3.3 The Pinch Grasp (PG) display

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Chapter 3

1D and 2D Haptic Interfaces based on MRFs

3.1 Introduction

In this chapter the process and the rules for designing and building a haptic device capable of properly energizing the MRFs are discussed. A heuristic approach to design and implement the MRF-based devices is reported and then analyzed. The starting point was to investigate on the possibility of using the smart fluids to mimic the compliance, damping and creep (in other terms, the rheology) of some materials in order to realize haptic displays. We envisioned two possible schemes for MRF- based displays, i.e. a Pinch Grasp (PG) device and an ”immersive” configuration.

In the first one, the MRF specimen is positioned in the air-gap of an electromagnet allowing pinch grasp manipulation. In the second scheme, a Haptic Black Box (HBB), free-hand display, a given volume of MRF was placed into a plastic box in such a way that a hand can be introduced to freely interact with the fluid. The magnetic field applied in the fluid can be controlled varying intensity over time and

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space, by means of suitable electromagnetic devices.

Both configurations have been designed to focus a magnetic flux into a specified region of the MRF, maximizing the magnetic field energy in this region and minimizing the energy lost in the other regions. An accurate magnetic field profiling permits to build figures with a given shape and compliance. A suitable control strategy was studied so as to make MRF capable of mimicking a wide range of rheological behaviours, within limits dictated by saturation effects in the fluid.

These approaches are motivated primarly by the fact that the viscoelasticity of biological tissues results to be compatible with the rheological behaviour of most commercial MRFs. The usage of MRF as haptic displays could be a viable solution in important applications such as surgical training in Minimally Invasive Surgery (MIS) and Open Surgery (OS). MIS is a technique that has become increasingly widespread in recent years with the aim of reducing the traumatic effect of some surgical operations: a number of general surgical operations, like gall-bladder and appendix removal, have been recently performed by using this technique [26, 87].

The reasons for such a fast growth in using this technique depend on many factors such as the risks reduction, disfigurement, patient pain, shorter immobilization (about 24 hours), shorter hospitalization (about 2-24 hours) and an earlier return to work (as early as 2 days). These advantages may be translated into a total health care cost reduction for commercial and governmental institutions as well as for the patient (see fig.3.1).

Nevertheless, MIS is still afflicted by substantial limitations. One of the most pressing limitations is the surgeon’s loss of tactile sensibility due to the transmission mechanism of the elongated tools used during the operation. Indeed, the surgeon may manipulate the patient’s organs and tissues by using only long tools, observing actions and movements on a monitor displaying the abdominal environment. He can neither touch nor see the organs directly and that restricts the application of this technique only to some specific fields. Diminished tactile sensibility causes a

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3.1 Introduction

Figure 3.1: A possible field of application of the MRF-based haptic interface in MIS or OS where the MRF reproduces the VE in tactile domain.

loss of surgeon palpation capability, particularly with regard to the compliance and viscosity of tissues. The PG display could be a helpful means for training, enhanc- ing the surgeon’s skills. In particular, by using sensorized surgical instruments [7], suitable signals could be acquired and utilized for feedback control of the haptic device. Similarly to MIS, a HBB display could be applied to OS simulators, whereby the operator would interact with virtual replicas of organs or complex surgical envi- ronments [9] (see fig.3.1). Although challenged by new developments in endoscopic technologies, traditional operative procedures often remain the only solution in most cases of surgical operations. Therefore, surgical training in open surgery is very important: for example, an entire abdominal cavity could be virtually and partially reproduced by using MRFs to mimic the inner organs.

The present work proposes a viable solution for surgical training, providing a haptic display capable of mimicking the softness of some viscoelastic materials, and,

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biological tissues manipulated by surgical tools.

3.2 Design Criteria

In order to guarantee high flexibility and fidelity of the control, a ferromagnetic source was chosen from among many solutions. The structures are composed of a series of metallic cores contoured in a way to form the interface between the device and MRF. Each device was designed satisfying some constraints:

• low reluctance flux conduit to focus magnetic flux into region of active mag- netic fluid, starting from liquid (without field) until the range of magnetic field saturation (0.5 − 0.6 T for MRF 132LD used in our applications);

• magnetic field inside the MRF specimen as uniform as possible, maximizing the magnetic energy in the fluid gap while minimizing the energy lost in the ferromagnetic conduit and other regions according to safe criteria);

• accessibility of the MRF specimen and reduction of rigid constraints to facili- tate the manipulation and experimental tests (psychophysical tests and stress relaxation analysis).

Two steps in the design and realization process were used:

1 At the beginning, by using Kirchoffs’s Law for magnetic circuits and the con- tinuity of magnetic normal flux at the interface, we determined approximately the flux density and the number of Ampere-Turns (AT) in order to obtain the desired operating magnetic field in the MRF specimen.

