Displacement sensor

A plurality of Hall ICs 34A, 34B are disposed around a magnetic rod 32 that is movable along a central axis. The Hall ICs are disposed in different positions in terms of both a straight line distance coordinate in the direction of the central axis and a rotational angle coordinate around the central axis. The amount of displacement of the magnetic rod 32 is calculated based on the average of output signals from the Hall ICs. Errors in the output signals due to a shift and tilt of the magnetic rod are detected and calibration of the measurement method is carried out.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a displacement sensor that comprises magnetic sensing devices such as Hall ICs, and a magnetic member that generates a magnetic field and is movable relative to the magnetic sensing devices, the displacement sensor outputting electrical signals from the magnetic sensing devices having a level corresponding to the amount of displacement of the magnetic member.

2. Description of the Related Art

A displacement sensor disclosed in Japanese Patent Application Laid-open No. 2000-258109 has a rod-shaped magnetic member (e.g. a magnet) joined to a mover, and two magnetic sensing devices are disposed in positions differing by 180° on the same circumference centered on a central axis of the magnetic member. Output signals from the two magnetic sensing devices are averaged, whereby errors in the output signals from the two magnetic sensing devices due to misalignment of the rod-shaped magnet in the radial direction cancel each other out, and hence the accuracy of position detection is improved.

With such a displacement sensor, it is generally desired to expand the range of amounts of displacement that can be detected (the detection range).

Moreover, the installation position of the magnetic member may be slightly misaligned from the proper position. This misalignment comprises two components, a shift in the central axis of the magnetic member in the radial direction from the proper position of the axis, and a tilt of the central axis of the magnetic member from the proper direction of the axis; in general, such a shift and tilt are present compounded together. Errors occur in the output signals from the magnetic sensing devices due to this shift and tilt. It is desirable for errors due to such misalignment to be detected when the displacement sensor is shipped out from the factory, during use or the like, and for the displacement sensor to be calibrated based on this. However, with the prior art described above, the errors in the output signals from the magnetic sensing devices cannot be detected in the case that a shift and tilt of the magnetic member are compounded together.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to expand the detection range of a displacement sensor.

It is another object of the present invention to enable errors in the output signals from magnetic sensing devices caused by a shift and tilt of a magnetic member to be detected in the case that the shift and tilt are present compounded together.

A displacement sensor according to the present invention comprises a plurality of magnetic sensing devices, and a magnetic member that is movable relative to the magnetic sensing devices along a prescribed reference axis, and forms, at the position of each of the magnetic sensing devices, a magnetic field having a strength that varies according to an amount of displacement in the movement direction. Moreover, under a cylindrical coordinate system comprising a straight line distance coordinate along the reference axis, a rotational angle coordinate centered on the reference axis, and a radial distance coordinate from the reference axis, the magnetic sensing devices are disposed in different positions in terms of the straight line distance coordinate, and each of the magnetic sensing devices outputs a signal having a level corresponding to the strength of the magnetic field at the respective location.

According to the above displacement sensor, a plurality of magnetic sensing devices that are disposed in different positions in terms of the straight line distance coordinate are used. As a result, the range of amounts of displacement that can be detected (the detection range) is expanded compared with the prior art.

With the above displacement sensor, there may be further provided an averaging circuit that receives the output signals from the magnetic sensing devices, and outputs an average signal having a level that is the average of the levels of these output signals. By using the average signal from this averaging circuit, amounts of displacement can be measured over the above-mentioned expanded detection range.

With the above displacement sensor, a magnetic member in which a magnet is housed and fixed inside a holder made of a nonmagnetic material may be used as the magnetic member. According to this constitution, the magnet, which is easily damaged, can be protected.

