A new magneto rheological skin for controlling pressure of haptic devices

ABSTRACT In this paper, a new skin tissue which can emulate the stiffness of several organs of human being is proposed and analyzed utilizing a magneto-rheological MR fluid.. Keywords:

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PROCEEDINGS OF SPIE

SPIEDigitalLibrary.org/conference-proceedings-of-spie

A new magneto-rheological skin for

controlling pressure of haptic devices

Xuan Phu Do, Tran Huy Thang Le, Byung Hyuk Kang,

Seung-Bok Choi

Xuan Phu Do, Tran Huy Thang Le, Byung Hyuk Kang, Seung-Bok Choi, "A new magneto-rheological skin for controlling pressure of haptic devices,"

Proc SPIE 10970, Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2019, 109702N (27 March 2019); doi:

10.1117/12.2512735 Event: SPIE Smart Structures + Nondestructive Evaluation, 2019, Denver, Colorado, United States

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A New Magneto-Rheological Skin for Controlling Pressure of Haptic

Devices

Do Xuan Phu1,2, Le Tran Huy Thang1, Byung-Hyuk Kang2, Seung-Bok Choi2*

1MediRobotics Lab, Department of Mechatronics and Sensor Systems Technology,

Vietnamese-German University, Vietnam

2Smart Structures and Systems Laboratory, Department of Mechanical Engineering, Inha University,

Incheon 402-751, Korea

*Corresponding Author: Seung-Bok Choi, Smart Structures and Systems Laboratory, Department of Mechanical Engineering, Inha University, Incheon 402-751, Korea

ABSTRACT

In this paper, a new skin tissue which can emulate the stiffness of several organs of human being is proposed and analyzed utilizing a magneto-rheological (MR) fluid It is called MR skin The proposed skin can be applied to the robot-assisted surgery manipulated by the haptic devices as a controllable tactile sensor In order to formulate the device, the valve networks are embedded inside the structure of the master actuator These valves use the flow mode and shear mode

of MR fluid for the pressure control The deformation equation of the MR skin is derived and the external force contacting to the MR skin is also analyzed After formulating, the proposed tactile display is optimized by using the finite element method In the optimization process, many different forces are applied to view different deformation of

MR skin with different pressures It is shown via the optimization that the results satisfy the initial requirements of the design This result directly indicates that the proposed MR skin structure is feasible in the manufacturing sense and applicable to haptic devices for robotic surgery

Keywords: Magnetorheological (MR) fluid, MR skin, MR actuator, optimization, pressure control, tactile device,

deformation change

1 INTRODUCTION

Recently, many pieces of research have been carried out on mimicking human skin via electronic as shown in Figure 1 Nonetheless, according to Wang et al [1] simulating the human skin using the tactile display with the ability to distinguish among a wide range of force values and have a rapid response is still challenging A tactile display is a device which can mimic the surface of an object including the texture, shape, roughness, and temperature By simulating the surface of the human’s organs, the device transmits small-scale information of the deformation of the skin to the operator’s hand [2] This application could be applied in minimally invasive surgery in order to provide a realistic environment for tele-tactile and high surgical precision Such devices could be developed using different approaches, including electromagnetic technologies, shape memory alloys, electrorheological fluid, magnetorheological (MR) fluid and pneumatic systems [3] Among these compliant tactile display, MR fluid possesses outstanding properties since it has low cost, low power consumption, and a simple configuration without any moving components by Yanju Liu et al

[4] MR fluid is a smart fluid which contains ferromagnetic particles - mostly micrometer scale – and oil carrier [5] In ambient condition, MR fluid behaves like Newtonian fluid When the free-flowing liquid is placed under a magnetic field, MR fluid immediately changes its density and transforms into the semi-solid state with a different mode of operations including valve mode, squeeze mode and shear mode These properties allow the user to control the behavior

of MR fluid by manipulating the magnetic field [6] Therefore, MR fluid is applied in a lot of different research fields and contributed to creating cutting-edge technology devices such as MR brake, MR damper and MR valve [7-8]

In this paper, MR skin is proposed as a new tactile display device with the ability to mimic different human organ’s surfaces MR skin consists of a valve system which deploys the behavior of MR fluid The optimal design of the valve is

Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2019, edited by Jerome P Lynch, Haiying Huang, Hoon Sohn, Kon-Well Wang,Proc of SPIE Vol 10970, 109702N

Proc of SPIE Vol 10970 109702N-1 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 31 Mar 2019

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undertaken to provide the user’s hand with realistic sensation MR skin has promising potential to apply in robotic master–slave surgical system such as the da VinciTM Surgical System [9] Due to the development of the surgical robotic system, the need for a tactile display device for better tactile sensations and spatially distributed information becomes more significant The structures of the next sections are organized as follows: Section 2 gives an overview of the MR skin configuration and the valve networks design Section 3 analyzes the desired parameters in order to emulate the real organs and also the displacement calculation Section 4 provides the optimization parameter and the result configuration, followed by conclusion

Figure 1 Recent applications of electronic skin

2 MR SKIN CONFIGURATION

2.1 MR Skin Design

To provide the user with the best experience, the width and length of the device are chosen to fit the dimension of a human hand When the user touches the device, the vibration stimulus would be recorded and classified base on different characteristics such as strength and location This design has a total of 25 cells and can be divided into 2 types As in

Figure 2, type 1 is cells contain only contain MR Fluid and type 2 is valve cells These cells are arranged alternately, each MR fluid cell is surrounded by 2 or 4 valve cell

