Position detection switch

Abstract

An area transition information generation unit (20) determines a piston area on the basis of the outputs of first to third MR sensors (12, 14, 16), updates area transition information when the piston area transitions between two continuous areas, and does not update the area transition information when the piston area jumps and transitions between two non-continuous areas. A switch signal generation unit (28) switches a switch signal on the basis of at least the area transition information.

Description

Position detection switch

[0001] The present invention relates to a position detection switch that detects the position of a piston of a fluid pressure cylinder.

[0002] 2. Description of the Related Art Conventionally, a position detection switch is known that detects the position of a piston by attaching a magnet (permanent magnet) to the piston and providing a magnetic sensor in a cylinder tube that detects the magnetic field of the magnet.

[0003] When such a position detection switch is used in an environment where an external magnetic field is present, the external magnetic field may cause the piston position to be erroneously detected. Position detection switches that do not erroneously detect the piston position even when an external magnetic field is applied are also known.

[0004] For example, Japanese Patent Laid-Open Publication No. 6-249605 describes a device in which two magnetic sensors are placed at separate positions along the direction of piston movement, and detects that the piston is in a specific position when the two magnetic sensors detect a magnetic field at a predetermined time interval or longer. When a strong external magnetic field from a welding machine acts on these magnetic sensors, the two magnetic sensors turn on almost simultaneously, making it possible to distinguish between the magnetic field of the magnet attached to the piston and the external magnetic field.

[0005] However, in the two magnetic sensors of JP-A-6-249605, if the spacing between them is small and the piston is moving fast, they will detect the magnetic field of the magnet attached to the piston almost simultaneously and turn on. If the spacing between the two magnetic sensors is increased to avoid this, the two magnetic sensors will not be able to simultaneously receive an external magnetic field of the same strength when the external magnetic field weakens.

[0006] The present invention aims to solve the above-mentioned problems.

[0007] A position detection switch according to the present invention includes first to third MR sensors that detect the magnetic field of a magnet attached to a piston of a fluid pressure cylinder, as well as an area transition information generator and a switch signal generator. The area transition information generator determines the piston area based on the outputs of the first to third MR sensors, and updates the area transition information when the piston area transitions between two consecutive areas, but does not update the area transition information when the piston area transitions between two non-consecutive areas. The switch signal generator switches a switch signal based on at least the area transition information, and the switch signal is in a predetermined state when the piston is within a predetermined operating range.

[0008] The position detection switch can determine whether the piston area has transitioned between two consecutive areas or between two non-consecutive areas, and can reliably distinguish an external magnetic field, such as a welding magnetic field, from the cylinder magnetic field to switch the switch signal.

[0009] Fig. 1 is a diagram showing the basic configuration of a position detection switch according to a first embodiment of the present invention, together with a fluid pressure cylinder. Fig. 2 is a diagram showing outputs of first to third MR sensors in the position detection switch of Fig. 1. Fig. 3 is a block diagram of the position detection switch of Fig. 1. Fig. 4 is a block diagram of a position detection switch according to a second embodiment of the present invention.

[0010] The present invention will be described below with reference to the accompanying drawings, which illustrate preferred embodiments of the present invention. In the following description, when terms relating to left and right directions are used, they refer to directions in the drawings for convenience and do not limit the actual arrangement of components, etc.

[0011] First Embodiment A position detection switch 10 according to a first embodiment of the present invention will be described with reference to Figures 1 to 3. A fluid pressure cylinder 40 to which the position detection switch 10 is attached is used, for example, to clamp a workpiece to be welded.

[0012] 1, the fluid pressure cylinder 40 includes a cylinder tube 42, a piston 44 slidably disposed within the cylinder tube 42, and a piston rod 46 connected to the piston 44. The right end of the cylinder tube 42 is sealed with a rod cover 48, and the left end of the cylinder tube 42 is sealed with a head cover 50. In the following description, the displacement direction (left-right direction) of the piston 44 is referred to as direction A. Furthermore, the displacement direction of the piston 44 toward the head cover 50 is referred to as direction A1, and the displacement direction of the piston 44 toward the rod cover 48 is referred to as direction A2.

[0013] An annular magnet (permanent magnet) 52 is attached to the outer periphery of the piston 44. The magnet 52 is magnetized in the A direction, with the left end face of the magnet 52 being the north pole and the right end face being the south pole. A magnetic field (magnetic flux) 54 is formed around the magnet 52, emanating from the left end face of the magnet 52, passing around the outside of the magnet 52 in the radial direction, and returning to the right end face of the magnet 52. To distinguish it from an external magnetic field, the magnetic field 54 generated by the magnet 52 is referred to as a "cylinder magnetic field." In this embodiment, the left end face of the magnet 52 will be described as being the north pole and the right end face as being the south pole, but the north and south poles may be reversed.