With the above displacement sensor, the magnetic sensing devices may be disposed in different positions in terms of not only the straight line distance coordinate but also the rotational angle coordinate. According to this constitution, even in the case that a shift and tilt of the magnetic member are compounded together, errors in the output signals from the magnetic sensing devices due to the shift and tilt can be detected based on the signals from the magnetic sensing devices. The detected errors can be used to correct a computational method for determining the amount of displacement from the output signals of the magnetic sensing devices (or to correct the amount of displacement determined).

With the above displacement sensor, two magnetic sensing devices out of the magnetic sensing devices maybe disposed in angular positions differing by 180° in terms of the rotational angle coordinate. Alternatively, three or more magnetic sensing devices may be disposed in positions differing by an angle obtained by dividing 360° equally by the number of the magnetic sensing devices in terms of the rotational angle coordinate. Alternatively, three or more magnetic sensing devices may be disposed in positions differing by 180° in terms of the rotational angle coordinate alternately following the order of arrangement of the magnetic sensing devices in terms of the straight line distance coordinate. Other variations of the arrangement of the plurality of magnetic sensing devices also exist.

In the case of disposing three or more magnetic sensing devices in different positions in terms of the straight line distance coordinate, the detection range can be expanded yet more.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1consists of a sectional drawing of mechanical parts and a block diagram of electrical parts showing the overall constitution of an embodiment of a displacement sensor according to the present invention. InFIG. 1, parts shown with diagonal hatching are parts made of a magnetic material. With the exception of a magnet28, parts shown with no hatching are parts made of a nonmagnetic material (e.g. nonmagnetic stainless steel, plastic, rubber etc.).

As shown inFIG. 1, the mechanical parts of the displacement sensor10comprise a sensor main body12and a movable plug14. The sensor main body12has a tubular main body housing16having openings at front and rear ends thereof, the rear end of the main body housing16being covered with a main body cap18. The main body housing16and the main body cap18are each made of a magnetic material, and constitute an outer shell of the sensor main body12, and have a function of magnetically shielding the inside of the sensor main body12from the outside.

A pressure-resistant sleeve20is inserted and fixed in the main body housing16from the opening at the front end of the main body housing16. The pressure-resistant sleeve20has an opening in a front end thereof, and has a long, thin, cylindrical internal space30therein enclosed by walls thereof. A representative use of the displacement sensor10is, for example, to detect the amount of displacement of hydraulic machinery, for example to detect the stroke of a hydraulic valve; in this use, the internal space30of the pressure-resistant sleeve20is filled with high-pressure hydraulic fluid, and hence the walls of the pressure-resistant sleeve20are subjected to a high hydraulic pressure. The pressure-resistant sleeve20is made of a sturdy nonmagnetic material (e.g. nonmagnetic stainless steel), and has a strength sufficient to be able to withstand the high hydraulic pressure from the internal space30.

The movable plug14is inserted into the internal space30of the pressure-resistant sleeve20from the opening at the front end of the pressure-resistant sleeve20. The movable plug14is ideally aligned with the pressure-resistant sleeve20such that a central axis of the movable plug14and a central axis22of the internal space30(hereinafter referred to as the ‘reference axis’) coincide perfectly. However, in actual practice, the central axis of the movable plug14may be shifted by a slight distance in the radial direction and tilted by a slight angle from the reference axis22, and this shift and tilt will cause an error in the displacement sensor10. The movable plug14is movable within a certain distance range along the reference axis22. The outside diameter of the part of the movable plug14that is inserted in the pressure-resistant sleeve20is slightly smaller than the inside diameter of the pressure-resistant sleeve20, whereby a small clearance is secured between the external surface of the movable plug14and the internal surface of the pressure-resistant sleeve20, so that the movable plug14can move smoothly.

