Patent Publication Number: US-2022220985-A1

Title: Fluid actuator, fluid actuator control method, and computer readable medium storing control program of fluid actuator

Description:
RELATED APPLICATIONS 
     The content of Japanese Patent Application No. 2021-003200, on the basis of which priority benefits are claimed in an accompanying application data sheet, is in its entirety incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     Certain embodiments of the present invention relate to a control technique for a fluid actuator. 
     Description of Related Art 
     In air actuators that use air as a working fluid and drive a drive target with the pressure (also referred to as driving pressure) of air, there is known a technique for emergency-stopping the drive when an abnormality occurs. 
     SUMMARY 
     According to an embodiment of the present invention, there is provided a fluid actuator including a first pressure sensor that measures a pressure of a working fluid that drives a drive target in a first drive direction; a second pressure sensor that measures the pressure of the working fluid that drives the drive target in a second drive direction different from the first drive direction; and an acceleration detection unit that detects an acceleration generated in the drive target on the basis of the pressure measured by the first pressure sensor and the pressure measured by the second pressure sensor. 
     According to this aspect, the acceleration generated in the drive target can be detected on the basis of the pressures measured by the two pressure sensors corresponding to the different drive directions. Accordingly, the drive target can be safely driven while being monitored such that the acceleration does not become excessive. 
     Another aspect of the present invention is a fluid actuator control method. This method includes measuring a pressure of a working fluid that drives a drive target in a first drive direction, by a first pressure sensor; measuring a pressure of the working fluid that drives the drive target in a second drive direction different from the first drive direction, by a second pressure sensor; and detecting an acceleration generated in the drive target on the basis of the pressure measured by the first pressure sensor and the pressure measured by the second pressure sensor. 
     In addition, optional combinations of the above components and those obtained by exchanging the expressions of the present invention with each other between methods, devices, systems, recording media, computer programs, and the like are also effective as aspects of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are views showing the concept of a fluid actuator of the present embodiment. 
         FIG. 2  is a perspective view of an air stage to which the fluid actuator of the present embodiment is applied. 
         FIG. 3  is a schematic sectional view of an air actuator. 
         FIG. 4  is a cross-sectional view of a servo valve. 
         FIG. 5  is a view showing the operation of the air stage during normal operation. 
         FIG. 6  is a view showing an example of drive limitation in a case where the translational acceleration along any of drive axes becomes excessive. 
         FIG. 7  is a view showing an example of drive limitation in a case where the rotational acceleration of a drive target becomes excessive. 
     
    
    
     DETAILED DESCRIPTION 
     At the time of emergency stop of the air actuators, even when the driving pressure is lowered or the driving pressure is applied in a direction opposite to a drive direction before the emergency stop, it is difficult to stop a drive target instantly due to the inertia of the drive target during driving. In a case where the drive target is driven at high speed, there is also a possibility that the drive target collides with other parts of the air actuator before the drive target stops. 
     The present invention has been made in view of such a situation, it is desirable to provide a fluid actuator capable of safely driving a drive target. 
     Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. In the description and drawings, the same or equivalent components, members, and processing are designated by the same reference numerals, and redundant descriptions will be appropriately omitted. The scales and shapes of the respective parts shown in the figures are set for convenience in order to facilitate the description, and should not be interpreted as limiting unless otherwise specified. The embodiments are merely examples and do not limit the scope of the present invention. All the features and combinations to be described in the embodiments are not necessarily essential to the invention. 
       FIGS. 1A and 1B  show the concept of a fluid actuator of the present embodiment.  FIG. 1A  shows a generalized concept, and  FIG. 1B  shows a concept according to a specific example described below. In  FIG. 1A , W is a drive target driven by the fluid actuator, and G represents the center of gravity thereof. The fluid actuator has at least one drive axis and, in the shown example, has two different drive axes A 1  and A 2 . The relationship between the two drive axes A 1  and A 2  is optional, and an angle θ formed in a case where the drive axes are provided in the same plane is optionally set (in a case where A 1  and A 2  are parallel to each other, θ=0°). Typically, as shown in  FIG. 1B , drive axes X, Y 1 , and Y 2  are parallel to each other (θ=0°) or perpendicular (θ=90°) to each other. A case where all the drive axes of the fluid actuator are in the same plane will be described below, but the concept of the invention is that the drive axes may not be in the same plane and may be at mutually twisted positions, for example. 
