Interaction with the hand

Chapter 6

Interaction with the hand

6.1 Introduction

The MRF-based devices, as described in previous sections, are equipped with a latex glove allowing an unconstrained interaction between operator’s hand, in the case of HBB, or fingers, in the case of PG display the the MRF. However, since the hand wearing a glove and the MRF have different magnetic characteristics, it was necessary to verify what happens during manipulation phases. For the sake of simplicity and taking into account that the formulation of the problem is similar for different devices, the analysis was performed using the PG model. The conside- rations and the solutions proposed can be can be easily extended and customized to support the HBB free-hand configurations.

6.1.1 Problem description and solutions

In order to investigate the interaction problem, a first analysis has been performed

simulating the insertion in the fluid of a little object with magnetic characteristics

different from those of the fluid. From a magnetic point of view, the MRF’s magnetic

Figure 6.1: Induction B along x-direction when an object with different magnetic permeability is inserted in the fluid.

permeability varies in a range between about 3 and 7 (µ

= 37), depending on the

operating point. The inserted object perturbs the magnetic path with an increase or

decrease of the magnetic flux density in the regions surrounding the object. fig.6.1

and fig.6.2 show the profiles of the magnetic flux density B in the central zone of the

fluid, along two orthogonal directions (parallel to the x and z axes), when the object

with µ

= 1 and µ

>> 37 (where µ

= 37 indicates the fluid) is positioned inside

the MRF. As it can be seen, for both cases, the magnetic field changes its value with

respect to the case of absence of the object. In order to investigate a more realistic

and relevant case, the system has been analyzed simulating the insertion in the fluid

of the operator’s fingers (µ

= 1) as shown in fig.6.3 and fig.6.4. Figs.6.5, 6.6, 6.7,

and fig.6.8, show the profiles of the magnetic field along two orthogonal directions

for both the simulated models. As it can be seen, the difference between the values

of field in the ideal case (absence of fingers inside the fluid) and in the analyzed case

(two fingers inside the fluid) is about 20 − 22%. Since the device should satisfy the

6.1 Introduction

Figure 6.2: Induction B along z-direction when an object with different magnetic permeability is inserted in the fluid.

Figure 6.3: Model of PG display with two fingers inserted inside the fluid.

Figure 6.4: Model of PG display with one finger inserted inside the fluid.

Figure 6.5: Induction B along x-direction when two fingers are inserted in the fluid.

6.1 Introduction

Figure 6.6: Induction B along z-direction when two fingers are inserted in the fluid.

requisite of a good field uniformity, also under different conditions, the perturbation due to the presence of objects in the fluid should be attenuated.

Among different solutions for the described problem, here two of these will be indicated. The first one is based on magnetic field sensors; such sensors, positioned in the fluid, are used in a feedback control system to modulate the current in the excitation coils and to compensate the field variations due to the introduction of some objects. Although such solution is quite general (it should be used either for µ

< µ

or µ

> µ

) its implementation requires an auxiliary control system that could be extremely complex due to the difficulties in positioning the magnetic sensors.

The second proposed solution, instead, is based on a proper choice of the material

that covers the inserted object. If such a material is able to make the reluctance

of magnetic paths around the object equal to that of the MRF, the insertion of

the object will not perturb the magnetic field. This solution is only applicable to

objects with µ

< µ

(that is the case of the operator’s fingers).

Figure 6.7: Induction B along x-direction when two fingers are inserted in the fluid.

Figure 6.8: Induction B along z-direction when two fingers are inserted in the fluid.

6.2 The proposed solution

Figure 6.9: 2D model of the system used to characterize the magnetic permeability of the auxiliary material.

6.2 The proposed solution

As written, the proposed solution is based on the fact that, with a proper covering, it is possible to modify the magnetic characteristics of the object to be introduce in the fluid in such a way that its magnetic reluctance is equal to that of the fluid.