[0014] The position detection switch 10 includes first to third MR sensors 12, 14, and 16, and is attached to the outside of a cylinder tube 42. The first to third MR sensors 12, 14, and 16 are magnetic sensors that use magnetoresistive elements and have the same output characteristics. The first to third MR sensors 12, 14, and 16 are arranged at equal intervals in the A direction.

[0015] FIG. 2 is a diagram showing the outputs V1 to V3 of the first to third MR sensors 12, 14, and 16 in response to the position X of the piston 44. The horizontal axis represents the position X of the piston 44, and the vertical axis represents the outputs V1 to V3 of the first to third MR sensors 12, 14, and 16. The output curve of the first MR sensor 12 is shown by a dotted line. The output curve of the second MR sensor 14 is shown by a solid line. The output curve of the third MR sensor 16 is shown by a dashed line. With respect to the A direction, when the center of the magnet 52 is located at a position corresponding to the second MR sensor 14 (see FIG. 1), the piston 44 is at the origin (X=0). When the piston 44 is displaced from the origin in the A2 direction, the value of X is positive, and when the piston 44 is displaced from the origin in the A1 direction, the value of X is negative.

[0016] The output V2 of the second MR sensor 14 has a maximum value Vmax when X = 0. The output V2 of the second MR sensor 14 is zero when X = Xc and X = -Xc, is positive in the range of -Xc < X < Xc, and is negative in the range of X < -Xc and X > Xc.

[0017] The output curve of the first MR sensor 12 is obtained by shifting the output curve of the second MR sensor 14 to the left by Xb. The output V1 of the first MR sensor 12 reaches a maximum value Vmax when X = -Xb. The output curve of the third MR sensor 16 is obtained by shifting the output curve of the second MR sensor 14 to the right by Xb. The output V3 of the third MR sensor 16 reaches a maximum value Vmax when X = Xb. If Xd = Xb + Xc, the output V1 of the first MR sensor 12 is zero when X = -Xd, and the output V3 of the third MR sensor 16 is zero when X = Xd.

[0018] The output curve of the first MR sensor 12 intersects with the output curve of the second MR sensor 14 at X = -Xa (-Xb < -Xa). At X = -Xa, the output V1 of the first MR sensor 12 and the output V2 of the second MR sensor 14 are both Va. The output curve of the third MR sensor 16 intersects with the output curve of the second MR sensor 14 at X = Xa (Xa < Xb). At X = Xa, the output V2 of the second MR sensor 14 and the output V3 of the third MR sensor 16 are both Va.

[0019] To determine whether the piston 44 is in the optimum position, the output V2 of the second MR sensor 14 is compared with the optimum position threshold value TH1. When the position X of the piston 44 is Xf or -Xf, the output V2 of the second MR sensor 14 is equal to the optimum position threshold value TH1. When V2 ≥ TH1, it is determined that the piston 44 is in the optimum position (-Xf ≤ X ≤ Xf). The fact that the piston 44 is in the optimum position is notified to the operator, for example, by lighting up a green indicator light (first indicator light). In this embodiment, Xf = Xb.

[0020] To determine whether the piston 44 is in a predetermined operating range (hereinafter simply referred to as the "operating range") including the optimal position, the outputs V1 to V3 of the first to third MR sensors 12, 14, and 16 are compared with an operating range threshold TH2. The operating range threshold TH2 is smaller than the optimal position threshold TH1. The output V2 of the second MR sensor 14 is equal to the operating range threshold TH2 when the position X of the piston 44 is Xg (Xf<Xg<Xc) and -Xg. The output V3 of the third MR sensor 16 is equal to the operating range threshold TH2 when the position X of the piston 44 is Xh (Xc<Xh<Xd). The output V1 of the first MR sensor 12 is equal to the operating range threshold TH2 when the position X of the piston 44 is -Xh.

[0021] When the position X of the piston 44 changes from −Xd to Xd, if the output V2 of the second MR sensor 14 becomes equal to or greater than TH2, it is determined that the piston 44 has entered its operating range. Thereafter, if the output V3 of the third MR sensor 16 falls below TH2, it is determined that the piston 44 has left its operating range. That is, when the piston 44 displaces in the A2 direction, the operating range is −Xg≦X≦Xh. When the position X of the piston 44 changes from Xd to −Xd, if the output V2 of the second MR sensor 14 becomes equal to or greater than TH2, it is determined that the piston 44 has entered its operating range. Then, if the output V1 of the first MR sensor 12 falls below TH2, it is determined that the piston 44 has left its operating range. That is, when the piston 44 displaces in the A1 direction, the operating range is −Xh≦X≦Xg. In this embodiment, the operating range is different when the piston 44 is displaced in the A1 direction and when it is displaced in the A2 direction, but the operating range may be the same when the piston 44 is displaced in the A1 direction and when it is displaced in the A2 direction.

