouch ofMR fluid was bonded to the surface of each of the two electromag-nets. The high density of the MR fluid tends to give the pouches anatural drop shape protruding approximately 10 mm from themagnet surface. These electromagnets were mounted, with theMR pouches facing each other, on the arms of a parallel jaw actu-ator. The actuator design uses a stepper motor with a linear bear-ing and a ball screw. This setup allows for a step resolution of0.031 mm/step and a maximum grip separation of 79 mm. Themaximum product width that this gripper can handle is 69 mm,in its current configuration. In Fig. 2 the gripper is shown in detail.

The magnets used are 50 mm in diameter with a thickness of27 mm and are powered with 24 V DC. In this project a MRF-140CG MR fluid (LORD Corporation, Cary, NC, USA) has been used.Up to a magnetic field strength of approximately 100 kA/m, theyield stress of this MR fluid increases almost linearly from 3 to44 kPa and then flattens out to around 60 kPa at 200 kA/m, seeTable 1 for further properties (Lord Corporation, 2008). For themagnetic field strength used in this study the MR fluids relativepermeability, l/l0, have been calculated as approximately 12. AFig. 2. The MR fluid gripper gripping a model strawberry. The right most gripperarm is stationary and equipped with a strain gauge force sensor. On the left thestepper motor and belt drive transmission is seen and in the middle the ball screwand linear ball bearing.problem with MR fluids is the tendency for the heavy particles tosettle and thus change the fluids properties. The MRF-140CG MRfluid has been designed to prevent hard settling and to be easilyredispersed (Lord Corporation, 2008). In this study this problemhas not been investigated as the pouches will be continuouslykneaded by the gripping action and the fluid therefore will be con-tinuously redispersed. Via a microcontroller, the gripper arm sepa-ration, speed, acceleration, deceleration, closing force and magnetscan be controlled.

As a gripping cycle is started the gripper arms closes in on theobject to be grasped. When the MR fluid filled pouches reach thesides of the product the pouches will start to deform, withoutstretching the pouch material, and shape to the products contoursas the low viscous MR fluid flows out of the way. The gripper armswill be stopped at a predefined position, leaving a gap, see Fig. 3,between the magnet surface and the product of 07.5 mm on eachside. This gripper arm separation is set individually for each prod-uct using the width or diameter parameters extracted by the visionsystem. Activating the electromagnets induces yield strength in theMR fluid and the product is now confined in the mould created. Atthis stage the product can be lifted, handled with a low forcespread over a large surface.

One of the gripper arms has been equipped with a strain gaugeforce sensor. This sensor allows for multiple gripping modes. Innormal gripping mode the arms are closed to a desired separationdepending on sample size and desired gap. This gripping uses verylow force but some products might require even more delicatehandling. A force limit can be used. Using the force sensor theFig. 3. Schematic description of the experimental setup. (a) Electromagnet, (b)actuator unit, (c) MR fluid filled pouch, (d) grip gap, (e) forming depth (FD) and (f)force lifting the INSTRON cross-bar.

A. Pettersson et al. / Journal of Food Engineering 98 (2010) 332338 335closing of the gripper arms can be regulated by a preset force limitand will move to the set position without passing this limit (forcemode gripping). A rule based decision is made for each productidentified by the vision system and the maximum force limit issent to the gripper. Since the force during gripping will dependon the viscosity and density of the MR fluid this will lead to longergripping times, to allow for the MR fluid to flow out of the way.

2.2. Robot station configuration

For the pick and place tests a KUKA KR5 sixx 6-axis robot (Augs-burg, Germany) has been used. The robot has a reach of 850 mmand is specified to a payload of 5 kg. Vision data are acquired usingan overhead mounted Cognex In-Sight 5400 camera (Natick, MA,USA), IP67 with PatMax software. Products are identified by thecamera using a sorting algorithm to find products of different sizesand elongation. Using this setup a mixture of apples, smaller car-rots, larger carrots, broccolis, tomatoes, strawberries and grapescan be identified and sorted from one picture. Data such as productposition coordinates, angle, width, diameter and length is ex-tracted and sent to the robot.

For each product targeted the robot positions the gripper overthe product and requests a grip appropriate for the type of productto be gripped. In this grip request, gripper separation and/or max-imal force to be used is specified individually for each product. Themaximum force and the separation are based on the identifiedproduct type and width. For example, strawberries are grippedusing only a small forming due to their relative frailness whilst ap-ples can use a larger forming depth (FD), i.e. a smaller gap, seeFig. 3, to achieve higher lift strength. The request is sent to the grip-per via a control PC using RS232 serial communication. In the grip-pers microcontroller the request is executed and the gripper closesto the specified force or distance, sending a signal back to the robotwhen the product is secured in the grip. The products are placedaccording to gripping information and to a preferred layout.

