Patent Publication Number: US-9891135-B2

Title: Fault detection system for actuator

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-142556 filed on Jul. 8, 2013, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a fault detection system for detecting an abnormality of an actuator, based on a movement time of a movable member in the actuator. 
     Description of the Related Art 
     With an actuator in which a movable member is displaced upon supply of a pressure fluid (e.g., a pressurized gas), maintenance typically is carried out prior to the occurrence of a fault, when the frequency of use or a number of operations (operating time, duration of use) thereof reaches a given empirically determined level. 
     However, conventionally, even with a normal actuator that is not currently suffering from deterioration, under routine maintenance, replacement of the actuator may take place unnecessarily, leading to an increase in costs. Further, in the market, it has been sought to enhance productivity of equipment in which the actuator is used, and to reduce the cost of products made by such equipment, by shortening the stroke time (tact) and thereby increasing the tact time of the movable actuator. For this purpose, it is desirable for the maintenance interval not to be set based on the judgment of an operator, but rather for the actuator to be managed automatically and numerically. 
     Further, in general, it is thought that deterioration of an actuator in which a pressure fluid is used occurs due to load conditions applied to the actuator, or time-based variations, i.e., aging, of a fluid pressure device such as pneumatic device or the like in which the actuator is included. Furthermore, assuming that the occurrence of a fault in the actuator can be detected prior to a failure of the actuator due to deterioration caused by changes in the tact time, the fluid pressure device can continue to be used until just prior to the end of its life cycle, thereby enabling the equipment to be operated with maximum efficiency. 
     Thus, instead of carrying out maintenance operations based on frequency of use or a number of operations (operating time, duration of use), various types of fault detection systems, which are equipped with a failure prediction function for detecting a fault or abnormality of the actuator automatically and numerically, have been proposed. 
     For example,  FIG. 9  illustrates a fault detection system  100  in which a fault of an actuator is detected by measuring variations in the flow rate and pressure of a pressure fluid. In such a fault detection system  100 , a pressure fluid is supplied selectively to an actuator  106  from a fluid pressure source  102  through a directional switching valve  104 . In the interior of the actuator  106 , which comprises a cylinder, a piston  110  to which there is connected a piston rod  108  is displaced in left and right directions of  FIG. 9  between one end  116  and another end  118  of the actuator  106 . 
     The directional switching valve  104  comprises a 4-way 5-port single-acting solenoid valve having a solenoid  112  and a spring  114 . More specifically, when the solenoid  112  is actuated by supplying an external control signal (operation command), the directional switching valve  104  supplies pressure fluid from the fluid pressure source  102  to the one end  116  of the actuator  106  through a port  120 , whereas the fluid (pressure fluid) at the other end  118  of the actuator  106  is discharged to the exterior through a port  122 . As a result, the piston  110  is displaced from the one end  116  to the other end  118 . 
     On the other hand, when supply of the control signal is stopped, under operation of the spring  114 , the directional switching valve  104  supplies the pressure fluid from the fluid pressure source  102  to the other end  118  through the port  122 , whereas the pressure fluid at the one end  116  is discharged to the exterior through the port  120 . As a result, the piston  110  is displaced from the other end  118  to the one end  116 . 
     Further, at an intermediate location of tubes  123 ,  125  that serve to connect the directional switching valve  104  and the ports  120 ,  122 , couplings  124 ,  126  are arranged respectively, which are constituted by parallel-connected throttle and check valves. 
     In this case, as shown by the dashed-line arrows in  FIG. 9 , there is a possibility of the pressure fluid to leak from respective portions of the fault detection system  100 . More specifically, external leakage of pressure fluid can take place (1) from the respective tubes  123 ,  125 ,  127  that are arranged between the fluid pressure source  102 , the directional switching valve  104  and the actuator  106 , (2) from the directional switching valve  104 , (3) from the piston rod  108  and from a non-illustrated packing provided between the cylinder and the piston rod  108 , and (4) from the couplings  124 ,  126 . Further, in the interior of the actuator  106  as well, there is a possibility for pressure fluid to leak between the one end  116  and the other end  118  via the piston  110  and a non-illustrated packing provided between the cylinder and the piston  110 . 
     Thus, in the fault detection system  100 , non-illustrated flow meters and pressure gauges are arranged in each of the tubes  123 ,  125 ,  127 , whereby the flow rate of the pressure fluid is measured by the respective flow meters, and the pressure of the pressure fluid is measured by the respective pressure gauges. Consequently, since variations in the flow rate and pressure of the pressure fluid can be measured, faults at locations where the pressure fluid leaks can be detected, and replacement of the affected components prior to a failure thereof can be performed. 
     On the other hand, the fault detection system  130  of  FIG. 10  detects an abnormality of an actuator  106  based on the stroke time of a piston  110 . In addition to the respective constituent elements of the aforementioned fault detection system  100 , the fault detection system  130  further includes a controller  132  such as a PLC (Programmable Logic Controller) or the like for supplying control signals to the solenoid  112  from an output  132   a , a first sensor  134  arranged on the one end  116  of the actuator  106 , and a second sensor  136  arranged on the other end  118  of the actuator  106 . The couplings  124 ,  126  referred to above (see  FIG. 9 ) are not provided in the fault detection system  130 . Further, in the fault detection system  130 , a silencer  138  is arranged in a discharge passageway for the pressure fluid, which is discharged from the one end  116  and the other end  118  of the actuator  106 . 
     The first sensor  134  detects the piston  110  upon displacement thereof to the one end  116 . The second sensor  136  detects the piston  110  upon displacement thereof to the other end  118 . Detection signals indicative of detection results of the piston  110  by the first sensor  134  and the second sensor  136  are input to an input  132   b  of the controller  132 . Thus, in the controller  132 , a time (stroke time of the piston  110 ), from an output time at which the control signal is output to the directional switching valve  104 , until an input time, at which the detection signal is input (i.e., a time at which displacement of the piston  110  is completed), is measured, and based on the measured stroke time, an abnormality of the actuator  106  is detected. 
     Further, similar to the fault detection system  130  of  FIG. 10 , techniques for detecting abnormalities of an actuator by using the stroke time of a movable member of the actuator are disclosed in Japanese Laid-Open Patent Publication No. 10-281113 (hereinafter referred to as Patent Document 1) and Japanese Laid-Open Patent Publication No. 2002-174358 (hereinafter referred to as Patent Document 2). 
     As disclosed in Patent Document 1, a velocity and a stroke time from initiation of an operation command of an actuator and a driven body are measured. The measured velocity and stroke time are compared with a reference velocity and stroke time during normal operation, and a judgment is made as to whether the actuator and the driven body are functioning normally. 
     As disclosed in Patent Document 2, a time from energization of a solenoid valve until the piston of a double-acting cylinder reaches a stroke end is measured as a stroke time, and a warning is issued if the measured stroke time becomes equal to or greater than a predetermined threshold value. 
     SUMMARY OF THE INVENTION 
     However, with the fault detection system  100  shown in  FIG. 9 , it is necessary for a fault such as deterioration or the like to be detected in a condition in which the equipment as a whole including the actuator  106  thereof is temporarily stopped. In other words, an abnormality of the actuator  106  cannot be detected during ongoing operation of the equipment. Accordingly, with the fault detection system  100 , there is a concern that productivity of the equipment will be reduced as a result of maintenance operations carried out thereon. 
     Further, with the fault detection system  130  shown in  FIG. 10 , through a combination of the controller  132 , the first sensor  134 , and the second sensor  136 , the responsiveness of the actuator  106  (stroke time or travel time of the piston  110 ) is measured. As a result, it is possible for faults of the actuator  106  to be detected during operation of the equipment. In this case, the accuracy of the response depends on the processing speed of the controller  132 . For this reason, in the event that a fault of a small sized actuator  106  which operates at high speed is to be detected, it is necessary to construct a measurement system including a controller  132  having a high processing speed, thus increasing the cost of the system. Further, since a PLC is used in the controller  132 , it is necessary for a user (operator) to construct the fault detection system  130 , as well as to create the controller program used by the PLC, which increases the burden on the operator. 
     Additionally, in the case that plural actuators are used in one set of equipment, the user must create controller programs for measuring the stroke times of all of the actuators, and set such programs in the PLC, which is time consuming. Further, since a large capacity memory and a PLC with advanced programming capabilities are needed for storing the controller programs and the measurement results, construction of the fault detection system  130  tends to be expensive. 