The movable plug14has as a main body thereof a cylindrical rod24that is made of a nonmagnetic material. A front end part24aof the rod24is joined to the object for which the amount of displacement is to be measured, for example the spool of a hydraulic valve. A rear-half part of the rod24inserted in the pressure-resistant sleeve20constitutes a hollow cylindrical holder24bhaving an opening in a rear end thereof, and the magnet28is inserted in this holder24b. A centering stopper31installed in an opening in a rear end of the holder24bstops up a gap between the rear end of the holder24band the rear end of the magnet28, whereby the magnet28is fixed in the holder24bso as to not move. Furthermore, the centering stopper31fixes the gap between the rear end of the holder24band the rear end of the magnet28around the circumference, thus fulfilling a centering role of making the central axis of the magnet28and the central axis of the rod24coincide. In the following description, the rear-half part32of the movable plug14(i.e. the part comprising the holder24b, the magnet28and the centering stopper31) is referred to as the ‘magnetic rod’. A magnetic field33due to the magnet28is formed around the outside of this magnetic rod32.

Here, let us consider a cylindrical coordinate system comprising a straight line distance coordinate in the direction along the reference axis22, a rotational angle coordinate centered on the reference axis22, and a radial distance coordinate from the reference axis22. It is desirable for the strength distribution of the magnetic field33along the straight line distance coordinate of this cylindrical coordinate system to be linear. With this objective, the magnet28has, for example, a spindle shape that tapers from the center toward the two ends.

A plurality of (e.g. two) magnetic sensing devices, for example Hall ICs34A and34B, are fixed to an outside surface of the pressure-resistant sleeve20in the sensor main body12. The locations of the two Hall ICs34A and34B differ from one another in terms of both the straight line distance coordinate and the rotational angle coordinate in the cylindrical coordinate system described above, but are in the same position in terms of the radial distance coordinate.FIG. 1shows a neutral state in which the movable plug14is positioned in the center of its range of movement. In this neutral state, the positions36A and36B along the straight line distance coordinate of the two Hall ICs34A and34B are each the same distance from a central position36of the distribution of the magnetic field33but in opposite directions. The two Hall ICs34A and34B respectively output voltage signals50A and50B having a level corresponding to the strength of the magnetic field component in the radial direction centered on the reference axis22of the magnetic field33in the location of that Hall IC34A or34B.

The voltage signals50A and50B outputted from the Hall ICs34A and34B are inputted via signal cables37into an information processing circuit42which is provided outside the sensor main body12. Moreover, the voltage signals50A and50B are also inputted into an averaging circuit40. The averaging circuit40outputs an average signal54having a level that is the average of the levels of the two inputted voltage signals50A and50B. This average signal54is also inputted into the information processing circuit42. The information processing circuit42calculates the amount of displacement of the movable plug14(and hence the object targeted for measurement) based on the average signal54using a method described later. Moreover, the information processing circuit42also carries out calibration of the processing of calculating the amount of displacement based on the voltage signals50A and50B (in particular the voltage levels obtained in the neutral state described earlier) from the Hall ICs34A and34B using a method described later. Note that the averaging circuit40maybe disposed outside the sensor main body12as shown inFIG. 1, but may alternatively be disposed inside the sensor main body12.

FIGS. 2A and 2Bshow examples of the arrangement of the two Hall ICs34A and34B shown inFIG. 1in terms of the rotational angle coordinate.

In the example ofFIG. 2A, the two Hall ICs34A and34B are disposed in positions such that 360° is divided equally by the number of Hall ICs (two), i.e. in positions differing by 180° in terms of the rotational angle coordinate, in other words in positions on opposite sides centered on the reference axis22. Alternatively, as shown inFIG. 2B, the Hall ICs34A and34B may be disposed in positions that differ by an angle other than 180° (e.g. 90° in the case ofFIG. 2B).

In the example of the displacement sensor10shown inFIG. 1, two Hall ICs are provided, but more Hall ICs than this (e.g. three, four, or more) may be provided, the Hall ICs being arranged in different positions to each other in terms of the straight line distance coordinate and the rotational angle coordinate described earlier.

FIGS. 2C to 2Eshow examples of the arrangement of the Hall ICs in terms of the rotational angle coordinate in the case that more than two Hall ICs, for example three Hall ICs34A,34B and34C, are provided.

