Patent Publication Number: US-2015061650-A1

Title: Method and arrangement and sensor for determing the postion of a component

Description:
The invention relates to a method according to the preamble of claim  1 , an arrangement for determining position according to the preamble of claim  6 , and a sensor according to the preamble of claim  16 . 
     U.S. Pat. No. 7,652,469 B2 describes an inductive position sensor having a spatially periodic scale with a series of conducting or permeable features at distance T, and a reading head with drive windings and sense windings, arranged facing the scale with a spatial period 2T along the scale. The windings are each divided into two identical winding elements having the same relative arrangement within two identical winding element patterns having a center-to-center distance along the scale of NT+T/2, wherein the windings are connected to one another in such a way that the winding element polarities in each winding are either opposed for drive windings or the same for sense windings. 
     U.S. Pat. No. 7,667,455 B2 relates to an annular magnetic encoder comprising a plurality of south magnetic poles and north magnetic poles, arranged alternately in an arrangement pattern. This case likewise utilizes a differential measuring method, in which a differential transformer is used as the magnetic flux sensor. 
     U.S. Pat. No. 8,004,277 B2 relates to an angular position sensor for determining angular position, comprising a shaft having a threaded portion and a structure for engaging an external arrangement. The shaft comprises a first permanent magnet. A nut is threaded onto the threaded portion and is formed from a first magnetic permeable material or comprises a second permanent magnet. At least one constraint is coupled to the nut for preventing rotational movement of the screw while allowing linear motion of the screw while the shaft rotates. A first magnetic sensor is positioned along a length of the threaded portion, for the purpose of measuring a linear position of the nut. A second magnetic sensor is provided for measuring an angular position of the shaft. Signal processing circuitry is coupled to the first magnetic sensor and the second magnetic sensor in order to determine parameters relating to an angular position of the rotating part. 
     U.S. Pat. No. 6,011,389 A relates to a current inducing position transducer having a low-power electronic circuit. The described sensor arrangement is embodied as a differential transformer. At least one transformer winding could also be designed as a primary winding and another winding as a secondary winding. 
     From DE 102 48 142 B3 a method for producing a magnetically scannable coding in a metallic component and a metallic component having a corresponding magnetically scannable coding is known. The coding comprises code elements which are produced in that structural changes that persist in the component are generated as having a magnetic conductivity different from that of the untreated material of the component. Said document also describes how the code elements are read out. In this process, the component is moved relative to the magnetic field sensor, which detects the different magnetic properties and/or magnetic flux changes between the code elements and the material encompassing said elements. In this manner, the code elements can be detected for the purpose of identifying or determining the position of the component. 
     The known sensor is a magnetic field sensor having a single coil, which can result in inaccuracies in practical use. 
     Proceeding from the above, the object of the present invention is to develop a method and an arrangement of the type described in the introductory part such that the accuracy of position determination is improved. 
     The object is attained according to the invention, i.e., in that the method is executed as a differential measurement method in which a differential transformer having a primary winding and two secondary windings is used as the magnetic flux sensor. The primary winding is excited with AC voltage, so that a differential signal can be picked up as an output signal at the secondary windings. The magnetically scannable structures, such as measurement strips, which are introduced into the metallic surface of the component by structural changes, such as selective hardening, can be detected by various physical methods. In particular, changes in magnetic permeability, such as eddy losses or other processes, can be used to detect selectively introduced structural changes. A differential measuring method according to the invention has proven particularly suitable for this purpose, because it enables continuous measurement and allows interference effects to be eliminated. 
     To achieve particularly high accuracy, magnetically scannable measurement strips, preferably extending as components of a scale, arranged equidistant along the longitudinal axis of the first component and transversely to the longitudinal axis, are used as the first magnetically scannable structure, and a continuous sine signal or cosine signal is generated during a relative movement of the measurement strip. 
     The primary winding of the differential transformer is excited with AC voltage and the secondary windings are preferably interconnected in phase. As a result, all in-phase signals are eliminated. Therefore, only differential signals occur as output signals. When a sensor head of this configuration moves along a scale of equidistant measurement strips, the output signal is continuously sinusoidal and highly stable, allowing very high resolution to be achieved. 