     Moreover, in the techniques disclosed in Patent Document 1 and Patent Document 2, similar problems to those of the fault detection system  130  of  FIG. 10  are raised. 
     The present invention seeks to resolve the aforementioned problems, and has the object of providing a fault detection system for an actuator, in which maintainability of the fault detection system can be enhanced, by easily detecting faults of the actuator without requiring stoppage of the equipment. 
     The fault detection system for an actuator according to the present invention enables detection of faults of the actuator based on a stroke time of a movable member of the actuator. The fault detection system includes the following first through ninth features. 
     More specifically, according to the first feature, the fault detection system is equipped with a first sensor, a second sensor, and a fault detecting device. The first sensor is disposed on one end of the actuator along a displacement direction of the movable member, and detects the movable member upon displacement thereof to the one end. The second sensor is disposed on another end of the actuator along the displacement direction, and detects the movable member upon displacement thereof to the other end. 
     The fault detecting device detects a fault of the actuator based on detection results of the first sensor and the second sensor. 
     More specifically, the fault detecting device further includes a stroke time calculator, a statistical computation processing unit, and a fault detector. The stroke time calculator calculates a stroke time required for the movable member to travel between the one end and the other end, based on each of the detection results. The statistical computation processing unit performs a predetermined statistical calculation with respect to the calculated stroke time. The fault detector detects whether or not a fault of the actuator has occurred, based on a processing result of the statistical computation processing unit. 
     According to the above-described first feature, the statistical calculation is carried out with respect to the stroke time of the movable member, and based on the processing result it is detected whether or not a fault of the actuator has occurred. Therefore, even during operation of equipment including the actuator, a fault of the actuator can be detected without requiring the equipment to be stopped. As a result, productivity of such equipment is maintained, and faults of the actuator can be detected in real time while the equipment remains online. 
     Further, a maintenance cycle, which heretofore has been set (defined) based on the operator&#39;s judgment, can be managed automatically and numerically. More specifically, even if maintenance operations are not carried out regularly by the operator, the fault detection system carries out maintenance automatically during operation of the equipment, and based on the stroke time, which serves as response information from the actuator, the occurrence of abnormalities of the actuator are determined easily. In addition, with the fault detection system, based on the processing result of the statistical calculation carried out with respect to the stroke time of the movable member, whether or not an abnormality of the actuator has occurred can be judged (managed) numerically. 
     As a result, according to the present invention, the number of processing steps required for maintenance can be reduced, the burden imposed on the operator can be mitigated significantly, and maintainability of the equipment including the actuator can be enhanced. Further, by being managed numerically, training and education of the operator in charge of such maintenance is facilitated. 
     Furthermore, since the stroke time of the movable member is calculated based on the detection results of the first sensor and the second sensor, existing sensors can be used without modification. More specifically, the fault detection system can be constructed merely by adding the fault detecting device with respect to conventional existing sensors. Accordingly, with the present invention, faults of the actuator can be detected easily and at low cost. 
     In the second feature of the present invention, the fault detecting device further includes a first storage unit in which the stroke time is stored, and a second storage unit in which the processing result is stored. In this case, preferably, the fault detector reads out at least the processing result that is stored in the second storage unit, and detects whether or not a fault of the actuator has occurred based on the read out processing result. 
     According to the second feature, since stroke times are stored (accumulated) in the first storage unit, even in the case that the movable member travels (moves reciprocally) between the one end and the other end, the statistical computation processing unit can sequentially read out the stroke times from the first storage unit, and carry out the statistical calculation thereon. Further, since processing results are stored (accumulated) in the second storage unit, the fault detector can suitably read out the processing results from the second storage unit, and carry out a detection process thereon. 
     In the third feature of the present invention, a normal stroke time of the movable member preferably is stored beforehand as a normal value in the first storage unit. In this case, the statistical computation processing unit calculates at least a deviation between the stroke time calculated by the stroke time calculator and the normal value, and stores the calculated deviation as a statistically calculated value in the second storage unit. Further, the fault detector reads out the statistically calculated value from the second storage unit, and judges whether or not a fault of the actuator has occurred based on the read out statistically calculated value. 
     According to the third feature, based on a comparison between the normal value, which is set beforehand, and the actually calculated stroke time, since it can be determined whether or not a fault of the actuator has occurred, the occurrence of a fault of the actuator can be judged accurately. More specifically, if the actuator becomes deteriorated, each time that the stroke time is calculated based on the respective detection results of the first sensor and the second sensor, the variability in the aforementioned deviation becomes greater. Thus, for example, if the deviation becomes greater than a predetermined threshold, it can easily be judged that a fault of the actuator has occurred. 
     Moreover, the normal stroke time of the movable member is defined as a stroke time of the movable member between the one end and the other end, in a state in which an abnormality such as deterioration or failure of the actuator is not occurring (e.g., an initial operating state of the actuator immediately after installation or replacement thereof). The normal stroke time may be set beforehand by an operator, or may be stored in the first storage unit at the time that the fault detecting device is manufactured. 
     Further, instead of the third feature, the fault detecting device can be configured to include the fourth feature of the invention, as noted below. 
     More specifically, according to the fourth feature, in the case that the first sensor and the second sensor detect the movable member each time that the movable member moves in the displacement direction, the stroke time calculator calculates and stores the stroke time in the first storage unit each time that respective detection results from the first sensor and the second sensor are input. 
     In this case, each time that the stroke time calculator calculates and stores the stroke time in the first storage unit, the statistical computation processing unit reads out data of all of the stroke times that are stored in the first storage unit, calculates an average value, a standard deviation, or a variance of the read out data, and stores the average value, the standard deviation, or the variance as a statistically calculated value in the second storage unit. 
     Additionally, it is preferable that an average value, a standard deviation, or a variance of a normal stroke time of the movable member is stored as a normal value in the second storage unit. 
     Consequently, each time that the statistical computation processing unit stores the statistically calculated value in the second storage unit, the fault detector reads out the statistically calculated value and the normal value from the second storage unit, and can detect whether or not a fault of the actuator has occurred based on a comparison between the statistically calculated value and the normal value. 
     Thus, according to the fourth feature, whether or not a fault of the actuator has occurred can be detected easily and in real time during operation of the actuator (i.e., during a time that the movable member moves reciprocally back and forth along the direction of displacement). More specifically, using actually calculated data of the stroke time, the statistical computation processing unit sequentially calculates an average value, a standard deviation, or a variance of the data, and stores the same as a statistically calculated value in the second storage unit. Further, based on a comparison between the statistically calculated value and the normal value that are stored in the second storage unit, the fault detector can judge sequentially whether a fault of the actuator has occurred. 
     Furthermore, if the actuator becomes deteriorated, each time that the stroke time is calculated based on the respective detection results of the first sensor and the second sensor, the variability in the aforementioned average value, the standard deviation, or the variance becomes greater. Thus, for example, if the average value, the standard deviation, or the variance becomes greater than a predetermined threshold, it can easily be judged that a fault of the actuator has occurred. 
     The fifth feature specifies in greater detail certain structural components of the fourth feature. More specifically, in the fifth feature, in the case that the movable member is moved reciprocally along the displacement direction at a fixed time period from an initial state of operation of the actuator, the stroke time calculator calculates the stroke time of the movable member and stores the calculated stroke time in the first storage unit, each time that respective detection results from the first sensor and the second sensor are input. 
     In this case, the statistical computation processing unit reads out the data of all of the stroke times that are stored in the first storage unit, calculates an average value, a standard deviation, or a variance of the read out data, and stores the average value, the standard deviation, or the variance as a normal value in the second storage unit. 
     According to the fifth feature, in an initial state of operation immediately after installation or replacement of the actuator in the equipment, the normal value is calculated automatically, and is stored in the second storage unit. Thus, setting of the normal value can be performed with high efficiency. 
     The sixth feature specifies in greater detail the second through fifth features of the present invention. 
     More specifically, in the sixth feature, the fault detection system further comprises a directional switching valve that selectively supplies a pressure fluid to the one end or the other end of the actuator, based on an external control signal supplied thereto. In this case, in accordance with the selective supply of the pressure fluid, the movable member is displaced in the displacement direction to the one end or the other end of the actuator. 
     In addition, the stroke time calculator calculates as the stroke time of the movable member a time difference between a first detection time from start of supply of the control signal to the directional switching valve until a time at which the movable member can no longer be detected by one of the first sensor and the second sensor, and a second detection time from the start of supply until a time at which detection of the movable member by the other of the first sensor and the second sensor is started. 
