Patent Publication Number: US-9423232-B2

Title: Touch probe with piezoelectric sensor and thermal insulation

Description:
TECHNICAL FIELD 
     The present invention relates to a touch probe for checking position or dimensions of a part in machine tools or measuring machines including a support frame with a protective casing and a bearing and locating area, a movable armset that is partly housed in the protective casing and includes an arm carrying a feeler adapted to touch the part to be checked, a thrust device arranged between the support frame and the movable armset for urging the movable armset against the bearing and locating area, a constraint and positioning system between the movable armset and the support frame at the bearing and locating area, and a detection device with a laminar piezoelectric transducer connected to the support frame and adapted to provide a signal in consequence of a force applied to the feeler. 
     BACKGROUND ART 
     Touch probes with movable armsets carrying feelers are used in coordinate measuring machines and in machine tools, in particular in machining centres and lathes, for checking pieces that have been or have to be machined, tools, machine tables, etc. In each one of such probes, the contact between the feeler and, for example, a mechanical piece is signalled by suitable devices that detect certain movements of the movable armset with respect to a casing and control the readout of transducers associated to the slides of the machine, the transducers providing measuring values with respect to a reference position or origin. 
     A detection and signalling device of a probe, like the probe described in the patent No. U.S. Pat. No. 5,299,360, can include an electric circuit and at least one associated switch that is mechanically actuated in consequence of displacements occurred between the movable arm and the casing and causes the closure or, more frequently, the opening of the circuit. 
     Other probes with detection devices including electric circuits placed at support and positioning systems are known for example from the patent No. U.S. Pat. No. 4,153,998. Other types of touch probes can include detection devices of very different kind and arrangement, among them strain or piezoelectric transducers. Patents publications No. U.S. Pat. Nos. 3,945,124, 4,177,568, GB-A-2049198, U.S. Pat. Nos. 4,462,162 and 4,972,594 show probes of this type. 
     Among them, the patent No. U.S. Pat. No. 4,972,594 shows a probe with an armset that includes arms and feelers and is connected to a fixed frame in a movable way. In particular, the armset is coupled to an intermediate support at mechanical reference abutments. The probe includes two detection devices: a laminar shaped piezoelectric transducer that generates a signal right after one of the feelers has touched the mechanical part to be checked, and an electric circuit closed by contacts in the above mentioned mechanical reference abutments. To avoid false signalling by the piezoelectric transducer, which is particularly sensitive to vibrations or other noises and to thermal variations, the signal of such piezoelectric transducer is considered as indicative of the contact actually occurred between the feeler and the mechanical part only if such contact is confirmed within a certain delay by the opening of the electric circuit caused by the armset movement and the consequent separation between the mechanical abutments. This procedure for ensuring immunity to noises of different types may cause problems due to the delay necessarily introduced in the contact detection. Such problems even increase when it is not possible to completely control the closing rate—set by the machine cycles—between the probe ad the part to be checked. 
     The piezoelectric transducer is—as already mentioned—thermosensitive, i.e. sensitive to thermal variations, and when changes in temperature occur it can produce electric signals that may cause false contact signalling. 
     DISCLOSURE OF THE INVENTION 
     The object of the present invention is to implement a touch probe that is particularly reliable and substantially immune to errors due to environmental noises such as thermal variations. 
     This and other objects and advantages are achieved by a touch probe according to the present invention as defined in claim  1 . 
     A touch probe according to the present invention includes a support frame with a protective casing and a bearing and locating area for a movable armset with an arm and a feeler. 
