Patent Publication Number: US-2011077890-A1

Title: Method for thermally compensating a gaging device and thermally compensated gaging station

Description:
TECHNICAL FIELD 
     The present invention relates to a method for thermally compensating a gaging device, and to a thermally compensated gaging station. 
     BACKGROUND ART 
     The information provided by a gaging device such as a position sensor is affected, among other things, by the environmental temperature, since a temperature variation causes so-called thermal drifts caused by both unavoidable thermal deformations in the metal component parts of the position sensor, and unavoidable variations in the electrical resistance of the electric circuits of the position sensor. For attempting to render the sensor less sensitive to the temperature variations, the position sensor can be implemented with materials having limited thermal deformations and limited electrical resistance variations. However, it is not possible to obtain a gaging device which be totally insensitive to the effects of the temperature variations. 
     In the high accuracy gaging devices and sensors it is known to carry out a compensation of the reading provided by the sensor as a function of the environmental temperature. For example, US patent US5689447A1 discloses a gage head or position sensor of the LVDT type, i.e. including an “LVDT” (Linear Variable Differential Transformer”) inductive transducer, wherein there occurs a thermal compensation of the reading provided by the sensor which takes into consideration the influence of the environmental temperature. US patents US6844720B1 and US6931749B2 discloses further examples of thermal compensation of a position sensor of the LVDT type. 
     However, the known methods (for example of the same type as the one described in patent US5689447A1) for determining the value of the thermal compensation coefficient involve quite remarkable approximations, and thus they do not enable to achieve a very accurate compensation. As a consequence, the known methods can not be applied to gaging applications requiring an extremely high accuracy. 
     DISCLOSURE OF THE INVENTION 
     Object of the present invention is to provide a method for thermally compensating a gaging device and a thermally compensated gaging station, which method and station do not present the above described disadvantages and can be easily and cheaply implemented. 
     According to the present invention there are provided a method for thermally compensating a gaging device and a thermally compensated gaging station according to what is claimed in the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present 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 front view, with some parts removed for sake of clarity, of a calibration station for a thermally compensated position sensor; 
         FIG. 2  is a simplified side view, with some parts removed for sake of clarity, of the calibration station of  FIG. 1 ; 
         FIG. 3  is a graph showing the time variation of the temperature of a position sensor which is located in the calibration station of  FIG. 1  during a phase of determining the value of a thermal compensation coefficient, and 
         FIG. 4  is a three dimensional graph showing an example of the values taken by a thermal compensation coefficient. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     In  FIG. 1 , the reference number  1  indicates, on the whole, a gaging device, e.g. a position sensor including a linear transducer of the LVDT (Linear Variable Differential Transformer) type, for instance of the same type as the one described in US patent US6931749B1. The gaging device or position sensor  1  includes a stationary part  2  and a movable element, more specifically a slider  3 , which carries a feeler and is movable with respect to the stationary part. The transducer of the position sensor  1  includes windings and a movable core (per se known and thus not illustrated in the attached sheets of drawings), connected to the stationary part  2  and to the movable element or slider  3 , respectively, and is adapted for providing an alternating electrical signal which has a variable intensity voltage and depends on the position of the movable slider  3 . The windings of the transducer of the position sensor  1  are part of an electric circuit which is schematically shown in  FIG. 1  with the reference number  4 , is fed with an alternating electrical voltage, and has a variable inductance depending on the position of the movable slider  3 . 
     The position sensor  1  includes a coupling cable and an electrical connector  5 , which is employed for forming an electrical connection between the transducer and a gaging unit  6  being adapted to detect the reading provided by the transducer of the position sensor  1  in order to determine the exact position of the slider  3  of the position sensor  1 . The gaging device or position sensor  1  and the corresponding gaging unit  6 , taken as a whole, form a gaging station. 
     The electrical connector  5  also includes a digital memory  7 , which can be read by the gaging unit  6 . Preferably, the digital memory  7  is fixed to the connector  5  in a permanent way (that is, the former is integrated in the connector  5  in a non-separable way). The electrical connector  5  includes a pair of feed terminals for feeding the position sensor  1  with an alternating feed voltage, a pair of analogue terminals providing an alternating electrical signal which has a variable intensity voltage and depends on the position of the movable slider  3 , and a pair of digital terminals that can be used for reading the content of the digital memory  7 . Obviously, the three pairs of terminals can share a single earth terminal, and thus there can be just four different terminals. According to different embodiments herein not illustrated, the digital memory  7  can be permanently connected to the casing or to another part of the sensor  1 , and/or it can include a wireless communication device, based for example on the transponder technology, for enabling to communicate with the gaging unit  6 ; in this latter embodiment the digital terminals can be obviously omitted. 