The problem is now reduced to find an auxiliary material (with a proper magnetic permeability) that will be used to cover the object to be inserted in the fluid.

In particular, thinking to an operator, it is necessary to find a proper material to manufacture the glove. The value of the magnetic permeability µ

of such auxiliary material has been calculated using the bi-dimensional scheme of fig.6.9.

Let us consider a domain of fluid that completely surrounds the object with its cover

of unknown magnetic permeability µ

, it can be subdivided in a proper number

of flux tubes (48 in our case). Then, calculating the magnetic reluctance of the

subdomain, composed of the object with the auxiliary material, and setting it equal

to the reluctance of the subdomain composed of only MRF, it leads to a quadratic equation where the unknown is the magnetic permeability µ

and the constants k

take into account the geometry and the physical characteristics of the system.

k

µ

+ k

µ

+ k

= 0 (6.1)

The solutions of such equation are: µ

= 10.7 and µ

=-0.23, with obvious choice for the first one.

6.2.1 The simulation results

As a first step, it is useful to verify numerically the result obtained in the previous section. A set of simulations of the system shows in fig.6.4, with different values of for the material that covers the fingers, has been carried out. The profile of field, plotted in fig.6.10 and fig.6.11, shows the accuracy of the obtained result.

Then, the obtained result has been applied to the Pinch Grasp device considering the insertion in the fluid of the operator’s fingers covered by a glove of a material with magnetic permeability. In fig.6.12, fig.6.13 and fig.6.14 are shown the profile of magnetic induction B, in the central zone of the MRF, respectively along the x, y and z direction, when two fingers are inside the fluid. As it can be seen, the value of B is not perturbed with respect to the ideal case (absence of fingers in the fluid).

6.2.2 Discussion

The analysis of the interaction between the operator and the MRF-based device,

showed the presence of some perturbations in terms of different values of magnetic

field inside the fluid. This problem has been investigated and a solution for the

attenuation of such perturbations has been proposed. The solution is based on the

use of an auxiliary material, with proper magnetic characteristics, that surround-

ing the object inserted in the fluid (for example the operator’s fingers), does not

6.2 The proposed solution

Figure 6.10: Induction B along x-direction when a finger covered with a material of different magnetic permeability is inserted in the fluid (see model in fig.6.4).

Figure 6.11: Induction B along z-direction when a finger covered with a material

of different magnetic permeability is inserted in the fluid (see model in fig.6.4).

Figure 6.12: Induction B along x-direction when two fingers covered with a material of magnetic permeability=10 are inserted in the fluid (see model in fig.6.3).

Figure 6.13: Induction B along y-direction when two fingers covered with a material

of magnetic permeability=10 are inserted in the fluid (see model in fig.6.3).

6.2 The proposed solution

Figure 6.14: Induction B along z-direction when two fingers covered with a material of magnetic permeability=10 are inserted in the fluid (see model in fig.6.3).

Figure 6.15: Magnetic field maps inside the fluid when two fingers covered with a

material of magnetic permeability=10 are inserted in the fluid.

alter the magnetic equilibrium of the system. The magnetic permeability of such a material has been calculated analytically using a bi-dimensional model and tested numerically by means of a set of simulations performed on the PG device. Results have shown a good attenuation of the perturbations introduced in the system ma- nipulating the fluid. Although from a magnetic point of view, the solution seems to show good results, the proposed solution should be deeply analyzed with reference to the tactile capability of the operator that should be able to perceive any changes of shape and consistence of the fluid when it is excited by means a magnetic field.

6.3 Risk analysis of MRF in VE

6.3.1 Risks in the technological developments

As a result of the FEM simulations, it seems useful to report some general conside- rations on the main problems that will be treated in the design of a new electroma- gnetic device able to satisfy the specified performance. Such observations will be useful to better know some solutions that will be used during the characterization of the design criteria of the device. The first observation deals with the dimensions of the MRF volume to be energized.