2 Next, the optimization of the electromagnetic design and prediction of rheological behaviour were considered by using a numerical analysis capable of take into account the nonlinearity of materials. We used two dedicated software based on Finite Element Method (FEM): the FEMM [40], we only used for a preliminary 2D study and MEGA for detailed 3D simulations.

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3.3 The Pinch Grasp (PG) display

Figure 3.2: Magnetization curve: setup (left) and material under testing (right).

The ferromagnetic cores require a high magnetic permeability and saturation level compatible with the features of the MRF. Typical materials convenient in MRF applications are steel, and in particular carbon-steel, having low level of carbon.

In particular in our applications we used AISI 1040 or AISI 1015 compatible with the aforementioned constraints. An investigative phase was carried out in order to individuate the materials with the suitable range of permeability (see fig.3.2) and with the specified percentage of carbon steel. A series of magnetic and ferromagnetic materials was tested and characterized in terms of normal magnetization curve (or B-H curve) (fig.3.3). Also the MRF was characterized in term of first magnetization curve.

3.3 The Pinch Grasp (PG) display

The initial idea was to develop a haptic display capable to allows users to simply manipulate by using their fingers (two fingers, thumb and forefinger) a reduced MRF volume.

In order to apply the desired magnetic field to the MRF specimen, a preliminary configuration for a non-immersive scheme (fig.3.5) was designed and realized. The

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Figure 3.3: Magnetization curve: results for AISI 1015 (left) and MRF 132 LD (right).

Figure 3.4: Design of the Pinch Grasp (PG) display.

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3.4 The Haptic Black Box (HBB) display

Figure 3.5: Pinch Grasp (PG) display: magnetic core without (left) and with (right) coils.

device cab be properly excited in order to mimic the compliance of the virtual objects [70].

We used in this case the carbon steel core AISI 1040. It composed of 40% carbon, and the structure was designed in order to obtain a low reluctance steel flux conduit to address the magnetic flux into MRF specimen. Taking into account Hopkinson’s law, the overall coils consists of 2400 turns of copper wire having 0.8 mm in diameter.

In such a way, by using a simple linear approximation, the maximum electric current is 1.26 A sufficient to produce the magnetic field desired.

3.4 The Haptic Black Box (HBB) display

In this section we report another MRF-prototype of a haptic display for whole- hand immersive exploration. The ideal haptic display would allow the operator to interact with the virtual object by freely moving his or her hand without mechanical constraints, exciting sensory receptors on the operator’s whole hand, rather than on just one or few fingertips or phalanges. A conceptually new typology of haptic display, the Haptic Black Box (HBB) display, is here proposed [71, 72]. A HBB can be imagined as a box in which the operator can poke his/her bare hand, and

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Figure 3.6: Haptic Black Box (HBB) concept.

where virtual objects materialize and move under the computer control according to interactions with the operator and the VE (fig.3.6). As above introduced, an application that would greatly benefit from the availability of a HBB display is clearly the training of operators to open surgery operations.

An implementation of an HBB would consist of a controlled volume in which the material properties at each point can be tuned independently by some non-intrusive means.

Clearly, a concept of a 3D-HBB is rather complicated at this illustrated stage, but, to progress towards such a challenging goal, MRFs represent an interesting and innovative technology enabling, at least, some simplified form of the 3D HBB.

3.4.1 Mechanical design and equipments

We have investigated several configurations of HBB and we propose here two possible ways to realize the HBB prototype. The first design envisioned was conceived as four walls of four solenoids placed side by side to form a box (see fig.3.7). In this configuration, with a horizontal solenoids’ structure, the workspace is a Cartesian plane with 16 ”quantized” points. Each point is identified by the intersection of

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Figure 3.7: Basic idea of the immersive scheme HBB arranged by using horizontal solenoids architecture (left) and mechanical design (right).

the extensions of the axes of four facing solenoid. All the solenoids are dimensioned according to mechanical, thermal and electrical criteria. Mechanical aspects con- cerned the size and the shape of the steel core; thermal considerations have been considered in order to avoid overheating inconveniences; and, finally, electrical eval- uations aimed at estimating the resistance and inductance of the coil such that the current density flowing into the coils was under limitations foreseen by regulations.