In the example ofFIG. 2C, the three Hall ICs34A,34B and34C are disposed in positions such that 360° is divided equally by the number of Hall ICs (three), i.e. in positions differing by 120° in terms of the rotational angle coordinate. In the example shown inFIGS. 2D and 2E, the three Hall ICs34A,34B and34C are disposed in positions that differ by 180° in terms of the rotational angle coordinate such as to alternate following the order of disposition in terms of the straight line distance coordinate.

In the case that three or more Hall ICs are provided as shown inFIGS. 2C to 2E, an average signal may be produced by averaging the levels of the output signals of all of the Hall ICs, with the amount of displacement being calculated using this average signal. Alternatively, an average signal may be produced for each pair of two Hall ICs out of the three or more Hall ICs, with the amount of displacement being calculated using these average signals. Any of various arrangements other than those of the examples shown inFIGS. 2A to 2Emay also be adopted.

FIG. 3shows the constitution of the averaging circuit40and the information processing circuit42.

As shown inFIG. 3, the averaging circuit40has a simple constitution comprising two input terminals for inputting the voltage signals50A and50B outputted from the Hall ICs34A and34B, one output terminal for outputting the average signal54, and two resistors R1and R2connected respectively between the two input terminals and the output terminal. The averaging circuit40can thus be created, for example, merely by inserting the resistors R1and R2in the output lines from the Hall ICs34A and34B. The output terminal of the averaging circuit40is joined to an analog input terminal of an A/D converter44in the information processing circuit42. The analog input terminal of the A/D converter44is connected to ground via a resistor R3.

Here, a quantitative explanation of the level of the average signal54outputted from the averaging circuit40is as follows. First, as shown inFIG. 3, the Hall ICs34A and34B are each equivalent to a cell generating a voltage corresponding to the magnetic field strength; let us take the output voltages to be E1and E2respectively. To simplify the explanation, let us assume that the resistors R1and R2have the same resistance value as each other.
E1=R1·i1+R3(i1+i2)  (1)
E2=R1·i2+R3(i1+i2)  (2)
Level of average signal 54=R3(i1+i2)  (3)
Therefore:
Level of average signal 54=(E1+E2)/2−R1(i1+i2)/2  (4)
The first term on the right hand side of equation (4) is the average of the levels of the outputs of the Hall ICs34A and34B, and the second term is an error.

Here, if the resistors R1and R2are set to have a resistance value sufficiently smaller than that of the resistor R3in accordance with the required accuracy, then the error will be sufficiently small as to be ignorable, and hence the required accuracy will be obtained. For example, the resistors R1and R2can be made to have a resistance value of a few hundred Ω, and the resistor R3can be made to have a resistance value of a few hundred Ω. As a specific example, considering the case that R1=R2=100Ω, R3=220Ω, and E1=E2=4V (incidentally, the output level of a Hall IC is generally approximately 1 to 4V), the above error will be 0.9 mV, which is very small compared with the average value of 4V. In this way, the output signals of the Hall ICs34A and34B can be averaged accurately using an averaging circuit40having an extremely simple constitution as shown inFIG. 3.

As shown inFIG. 3, the information processing circuit42has the A/D converter44, a displacement calculating section46, a voltage-displacement table47, a mechanical control section48, and a correcting section49. The A/D converter44converts the average signal54which gives the analog average voltage into average voltage data57which gives the digital average voltage. The voltage-displacement table47has stored therein amounts of displacement corresponding respectively to various average voltage values that the average voltage data57could take. The displacement calculating section46refers to the voltage-displacement table47, and converts the average voltage data57into displacement data58which gives the corresponding amount of displacement. The mechanical control section48controls machinery (e.g. hydraulic machinery), not shown in the drawings, based on the displacement data58.