2.3. Methods

To gain knowledge of what products are likely to be able to behandled the maximal lift force was measured. For this test a set ofmodel products fabricated out of wood were used for convenience.The models were made to resemble the shapes of an apple (D69 mm), a tomato (D 55 mm), two strawberries (D 36 mm, L36 mm and D 26 mm, L 30 mm) and a carrot (L 130 mm D1, thickend, 26 mm D2, 17 mm, thin end). Since the variations in sizes andshapes for natural products are high, these measurements can onlybe used as guidelines. For most food products, and other non-mag-netic materials, the permeability is very close to that of water orair. The permeability differences between model products of woodand real products will therefore have minimal impact on the gripforce measured.

To measure the grippers lift force for various shapes and gapsettings the gripper was connected to a 100 N load cell mountedon the cross-bar of an INSTRON 4301 material testing device (Nor-wood, MA, USA), a schematic description of the experimental setupis shown in Fig. 3. The cross-bar can move vertically up or downand the force moving the cross-bar is regulated to increase witha constant rate of 30 N/min during testing. As the gripper ismounted under the cross-bar it can grip objects placed on the sur-face below it. The load cell measures the vertical forces exerted onthe gripper connected to it. The object is fastened to the surface be-low the gripper, gripped by the gripper and then the lifting force isincreased acting on the cross-bar. A forcedisplacement diagram isthen recorded. At the start of the test, as the product is firmly heldby the gripper (and secured to the surface under it), the force forc-ing the gripper up will increase sharply without the gripper mov-ing upwards. When the force that is lifting the cross-barcontinues to increase, the gripper will slowly start to move up-wards with the cross-bar as the product slips in its grip. Whenthe product had moved 2 mm in the grip (the gripper had moved2 mm upwards), the test was terminated. The force needed to dis-place the product 1 mm, within the grip, was recorded as the grip-pers maximal lift force. A product displacement of 1 mm in the gripduring handling was considered acceptable with the motivationfrom Wallin (1997) where a positioning accuracy of a few millime-tres is considered sufficient for most food handling applications.

Various gap settings have been tested with the model products.A gap of 0 mm would indicate that the surfaces of both magnetsare in contact with the sides of the product and the MR fluid filledpouches completely compressed. At a gap setting of 10 mm there isa MR fluid filled spacing between the product and the magnet sur-face of 5 mm on each side.

The test is considered to give a good estimate of the loads thegripper is able to handle as it is similar to the action of a robot sta-tion where the object is gripped and lifted.

A 2D simulation of the magnets magnetic field strength hasbeen performed using Visimag software (Visimag, http://www.viz-imag.com). The physical dimensions of the magnets were used inthe model and the relative permeability, l/l0, was set to 500 (mildsteel or free cutting steel). The MR fluid was modelled as a 7 mmhomogenous block on the surface of the magnet. From the productdata sheet of the MR fluid the relative permeability, l/l0, was cal-culated as 12 for the field strength used. With the data collectedfrom the simulation the yield stress of the MR fluid can be esti-mated and a theoretical lift force has been calculated.

Using the force sensor in the gripper arm the forces exerted onthe product during gripping were measured. Gripping force wasmeasured on a model strawberry for a range of closing speeds.Information of force and gripping time were recorded.

For the pick and place evaluation the gripper was implementedas a part of the robot station. A mixture of products was spread outon a table positioned under the vision camera and the robot stationwas started. When the robot had picked all identified productsavailable, a new load of products were spread out on the tableand a signal was sent to the camera using a manual switch, initiat-ing a new image capture and the following automated picking andplacing. The grippers ability to handle the different products usingthe vision data was observed and the products were afterwardsvisually inspected to detect denting or bruising.3. Results

3.1. Grip strength

The results from the grip strength test are shown in Fig. 4. Val-ues are presented both with and without the magnets activated asa comparison. It can be seen that a decrease in gap leads to an in-crease in grip strength.

Partially this is an effect of the increased amount of MR fluidtrapped under the products curvature when the gap is smaller. Itis the MR fluid trapped under the product that supports the weightof the product when the magnets are activated. However, the mainreason for the increase in grip strength is considered to be an effectof the increase in magnetic field strength closer to the magnet sur-face. The increase in field strength results in an increase in the MRfluids yield strength and the higher yield strength can in turn sup-port higher loads. For the object to move in the grip the MR fluidunder it must flow to the sides which require a higher force asthe yield strength increases. It can also be seen that larger productsgenerate higher grip strength. Again this is an effect of the amountof fluid trapped under the object gripped.

http://www.vizimag.comhttp://www.vizimag.com

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Fig. 4. The figure shows the grip strength measured for a set of model product with(M) and without the magnets activated. 3, 5, 10 and 15 mm gap settings have beentested for all products except apple where only 5, 10 and 15 mm have been used.The error bars indicates the standard deviation.

