Electromagnetic Thimble for Blind : Design and Investigation

Special electromagnetic thimble is developed and investigated, allowing the blind to perceive specific 2D graphic information. Electromagnetic thimble is based on the idea, that controllable friction force between thimble and ferromagnetic layer of the screen is related to position of contours or lines and intensity or color of background. Electromagnet with axial symmetry is optimal in respect to the magnetic flux leakage. The magnetic circuit of electromagnet was explored and the model of thimble was designed and investigated. By varying the excitation current in interval [4 mA–14 mA], the attraction force has been varied in interval [0,7 N–6 N]. Initial experiments, supporting the feasibility of the method, were performed by employing blind operator. DOI: http://dx.doi.org/10.5755/j01.eee.19.10.2399


I. INTRODUCTION
Lately a lot of research activity was devoted to the development of a touch screen with tactile feedback [1]- [4].An initial research results indicate that vibration of the screen or touch screen that plays "sticky", could make for a better sensory experience on a smooth touch surface, providing texture illusion or "Programmable friction" [5], [6].The blind person, placing the finger on special thimble and scanning with it the surface of PC tactile screen, perceives specific 2D graphic information in case, when friction force between thimble and screen is related to position of contours or lines and intensity or color of background.While friction force is controlled by applying signal U(t) to the thimble, whereas the frequency and amplitude of the signal U(t) is correlated with the color and intensity of lines or contours.
In Fig. 1 two possibilities of electromagnetic thimble realization are presented.In Fig. 1(a), the thimble is placed on a thin ferromagnetic film attached to the PC screen, Attaching the ferromagnetic film on the surface of PC screen reduces the tactile properties of the screen.As shown in [7], the frequency range of this kind perception lies for most operators between 2 Hz and 250 Hz allowing them after some training to differentiate among six main colors while intensity of the line or background is related to constant component of friction force.The thimble interacts with ferromagnetic film surely in the case shown in Fig. 1(a), if the film thickness is not more than 1 mm.
In this paper we investigate the case with ferromagnetic film on the tactile screen (Fig. 1(a)) because in the case ferromagnetic under screen (Fig. 1(b)) a lot more power is needed for magnetic field excitation.We propose design of the thimble with minimal magnetic flux leakage, investigate the magnetic circuit of system thimble -ferromagnetic film, propose the technique of the real thimble realization and present the thimble experimental investigation results.

II. ELECTROMAGNET SHAPE SELECTION
Let us consider magnetic circuit with cross-section, shown in the Fig. 2. It is composed of  shape ferromagnetic core EMM with mean length of magnetic line equal to lm, of the part of magnetic basic plane MP where the magnetic flux lines are distributed (with the mean length equal to lp) and some intermediate layer lt between the magnetic base and the bottom of the electromagnet core.The major part of the magnetic flux is concentrated in the core and basic plane.But some part of magnetic flux created by the coil EC is distributed near the core in the both sides of the core.
The more effective design of electromagnet is with the round electromagnet core.It is shown in Fig. 3.Such design could be performed turning in 360 o around the axis superposed with the left side of cross-section the crosssection of the magnet, presented in the Fig. 2. In this electromagnet the leakage of magnetic flux through the sidelong surfaces is impossible.Only a little part of magnetic flux closes near the magnetic circuit.It is very actually when we want to reduce the thickness of basic plate.Therefore this electromagnet's design is economically the best.

III. INTERACTION BETWEEN FINGER AND ELECTROMAGNET
This interaction is shown in Fig. 2. The finger FN acts on the electromagnet with the force Ft parallel to the base MP.It can be expressed: , where N is the normal force, directed perpendicular to the base MP, f is a coefficient of friction between the bottom of the electromagnet and the basic plane.The electromagnet touches the basic plate by two areas Ac and Ap (Fig. 3).
The normal force N is composed of three components: the weight force P, the electromagnet attraction force T and finger pressing force Fn: . For blind the variation N is important.We suppose that pressing force is constant as the weight force.Therefore the variation of force N arises for the electromagnet attraction force variation T, N  T.This variation can be caused by putting the electric current Ie to the electromagnet coil or varying its value.The current variation Ie is related with magnetic flux variation .In the rest part of this paper by T we note the variation of attraction force and by  the magnetic flux variation.
Attraction force T has two components which act to central and peripheral parts of thimble core.The magnetic flux  is the same in both areas.Force T depends on  2 ), where 0  410 -7 H/m -vacuum permeability, Ac and Ap are the areas of central and peripheral parts cross-sections of magnetic core, correspondingly.These areas can be expressed as follows ), ( where rc is radius of magnetic core central part, ri -inner and ro -outer radii of core peripheral part, correspondingly.

