Abstract:
Head for checking linear dimensions of parts in machine tools or measuring machines, with a casing, a movable arm-set including an arm carrying a feeler for contacting the part to be checked, a biasing device arranged between the casing and the movable arm-set, a detecting device for providing a signal depending on the position of the movable arm-set, and two constraining systems—between the casing and the movable arm-set—both totally with force closure under the action of the biasing device, that eliminate, in a univocal way, the six degrees of freedom of the movable arm-set, one of the two constraining systems featuring a structure with rotational symmetry.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a head for checking linear dimensions of parts in machine tools or measuring machines, with a support structure, a movable arm-set including an. arm carrying a feeler for contacting the part to be checked, a bias device arranged between the support structure and the movable arm-set, a first constraining system having a structure with rotational symmetry about a first geometrical axis and a second constraining system of a type different from that of the first constraining system, the first and the second constraining system being arranged between the support structure and the movable arm-set, for eliminating, under the action of the bias device, the six degrees of freedom of the movable arm-set, the degrees of freedom eliminated by the first constraining system including the translation of the movable arm-set along said first geometrical axis, and a detecting device for providing a signal depending on the position of the movable arm-set. 
     2. Description of the Related Art 
     Contact detecting heads, or touch trigger probes, and measuring heads are used in co-ordinate measuring machines and machine tools, particularly machining centres and turning machines, for checking machined or being machined parts, tools, machine tables, etc. These heads generally include a movable arm-set with an arm carrying one or more feelers, a biasing device for urging the arm-set against a support structure, and a detecting device with one or more switches, or other devices for providing a triggering signal, or with position transducers. 
     In connection with contact detecting heads, the displacement of the feeler due to the contact with the part causes triggering of the detecting device, that in turn controls the reading of transducers associated with the machine slides, that provide measurement values with respect to a reference position or origin. 
     Basic requirements for these heads are the repeatability i.e., the correspondence among determined positions of the feeler and the triggering of the detecting device or, for the measuring heads, the values of the signals of the transducers of the head, reliability, sturdiness, small overall dimensions and a limited cost. 
     An indicative value of the acceptable repeatability error for these heads is of 1 μm, or less. 
     Some of the heads are anisotropic with respect to triggering of the detecting device as a consequence of transversal displacements of the feeler. On this regard, it is pointed out that normally the transversal displacements are not purely translational, but correspond, for example, to rotational displacements of the movable arm-set. However, in view of the small entity of the feeler displacements, it is customary to refer to feeler displacements along the directions of the transversal axes. 
     Substantially, the concept of anisotropy consists in that, as the direction of the transversal displacement changes, the triggering of the detecting device takes place in correspondence with different eccentricity values of the feeler with respect to the longitudinal geometric axis of the head. 
     An example of a head strongly anisotropic is illustrated in FIGS. 1 to  3  of U.S. Pat. No. 4,153,998. 
     Another anisotropic head is described in the Japanese patent application laid-open with number 63-263406. 
     Other known heads, as those described in U.S. Pat. No. 5,299,360, GB-A-2,205,650 and inventor&#39;s certificate SU-A-1516786, and some of those described in U.S. Pat. No. 5,146,691, in particular the head shown in FIGS. 1 to  3  of the latter patent, are, at least conceptually, of isotropic type. 
     Further differences among the conventional heads relate to the systems for constraining the movable arm-set to the support structure. 
     As well known, a rigid body free in space has six degrees of freedom that, with reference to a Cartesian co-ordinate system, consist in the possibility of performing translations along the axes X, Y and Z and rotations about the same axes. 
     As an example, the movable arm-set of the head described in U.S. Pat. No. 5,299,360 has, in the absence of forces acting on the feeler, two degrees of freedom (possibility of rotating about the X and Y axes). The movable arm-set of the head shown in FIGS. 1 to  3  of U.S. Pat. No. 4,153,998, that is coupled to the support structure through a constraining system constituted by the so-called Boys&#39; joint (three cylindrical elements fixed to the arm-set and three pairs of balls fixed to the support structure), does not feature, still in absence of forces acting on the feeler, any degree of freedom. 