     The gaging unit  6  is adapted for determining a value of a thermal compensation coefficient K as a function of both the current temperature T of the position sensor  1  and the reading X of the position sensor  1  (that is, of the position of the slider  3  of the position sensor  1 ). In order to perform a correct reading of the position of the slider  3  of the position sensor  1 , the gaging unit  6  detects the reading X of the position sensor  1 , detects the current temperature T of the position sensor  1 , determines a current value of the thermal compensation coefficient K and compensates the reading X of the position sensor  1  by applying the current value of the thermal compensation coefficient K. It is important to point out that the thermal compensation coefficient K can be of the additive type, which means that it can be algebraically added to the reading X of the position sensor  1 , or it can be of the multiplicative type, which means that the reading X of the position sensor  1  can be multiplied by it. 
     According to a preferred embodiment, the gaging unit  6  detects the current temperature T of the position sensor  1  as a function of the current electrical resistance of the electric circuit  4  of the transducer of the position sensor  1 ; in other words, the gaging unit  6  feeds the electric circuit  4  of the transducer of the position sensor  1  with a direct feed voltage which enables to determine a value of the current electrical resistance of the electric circuit  4  and does not affect in any way the alternating electrical signal which has a variable intensity voltage depending on the position of the movable slider  3 . 
     The digital memory  7  stores a table  9  of the compensation coefficient K including a plurality of triads of values, each of them providing the value of the compensation coefficient K at a determined value of the temperature T of the position sensor  1  and at a determined value of the reading X of the position sensor  1 . According to a possible embodiment, the table  9  of the compensation coefficient K includes twenty determined triads of values each triad indicating the value of the compensation coefficient K in correspondence of one out of four different values of temperature T of the position sensor  1  (typically 10° C., 20° C., 30° C., and 40° C.) and of one out of five different values of the reading X of the position sensor  1 . The five different values of the reading X of the position sensor  1  correspond to two end positions of the position sensor  1 , to a central position of the position sensor  1 , and to two intermediate positions of the position sensor  1 , each of the latter being comprised between the central position of the position sensor  1  and a respective end position of the position sensor  1 . 
     When the current temperature T of the position sensor  1  is comprised between two adjacent values in the table  9 , and/or the current reading X of the position sensor  1  is comprised between two adjacent values in the table  9 , a mathematical interpolation operation is carried out (for example using Lagrange polynomials) for calculating the value of the corresponding compensation coefficient K. 
     In the graph of  FIG. 4 , the triads of values of the table  9  correspond to the points of a surface S enabling to identify the compensation coefficient K to be used for thermally compensating a certain reading X of the position sensor  1  at a certain temperature T. 
     The table  9  of the compensation coefficient K can be generated for each position sensor  1 . In this way, the values of the compensation coefficients K included in the table  9  are more accurate, since they take into consideration all the specific features of the single position sensor  1 , but the downside is that it is necessary to undergo each position sensor  1  to a calibration operation. As an alternative, the table  9  of the compensation coefficient K can be generated for a certain family of position sensors  1 . In this way it is not necessary to undergo each position sensor  1  to a specific calibration operation, but the values of the compensation coefficients K included in the table  9  show average values of the specific family of position sensors  1  instead of the actual values of each position sensor  1 . 
     According to an equivalent embodiment, the digital memory  7  does not store the values of the single triads of values of the compensation coefficients K, but it stores values of parameters of a function (for example a polynomial function) which interpolates the triads of values of the compensation coefficients K. This function is adapted to provide the value of the compensation coefficient K as a function of both the value of the temperature T of the position sensor  1  and the reading X of the position sensor  1 . 
     A calibration operation for generating the table  9  of the compensation coefficient K is described herebelow. 
     For generating the table  9  of the compensation coefficient K, the position sensor  1  is located in a calibration station  10  which is housed inside a climatic chamber wherein the environmental temperature can be very accurately adjusted. The calibration station  10  includes a C-shaped locking device  11  comprising an upper element  12  to which the stationary part  2  of the position sensor  1  is fixed by means of screws  13 , and a lower element  14  cooperating with the slider  3  of the position sensor  1 . In particular, the lower element  14  includes a screw  15  which is screwed through a threaded through hole  16  and forms an abutment against which a free end of the slider  3  of the position sensor  1  leans. By screwing and unscrewing the screw  15  into the hole  16 , the axial position of the screw  15  varies, and thus the relative position between the slider  3  of the sensor position  1  and the stationary part  2  varies, too. 
     It should be noted that the screw  15  enables to lock the position sensor  1  (that is, the slider  3  of the position sensor  1 ) at a desired calibrating position. 
     Once the position sensor  1  has been located in the calibration station  10 , at each predetermined calibration position the readings X of the position sensor  1  that will be inserted in the triads of values of the table  9  of the compensation coefficient K are detected. More specifically, the position sensor  1  (that is, the slider  3  of the position sensor  1 ) is located and locked at each predetermined calibration position which is identified by means of the reading X of the position sensor  1 . It is not necessary to exactly locate and lock the position sensor  1  at each predetermined calibration position (this would be a very difficult operation since an accuracy in the order of micron is required), but it is sufficient to locate and lock the position sensor  1  in a neighborhood of the predetermined calibration position. For this reason, once the position sensor  1  has been located and locked at a predetermined calibration position, the correspondent reading X of the position sensor  1  is subsequently detected at a known and predetermined reference temperature T ref —as described hereinafter in more detail—for determining the actual calibration position (which is comprised in a neighborhood of the predetermined calibration position, but which exactly corresponds to the predetermined calibration position just in rare and accidental cases). 