     One direction along the drive axis A 1  is referred to as a first drive direction, a direction opposite to the first drive direction on the same drive axis A 1  is referred to as a third drive direction, one direction along the drive axis A 2  is referred to as a second drive direction, and a direction opposite to the second drive direction on the same drive axis A 2  is referred to as a fourth drive direction. In this way, the “drive directions” are defined by the drive axes and the directions on the drive axes. On the drive axis A 1 , a pressure P 1  along the first drive direction and a pressure P 3  along the third drive direction are applied to a drive target W. On the drive axis A 2 , a pressure P 2  along the second drive direction and a pressure P 4  along the fourth drive direction are applied to the drive target W. By combining the pressures P 1  to P 4 , the drive target W can be optionally driven in the same plane. In other words, the combination of the pressures P 1  to P 4  causes an optional acceleration in the drive target W. For example, the combination of the pressures P 1  and P 3  along the drive axis A 1  causes translational acceleration along the drive axis A 1 , and the combination of the pressures P 2  and P 4  along the drive axis A 2  causes translational acceleration along the drive axis A 2 . Additionally, the combination of pressures between the different drive axes A 1  and A 2  (for example, P 1  and P 2 ) causes rotational acceleration (angular acceleration) in addition to the translational acceleration. The fluid actuator of the present embodiment improves safety during driving by monitoring various accelerations generated in the drive target W by the pressures P 1  to P 4 . 
     In  FIG. 1B , the three drive axes X, Y 1 , and Y 2  are provided. The drive axis X passes through the center of gravity G of the drive target W. The drive axes Y 1  and Y 2  are provided with the drive target W sandwiched therebetween and are parallel to each other and perpendicular to the drive axis X. Hereinafter, the drive axes X, Y 1 , and Y 2  are also referred to as an X axis, a Y 1  axis, and a Y 2  axis, respectively, and the respective directions are also referred to as an X direction, a Y 1  direction, and a Y 2  direction. Additionally, the drive axes Y 1  and Y 2  are collectively referred to as a Y axis, and the direction thereof is also referred to as a Y direction. On the drive axis X, a pressure PX− along a drive direction from a positive side to a negative side in the X direction and a pressure PX+ along a drive direction from the negative side to the positive side in the X direction are applied to the drive target W. On the drive axis Y 1 , a pressure PY 1 − along a drive direction from a positive side to a negative side in the Y 1  direction and a pressure PY 1 + along a drive direction from the negative side to the positive side in the Y 1  direction are applied to the drive target W. On the drive axis Y 2 , a pressure PY 2 − along a drive direction from a positive side to a negative side in the Y 2  direction and a pressure PY 2 + along a drive direction from the negative side to the positive side in the Y 2  direction are applied to the drive target W. 
     Combinations (PX−, PX+), (PY 1 −, PY 1 +), (PY 2 −, PY 2 +) of pressures along the respective drive axes X, Y 1 , and Y 2  cause translational accelerations along the respective drive axes X, Y 1 , and Y 2 . Additionally, the combinations of pressures between the different drive axes X, Y 1 , and Y 2  cause rotational accelerations in addition to the translational accelerations. The rotational accelerations are particularly caused by combinations of pressures on the Y 1  axis and the Y 2  axis. For example, a combination of PY 1 − and PY 2 + causes a counterclockwise rotational acceleration in  FIGS. 1A and 1B , and a combination of PY 1 + and PY 2 − causes a clockwise rotational acceleration in  FIGS. 1A and 1B . Additionally, a combination of PY 1 − and PY 2 − causes a rotational acceleration according to a magnitude relationship thereof. That is, a counterclockwise rotational acceleration is generated in a case where PY 1 − is larger than PY 2 −, and a clockwise rotational acceleration is generated in a case where PY 1 − is smaller than PY 2 −. Similarly, a combination of PY 1 + and PY 2 + also causes rotational acceleration according to the magnitude relationship thereof. That is, a clockwise rotational acceleration is generated in a case where PY 1 + is larger than PY 2 +, and a counterclockwise rotational acceleration is generated in a case where PY 1 + is smaller than PY 2 +. The fluid actuator of the present embodiment monitors the translational accelerations along the respective drive axes X, Y 1 , and Y 2  on the basis of the comparison of opposite pressures on the respective drive axes X, Y 1 , and Y 2  and also monitors the rotational accelerations on the basis of the comparison of pressures on the Y 1  axis and the Y 2  axis, thereby improving the safety during driving. In addition, the acceleration generated in the drive target W can be uniquely obtained by mechanical calculation based on the measured values of the pressures PX−, PX+, PY 1 −, PY 1 +, PY 2 −, and PY 2 +, and the relative positions of the drive target W with respect to the drive axes X, Y 1 , and Y 2  (measurable by position sensors  140  and  142  described below). Accordingly, instead of individually comparing the measured values of the two pressures as described above, the acceleration may be obtained at once by using a function having the respective measured values as variables. As is clear from the above description, the present invention is suitable for a multi-axis fluid actuator having a plurality of drive axes. 