The relative permeability of such fluids (µ

between 3 and 7) is comparable to that of the air ( µ

= 1) and, consequently, any direction of closing of magnetic flux has a huge ”airgap” and magnetic reluctance. However, this ”airgap” is needed in order to allow an easy accessibility to the fluid to perform the psychophysical tests.

Generally, the presence of a huge airgap involves thermal problems linked to the

heat dissipation due to the excitation coils and introduces an upper limit to the mag-

netic field obtained by means of ”resistive electromagnets” [38] (so called to distinct

them from the ”superconducting electromagnets” in which the electrical resistivity

is practically zero). In such a case, the reduction of all the pathways of closing of

magnetic flux and the increase of the heat dissipation surfaces (or the introduction

6.3 Risk analysis of MRF in VE

of ventilation systems) allow to obtain a compact and light electromagnetic system.

Another problem to be taken into account in the design is the saturation of the used ferromagnetic materials whose non-linearity compromises the performance of the whole system. The choice of special materials could attenuate this problem.

A possible solution to overcome the main problems arising from traditional elec- tromagnetic systems is to use a superconducting system; however this solution is complex and expensive. However, not going into details, the main characteristics of superconducting materials are due to the possibility to reduce their electrical resistivity to values practically zero, if submitted to very low temperatures (for ex- ample, the aluminum becomes superconducting material at −271.9

C, that is 1.02 K). The advantages in the use of these materials are due to the weight reduction of the whole system caused by the low losses of the coils. Furthermore, it is possible to increase the number of amperturns due to the increase of the permissible current density (the current density limit for traditional conductors is about 2 − 3 A/mm

, for superconducting materials such limit could reach 2000 − 3000 A/mm

) [13].

Even if the required power to maintain the coils at constant temperature under the superconductive threshold is not so high (for the liquid helium, to obtain 1 Watt of cooling is necessary to spend about 800 W; the nitrogen needs about 10 W only), the cooling system is complex. The recent improvements in new superconducting materials with high critical temperature (50 − 60 K) could reduce complexity and costs of such systems.

A brief discussion is due to the use of permanent magnetic materials. Recent

improvements allow obtaining permanent magnets with a value of remnant field

Brem of about 1.45 T and coercive field Hci of about 1.2 MA/m [80]. Their use

to build other kind of systems, for example Magnetic Resonance Imaging systems,

allows a real reduction of weight and complexity of such devices [31]. However,

the application of permanent magnets to excite MRF involves some difficulties for

the field magnitude modulation in the range 0 − 0.5 T. The only way to change

the value of field in a point when permanent magnets are present in a system is a mechanical modulation; that is, the increase of decrease of the field in a specified area is possible by acting on the physical distance between the permanent magnet and that area. However, this way of control involves the presence of fast and precise electromechanical devices with a sure increase of the complexity of the whole system.

In spite of this disadvantage, if during the design of the new device their contribution had to favor the reduction of the dimensions of the system, they will be taken into account as field sources. Finally, a last consideration is due to a further development of the device to obtain a three-dimensional spatial resolution of the flux density B in MRF. Since the divergence of B is zero, it is impossible to obtain a desired value of field in a specified point of the space and contemporarily to have zero magnetic fields in a domain, as ample as you like, containing that point. Reasoning ab absurdo, if it is possible such configuration of field, it would have a domain with a ”well”

or a source of field, so violating the field solenoidality. However, although is not possible to excite isolated volumes inside the fluid without interesting the adjacent zones of fluid, it is possible to vary the field along the three spatial directions in order to obtain, in a specified area, a value of field bigger than in other zones as schematically shown in Fig.6.16. If three tubes of magnetic flux are located in the MRF with axes parallel to the 3 coordinated directions x, y and z and characterized by a value of field bx = by = bz = 0.5 T, in the central volume (intersection of the three tubes) it will have a flux of magnitude 0.866 T, that is times the value of each tube of flux. A real three-dimensional field resolution in a specified volume of MRF, could be obtained combining the values of field along the x,y and z axis. In this case, an advantage in order to obtain a better 3D resolution could come from the existence of MRF in which the transition between liquid and solid state happens in a more marked way with a field variation of √