336 A. Pettersson et al. / Journal of Food Engineering 98 (2010) 332338If the magnets are activated the achievable grip strength in-creases between 24% and 110% as compared to gripping withoutactivating the magnets. The reason to why the grip strength is ashigh as it is, when not using the magnets, is considered to be dueto the rather high friction between the product and the MR filledpouches. This friction causes the product to be quite well grippedand hanging in the rubber pouch surface. However, the mobilityis much higher sideways for the product when not activating themagnets. The largest improvement in grip strength can be seenwhen using the smallest gap. This is likely due to the higher mag-netic field strength at small gaps. It is suggested that the grippingwould be improved by using less amount of MR fluid and aim touse as small gaps as possible. This would decrease the force ex-erted on the object at gripping even further and increase grippingstrength as the gap gets smaller. In Table 2 it can clearly be seenthat the grip strength is enough for all products tested.

It is important to consider the high accelerations of robots whendimensioning a gripper. Some of the fastest robots today can use1015 G of acceleration, requiring a secure grip on the product.The maximal allowable acceleration can be calculated using Eq.(1) and the maximal lift strength from Fig. 4:

F m a 1

For a tomato with an average weight of 80 g and a grip gap of3 mm the maximal acceleration expressed in G is 11 G when acti-vating the magnets and 5 G without activating the magnets. It isclear that the MR fluid in combination with the magnetic field en-ables higher handling speeds.

3.2. Magnetic field simulation

The magnetic field strength was simulated at four distancesfrom the magnet surface: 1.5, 2.75, 5 and 7 mm. In Fig. 5 it canTable 2MR gripper grip strength compared with real product masses. For apple data a 5 mmgap has been used and for the others a 3 mm gap.

Product Typical productmass (g)

MR gripper lift capacity(Experimental data) (g)

Apple 150 950Tomato 80 900Carrot 75100 660Strawberry 512 440be seen that the field strength is strongest close to the magnet sur-face and weakest closest to the pole. At transitions from the pole tothe coil, field strength peaks can be seen but these are most pro-nounced at small distances from the surface. These simulationswere made with the MR fluid pouches modelled as a rectangularblock on the magnets surface. When a product is pressed intothe MR fluid pouches the geometry of the pouches will change aswill the shape of the magnetic field. However, as the permeabilityis higher in the MR fluid than in the surrounding air the magneticfield will mainly flow through the pouches, reducing the change inmagnetic field strength. Data from the simulations have been usedto calculate the gripping force when gripping the model apple witha gap setting of 5 mm. Using Eq. (2), the surface that the pouchescover of the lower side of the apple can be calculated if the areais divided by 2 (using only the lower half). This results in a contactarea of 1650 mm2 using the radius of the apple and the formingdepth (FD), see Fig. 3, of 7.5 mm (the pouch protrudes 10 mm fromthe magnet surface 2.5 mm from the gap setting). From Fig. 5, anestimated field strength value of 18 kA/m results in a yield stress ofapproximately 11.5 kPa. Multiplying the yield stress with the con-tact area results in an estimated lift force of 19 N. This value ishigher than the experimental value of 9.5 N which is likely dueto differences in the model and the real prototype. It is also be-lieved that minute bending of the grippers arms during the testing,might lead to increased movement and lower grip forces, when theproduct is forced down in the grip and gripper arms forced apart.

3.3. Gripping impact force

The different gripping modes will affect the gripping cycle indifferent ways. If position control is used a higher closing speedwill increase the peak force exerted on the object but will also de-crease gripping time as shown in Fig. 6. The force exerted on theobject is due to the displacement/flow of the fluid as the objectis pressed into the MR filled pouches. In Fig. 6 the data from a forcemode gripping, described in Section 2.1, is also presented. Here itcan be seen that the gripping can be performed with reduced force,in this case limited to less than 2.9 N exerted on the object. How-ever the gripping time will be extended.

In Fig. 7, the gripping force data, from the gripping of the largemodel strawberry, have been divided with the area in contact withthe object during the gripping process at 95 mm/s. The contactarea has been estimated to that of a sphere with the same diameteras the model strawberry using Eq. (2):

S r FD p 2 2

where r is the radius, S the surface area and FD the distance thesphere is pressed into the MR filled pouches. It has been difficultto find data of the forces at which a strawberry is bruised. However,puncture tests have shown that a strawberry skin can be puncturedwith as little as 75 kPa and up to 187 kPa using a 2 mm diameterprobe (Monma et al., 1977). When comparing this data with theforces exerted on the product by the gripper it is still clear thatthe gripper exerts, by far, too little force on the product to risk punc-ture. However, bruising will likely appear before puncture and fur-ther testing is needed to guarantee that no bruising has occurred.