     Once the position sensor  1  (that is the slider  3  of the position sensor  1 ) is located and locked at one of the predetermined calibration positions, first of all the corresponding reading X of the position sensor  1  is detected at the temperature T ref  of the position sensor  1 ; in other words, the temperature T of the position sensor  1  (that is the internal temperature of the climatic chamber housing the calibration station  10 ) is adjusted so as to be equal to the reference temperature T ref , as already stated hereinbefore, and when the current temperature T of the position sensor  1  is equal to the reference temperature T ref  and is in steady state, there is detected the value of the reading X of the position sensor  1  at the reference temperature T ref . Subsequently, the temperature T of the position sensor  1  (which means the internal temperature of the climatic chamber housing the calibration station  10 ) is varied step by step so that the current temperature T of the position sensor  1  takes all the preset values (typically 10° C., 20° C., 30° C., and 40° C.) in steady state.  FIG. 3  is a graph showing an example of the step-by-step time variation of the current temperature of the position sensor  1  located in the calibration station  10 . Preferably each value of the current temperature T of the position sensor  1  is maintained for three hours so that all the components of the position sensor  1  can be thermally settled down. At each step of the current temperature T of the position sensor  1  and when the current temperature T of the position sensor  1  is in steady state, the value of the reading X of the position sensor  1  is detected, and by comparing the latter with the reading X of the position sensor  1  at the reference temperature T ref , the value of the compensation coefficient K is determined. In this way there are determined the three values of the temperature T, the reading X of the position sensor  1  and the compensation coefficient K for generating a corresponding triad of values. More specifically, the triad of values is determined at the end of the step of the current temperature T of the position sensor  1 , which means when the thermal settling down of all the components of the position sensor  1  has occurred. According to a preferred embodiment of the present invention, the coefficient K is of the additive type, it has a mathematical sign (which means it can be a positive or a negative value) and it is calculated as the difference between the reading X of the position sensor  1  at the current temperature and the reading X of the position sensor  1  at the reference temperature T ref . 
     Once the step-by-step time variation of the current temperature of the sensor position  1  has ended, the position sensor  1  (that is, the slider  3  of the position sensor  1 ) is located at a new predetermined calibration position that is detected by a new reading X of the position sensor  1  at the reference temperature T ref  until all the predetermined calibration positions are completed. According to a preferred embodiment which is illustrated in detail in the graph of  FIG. 3 , once the position sensor  1  (that is, the slider  3  of the position sensor  1 ) has been located in a calibration position, the position sensor  1  is subjected to a thermal settling cycle so that the temperature T of the position sensor  1  varies between the preset minimal value and the preset maximal value (which means between 10° C. and 40° C.). The object of said thermal settling cycle is to enable a settling of the mechanical hysteresis of all the components of the position sensor  1 . Moreover, according to a preferred embodiment, the current temperature T of the position sensor  1  is detected as a function of the current electrical resistance of the electric circuit  4  of the transducer of the position sensor  1 . More specifically, the electric circuit  4  of the transducer of the position sensor  1  is fed with a continuous feed voltage which enables to determine a value of the current electrical resistance of a component of the electric circuit  4 . This does not affect in any way the alternating electrical signal the intensity voltage of which can vary depending on the position of the movable slider  3 . It should be noted that the current temperature T of the position sensor  1  is detected as a function of the current electrical resistance of the electric circuit  4  of the transducer of the position sensor  1  during both the calibration operation for generating the table  9  of the compensation coefficient K and the actual working of the position sensor  1 . In this way, by using the same method and the same components for detecting the current temperature T of the position sensor  1 , possible systematic errors introduced during the detection of the current temperature T of the position sensor  1  similarly repeat during both the generation of the compensation coefficients K and the usage of the compensation coefficient K, and thus they do not affect the proper thermal compensation proceeding. 
     According to a different embodiment, the current temperature T of the position sensor  1  can be detected by means of a temperature sensor (for instance a thermistor or a thermocouple) which is separate and independent from the electric circuit  4 , and can be fixed to the stationary part  2  of the position sensor  1 . 
     In the above described example, the gaging device is a position sensor  1  having a feeler carried by an axially movable slider  3  and including an inductive linear transducer of the LVDT type. According to possible alternative embodiments of the invention, the gaging device can have different mechanical features and/or can include an inductive linear transducer of a different kind (for example a “Half Bridge” or HBT transducer) or a non-inductive linear transducer. As a possible mechanical alternative, a feeler can be carried by a movable element adapted to pivot about a fulcrum with respect to a stationary part, substantially as shown in the gaging head of the above mentioned patent US5689447. 
     The above described compensation method provides many advantages since it can be easily and cheaply implemented, and, above all, it enables to obtain a very accurate compensation which can be also applied to gaging applications requiring an extremely high accuracy.