Patent Description:
Conventionally, a fluid pressure cylinder equipped with a sensor for detecting a magnetic force of a magnet mounted on a piston, and detecting a position of the piston has been known.

For example, in <CIT>, a fluid pressure cylinder position detecting device is disclosed in which a pair of magnets are mounted on a piston, and two proximity switches are arranged on an outer side of a cylinder tube. In such a fluid pressure cylinder position detecting device, by using a pair of magnets with the same poles facing toward each other, the peak of a magnetic force to be detected is made larger.

<CIT> describes a fluid pressure cylinder movement time sensor for measuring a time required for a piston to move a piston by a predetermined stroke. The sensor includes a magnetic sensor for detecting magnetic flux of a magnet mounted on the piston.

However, the above-described fluid pressure cylinder position detecting device requires two proximity sensors (magnetic sensors). In general, if there are a plurality of positions of the sensor to be detected, it becomes necessary for a number of magnetic sensors corresponding thereto to be provided. Accordingly, in the case of measuring the time required for the piston to move from a predetermined first position to a predetermined second position, two magnetic sensors are required.

The present invention has the object of solving the aforementioned problem.

A fluid pressure cylinder according to claim <NUM> of the invention comprises a piston and a fluid pressure cylinder movement time sensor, the fluid pressure cylinder movement time sensor comprising a single magnetic sensor configured to detect a magnetic flux density of a magnet mounted on the piston, the fluid pressure cylinder movement time sensor being configured to measure a time required for the piston to move by a predetermined stroke, wherein: the magnet is arranged so that a direction of magnetization thereof coincides with a direction of movement of the piston, and the magnetic sensor is configured to detect the magnetic flux density in a direction perpendicular to the direction of movement of the piston; and the fluid pressure cylinder movement time sensor is configured to monitor a relationship between a position of the magnet along the direction of movement of the piston and the magnetic flux density detected by the magnetic sensor, the relationship being represented by a curve including a positive local maximum value and a negative local minimum value, the fluid pressure cylinder movement time sensor being configured to define a positive value smaller than the positive local maximum value as a first threshold value, and a negative value larger than the negative local minimum value as a second threshold value, a point in time at which the magnetic flux density detected by the magnetic sensor exceeds or falls below the first threshold value as one of a starting point or an end point, a point in time at which the magnetic flux density detected by the magnetic sensor exceeds or falls below the second threshold value as another one of the starting point or the end point, and the fluid pressure cylinder movement time sensor measures a time from the starting point to the end point.

According to the above-described fluid pressure cylinder movement time sensor, the magnetic flux density detected by a single uniaxial magnetic sensor is compared with the first threshold value and the second threshold value, whereby the starting point and the end point when the piston moves by the predetermined stroke can be determined.

The fluid pressure cylinder movement time sensor according to the present invention is capable of determining the starting point and the end point based on the magnetic flux density detected by such a single uniaxial magnetic sensor. Accordingly, using a magnetic sensor having a simple configuration, it is possible to measure the time required for the piston to move by the predetermined stroke.

A basic configuration of a fluid pressure cylinder <NUM> equipped with a fluid pressure cylinder movement time sensor <NUM> according to an embodiment of the present invention is shown in <FIG>. A permanent magnet (magnet) <NUM> is mounted on a piston <NUM> of the fluid pressure cylinder <NUM>. A direction of magnetization of the permanent magnet <NUM> coincides with a direction of movement (X direction) of the piston <NUM>. The fluid pressure cylinder movement time sensor <NUM> is equipped with a magnetic sensor <NUM> capable of detecting a magnetic flux density of the permanent magnet <NUM>, and is arranged on an outer side of a cylinder tube <NUM> of the fluid pressure cylinder <NUM>.

The magnetic sensor <NUM> is a uniaxial magnetic sensor constituted from a Hall IC or the like. The magnetic sensor <NUM> detects a magnetic flux density in a direction (Y direction) perpendicular to the direction of movement of the piston <NUM>, and does not detect a magnetic flux density in the direction of movement of the piston <NUM>. The fluid pressure cylinder movement time sensor <NUM> including the magnetic sensor <NUM> is attached to the cylinder tube <NUM> in a state in which the position thereof along the longitudinal direction (X direction) of the cylinder tube <NUM> can be adjusted. The fluid pressure cylinder movement time sensor <NUM> includes a control unit, a computation unit, a storage unit, and the like, which are constituted from a non-illustrated circuit board.