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(a) (b)

Figure 2 Proposed design of MR skin: a) MR skin configuration, b) MR skin top view

The MR fluid cell is able to exert forces normal to the MR skin surface against the applied forces The strength of the palpation force could be controlled by the intensity of the magnetic flux This feature is manipulated by the electromagnet under the MR skin surface The coils setup can be adjusted to create the clock-wise or counter clock-wise magnetic field lines At the initial state, the MR skin has a flat surface, as in Figure 2a The fluid inside the device has a pressure equal to the ambient pressure:

Below the MR skin’s surface is a shock absorbing system This system connects with the frame underneath to stabilize the structure There is a displacement sensor installed under each valve cell This sensor will provide accurate information about the position and how long the force last

2.2 Valve design

As discussed above, the MR skin consists of valve cells and MR fluid cell The valve system contains 12 identical valves The valve's cross section has an isosceles trapezoidal shape Due to the simple structure, its execution requires fewer accessories and no moving parts Besides, this configuration allows the valve to work more efficiently when the magnetic field is applied

Figure 3 The valve system design of the MR skin

3 MR SKIN DEFORMATION ANALYSIS

3.1 Desired palpation force

To imitate the human organ, numerous factors need to be considered Firstly, how the strength and frequency of the applied force affect the deformation are examined Secondly, the environmental elements such as the temperature or texture of the organs are studied However, at the initial stage of creating the MR skin, only the palpation force and the displacement of the MR skin will be analyzed

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Table 1 Palpation force at 5 body parts [2]

Organ Palpation Force (N)

Hand 0.6 – 0.9 Neck 0.6 – 1.2 Abdomen 0.4 – 1.3 Thigh 0.7 – 1.2 Back 1.0 – 1.5 Palpation force is the reaction force between the MR skin and the human organ A dynamic force sensor is used to measure this value An experiment was carried out on five different external human organs The result is recorded in Table 1 by Han et al [2] The palpation force oscillates approximately between 0.4 to 1.5N Thus, MR skin needs to be able to exert a force normal to the tactile display surface with the value ranging from 0 to 2N

3.2 Desired displacement value

To provide the true sensation for the user when touching the MR skin, the displacement of the MR skin's surface also needs to be considered In 2015, an experiment was carried out by Ogawa et al on the stomach to evaluate the stiffness of the organ palpation device [10] The result is shown in Figure 4 From the diagram, the displacement value approximately 0.004m will generate a reaction force value around 1.2N As discussed above, our device needs to produce a palpation value equal to 1.5N Hence, to satisfy our design purposed, a displacement value equal to 0.005m is chosen

Figure 4 Organ displacement when applied force: (a) Position response, (b) Force response [10]

3.3 Working principle

At the initial state, a high current intensity is used to activate the magnetic field The relation between the magnetic field and the current intensity is:

Where is vacuum permeability which is equal to 4 × 10 and is the number of coil turns The fluid inside and near the valve cells region would be in flow mode as in Figure 5 As the same time, the liquid in the MR fluid cell not affected by the magnetic field would behave like a Newtonian fluid In this case, gradually decrease the magnetic field strength would result in changing the speed of the displacement and the reaction force of the MR skin when the user touches the surface The volume flow of the liquid through the valve during the transition of MR fluid from liquid to solid is calculated as [11]:

= ∆ (20 − ∆ + 15ℎ ∆ + 2ℎ ∆ (1)

Where is the critical shear yield stress of MR fluid and n is the apparent viscosity

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For the simplicity of calculation, suppose that the force applied by the user's hand spread equally over the MR fluid cell Besides, only the deformation of one cell's surface will be analyzed The displacement of the skin's surface can be derived as follow:

Where, is the displacement, is the pressure, is the moment of inertia and E is elastic modulus of the MR skin’s

surface To mimic the human tissue, the elastic modulus could be calculated using equation 3 [12]

Where, is the Poisson’s ratio, is the thermal expansion coefficient, is the intensity of the continuously distributed load and is the component of displacement

P

Figure 5 The flow mode of MR fluid

4 OPTIMIZATION AND RESULT

In this design, a valve network is embedded inside the tactile display to provide the best experience for the user The deformation of the MR skin and the elastic modulus of the MR skin surface's material imitate these characteristics of the human organ Commercial software - ANSYS 12 is used to solve the magnetic loop and program for the optimization The process of optimizing the MR skin design concentrates around four parameters related to the dimension of the MR skin surface and MR fluid cell These parameters have a significant role in determining the flow of MR fluid through the valve cell and the displacement of the MR skin when the user touches its surface Table 2 below shows the initial values and the optimized value of these parameters using finite element method: With these parameters, the displacement and the palpation force value equal to 5mm and 2N respectively could be achieved The velocity of the deformation could be controlled by the current intensity Besides, the coil’s arrangement also affects the MR fluid behavior Figure 6 shows the magnetic field inside the network with different coil's set up From the simulation, the first configuration clearly shows better result

Table 1 Palpation force at 5 body parts

Parameter Initial Value (mm) Optimized Value (mm)

Surface Thickness 0.5 0.12 Cell Depth 30 40 Cell Length 30 24 Cell Height 30 25

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[10] Kenji Ogawa, Kouhei Ohnishi, “Evaluation of stiffness calculation for organ palpation device with electro-hydraulic transmission system based remote actuator”,Proc INDIN 13, 378-383 (2015)

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