Voltage signals50A and50B outputted from the Hall ICs34A and34B, in particular the voltage signals50A and50B when the displacement sensor10is in the neutral state described earlier, are inputted into the correcting section49. Based on the inputted voltage signals50A and50B in the neutral state, the correcting section49then calculates the errors in the voltage signals50A and50B caused by the shift and tilt of the magnetic rod32from the reference axis22. Based on the calculated errors, the correcting section49then corrects the voltage-displacement table47such that the amount of displacement corresponding to each average voltage becomes correct. The method of calculating the errors will be described later with reference toFIG. 6.

Following is a description of the operation under the constitution described above.

FIG. 4consists of graphs for explaining how the range of amounts of displacement that can be detected (the detection range) is expanded by using the average signal54. InFIG. 4, the point where the amount of displacement is zero indicates the neutral state.

FIG. 4Ashows the output signal50of a displacement sensor having one Hall IC, and the maximum detection range (hereinafter referred to as the ‘basic detection range’)52therefor. The range over which the slope of the output signal150is substantially non-zero is the basic detection range52. With the displacement sensor described in Japanese Patent Application Laid-open No. 2000-258109, two Hall ICs are used, but these two Hall ICs are in the same position in terms of the straight line distance coordinate, and hence the detection range is the same as the basic detection range52for a single Hall IC shown inFIG. 4A.

FIG. 4Bshows the average signal54and the detection range56for the displacement sensor10according to the present invention having two Hall ICs34A and34B that are disposed in different positions in terms of the straight line distance coordinate and the rotational angle coordinate as shown inFIG. 1.

As shown inFIG. 4B, the curves of the output signals50A and50B from the two Hall ICs34A and34B are each shifted by a certain amount of displacement from the zero point of the amount of displacement but in opposite directions to one another. The detection range56for the average signal54is thus expanded on each side by this certain amount of displacement compared with the basic detection range52ofFIG. 4A. Furthermore, with a constitution in which three or more Hall ICs are disposed indifferent positions in terms of the straight line distance coordinate as shown inFIGS. 2C to 2E, the detection range can be expanded yet more.

FIG. 5shows examples of the shift and tilt of the magnetic rod32relative to the reference axis22. Specifically,FIG. 5Ashows an example in which the magnetic rod32is shifted by a distance a from the reference axis22so as to become closer to the second Hall IC34B.FIG. 5Bshows an example in which the magnetic rod32is tilted by an angle b from the reference axis22such that the N pole of the magnetic rod32becomes closer to the second Hall IC34B.

Moreover,FIG. 6Ashows the changes in the output signals50A and50B from the two Hall ICs34A and34B around the neutral state caused by the shift shown inFIG. 5A.FIG. 6Bshows the changes in the output signals50A and50B from the two Hall ICs34A and34B around the neutral state caused by the tilt shown inFIG. 5B.

If a shift as shown inFIG. 5Aarises, then the magnetic rod32as a whole moves away from the first Hall IC34A and closer to the second Hall IC34B. As a result, as shown inFIG. 6A, around the neutral state (the zero point of the amount of displacement), the output signal from the first Hall IC34A becomes like the output signal50Aa, with the slope becoming smaller than for the normal output signal50A. As a result, in the neutral state, the voltage level of the output signal50Aa affected by the shift becomes higher by an error ΔVa than the normal voltage level VA. On the other hand, around the neutral state, the output signal from the second Hall IC34B becomes like the output signal50Ba, with the slope becoming greater than for the normal output signal50B. As a result, in the neutral state, the voltage level of the output signal50Ba affected by the shift becomes higher by an error ΔVa than the normal voltage level VB.