       FIG. 2  is a perspective view of an air stage to which the fluid actuator of the present embodiment is applied. An air stage  100  mainly includes a platen  102 , an anti-vibration table  104 , an anti-vibration device  106 , a workpiece table  110 , one X-axis air actuator  120  extending along the X axis, and two Y-axis air actuators  130 A and  130 B (hereinafter, collectively referred to as a Y-axis air actuator  130 ) extending along the Y axis. The platen  102  is supported by the anti-vibration table  104 . The X-axis air actuator  120  and the Y-axis air actuators  130 A and  130 B form an H shape as viewed from above. The anti-vibration device  106  absorbs the force caused by the movement of the X-axis air actuator  120  and the Y-axis air actuators  130 A and  130 B and the vibration from a floor and suppresses the vibration of the platen  102 . 
     The X-axis air actuator  120  and the Y-axis air actuator  130  are fluid actuators that drive the workpiece table  110 , which is a drive target, along the X axis and the Y axis, respectively, by using air, which is a gas, as a working fluid. The X-axis air actuator  120  has a guide (square shaft)  122 , a slider  124 , and a servo valve  126  (not shown). Similarly, the Y-axis air actuators  130 A and  130 B each have a guide  132 , a slider  134 , and a servo valve  136 , respectively. Both ends of the X-axis guide  122  are respectively supported by the sliders  134  of the Y-axis air actuators  130 A and  130 B. The slider  124  moves in the X direction along the guide  122 . The X-axis air actuator  120  moves in the Y direction along the guide  132  as the slider  134  moves. In this way, the air stage  100  moves the workpiece table  110  together with the slider  124  in the XY plane. The workpiece table  110 , the X-axis air actuator  120 , and the Y-axis air actuators  130 A and  130 B are placed in a vacuum environment covered with a casing  108 . 
     In the X-axis air actuator  120 , the slider  124  constitutes a first drive unit that drives the workpiece table  110  serving as a drive target along the guide  122  that constitutes the X axis, which is a first drive axis. In the Y-axis air actuator  130 B, the slider  134  constitutes a second drive unit that drives the workpiece table  110  serving as a drive target along the guide  132  that constitutes the Y 1  axis, which is a second drive axis. Similarly, in the Y-axis air actuator  130 A, the slider  134  constitutes a third drive unit that drives the workpiece table  110  serving as a drive target along the guide  132  that constitutes the Y 2  axis, which is a third drive axis parallel to the Y 1  axis. The Y-axis air actuators  130 A and  130 B are provided with the workpiece table  110  interposed therebetween. Additionally, the servo valves  126  and  136  constitute a driving pressure generating unit that supplies air at a pressure commanded by the controller  200  ( FIG. 3 ) to the sliders  124  and  134 . 
     The position sensor  140  measures the position of the workpiece table  110  in the X direction. Additionally, the position sensor  142  measures the position of the workpiece table  110  in the Y direction. By differentiating the measured positions in the X and Y directions with respect to time, velocities in the X direction and the Y direction can be obtained. Additionally, by differentiating the velocities in the X direction and the Y direction with respect to time, accelerations in the X and Y directions can be obtained. 