     According to the sixth feature, by calculating as the stroke time the time difference between the first detection time and the second detection time, the stroke time can be calculated easily and reliably. 
     In the seventh feature, the stroke time calculator stores the first detection time, the second detection time, and the stroke time in the first storage unit. Further, the statistical computation processing unit performs a predetermined statistical calculation with respect to the first detection time, and stores a processing result of the statistical calculation in the second storage unit. Consequently, the fault detector reads out the processing result with respect to the first detection time that is stored in the second storage unit, and based on the read out processing result, is capable of detecting whether or not a fault has occurred at a location between the directional switching valve and the actuator. 
     In this manner, according to the seventh feature, in addition to detecting a fault of the actuator, a fault that occurs between the directional switching valve and the actuator can be detected. The statistical calculation performed with respect to the first detection time may be the same process (calculation of an average value, a standard deviation, or a variance) as the statistical calculation performed with respect to the stroke time. 
     In the eighth feature of the invention, the fault detection system further comprises a controller that supplies the control signal to the directional switching valve. In this case, the fault detecting device further includes an output processor that supplies the control signal from the controller to the directional switching valve, and outputs the detection result of the fault detector to the controller. 
     According to the eighth feature, the controller, which is made up from a PLC or the like, supplies the control signal to the directional switching valve through the fault detecting device, whereas the controller receives the detection result from the fault detecting device. As a result, the controller is capable of grasping (detecting) a fault of the actuator in an online state, and based on the detection result, can take an appropriate action such as stopping supply of the control signal. 
     Further, in the fault detecting device, a malfunction of the actuator is detected, and the detection result alone is output to the controller. Therefore, it is unnecessary for the operator to create a control program for use by the controller in order to detect a malfunction of the actuator. As a result, the load imposed on the operator to construct the fault detection system can be reduced. 
     In the ninth feature of the invention, the fault detection system further includes a display device that displays the stroke time stored in the first storage unit, the processing result stored in the second storage unit, and the detection result of the fault detector. 
     In accordance with the ninth feature, by visually confirming the content displayed on the display device, the operator can grasp the occurrence of a fault of the actuator, and can quickly carry out an appropriate action such as halting operation of the equipment, replacing the actuator, etc. 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings, in which a preferred embodiment of the present invention is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a fault detection system for an actuator according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram showing a partial modification of the fault detection system of  FIG. 1 ; 
         FIG. 3  is a block diagram of the failure detecting device shown in  FIGS. 1 and 2 ; 
         FIG. 4  is a graph showing a difference between a normal component and a component in which a fault is occurring, in relation to the actuator shown in  FIGS. 1 and 2 ; 
         FIG. 5  is a timing chart showing operations of the fault detection system of  FIGS. 1 and 2 ; 
         FIG. 6  is a schematic diagram showing a partial modification of the fault detection system of  FIG. 1 ; 
         FIG. 7  is a schematic diagram showing a partial modification of the fault detection system of  FIG. 2 ; 
         FIG. 8  is a timing chart showing a case in which the fault detection system of  FIGS. 1, 2, 6 and 7  is applied to the system of Patent Document 2; 
         FIG. 9  is a schematic diagram of a fault detection system for an actuator according to one conventional technique; and 
         FIG. 10  is a schematic diagram of a fault detection system for an actuator according to another conventional technique. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of a fault detection system for an actuator according to the present invention will be described in detail below with reference to the accompanying drawings. 
     [Overall Configuration of Fault Detection System] 
     As shown in  FIG. 1 , a fault detection system  10  for an actuator according to the present embodiment (hereinafter referred to as a fault detection system  10  according to the present embodiment) is equipped with a controller  12  such as a PLC or the like, a failure detecting device  14  (fault detecting device), a directional switching valve  16  comprising a 4-way, 5-port double-acting solenoid valve, an actuator  18  such as a fluid pressure cylinder or the like, and a first sensor  20  (first sensor) and a second sensor  22  (second sensor), which are arranged on an outer circumferential surface of the actuator  18 . 
     The fault detection system  10  is incorporated in non-illustrated equipment to make up a system that is equipped with a failure prediction function, which is capable of automatically detecting a fault such as deterioration or failure of the actuator  18 , etc., during operation of the equipment (i.e., during manufacturing of products) without requiring stoppage of the equipment. 
     The controller  12  includes an output unit  12   a , an input unit  12   b , and a detection result input unit  12   c . The output unit  12   a  supplies control signals (control commands) to solenoids  16   a ,  16   b  of the directional switching valve  16  through the failure detecting device  14 . Detection signals, which are indicative of the detection results produced by the first sensor  20  and the second sensor  22 , are input through the failure detecting device  14  to the input unit  12   b . A detection signal, which is indicative of the occurrence or non-occurrence (detection result) of a fault of the actuator  18  that is judged based on respective detection signals in the failure detecting device  14 , is input to the detection result input unit  12   c.    
     The directional switching valve  16 , by means of control signals supplied from the controller  12  through the failure detecting device  14  to the solenoids  16   a ,  16   b , selectively outputs or supplies a pressure fluid, which is supplied from a fluid pressure source  24 , to one end  26  or another end  28  of the actuator  18 . More specifically, if the control signal is supplied to the solenoid  16   a , among the two blocks shown for the directional switching valve  16  in  FIG. 1 , the directional switching valve  16  is placed in a state in which the upper side block is selected. Further, if the control signal is supplied to the solenoid  16   b , the directional switching valve  16  is placed in a state in which the lower side block is selected. 
     As discussed previously, the actuator  18  comprises a fluid pressure cylinder in which a piston  32  (movable member), to which a piston rod  30  is connected, is displaced in directions to the left and right (displacement direction) of  FIG. 1 , by supplying the pressure fluid from the directional switching valve  16 . 
     As noted above, when the control signal is supplied to the solenoid  16   a , thereby exciting the solenoid  16   a , the directional switching valve  16  is placed in a state in which the upper side block is selected. Consequently, pressure fluid from the fluid pressure source  24  is supplied through the directional switching valve  16 , a tube  33  and a port  36  to the one end  26  of the actuator  18 , together with the pressure fluid in the other end  28  being discharged to the exterior from the other end  28  through a port  38 , a tube  35 , and the directional switching valve  16 . As a result, the piston  32  and the piston rod  30  are displaced in unison from the one end  26  to the other end  28 . 
     Further, when the control signal is supplied to the solenoid  16   b , thereby exciting the solenoid  16   b , the directional switching valve  16  is placed in a state in which the lower side block is selected. Consequently, pressure fluid from the fluid pressure source  24  is supplied through the directional switching valve  16 , the tube  35  and the port  38  to the other end  28  of the actuator  18 , together with the pressure fluid in the one end  26  being discharged to the exterior from the one end  26  through the port  36 , the tube  33 , and the directional switching valve  16 . As a result, the piston  32  and the piston rod  30  are displaced in unison from the other end  28  to the one end  26 . 
     Accordingly, by supplying control signals from the controller  12  alternately to the solenoid  16   a  and the solenoid  16   b  through the failure detecting device  14 , the piston  32  and the piston rod  30  can be moved reciprocally between the one end  26  and the other end  28  in the left and right directions of  FIG. 1 . 
     A silencer  34  is disposed on a tip end of the discharge passageway for the pressure fluid that extends from the one end  26  or the other end  28 . 
     The first sensor  20  is disposed on an outer circumferential surface on the one end  26  side of the fluid pressure cylinder that makes up the actuator  18 , whereas the second sensor  22  is disposed on an outer circumferential surface on the other end  28  side of the fluid pressure cylinder. The first sensor  20  and the second sensor  22  are constituted by limit switches or magnetic switches, which detect the piston  32  when the piston  32  is displaced to positions confronting the first sensor  20  and the second sensor  22 , and output detection results as detection signals to the failure detecting device  14 . Further, by displacement of the piston  32 , when the piston  32  is not located in confronting relation to the first sensor  20  and the second sensor  22 , output of the detection signals from the first sensor  20  and the second sensor  22  is stopped. 
     As will be described later, using the detection signals from the first sensor  20  and the second sensor  22 , the failure detecting device  14  calculates the stroke time of the piston  32  as the piston  32  travels between the one end  26  and the other end  28 , and based on the calculated stroke time, the failure detecting device  14  detects the occurrence or non-occurrence of a fault such as deterioration or failure, etc., of the actuator  18 . Additionally, the failure detecting device  14  outputs a detection result, which is indicative of the occurrence of a fault in the actuator  18 , as a detection signal to the detection result input unit  12   c . More specifically, the failure detecting device monitors in real time the control signal supplied from the controller  12  as well as the respective detection signals output from the first sensor  20  and the second sensor  22 , such that during operation of the equipment, a detection process for detecting a fault of the actuator  18 , etc., can be carried out continuously. 