3.4. Pick and place evaluation

In Fig. 8, the starting point of the pick and place test with realproducts is shown. Randomly mixed and positioned products areplaced under the vision system camera. When an image has beenacquired it is evaluated by the vision system, the data is sent tothe robot and queued up for picking. Using the extracted informa-tion of the object position, object type and object width the robotgrips the object with the MR fluid gripper. The current robot

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Fig. 5. The figure shows the results from a 2D simulation of the magnetic field strength. The magnetic field strength, H (A/m), have been plotted, from the midpoint of themagnet out to the rim, at a distance of 1.5, 2.75, 5 and 7 mm from the gripper surface.

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Fig. 8. Example of how the products are presented in front of the vision camera. Onthis picture a mixture of tomatoes, carrots, strawberries, broccoli and grapes ispresent.

Fig. 9. The mixture of fruits and vegetables has been sorted and assembled onto theoutgoing feed using a robot station equipped with the universal MR gripper.

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A. Pettersson et al. / Journal of Food Engineering 98 (2010) 332338 337station setup allowed for accurate positioning of the gripper by therobot and to set correct gripper separation using the vision systemdata. Each product is gripped using settings suitable for thatproduct. The products are then placed according to the preferredand pre programmed layout, see Fig. 9. All products werehandled repetitively during this test and no visual damage wasdetected.3.5. General observations

With optimization of the magnets and the actuator mechanismit is likely it is possible to reduce current gripping times, increase

338 A. Pettersson et al. / Journal of Food Engineering 98 (2010) 332338grip strength and reduce magnet size. The rubber pouch for the MRfluid could be greatly improved to reduce size and facilitate theflow of the fluid. This would increase the grippers use as bulkinesscan be a problem for some applications.

A main advantage of the gripper is that the force at gripping isspread evenly over a large surface by the rubber pouches, greatlyreducing the risk of bruising. Compared with other soft materials,that can be used on gripper surfaces, the fluid filled pouches havean unbeatable ability to maximize contact surface area.

If the product is correctly identified by the vision system thegripping is usually successful. Problems arise if the product hasbeen identified as narrower or wider, often due to light settingproblems.

The broccoli was difficult both to identify correctly and to grip.Due to the spongy character of broccoli the gripper does not formso well to this product and it needs to be clamped instead. If theproduct is clamped there are still problems with releasing it sinceit expands as the arms separates and tends to stay in the grip ordrop out somewhere during robot motion. The broccoli can be han-dled with the gripper but it needs another gripping rule strategy.

Furthermore, it is important to dimension the gripper for the in-tended sizes and shapes to be handled, as even a universal gripperhave a limited range. In this study apple, carrots, strawberries andtomatoes have been the targeted products. Too small or large prod-ucts will not allow for enough MR fluid to flow under the productand the resulting lift force will not be sufficient.4. Conclusions and future work

The gripper has been shown to be able to handle a variety ofshapes and sizes. From the results in this studied the gripper canbe used for very delicate handling even at high handling speeds.Depending on how delicate the products are this gripper only adds0.41.3 s to the cycle time.

The robot station demonstrated in this article allows for onesingle robot to e.g. fill fruit trays with different fruits instead ofusing one robot or one gripper per fruit type. A setup like this is be-lieved to allow for very short product changeover times allowingfor quick response to consumer trends.

Our study has further confirmed that the accuracy, in regard tovision system and mechanical systems, is satisfactory within thecurrent application.

To make the gripper meet the hygiene requirements in the foodindustry encapsulation of the actuator mechanism, to allow forhose down washing, will be investigated. Size and shape of themagnets and the MR fluid pouches should be optimized. To reducethe bulkiness of the gripper arms a permanent magnet solution hasbeen suggested and will be further investigated.Performance at high speed pick and place operation and con-veyor picking will be evaluated.

Acknowledgment

This study has been carried out with financial support from theCommission of the European Communities, Framework 6, Priority5 Food Quality and Safety, Integrated Project NovelQ FP6-CT-2006-015710.

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http://www.robotics.orghttp://www.robotics.orghttp://www.nyteknik.se/art/50562http://www.nyteknik.se/art/50562http://www.lordfulfillment.com/upload/DS7012.pdfhttp://www.lordfulfillment.com/upload/DS7012.pdf

Design of a magnetorheological robot gripper for handling of delicate food products with varying shapesIntroductionGripper design and experimental setupDesigning a magnetorheological (MR) fluid gripperRobot station configurationMethods

ResultsGrip strengthMagnetic field simulationGripping impact forcePick and place evaluationGeneral observations

Conclusions and future workAcknowledgmentReferences