SOFT ROBOTICS – SOFT ARTIFICIALSKIN Abstract Softrobotics is a fairly new emerging field which deals which robotic systemssimilar to organisational structure of living organisms. Biologically inspiredrobotics use sensors and materials which are very similar to that of a livingorganism. One such application is soft artificial skin sensor. This paper showsthe progress in development of the artificial skin sensor. 1.Introduction 1 The field of robotics has developedover the years and robotics play an essential role in our daily lives.
Robotsare used in fields such as medical and automation industry. Conventional robotsare made using hard and bulky materials such as aluminium and steel whereas thedevelopment in the field of soft robotics has led to emergence of technologyand products which are similar to body structure of biological organisms suchas their skin, muscles and organs. Onesuch sensor is the soft artificial skin sensor. This sensor is used to detectthe external strain and force applied to it. This sensor will make the robotmore responsive while performing actions such as assembly and manipulation 2. The most common approach in designing a softskin sensor is use of contact sensors and embedded microfluidic channels fordetection of changes in structure of the sensors 2. Soft artificial skinsensors which are capable of detection shear forces provide an added advantageof giving comfort to the user of wearable devices 2.These forces areresponsible for detection of hand motion which would give an added advantage ofdeveloping sensors for robotic arm.
2. Design An artificialskin was developed using rubber silicone for the sensor layers. Rubber siliconehas very high stretching and compressing properties 2. The skin includedthree layers of silicone. the first and second layer consisted of microchannels which are induced with a conductive fluid, EGaIn (Eutectic Gallium-Indium)2. The third layer consisted of micro channels in a circular shape which isresponsive to external pressure 2. The second layer is placed perpendicularto first layer to detect strain. All the layers are connected to each otherusing interconnects 2.
When externalforce is applied onto the artificial skin the microfluidic channels changeform. These changes affect the cross – sectional area and lengths of thechannels due to which changes occur in electrical resistance 2. The paper 2showed that the skin sense strain and pressure. The skin detects the strainwhen the cross- sectional areas of the channels decreased and channel lengthincreased 2. The resistance and pressure are related and can be derivedtheoretically using the equation (4) in 2.An artificialskin was designed to be worn in 3. It is made of silicone rubber whichresembles a human hand. The glove has micro-channels which contain conductive liquidsand these channels helped in sensing changes in hand motion 3.
Since theglove is modelled after a human hand it had twenty-four degrees of freedom 3.Two sensors were placed on each of the four long fingers, one sensor on eachjoint of each finger 3. The thumb had three sensors, two on the joints on thefinger and one sensor between the thumb and the index finger 3. In this waythe skin can detect motions of the hand 3.Themicrofluidic channels had two types of conductive fluid flowing through them,i.e. ionic fluid for pressure sensing and liquid metal for wiring. The ions inthe ionic fluid served as charge carriers, which helped in conductance 3.
Aninstrumentation circuit was used in 3 to avoid polarization of electrodes.Due to polarization the sensors were not able to give impedance output whichwas dependable 3. The fabricgloves presented in 4 have similar construction as that of 3. In 4 thetwenty-four degrees of freedom of a human hand has been reduced to 19 degrees offreedom by considering the joints which are responsible for movement of thehand during the actions of handling and grasping.
Sensors for detection ofpressure and strain are used in the glove 4. The changes in the shape of theliquid – metal channels are responsible for changes in the electricalresistance and cross – section of the channel length on which the working ofsensor is based on 4. The change in resistance is dependent on the propertiesof the material, channel and elastomer properties and external forces 4. Thestrain and change in resistance is given by (3) in 4. In 5 thesensor is made of a flexible elastomer and can measure the force in threedirections (x, y and z axis). Theconductive fluid flows through the micro-channels which is EutecticGallium-Indium 5.
A force transmission component is placed in the siliconerubber which is made using 3D printer 5. Two layers of the silicone rubbermake up the sensor, the first layer consists of the force post and the secondlayer consists of the micro-channels 5. The three sensors required for thesensing the three in-plane forces placed 120 degrees to each other 5. Whenexternal pressure is applied to the sensor in normal direction, the deformationof channels results in changes in electrical resistance 5. These changes helpin determining of the three forces 5.The paper 6shows working of two sensor prototypes. The first prototype is based on paper5, which could measure three forces. In the second prototype the conductivemetal liquid EGaIn (Eutectic Gallium-Indium) channel wiring are replaced bycopper flex circuit 6.
This circuit avoids bogus sensor indications caused byfaulty mechanical trigger signals given to region of wiring.The paper 7elaborates on a sensor which can withstand strain. The sensor in this paper usestwo different conductive fluids which have distinguishable resistive values7.
The two conductive liquids used are ionic solution of sodium chloride and EutecticGallium- Indium. For the two liquids, to be conductive an electrode was used asan interface between the liquids. The electrode is made of silicone with microand nano-particle doping 7. A low voltageamplifier was used to design an oscillator circuit which has an advantage ofusing DC supply to produce oscillatory pulses 7. When the sensor is deformedthe resistance increases and the oscillation decreases 7.
