• to obtain a proper field within the MRF a closed magnetic structure is nec- essary in order to reduce the magnetic reluctance of the device and to better address the magnetic flux;

5.1 Introduction

Some simulations of the HBB-I fig.5.1 were carried out in order to verify performance and to obtain some design criteria. Such an analysis raised following considerations:

• to obtain a proper field within the MRF a closed magnetic structure is nec- essary in order to reduce the magnetic reluctance of the device and to better address the magnetic flux;

• to achieve a suitable 2D spatial resolution, the number of ferromagnetic cores around the MRF box should be increased and properly operated in order to select the fluid zones to be excited;

• to increase the spatial resolution, the cores should be architecturally changed.

Then, taking into account the results of the preliminary simulations and the

previous considerations, new systems for the energization of MRFs were designed

Figure 5.1: Haptic Black Box I (HBB-I) prototype during experimental session.

and simulated by means of a Finite Elements code called MEGA.

Although these displays could be designed with the use of superconducting mate- rials or permanent magnets, for the purposes of the present analysis we considered, for simplicity, only standard materials.

The simulation results so obtained, have shown that the new proposed elec- tromagnetic devices have better performance than the previous one and they can create a proper field of about 0.5 T and a suitable spatial resolution in the plastic box containing the MRF.

However the models hereto described and simulated present a relevant short- coming because they work only in 2D. On the basis of these considerations, a new advanced device HBB-II towards 3D HBB free-hand exploration and capable of overcoming these limitations was designed and built. The novel device presents a cylindrical-shaped plastic box containing the MRF and a series of ferromagnetic cores, positioned in aureole shape around the plastic box and used to dynamically address the magnetic flux inside the fluid.

A hand wearing a latex glove can be inserted in the cylindrical plastic box

and the operator can interact with various shapes and compliance perceiving and

discriminating different virtual objects.

Figure 5.2: Architecture of the new HBB-II device without coils.

5.2 Detailed description of the HBB-II

The architecture of this new 3D display is reported in fig.5.2, where a schematic view of the device without coils of with its main dimensions is shown.

It is possible to decompose the whole system in four main parts.

• The plastic box is used to contain the MRF and it is cylindrically shaped

for a better symmetry of the system. Then, in order to allow to freely handle

the fluid, the box is internally equipped with a latex glove. However, since

the dimensions of the box have to respect a compromise between an easy

accessibility to the fluid, and a reduction of magnetic reluctance, it has a

circular base with a diameter of about 15 cm and an height of about 50 cm.

Figure 5.3: Mechanical design of the HBB2 with coils.

• The ferromagnetic structure is used to close and to address the magnetic flux. It is composed of 10 vertical columns bolted to an iron circular plate and a series of 72 ”pistons”, properly positioned in the system and free to move along a fixed trajectory with respect to the plastic box containing the MRF. Twenty two of such pistons are arranged in a circular matrix form below the box’s base; the remainder fifty, arranged in series of 10 × 5 are placed in aureole form around the lateral surface of the plastic box. They are constrained to slide in special holes present in the superior part of each column.

All the used cores are composed of ferromagnetic material AISI 1040 having

enough magnetic permeability and suitable magnetic saturation-threshold to

reduce the transversal sections. Then, taking into account such a saturation

threshold, the iron plate has a diameter of about 30 cm and a height of about

1.5 cm; each column has a base diameter of about 4.5 cm and a height of

Figure 5.4: FEM modeling and partial representation of HBB-II display.

about 35 cm; and each piston has a base diameter of about 2 cm and a height of about 15 cm.

• The coils system needs to produce the proper magnetic field for the energiza- tion of the MRF. In the system there are two types of coils: that positioned around the inferior part of the columns and used to create the main mag- netic field, so-called primary-coils, and that positioned around the 72 pistons and used for a fine control field resolution, so-called secondary-coils. Each primary-coil is built with about 5500 AT of enamelled copper wire, with a low thermal resistivity, arranged in 11 layers of 50 turns around a hollowed plastic cylindrical support of an inner diameter of 46 mm and total length of 110 mm. The electric resistance of each primary-coil is 0.58 Ω at 27

C.

The secondary-coils consist, instead, of about 2700 AT, arranged in 5 layers

of 54 turns around a hollowed plastic cylindrical support of an inner diame-

ter of 21 mm and total length of 150 mm. The electrical resistance of each

secondary-coil is 0.1 Ω at 27

C.

All the coils are connected to an external electronic power system to obtain the desired magnetic field in different regions of the fluid.

• The control system deals the current in each coil for a double purpose.

From an electrical point of view it adjusts the value of current for a direct modulation of the magnetic field; from a mechanical point of view, the current in each coil allows to move the piston, inserted in the plastic cylindrical sup- port as in a classical solenoid with a plunger inside it, along a fixed trajectory starting from the column and ending at the lateral surface of the plastic box containing the MRF. The piston return in its initial position by the effect of a constrained spring.