     The systems constraining the movable arm-set with respect to the support structure can feature force closure and/or deformations. 
     In the kinematic constraint systems (shape constraints with force closure), the degrees of freedom (one or more) are eliminated under the action of forces, for example elastic forces, that “close” conceptually rigid elements of the system, maintaining them into contact. When the feeler is biased by forces having values higher than the above mentioned elastic forces, it displaces and eliminates the concerned constraint, without causing (conceptually, namely with reference to ideal rigid bodies) deformations of the constraint system. An example of a head with constraint system with full force closure is the already mentioned head shown in FIGS. 1 to  3  of U.S. Pat. No. 4,153,998, that features a Boys&#39; joint, i.e. a system typically featuring an anisotropic structure. 
     On the contrary, in connection with constraints featuring deformations, like the leaf spring of the head described in GB-A-2,205,650, the displacement of the feeler occurs due to the elastic deformation of one or more elements of the constraint system. 
     The constraint systems featuring deformations involve some problems, for example because the deformations must be limited, in order to avoid that they become permanent, and because these systems normally feature poor ruggedness. In particular, drawbacks of this type arise when the feeler must have the possibility of performing large movements, for example in contact detecting heads used in machine tools requiring very fast measurement cycles. In similar situations, the head must allow large values of the “extra-travel” of the feeler after the generation of the triggering signal. 
     Further drawbacks occurring in some conventional heads derive from the fact that the constraint systems used in these heads would give rise to situations of over-constraint, because the constraints would be in excess with respect to those needed to eliminate one or more degrees of freedom of the movable arm-set. For example, if a head features two independent constraints with respect to the axial translation of the movable arm-set, it is necessary to add a de-coupling device for eliminating the effect of one of the constraints on the feeler, in order to avoid instability problems and thus poor repeatability of the head. The elimination of the superabundant constraints involves complications in the head structure and other drawbacks (use of de-coupling elements based on deformations, arm-sets consisting of a plurality of mutually movable elements, need of a plurality of biasing devices). Similar situations occur in the heads described in the already mentioned patent U.S. Pat. No. 5,146,691. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a contact detecting head, or a measuring head, permitting advantageous compromises, with respect to the prior art, in connection with the characteristics of repeatability, sturdiness, isotropy, reliability, simplicity of structure, possibility of use both in metrology department and in workshop environment, or some of these characteristics. 
     A favourable compromise of relatively general character is obtained by a head of the type initially mentioned, that corresponds to that described in the patent application GB-A2,205,650, in which the first constraining system and the second constraining system are totally with force closure. 
     More generally, the invention relates to characteristics regarding the structure and the functions of the constraining systems, the bias device and the detecting device, considered singly or in combination with other characteristics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in detail with reference to the annexed drawings, given for exemplary and non limiting purpose, wherein: 
     FIG. 1 is a longitudinal section, along path I—I in FIG. 2, of a contact detecting head, according to the preferred embodiment of the invention; 
     FIG. 2 is a partial cross-section of the head shown in FIG. 1, along path II—II of FIG. 1; 
     FIG. 3 is a longitudinal section, along path III—III of FIG. 4, of a contact detecting head according to a variant with respect to FIGS. 1 and 2, shown without the detecting device; 
     FIG. 4 is a partial cross-section of the head of FIG. 3, along path IV—IV of FIG. 3; 
     FIG. 5 is a longitudinal section, along path V—V of FIG. 7, of a contact detecting head according to another variant with respect to FIGS. 1 and 2, shown without the detecting device; 
     FIG. 6 is a simplified cross-section, along path VI—VI in FIG. 5, showing some details of the head of FIG. 5; 
     FIG. 7 is a cross-sectional view, along the plane passing through path VII—VII in FIG. 5, showing a spring of the head of FIGS. 5 and 6; 
     FIG. 8 is a longitudinal view, partially in section, of a contact detecting head according to another variant with respect to FIGS. 1 and 2, shown without the detecting device; 
     FIG. 9 is a cross-sectional view along the plane passing through path IX—IX in FIG. 8, showing some details of the head of FIG. 8; 
     FIG. 10 is a longitudinal section showing a portion of a head according to another embodiment of the invention; 
     FIG. 11 is a longitudinal section showing a head according to a variant with respect to the head of FIG. 10; 
     FIG. 12 partially and schematically shows a head with detecting device of resistive type; 
     FIG. 13 shows a detecting circuit of the head of FIG. 12; and 
     FIG. 14 shows in a schematic and partial way a head with an opto-electronic detecting device. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The head of FIGS. 1 and 2 comprises a support structure, or casing  1  including a first member  2 , substantially cylindrical and having an upper base  3 , and a second member  4 , substantially annular, with a lower base  5 . The first and the second member  2 ,  4  are secured to each other, in correspondence with the ends opposite to the bases  3  and  5 , by a fixed or detachable coupling  6  represented in a very schematic way. 