Since resistanceand oscillations are dependent on each other, the relationship can be given as(3) in 7. The input to a counter of digital processor gives the oscillatorfrequency 7.The artificialskin demonstrated in the paper 8 is able to detect the pressure and strainapplied to it, at the same time. The notion of sensor’s that are capable ofdetecting strain and pressure is used in the paper 8.
These sensors consistof micro channels with eutectic gallium- indium liquid flowing through them forconductivity 8. The skin is designed to have three sensor layers that arefabricated using silicone rubber 8. The silicone rubber has elasticproperties (stretchable and soft) 8. The first and second layer consists ofchannels which are responsive to strain and pressure while the third layer isresponsive only to pressure. The first and the second layer are place at ninetydegrees to each other to be responsive to strain along the axis 8. This waythe sensor is able to detect the strain and pressure in x, y and z directions.
To make this circuit as a whole, the layers are connected using interconnects8.3. Fabrication and InstrumentationThree stepswere followed for fabrication of artificial soft skin sensor done 2. Thesesteps are as follows:1.
Casting: The three sensor layers were developed with help of plasticmolds, which were made using a 3D printer 2. The molten silicone was pouredinto the molds and left to harden at a temperature of 60 degree Celsius fortime duration more than three hours 2.2. Bonding: The three hardened silicone layers were bonded usingliquid silicone 2. To avoid blockage of the interconnect holes a plasticpiece is introduced into the hole.
The plastic piece is removed aftercompletion of the bonding process 2. To avoid liquid silicone from blockingthe micro channels, partial curing is done at 60 degrees Celsius. The process ofuniform silicone coating of silicone (i.e. spin-coating) and bonding isrepeated to get one sensor structure 2.3.
EGaIn Injection: EGaIn is injected into the micro channelsusing one of the syringe and the other syringe is used to remove the air trappedin the micro channels 2. After combining all the three layers, EGaIn isinjected. The duration of this process is approximately 60 seconds 2. An electrode is used forwiring connections 2. The holes made by the wire connections is closed usingsilicone rubber 2. The micro channels are of the length 2.25 m and thethickness of the complete prototype is approximately 3.
5 mm 2.The sensor isacts as an input device when interfaced with computer. Whenever the sensor isdeformed due to application of pressure or strain, there are voltage dropsacross the three layers of the sensor 2. The voltage drops are amplified bythe instrumentation amplifier 2.
The output of the instrumentation amplifierserves as input to a microcontroller where changes in resistance is detected2. These changes aretaken by a MATLB program and a virtual model of the sensor is generated 2. Thedisadvantage of the sensor prototype in 2 is that it is unable to detectpressure and strain at the same time 2.
The glove prototype manufacturing process 3 consists ofprocesses similar to that of fabrication process in 2. However, the mold hastwo components, a sensor bulge on the base and microchannel patterning and sidewall to decide the limit of the skin to used 3. The side wall and the base isfixed together using screws 3. Since the glove is designed similar to human hand,the joints which are responsible for motion are selected and the channels areplaced on these joints 3.
The molten elastomer undergoes the process ofcutting in the 3d printed mold 3. Silver threads are used between the twoconductive liquids as interfaces 3. The liquids were introduced into the skinwith the help of two syringes, one for injecting the liquids and the other forremoval of air in the channels 3. The glove is interfaced with an instrumentation circuitthrough which the electrical changes in the glove are measured and the sensorresponse is observed. The circuit consisted of chip which had a microcontroller,digital and analog components which are programmable and amplifiers 3. Amultiplexer was used so that the chip could be individually connected to any ofthe sensors present on the joints 3. The response of the sensors is detectedand read by a MATLAB program 3.
Thefabrication process in 4 is same as that of fabrication in 2. The processinvolves four steps, “1. Functional component embedding, 2. Silicone casting,3. Layer bonding, 4. Conductive liquid injection,” 4.
Three methods areconsidered in 4 for obtaining the desired bond in the elastomer layers of theglove of which mechanical method has been used 4. The sensors are casted intoa glove form using process of encapsulation 4. Encapsulation provides alayout for the sensors to be placed in the glove form 4.
The glove was testedand was able to detect both pressure and hand motion at the same time 4.The sensorused in 56 has been fabricated using set similar to that of 3 and 4.However, the instrumentation setup differs. Firstly, a known load of “6-axisforce and torque sensor”, 56 has been applied to the sensor. Further thesensor is connected to a PC so that the data can be read by MATLAB 5.
Thesensor in 5 has a maximum capacity load of 13.3 N and range of 9.5 mm whilein 6 the sensor has a load capacity of around 44.1 N and range of 13 mm. Theforces are applied to the sensor in in plane force directions (x, y and zdirections) 56.