     According to a preferred procedure, it is provided that the position of the first component in relation to the second component is determined absolutely, wherein a second magnetically scannable structure arranged on the surface of the first component is detected by means of at least one second magnetic flux sensor connected to the second component, and that two magnetically scannable measurement strips, extending along the longitudinal axis in a V-shape, symmetrically to the longitudinal axis of the component, are used as the second magnetically scannable structure. 
     If a blank space and a measurement strip of the above-mentioned scale are viewed as sectors, for example, a so-called sector pointer is proposed for measuring absolute value. Said sector pointer is implemented according to the invention by the second measurement strip arrangement in the form of a linear measurement system, which requires substantially lower resolution as compared with the first scale. 
     The second measurement strip arrangement can be scanned using the same differential transformer as the magnetic flux sensor, specifically for long distances, but with lower resolution. The measurement strips are preferably embodied as selectively hardened strips on the metallic surface of the component, extending at an angle in the longitudinal direction. 
     A further preferred procedure is characterized in that, when the component moves linearly, the measurement strips are moved transversely to a sensor surface of the second magnetic flux sensor along the sensor surface, so that a continuous output signal that is dependent on the position of the first component is generated in the second magnetic flux sensor. Thus the absolute position of the first component can be determined from the signal sequence of the first magnetic flux sensor and the continuous output signal of the second magnetic flux sensor. To further improve resolution, it is proposed that at least two of the first magnetic flux sensors are arranged spatially offset from one another such that, when the component moves linearly, a first magnetic flux sensor at the front generates a sine signal and a first magnetic flux sensor at the rear generates a cosine signal. 
     The invention further relates to an arrangement for determining a position of a first component, such as a piston rod, in relation to a second component, such as a hydraulic or pneumatic cylinder, said arrangement comprising at least one first magnetic flux sensor connected to the second component for detecting a first magnetically scannable structure, such as measurement strips, formed on a surface of the first component, during a relative movement between the first component and the magnetic flux sensor. This arrangement is characterized in that the at least one first magnetic flux sensor is embodied as a differential transformer. 
     The differential transformer comprises a primary winding and two secondary windings, wherein the primary winding is connected to an alternating current generator, and wherein the two secondary windings are preferably interconnected in phase and connected to an evaluation unit. 
     The system for measuring the absolute position of the component preferably has at least one second magnetic flux sensor, which detects a second magnetically scannable measurement strip structure extending along a longitudinal axis of the component, wherein the second magnetically scannable measurement strip arrangement comprises a V-shaped measurement strip extending along the longitudinal axis, which traverses a sensor surface of the second magnetic flux sensor during linear movement of the component. 
     A preferred embodiment of the arrangement is characterized in that primary and secondary windings of the differential transformer are embodied as inductors arranged within a single plane, and positioned adjacent to one another on a substrate so as to form a sensor surface, wherein the magnetic axes thereof extend parallel to one another. In said embodiment, the inductor is designed as an SMD (surface mounted device) inductor or miniature inductor. 
     According to a further preferred embodiment, it is provided that the arrangement is a hydraulic, electric, pneumatic or other drive, wherein the first component is embodied as a piston rod and wherein the second component is embodied as a hydraulic, electric, pneumatic or other cylinder within which the piston rod is displaceable longitudinally. 
     In this embodiment, the magnetic flux sensor can be is arranged on an inner edge of a sensor holder which is arranged on the hydraulic or pneumatic cylinder, bordering the surface of the piston rod for the purpose of detecting the arrangement of the measurement strips. The sensor holder is preferably embodied as a sensor ring that is connected to the end face of the cylinder. 
     For determining absolute position, it is provided that the first and second magnetically scannable measurement strip arrangements are formed on diametrically opposite surfaces of the piston rod, and that the first and second magnetic flux sensors are arranged at diametrically opposite positions on the sensor holder that encompasses the end face, wherein each of the first and second magnetic flux sensors is arranged in a recess in an inner surface of the sensor holder. 
     Even higher resolution is achieved by providing a plurality of first or second magnetic flux sensors, which are offset spatially such that, when the first component moves linearly, a sine signal and a cosine signal are generated as output signals. 
     The invention further relates to a sensor for detecting a magnetically scannable measurement strip structure, formed on a metallic surface of a component, during a relative movement between the sensor and the component, wherein the sensor comprises at least one transformer winding. The sensor is preferably embodied as a differential transformer. Said transformer comprises at least one transformer winding as a primary winding and two additional windings as secondary windings, wherein the primary and secondary windings can be embodied as SMD inductors or miniature inductors, and are arranged magnetically coupled and flat on a substrate. 