     The movable arm-set  7  of the head comprises an arm  9  carrying at an end external to casing  1  a feeler  10  and at the other end a member having the shape of a circular disc  11 , with an annular portion  12  protruding towards feeler  10 . 
     Three plates or vanes  13  with plane walls are fixed to the arm-set  7 , between the lower surface of disc  11  and the upper portion of arm  9 . The walls of plates  13  lie in radial directions and along the longitudinal geometric axis Z of the head and are angularly spaced apart, with respect to the Z axis, at 120° from one another. 
     Three other plates or vanes  14 , with flat radial walls, parallel to the Z axis, are fixed to member  4 , angularly spaced apart at 120° from one another. 
     A double-coiled spring  15  is arranged between the lower surface of base  3  and the upper surface of disc  11 , and is pre-loaded so as to apply to movable arm-set  7  an axial and torsional pre-load, as shown by the arrows in FIGS. 1 and 2, indicating that the torsional pre-load applies a rotation moment, in clockwise direction, to movable arm-set  7 . 
     Three intermediate rolling elements, constituted by free balls  16 , are arranged between adjacent surfaces of respective plates  13 ,  14 . Due to the torsional pre-load applied by spring  15 , the three balls  16  remain trapped between the pairs of plates  13 ,  14  and a cylindrical internal wall  17  of member  4 . A transversal flat surface  18  of member  4  provides an axial limit stop for balls  16 . Another axial limit stop for balls  16  is provided by the lower surface of disc  11 . 
     An annular flat zone  20 , i.e. with the shape of an annulus, at the lower surface of portion  12 , is biased by spring  15  into contact with a corresponding annular flat zone  21  at the upper portion of member  4 , so as to provide a first constraining system, with force closure, of the movable arm-set  7  with respect to casing  1 . This constraining system is closed due to the axial pre-load applied by spring  15  and eliminates three degrees of freedom, relating to the translation along the longitudinal axis Z and rotations about the transversal axes X and Y of a Cartesian co-ordinate system. 
     A second constraining system, it too with force closure, includes plates  13 ,  14 , balls  16 , and wall  17 . The second constraining system is closed due to the torsional pre-load applied by spring  15  and eliminates the other three degrees of freedom of movable arm-set  7 , relating to translations along the X and Y axes and the rotation about the Z axis. 
     If an axial force, namely along the Z axis, having a value higher than that of the axial pre-load provided by spring  15 , is applied to feeler  10  due to contact with a part to be checked, movable arm-set  7  undergoes a translational displacement along Z and annular zone  20  detaches from annular flat zone  21 . 