       FIG. 3  is a schematic sectional view of the air actuator. Specifically, a longitudinal section of the X-axis guide  122  at the center in the Y direction is schematically shown. 
     A hydrostatic bearing is formed between the guide  122  and the slider  124 , and the slider  124  floats from the guide  122  and is movable in the X direction in complete non-contact, due to the air pressure constantly supplied between an outer peripheral surface of the guide  122  and an inner peripheral surface of the slider  124 . In addition, although not shown, the workpiece table  110  is fixed to a +Z-side surface of the slider  124  and moves integrally with the slider  124  along the X axis. 
     The slider  124  is provided with a servo chamber  150  that is an internal space. The servo chamber  150  is partitioned into a positive-side chamber  152  and a negative-side chamber  154  by a pressure-receiving plate  123  fixed to the guide  122 . 
     The X-axis air actuator  120  includes a positive-side servo valve  126 P and a negative-side servo valve  126 N that are respectively disposed on the positive side and the negative side of the X axis. The slider  124  is driven by the positive-side servo valve  126 P and the negative-side servo valve  126 N. The positive-side servo valve  126 P and the negative-side servo valve  126 N control the intake/exhaust amount of the positive-side chamber  152  and the negative-side chamber  154  depending on the position of a spool to be described below. The positive-side servo valve  126 P communicates with the positive-side chamber  152  via a positive-side pipe  128 P. The negative-side servo valve  126 N communicates with the negative-side chamber  154  via a negative-side pipe  128 N. 
     The X-axis air actuator  120  controls the positive-side servo valve  126 P and the negative-side servo valve  126 N to generate a differential pressure in the positive-side chamber  152  and the negative-side chamber  154 . The velocity and acceleration of the slider  124  with respect to the guide  122  are controlled by the differential pressure. 
     The positive-side servo valve  126 P and the negative-side servo valve  126 N are connected to a pump  146  as an air supply source via a positive-side air supply pipe  144 P and a negative-side air supply pipe  144 N, respectively. Additionally, the positive-side servo valve  126 P and the negative-side servo valve  126 N discharge air to the outside of a casing  108  via a positive-side air discharge pipe  148 P and a negative-side air discharge pipe  148 N, respectively. The air from the pump  146  is supplied to the positive-side chamber  152  via the positive-side air supply pipe  144 P, the positive-side servo valve  126 P, and the positive-side pipe  128 P. That is, the positive-side air supply pipe  144 P, the positive-side servo valve  126 P, and the positive-side pipe  128 P constitute a positive-side air supply flow path. Similarly, the air from the pump  146  is supplied to the negative-side chamber  154  via the negative-side air supply pipe  144 N, the negative-side servo valve  126 N, and the negative-side pipe  128 N. That is, the negative-side air supply pipe  144 N, the negative-side servo valve  126 N, and the negative-side pipe  128 N constitute a negative-side air supply flow path. The air in the positive-side chamber  152  is discharged to the outside via the positive-side pipe  128 P, the positive-side servo valve  126 P, and the positive-side air discharge pipe  148 P. That is, the positive-side pipe  128 P, the positive-side servo valve  126 P, and the positive-side air discharge pipe  148 P constitute a positive-side air discharge flow path. Similarly, the air in the negative-side chamber  154  is discharged to the outside through the negative-side pipe  128 N, the negative-side servo valve  126 N, and the negative-side air discharge pipe  148 N. That is, the negative-side pipe  128 N, the negative-side servo valve  126 N, and the negative-side air discharge pipe  148 N constitute a negative-side air discharge flow path. 
     The air stage  100  includes the controller  200  that controls the positive-side servo valve  126 P and the negative-side servo valve  126 N. Although the X-axis air actuator  120  has been described above as an example, the Y-axis air actuator  130  can be similarly configured. The controller  200  controls the positive-side servo valve and the negative-side servo valve of all the air actuator  120 ,  130 A, and  130 B. 
       FIG. 4  is a cross-sectional view of the servo valve. Here, since the configurations of the positive-side servo valve  126 P and the negative-side servo valve  126 N are the same, the valves will be collectively described as a servo valve  126 . Additionally, with respect to the configuration of respective parts of the servo valve  126 , the terms “positive side” and “negative side” and the reference numerals “N” and “P” are omitted. 