     Each of the SMD inductors preferably has a magnetic axis, and the inductors are arranged adjacent to one another such that the magnetic axes extend parallel to one another. 
    
    
     
       Additional details, advantages and features of the invention are described not only in the claims, in the features specified therein—alone and/or in combination,—but also in the following description of one of the preferred embodiment examples depicted in the set of drawings. 
       The drawings show: 
         FIG. 1   a  a plan view of a hydraulic drive with hydraulic cylinder and piston rod, 
         FIG. 1   b  a bottom view of the hydraulic drive with hydraulic cylinder and piston rod, 
         FIG. 2  a wiring diagram of a magnetic flux sensor configured as a differential transformer, 
         FIG. 3  a schematic illustration of a sensor head of the magnetic flux sensor in relation to a magnetically scannable measurement strip of the piston rod, 
         FIG. 4  a schematic illustration of a measurement strip that extends in the longitudinal direction of the piston rod and a sensor head in different positions, 
         FIG. 5   a  a front view of a sensor ring for holding the magnetic flux sensors and 
         FIG. 5   b  a sectional illustration of the sensor ring. 
     
    
    
       FIG. 1   a  shows a plan view of a hydraulic drive  10 , comprising a hydraulic cylinder  12  in which a piston rod  14  is arranged so as to be movable along a longitudinal axis  16 . A first magnetically scannable structure  20  in the form of equidistant measurement strips  34  is positioned on a metallic surface  18  of the piston rod  14 . These strips are detected by means of a first magnetic flux sensor  22 , which is arranged stationarily on the hydraulic cylinder  12 , preferably in an end-face sensor ring  24 . 
     For determining absolute position, a second magnetically scannable structure  28  comprising first and second measurement strips  30  extending preferably in a V-shape along the longitudinal axis  16  is provided on a bottom side  26  of the cylindrical piston rod  14 . The measurement strips  30  extend at an angle α relative to the longitudinal axis  16 , with a measuring within a range of 0.2°≦α≦20°, preferably with α=1° to 2°. 
     The first magnetically scannable structure  34 , which comprises individual measurement strips  20  arranged equidistant from one another, is produced by thermal structural change, e.g. by the selective hardening of the surface  18  of the piston rod  14 , which is made of steel. The structural change can be generated by selective laser hardening or similar methods, such as electron beam hardening or selective soft annealing. Another option consists in introducing microgrooves into the surface of the piston rod  14  or by other structure modifying measures, and filling said microgrooves with a material, the permeability and/or magnetic property of which is different from that of the material of the surface. Once the measurement strip  34 ,  30  has been introduced into the piston material, the surface  18 ,  26  of the piston rod  14  is ground and hard-faced, e.g. with chromium or nickel-chromium. 
     The measurement strips  30  of the second structure  28  are detected by means of a second magnetic flux sensor  32 . 
     According to the invention, the magnetically scannable structures  34 ,  30  are detected by means of a differential measuring method. This has the advantage of increasing accuracy and eliminating interference effects. 
     The magnetic flux sensor  22  or  32  for implementing the method is represented purely schematically in  FIG. 2 . According to the invention, the magnetic flux sensor  22 ,  32  is embodied as a differential transformer  34 . The differential transformer  34  comprises a primary winding  36  which is connected to an AC voltage generator  38 , and a sine signal is preferably excited with AC voltage. 
     The differential transformer  34  further comprises outer secondary windings  40 ,  42 , which are magnetically coupled to the primary winding  26 , and which are interconnected in phase via a connection  44 . Outputs  46 ,  48  of the respective secondary windings  40 ,  42  are connected to an evaluation circuit  50  such as an evaluation amplifier. 
       FIG. 3  shows a schematic plan view of a sensor head  52  of differential transformer  36 ,  40 ,  42  in relation to measurement strips  34  according to  FIG. 1   a.  Measurement strips  34  are arranged at equidistant spacing A in the longitudinal direction to form a scale, wherein spacing A is preferably within a range of 1 mm≦A≦20 mm, preferably with A=5 mm. The width B of measurement strips  34  is within a range of 0.5 mm≦B≦10 mm, preferably with B=5 mm. 