     The structure of the head is made in such a way that the triangle having its three vertexes coinciding with the points of contact of each ball  16  with plates  13  and  14  and wall  17  is an isosceles triangle, with its base defined by the points of contact with plate  14  (stationary) and wall  17  (it too stationary). Therefore, during the translational displacement, balls  16  roll on the respective plates  13 ,  14  and wall  17 . Since the motion of balls  16  is of pure rolling type, it involves frictional forces of negligible value. 
     If on the contrary a sufficient radial force, namely lying in a meridian plane of the X, Y, Z system, is applied to feeler  10  due to contact with a part to be checked, movable arm-set  7  undergoes a tilting displacement on a point of contact between annular zone  20  and annular flat zone  21  and, apart from this point, annular flat zone  21  detaches from annular zone  20 . During this tilting motion balls  16  roll on the respective plates  13 ,  14  and the wall  17 . 
     A sealing and protection gasket  22  is arranged between movable arm-set  9  and base  4 . 
     The disengagement of the first constraining system, i.e. the total or partial detachment of annular zone  21  from annular zone  20 , is detected by a detecting device connected to an alternating current supply and including a capacitor with two annular plates  23 ,  24 , arranged in toroidal recesses  25 ,  26  obtained in annular portion  12  and in member  4 . Recesses  25  and  26  are filled by a dielectric material and are crossed by conductors  27 ,  28  connected, respectively, to plates  23 ,  24  and to a printed circuit  29  fixed to upper base  3 . 
     Any displacement of movable arm-set  7  with respect to casing  1  causes a variation of the capacitance of the capacitor including plates  23 ,  24 , that is detected by an external circuit connected with printed circuit  29 . 
     It is evident that a circuit with inductive coupling, connected to a detecting circuit properly modified, can be substituted for the capacitor including plates  23 ,  24 . 
     In FIGS. 3 and 4, that refer to a variant with respect to the head of FIGS. 1 and 2, elements equal or equivalent to those shown in FIGS. 1 and 2 are marked with the same reference numbers. 
     As per FIGS. 1 and 2, disc  11  and the upper portion of member  4  comprise two flat annular zones  20  and  21  that define a first constraining system adapted to eliminate three degrees of freedom of the movable arm-set  32  (translation along the Z axis and rotations about the X and Y axes). 
     A second constraining system is defined by a first tern of cylindrical pins  34  and a second tern of cylindrical pins  35 . Pins  34  are fixed to member  4 , are arranged substantially along the direction of longitudinal axis Z and are angularly spaced apart at 120° from one another. Pins  35  are fixed to disc  11 , are arranged along the direction of longitudinal axis Z and are also angularly spaced apart at 120° from one another. 
     Three balls  16  are arranged between respective pairs of pins  34  and  35 . 
     Due to the pre-load applied by spring  15  each ball  16  remains trapped between a pairs of pins  34  and  35  and the upper portion of arm  9 , into contact with a point of a stationary element (a pin  34 ) and two points of movable elements (a pin  35  and arm  9 ). 
     The structure of the head is made in such a way that the triangle having its three vertexes coinciding with the points of contact of each ball  16  with pins  34  and  35  and movable arm  9  is an isosceles triangle, with its base defined by the points of contact with pin  35  (movable) and arm  9  (it too movable). 
     When a force with a sufficient value is applied to feeler  10 , movable arm-set  32  undergoes displacements similar to those described with reference to movable arm-set  7  of the head shown in FIGS. 1 and 2. 
     The head according to FIGS. 5 to  7 , in which elements equal or equivalent to elements of the heads described before are marked with the same reference numbers, comprises two springs, rather than a single spring adapted to apply a pre-load both axial and torsional. A compression spring  37  is arranged between the lower surface of base  3  and the upper surface of disc  11 , while a torsion spring  38 , substantially flat, is arranged between arm  9  and lower base  5 . Spring  38  has a central portion locked within a hole of arm  9  and two spiral coils with their ends fixed within axial holes obtained in base  5 . 
     Three radial pins  39 , fixed to casing  1 , at 120° from one another, and three pins  40 , substantially longitudinal, fixed to disc  11 , they too at 120° from one another, provide constraints with respect to translations along axes X and Y and rotation about the Z axis. Due to the action of springs  37  and  38  each pin  40  is biased into contact with a corresponding pin  39 . 
     When a force having a sufficient value is applied to feeler  10 , the movable arm-set of the head of FIGS. 5 to  7  undergoes displacements similar to those of the arm-sets of the heads shown in the preceding figures. However, the mutual displacements among the pairs of pins  39  and  40  involve sliding frictions. In order to reduce the frictions, it is possible to use three ball bearings, not shown, having the inner rings keyed to the stationary pins  39 , respectively, and the outer rings in contact with the movable pins  40 . 
     The head shown in FIGS. 8 and 9, in order to provide constraints with respect to translations along the X and Y axes and rotation about the Z axis, comprises three struts  42  having conical ends housed in corresponding seats obtained in longitudinal pins  43  fixed to lower base  5  and in pins  44 , substantially longitudinal, fixed to disc  11 . 
     The ends of struts  42  are maintained into contact with the corresponding seats due to the combined action of a compression spring  46 , arranged between the lower surface of base  5  and the upper surface of disc  11 , and of three return springs  47  having their ends fixed to respective pins  43  and  44 . It is evident that the arrangement of springs  47  gives rise to a torsional pre-load applied to the movable arm-set of the head, as indicated by the arrow in FIG.  9 . 
     FIG. 10 partially shows another embodiment of the invention, that mainly differs from those of FIGS. 1 to  9  in that the first constraining system, including plane annular surfaces, substantially with the shape of annuluses, is replaced by another constraining system, also having a structure with symmetry of rotation, but with surfaces inclined to the Z axis. In particular, the lower base of casing  50  of the probe defines, in correspondence with an opening for the passage of movable arm  51 , a surface with the shape of a truncated cone  52  that co-operates with an element  53  having the shape of a hemisphere or of a spherical sector, fixed to movable arm  51  . 
     This type of coupling is adapted, per se, to eliminate, due to the action of a suitable bias device, three degrees of freedom of the movable arm-set, consisting of translations along the X, Y and Z axes. 
     A second constraining system is constituted by a first tern of cylindrical pins  55  arranged horizontally and radially and fixed to casing  50  and by a second tern of cylindrical pins  56 , fixed to a disc  57  coupled to an extension  58  of movable arm  51 , and arranged substantially along the Z axis and, as for the first tern, at 120° from one another. 
     A double-coiled spring  60  with the ends fixed to casing  50  and element  53  provides a pre-load both axial and torsional, for maintaining into contact the corresponding pairs of stationary and movable pins  55  and  56 . 
     Per se, the second constraining system is adapted to prevent the rotation of the movable arm-set about the Z axis and translations along the X and Y axes. 
     The second constraining system is arranged at a substantial longitudinal distance from the first constraining system: in particular, the distance from the point of contact between two pins  55 ,  56  to the plane containing the theoretical circumference of contact between the surface with the shape of a truncated cone  52  and element  53  is some times larger than the radius of the circumference of contact, for example 10 times larger. 
     Therefore, in view of well-known theorems from theoretic mechanics, notwithstanding both the first constraining system and the second constraining system prevent, singly, translations along the X and Y axes, this does not originate problems of superabundance of constraints, namely of over-constraint, because the combined effect of the two pairs of constraints relating to translations along X and Y is to provide single constraints with respect to translations along X and Y and additionally two constraints with respect to rotations about the same axes X and Y. 
     As for the head of the FIGS. 5 to  7 , three ball bearings, not shown, can be arranged in the head of FIG. 10, too, in order to prevent sliding friction among pins  55  and  56 . 
     As a consequence of the application to feeler  61  of a force having sufficient value and acting along the radial direction, element  53  partially disengages from the surface having the shape of a truncated cone  52 , sliding on it, conceptually with a single point of contact. 