     The servo valve  126  includes a main body  160 , a spool  162  disposed in the main body  160 , a motor  164 , and a position sensor  166 . The servo valve  126  is a three-way valve including three ports  168 A,  168 B, and  168 C. The servo valve  126  switches a connection point of the port  168 C between the port  168 A or the port  168 B depending on the position of the spool  162 . The spool  162  is disposed in a flow path extending along the Z axis inside the main body  160  and is movable along the Z axis. The position of the spool  162  changes depending on the driving amount of the motor  164 . The position sensor  166  measures the position of the spool  162 . The two ports  168 A and  168 B lined up along the Z axis are provided on one side surface of the main body  160 . The port  168 A on the +Z side is connected to an air discharge pipe  148 , and the port  168 B on the −Z side is connected to an air supply pipe  144 . The port  168 A may be connected to the air supply pipe  144  and the port  168 B may be connected to the air discharge pipe  148 . The port  168 C provided on the other side surface of the main body  160  is connected to a pipe  128 . The measurement result of the position sensor  166  is supplied to an amplifier unit AU of the controller  200 . The controller  200  detects the position of the spool  162  on the basis of the measurement result acquired by the amplifier unit AU, and controls the motor  164  on the basis of the position of the spool  162 . As the controller  200  drives the motor  164  to control the position of the spool  162 , the air supplied from the pump  146  is supplied to the servo chamber  150  through the servo valve  126 , or the air in the servo chamber  150  is discharged to the outside through the servo valve  126 . In  FIG. 4 , the servo valve  126  is disposed such that the spool  162  moves along the Z axis, but the direction in which the servo valve  126  is disposed is not limited to this. 
     Subsequently, the operation of the air stage  100  during normal operation will be described.  FIG. 5  shows time-dependent changes of a velocity v of the slider  124 , an acceleration α of the slider  124 , and a pressure P in the servo chamber  150  during normal operation. 
     In a case where the slider  124  is moved to the positive side with reference to  FIGS. 3 to 5 , the controller  200  moves the spool  162  of the positive-side servo valve  126 P to close the port  168 A connected to the positive-side air discharge pipe  148 P and open the port  168 B connected to the positive-side air supply pipe  144 P. At the same time, the controller  200  moves the spool  162  of the negative-side servo valve  126 N to open the port  168 A connected to the negative-side air discharge pipe  148 N and close the port  168 B connected to the negative-side air supply pipe  144 N. Accordingly, air is supplied into the positive-side chamber  152  to increase the pressure P+, and air is discharged from the negative-side chamber  154  to decrease the pressure P− (time t 0 ). When a differential pressure is generated between the pressure P+ and the pressure P−, the acceleration a increases and the slider  124  accelerates (time t 0  to t 1 ). The controller  200  controls the positive-side servo valve  126 P and the negative-side servo valve  126 N such that the differential pressure between the pressure P+ and the pressure P− becomes zero when the velocity v of the slider  124  reaches a predetermined velocity v 1  (time t 1  to t 2 ). When the differential pressure becomes zero, the slider  124  moves at a constant speed. 
     Subsequently, the controller  200  decelerates the slider  124  such that the velocity v becomes zero when the slider  124  reaches a target position. In this case, the controller  200  moves the spool  162  of the positive-side servo valve  126 P to open the port  168 A connected to the positive-side air discharge pipe  148 P and close the port  168 B connected to the positive-side air supply pipe  144 P. At the same time, the controller  200  moves the spool  162  of the negative-side servo valve  126 N to close the port  168 A connected to the negative-side air discharge pipe  148 N and open the port  168 B connected to the negative-side air supply pipe  144 N. Accordingly, air is discharged from the positive-side chamber  152  to reduce the pressure P+, and air is supplied to the negative-side chamber  154  to increase the pressure P−. When a differential pressure is generated between the pressure P+ and the pressure P−, the acceleration α decreases and the slider  124  decelerates (time t 2  to t 3 ). The controller  200  stops the slider  124  by setting the differential pressure to zero when the slider  124  reaches the target position (time t 3 ). 
     Subsequently, the features of the air stage  100  will be described. 