IV. INVESTIGATION OF ROUND ELECTROMAGNET MAGNETIC CIRCUIT
The analysis was performed for magnetic circuit of thimble presented in Fig. 1(a), supposing that air gap is small and can be not evaluated.Cross-section of the magnetic circuit for round electromagnet in which the magnetic flux  circulates is shown in the Fig. 3.It is composed of electromagnet core and basic plate.The electromagnet core has three parts with different geometrical parameters.The magnetomotive force is equal to MNIe, where N is number of coil W turns, Ie is the coil current.The equivalent electrical circuit of this magnetic circuit is shown in Fig. 4. The magnetic resistance Rmc presents the geometrical and magnetic properties of cylindrical central part of magnetic core.The magnetic resistance Rmu presents the upper side U of core with thickness c (Fig. 3) which joints the central and peripheral parts.The magnetic resistance of peripheral cylindrical part is noted as Rmp.The magnetic resistance of basic plate is noted as Rmb.Magnetic voltages Umc, Umu, Ump and Umb, correspondingly, fall in presented resistances.
Supposing that magnetic flux is distributed uniformly in all volume of magnetic resistances Rmc and Rmp we express magnetic voltages Umc and Ump as follows: The values r(Bc) and r(Bp) can be found of magnetisation curve B(H) for chosen ferromagnetic material [8], evaluating that Bc  /Ac and Bp  /Ap.The mean magnetic lines lengths of magnetic core central lc and peripheral lp parts are the same where h is height of central core part, c and d are thicknesses of upper side and basic plate, correspondingly.
In basic plate and in upper side of magnetic core the magnetic flux is non-uniform because the cross-section areas vary along the radius r in both volumes.Therefore, B and r vary, too.The mean lines of magnetic flux lay between the radii from 0,5rc to 0,5(r0 + ri).For any r in this interval the cross-section area of upper side or basic plate is Ab(r)  Au(r)  2  rx, where x  d for the basic plate and x  c for the upper side of the core.
Supposing that the directions of vector B and radius r coincide, we can express the magnetic voltages Umu and Umb Evaluating geometrical sense of the definite integral we can write finely , ln 2 where Qm is the mean value of function Q  1/r(lnr) in the interval [rmin  0,5rc, rmax  (0,5ro + 0,5ri)].
We obtain the function Q  1/r(lnr) from the magnetisation curve B(H).The B(r) value in upper side and basic plate depends on r by this relation The minimal flux densities Bbmin, Bumin are in cross-section with maximal radius rmax, the maximal flux densities Bbmax, Bumax are in cross-section with minimal radius rmin.Substituting rmax and rmin into (10) and evaluating that x  d for basic plate and x  c for upper side we obtain: The procedure of Qm calculation is explained below.

V. REALIZATION OF THIMBLE AND MAGNETIC CIRCUIT CALCULATION
The thimble geometrical parameters were chosen as follows: ro  10,57 mm, ri  9,74 mm, rc  2,6 mm, c  1 mm, h  5 mm.Such parameters are suitable with finger dimensions.The thimble is convenient for by finger on the plane.Again the dimensions of inner volume of thimble must be sufficient for excitation coil placement.
The thimble must be made of the soft magnetic material with narrow hysteresis loop and little coercivity.The best steel suitable for electromagnets is carbon steel AISI 1020.But in the basic plate and upper core side the ratio of maximal and minimal values of magnetic flux densities is Bmax/Bmin  rmax/rmin  7,8.In this diapason of magnetic flux densities the relative permeability r of AISI 1020 varies in very large limits.The magnetic circuit becomes very nonlinear.By this reason the steel AISI 1030 was used.Basic plate was made of the same steel AISI 1030 with thickness b  1 mm.
The preliminary experimental investigation shows, that graphic information can be presented to the blind surely when the force Ft which acts to the blind finger varies in interval [0,5; 2,5 N].The weight force is 0,68 N. Evaluating the friction coefficient f  0,38 for the pair composed of two metallic surfaces of steel AISI 1030, we obtain that attraction force must be about T  6 N. By (2) and ( 3) the area of central core part is Ac  21,2 mm 2 and the area of the peripheral part is Ap  53,0 mm 2 .From ( 1 The magnetic flux densities in central and peripheral parts are: Bc  /Ac  0,71 T, Bp  /Ap  0,28 T. From data presented for AISI 1030 in [8] we find r(Bc)  510 and r(Bp)  850.The magnetic voltages Umc and Ump from ( 5) and ( 6) are:  In Fig. 5 it is shown the dependence Q  (1/r)*10 3 on B in logarithmic scale.From ( 11)-( 14) we obtain Bmax  1,84 T and Bmin  0,23 T. The dependence Q (lnB) was calculated of data presented in [8] for AISI 1030.By broken dashed lines it is presented the mean values of Q(lnB) for six intervals with ratio of maximal and minimal values is Bmax/Bmin  2 .The mean value of Q calculated for all six intervals is 2,77.The mean relative permeability is r  358 in B interval [0,23T-1,84T].Substituting calculated values , r and geometrical parameters in (7) we obtain Umb  Umu  10,9 A. Needed value of magnetomotive force is 28, 7 A.
To obtain this magnetomotive force the coil with N  2050 turns of 0,07 mm diameter copper wire was winded.The value of M  NIe  28,7 A will be reached when Ie  0,014 A. Time constant of excitation coil is small and information for blind can be presented not only by force variation but and by frequency variation in interval [2 Hz-250 Hz].