For prevention of the two layers of sensor sliding awaywhen forces were applied onto it, sandpaper was glued to the surface 56. The sensor in 7 was fabricated usingCO2 laser cutting. Three molds were prepared for shapingof interfaces, determination of micro-channels and a final layer 7. The widthof the channels can be determined by keeping a check on power and laser speed7.
The elastomer consists of conductive particles which were introduced bythe process of mixing. The foundation of surface between the two conductiveliquids is “platinum cure silicone”, 7. For successful bonding of theelastomer and Ecoflex, process of spin coating was used while elastomer wasplaced in an oven 7. The last step was injection of the conductive liquidsusing two syringes into the channels 7. After this step Ecoflex is used toclose holes made when wires were placed into the micro-channels 7. Thesewires are used to connect the sensor to the PCB 7. While testingthe design for its response and conductivity, it was observed that there was waterevaporation which resulted in slight change in resistance of the conductivesolutions 7.
Further more strain and pressure were applied for testing theresponse of micro-channels which did not cause any noticeable change in theresistance of the sensor 7.Fabricationprocess of sensor in 8 is similar to fabrication process in 5,6. Theprocess follows the steps of casting, bonding and conductive liquid (EGaIn)injection 8. Three sensor layers are cast using 3D printed molds.
The processof bonding takes place with the help of spin coating of silicone. The finalsteps involve insertion of the EGaIn into the micro-channels 8. The sensorlayers are connected to each other using interconnects 8.
Theinstrumentation used to connect the sensor to computer involves usage ofamplifiers 8. These amplifiers detect the voltage difference between thethree sensor layers 8. The detected values are given as inputs to themicrocontroller. The robustness of the sensor was measured by applying strainand pressure in x, y and z directions 8. The response was distinguishable inall the three cases; therefore, the sensor can detect strain, pressure alongwith the different types trigger source 8.Thefabrication process introduced in 9 can used to print intricate microchannels. This construction of the sensors involves two sections, first sectionbeing, fabrication of micro fluidic channels and the second being constructionof electrical components for sensing element 9.
The fabrication of thecomplete involves five steps. The steps are as follows:1. Printing of the mold using inkjet: Using inkjet printing for determination of the micro-channels is muchmore environment friendly and cost effective than usual soft photolithographyprocess 9.
The glass on which the moldis printed is treated with an acetone solution and UV– ozone so that the inkdrops can adhere to the glass better 9. The holes present for the movement ofthe fluids were reserved by placing a 1.5 mm holder 9.
2. Channel patterning: The channels were patterned using a silicone elastomer kit 9. The kitconsists of two mixing agents which were mixed in a ratio of 10:1and was filledinto a container until the liquid overflowed 9. The glass used in theprevious step is used to cover the container 9. The pattern is obtained onthe PDMS sheet after the container cools down 9. 3. Printing of isolation and metallization characteristics usinginkjet: Using silver ink, the metallization characteristics were printed onto apolyethylene terephthalate (PET) sheets 9. These sheets can be replaced bypaper, silicon, LCP (liquid crystal polymer) and glass 9.
To obtain an idealresistance of sheet, silver patterns were printed four times 9. The isolationlayer is made of the material SU-8 due to its properties of high chemicalresistance 9.4. Channel Sealing: Van Der Waals forces are considered in sealing the PDMS with a smoothsurface 9. The PDMS and PET are bonded together tightly in such a way thatthere is no presence of air bubbles 9.
This prevents conductive fluiddrainage due to high pressure 9. The ability of peeling of the microfluidiclayer of the sensor, helps in replacement if needed, and also helps in makingchanges to the sensitivity 9. The two basic properties of a wearable sensor aresensitivity and flexibility 9.
For measurement of the sensitivity factor, aconsiderable number of liquids were used with minimum ground interference 9.It was observed that the variations in attenuation and bandwidth are due to thedielectric losses due of the different liquids 9. Very small amount of liquidis required for the sensitivity measurement due to the small size of thechannels 9. To check the flexibility of the sensor, it was folded to the sizeof four cylinders with different measurements 9. It was concluded that theprototype is sensitive as well as flexible 9. CONCLUSIONAll the papershave demonstrated use of biologically similar materials to implement thesensors used in the porotypes.
These sensors can find application in areas ofhumanoid, haptics, smart skins, health care and machine- human interactions.The manufacturing process of these sensors may include 3D printing, Ink-jettingor soft lithography. The sensor prototypes have been designed using a veryelastic material like silicone rubber through which microfluidic channels run.These channels have conductive liquid flowing through them which can be EGaInor a mixture of ionic solution and EGaIn.
When these channels undergodeformation, there is change in electrical resistance of the conductive liquid.The change in resistance is measured and transferred to the interfacedprocessor. These readings are read by MATLAB and simulation is displayed. Theseprototypes can be designed to be adapt to different users.
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