     Returning to  FIG. 3 , the X-axis air actuator  120  includes a positive-side pressure sensor  129 P provided on the positive-side pipe  128 P and a negative-side pressure sensor  129 N provided on the negative-side pipe  128 N. The positive-side pressure sensor  129 P measures the pressure PX+ of the air that drives the slider  124  in the + X direction. The negative-side pressure sensor  129 N measures the pressure PX− of the air that drives the slider  124  in the −X direction. 
     The pressure PX+ is equivalent to the pressure P+ in the positive-side chamber  152  in  FIG. 5 , and the pressure PX− is equivalent to the pressure P− in the negative-side chamber  154  in  FIG. 5 . As described with respect to  FIG. 5 , when the pressure P+ (PX+) in the positive-side chamber  152  rises (time t 0  to t 1 ), the slider  124  accelerates in the + X direction, and when the pressure P− (PX−) in the negative-side chamber  154  rises (time t 2  to t 3 ), the slider  124  accelerates in the −X direction. In this way, in order to schematically show that the pressure PX+ is the driving pressure for driving the slider  124  in the + X direction, the pressure PX+ is represented by a vector in the + X direction in  FIG. 3 . Similarly, in order to schematically show that the pressure PX− is the driving pressure for driving the slider  124  in the −X direction, the pressure PX− is represented by a vector in the −X direction in  FIG. 3 . 
     The pressures PX+ and PX− in the X direction are represented in  FIG. 1B  to the same effect. The same applies to the pressures PY 1 +, PY 1 −, PY 2 +, and PY 2 − in the Y direction shown in  FIG. 1B . That is, in the Y-axis air actuator  130 B constituting the Y 1  axis, the pressure PY 1 + is the driving pressure for driving the slider  134  in the + Y direction, and the pressure PY 1 − is the driving pressure for driving the slider  134  in the −Y direction. Similarly, in the Y-axis air actuator  130 A constituting the Y 2  axis, the pressure PY 2 + is the driving pressure for driving the slider  134  in the + Y direction, and the pressure PY 2 − is the driving pressure for driving the slider  134  in the −Y direction. The driving pressures PY 1 +, PY 1 −, PY 2 +, and PY 2 − in the Y direction are individually measured by pressure sensors similar to the pressure sensors  129 P and  129 N shown in  FIG. 3 . 
     In  FIG. 3 , the controller  200  common to the X-axis air actuator  120  and the Y-axis air actuators  130 A and  130 B includes an acceleration detection unit  210  and a drive limiting unit  220 . The acceleration detection unit  210  detects the acceleration generated in the slider  124  and the workpiece table  110  on the basis of the driving pressures PX+, PX−, PY 1 +, PY 1 −, PY 2 +, and PY 2 − measured by the respective pressure sensors in the respective drive directions. The drive limiting unit  220  limits the driving of the slider  124  and the workpiece table  110  in a case where the acceleration detected by the acceleration detection unit  210  exceeds a predetermined threshold. 
     With reference to  FIG. 1B , accelerations in respective directions generated in the drive target W detected by the acceleration detection unit  210  will be described. On the basis of the equation of motion, the translational acceleration is represented by “force/mass” and the rotational acceleration is represented by “torque/moment of inertia”. The force in each drive direction is obtained by multiplying the pressure caused by the air by the cross-sectional area. In the following, it is assumed that the cross-sectional areas of air in the respective directions are equal as S. In this case, a resultant force FX in the X direction is ((PX+)−(PX−)) S, and a resultant force FY in the Y direction is ((PY 1 +)+ (PY 2 +)−(PY 1 −)−(PY 2 −)) S. The origin when considering the rotary motion can be optionally set, but for example, as shown in the figure, the point O on the Y 1  axis is set as the origin. The torque N around an origin O is the sum of torques obtained multiplying a force caused by each driving pressure by each vertical distance (arm lengths) from the origin O. 
     The drive target Win the translational motion in the X direction is the slider  124 , the workpiece table  110 , and a placed object placed on the workpiece table  110 , and the total mass of these objects is m. Additionally, in the translational motion in the Y direction, since the entire X-axis air actuator  120  including the above is driven, m+M including the residual mass M becomes the mass of the drive target W. Even in the rotary motion, the rotation of the entire X-axis air actuator  120  becomes a problem. Therefore, m+M is the mass of the drive target W. The moment of inertia I around the origin O is obtained by approximating the drive target W of mass m+M with an appropriate number of mass points and the sum of moments of inertia obtained by multiplying the mass of each mass point by the squared of each vertical distance (arm length) from the origin O. 