[0022] When it is determined that the piston 44 has entered the operating range, the switch signal SW changes from off to on. When it is determined that the piston 44 has left the operating range, the switch signal SW changes from on to off. When the switch signal SW is on and the piston 44 is not in the optimal position, for example, a red indicator light (second indicator light) is illuminated. The illumination of the red indicator light allows the operator to recognize that the piston 44 has deviated from the optimal position. Possible causes of the piston 44 being deviated from the optimal position include the piston 44 being in an unstable region, and vibration causing the mounting position of the position detection switch 10 relative to the cylinder tube 42 to shift. The relationship between the operating range and the switch signal SW may be reversed from the above. That is, when it is determined that the piston 44 has entered the operating range, the switch signal SW changes from on to off, and when it is determined that the piston 44 has left the operating range, the switch signal SW changes from off to on. In short, it is sufficient if it is possible to determine whether the piston 44 has entered the operating range or whether the piston 44 has left the operating range using the switch signal SW.

[0023] With respect to the position X of the piston 44, the range from -Xd to Xd is divided into six consecutive areas. -Xd≦X<-Xc is the first area A1, -Xc≦X<-Xa is the second area A2, -Xa≦X<0 is the third area A3, 0≦X≦Xa is the fourth area A4, Xa<X≦Xc is the fifth area A5, and Xc<X≦Xd is the sixth area A6. The position -Xh of the piston 44 described above is included in the first area A1. The position -Xg of the piston 44 described above is included in the second area A2. The position Xg of the piston 44 described above is included in the fifth area A5. The position Xh of the piston 44 described above is included in the sixth area A6.

[0024] Whether the piston 44 is in the first to sixth areas A1 to A6 is determined based on the outputs V1 to V3 of the first to third MR sensors 12, 14, and 16. Specifically, when V1 ≥ 0 and V2 < 0 are satisfied, the piston 44 is determined to be in the first area A1. When V3 < V2 < V1 and V2 ≥ 0 are satisfied, the piston 44 is determined to be in the second area A2. When V3 < V1 ≤ V2 and V1 > 0 are satisfied, the piston 44 is determined to be in the third area A3. When V1 ≤ V3 ≤ V2 and V3 > 0 are satisfied, the piston 44 is determined to be in the fourth area A4. When V1 < V2 < V3 and V2 ≥ 0 are satisfied, the piston 44 is determined to be in the fifth area A5. When V2 < 0 and V3 ≥ 0 are satisfied, the piston 44 is determined to be in the sixth area A6.

[0025] Whether the piston 44 is in a seventh area A7, which is an area other than the first to sixth areas A1 to A6, is also determined based on the outputs V1 to V3 of the first to third MR sensors 12, 14, and 16. That is, whether the piston 44 is in the area where X<-Xd or the area where X>Xd is also determined based on the outputs V1 to V3 of the first to third MR sensors 12, 14, and 16. Specifically, when V1, V2, and V3 are all negative, the piston 44 is determined to be in the seventh area A7. Alternatively, the piston 44 may be determined to be in the seventh area A7 when the absolute values ​​of V1, V2, and V3 are all smaller than a predetermined threshold value. In this case, the predetermined threshold value is smaller than the operating range threshold value TH2.

[0026] The method for determining which of the first to seventh areas A1 to A7 the piston 44 is in based on the outputs V1 to V3 of the first to third MR sensors 12, 14, and 16 is not limited to the above specific example. For example, V3<V2<V1 may be added as a condition for determining that the piston 44 is in the first area A1 in addition to V1≧0 and V2<0. Also, in this embodiment, the areas in which the piston 44 is located (hereinafter referred to as "piston areas") are divided into seven, but the number of piston areas is not limited to this.

[0027] The determination of which of the first to seventh areas A1 to A7 the piston area is is performed at each predetermined control cycle. Suppose that in one control cycle, the piston 44 is determined to be in one of the first to seventh areas A1 to A7, and in the next control cycle, the piston 44 is determined to be in another area. In other words, suppose that the newly determined piston area (hereinafter referred to as the "current piston area") differs from the previously determined piston area (hereinafter referred to as the "previous piston area"), and the piston area has changed.