By tuning the current into each coil, figures with a given shape and compliance can be realized. This configuration presents an inconvenience due to the fact that the region closer to the walls is exposed to a greater magnetic field. This is due to the fact that the magnetic field generated by a solenoid is concentrated in a nearly uniform field inside the solenoid, while the field outside is weaker and decreases with the distance from the axis. In this way, the figures we can reproduce tend to be stiffer at the edges. A numerical 2D FEM simulation of the distribution of the magnetic field, when four facing solenoids are activated, is reported in fig.3.8(left). In fig.3.8(right) the realized system during a manipulation task is shown. It is worth noting that figures like cross or shape running from one edge to the another are well defined and easily reproduced. To overcome this limitation we adopted such as HBB-I a second configuration where the same solenoids are vertically placed below the box fig.3.9. In this case every quantized point is identified by the ideal prolon-

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Figure 3.8: HBB-I: simulation (left) and prototype (right) arranged by using horizontal solenoids during a tactile task.

Figure 3.9: HBB-I: design (left) and prototype (right) arranged by using vertical solenoids during a tactile task.

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gation of the solenoid’s axis in the box and the magnetic field may be distributed throughout the box surface.

3.4.2 Energization of the HBB-I

Due to the presence of an airgap in the system, it is possible to perform a first calculation, with indicative purposes of the Amper-turns (AT) capable to produce a flux density B ∈ [0.45, 0.5] T in a specified region of the MRF. Such a calculation, independent from the shape of the device, is based on the magnetic circuit theory and it presupposes to consider negligible the magnetic reluctance of the steel cores present in the system. Taking into account Hopkinson’s law, it’s possible to evaluate the number of AT:

N I = <Φ (3.1)

where NI represent the number of AT, and, in particular, N is the number of turns coil, and I is the electric current flowing in the coils. < represents the magnetic reluctance of the whole circuit and Φ the magnetic flux obtained by the product of the induction B and the surface S (Φ = B · S).

Now, from an analytical point of view, by considering a box of 5 cm height (filled with MRF having µr = 5) and a volume of air above the box of 10 cm (to allow the fluid accessibility), the value of the reluctance of the circuit can be calculated as follows:

< = <f + <a = lf

µ0µrS + la

µ0S ≈ 8.7510⁴

S (3.2)

substituting this value in (3.1) and taking into account the relationship between Φ, B, and S, the number of AT becomes:

N I = 8.7510⁴

S 0.5S = 44000 AT (3.3)

Such value, although only indicative, was used to define the new prototypal devices.

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Figure 3.10: Coil and its design for HBB-I.

3.4.3 Hardware and Software arrangements

The preliminary prototype of HBB device, called HBB-I, is shown in fig.3.9. It is composed of 16 excitation coils vertically placed below the plastic box with a base of 20 cm × 20 cm, which contains the MRF, arranged in a matrix form of 4 × 4 below a plastic box. z Each coil is built with 305 turns of enameled copper wire, marked Autovex180 Pirelli, with a low thermal resistivity, arranged in 5 layer of 61 turns around a cylindrical carbon steel AISI 1015 core of a diameter of 21 mm.

The electrical resistance of a coil is 0.25 Ω at 27^o C, and the inductance is 5.25 mH at a frequency of 50 Hz. The main dimensions and the characteristics of the coils in the mechanical design are shown in fig.3.10. The AISI 1015 ferromagnetic core is commonly used for screws and bolts production. Indeed, it shows a good magnetic relative permeability around thousand, a higher saturation level threshold (about 2.3 T) than AISI 1040 and low hysteresis. These properties arise from low carbon level (typical value is about 0.15%) and negligible nickel level. Turns are not into direct contact with the core, but they are separated by a rubber support covered with a layer of Nomex which is a special insulating material for voltage transformers. This precaution assures a good thermal and electrical isolation. In order to minimize the dispersed flow beyond the workspace and to guarantee the safety of the user, the solenoids are connected together by means of an AISI 1015

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steel ring.

By separately tuning the current flowing into the 16 coils, it is possible to reproduce figures with desired shape and compliance.

A rough control system was obtained by manually acting on 32 switches: every couple of switch allows to establish the current direction in the coils and to turn on and off each solenoid.

A real time control strategy is obtained by using two acquisition cards interfaced to a computer. One is used as input card to acquire signals from Hall sensors (witch allow to control the magnetic field) and the other as output with 16 analogical channels. The output signals are used to drive a power circuit able to generate high electric current for the solenoids. Basically, the electronic driver circuit is a voltage-current converter, where a high gain operational amplifier supplies a final push-pull section implemented by using C-Mos (IRF-540 and complementary IRF- 9540) components. As Hall Sensor we choose UGN3503U, produced by Allegro, for reliability and cheapness. Moreover, it provides a voltage signal proportional to the magnetic flux enclosed by the sensors plan.

The system has been controlled by using a specific model elaborate in XPC- target Matlab tool in order to realize a LAN-remote control of the device. Fig.3.11 shows the HBB-I device, with hardware arrangements. It was presented at Eurohap- tics 2004 International Conference in Munich, as a haptic free-hand demonstrator, gaining wide consensus.

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Figure 3.11: The prototypal demonstrator HBB-I display.