Moreover, if a tilt as shown inFIG. 5Barises, then the S pole of the magnetic rod32moves closer to the first Hall IC34A, and the N pole moves closer to the second Hall IC34B. As a result, as shown inFIG. 6B, around the neutral state, the output signal from the first Hall IC34A becomes like the output signal50Ab, with the slope becoming greater than for the normal output signal50A. As a result, in the neutral state, the voltage level of the output signal50Ab affected by the tilt becomes lower by an error ΔVb than the normal voltage level VA. On the other hand, around the neutral state, the output signal from the second Hall IC34B becomes like the output signal50Bb, with the slope becoming greater than for the normal output signal50B. As a result, in the neutral state, the voltage level of the output signal50Bb affected by the tilt becomes higher by an error ΔVb than the normal voltage level VB.

Consequently, in the case that the shift and tilt shown inFIGS. 5A and 5Bare compounded together, in the neutral state, the level of the output signal from the first Hall IC34A becomes ‘VA+ΔVa−ΔVb’, and the level of the output signal from the second Hall IC34B becomes ‘VB+ΔVa+ΔVb’.

Focusing on this, the correcting section49shown inFIG. 3adds together the voltage levels ‘VA+ΔVa−ΔVb’ and ‘VB+ΔVa+ΔVb’ of the output signals from the two Hall ICs34A and34B in the neutral state to obtain ‘(VA+VB)+2ΔVa’, and then subtracts therefrom the sum ‘VA+VB’ of the voltage levels in the neutral state at a normal time which has been preset, thus determining the voltage error ΔVa due to the shift. Moreover, the correcting section49calculates the difference between the voltage levels ‘VA+ΔVa−ΔVb’ and ‘VB+ΔVa+ΔVb’ of the output signals from the two Hall ICs34A and34B in the neutral state to obtain ‘(VA−VB)−2ΔVb’, and then subtracts therefrom the difference ‘VA−VB’ between the voltage levels in the neutral state at a normal time which has been preset, thus determining the voltage error ΔVb due to the tilt. Using data or a program that defines the relationship between various voltage errors ΔVa and ΔVb and correction amounts for the voltage-displacement table47as determined in advance either empirically or theoretically, the correcting section49then corrects the voltage-displacement table47in accordance with the determined voltage errors ΔVa and ΔVb. Through this correction, errors due to the effects of the shift and tilt are kept down, and hence the amount of displacement can be measured with high accuracy.

An embodiment of the present invention has been described above; however, this embodiment is merely an example for describing the present invention, and the scope of the present invention is not intended to be limited to only this embodiment. The present invention can be implemented in various other ways so long as the gist of the present invention is not deviated from.

For example, as the constitution of the magnetic rod, instead of a constitution in which a rod-shaped permanent magnet is used as described above, a constitution may be used in which ring-shaped permanent magnets are set around the outside at both ends of a rod-shaped magnetic core member, and the shape of the magnetic core member is designed such that a linear magnetic field strength distribution is obtained.

Moreover, in the case of a constitution in which three or more magnetic sensing devices34A,34B and34C are disposed in different positions in terms of the straight line distance coordinate as shown inFIGS. 2C to 2E, it may be made such that these three or more magnetic sensing devices34A,34B and34C are categorized into pairs of magnetic sensing devices that are adjacent to one another in terms of the straight line distance coordinate, for example a first pair comprising the first and second magnetic sensing devices34A and34B and a second pair comprising the second and third magnetic sensing devices34B and34C, and measurement of the amount of displacement is carried out based on the average signal as shown inFIG. 3for each pair. It can then be made to be such that, for example, the amount of displacement in a detection zone covered by the first pair is measured based on the average signal from the first pair, and the amount of displacement in a detection zone covered by the second pair is measured based on the average signal from the second pair, i.e. the amount of displacement in the detection zone covered by each pair is determined using the signals from that pair. As a result, measurement of the amount of displacement can be carried out over a long distance comprising the detection ranges of the plurality of pairs joined together.

Moreover, in the embodiment described above, a constitution has been adopted in which the magnetic sensing devices are fixed, and the magnetic rod moves together with the object targeted for measurement. Instead of this, a constitution may be adopted in which the magnetic rod is fixed, and the magnetic sensing devices move together with the object targeted for measurement.