     A microswitch  62  with its casing fixed to the upper base of the probe casing  50  is provided for detecting the displacements of feeler  61 . When feeler  61  is not under the action of any force, the end of the movable stem  63  of microswitch  62  is arranged at a small distance from an abutment plate  64  fixed to disc  57 . Of course, this clearance gives rise to a certain pre-stroke before the activation of microswitch  62 . 
     The head shown in FIG. 11 is conceptually similar to that of FIG. 10, but is not subjected to sliding friction. 
     For this purpose, movable arm-set  65  comprises a plate  66  carrying arm  51  and a block  67  coupled to plate  66  by four studs  68 . An end of the torsion and compression spring  60  is fixed to the upper portion of block  67 , while the lower portion of block  67  defines a surface having the shape of a truncated cone  70 , facing surface  52 . A ball  72  is arranged between the surfaces having the shapes of truncated cones  52  and  70  and, as a consequence of lateral displacements of feeler  61 , can move with respect to surfaces  52  and  70 , with pure rolling motion. 
     Casing  50  of the head includes an internal annular flange  74  that caries three longitudinal  30  pins  75 , arranged at 120° from one another. Three balls  77  are arranged between pins  75  and pins  56  and permit movable pins  56  to displace with respect to stationary pins  75  without any sliding friction. 
     It can be noticed that, in connection with the heads shown in FIGS. 3 to  9 , for simplification sake, no detecting device for detecting the displacement of the feeler, and/or the entity of the displacement, has been described. 
     However, for all of the heads shown in FIGS. 1 to  11 , it is possible to use the capacitive device described with reference to FIGS. 1 and 2, or the microswitch device used in the heads of FIGS. 10 and 11, or devices of other types. 
     Moreover, it is possible to use detecting devices and measurement transducers, such as devices with one or more linear variable differential transducers, or with transducers of other types. 
     FIGS. 12 and 13 refer to a head or probe with a resistive detecting device, which can be used for all of the embodiments shown in FIGS. 1 to  11 . For the sake of simplification, the constraining systems of the probe of FIG. 12 are shown only partially. 
     The probe partially shown in FIG. 12 comprises a support and protection structure with a casing  81 , having a lower base  82  and an upper base  83 . Casing  81  has a substantially cylindrical shape and defines a longitudinal geometric axis (Z axis in a Cartesian co-ordinate system). 
     A movable arm-set  84  is partially housed within casing  81  and includes a support element  85 , an arm  86  coupled to support element  85  and partially protruding from casing  81  through a hole of the lower base  82 , and a feeler  88  fixed to a free end of arm  86 . Support element  85  comprises an upper base  90 , a cylindrical portion  91  ending with an annular end, a ring  92 , made of electrically insulating material, fixed to the annular end, and another ring  93  made of electrically resistive material, fixed to ring  92 . 
     A third ring  95 , made of insulating material, is internally fixed to the lower base  82  of casing  81  and a fourth ring  96 , made of electrically resistive material, is fixed on the third ring  95 . 
     A biasing device comprises a compression spring  97  arranged between the upper base  83  of casing  81  and support element  85 , for urging the lower annular surface  98  of ring  93  into contact with the upper annular surface  99  of ring  96 . 
     When arm-set  84  is in rest condition, i.e. in the absence of forces acting on feeler  88 , the constraining device including the lower annular surface  98  of ring  93  and the upper annular surface  99  of ring  96  is closed by the force provided by spring  97  and prevents displacements of movable arm-set  84  in connection with the translation along the longitudinal axis (Z axis) of the probe and displacements of rotation about the transversal axes X and Y. 
     The constraining system of the movable arm-set  84  comprises further constraining means for preventing translations of the movable arm-set along the transversal axes X and Y and displacements of rotation about the longitudinal axis Z. For the sake of simplification, the further constraining means are not shown in the drawings. They can be made in different ways, for example (also in connection with the bias device) in accordance with FIGS. 1 to  11 . 