     The movable armset is urged against the support frame in a defined position at the bearing and locating area by a constraint and positioning system with an isostatic rest system including, for example, V-shaped seats in three support areas, and associated radial elements in the movable armset. A detection device includes a laminar piezoelectric transducer, preferably made of polyvinylidene fluoride, which is connected to the support frame at the bearing and locating area. A thermal insulation system is located between the protective casing and the transducer at the bearing and locating area, to make the piezoelectric transducer substantially insensitive to sudden changes in temperature. The thermal insulation system preferably include elements made of thermal insulating material, such as fibreglass, between the piezoelectric transducer and the support frame. The protective casing is preferably made of an iron-nickel alloy featuring low coefficient of thermal expansion, such as Invar®. The thermal insulation system can further include a hollow body housing the protective casing and defining a suitable thermal insulating hollow space. Preferably, the transducer includes three separated sensitive sectors placed at the three support areas. According to a preferred embodiment of the invention, the probe includes a second detection device, for example an electric circuit positioned at the isostatic rest system between the movable armset and the support frame. The signalling of such second detection device can be used in addition to that one of the piezoelectric transducer in order to confirm the latter, for example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is now described with reference to the enclosed sheets of drawings given by way of non limiting example, wherein: 
         FIG. 1  is a simplified longitudinal section of a touch probe according to the invention, 
         FIG. 2A  is a perspective view of a component of the touch probe of  FIG. 1 , shown in an enlarged scale with respect to  FIG. 1 , 
         FIGS. 2B and 2C  are perspective, exploded views of the component of  FIG. 2A  from two different angles, 
         FIG. 3  is a circuit schematic diagram of processing means with components of the touch probe of  FIG. 1 , 
         FIG. 4  is a circuit schematic diagram of a processing system of a signal in a probe according to the invention, 
         FIG. 5  is a circuit schematic diagram of a different processing system in a probe according to the invention, 
         FIG. 6  is a graphic representation of the trend of electric signals relative to the processing system of  FIG. 5 , 
         FIG. 7  is a simplified, longitudinal section of a touch probe, with some parts shown in view, according to a different embodiment of the invention, 
         FIG. 8  is a simplified, longitudinal section of a touch probe, with some parts shown in view, according to a third, different embodiment of the invention, 
         FIG. 9  is a simplified, perspective, longitudinal section of a touch probe according to a further embodiment of the invention, 
         FIG. 10  is a circuit schematic diagram of a processing aspect of the signal of a probe according to the invention, and 
         FIG. 11  is a circuit schematic diagram of a known solution concerning the signal processing of a piezoelectric transducer. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The section of  FIG. 1  shows, in a very simplified way, a touch probe  1 , including a support frame  2  with a protective casing  3 —substantially cylindrical shaped and composed by, for example, different portions assembled to each other—which defines a longitudinal geometric axis, and a movable armset  5 , partly housed in the protective casing  3  and properly supported. The support frame  2  includes a substantially cylindrical, internal component  6 , which is fixed to the protective casing  3  by means of a closing plate  8 . More specifically, the closing plate  8  is screwed to the protective casing  3  and axially urges and keeps the internal component in place  6 . The internal component  6  has a bearing and locating area  7  for the movable armset  5  and the latter includes an arm  9  carrying a feeler  11  adapted to touch a part to be checked, which is shown in simplified way in  FIG. 1  and marked with the reference number  13 . A thrust device with a compression spring  15  is arranged between the support frame  2  and the movable armset  5  and urges the latter against the bearing and locating area  7 . A constraint and positioning system  17  between the movable armset  5  and the support frame  2  is placed at the bearing and locating area  7  and includes an isostatic rest system with three support areas, each with a couple of balls—just one ball being visible in the figure and marked with the reference number  19 —fixed to the support frame  2 , more specifically to the internal component  6 , and defining a V-shaped seat—marked in the figure with the same reference number  19 —and a radial element  21 , for example cylindrical shaped, defined in the movable armset  5 . In the inoperative condition of the probe shown in  FIG. 1 , that is when there is no contact between the feeler  11  and the part to be checked  13 , the three radial elements  21  are arranged, thanks to the action of the spring  15 , in the V-shaped seats defined by the balls  19 , and thus define the position of the movable armset  5  with respect to the support frame  2  in a unique way. 
     According to alternative solutions, the V-shaped seats and the radial elements can be positioned in the movable armset  5  and the support frame  2 , respectively, or the constraint and positioning system can be made in a different way, for example with a different isostatic rest system such as the one known as Kelvin coupling. 