VI. EXPERIMENTAL INVESTIGATION OF THIMBLE
The experimental investigation of thimble was performed on special stand designed for friction force measurement.
The experiment was made after treatment of basic plate surface by two different materials.The static friction force was measured moving the thimble with constant velocity v = 0,08 mm/s along the basic plate.The results were fixed for three different values of normal force: N  0, N  0,49 N, N  0,98 N when current in the thimble coil was absent I  0 and when the thimble was excited by maximal current Ie  0,014 A. The results are presented in the Table I.
Attraction force T depends on excitation current nonlinearly.The steepness of this dependence decreases for maximal current values because steel saturates.Decrease of steepness for low values of current can be explained by Fig. 7.There the dependence of magnetomotive force and sum of magnetic voltages on excitation current is presented.
Lesser sum of magnetic voltages than magnetomotive force shows that some magnetic voltage falls in the air gap between basic plate and magnetic core which we are not evaluated in magnetic circuit analysis.But it is actually for values of current less than 8 mA only.Attraction force dependence on the coil current was investigated, too.The results are presented in the Fig. 6 and Fig. 7. Initial experiments were made by employing one blind operator; the task was to find the door in a room (laboratory), using ferromagnetic pad and computer mouse, attached to electromagnetic thimble.There were well expressed contours of the white door in rather grey wall and blind operator after some time found the right direction.He felt well the force magnitude variation and the frequency variation.Therefore the magnetic thimble can be used to give the 2D graphic information for blind person quite effectively.

VII. CONCLUSIONS
Specific 2D graphic information from PC can be presented for blind person on tactile screen.The original electromagnetic thimble on ferromagnetic film is purposed to perceive this information by friction force varying.Electromagnet with axial symmetry is optimal for thimble in respect to the magnetic flux leakage.The analysis of magnetic circuit was performed.The carbon steel with soft magnetisation characteristic for thimble realisation must be used.The real thimble was presented and investigated.By varying the excitation current in interval [4 mA-14 mA], the attraction force has been varied in interval [0,7 N-6 N].The experimental investigation shows that proposed thimble can be used for the blind orientation effectively.The information can be presented by the force magnitude or by the frequency variation.

Fig. 1 (
b) presents another -attaching the ferromagnetic layer under the screen does not affect tactile properties of the screen, but due to Manuscript received February 5, 2013; accepted November 14, 2013.This research was funded by a grant (No.MIP-042/2011) from the Research Council of Lithuania and Nr.31V-38 (TactilApps) from Agency for Science, Innovation and Technology.This research was performed in cooperation with the Institutions.

gap 1 Fig. 1 .
, the amplitude of the harmonic signal U(t) should be increased.Two cases of thimble, interacting with ferromagnetic film.

Fig. 3 .
Fig.3.The round electromagnet: a -cross-section along the electromagnet axis; a΄ -cross-section perpendicular to electromagnet axis.

Fig. 4 .
Fig. 4. The magnetic circuit of the round electromagnet.Let we know the attraction force T must be created by electromagnet.From (1) we can find magnetic flux  needed to create this attraction force.The problem is to find the magnetomotive force NIe.By Kirchhoff's voltage equation logarithm taking we have lnB  -Klnr, (K  /2x  const).Therefore the mean value Qm of function Q = 1/r(lnr) calculated in the interval [rmin, rmax] coincides with the mean value of function Q = 1/r(lnB) calculated in the interval [Bb(u)min, Bb(u)max].Bbmin, Bbmax and Bumin, Bumax are the minimal and maximal values of magnetic flux density in basic plate and in the upper side, correspondingly.
) we get the value of flux  which circulates in the magnetic circuit of the thimble

Fi 6 .
Dependence the magnetic flux (a) and attraction force T (b) on the coil excitation current.The nonlinearity of function T(I) can be eliminate easily by computer.We will have the linear dependence the attraction force on control action u if we relate control action with needed excitation current I (see Fig. 6(b)).

Fig. 7 .
Fig. 7.The dependance of magnetomotive force and sum of magnetic voltages on the current.

TABLE I .
RESULTS OF FRICTION FORCE MEASUREMENT.