[0028] When the piston area transitions due to a change in the cylinder magnetic field, the current piston area is an area that is continuous with the previous piston area. On the other hand, when the piston area transitions due to a welding magnetic field, the current piston area is an area that is not continuous with the previous piston area. When the welding magnetic field acts on the first to third MR sensors 12, 14, and 16, the outputs V1 to V3 of the first to third MR sensors 12, 14, and 16 change suddenly, so that in a normal control cycle, the transition appears to jump between two discontinuous areas. By determining whether the piston area transitioned between two contiguous areas or between two discontinuous areas, it is possible to distinguish an external magnetic field such as a welding magnetic field from the cylinder magnetic field.

[0029] The operation of the position detection switch 10 will be described in more detail with reference to the block diagram of Fig. 3. In addition to the first to third MR sensors 12, 14, and 16, the position detection switch 10 includes a control unit 18, an area transition information generation unit 20, first to third comparison units 22, 24, and 26, a switch signal generation unit 28, an optimum position determination unit 30, first to third diagnosis units 32, 34, and 36, and a communication unit 38. An optimum position threshold value TH1 and an operating range threshold value TH2 are held in the control unit 18.

[0030] The area transition information generating unit 20 includes an area determining unit 20a, an area transition determining unit 20b, and an area transition information storage unit 20c. The outputs V1 to V3 of the first to third MR sensors 12, 14, and 16 are input to the area determining unit 20a for each control cycle. The area determining unit 20a determines which of the first to seventh areas A1 to A7 the piston area is based on the outputs V1 to V3 of the first to third MR sensors 12, 14, and 16, and sends the current piston area to the area transition determining unit 20b. Note that if the piston area is not determined to be one of the first to seventh areas A1 to A7, it is treated as an error.

[0031] The area transition determination unit 20b stores the previous piston area and compares the current piston area with the previous piston area. If the current piston area is different from the previous piston area and is an area contiguous with the previous piston area, the area transition determination unit 20b updates the area transition information TR stored in the area transition information storage unit 20c. If the current piston area is an area not contiguous with the previous piston area, the area transition information TR is not updated. The area transition determination unit 20b outputs the area transition information TR stored in the area transition information storage unit 20c to the switch signal generation unit 28 for each control cycle.

[0032] For example, suppose that the piston area transitions from the first area A1 to the second area A2 in the nth control cycle and remains in the second area A2 in the (n+1)th control cycle. In this case, the area transition information TR output in the nth control cycle and the area transition information TR output in the (n+1)th control cycle both indicate that the piston area has transitioned from the first area A1 to the second area A2. That is, the area transition determination unit 20b outputs the latest area transition information TR to the switch signal generation unit 28 for each control cycle.

[0033] Here, information indicating that the piston area has transitioned from the first area A1 to the second area A2 is denoted as TR12, and information indicating that the piston area has transitioned from the second area A2 to the first area A1 is denoted as TR21. The same applies to information indicating transitions between other combinations of two areas. There are a total of 14 types of area transition information TR: TR71, TR12, TR23, TR34, TR45, TR56, TR67, TR76, TR65, TR54, TR43, TR32, TR21, and TR17.

[0034] The output V1 of the first MR sensor 12 is input to the first comparison unit 22 for each control cycle. The first comparison unit 22 holds an operating range threshold TH2 input in advance from the control unit 18. The first comparison unit 22 compares the output V1 of the first MR sensor 12 with the operating range threshold TH2 and outputs a first comparison signal C1 to the switch signal generation unit 28. When the output V1 of the first MR sensor 12 is less than the operating range threshold TH2, the first comparison signal C1 is 1. When the output V1 of the first MR sensor 12 is equal to or greater than the operating range threshold TH2, the first comparison signal C1 is 0.

[0035] The output V2 of the second MR sensor 14 is input to the second comparison unit 24 for each control cycle. The second comparison unit 24 holds an operating range threshold TH2 input in advance from the control unit 18. The second comparison unit 24 compares the output V2 of the second MR sensor 14 with the operating range threshold TH2 and outputs a second comparison signal C2 to the switch signal generation unit 28. When the output V2 of the second MR sensor 14 is equal to or greater than the operating range threshold TH2, the second comparison signal C2 is 1, and when the output V2 of the second MR sensor 14 is less than the operating range threshold TH2, the second comparison signal C2 is 0.

[0036] The output V3 of the third MR sensor 16 is input to the third comparison unit 26 every control cycle. The third comparison unit 26 holds an operating range threshold TH2 input in advance from the control unit 18. The third comparison unit 26 compares the output V3 of the third MR sensor 16 with the operating range threshold TH2 and outputs a third comparison signal C3 to the switch signal generation unit 28. When the output V3 of the third MR sensor 16 is less than the operating range threshold TH2, the third comparison signal C3 is 1. When the output V3 of the third MR sensor 16 is equal to or greater than the operating range threshold TH2, the third comparison signal C3 is 0.