     A detection device  23  includes at least one laminar piezoelectric element or transducer  25  that is arranged substantially on a plane perpendicular to the longitudinal geometric axis of the protective casing  3 , and connected to the support frame  2  at the bearing and locating area  7 . In particular, according to the preferred embodiment shown in  FIGS. 2A, 2B and 2C , the detection device  23  includes two annular circuit boards  24  and  26 , made of electrically insulating material such as fibreglass, between which the laminar piezoelectric transducer is tightly held and positioned. One side of each of the circuit boards  24  and  26 , more specifically the side contacting the laminar piezoelectric transducer  25 , has a conductive surface (for example made of copper or gold, according to known techniques) divided into three, substantially distinct, angular sectors  24 ′,  24 ″,  24 ″′ and  26 ′,  26 ″,  26 ″′, respectively. In this way, there are three sensitive sectors  23 ′,  23 ″,  23 ″′ of the detection device  23  that is fixed to an area of the support frame  2  in such a way that each of the sensitive sectors  23 ′,  23 ″,  23 ″′ is substantially centered with respect to one of the three support areas, i.e. with respect to one of the couples of balls  19  in the example of  FIG. 1 . 
     The present invention can include sensitive sectors  23 ′,  23 ″,  23 ″′ that are different in number and/or have a different construction of the circuit boards  24 ,  26  and/or of the laminar piezoelectric transducer  25 . For example the latter can be differently shaped or obtained from particular machining processes, per se known. 
     The detection device  23  is pressure locked between the internal component  6  and another area of the support frame  2  and at least one element  28  made of thermal insulating material is placed therebetween. In particular, the internal component  6 , urged by the closing plate  8 , urges in turn the detection device  23  on the side with the circuit board  24  against the element  28  made of thermal insulating material, and thus locks the detection device  23  to the support frame  2 . The element  28 , made for example of fibreglass, implements a thermal insulation system between the protective casing  3  and the laminar piezoelectric transducer  25  at the bearing and locating area  7 . The element  28  can be annular shaped or divided into distinct parts that are arranged at different areas of the circuit board  26 . 
     According to alternative methods for connecting the detection device  23  to the support frame, the circuit board  24  is glued to the internal component  6  at the bearing and locating area  7 , and/or the circuit board  26  is glued to the element  28  made of thermal insulating material. Processing means in signal conditioning electronics  30  include a microprocessor  37  ( FIG. 3 ) and are arranged in the protective casing  3  and connected to the circuit boards  24  and  26  of the detection device  23  by means of electric leads that are schematically shown in  FIG. 3  with couples of wires  32 , each connected to a terminal in the circuit board  24  and to a terminal in the circuit board  26 , both terminals being arranged at one of the three sensitive sectors  23 ′, 23 ″, 23 ″′. 
     The laminar piezoelectric transducer  25  is obtained by a sheet made of piezoelectric plastic material, preferably made of polymeric material such as polyvinylidene fluoride or PVDF. The characteristics of this material are such that it features a very high sensitivity in generating electric charges in consequence of compression or decompression phenomena to which the material itself is subject. Moreover, PVDF is a material featuring a substantially unlimited compressibility, thus mechanical restraints are not necessary. The circuit boards  24  and  26  detect and transmit such electric charges to the signal conditioning electronics  30  wherein, as schematically shown in  FIG. 3 , the signals in output from respective differential charge amplifiers  33  are converted into a digital number (as schematically shown in  FIG. 3  by means of the functional blocks  31 ) and the relative absolute values are add up in an adder implemented by a software module  35  for obtaining a single force signal M, whose value is indicative of the overall force variation applied to the laminar piezoelectric transducer  25 . The microprocessor  37  receives as input the force signal M and, depending on it, emits a touch signal T indicative of the touch occurred, which is transmitted in a known way, by means of electric wires or contactless systems, to an external interface unit not shown in the figures. 
     In the embodiment according to  FIG. 1 , the material used for the protective casing  3  is an iron-nickel alloy featuring extremely low coefficient of thermal expansion, typically about 1 ppm/C.° such as Invar® (registered trademark in USA and other countries). 
     The probe  1  operates as follows. 