3, as shown in fig.6.17. However,

the two-dimensional resolution, linked to the x-y axes of the MRF box (with x and

y placed on the base of the box and z, orthogonal in direction of the box height),

6.3 Risk analysis of MRF in VE

Figure 6.16: Three-dimensional excitation with different values of field.

Figure 6.17: Rheological properties vs applied field.

allows to excite volumes of MRF in parallelepiped shape in which it is possible to modify the dimension along x and y axes, maintaining a constant height along z direction (to modify the height of the excited fluid is necessary to vary the quantity of MRF in the box). The flux lines closed along z direction allow satisfying the field solenoidality.

6.3.2 Safety analysis

Safety aspects of a haptic prototypes based on new materials, such as controllable fluids and specially magnetorheological fluids, should be considered. In this scenario safety criteria, according to risk of potential users are very important in order to provide some mechanical, electromagnetic, thermal, chemical considerations in design and/or realization phases. In particular, because the systems described are used as haptic displays, they are physically in contact, through a fine medium such as latex sleeve, with the users. In our case we considered the analysis of haptic devices based on MRFs. Initially, some considerations should be provided on chemical reactions and toxicity of MRFs. Referring to datasheet of the selected MRF 132LD [43], it isn’t dangerous, but clearly can be used with some caution.

The main point regards the relevant magnitude of magnetic field (necessary for the MRFs to be properly excited) in terms of potential dangerous biological effects.

The nature of the interaction of an electromagnetic source with biological material

and human tissues and organs depends on magnitude but also on the frequency

of the source. In fact, as the frequency increases, the electromagnetic phenomena

show microscopic features (by developing photonic energy). Many of the biological

effects depend on photon energy. When a wavelength of electromagnetic source

is smaller or larger than the human body, respectively dangerous currents occurs

or not occurs. So at low frequency (from DC to 50 − 60 Hz) the photonic effect

is negligible. There is not reason to suspect that static and dynamic fields (of

course at low frequency) could cause or produce human health problems and it’s

6.3 Risk analysis of MRF in VE

justified by public epidemiological studies [62]. On the other hand specific bounds of static and/or dynamic magnetic field exposures to the general public are fixed in magnetic resonance imaging (MRI), used for medical examination, where the fields range is from 150 mT to 2 T (also until 4 T). Briefly, referring to these cases, to preserve patients’ safety, some specifications are required [32]. The following data are indicative because currently still in research phase.

• STATIC MAGNETIC FIELD and static Magneto-Mechanical Effects: the maximum magnetic induction field torelated is 2 T. This value did show spe- cific problems related to human interaction.

• DYNAMIC MAGNETIC FIELD at low frequency: 20 T/s is the threshold, fixed for dynamic interactions with humans (concerning the electrodynamic and magnetohydrodinamic effects).

Finally, in haptic prototypes we can use the same safety constraints to reduce

the interaction risks. Accordingly the maximum magnetic field generated, 0.6 T,

compatible with saturation level of MRF, is smaller than 2 T. On the other hand in

dynamic simulations (for example to modulate compliance and to generate dynamic

shapes) low frequency signals (until 60 Hz) and compatible levels of magnetic field

magnitude have been used. Anyway, we included a magnetic cage such as magnetic

ringin the operating prototypes and a magnetic sheet in some simulated designs, not

only for the increasing of spatial resolution inside the fluid, but also to limit their

exposure to as little as possible, specially outside of the box. In the simulation phase

the value of magnetic field in the hand model has been estimated about 8.5 times

smaller MRFs magnetic field and, of course, compatible with safe specifications.