[0037] The switch signal generator 28 generates a switch signal SW based on the area transition information TR and the first to third comparison signals C1 to C3, and outputs the switch signal SW to the controller 18 for each control period.

[0038] Consider the case where the piston 44 is displaced in the A2 direction from the area where X<-Xd to the area where X>Xd. If the area transition information TR is TR71, the switch signal SW is off. If the area transition information TR is TR12 and the second comparison signal C2 is 0, the switch signal SW is off. If the area transition information TR is TR12 and the second comparison signal C2 is 1, the switch signal SW is on. If the area transition information TR is TR23, TR34, or TR45, the switch signal SW is on. If the area transition information TR is TR56 and the third comparison signal C3 is 0, the switch signal SW is on. If the area transition information TR is TR56 and the third comparison signal C3 is 1, the switch signal SW is off. If the area transition information TR is TR67, the switch signal SW is off.

[0039] Consider the case where the piston 44 is displaced in the A1 direction from the area where X > Xd to the area where X < -Xd. If the area transition information TR is TR76, the switch signal SW is off. If the area transition information TR is TR65 and the second comparison signal C2 is 0, the switch signal SW is off. If the area transition information TR is TR65 and the second comparison signal C2 is 1, the switch signal SW is on. If the area transition information TR is TR54, TR43, or TR32, the switch signal SW is on. If the area transition information TR is TR21 and the first comparison signal C1 is 0, the switch signal SW is on. If the area transition information TR is TR21 and the first comparison signal C1 is 1, the switch signal SW is off. If the area transition information TR is TR17, the switch signal SW is off.

[0040] As can be understood from the above description, the second comparison signal C2 is used to switch the switch signal SW from OFF to ON when the piston 44 is displaced in the A1 direction and in the A2 direction. The first comparison signal C1 is used to switch the switch signal SW from ON to OFF when the piston 44 is displaced in the A1 direction. The third comparison signal C3 is used to switch the switch signal SW from ON to OFF when the piston 44 is displaced in the A2 direction. The on / off operation of the switch signal SW may be reversed.

[0041] The output V2 of the second MR sensor 14 is input to the optimum position determination unit 30 for each control cycle. The optimum position determination unit 30 holds an optimum position threshold value TH1 input in advance from the control unit 18. The optimum position determination unit 30 compares the output V2 of the second MR sensor 14 with the optimum position threshold value TH1 and generates an optimum position signal G. The optimum position determination unit 30 outputs the optimum position signal G to the control unit 18 for each control cycle. When the output V2 of the second MR sensor 14 is equal to or greater than the optimum position threshold value TH1, the optimum position signal G is 1, and when the output V2 of the second MR sensor 14 is less than the optimum position threshold value TH1, the optimum position signal G is 0.

[0042] The control unit 18 is capable of bidirectional communication with the outside via the communication unit 38. In response to the switch signal SW received from the switch signal generation unit 28, the control unit 18 outputs a signal SW' used to control external devices operating in conjunction with the fluid pressure cylinder 40 to the outside via the communication unit 38. The control unit 18 outputs a signal to control an indicator light (not shown) based on the switch signal SW and the optimum position signal G. When the switch signal SW is in a predetermined on or off state and the optimum position signal G is 1, the control unit 18 outputs a command signal G1 to turn the indicator light green. When the switch signal SW is in a predetermined on or off state and the optimum position signal G is 0, the control unit 18 outputs a command signal G2 to turn the indicator light red.

[0043] The settings of the optimum position threshold TH1 and the operating range threshold TH2 can be changed externally for the position detection switch 10. When the control unit 18 receives data related to the change in the setting of the optimum position threshold TH1 via the communication unit 38, it outputs the new optimum position threshold TH1 to the optimum position determination unit 30. When the control unit 18 receives data related to the change in the setting of the operating range threshold TH2 via the communication unit 38, it outputs the new operating range threshold TH2 to the first to third comparison units 22, 24, and 26.

[0044] The magnet 52 loses its magnetic force (demagnetizes) over time. In particular, the magnet 52 demagnetizes when exposed to a welding magnetic field or when placed in a high-temperature environment for an extended period of time. When such events occur, the maximum values ​​Vmax of the outputs V1 to V3 of the first to third MR sensors 12, 14, and 16 may decrease to a level that does not exceed the optimum position threshold value TH1.

[0045] Therefore, first to third diagnostic units 32, 34, 36 are provided for the purpose of monitoring whether the maximum value Vmax of the outputs V1 to V3 of the first to third MR sensors 12, 14, 16 has decreased by a predetermined amount or more due to aging or the like. The output V1 of the first MR sensor 12 is input to the first diagnostic unit 32, the output V2 of the second MR sensor 14 is input to the second diagnostic unit 34, and the output V3 of the third MR sensor 16 is input to the third diagnostic unit 36. The processing details of the first diagnostic unit 32 will be explained below, but the processing details of the second diagnostic unit 34 and the third diagnostic unit 36 ​​are similar.

[0046] The first diagnostic unit 32 has a maximum value determination unit 32a, a maximum value storage unit 32b, and a monitoring unit 32c. Every time the output V1 of the first MR sensor 12 changes from an increase to a decrease, the maximum value determination unit 32a sends the output at that time (the output V1 immediately before the change to a decrease) to the maximum value storage unit 32b as a maximum value Vmax. The maximum value storage unit 32b stores the maximum value Vmax data received from the maximum value determination unit 32a in chronological order.

[0047] The monitoring unit 32c periodically calculates the average value of a predetermined number of most recent data out of the many maximum value Vmax data stored in the maximum value storage unit 32b, and compares this with the average value of the initial predetermined number of data. If it determines that the average value of the most recent predetermined number of data is smaller than the average value of the initial predetermined number of data by a predetermined value or more, it outputs a warning signal E1 to the control unit 18.

[0048] When the control unit 18 receives the warning signal E1 from the monitoring unit 32c of the first diagnosis unit 32, it outputs a warning signal E1' to the outside via the communication unit 38 to notify that there is a high probability that demagnetization has occurred in the magnet 52. This can prompt the worker to perform maintenance such as part replacement or to adjust the optimum position threshold value TH1.

[0049] In this embodiment, the first diagnostic unit 32 monitors the maximum value Vmax of the output V1 of the first MR sensor 12, the second diagnostic unit 34 monitors the maximum value Vmax of the output V2 of the second MR sensor 14, and the third diagnostic unit 36 ​​monitors the maximum value Vmax of the output V3 of the third MR sensor 16. However, a common diagnostic unit may comprehensively monitor the maximum values ​​Vmax of the outputs V1 to V3 of the first to third MR sensors 12, 14, and 16. If the maximum values ​​Vmax of multiple outputs V1 to V3 of the first to third MR sensors 12, 14, and 16 decrease by a predetermined value or more, it is highly likely that demagnetization of the magnet 52 is the cause. If the maximum value Vmax of only one output V1 to V3 of the first to third MR sensors 12, 14, and 16 decreases by a predetermined value or more, it is highly likely that a decrease in the sensitivity of that MR sensor is the cause. It is more efficient if the common diagnostic unit distinguishes between these events and outputs a warning signal.

[0050] According to this embodiment, the switch signal SW is switched based on at least the area transition information TR, which is information based on the transition of the piston area between two consecutive areas. Therefore, the switch signal SW can be switched by reliably distinguishing an external magnetic field such as a welding magnetic field from the cylinder magnetic field.

[0051] Second Embodiment A position detection switch 60 according to a second embodiment of the present invention will be described with reference to Fig. 4. The position detection switch 60 differs from the position detection switch 10 according to the first embodiment in that it includes a time measurement unit 62 and adds a time-related condition as a condition for switching the switch signal SW. Note that components that are the same as or equivalent to those in the position detection switch 10 according to the first embodiment are given the same reference numerals, and detailed descriptions thereof will be omitted.

[0052] The outputs V1 to V3 of the first to third MR sensors 12, 14, 16 are input to a time measurement unit 62 for each control cycle. The time measurement unit 62 holds a motion range threshold TH2 input in advance from the control unit 18. The time measurement unit 62 includes first to fourth time measurement units 62a to 62d.

[0053] The first time measurement unit 62a measures the time from when the output V1 of the first MR sensor 12 becomes equal to or greater than the threshold value TH2 to when the output V2 of the second MR sensor 14 becomes equal to or greater than the threshold value TH2. That is, the first time measurement unit 62a measures the time required for the piston 44 to be displaced in the A2 direction from -Xh to -Xg (hereinafter referred to as the "first displacement time"). The first time measurement unit 62a determines whether the first displacement time falls within a predetermined range, and the time measurement unit 62 outputs a determination signal J1 to the switch signal generation unit 28 for each control cycle. When it is determined that the first displacement time falls within the predetermined range, the determination signal J1 is 1; otherwise, the determination signal J1 is 0.

[0054] The second time measurement unit 62b measures the time from when the output V2 of the second MR sensor 14 falls below the threshold value TH2 to when the output V3 of the third MR sensor 16 falls below the threshold value TH2. That is, the second time measurement unit 62b measures the time required for the piston 44 to be displaced in the A2 direction from Xg to Xh (hereinafter referred to as the "second displacement time"). The second time measurement unit 62b determines whether the second displacement time falls within a predetermined range, and the time measurement unit 62 outputs a determination signal J2 to the switch signal generation unit 28 for each control cycle. When it is determined that the second displacement time falls within the predetermined range, the determination signal J2 is 1; otherwise, the determination signal J2 is 0.

[0055] The third time measurement unit 62c measures the time from when the output V3 of the third MR sensor 16 becomes equal to or greater than the threshold value TH2 to when the output V2 of the second MR sensor 14 becomes equal to or greater than the threshold value TH2. That is, the third time measurement unit 62c measures the time required for the piston 44 to be displaced in the A1 direction from Xh to Xg (hereinafter referred to as the "third displacement time"). The third time measurement unit 62c determines whether the third displacement time falls within a predetermined range, and the time measurement unit 62 outputs a determination signal J3 to the switch signal generation unit 28 for each control cycle. When it is determined that the third displacement time falls within the predetermined range, the determination signal J3 is 1; otherwise, the determination signal J3 is 0.

[0056] The fourth time measurement unit 62d measures the time from when the output V2 of the second MR sensor 14 falls below the threshold value TH2 to when the output V1 of the first MR sensor 12 falls below the threshold value TH2. That is, the fourth time measurement unit 62d measures the time required for the piston 44 to be displaced in the A1 direction from -Xg to -Xh (hereinafter referred to as the "fourth displacement time"). The fourth time measurement unit 62d determines whether the fourth displacement time falls within a predetermined range, and the time measurement unit 62 outputs a determination signal J4 to the switch signal generation unit 28 for each control cycle. When it is determined that the fourth displacement time falls within the predetermined range, the determination signal J4 is 1; otherwise, the determination signal J4 is 0.

[0057] The predetermined ranges for the first to fourth displacement times are defined using at least the lower limit values ​​that can be assumed from a normal piston speed. If the measured first to fourth displacement times are less than the lower limit values, the switch signal SW is not switched. This more reliably prevents the switch signal SW from being switched by an external magnetic field, such as a welding magnetic field. This effect will be described further below. Note that the predetermined ranges for the first to fourth displacement times may be defined using upper and lower limit values ​​that can be assumed from a normal piston speed, taking into account the warning signal E4 described below.

[0058] The switch signal generator 28 generates a switch signal SW based on the area transition information TR, the first to third comparison signals C1 to C3, and the determination signals J1 to J4, and outputs the switch signal SW to the control unit 18 for each control period.

[0059] Consider the case where the piston 44 is displaced in the A2 direction from the area where X<-Xd to the area where X>Xd. When the area transition information TR is TR71, the switch signal SW is off. When the area transition information TR is TR12 and the second comparison signal C2 is 0, the switch signal SW is off. When the area transition information TR is TR12, the second comparison signal C2 is 1, and the determination signal J1 is 0, the switch signal SW is off. When the area transition information TR is TR12, the second comparison signal C2 is 1, and the determination signal J1 is 1, the switch signal SW is on.

[0060] When the area transition information TR is TR23, TR34, or TR45, the switch signal SW is on. When the area transition information TR is TR56 and the third comparison signal C3 is 0, the switch signal SW is on. When the area transition information TR is TR56, the third comparison signal C3 is 1, and the determination signal J2 is 0, the switch signal SW is on. When the area transition information TR is TR56, the third comparison signal C3 is 1, and the determination signal J2 is 1, the switch signal SW is off. When the area transition information TR is TR67, the switch signal SW is off.

[0061] Consider the case where the piston 44 is displaced in the A1 direction from the area where X > Xd to the area where X < -Xd. When the area transition information TR is TR76, the switch signal SW is off. When the area transition information TR is TR65 and the second comparison signal C2 is 0, the switch signal SW is off. When the area transition information TR is TR65, the second comparison signal C2 is 1, and the determination signal J3 is 0, the switch signal SW is off. When the area transition information TR is TR65, the second comparison signal C2 is 1, and the determination signal J3 is 1, the switch signal SW is on.

[0062] When the area transition information TR is TR54, TR43, or TR32, the switch signal SW is on. When the area transition information TR is TR21 and the first comparison signal C1 is 0, the switch signal SW is on. When the area transition information TR is TR21, the first comparison signal C1 is 1, and the determination signal J4 is 0, the switch signal SW is on. When the area transition information TR is TR21, the first comparison signal C1 is 1, and the determination signal J4 is 1, the switch signal SW is off. When the area transition information TR is TR17, the switch signal SW is off.

[0063] As can be understood from the above explanation, the determination signal J1 is used to switch the switch signal SW from OFF to ON when the piston 44 is displaced in the A2 direction. The determination signal J2 is used to switch the switch signal SW from ON to OFF when the piston 44 is displaced in the A2 direction. The determination signal J3 is used to switch the switch signal SW from OFF to ON when the piston 44 is displaced in the A1 direction. The determination signal J4 is used to switch the switch signal SW from ON to OFF when the piston 44 is displaced in the A1 direction. The on / off operation of the switch signal SW may be reversed.

[0064] If the measured first to fourth displacement times are not within a predetermined range, this may be because the air pressure supplied to the fluid pressure cylinder 40 is outside the appropriate range, or because a component of the fluid pressure cylinder 40 has deteriorated. When the time measurement unit 62 determines that the measured first to fourth displacement times are not within the predetermined range, it outputs a warning signal E4 to the control unit 18. When the control unit 18 receives the warning signal E4 from the time measurement unit 62, it outputs a warning signal E4' to the outside via the communication unit 38 to notify the user of the possibility of an inappropriate supply air pressure or deterioration of the fluid pressure cylinder 40.

[0065] According to this embodiment, the switch signal SW is switched based on at least the area transition information TR and the determination signals J1 to J4. The area transition information TR is information based on the transition of the piston area between two consecutive areas, and the determination signals J1 to J4 are information that takes into account the time required for the piston 44 to make a predetermined displacement. This makes it possible to more reliably distinguish an external magnetic field, such as a welding magnetic field, from the cylinder magnetic field and switch the switch signal SW accordingly.

[0066] The position detection switch according to the present invention is not limited to the above-described embodiment, and various configurations can be adopted without departing from the gist of the present invention.

Claims

1. A position detection switch (10, 60) comprising first to third MR sensors (12, 14, 16) for detecting the magnetic field of a magnet (52) attached to a piston (44) of a fluid pressure cylinder (40), as well as an area transition information generation unit (20) and a switch signal generation unit (28), wherein the area transition information generation unit determines a piston area based on the output of the first to third MR sensors, and updates the area transition information when the piston area transitions between two consecutive areas, but does not update the area transition information when the piston area transitions between two discontinuous areas, and the switch signal generation unit switches a switch signal based on at least the area transition information, and the switch signal is in a predetermined state when the piston is within a predetermined operating range.

2. A position detection switch as claimed in claim 1, comprising a first comparison section (22) which compares the output of the first MR sensor with an operating range threshold and outputs a first comparison signal, a second comparison section (24) which compares the output of the second MR sensor with the operating range threshold and outputs a second comparison signal, and a third comparison section (26) which compares the output of the third MR sensor with the operating range threshold and outputs a third comparison signal, and the switch signal generation section switches the switch signal based on the area transition information and the first to third comparison signals.

3. A position detection switch according to claim 1, wherein the first to third MR sensors have the same output characteristics and are arranged at equal intervals along the displacement direction of the piston.

4. A position detection switch according to claim 1, wherein the piston area is determined based on the relative magnitudes of the outputs of the first to third MR sensors.

5. A position detection switch as claimed in claim 1, further comprising an optimum position determination section (30) for comparing the output of said second MR sensor with an optimum position threshold value to determine whether or not said piston is in an optimum position, said optimum position being included in said operating range.

6. A position detection switch as claimed in claim 5, which outputs a signal to turn on a first indicator light when the piston is in the optimum position, and outputs a signal to turn on a second indicator light when the piston is within the operating range but not in the optimum position.

7. A position detection switch as claimed in claim 1, comprising a first comparison section (22) for comparing the output of said first MR sensor with an operating range threshold and outputting a first comparison signal, a second comparison section (24) for comparing the output of said second MR sensor with said operating range threshold and outputting a second comparison signal, a third comparison section (26) for comparing the output of said third MR sensor with said operating range threshold and outputting a third comparison signal, and a time measurement section (62), wherein said time measurement section measures, for two adjacent MR sensors among said first to third MR sensors, the time from when the output of one MR sensor becomes equal to or exceeds the operating range threshold to when the output of the other MR sensor becomes equal to or exceeds the operating range threshold, or the time from when the output of one MR sensor becomes less than the operating range threshold to when the output of the other MR sensor becomes less than the operating range threshold, and judges whether or not the measured time is within a predetermined range and outputs a judgement signal; The switch signal generating section is a position detection switch that switches the switch signal based on the area transition information, the first to third comparison signals, and the determination signal.

8. A position detection switch as claimed in claim 7, which outputs a warning signal if the measured time is not within the specified range.

9. A position detection switch according to claim 1, further comprising a diagnostic section (32, 34, 36) for monitoring whether or not the maximum value of the outputs of said first to third MR sensors has decreased by a predetermined amount or more.

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