     Further to mutual movements between the probe  1  and the part to be checked  13 , for example along a transversal direction marked with the double arrow X in  FIG. 1 , the feeler  11  contacts the part  13 , and a force is applied on the movable armset  5  and transmitted to the constraint and positioning system  17 . At one or two out of the three support areas, the radial element  21  applies a thrust to the respective couple of balls  19  and, at the same time, the thrust applied by the spring  15  is released at the other support area(-s). The thrust action and the release action are transmitted to the detection device  23 , more specifically to the laminar piezoelectric transducer  25  that undergoes a compression or a decompression at the sensitive sectors  23 ′, 23 ″, 23 ″′, and thus generates electric charges that are detected by means of the terminals placed in the circuit boards  24  and  26 , as hereinbefore briefly disclosed. The consequent force signal M, indicative of the force variation applied to the laminar piezoelectric transducer  25 , is processed in the microprocessor  37 , and in particular it is detected when a determined threshold value is exceeded and, immediately or after a certain delay, the touch signal T is emitted. 
     Similar effects on the laminar piezoelectric transducer  25  are caused by mutual contacts between the feeler  11  and the part to be checked  13  along directions differing from X, for example a contact occurring along the direction Z, in consequence of which, at least in theory, the thrust is released at the three support areas and the laminar piezoelectric transducer  25  thus undergoes a decompression at the sensitive sectors  23 ′, 23 ″, 23 ″′. 
       FIG. 4  schematically shows a possible expedient in the signal conditioning electronics  30  to prevent thermal variations undergone by the probe  1  from negatively affecting the performance of the probe  1  and thus providing false signalling. Indeed, the material of which the laminar piezoelectric transducer  25  is made can suffer such thermal variations and cause the generation of consequent, unwanted electric charges. In the solution with a processing system  40  according to the simple diagram of  FIG. 4 , the signal St provided by the laminar piezoelectric transducer, more specifically generated at each one of the sensitive sectors  23 ′, 23 ″ and  23 ″′ and digitized is sent to the entry of a filtering component, more specifically a low-pass filter  34 , implemented in this case as a software module, featuring for example a cut-off frequency of about 1 Hz, and also to a numeric subtracting unit  36 , implemented as a software module as well. The unit  36  outputs a signal Sd representing the difference, in absolute value, between such signal St and a filtered signal Sf in output from the low-pass filter  34 . In this way, when the signal St undergoes gradual variations, like the ones typically caused by changes in temperature, the signal Sf undergoes the same variations and there is no signalling because the signal Sd in output from the unit  36  is substantially equal to zero. Whereas, when the variation of the signal St is sudden, as it happens in consequence of the contact between the feeler  11  and the part to be checked  13 , a corresponding variation does not immediately occur in the signal Sf that remains unchanged and, as a consequence, the signal Sd in output from the unit  36  undergoes a sudden variation. Therefore, only in this case a force signal M different from zero will reach the microprocessor  37 , enabling a correct signalling and the consequent generation of the touch signal T. 
     It should be noted that the thermal insulation system with the element  28  made of thermal insulating material, for example fibreglass, prevent sudden environmental, thermal variations, such as those caused by coolant spurts affecting the protective casing  3 , from being transmitted to the laminar piezoelectric transducer  25  in a sudden way. This ensures that the protective action of the expedient of  FIG. 4  is proper and reliable. 
     Moreover, the use of material featuring low coefficient of thermal expansion, such as Invar®, for the protective casing  3  prevents environmental thermal variations from causing changes in the dimensions of the protective casing  3 . Indeed such changes in dimensions could apply voltage to the laminar piezoelectric transducer  25  and cause further false signalling. 
     As an alternative to the expedient of  FIG. 4  (in which the circuit diagram shows functional elements that are preferably implemented via software) there are other possible protection systems against environmental noises, more specifically noises caused by thermal variations. 