5.2.1 FEM analysis of the HBB-II

In this new configuration, in order to reduce the reluctance of the magnetic path, opportune ferromagnetic yokes and cores have been introduced at strategic posi- tions. To increase the spatial resolution, more solenoids, having smaller sizes has been added, suitably arranged into a three-dimensional architecture in order to create 3D virtual objects. A further improvement concerned the choice of special materials AISI 1040 equipped with rubber whose nonlinearity is more attenuated.

Fig.5.3 shows the mechanical design of HBB-II with all the coils.

Fig.5.4 shows the FE sub-modeling used for the numerical simulation of the HBB-II device.

Figs. 5.5, 5.6, 5.10 report some numerical simulation of different sections.

In particular fig.5.5 reports the flux density in a generic plane of the fluid when

2 pistons operate. In fig.5.9 is reported the flux density at the base of the fluid

when the central vertical piston operate and in fig.5.6 the flux density at the middle

plane of the fluid when 2 opposite pistons operate is illustrated. Finally, fig.5.10

shows the flux density at the middle plane of the fluid when all the pistons of a

plane operate.

Figure 5.5: Numerical simulation of HBB-II display: flux density in a generic plane of the fluid when 2 pistons operate.

5.2.2 Working procedure

The HBB-II working implements the procedures described in the section 4.5.2. Once the choice of the MRF region to be invested by the magnetic field has been done, for example to obtain a little hemisphere of MRF at the basis of the box, or along its lateral surface, otherwise a cylinder of MRF with the axis along the radial direction at different height in the plastic box, or whatever, the corresponding pistons must be activated impulsively feeding the relative secondary-coils. Then, tuning the current in the primary-coils belonging to the cores that compose the magnetic path, it is possible to change the rheology of the fluid and to obtain different compliance.

When all the pistons are at rest (far from the plastic box), the reluctance of the

magnetic path, closed along the line B − B

and shown in fig.5.7, is very high and

the value of flux in the fluid is neglectable; on the contrary, when two or more

pistons are “in action” (close to the plastic box), the gap along the magnetic path

A − A

is reduced implying an increase of magnetic flux in the volume of fluid

corresponding to the active pistons. In summary, the modulus of the magnetic

field in a specified portion of the MRF and its spatial resolution can be controlled

Figure 5.6: Numerical simulation of HBB-II display: flux density at the middle plane of the fluid when 2 opposite pistons operate.

acting both electrically, varying the value of the current flowing into some coils, and mechanically, moving the pistons. In such a way, it is possible to reconstruct many objects of different shapes in different zones within the box containing the fluid.

A particular of the mechanical arrangements (see fig.5.3), the secondary coil system to active the pistons, is shown in fig.5.8. Each piston is equipped with an auxiliary mechanical system fig.5.8, capable to move each piston along its axial di- rection when electrically unexcitated. Specifically the auxiliary system is composed of two carbon steel mass attached to a spring. The mass are allowed to move in the axial direction. The attractive magnetic force between the pistons and the MRF was simulated and its value is approximately 3.3 N for each active piston. The non- magnetic spring of aluminum was designed and realized according to this specify.

The 3D free-hand prototype HBB-II was presented at World Haptics Conference

2005, in Pisa, as 3D haptic free-hand demonstrator. The haptic environment that

control the HBB-II was implemented by developing a tool in Visual C++ in order

Figure 5.7: MRF energization principle of HBB-II: particular of pistons mechanism.

to decide and to give the control strategy. The software was designed taking into account typical characteristics of a VE, including real time and feedback control, by means a proper interface. The tool consists of a simple dialog based on Graphical User Interface (GUI). The core of the tool is a haptic thread which runs at 1 Khz and allows to fix the energization of the HBB-II device. By setting the run-time the parameters (electric current in each coil) and by using a virtual button and on the GUI, it is possible to start and to stop the experimental session. In this case a low-level control was implemented by using a real-time class in order to synchronize the movements with the a 3D Open GL simulation of the model and to realize the required haptic timing.

In fig.5.12 a view of the screenshot realized in Visual C++ code and Open GL

to control the operatively the device, and a picture of the complete final HBB-II

device is shown and is reported in fig.5.11.

Figure 5.8: Particular of secondary-coils system equipped with auxiliary system.

Figure 5.9: Numerical simulation of HBB-II display: flux density at the base of the

fluid when the central vertical piston operate.

Figure 5.10: Numerical simulation of HBB-II display: flux density at the middle plane of the fluid when all the pistons of the plane operate.

Figure 5.11: HBB-II: Visual C++ GUI screenshot.

Figure 5.12: HBB-II: picture of the final prototype.