       FIG. 1  shows a situation in which the controller  12  includes the output unit  12   a , the input unit  12   b , and the detection result input unit  12   c , in such a manner that between the controller  12  and the failure detecting device  14 , various signals are transmitted and received by way of parallel communications. However, the fault detection system  10  is not limited to the configuration shown in  FIG. 1 . As shown in  FIG. 2 , a communications unit  39  may be provided in the controller  12 , and serial connections may be made through a field bus or the like between the communications unit  39  and the failure detecting device  14 , whereby the various signals may be transmitted and received by way of serial communications. 
     [Configuration of Failure Detecting Device] 
     As shown in  FIG. 3 , the failure detecting device  14  includes a sensor input unit  40 , an output signal input unit  42 , a detection time calculator  44  (stroke time calculator), an internal timer  46 , a data storage processor  48 , a first data storage unit  50  (first storage unit), a statistical processor  52  (statistical computation processing unit), a second data storage unit  54  (second storage unit), a fault response detector  56  (fault detector), a display processor  58 , a display device  60 , an output processor  62 , and an operation input unit  64 . 
     The detection signals from the first sensor  20  and the second sensor  22  (see  FIGS. 1 and 2 ) are input to the sensor input unit  40 . When the detection signal from either the first sensor  20  or the second sensor  22  is input thereto (i.e., when the signal level of the detection signal switches from a low level to a high level), the sensor input unit  40  detects the rising edge of the detection signal, and outputs a detection result thereof to the detection time calculator  44 . Further, when the input of the detection signal from either the first sensor  20  or the second sensor  22  is stopped (i.e., when the signal level of the detection signal switches from a high level to a low level), the sensor input unit  40  detects the falling edge of the detection signal, and outputs a detection result thereof to the detection time calculator  44 . 
     The control signal, which is output from the output unit  12   a  of the controller  12 , is input to the output signal input unit  42 . The output signal input unit  42  outputs the control signal that has been input thereto to the detection time calculator  44  and the output processor  62 . The output processor  62  outputs the control signal that has been input thereto to the solenoid  16   a  or the solenoid  16   b.    
     The detection time calculator  44 , using a timer function of the internal timer  46 , calculates a first time T1 (first detection time) from a time at which the control signal is input until a time at which a detection result of the falling edge is input. Further, the detection time calculator  44  calculates a second time T2 (second detection time) from a time at which the control signal is input until a time at which a detection result of the rising edge is input. In addition, the detection time calculator  44  calculates, as the stroke time T3 of the piston  32  between the one end  26  and the other end  28 , a time difference (T2−T1) between the first time T1 and the second time T2. The first time T1, the second time T2, and the stroke time T3 are defined in the manner described below, responsive to the displacement direction of the piston  32  between the one end  26  and the other end  28 . 
     In the case that the piston  32 , at a position on the side of the one end  26 , is displaced toward the other end  28  as a result of the control signal being supplied to the solenoid  16   a  from the output processor  62 , the first sensor  20  eventually becomes incapable of detecting the piston  32  after elapse of a predetermined time from supply of the control signal to the solenoid  16   a , whereupon the detection signal ceases to be output. Consequently, the sensor input unit  40  is capable of detecting the falling edge of the detection signal from the first sensor  20 . 
     Accordingly, assuming that the time delay required to supply the control signal between the controller  12  and the solenoid  16   a  is small, the first time T1 when the piston  32  is displaced from the one end  26  toward the other end  28  can be regarded as a time period from the time at which the control signal starts to be supplied from the controller  12  until the time at which the first sensor  20  can no longer detect the piston  32 . 
     Further, upon the piston  32  and the second sensor  22  being brought into confronting relation to each other by movement of the piston  32  to the other end  28 , the second sensor  22  detects the piston  32 , and output of the detection signal is started. Consequently, the sensor input unit  40  is capable of detecting the rising edge of the detection signal from the second sensor  22 . Accordingly, the second time T2 when the piston  32  is displaced from the one end  26  toward the other end  28  can be regarded as a time period from the time at which the control signal starts to be supplied from the controller  12  until the time at which the second sensor  22  starts to detect the piston  32 . 
     For this reason, the stroke time T3 of the piston  32 , when the piston  32  is displaced from the one end  26  toward the other end  28 , becomes the time period between the time of the falling edge of the detection signal from the first sensor  20  to the time of the rising edge of the detection signal from the second sensor  22 . 
     On the other hand, in the case that the piston  32 , which is now located on the side of the other end  28 , is displaced toward the one end  26  as a result of the control signal being supplied to the solenoid  16   b  from the output processor  62 , the second sensor  22  eventually becomes incapable of detecting the piston  32  after elapse of a predetermined time from supply of the control signal to the solenoid  16   b , whereupon the detection signal ceases to be output. Consequently, the sensor input unit  40  is capable of detecting the falling edge of the detection signal from the second sensor  22 . 
     Accordingly, assuming that the time delay required to supply the control signal between the controller  12  and the solenoid  16   b  is small, the first time T1 when the piston  32  is displaced from the other end  28  toward the one end  26  can be regarded as a time period from the time at which the control signal starts to be supplied from the controller  12  until the time at which the second sensor  22  can no longer detect the piston  32 . 
     Further, upon the piston  32  and the first sensor  20  being brought into confronting relation to each other by movement of the piston  32  to the one end  26 , the first sensor  20  detects the piston  32 , and output of the detection signal is started. Consequently, the sensor input unit  40  is capable of detecting the rising edge of the detection signal from the first sensor  20 . Accordingly, the second time T2 when the piston  32  is displaced from the other end  28  toward the one end  26  can be regarded as a time period from the time at which the control signal starts to be supplied from the controller  12  until the time at which the first sensor  20  starts to detect the piston  32 . 
     For this reason, the stroke time T3 of the piston  32 , when the piston  32  is displaced from the other end  28  toward the one end  26 , becomes the time period between the time of the falling edge of the detection signal from the second sensor  22  to the time of the rising edge of the detection signal from the first sensor  20 . 
     The first time T1, the second time T2, and the stroke time T3, which are calculated in the foregoing manner, are output to the data storage processor  48  from the detection time calculator  44 . The data storage processor  48  stores (accumulates) data concerning the first time T1, the second time T2, and the stroke time T3 in the first data storage unit  50 . 
     As described above, the fault detection system  10  is incorporated in an assembly of equipment (not shown). In this case, by supplying control signals alternately to the solenoids  16   a ,  16   b  from the controller  12  and through the failure detecting device  14 , the directional switching valve  16  is operated to supply pressure fluid selectively to the one end  26  and the other end  28  of the actuator  18 . Accordingly, the piston  32  is moved reciprocally in the left and right directions of  FIGS. 1 and 2  in the interior of the actuator  18 . 
     Therefore, under actual usage, the first sensor  20  and the second sensor  22  detect the piston  32  respectively, and detection signals indicative of detection results therefrom are output to the sensor input unit  40 . The sensor input unit  40  detects the falling edge and the rising edge of each of the detection signals, and outputs detection results to the detection time calculator  44 . Accordingly, each time that control signals are supplied alternately to the solenoids  16   a ,  16   b , the detection time calculator  44  calculates the first time T1, the second time T2, and the stroke time T3, and the data storage processor  48  sequentially stores the first time T1, the second time T2, and the stroke time T3, which have been calculated, in the first data storage unit  50 . 
     The statistical processor  52  reads out through the data storage processor  48  all of the data concerning the first time T1 and the stroke time T3 that are stored in the first data storage unit  50 , and carries out a predetermined statistical computation process with respect to the data of the first time T1 and the stroke time T3, which have been read out. The result of the statistical computation process is stored as a statistically calculated value in the second data storage unit  54 . The fault response detector  56  reads out at least the statistically calculated value that is stored in the second data storage unit  54 , and if the read out statistically calculated value exceeds a predetermined threshold, it is judged that a fault of the actuator  18  or the like has occurred. The judgment result of the fault response detector  56  is output to the display processor  58  and the output processor  62 . The display processor  58  performs a predetermined display-related process with respect to the judgment result, and displays the judgment result on the display device  60 . On the other hand, the output processor  62  outputs the judgment result that was input thereto as a detection signal to the controller  12 . 