     One of such systems that can be included in the signal conditioning electronics  30  is schematically shown in  FIGS. 5 and 6 . A processing system  55  for processing the signal provided by the laminar piezoelectric transducer  25  includes devices, preferably implemented via software, that enable to gradually change in time a threshold value S, with which the value of the force signal M is compared in the microprocessor  37 , when thermal variations giving rise to unwanted electric charges in the laminar piezoelectric transducer  25  occur. In the diagram of  FIG. 5 , an increment adder represented by the blocks  50  and  51  provides the sum of a plurality of values detected for the force signal M, in particular of the most recent n values detected, according to a “FIFO” (First-IN First-OUT) sequence. Processing units  52  and  54  provide the average of such n detected values and the threshold value S that is processed and modified, respectively, depending on the calculated average. A comparator  56  compares the current force signal M with such threshold value S. According to the processing system  55 , an average of the values of the force signal M is carried out as these values are collected, for example the last 8192 values (that is n=8192, considering a typical sampling frequency of 100 khz) are averaged, and the threshold value S is dynamically varied depending on this average. As a consequence, slow, gradual variations of the force signal M, like those caused by thermal drift phenomena or unwanted capacitive effects, correspond to variations of the threshold value S too, which prevent the first one (M) from exceeding the second one (S). As hereinbefore described with reference to figure and to the signal St, in consequence of the contact between the feeler  11  and the part to be checked  13 , there is a sudden variation of the signals in output from the laminar piezoelectric transducer  25  and thus a variation of the force signal M that is as sudden as the one of the output signals. In this case, the threshold value S undergoes small and slow variations in time and thus it is exceeded by the force signal M. As a consequence, the microprocessor  37  generates the touch signal T. The diagrams of  FIG. 6  schematically show how the threshold value S dynamically varies in time owing to slow, gradual variations of the force signal M caused by an increase in temperature. At an instant t o , further to the contact between the feeler  11  and the part to be checked  13 , there is a sudden variation of the force signal M that exceeds the threshold value S and causes the generation of the touch signal T. 
     Even the solution to which  FIGS. 5 and 6  refer properly operates in case of gradual thermal variations, and the substantial immunity to sudden changes in temperature is ensured by the thermal insulation system including the element  28  (integral or divided in different parts) made of thermal insulating material that is arranged at the bearing and locating area. 
     An alternative solution of a probe  1 ′ according to the invention is shown in  FIG. 7 . The probe  1 ′ of  FIG. 7  is substantially identical to the probe  1  of  FIG. 1  and thus the description of like parts, marked with the same reference numbers, is not repeated. The only difference is that there is a second detection device, which is per se known and similar to what has been described in the first part of the description with reference to the patent No. U.S. Pat. No. 4,972,594. In particular, an electric circuit is positioned at the constraint and positioning system  17 , such electric circuit being closed when the three radial elements  21  are housed in the V-shaped seats and contact the respective couples of balls  19 . The electric circuit of the second detection device includes electric leads that connect the electric circuit to the signal conditioning electronics  30 , such leads being shown in as simplified way in  FIG. 7  with a couple of wires  60 . The signal of the second detection device is handled in the microprocessor  37  in order to confirm, after a preset time interval (for example a tenth of milliseconds) that the force signal M generated by the laminar piezoelectric transducer  25  actually corresponds to a contact occurred between the feeler  11  and the part to be checked  13 . Moreover, as after a pulse signal is emitted it is no longer possible to receive from the laminar piezoelectric transducer  25  information about the system status (in particular to know whether the movable armset  5  is deflected with respect to the support frame  2  or not), such information can be obtained from the second detection device. 
     Therefore, while in the probe  1  of  FIG. 1  the constraint and positioning system  17  only serves as so-called “overstroke”, i.e. the possibility for the movable armset  5 , integral with the feeler  11 , of displacing with respect to the support frame for avoiding excessive stresses and breakages consequent to the contact between the probe  1  and the part to be checked  13 , in the probe  1 ′ of  FIG. 7  the displacements of the movable armset  5  with respect to the support frame  2  are connected also to the operation of the second detection device, whose signalling is used by the microprocessor  37  for the proper emission of the touch signal T. 
       FIG. 8  shows a probe  1 ″ according to a different embodiment of the invention. The probe  1 ″ includes many of the components of the probes  1  and  1 ′, thus the description of like components, marked with the same reference numbers, is not repeated.  FIG. 8  shows also a protective cap  4 , not sectioned, which is connected to the support frame  2  and includes, among other things, sealing gaskets, such cap being omitted in the simplified sections of  FIGS. 1 and 7 . The main difference of the probe  1 ″ from the probes  1  and  1 ′ is the presence of a further component of the thermal insulation system, i.e. an insulating hollow body  64  that has one end  65  connected to the protective casing  3 . The insulating hollow body  64  includes a substantially cylindrical recess housing the protective casing  3  so as to define, at least in one longitudinal length including the section in which the laminar piezoelectric transducer  25  lies, a thermal insulating hollow space  66  between the protective casing  3  and the insulating hollow body  64 . In the example of  FIG. 8 , the thermal insulating hollow space  66  extends even transversally at the end opposite to the locking end  65 . An air cushion that forms in the thermal insulating hollow space  66  (for example with a thickness of a few tenths of mm) enables the thermal insulation of the probe where the detecting device  23  is placed, and prevents sudden environmental thermal variations, such as those caused by coolant spurts, from being suddenly transmitted to the laminar piezoelectric transducer  25 . In this way, possible thermal variations reach the laminar piezoelectric transducer  25  in a gradual way, and the negative effects can be easily avoided thanks to systems such as those illustrated with reference to  FIG. 4  and to  FIGS. 5 and 6 . 