     In the embodiment example represented here, primary winding  36  and secondary windings  40 ,  42  are embodied as SMD (surface mounted device) inductors. Each of SMD inductors  36 ,  40 ,  42  comprises a magnetic core  54 ,  56 ,  58 , with each core bearing a coil  60 ,  62 ,  64 . Said coils are arranged on a support within a single plane. 
     Magnetic cores  54 ,  56 ,  58  are arranged with their magnetic axes parallel to one another and close one behind the other in the direction of movement of the piston rod  14 , wherein transformer cores  54 ,  56 ,  58  are aligned parallel to measurement strips  34 . 
     It is further provided that the width C of SMD inductors  36 ,  40 ,  42  is within a range of 0.5 mm≦C≦10 mm. Moreover, the length D of the SMD inductors is within a range of 1 mm≦D≦20 mm. 
     As described above, differential transformer  34  comprises primary winding  36  and secondary windings  40 ,  42 , wherein the latter are connected in phase opposition or in phase, depending on the evaluation method that is applied (differential or non-differential). The steady components of the voltages are thereby eliminated at connections  46 ,  48 . The resulting voltage is then precisely zero when both windings and the entire structure are symmetrically structured. If this symmetry is altered, e.g. by movements of magnetic flux sensor  22 ,  32  relative to measurement strips  34 ,  30 , an output voltage will result, the amplitude of which indicates the degree of asymmetry. An AC voltage of constant amplitude and constant frequency is present at primary winding  36 , with the frequency thereof generally ranging from 50 Hz to 500 kHz. 
     When piston rod  14  is moved relative to stationary magnetic flux sensor  22 ,  32  along the magnetically scannable measurement strips, the coupling factors between the windings are altered. For example, if primary winding  36  is located directly above a measurement strip  34 , then the arrangement is symmetrical, the voltages of secondary windings are canceled out, and no output signal is generated. As soon as piston rod  14  is displaced, an uneven magnetic coupling is present, and as a result, an output voltage is generated at the secondary windings. A directional signal can be generated by a correlation with the excitation voltage. The output signal is sinusoidal and highly stable, and therefore, very high resolution can be achieved. 
       FIG. 3  shows sensor head arrangement  22 ,  32 , according to the principle of differential transformer  34  represented in  FIG. 2 , over selectively hardened measurement strip  34 . In each case, a measurement strip  34  and an associated blank space  62  together form a sector. 
     With the method described thus far, however, an absolute value measurement cannot be achieved. 
     According to the invention, a so-called sector pointer is proposed for absolute displacement measurement. This measurement is implemented using the second magnetically scannable structure  28  arranged on bottom side  26  of piston  14 . Said structure comprises two measurement strips  30  extending at an angle along longitudinal axis  16  of piston  14 . Measurement strip  30  is represented in various positions in  FIG. 4  together with second magnetic flux sensor  32 . 
     The displacement measuring system according to  FIG. 4  has lower resolution but a substantially larger measuring range than the displacement measuring system according to  FIG. 3 . The structure of magnetic flux sensor  32  corresponds to that of magnetic flux sensor  22 . However, one sensor surface  66  of magnetic flux sensor  32  is arranged transversely to longitudinal axis  16 , so that when piston rod  14  moves in a linear fashion along magnetic flux sensor  32 , measurement strip  30  passes transversely over sensor head  66  in the direction of arrow  68 , for example, wherein a sinusoidal output signal is generated on the basis of the steady coupling changes. 
     In combination, the measuring systems represented in  FIG. 3  and  FIG. 4  form a highly precise, absolute displacement measuring system. 
       FIG. 5   a  shows a front view of a sensor ring  68  for holding sensors  22 ,  32 , while  FIG. 5   b  shows sensor ring  68  in a sectional illustration. Magnetic flux sensor  32  comprises the three SMD inductors, which are arranged one in behind the other in the direction of longitudinal axis  16 , while magnetic flux sensor  32  extends along an inner edge  70  of sensor ring  68  in the circumferential direction. Sensor ring  68  enables the drive system according to  FIG. 1  to be fitted in a simple manner, including post-market, with a displacement measuring system, wherein sensor ring  24  is secured to an end face of cylinder  12 , coaxially to piston rod  14 .