     Next, descriptions shall be given of two detailed examples first detailed example, second detailed example in relation to the statistical computation process carried out in the statistical processor  52 , and the judgment process carried out in the fault response detector  56 . 
     First Detailed Example 
     In the first detailed example, the statistical computation process, which is performed by the statistical processor  52 , is carried out using predetermined values and the first time T1 or the stroke time T3, and the judgment process by the fault response detector  56  is carried out using the statistically calculated value that has been obtained by the statistical computation process. 
     In the first detailed example, the predetermined values are defined by a normal stroke time T3n and a normal first time T1n (normal value) for the piston  32 . In this case, the normal stroke time T3 n is defined as the stroke time T3 of the piston  32  between the one end  26  and the other end  28 , in a state in which an abnormality such as deterioration or failure of the actuator  18  is not occurring (e.g., an initial operating state of the actuator  18  immediately after installation or replacement thereof). Further, the normal first time T1n is defined as the first time T1 in a condition in which an abnormality such as deterioration or failure of the actuator  18  is not occurring. 
     More specifically, as shown in  FIG. 4 , in the case of a normal actuator  18  (indicated by the term “normal product” shown by the solid line) in which an abnormality such as deterioration or failure is not occurring, irrespective of the number of times that the piston  32  is operated, the stroke time T3 (indicated by the term “stroke time” in  FIG. 4 ) is substantially constant. On the other hand, in the case of an actuator  18  in which a fault is occurring (indicated by the term “faulty product 1” shown by the dashed line, and the term “faulty product 2” shown by the one-dot-chain line), if the number of times that the piston  32  is operated goes beyond a predetermined number, the stroke time T3 becomes longer in comparison with that of the normal product. 
     In other words, in the case of the faulty product 1 and the faulty product 2, in comparison with the normal product, the variability of the stroke time T3 becomes greater when the number of times that the piston  32  is operated increases. Consequently, the deviation between the stroke time T3 of the normal product and the stroke times T3 of the faulty product 1 and the faulty product 2 becomes greater as the number of operations increases. Further, concerning the average value, the standard deviation, and the variance of the stroke times T3 of the faulty product 1 and the faulty product 2, as the number of operations increases, it is to be expected that the variability with respect to the average value, the standard deviation, and the variance of the stroke time T3 of the normal product will also become greater. 
     Thus, in the present embodiment, in the case that the normal stroke time T3 n and the normal first time T1n are known beforehand, the normal stroke time T3 n and the normal first time T1n can be preset beforehand by the operator through use of the operation input unit  64 , which is constituted by a numeric keypad or the like. The normal stroke time T3 n and the normal first time T1n, which are set in the foregoing manner, are stored as normal values in the first data storage unit  50 . Further, the normal stroke time T3 n and the normal first time T1n are displayed on the display device  60  through the display processor  58 , and also are output to the controller  12  through the output processor  62 . 
     In the first detailed example, in accordance with the reciprocal movement of the piston  32  in the left and right directions of  FIGS. 1 and 2 , the first time T1 and the stroke time T3 are calculated sequentially by the detection time calculator  44 . Each time that the first time T1 and the stroke time T3, which are calculated as described above, are stored in the first data storage unit  50 , the statistical processor  52  reads out from the first data storage unit  50  the currently stored first time T1 and the stroke time T3, and the normal first time T1n and the normal stroke time T3 n that have been stored beforehand. 
     Next, the statistical processor  52  calculates an absolute value εT1 (=|T1−T1n|) of the deviation between the first time T1 and the normal first time T1n, and an absolute value εT3 (=|T3−T3n|) of the deviation between the stroke time T3 and the normal stroke time T3n. The calculated absolute values εT1, εT3 are stored respectively as statistically calculated values in the second data storage unit  54 . 
     Next, the fault response detector  56  reads out each of the absolute values εT1, εT3 that are stored in the second data storage unit  54 , and compares the read out absolute values εT1, εT3 respectively with threshold values TH1, TH3. 
     In this case, if the absolute value εT3 lies within the threshold value TH3 (εT3≦TH3), since the stroke time T3 is indicative of normal response information, the fault response detector  56  judges that the actuator  18  is functioning normally. On the other hand, if the absolute value εT3 exceeds the threshold value TH3 (εT3&gt;TH3), since the stroke time T3 is indicative of abnormal response information, the fault response detector  56  judges that the actuator  18  is suffering from a fault. 
     Further, if the absolute value εT1 lies within the threshold value TH1 (εT1&lt;TH1), since the first time T1 is indicative of normal response information, the fault response detector  56  judges that the tubes  33 ,  35  between the directional switching valve  16  and the actuator  18  are behaving normally. On the other hand, if the absolute value εT1 exceeds the threshold value TH1 (εT1&gt;TH1), since the first time T1 is judged to be indicative of abnormal response information, the fault response detector  56  judges that a fault has occurred in the tubes  33 ,  35  between the directional switching valve  16  and the actuator  18 . 
     Since the above judgment results from the fault response detector  56  are displayed on the display device  60  through the display processor  58 , the occurrence or non-occurrence of a fault of the actuator  18 , or the occurrence or non-occurrence of a fault in the tubes  33 ,  35  between the directional switching valve  16  and the actuator  18  can be grasped. As noted above, the first times T1, T1n, the second time T2, and the stroke times T3, T3 n are stored in the first data storage unit  50 , and the absolute values εT1, εT3 are stored in the second data storage unit  54 . Therefore, together with the judgment results from the fault response detector  56 , the first times T1, T1n, the second time T2, the stroke times T3, T3n, the absolute values εT1, εT3, and the threshold values TH1, TH3 may also be displayed in the display device  60 . 
     Further, the above judgment results from the fault response detector  56  are output as detection signals to the controller  12  through the output processor  62 . Therefore, if there is a judgment result indicative of a fault of the actuator  18 , or indicative of a fault of the tubes  33 ,  35  between the directional switching valve  16  and the actuator  18 , the controller  12  stops the supply of control signals to the solenoids  16   a ,  16   b . In this case, along with the detection signal, the output processor  62  may output to the controller  12  various information pertaining to the first times T1, T1n, the second time T2, the stroke times T3, T3n, the absolute values εT1, εT3, and the threshold values TH1, TH3. 
     Second Detailed Example 
     As shown in  FIG. 4 , as the number of times that the piston  32  is operated increases, in comparison with the normal product, the variability of the stroke time T3 of the faulty product 1 and the faulty product 2 becomes greater. Accordingly, concerning the average value, the standard deviation, and the variance of the stroke times T3 of the faulty product 1 and the faulty product 2, as the number of operations increases, it is to be expected that the variability with respect to the average value, the standard deviation, and the variance of the stroke time T3 of the normal product will also become greater. 
     Thus, in the second detailed example, a statistical computation process is carried out by the statistical processor  52  with respect to the first time T1 and the stroke time T3. The fault response detector  56  compares statistically calculated values, which are obtained by the statistical computation process, with normal values that have been obtained beforehand by the statistical computation process, whereby a judgment is made concerning the occurrence or non-occurrence of a fault of the actuator  18 . 
     More specifically, in the second detailed example, the statistically calculated values, which are obtained by the statistical computation process with respect to the first time T1 and the stroke time T3, are defined by an average value AVE1, a standard deviation σ1, or a variance σ1 2  of the first time T1, and an average value AVE3, a standard deviation σ3, or a variance σ3 2  of the stroke time T3. 
     Further, the normal values, which are obtained beforehand by the statistical computation process, are defined by average values AVE1n, AVE3n, standard deviations σ1n, σ3n, or variances σ1n 2 , σ3n 2 , which are calculated by the statistical computation process with respect to first times T1 and stroke times T3 that are obtained inside of a given calibration time, wherein the calibration time is taken as a fixed period from an initial operation state in relation to a normal actuator  18 . Thus, the normal values in the second detailed example are defined by an average value, a standard deviation, or a variance corresponding to the first time T1 and the stroke time T3 of the normal product of  FIG. 4 . Such normal values are stored in the second data storage unit  54 . 
     In addition, in the second detailed example, in accordance with reciprocal movement of the piston  32  in the left and right directions of  FIGS. 1 and 2 , the first time T1 and the stroke time T3 are calculated sequentially by the detection time calculator  44 . Each time that the first time T1 and the stroke time T3, which are calculated as described above, are stored in the first data storage unit  50 , the statistical processor  52  reads out data of all of the first times T1 and the stroke times T3 that are stored in the first data storage unit  50 . 
     Next, using the read out data of all of the first times T1, the statistical processor  52  calculates the average value AVE1, the standard deviation σ1, or the variance σ1 2 , and using the data of all of the stroke times T3, the statistical processor  52  calculates the average value AVE3, the standard deviation σ3, or the variance σ3 2 . The calculated average values AVE1, AVE3, the standard deviations σ1, σ3, or the variances σ1 2 , σ3 2  are temporarily stored, respectively, as statistically calculated values in the second data storage unit  54 . 
     Next, the statistical processor  52  reads out from the second data storage unit  54  the average values AVE1n, AVE3n, the standard deviations σ1n, σ3n, or the variances σ1n 2 , σ3n 2 . 
     Next, the statistical processor  52  calculates absolute values εAVE1 (=|AVE1−AVE1n|), εAVE3 (=|AVE3−AVE3n|) of the deviations between the average values AVE1, AVE3 and the average values AVE1n, AVE3n, or calculates absolute values εσ1 (=|σ1−σ1n|), εσ3 (=|(σ3−σ3n|) of the deviations between the standard deviations σ1, σ3 and the standard deviations σ1n, σ3n, or calculates absolute values εσ1 2  (=|σ1 2 −σ1n 2 |), εσ3 2  (=|σ3 2 −σ3n 2 |) of the deviations between the variances σ1 2 , σ3 2  and the variances σ1n 2 , σ3n 2 . 
     The calculated absolute values εAVE1, εAVE3, the absolute values εσ1, εσ3, or the absolute values εσ1 2 , εσ3 2  are stored, respectively, as statistically calculated values in the second data storage unit  54 . 
     The fault response detector  56  compares the respective absolute values εAVE1, εAVE3, the respective absolute values εσ1, εσ3, or the respective absolute values εσ1 2 , εσ3 2  that are stored in the second data storage unit  54  with the predetermined threshold values THAVE1, THAVE3, the threshold values THσ1, THσ3, or the threshold values THσ1 2 , THσ3 2 . 
     In this case, if the absolute value εAVE3, εσ3, or εσ3 2  lies within the threshold value THAVE3, THσ3, or THσ3 2  (εAVE3≦THAVE3, εσ3≦THσ3, or εσ3 2 ≦THσ3 2 ), since the stroke time T3 is indicative of normal response information, the fault response detector  56  judges that the actuator  18  is functioning normally. On the other hand, if the absolute value εAVE3, εσ3, or εσ3 2  exceeds the threshold value THAVE3, THσ3, or THσ3 2  (εAVE3&gt;THAVE3, εσ3&gt;THσ3, or εσ3 2 &gt;THσ3 2 ), since the stroke time T3 is indicative of abnormal response information, the fault response detector  56  judges that the actuator  18  is suffering from a fault. 
     Further, if the absolute value εAVE1, εσ1, or εσ1 2  lies within the threshold value THAVE1, THσ1, or THσ1 2  (εAVE1≦THAVE1, εσ1≦THσ1, or εσ1 2 ≦THσ1 2 ), since the first time T1 is indicative of normal response information, the fault response detector  56  judges that the tube  33  and the tube  35  between the directional switching valve  16  and the actuator  18  are functioning normally. On the other hand, if the absolute value εAVE1, εσ1, or εσ1 2  exceeds the threshold value THAVE1, THσ1, or THσ1 2  (εAVE1&gt;THAVE1, εσ1&gt;THσ1, or εσ1 2 &gt;THσ1 2 ), since the first time T1 is indicative of abnormal response information, the fault response detector  56  judges that the tube  33  and the tube  35  between the directional switching valve  16  and the actuator  18  are suffering from a fault. 
     Further, in the second detailed example as well, since the above judgment results from the fault response detector  56  are displayed on the display device  60  through the display processor  58 , the operator can grasp the occurrence or non-occurrence of a fault of the actuator  18 , or the occurrence or non-occurrence of a fault in the tubes  33 ,  35  between the directional switching valve  16  and the actuator  18 . Further, also in the second detailed example, together with the judgment results from the fault response detector  56 , the display device  60  may also display the first time T1, the second time T2, and the stroke time T3, and the aforementioned statistically calculated values and threshold values. 
     Furthermore, since the aforementioned judgment result from the fault response detector  56  is output as a detection signal to the controller  12  through the output processor  62 , in the event of a judgment result, which is indicative of a fault of the actuator  18 , or a fault of the tubes  33 ,  35  between the directional switching valve  16  and the actuator  18 , the controller  12  can stop the supply of control signals to the solenoids  16   a ,  16   b . In this case, along with the detection signal, the output processor  62  may output to the controller  12  various information pertaining to the first time T1, the second time T2, the stroke time T3, and the aforementioned statistically calculated values and threshold values. 
     Further, in the second detailed example, the statistical processor  52  may calculate only the respective average values AVE1, AVE3, the respective standard deviations σ1, σ3 or the respective variances σ1 2 , σ3 2 , and store such calculations as statistically calculated values in the second data storage unit  54 . In this case, the fault response detector  56  may read out from the second data storage unit  54  the respective average values AVE1, AVE3, the respective standard deviations σ1, σ3, or the respective variances σ1 2 , σ3 2 , and the normal average values AVE1n, AVE3n, the standard deviations σ1n, σ3n, or the variances σ1n 2 , σ3n 2 , and may calculate the absolute value of the deviation between the read out statistically calculated values and the normal values. Accordingly, by comparing the absolute value of the calculated deviation with predetermined thresholds, a fault of the actuator  18 , or a fault of the tubes  33 ,  35  between the directional switching valve  16  and the actuator  18  can be detected. 
     Further, in the second detailed example, the average values AVE1n, AVE3n, the standard deviations σ1n, σ3n, or the variances σ1n 2 , σ3n 2 , which are stored as normal values in the second data storage unit  54 , can easily be acquired by assuming, as a calibration time, a fixed period from a state of initial operation of a normal actuator  18 , and then applying the calculation method for the aforementioned average values AVE1, AVE3, the standard deviations σ1, σ3, or the variances σ1 2 , σ3 2  within the calibration time period. 
     However, if the normal values are acquired, after data of all of the first times T1 and the stroke times T3 within the calibration time period have been stored in the first data storage unit  50 , the statistical processor  52  may read out data of all of the first times T1 and the stroke times T3 that are stored in the first data storage unit  50 , and then may calculate the average values AVE1n, AVE3n, the standard deviations σ1n, σ3n, or the variances σ1n 2 , σ3n 2  of the read out data, and store such calculated values in the second data storage unit  54 . 
     Further, although in the above-described first detailed example, an operator sets the normal values by manipulating the operation input unit  64 , similar to the case of the second detailed example, the average values AVE1, AVE3 may be acquired during the calibration time period, and such average values AVE1, AVE3 may be set as normal values in the first data storage unit  50 . 
     [Operations of the Fault Detection System] 
     The fault detection system  10  according to the present embodiment is configured as described above. Next, operations of the fault detection system  10  will be described with reference to  FIG. 5 . In providing such operational descriptions, reference will also be made to  FIGS. 1 through 4  as necessary. 
       FIG. 5  is a timing chart showing operations during one round trip of the piston  32 , in which the piston  32 , which is positioned at the one end  26 , is displaced to the other end  28 , and thereafter, the piston  32  is displaced back to the one end  26 . 
     In this case, the labels “Solenoid Valve A” and “Solenoid Valve B” indicate operations (excitation, non-excitation) of the solenoids  16   a ,  16   b , and more specifically, show waveforms of the control signals supplied to the solenoids  16   a ,  16   b . Further, the labels “Pressure A” and “Pressure B” indicate, respectively, internal pressures in the actuator  18  at the one end  26  and the other end  28 . Furthermore, the time periods T1f, T2f, T3f indicate, respectively, the first time T1, the second time T2, and the stroke time T3 when the piston  32  is displaced from the one end  26  toward the other end  28 . Still further, the time periods T1r, T2r, T3r indicate, respectively, the first time T1, the second time T2, and the stroke time T3 when the piston  32  is displaced from the other end  28  toward the one end  26 . 
     At time t0, when a control signal is sent from the controller  12  to the solenoid  16   a  through the failure detecting device  14  (i.e., when the signal level of the control signal changes from a low level to a high level), the directional switching valve  16  is switched to a condition in which the upper side block thereof shown in  FIGS. 1 and 2  is selected. Consequently, pressure fluid is supplied from the fluid pressure source  24  to the one end  26  of the actuator  18  through the directional switching valve  16 , the tube  33 , and the port  36 , together with pressure fluid being discharged to the exterior from the other end  28  through the port  38 , the tube  35 , the directional switching valve  16 , and the silencer  34 . As a result, the pressure inside the one end  26  rises abruptly from time t1 and thereafter increases gradually. On the other hand, the pressure inside the other end  28  decreases rapidly from time t1 and thereafter becomes substantially constant. 
     Further, by supplying the pressure fluid from the directional switching valve  16  to the one end  26 , from time t2, the piston  32 , which is positioned at the one end  26 , is displaced toward the other end  28 . As a result, at time t2, the first sensor  20  becomes incapable of detecting the piston  32 , and output of the detection signal to the sensor input unit  40  of the failure detecting device  14  is stopped (the level of the detection signal changes from a high level to a low level). 
     Accordingly, the sensor input unit  40  detects the falling edge of the detection signal from the first sensor  20 , and outputs the detection result to the detection time calculator  44 . Using the timer function of the interior timer  46 , the detection time calculator  44  calculates, as the first time T1f, the time period from time t0 at which the control signal is supplied to the solenoid  16   a  to time t2 at which the falling edge is detected. The calculated first time T1f is stored in the first data storage unit  50  through the data storage processor  48 . 
     Thereafter, when the piston  32  is displaced to the other end  28 , the second sensor  22  detects the piston  32  at time t3, and a detection signal thereof is output to the sensor input unit  40  (the level of the detection signal changes from a low level to a high level). Accordingly, the sensor input unit  40  detects the rising edge of the detection signal from the second sensor  22 , and outputs the detection result to the detection time calculator  44 . Using the timer function of the interior timer  46 , the detection time calculator  44  calculates, as the second time T2f, the time period from time t0 to time t3 at which the rising edge is detected, and together therewith, calculates, as the stroke time T3f (=T2f−T1f) of the piston  32 , the difference in time between the first time T1f and the second time T2f. The second time T2f and the stroke time T3f, which have been calculated, are stored in the first data storage unit  50  through the data storage processor  48 . 
     Consequently, the statistical processor  52  reads out the first time T1f and the stroke time T3f, etc., from the first data storage unit  50 , and carries out the predetermined statistical computation process of the aforementioned first detailed example or the second detailed example with respect to the first time T1f and the stroke time T3f, etc., which have been read out. After the computation process is carried out, the statistically calculated values can be stored in the second data storage unit  54 . 
     Further, the fault response detector  56  reads out the statistically calculated value from the second data storage unit  54 , and compares the read out statistically calculated value with a predetermined threshold, whereby it can be judged whether or not a fault of the actuator  18 , or a fault of the tube  33  between the directional switching valve  16  and the port  36  of the actuator  18  has occurred. More specifically, if the statistically calculated value concerning the stroke time T3f exceeds the threshold value, the fault response detector  56  judges that the actuator  18  is suffering from a fault such as deterioration or failure of the actuator  18 . Further, if the statistically calculated value concerning the first time T1f exceeds the threshold value, the fault response detector  56  judges that a fault has occurred in the tube  33  between the directional switching valve  16  and the port  36 . Further, the above judgment results from the fault response detector  56  are displayed on the display device  60 , and are output as detection signals to the controller  12  from the output processor  62 . 
     As shown in  FIGS. 1 and 2 , since the directional switching valve  16  is a double-acting solenoid valve, even if supply of the control signal to the solenoid  16   a  is stopped at time t4, the state of the directional switching valve  16  can be maintained. 
     Thereafter, at time t5, when a control signal is sent from the controller  12  to the solenoid  16   b  through the failure detecting device  14 , the directional switching valve  16  is switched to the condition in which the lower side block thereof shown in  FIG. 1  is selected. Consequently, pressure fluid is supplied from the fluid pressure source  24  to the other end  28  of the actuator  18  through the directional switching valve  16 , the tube  35 , and the port  38 , together with pressure fluid being discharged to the exterior from the one end  26  through the port  36 , the tube  33 , the directional switching valve  16 , and the silencer  34 . As a result, the pressure inside the other end  28  rises abruptly from time t6 and thereafter settles at a constant value. On the other hand, the pressure inside the one end  26  falls abruptly from time t6 and thereafter decreases gradually. 
     Further, by supplying the pressure fluid from the directional switching valve  16  to the other end  28 , from time t7, the piston  32 , which is positioned at the other end  28 , is displaced toward the one end  26 . As a result, at time t7, the second sensor  22  becomes incapable of detecting the piston  32 , and output of the detection signal with respect to the sensor input unit  40  of the failure detecting device  14  is stopped. 
     Accordingly, the sensor input unit  40  detects the falling edge of the detection signal from the second sensor  22 , and outputs the detection result to the detection time calculator  44 . Using the timer function of the interior timer  46 , the detection time calculator  44  calculates, as the first time T1r, the time period from time t5 at which the control signal is supplied to the solenoid  16   b  to time t7 at which the falling edge is detected. The calculated first time T1r is stored in the first data storage unit  50  through the data storage processor  48 . 
     Thereafter, when the piston  32  is displaced to the one end  26 , the first sensor  20  detects the piston  32  at time t8, and a detection signal thereof is output to the sensor input unit  40 . Accordingly, the sensor input unit  40  detects the rising edge of the detection signal from the first sensor  20 , and outputs the detection result to the detection time calculator  44 . Using the timer function of the interior timer  46 , the detection time calculator  44  calculates, as the second time T2r, the time period from time t5 to time t8 at which the rising edge is detected, and together therewith, calculates, as the stroke time T3r (=T2r−T1r) of the piston  32 , the difference in time between the first time T1r and the second time T2r. The second time T2r and the stroke time T3r, which have been calculated, are stored in the first data storage unit  50  through the data storage processor  48 . 
     Consequently, the statistical processor  52  reads out the first time T1r and the stroke time T3r, etc., from the first data storage unit  50 , and carries out the predetermined statistical computation process of the first detailed example or the second detailed example with respect to the first time T1r and the stroke time T3r, etc., which have been read out. After the computation process is carried out, the statistically calculated values can be stored in the second data storage unit  54 . 
     Further, the fault response detector  56  reads out the statistically calculated value from the second data storage unit  54 , and compares the read out statistically calculated value with a predetermined threshold, whereby it can be judged whether or not a fault of the actuator  18 , or a fault of the tube  35  between the directional switching valve  16  and the port  38  of the actuator  18  has occurred. More specifically, if the statistically calculated value concerning the stroke time T3r exceeds the threshold value, the fault response detector  56  judges that the actuator  18  is suffering from a fault such as deterioration or failure of the actuator  18 . Further, if the statistically calculated value concerning the first time T1r exceeds the threshold value, the fault response detector  56  judges that a fault has occurred in the tube  35  between the directional switching valve  16  and the port  38 . Further, the above judgment results from the fault response detector  56  are displayed on the display device  60 , and are output as detection signals to the controller  12  from the output processor  62 . 
     In this manner, in accordance with the piston  32  making one round trip between the one end  26  and the other end  28 , a detection process for detecting a fault of the actuator  18 , and a detection process for detecting a fault in the tubes  33 ,  35  between the directional switching valve  16  and the ports  36 ,  38  of the actuator  18  can be carried out. Consequently, with the fault detection system  10 , even during operation of the equipment including the actuator  18 , detection of the above-described faults can be performed. 
     Modifications of the Present Embodiment 
       FIGS. 6 and 7  shows a configuration in which the double-acting directional switching valve  16  shown in  FIGS. 1 and 2  is replaced by a single-acting directional switching valve  16 . Accordingly, the solenoid  16   b  is replaced by a spring  16   c . With the configuration of  FIGS. 6 and 7 , when a control signal is supplied to the solenoid  16   a , the solenoid  16   a  is excited, and the directional switching valve  16  is placed in a state in which the upper side block is selected. On the other hand, if supply of the control signal to the solenoid  16   a  is stopped, the solenoid  16   a  becomes demagnetized, and under the action of the spring  16   c , a state is brought about in which the lower side block is selected. 
     The operations of the configuration of  FIGS. 6 and 7  differ from the operations of the configuration of  FIGS. 1 and 2 , in that, as shown by the one-dot chain lines in the timing chart of  FIG. 5 , supply of the control signal to the solenoid  16   a  is stopped at time t5, whereas, since the solenoid  16   b  is not provided, no control signal is supplied to the solenoid  16   b . More specifically, apart from the feature that the solenoid  16   b  is replaced by the spring  16   c , the configuration of  FIGS. 6 and 7  is the same as the configuration of  FIGS. 1 and 2 , and thus operates substantially in the same manner as the configuration of  FIGS. 1 and 2 . 
       FIG. 8  is a timing chart showing a case in which the fault detection system  10  is applied to the system of Patent Document 2. The timing chart of  FIG. 8  adds detection signals from the first sensor  20  and the second sensor  22  to the timing chart that is shown in FIG. 10 of Patent Document 2. Therefore, since explanations concerning time-based changes of pressure and displacement of the piston  32  are disclosed in Patent Document 2, which is expressly incorporated herein by reference, details of such features will be omitted from the present specification. 
     The time periods T4 to T7 correspond respectively to the features “Valve Actuation Delay”, “Charge Region (Filling Region)”, “Acceleration Region”, and “Constant Velocity Region” disclosed in Patent Document 2. Further, time t9 corresponds to time t2 in  FIG. 5 , time t10 is a time at which a change takes place from the acceleration region to the constant velocity region, and time t11 is a time at which the piston  32  arrives and stops at the other end  28 . 
     In this manner, by adding the failure detecting device  14  that makes up the fault detection system  10  to the conventional system, a fault of the actuator  18 , or a fault of the tubes  33 ,  35  between the directional switching valve  16  and the actuator  18  can be detected easily and readily, and the fault detection system  10  can be constructed at low cost. 
     Advantages of the Present Embodiment 
     As has been described above, with the fault detection system  10  according to the present embodiment, a statistical computation process is carried out with respect to the stroke times T3, T3f, T3r of the piston  32 , and based on the processing result, it is detected whether or not a fault of the actuator  18  has occurred. Therefore, even during operation of equipment including the actuator  18 , a fault of the actuator  18  can be detected without requiring the equipment to be stopped. As a result, productivity of such equipment is maintained, and faults of the actuator  18  can be detected in real time while the equipment remains online. 
     Further, a maintenance cycle, which heretofore has been set (defined) based on the operator&#39;s judgment, can be managed automatically and numerically. More specifically, even if maintenance operations are not carried out regularly by the operator, the fault detection system  10  carries out maintenance automatically during operation of the equipment, and based on the stroke times T3, T3f, T3r, which serve as response information from the actuator  18 , the occurrence of abnormalities of the actuator  18  are determined. In addition, with the fault detection system  10 , based on the processing result of the statistical calculation carried out with respect to the stroke times T3, T3f, T3r, whether or not an abnormality of the actuator  18  has occurred can be judged (managed) numerically. 
     As a result, according to the present embodiment, the number of processing steps required for maintenance can be reduced, the burden imposed on the operator can be mitigated significantly, and maintainability of the equipment including the actuator  18  can be enhanced. Further, by being managed numerically, training and education of the operator in charge of such maintenance is facilitated. 
     Furthermore, since the stroke times T3, T3f, T3r are calculated based on the detection results of the first sensor  20  and the second sensor  22 , existing sensors (limit switches, magnetic sensors) can be used without modification. More specifically, the fault detection system  10  can be constructed merely by adding the fault detecting device  14  with respect to conventional existing sensors. Accordingly, with the present invention, an abnormality or fault of the actuator  18  can be detected easily and at low cost. 
     Further, since the stroke times T3, T3f, T3r are stored (accumulated) in the first data storage unit  50 , even in the case that the piston  32  moves reciprocally between the one end  26  and the other end  28 , the statistical processor  52  can sequentially read out the stroke times T3, T3f, T3r from the first data storage unit  50 , and carry out statistical calculations thereon. Further, since processing results are stored (accumulated) in the second data storage unit  54 , the fault response detector  56  can suitably read out the processing results from the second data storage unit  54 , and carry out detection processing thereon. 
     Further, in the first detailed example, based on a comparison between the normal value (the normal stroke time T3n), which is set beforehand, and the actually calculated stroke times T3, T3f, T3r, since it can be determined whether or not a fault of the actuator  18  has occurred, the occurrence of a fault of the actuator  18  can be judged accurately. More specifically, if the actuator  18  becomes deteriorated, each time that the stroke times T3, T3f, T3r are calculated based on the respective detection signals of the first sensor  20  and the second sensor  22 , the variability in the absolute value εT3 of the deviation becomes greater. Thus, for example, if the absolute value εT3 of the deviation becomes greater than a predetermined threshold TH3, it can easily be judged that a fault of the actuator  18  has occurred. 
     The normal stroke time T3 n is defined as the stroke time T3 of the piston  32  between the one end  26  and the other end  28 , in a state in which an abnormality such as deterioration or failure of the actuator  18  is not occurring (e.g., an initial operating state of the actuator  18  immediately after installation or replacement thereof). The normal stroke time T3 n may be set beforehand by the operator, or may be stored in the first data storage unit  50  when the failure detecting device  14  is manufactured. 
     On the other hand, according to the second detailed example, whether or not a fault of the actuator  18  has occurred can be detected easily and in real time during operation of the actuator  18  (i.e., during reciprocal movement of the piston  32 ). More specifically, using actually calculated data of the stroke times T3, T3f, T3r, the statistical processor  52  sequentially calculates an average value AVE3, a standard deviation σ3, or a variance σ3 2  of the data, and stores the same as a statistically calculated value in the second data storage unit  54 . 
     Further, based on a comparison between the statistically calculated value (the average value AVE3, the standard deviation σ3, or the variance σ3 2 ) and the normal value (the average value AVE3n, the standard deviation σ3n, or the variance σ3n 2 ) that are stored in the second data storage unit  54 , and more specifically, by comparing the absolute value εAVE3, εσ3, or εσ3 2  of the deviation between the statistically calculated value and the normal value with the predetermined threshold value THAVE3, THσ3, or THσ3 2 , the fault response detector  56  can judge sequentially whether a fault of the actuator  18  has occurred. 
     Further, if the actuator  18  becomes deteriorated, each time that the stroke time T3 is calculated based on the respective detection signals of the first sensor  20  and the second sensor  22 , the variability in the average value AVE3, the standard deviation σ3, or the variance σ3 2  becomes greater. Thus, for example, if the absolute values εAVE3, εσ3, εσ3 2  corresponding to the average value AVE3, the standard deviation σ3, or the variance σ3 2  become greater than the predetermined thresholds THAVE3, THσ3, THσ3 2 , it can easily be judged that a fault of the actuator  18  has occurred. 
     According to the second detailed example, by setting a calibration time to a fixed time period in an initial state of operation immediately after installation or replacement of the actuator  18  in the equipment, the normal value is calculated automatically, and is stored in the second data storage unit  54 . Thus, setting of the normal value can be performed with high efficiency. 
     Further, by calculating as the stroke time T3 the time difference between the first time T1 and the second time T2, the stroke time T3 can be calculated easily and reliably. 
     Furthermore, in addition to a fault of the actuator  18 , with the fault detection system  10 , using the first time T1, a fault in the tubes  33 ,  35  between the directional switching valve  16  and the actuator  18  can be detected. The statistical calculation performed with respect to the first time T1 may be the same process (calculation of an average value, a standard deviation, or a variance) as the statistical calculation performed with respect to the stroke times T3, T3f, T3r. 
     Further, the controller  12 , which is made up from a PLC or the like, supplies control signals to the solenoids  16   a ,  16   b  of the directional switching valve  16  through the failure detecting device  14 , whereas the detection result of a fault in the actuator  18 , etc., is input thereto as a detection signal from the failure detecting device  14 . As a result, the controller  12  is capable of grasping (detecting) a fault of the actuator  18  or the like in an online state, and based on the detection result, can take an appropriate action such as stopping supply of the control signal. 
     Further, since the failure detecting device  14  detects a fault of the actuator  18 , etc., and outputs the detection signal to the controller  12 , it is unnecessary for the operator to create a control program for use by the controller  12  in order to detect a malfunction of the actuator  18  or the like. As a result, the load imposed on the operator to construct the fault detection system  10  can be reduced. 
     Further, by visually confirming the content displayed on the display device  60 , the operator can grasp the occurrence or the like of a fault of the actuator  18 , and can quickly carry out an appropriate action such as halting operation of the equipment, replacing the actuator  18 , etc. 
     The fault detection system  10  according to the present invention is not limited to the embodiments described above, and various modified or additional structures may be adopted therein without deviating from the scope of the invention as set forth in the appended claims.