     The thermal insulation system including the insulating hollow body  64  might substitute the element  28  made of fibreglass included in the embodiments previously described, but generally, in the embodiment of the present invention as shown in  FIG. 8 , both the thermal insulation systems are provided. 
     The insulating hollow body  64  can be made of various materials, for example steel or an iron-nickel alloy featuring low coefficient of thermal expansion, such as Invar®. 
     Another embodiment among the possible different implementations of a touch probe according to the invention is shown in  FIG. 9 . The probe  1 ″′ has features that are mostly similar to those of the probes  1 ,  1 ′ and  1 ″ of the  FIGS. 1, 7 and 8  and are not here disclosed in detail. 
     The probe  1 ″′ includes, like the probe  1 ′ of  FIG. 7 , a second detection device implemented, in this case, with a microswitch  74  that is shown not sectioned in  FIG. 9  and is connected to the signal conditioning electronics  30  by means of leads sketched and marked with the reference number  75 . The microswitch  74 , per se known, includes a transmission shaft  76 , axially movable, which is coupled to the movable armset  5  for transmitting displacements of the latter and causing the microswitch  74  to consequently emit a signal. Such signal of the second detection device, in this case consisting in the microswitch  74 , is used by the microprocessor  37  as previously described with reference to the probe  1 ′ of  FIG. 7 , to properly emit the touch signal T. 
     The microswitch  74  can be provided in a touch probe with characteristics differing from the probes shown in  FIG. 9 . The probe can include, for example, a thermal insulation system only with the element  28  made of thermal insulating material (like the probes  1  and  1 ′ of  FIGS. 1 and 7 ).  FIG. 10  refers to the particular, advantageous implementation shown in  FIG. 3 , where both the circuit boards  24 ,  26  are electrically connected to each differential charge amplifier  33 . More specifically, each of the terminals in the circuit boards  24  and  26 , here marked with the reference numbers  24   N  and  26   N , placed at the ends of the piezoelectric element  25  at one of the sensitive sectors  23 ′,  23 ″ and  23 ″′, is connected to a charge amplifier unit  84  and  86  that generates a signal responsive to the electric charge Q (of the order of a few picocoulombs) provided at the respective terminal  24   N  and  26   N . The absolute value of the difference of the signals in output from the charge amplifier units  84  and  86 , provided for example by a differential amplifier  88 , represents the signal generated by the piezoelectric element  25  at one of the sensitive sectors  23 ′,  23 ″ and  23 ″ and is aimed to be digitalized and added to the other signals, according to the diagram of  FIG. 3 , after any further processing, such as the one of  FIG. 4 , which may be carried out. 
     With respect to a known embodiment, such as the one sketched in  FIG. 11 , wherein one of the terminals at the ends of the piezoelectric element is connected to a ground, the differential implementation of  FIG. 10 , according to the invention, is particularly advantageous as it enables to dramatically reduce possible electrical noises. 
     Indeed, in applications of the probe according to the invention—in one of the embodiments  1 , 1 ′, 1 ″, 1 ″′ here shown and in other possible embodiments—on machine tools, the ground is typically the casing of the machine tool itself that usually features a lot of electrical noise. When employing a known implementation such as that one of  FIG. 11 , electrical noises generated in consequence of the grounding can give rise to a critical application. A differential implementation according to the invention, such as the one schematically shown in  FIG. 10 , is substantially immune to this type of noises, as the obtained signal does not depend on the characteristics of the ground used. 
     Other embodiments for a touch probe according to the present invention can be provided, with alternative implementations of the known parts concerning the mechanical structure and/or the processing of the signals involved.