A relationship between the position of the permanent magnet <NUM> along the direction of movement of the piston <NUM> and the magnetic flux density detected by the magnetic sensor <NUM> is shown in <FIG>. Hereinafter, the position of the permanent magnet <NUM> along the direction of movement of the piston <NUM> is referred to as a "piston position". Further, the magnetic flux density detected by the magnetic sensor <NUM> is referred to as a "detected magnetic flux density". The piston position when the detected magnetic flux density is zero is set to zero.

In a negative region of the piston position, when the piston <NUM> departs from a position located far away from the origin and moves until reaching the origin, the detected magnetic flux density becomes gradually larger within a range of positive values, and after having assumed a local maximum value M1, decreases in a substantially linear manner to zero. In a positive region of the piston position, when the piston <NUM> departs from a position located far away from the origin and moves until reaching the origin, the detected magnetic flux density becomes gradually smaller within a range of negative values, and after having assumed a local minimum value M2, increases in a substantially linear manner to zero.

The detected magnetic flux density changes in a substantially linear manner between a vicinity of the local maximum value and a vicinity of the local minimum value. According to the present invention, for the sake of convenience, the relationship between the piston position and the detected magnetic flux density is represented by a curve in which the linearly changing portion is included. The curve representing the relationship between the piston position and the detected magnetic flux density is symmetrical with respect to the origin. Moreover, in <FIG>, the detected magnetic flux density assumes positive values in the negative region of the piston position, and the detected magnetic flux density assumes negative values in the positive region of the piston position. In the case that the directions of the magnetic poles of the permanent magnet <NUM> are reversed, the detected magnetic flux density assumes negative values in the negative region of the piston position, and the detected magnetic flux density assumes positive values in the positive region of the piston position. Hereinafter, although a description is given on the basis of the curve shown in <FIG>, the same situation also applies in a case in which the directions of the magnetic poles of the permanent magnet <NUM> are reversed.

The fluid pressure cylinder of the present invention comprises a sensor that measures the time required for the piston <NUM> to move by a predetermined stroke (hereinafter, referred to as a "movement time (or stroke time) of the piston <NUM>"). It is important how to determine the starting point and the end point on a time axis when the piston <NUM> moves by the predetermined stroke. Hereinafter, with reference to <FIG>, a description will be given of a method for determining the starting point and the end point using two threshold values of the magnetic flux density.

In <FIG>, a relationship between the piston position and the detected magnetic flux density is shown with specific numerical values, and two threshold values concerning the magnetic flux density are also shown. In the example shown in <FIG>, when the piston position is -<NUM> (millimeters), the detected magnetic flux density becomes a local maximum, and a local maximum value M1 is <NUM> mT (millitesla). Further, when the piston position is <NUM>, the detected magnetic flux density becomes a local minimum, and a local minimum value M2 is -<NUM> mT. The absolute value of the local minimum value M2 is equivalent to the absolute value of the local maximum value M1.

A positive value that is smaller than the local maximum value M1 is set as a first threshold value T1, and a negative value that is larger than the local maximum value M2 is set as a second threshold value T2. The absolute value of the second threshold value T2 may be the same value as the absolute value of the first threshold value T1, or may be a value that differs from the absolute value of the first threshold value T1. In the example shown in <FIG>, the first threshold value T1 is <NUM> mT, the second threshold value T2 is -<NUM> mT, and the absolute value of the second threshold value T2 is equivalent to the absolute value of the first threshold value T1.

Before and after the piston position at which the detected magnetic flux density becomes the local maximum, the detected magnetic flux density becomes equivalent to the first threshold value T1. More specifically, when the piston position is a first position P1 and a second position P2, the detected magnetic flux density becomes equivalent to the first threshold value T1. Further, before and after the piston position at which the detected magnetic flux density becomes the local minimum, the detected magnetic flux density becomes equivalent to the second threshold value T2. More specifically, when the piston position is a third position P3 and a fourth position P4, the detected magnetic flux density becomes equivalent to the second threshold value T2. In the example shown in <FIG>, the first position P1 is -<NUM>, the second position P2 is -<NUM>, the third position P3 is <NUM>, and the fourth position P4 is <NUM>.

First, a case will be considered in which the piston <NUM> moves from the negative region toward the positive region of the piston position. As noted previously, although the detected magnetic flux density is equivalent to the first threshold value T1 at the two piston positions of the first position P1 and the second position P2, at which one of these two piston positions, the detected magnetic flux density has become equivalent to the first threshold value T1, can be determined by monitoring a change in the detected magnetic flux density. More specifically, when the detected magnetic flux density has changed from a state of being smaller than the first threshold value T1 to a state of being larger than the first threshold value T1, it can be determined that the piston <NUM> has passed through the first position P1. When the detected magnetic flux density has changed from a state of being larger than the first threshold value T1 to a state of being smaller than the first threshold value T1, it can be determined that the piston <NUM> has passed through the second position P2.

Further, although the detected magnetic flux density is equivalent to the second threshold value T2 at the two piston positions of the third position P3 and the fourth position P4, at which one of these two piston positions, the detected magnetic flux density has become equivalent to the second threshold value T2, can be determined by monitoring a change in the detected magnetic flux density. More specifically, when the detected magnetic flux density has changed from a state of being larger than the second threshold value T2 to a state of being smaller than the second threshold value T2, it can be determined that the piston <NUM> has passed through the third position P3. When the detected magnetic flux density has changed from a state of being smaller than the second threshold value T2 to a state of being larger than the second threshold value T2, it can be determined that the piston <NUM> has passed through the fourth position P4.

Since the piston <NUM> moves from the negative region toward the positive region, a point in time at which the piston <NUM> has passed through the first position P1, or alternatively, a point in time at which the piston <NUM> has passed through the second position P2 is determined to be the starting point. A point in time at which the piston <NUM> has passed through the third position P3, or alternatively, a point in time at which the piston <NUM> has passed through the fourth position P4 is determined to be the end point. Then, by measuring the time from the starting point to the end point, the movement time of the piston <NUM> can be measured.

A combination of the aforementioned two starting points and the aforementioned two end points is arbitrary, and there are a total of four of such combinations. From among the four combinations, in the case that the first position P1 is the starting point and the fourth position P4 is the end point, the distance between the piston position at the starting point and the piston position at the end point becomes maximum. Further, from among the four combinations, in the case that the second position P2 is the starting point and the third position P3 is the end point, the distance between the piston position at the starting point and the piston position at the end point becomes minimum. Hereinafter, the piston position at the starting point is referred to as a "starting point position", and the piston position at the end point is referred to as an "end point position".

Next, a case will be considered in which the piston <NUM> moves from the positive region toward the negative region of the piston position. Although the detected magnetic flux density is equivalent to the second threshold value T2 at the two piston positions of the fourth position P4 and the third position P3, at which one of these two piston positions, the detected magnetic flux density has become equivalent to the second threshold value T2, can be determined by monitoring a change in the detected magnetic flux density. More specifically, when the detected magnetic flux density has changed from a state of being larger than the second threshold value T2 to a state of being smaller than the second threshold value T2, it can be determined that the piston <NUM> has passed through the fourth position P4. When the detected magnetic flux density has changed from a state of being smaller than the second threshold value T2 to a state of being larger than the second threshold value T2, it can be determined that the piston <NUM> has passed through the third position P3.

Further, although the detected magnetic flux density is equivalent to the first threshold value T1 at the two piston positions of the second position P2 and the first position P1, at which one of these two piston positions, the detected magnetic flux density has become equivalent to the first threshold value T1, can be determined by monitoring a change in the detected magnetic flux density. More specifically, when the detected magnetic flux density has changed from a state of being smaller than the first threshold value T1 to a state of being larger than the first threshold value T1, it can be determined that the piston <NUM> has passed through the second position P2. When the detected magnetic flux density has changed from a state of being larger than the first threshold value T1 to a state of being smaller than the first threshold value T1, it can be determined that the piston <NUM> has passed through the first position P1.

Since the piston <NUM> moves from the positive region toward the negative region, a point in time at which the piston <NUM> has passed through the fourth position P4, or alternatively, a point in time at which the piston <NUM> has passed through the third position P3 is determined to be the starting point. A point in time at which the piston <NUM> has passed through the second position P2, or alternatively, a point in time at which the piston <NUM> has passed through the first position P1 is determined to be the end point. Then, by measuring the time from the starting point to the end point, the movement time of the piston <NUM> can be measured.

A combination of the aforementioned two starting points and the aforementioned two end points is arbitrary, and there are a total of four of such combinations. From among the four combinations, in the case that the fourth position P4 is the starting point and the first position P1 is the end point, the distance between the piston position at the starting point and the piston position at the end point becomes maximum. Further, from among the four combinations, in the case that the third position P3 is the starting point and the second position P2 is the end point, the distance between the piston position at the starting point and the piston position at the end point becomes minimum.

Next, a description will be given concerning a method for setting determination conditions for the starting point and the end point including the first threshold value T1 and the second threshold value T2, in the case that the starting point position and the end point position are specified. It is taken as a premise that the magnetic sensor <NUM> is arranged between the specified starting point position and the specified end point position.

The fluid pressure cylinder <NUM> is driven, whereby the piston <NUM> is actually moved from the specified starting point position to the specified end point position, and the detected magnetic flux density is monitored. Moreover, although the starting point position and the end point position are piston positions, such positions can be easily understood, for example, when the position of an end portion of a piston rod <NUM> that extends to the exterior from the cylinder tube <NUM> is considered, instead of the piston position.

The sign of the detected magnetic flux density at the specified end point position differs from the sign of the detected magnetic flux density at the specified starting point position. Hereinafter, explanations will be provided separately for a case in which the former detected magnetic flux density is a positive value and the latter detected magnetic flux density is a negative value, and a case in which the former detected magnetic flux density is a negative value and the latter detected magnetic flux density is a positive value.

In the case that the detected magnetic flux density at the specified starting point position is a positive value and the detected magnetic flux density at the specified end point position is a negative value, the former is set as the first threshold value T1, and the latter is set as the second threshold value T2. If the detected magnetic flux density has increased immediately after the piston <NUM> has separated away from the specified starting point position, a condition that "the detected magnetic flux density exceeds the first threshold value T1" is set as the determination condition for the starting point. If the detected magnetic flux density has decreased immediately after the piston <NUM> has separated away from the specified starting point position, a condition that "the detected magnetic flux density falls below the first threshold value T1" is set as the determination condition for the starting point.

Further, if the piston <NUM> has reached the specified end point position while the detected magnetic flux density is decreasing, a condition that "the detected magnetic flux density falls below the second threshold value T2" is set as the determination condition for the end point. If the piston <NUM> has reached the specified end point position while the detected magnetic flux density is increasing, a condition that "the detected magnetic flux density exceeds the second threshold value T2" is set as the determination condition for the end point.

On the other hand, in the case that the detected magnetic flux density at the specified starting point position is a negative value and the detected magnetic flux density at the specified end point position is a positive value, the former is set as the second threshold value T2, and the latter is set as the first threshold value T1. If the detected magnetic flux density has decreased immediately after the piston <NUM> has separated away from the specified starting point position, a condition that "the detected magnetic flux density falls below the second threshold value T2" is set as the determination condition for the starting point. If the detected magnetic flux density has increased immediately after the piston <NUM> has separated away from the specified starting point position, a condition that "the detected magnetic flux density exceeds the second threshold value T2" is set as the determination condition for the starting point.

Further, if the piston <NUM> has reached the specified end point position while the detected magnetic flux density is increasing, a condition that "the detected magnetic flux density exceeds the first threshold value T1" is set as the determination condition for the end point. If the piston <NUM> has reached the specified end point position while the detected magnetic flux density is decreasing, a condition that "the detected magnetic flux density falls below the first threshold value T1" is set as the determination condition for the end point.

In this manner, the determination conditions for the starting point and the end point can be obtained by actually moving the piston <NUM> from the specified starting point position to the specified end point position, and monitoring the detected magnetic flux density. Including the first threshold value T1 and the second threshold value T2, these determination conditions for the starting point and the end point are stored in the storage unit or the control unit of the fluid pressure cylinder movement time sensor <NUM>. Consequently, thereafter, the movement time of the piston <NUM> can be easily calculated in the computation unit of the fluid pressure cylinder movement time sensor <NUM>.

While referring to <FIG>, a specific example will be described of setting the determination conditions for the starting point and the end point according to the above-described method. As the starting point position, P1' (-<NUM>), which corresponds to the aforementioned first position P1, is specified by a user. Further, as the end point position, P3' (<NUM>), which corresponds to the aforementioned third position P3, is specified by the user. When designating the positions P1' and P3', the user does not need to be aware of their numerical values, and for example, the user may simply specify two locations of the end portion of the piston rod <NUM>. However, it is necessary that the magnetic sensor <NUM> be arranged at least between the positions P1' and P3'.

The piston <NUM> was actually moved from the position P1' to the position P3', and the detected magnetic flux density was monitored. It was monitored that the detected magnetic flux density was <NUM> mT when the piston position was the position P1', and the detected magnetic flux density exceeded <NUM> mT immediately after the piston <NUM> separated away from the position P1'. In this case, the first threshold value T1 is set to <NUM> mT, and a condition that "the detected magnetic flux density exceeds the first threshold value T1" is set as the determination condition for the starting point. Further, it was monitored that the detected magnetic flux density when the piston position was the position P3' was -<NUM> mT, and the piston <NUM> reached the position P3' while the detected magnetic flux density was decreasing. In this case, the second threshold value T2 is set to -<NUM> mT, and a condition that "the detected magnetic flux density falls below the second threshold value T2" is set as the determination condition for the end point.

In the case that there are a plurality of sets of the specified starting point position and the specified end point position, the determination conditions for the starting points and the end points are set for each of such sets. For example, a case may be presented in which it is desired to measure not only the time required for the piston <NUM> to move by the predetermined stroke in one direction, but also to measure the time required for the piston <NUM> to move by the same stroke in the opposite direction.

It is also possible to obtain the piston speed (an average value of the piston speed) on the basis of the movement time of the piston <NUM>. More specifically, the reciprocal of the movement time of the piston <NUM> is multiplied by a constant corresponding to the distance from the starting point position to the end point position. According to this method, the piston speed can be calculated by the computation unit of the fluid pressure cylinder movement time sensor <NUM>. Further, if the piston speed is obtained for a plurality of continuous segments, then a change in the piston speed can be detected.

Next, while referring to <FIG>, a description will be given concerning a first mode of use of the fluid pressure cylinder movement time sensor <NUM> of the fluid pressure cylinder according to the present invention. The first mode of use is a mode that is used to determine whether or not a buffering action of a shock absorber <NUM> that serves to decelerate the piston <NUM> is appropriate. Moreover, in <FIG>, the same or equivalent members as those in the basic configuration shown in <FIG> are designated by the same reference numerals.

In the example shown in <FIG>, the shock absorber <NUM> is a fluid type shock absorber, which exerts a buffering action in the vicinity of a stroke end of the piston <NUM> when the piston rod <NUM> is pulled in. The magnetic sensor <NUM> is arranged at a position where it is capable of effectively detecting a change in the magnetic flux density of the permanent magnet <NUM> that is mounted on the piston <NUM> in the vicinity of the stroke end. Determination conditions for the starting point and the end point are appropriately set, and the movement time of the piston <NUM> is measured. Moreover, reference numeral <NUM> indicates an end plate that is fixed to the end portion of the piston rod <NUM>.

In the case that the buffering action by the shock absorber <NUM> is appropriate, the movement time of the piston <NUM> lies within a range of a reference time period defined by a predetermined upper limit value and a predetermined lower limit value. On the other hand, in the case that the buffering action of the shock absorber <NUM> is too strong, the movement time of the piston <NUM> becomes longer than the upper limit value. Further, in the case that the buffering action of the shock absorber <NUM> is too weak, the movement time of the piston <NUM> becomes shorter than the lower limit value. When the movement time of the piston <NUM> lies outside of the reference time period, it can be determined that the buffering action of the shock absorber <NUM> is either too strong or too weak. Then, a notification is issued to the user that the shock absorber <NUM> needs to be adjusted.

Next, while referring to <FIG>, a description will be given concerning a second mode of use of the fluid pressure cylinder movement time sensor <NUM> of the fluid pressure cylinder according to the present invention. The second mode of use is a mode that is used to obtain the piston speed on the basis of the movement time of the piston <NUM> in order for the user to adjust a speed controller <NUM> provided in the fluid pressure cylinder <NUM>. Moreover, in <FIG>, the same or equivalent members as those in the basic configuration shown in <FIG> are designated by the same reference numerals.

In the example shown in <FIG>, the speed controller <NUM> is a meter-out variable throttle valve, which throttles the flow rate of the air discharged at a time when the piston rod <NUM> of the fluid pressure cylinder <NUM> is pushed out. The speed controller <NUM> is arranged in a discharge port (not shown) of the fluid pressure cylinder <NUM>. The magnetic sensor <NUM> is arranged at a position where it is capable of detecting a change in the magnetic flux density of the permanent magnet <NUM> that is mounted on the piston <NUM> when the piston rod <NUM> is pushed out. Determination conditions for the starting point and the end point are appropriately set, and the movement time of the piston <NUM> is measured.

Using a constant corresponding to the distance from the starting point position to the end point position, the movement time of the piston <NUM> is converted into a piston speed, and the piston speed is displayed on a display (not shown). The user observes the displayed piston speed, and in the case that the piston speed deviates from a desired value, the user manually operates the speed controller <NUM>, and adjusts a flow path area for the air.

Next, while referring to <FIG>, a description will be given concerning a third mode of use of the fluid pressure cylinder movement time sensor <NUM> of the fluid pressure cylinder according to the present invention. The third mode of use is a mode that is used to detect that an elastic body <NUM> has been clamped in the fluid pressure cylinder <NUM>, which clamps the elastic body <NUM> while compressing the elastic body <NUM>. Moreover, in <FIG>, the same or equivalent members as those in the basic configuration shown in <FIG> are designated by the same reference numerals.

In the example shown in <FIG>, when the piston <NUM> is driven by a predetermined amount in a direction to push the piston rod <NUM> out, an end plate <NUM> that is fixed to the end portion of the piston rod <NUM> comes into contact with the elastic body <NUM>. When the piston rod <NUM> comes into contact with the elastic body <NUM>, the piston rod <NUM> receives a reaction force from the elastic body <NUM>, and the piston speed is decelerated. The magnetic sensor <NUM> is arranged at a position where it is capable of detecting a change in the magnetic flux density of the permanent magnet <NUM> that is mounted on the piston <NUM>, within a range in which at least a position where the piston rod <NUM> comes into contact with the elastic body <NUM> is included.

Determination conditions are set for a plurality of sets of the starting point and the end point so that the piston speed can be obtained for a plurality of continuous segments, and the time required for the piston <NUM> to move in each of such segments is measured. Then, using a constant corresponding to the distance of each of such segments, the time required for the piston <NUM> to move in each of the segments is converted into the piston speed. When the piston speed is significantly reduced in adjacent segments, it can be determined that the elastic body <NUM> has been clamped by the fluid pressure cylinder <NUM>.

In addition to the first to third modes of use, various other modes of use of the fluid pressure cylinder movement time sensor <NUM> of the fluid pressure cylinder according to the present invention may be considered. For example, a mode may be considered that is used in the case that the movement time of the piston <NUM> is compared with the reference time period, and when the movement time of the piston <NUM> deviates from the reference time period, a notification is issued to the user that some type of abnormality has occurred. The cause of the abnormality can be assumed in accordance with the mode of use of the fluid pressure cylinder <NUM>. For example, in the case that transportation is carried out by the fluid pressure cylinder <NUM>, since there is a high likelihood that foreign matter being caught therein or dropping of the transported object will become a cause of the abnormality, a notification to that effect may be issued to the user.

Claim 1:
A fluid pressure cylinder (<NUM>) comprising a piston (<NUM>) and a fluid pressure cylinder movement time sensor (<NUM>), the fluid pressure cylinder movement time sensor (<NUM>) comprising a single magnetic sensor (<NUM>) configured to detect a magnetic flux density of a magnet (<NUM>) mounted on the piston (<NUM>), the fluid pressure cylinder movement time sensor being configured to measure a time required for the piston to move by a predetermined stroke, wherein:
the magnet is arranged so that a direction of magnetization thereof coincides with a direction of movement of the piston (<NUM>), and the magnetic sensor is configured to detect the magnetic flux density in a direction perpendicular to the direction of movement of the piston (<NUM>); and
the fluid pressure cylinder movement time sensor (<NUM>) is configured to monitor a relationship between a position of the magnet along the direction of movement of the piston (<NUM>) and the magnetic flux density detected by the magnetic sensor (<NUM>), the relationship being represented by a curve including a positive local maximum value (M1) and a negative local minimum value (M2),
the fluid pressure cylinder movement time sensor (<NUM>) being configured to define a positive value smaller than the positive local maximum value (M1) as a first threshold value (T1), and a negative value larger than the negative local minimum value (M2) as a second threshold value (T2), a point in time at which the magnetic flux density detected by the magnetic sensor (<NUM>) exceeds or falls below the first threshold value (T1) as one of a starting point or an end point, a point in time at which the magnetic flux density detected by the magnetic sensor (<NUM>) exceeds or falls below the second threshold value (T2) as another one of the starting point or the end point, and the fluid pressure cylinder movement time sensor (<NUM>) measures a time from the starting point to the end point.