     On the basis of the above respective elements, the accelerations in the respective directions can be obtained as follows.
         Translational acceleration αX in X direction: FX/m   Translational acceleration αY in Y direction: FY/(m+M)   Rotational acceleration αθ origin O: N/I       

     The drive limiting unit  220  of  FIG. 3  limits the drive of the drive target W in a case where the acceleration in each of the above directions exceeds a predetermined threshold and becomes excessive. For example, in a case the acceleration in any direction becomes excessive, all the servo valves  126  and  136  of the air stage  100  are connected to the air discharge pipe  148  to perform the emergency exhaust. Accordingly, the pressure of the air in the air stage  100  drops sharply, and the air stage  100  can be safely stopped. In addition, instead of connecting all the servo valves to the air discharge pipe, only the servo valves that contribute to a drive direction in which an excessive acceleration is detected may be connected to the air discharge pipe to perform the emergency exhaust. Additionally, by providing the pipe  128  with an exhaust valve that is opened at the time of the emergency exhaust, the pipe  128  may be configured to perform the emergency exhaust. Moreover, the drive limiting unit  220  may send an emergency control command for generating a driving pressure in the direction in which the excessive acceleration is offset to the servo valves  126  and  136 , instead of performing the emergency exhaust. 
       FIGS. 6 and 7  show an example of drive limitation by the drive limiting unit  220 .  FIG. 6  shows an example of drive limitation in a case where the translational acceleration along any drive axis of the X axis, Y 1  axis, and Y 2  axis becomes excessive, and shows the time-dependent changes of the velocity v of the sliders  124  and  134 , the translational acceleration a of the slider  124  and  134 , and the pressure P in the servo chamber  150 , similar to  FIG. 5 . A threshold vT is set for the velocity v, and when the threshold vT is exceeded, the air stage  100  is emergency-stopped. The time when the velocity v reaches the threshold vT and the emergency exhaust of the servo valves  126  and  136  starts is defined as tv 0 , and the time when the emergency exhaust is completed is defined as tv 1 . A threshold αT is set for the translational acceleration α, and when the threshold αT is exceeded, the air stage  100  is emergency-stopped. The time when the translational acceleration α reaches the threshold αT and the emergency exhaust of the servo valves  126  and  136  starts is defined as tα 0  and the time when the emergency exhaust is completed is defined as tα 1 . 
     As is clear from the figure, the air stage  100  earlier than the threshold control based on the velocity v can be emergency-stopped by the threshold control based on the translational acceleration a (tα 1 &lt;tv 1 ). Additionally, in the threshold control based on the velocity v, the velocity v of the drive target W is as high as vT at the time tv 0  when the emergency exhaust starts. For this reason, even when the emergency exhaust is performed from the time tv 0  and the translational acceleration α becomes zero at the time tORDOv 1 , time is further required until the drive target W finally stops due to the inertia of the drive target W during high-speed movement. In contrast, in the threshold control based on the translational acceleration α, the velocity v of the drive target W is almost zero at the time tα 0  when the emergency exhaust starts. For this reason, when the emergency exhaust is performed from time tα 0  and the translational acceleration a becomes zero at time tad, the drive target W during low-speed movement finally stops soon. In this way, according to the threshold control based on the translational acceleration α, the emergency exhaust can be started before the velocity v of the drive target W becomes high. Thus, the air stage  100  can be rapidly and safely emergency-stopped. In particular, in the air stage  100  in which the drive target W is driven in a state where the air stage have floated due to the pressure of air, it is difficult to easily stop the drive target W once the speed becomes high. Therefore, this point is extremely important. 