     Also in the head of FIG. 12 (and similarly in the heads of FIGS. 5,  8 ,  10  and  11 ) one or more gaskets, or similar sealing and protection elements, not shown in the drawings, are fixed between movable arm  86  and the lower base  82  of casing  81 . 
     With reference to FIG. 13, an electronic detecting device comprises a generator, or direct current source  101 , which applies a difference of electric potential between two contacts  102 ,  103 , connected to diametrically opposite points of ring  96 , and a detecting circuit  110 , which receives the voltage existing between two contacts  104 ,  105  connected with diametrically opposite points of ring  93  located (in the condition of FIG. 12) on the same meridian plane containing the points connected with contacts  102 ,  103 . 
     Detecting circuit  110 , that can be arranged in a control, display and supply unit  111 , comprises a comparator  112 , fed by a voltage source  113  and having the inverting input connected with contact  104  and the non-inverting input connected with contact  105 , through a resistor  114  of a voltage divider including another resistor  115 . 
     Annular surfaces  98 ,  99  are accurately lapped so as to approach the theoretical condition of mutual contact on their whole facing areas when feeler  88  is not subjected to forces. 
     In this condition, electric current flows though ring  93 . A difference of potential (lower than that present across contacts  102  and  103 , but in any case sufficient for the purposes described below) is present across contacts  104  and  105 . 
     In substance, it can be said that, from an electrical point of view, rings  93  and  96  are “substantially” in parallel. On this regard, it can be commented that, since the contact is distributed, the wording “in parallel” would not be totally proper. 
     Due to the mutual approach between the probe and part  117  along the Z axis and the contact of feeler  88  against part  117 , movable arm-set  84  can translate, against the bias provided by spring  97 , and ring  93  totally detaches from ring  96 , so that the difference of potential across contacts  104  and  105  becomes equal to zero. 
     In the case of mutual approach and contact along a transversal direction, for example along the X direction, as shown in FIG. 12, movable arm-set  84  tilts on a point of surface  99  and in this case, too, the current flow through ring  93  ceases and the difference of potential across contacts  104  and  105  become equal to zero. 
     Comparator  112  compares a threshold value defined by voltage divider  114 ,  115 , fed by voltage source  113 , with the difference of potential across contacts  104  and  105 . When arm  86  is not deflected the signal at the output  116  of comparator  112  is at low level, while when arm  86  is deflected the signal at output  116  is at high level. 
     FIG. 14 refers to a probe with an opto-electronic detecting device, that can be used for all of the embodiments shown in FIGS. 1 to  11 . 
     The probe schematically shown in FIG. 14 comprises support and protection means with a casing  121 , having substantially a cylindrical shape and defining a longitudinal geometric axis Z, comprising a lower base, defining an internal support surface  136  substantially flat and annular, and an upper base. A movable arm-set  125  is partially arranged within casing  121  and includes a support element  127  defining a portion or annular edge  128 , substantially with rotational symmetry, an arm  131  coupled to support element  127  and partially protruding with respect to casing  121  through a hole in the lower base, and a feeler  133  secured to a free end of arm  131 . 
     A biasing device includes a compression spring  137  arranged between surfaces of the upper base of casing  121  and of support element  127  for urging annular edge  128  and support surface  136  into mutual contact when arm-set  125  is in rest condition, in absence of contact between feeler  133  and a part  135  to be checked. 
     Centering and anti-rotation devices (for example, similar to those of the heads of FIGS. 1 to  11 , also with respect to the bias device) are arranged, for example between movable arm-set  125  and casing  121 , in order to prevent mutual displacements of transversal translation and rotation about the longitudinal axis Z. These devices are not shown in FIG. 14, for the sake of simplification. 
     A detecting device, of opto-electronic type, comprises emitter devices and receiver devices. In particular, two light emitting diodes  141 ,  143 , or “LEDs”, are fixed to the lower base of casing  121 , arranged in diametrically opposite positions, substantially in correspondence of the plane defined by annular surface  136 . Two receiving photodiodes  145 ,  147  are also fixed to the lower base of casing  121 , substantially on the same plane of LEDs  141 ,  143  and facing them. The mutual arrangement of the elements of each of the pairs LED/photodiode  141 / 145  and  143 / 147  is such that the light beam emitted by the LED is directed toward the corresponding photodiode along a determined path that consists of a section transversal with respect to the longitudinal axis Z of casing  121 . 
     The two LEDs  141 ,  143  and the two photodiodes  145 ,  147  are electrically connected to a supply and processing unit  151 , by means of cables schematised in FIG.  14  and marked with reference numeral  150 . 
     A sealing gasket  157  has its ends fixed to the lower base of casing  121  and arm  131 , respectively, and besides protecting the internal arm-set of the probe from foreign matter prevents the passage of light into the probe. 
     The two LEDs  141  and  143 , connected in series, can be fed by direct current or pulse current. 
     The operation of the probe of FIG. 14, in the case of supply of LEDs  141  and  143  by direct current, is as follows. 
     In absence of contact between feeler  133  and the part  135  to be measured, arm-set  125  is in a rest condition in which annular edge  128  and support surface  136  are into mutual annular contact due to the bias provided by spring  137 . In this condition, the light beams emitted by LEDs  141 ,  143  do not reach photodiodes  145 ,  147 , because impinge on edge  128 . 
     During a mutual movement between the probe and part  135  to be checked, along radial direction X, after the contact, at a time t 1 , between feeler  133  and a surface of part  135 , arm  131  and the whole movable arm-set  125  tilt with respect to casing  121  and edge  128  partially rises from annular surface  136 , permitting the passage of the light beam emitted by one of the LEDs, for example LED  143 . The light beam impinges upon the corresponding photodiode  147 , which generates a signal V I , that is amplified by an amplifier and compared, in a comparator, with a threshold V T . When the value V T  is exceeded, at a time t 1 +τ, there is generated an output signal that signals the occurrence of the contact between feeler  133  and part  135 . 
     In the case of longitudinal movement between the probe and the part  135 , and of contact between feeler  133  and a transversal surface of the part  135  along the Z direction, arm  131  and movable arm-set  125  rise and cause edge  128  to detach from annular surface  136 . In this condition, the light beams emitted by LEDs  141 ,  143  reach the corresponding photodiodes  145 ,  147  and the processing of the signals V I  provided by the latter takes place as previously described. 
     In the probe of FIG. 14 it is possible to introduce the other modifications and variants. In particular, it is possible to use emitters and receivers of types different from those previously described. For example, the photodiodes can be replaced by CCD devices (“Charge Coupled Devices”) or phototransistors. Moreover, it is possible to use non-optical systems, for example other radiating systems, such as systems based on ultrasounds or microwaves, in place of the couplings LED-photodiode. 
     Of course, further changes may be introduced into the embodiments described and illustrated, without departing from the scope of the invention. 
     For example, in the probes shown in FIGS. 1 to  4 ,  10  and  11  rather than a torsion and compression spring  15 , it is possible to use two springs similarly to what has been described for the embodiment shown in FIGS. 5 to  7 , and vice versa. 
     In order to reduce a possible anisotropic behaviour of the heads of FIGS. 1 to  9 , it is suitable that the width of the annular contact zone between annular surface  20  and annular surface  21  be sensibly lower than the inner radius of the same annular zone, so that, from a practical point of view, the contact zone can be considered equivalent to a circumference. On this regard, it is pointed out that the dimensions in the figures should not be considered significant. Similar comments apply for the head of FIGS. 12 to  14 . 
     Thus, all of the embodiments shown in FIGS. 1 to  11  feature heads with isostatic coupling of the movable arm-set, conceptual isotropy with respect to transversal displacements of the feeler, for both a contact detecting probe and a measuring probe, and constraints totally with force closure.