     In addition, the translational acceleration α can also be obtained by second-order differentiating the positions measured by the position sensors  140  and  142  with respect to time. However, since it is necessary to accumulate measurement data for a certain period of time for differential calculation, it may not be suitable for the above-mentioned situation having a high emergency. On the other hand, as described with respect to  FIG. 1B , according to the driving pressures PX+, PX−, PY 1 +, PY 1 −, PY 2 +, and PY 2 − measured by the pressure sensors, the translational accelerations αX and αY can be directly calculated. Thus, the stop processing of the air stage  100  can be rapidly started even in a situation having a high emergency. Additionally, even in a case where the position sensor  140  and  142  fail, when the pressure sensor is normally operating, the emergency stop processing can be performed. Thus, the robustness of the system is improved. 
       FIG. 7  shows an example of drive limitation in a case where the rotational acceleration of the drive target W becomes excessive, and shows the time-dependent changes of the driving pressure PY 1  of the Y-axis air actuator  130 B constituting the Y 1  axis and the driving pressure PY 2  of the Y-axis air actuator  130 A constituting the Y 2  axis. As described with respect to  FIG. 1B , the rotational acceleration can be accurately calculated by the mechanical calculation based on the driving pressures PX+, PX−, PY 1 +, PY 1 −, PY 2 +, PY 2 − measured by the pressure sensors and the relative positions of the drive target W with respect to the drive axes X, Y 1 , and Y 2 . However, in the example of this figure, the generation of undesired rotational acceleration is simply detected on the basis of the comparison between the pressures PY 1 + and PY 2 + in the positive directions of the respective axes and the comparison between the pressures PY 1 − and PY 2 − in the negative directions of the respective axes. 
     During normal operation when no rotational acceleration is generated, the translational acceleration generated on the Y 1  axis and the translational acceleration generated on the Y 2  axis are equal to each other. Accordingly, the X-axis air actuator  120  as the drive target in the Y direction is driven in the Y direction while maintaining a state where the X-axis air actuator  120  is parallel to the X direction and perpendicular to the Y direction. In this case, the graphs of PY 1  and PY 2  in  FIG. 7  become the same. Specifically, as shown in the graph of PY 2 , PY 2 + and PY 2 − when a differential pressure (translational acceleration in the Y direction) is generated change in opposite directions by the same amount ΔP with respect to an initial pressure P 0 . However, in the shown example, an abnormality occurs in the driving pressure PY 1 + in the positive direction of PY 1 , and a change of an amount ΔP′ larger than the desired amount of change ΔP is observed. In this case, the acceleration detection unit  210  performs the comparison between PY 1 + and PY 2 + and the comparison between PY 1 − and PY 2 −, respectively. In the former comparison, a differential pressure of ΔP′−ΔP is detected between PY 1 + and PY 2 +. In the latter comparison, since PY 1 − and PY 2 − are the same, no differential pressure is detected. On the basis of these comparisons, the acceleration detection unit  210  detects that the rotational acceleration in the clockwise direction in  FIG. 1B  is generated because of PY 1 +&gt;PY 2 +. Since the driving for causing the rotational acceleration is not assumed in the air stage  100  of the present embodiment, the drive limiting unit  220  has a substantially zero threshold for the rotational acceleration. Accordingly, as shown in  FIG. 7 , in a case where the pressures on the Y 1  axis and the Y 2  axis are unbalanced, the drive limiting unit  220  is determined to be abnormal and the air stage  100  is emergency-stopped. Similarly to  FIG. 6 , the time when the emergency exhaust of the servo valve  136  starts is defined as tα 0 , and the time when the emergency exhaust is completed is defined as tad. 
     The present invention has been described above on the basis of the embodiment. The embodiment is an example, and it will be understood by those skilled in the art that various modification examples are possible for the combinations of these respective components and the respective processing processes and that such modification examples are also within the scope of the present invention. 
     In the embodiment, the air actuator using air as the working fluid has been described, but the fluid actuator of the present invention may use a fluid other than this as the working fluid. For example, a hydraulic actuator using oil as the working fluid, a hydraulic actuator using water as the working fluid, or a gas actuator using an optional gas other than air as the working fluid may be used. 
     In addition, the functional configurations of the respective devices described in the embodiment can be realized by hardware resources or software resources or by the collaboration between the hardware resources and the software resources. Processors, ROMs, RAMs, and other LSIs can be used as the hardware resources. Programs such as operating systems and applications can be used as the software resources. 
     It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention.