Patent Publication Number: US-7210394-B2

Title: Method and apparatus for controlling air cylinder

Description:
TECHNICAL FIELD 
   The present invention relates to a method and an apparatus for controlling an air cylinder by an air servo valve. 
   BACKGROUND ART 
     FIG. 4  illustrates an example of basic connection of an apparatus for controlling a thrust force of an air cylinder by an air servo valve. In the figure, with respect to reference numbers,  1  denotes an air cylinder,  2  denotes a three-position air servo valve connected to a head-side pressure chamber  1   a  of the air cylinder  1 ,  3  denotes a pressure air source connected to the air servo valve  2  and a rod-side pressure chamber  1   b  through a regulator  4 ,  5  denotes a controller controlling the air servo valve  2  by a PID adjuster  5   a  (see  FIG. 5 ),  6  denotes a pressure sensor detecting an air pressure in the head-side pressure chamber  1   a  and feeding back a signal of the detected pressure to the controller  5 , and  7  denotes a position sensor detecting the position of a piston  1   c  of the air cylinder  1 . 
   In the apparatus, when the air servo valve  2  is switched to a first position, shown on the left of the figure, by the controller  5 , and a pressure air is fed to the head-side pressure chamber  1   a  of the air cylinder  1 , the piston  1   c  and a rod  1   d  of the air cylinder  1  move forward to the right in the figure. On this occasion, a pressure in the head-side pressure chamber  1   a  is detected by the pressure sensor  6 , the position of the piston  1   c  is detected by the position sensor  7 , and respective detection signals are fed back to the controller  5 . Then, by applying a necessary gain (amplification) on a difference between a pressure instruction value and the detection value of the pressure in the PID adjuster  5   a  of the controller  5 , and controlling the air servo valve  2 , thrust control in accordance with the position of the piston  1   c  is performed. On this occasion, the air servo valve  2  is opened at the degree of opening in accordance a control signal having the gain applied thereon, and the pressure in the pressure chamber  1   a  of the cylinder  1  is controlled with a rate of airflow according to the degree of opening. 
     FIG. 5  is a block diagram of the apparatus, for controlling a thrust force of the air cylinder  1  by controlling the pressure in the pressure chamber  1   a . With respect to reference characters in the figure, “Pi” is an instruction value, “Kp” is a proportional gain of the PID adjuster  5   a , “G(S)” is a transfer function of the air servo valve  2 , “V” is a volume of the pressure chamber, “1/VS” is a transfer function of the air cylinder  1 , “a” is a constant, “T” is a time constant, “s” is Laplace operator, “Q” is a manipulated variable, “Po” is a controlled variable, and “Kc” is a feedback gain. The block diagram will be described in detail later, in association with the description of the present invention. 
   DISCLOSURE OF THE INVENTION 
   Meanwhile, in the case of controlling an air cylinder by the air servo valve as described above, it is difficult to accurately control the air cylinder with a known control type, because of poor response due to a steady-state difference between an instruction value and a measurement value, and to influence of a disturbance. Especially, since the volume (tank volume) of the pressure chamber serving as a load changes dramatically in accordance with a position change of the piston, there is a problem that the control of the air cylinder is not smoothly performed. Also, there is a problem that a small or large volume of the pressure chamber causes an unstable or poor response control system, respectively. 
   Accordingly, in order to solve the problems of the known control type, the object of the present invention is to provide an innovative control technique with which an air cylinder is accurately controlled since a steady-state difference is reduced, and a disturbance is also less influenced so as to improve response and stability. 
   Means for Solving the Problems 
   In order to achieve the above object, the present invention provides a control method for controlling an air cylinder such that air is supplied to and exhausted from at least one pressure chamber of the air cylinder by at least one air servo valve; a pressure in the pressure chamber is detected by a pressure sensor; a signal of the detected pressure is fed back to a controller; and the degree of opening of the air servo valve is adjusted by at least one PID adjuster of the controller on the basis of a difference between an instruction value and a detection value, wherein a displacement of a rod of the air cylinder is detected by a displacement sensor; and only a gain of the PID adjuster is always changed on the basis of a detection signal of the displacement. 
   In this case, a proportional gain may be changed in proportion to the displacement of the rod. 
   According to the present invention, air is possibly supplied to and exhausted from the two head-side and rod-side pressure chambers of the air cylinder by the two respective air servo valves, and gains of the PID adjusters corresponding to the respective air servo valves may be changed in accordance with a detection signal of the displacement from the displacement sensor. 
   Also, in order to implement the foregoing method, the present invention provides a control apparatus of an air cylinder, which includes an air cylinder; at least one air servo valve supplying and exhausting air to and from at least one pressure chamber of the air cylinder; a pressure sensor detecting a pressure in the pressure chamber; a displacement sensor detecting a displacement of a rod of the air cylinder; and a controller controlling the air servo valve with at least one PID adjuster on the basis of a difference between a detection value of the pressure fed back from the pressure sensor and an instruction value, wherein a gain of the PID adjuster is always changed in accordance with a detection signal of the displacement from the displacement sensor. 
   In this case, the proportional gain may be changed in proportion to the displacement of the rod. 
   Further, the present invention also provide a control apparatus including the two air servo valves and the two pressure sensors, each pair connected to either one of the head-side and rod-side pressure chambers of the air cylinder; the two PID adjusters corresponding to the respective air servo valves; and the single displacement sensor. 
   According to the present invention, since the displacement of the rod of the air cylinder is detected by the displacement sensor, and only the gain of the PID adjuster is always changed on the basis of a detection signal of the displacement, even when the volume of the pressure chamber of the air cylinder is dramatically changed, or even when the volume of the chamber is small or large, due to controllability similar to that of adaptive control; a steady-state difference is reduced, and a disturbance is also less influenced, whereby response and stability are improved, resulting in accurately controlling the air cylinder. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is the overall connection diagram of a cylinder control apparatus according to an embodiment of the present invention. 
       FIG. 2  is an example time diagram illustrating a control method according to the present invention. 
       FIG. 3  is a block diagram of a head-side control system of the control apparatus shown in  FIG. 1 . 
       FIG. 4  is a connection diagram of a known cylinder control apparatus. 
       FIG. 5  is a block diagram of the control apparatus shown in  FIG. 4 . 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
     FIG. 1  shows a cylinder control apparatus according to an embodiment of the present invention, in which an air cylinder  10  is used as a welding air servo gun by way of example. 
   More particularly, the control apparatus includes the air cylinder  10  making up the welding gun; head-side and rod-side air servo valves  20  and  30  respectively connected to head-side and rod-side pressure chambers  11  and  12  of the air cylinder  10 ; a controller  40  outputting a control signal to these air servo valves  20  and  30 ; and an external controller  50  externally providing an instruction to the controller  40  and controlling both air servo valves  20  and  30  with the controller  40  so as to bring the air cylinder  10  in a desired operating state. 
   Also, the air cylinder  10  includes a cylinder tube  13 ; a piston  14  slidably inserted in the cylinder tube  13 ; and a piston rod  15  connected to the piston  14 , and a workpiece is clamped by the piston rod  15 . The cylinder tube  13  is a sealed cylindrical body and includes its head-side and rod-side pressure chambers  11  and  12  having the piston  14  interposed therebetween. The piston rod  15  extends hermetically through the cylinder tube  13  and to the outside. The piston rod  15  has one electrode member of the welding gun (not shown) placed at the end of the externally extended part thereof. 
   Air at a necessary pressure is fed to/discharged from the head-side pressure chamber  11  from/to the head-side air servo valve  20  through a flow path  22 , and the head-side pressure chamber  11  has a head-side pressure sensor  23  connected thereto, for detecting its air pressure. Also, the head-side pressure chamber  11  has a probe  26  of a displacement sensor  25  disposed therein, inserted in the piston  14  from the head cover side, for detecting the drive position of the piston  14 . Detection signals of the pressure and the displacement respectively detected by the head-side pressure sensor  23  and the displacement sensor  25  are fed back to the controller  40 . 
   On the other hand, air is fed to/discharged from the rod-side pressure chamber  12  from/to the head-side air servo valve  30  through a flow path  32 , and the rod-side pressure chamber  12  has a rod-side pressure sensor  33  connected thereto, for detecting a pressure of the air. A signal of the detected pressure from the rod-side pressure sensor  33  is fed back to the controller  40 . 
   Each of the head-side and rod-side air servo valves  20  and  30  is a three-position, three-port valves practically having the same structure as each other, including a supply port introducing air from an air supply source  41 , an output port outputting it, and an exhaust port exhausting it, and, if needed, opens each port at the degree of opening in accordance with an output signal from the controller  40  so as to feed the controlled pressure air to the corresponding pressure chamber. 
   As described above, signals of the detected pressures from the head-side and rod-side pressure sensors  23  and  33  and a detected position signal from the displacement sensor  25  are fed back to the controller  40 . Also, operation modes of the piston  14  and instruction values of air pressures in both pressure chambers  11  and  12  in accordance with the operational position of the piston  14  and so forth are set with respect to a time diagram and stored in the controller  40 . Thus, on the basis of instruction signals received from an external computer  50 , the detection values fed back from the corresponding pressure sensors  23  and  33  and the instruction values are respectively compared by PID adjusters of head-side and rod-side control units  40   a  and  40   b  of the controller  40 , necessary gains (amplifications) are applied on these differences, and the corresponding head-side and rod-side air servo valves  20  and  30  are controlled with these signals. On this occasion, the air servo valves  20  and  30  are opened at the degrees of opening in accordance with the corresponding gain-applied control signals, pressures Ph and Pr in respective pressure chambers  11  and  12  of the air cylinder  10  are controlled with rates of airflow in accordance with the degrees of opening, and a difference between these pressures is outputted as a thrust force. 
   Accordingly, the head-side air servo valve  20 , the head-side pressure sensor  23 , and the head-side control unit  40   a  make up a head-side control system  60 A, and the rod-side air servo valve  30 , the rod-side pressure sensor  33 , and the rod-side control unit  40   b  make up a rod-side control system  60 B. 
   Meanwhile, reference numbers  24  and  34  in the figure denote pressure sensors disposed in the flow paths  22  and  32  extending from the air servo valves  20  and  30  to the corresponding pressure chambers. 
     FIGS. 2(A) to 2(C)  illustrate an example control operation of the air cylinder  10  with respect to a time diagram.  FIG. 2(A)  illustrates changes of input signals Vh and Vr applied on respective air servo valves  20  and  30 , starting from an arbitrary stop position of the air cylinder  10 ,  FIG. 2(B)  illustrates a change in piston stroke X, and  FIG. 2(C)  illustrates changes in pressures Ph and Pr in the head-side and rod-side pressure chambers  11  and  12  of the air cylinder  10 . 
   In  FIG. 2(A) , at time t 1 , while one input signal shown by curve Vh is applied on the head-side air servo valve  20 , so as to fully or nearly fully open the air supply side of the air servo valve  20 , and the other input signal shown by curve Vr is applied on the rod-side air servo valve  30  so as to fully open the air exhaust side of the air servo valve  30 . 
   Hence, as shown in  FIG. 2(B) , the piston  14  located at an arbitrary stop position (Xa) is driven from that position towards a clamp position (Xo) serving as a target position Xt, at which a workpiece is clamped. 
   When the piston  14  is driven as described above and operated for clamping, the pressure of the head-side air servo valve  20  is controlled as shown in the figure, and that of the rod-side air servo valve  30  is controlled such that, by maintaining the degree of air servo valve opening so as to correspond to an input signal (a·ΔX, wherein “a” is a constant) in proportion to a difference (ΔX=X−Xo), the piston speed of the cylinder can be smoothly reduced as the piston comes closer to the clamp position of the workpiece. 
   Meanwhile, reduction in the degree of opening of the head-side air servo valve  20  is also needed in accordance with the difference ΔX. 
   When the piston speed is reduced to a satisfactory degree and the piston comes close to the clamp position of the workpiece also to a satisfactory degree, by fixing the degree of air servo valve opening (ΔV) of the rod-side air servo valve  30  at a fine constant value from the time when the piston reaches a set position (Xc), a clamping member comes into contact with the workpiece at a constant and low speed. 
     FIG. 3  is a block diagram of the head-side control system  60 A of the control apparatus controlling the pressure of the head-side pressure chamber  11 . The head-side control system  60 A has a structure in which, while the pressure of the head-side pressure chamber  11  is controlled as described above, a detection signal of the displacement of the rod detected by the displacement sensor  25  is fed back to the head-side control unit  40   a  at the same time, and, on the basis of a detection value K of the signal, a gain Kp of a PID adjuster  40   a ′ (not shown) is always changed in accordance with a change in a cylinder volume (a volume of the head-side pressure chamber) V. 
   In the meantime, since the basic pressure control performed by the control system  60 A is practically the same as that performed by a known apparatus shown in  FIGS. 4 and 5 , the basic part thereof will be described with reference to a block diagram of the known apparatus shown in  FIG. 5 . 
   A transfer function of the overall block diagram of the known apparatus is given by Expression (1). 
   
     
       
         
           
             
               
                 [ 
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 ] 
               
             
             
               
                   
               
             
           
           
             
               
                 
                   
                     
                       P 
                       o 
                     
                     ⁡ 
                     
                       ( 
                       S 
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   Also, in the foregoing Expression (1), when approximation with a first order system is applied on a transfer function of a valve so as to simplify it, it is given by G(S)=a/(1+T·s). As a result, Expression (1) is given by Expression (2), and its right side is given by Expression (3). 
   
     
       
         
           
             
               
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                   Expression 
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                   Expression 
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                   3 
                 
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   A transfer function of an output pressure outputted to the air cylinder  10  is expressed by a secondary order system with respect to a pressure instruction value inputted in the PID adjuster and is given by the following Expression (4). 
   
     
       
         
           
             
               
                 [ 
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
                 ] 
               
             
             
               
                   
               
             
           
           
             
               
                 K 
                 · 
                 
                   1 
                   
                     
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                       2 
                     
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                       ⁢ 
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                       ⁢ 
                       
                           
                       
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                         n 
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                       ⁢ 
                       
                           
                       
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                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   In the above expression, ωn and ζ denote an undamped natural angular frequency and a damping coefficient and are given by the following Expressions (5) and (6), respectively. 
   [Expression 5]
 
ω n =√{square root over (( K   c   ·K   p   ·a )/( T·V ))}{square root over (( K   c   ·K   p   ·a )/( T·V ))}  (5)
 
   
     
       
         
           
             
               
                 [ 
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
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                 = 
                 
                   
                     1 
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                 ( 
                 6 
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   From these expressions, it is understood that the undamped natural angular frequency on and the damping coefficient ζ depend heavily on the volume of the cylinder. 
   Since the volume of the pressure chamber of the cylinder varies depending on the position of the piston, and this causes the undamped natural angular frequency ωon and the damping coefficient ζ to vary, the controllability of the cylinder control also changes, and the response of the control is poor because of being easily influenced by, for example, the steady-state difference between the instruction and measurement values, and a disturbance, thereby resulting in a difficulty in achieving accurate control. 
   However, upon focusing attention on the foregoing Expressions (5) and (6), it is understood that the cylinder volume V and the gain Kp of the PID adjuster are included in the denominator and numerator or the numerator and denominator of the corresponding expression. As such, when the gain Kp is adjusted in accordance with a change in the cylinder volume so as to make Kp/V constant, the undamped natural angular frequency and the damping coefficient do not vary, thereby resulting in constant controllability. 
   From such a viewpoint, according to the present invention, as shown in  FIG. 3 , a signal of the displacement of the rod detected by the displacement sensor  25  is fed back to the head-side control unit  40   a  so that the gain Kp of the PID adjuster  40   a ′ is always changed in accordance with the detection value K of the signal. As a concrete method, it is sufficient that the detection value K of the displacement is multiplied by a value of the gain. 
   With this arrangement, even when the volume of the pressure chamber of the air cylinder  10  varies dramatically, due to controllability excellent the same as that of adaptive control, a steady-state difference and a disturbance are reliably prevented from occurrence, thereby achieving excellent response regardless of the position of the rod. 
   Meanwhile, while the gain of the PID adjuster of the head-side control system is changed in the foregoing embodiment, the same control can be performed for the rod-side control system. 
   Also, by using a speed sensor or an acceleration sensor as the displacement sensor for detecting a speed or an acceleration of the rod so as to serve as a displacement signal, the same control can also be performed. 
   Meanwhile, it can be understood that the art controlling the air pressure of the pressure chamber of the air cylinder  10  according to the above-described method is applicable no only to thrust control of the air cylinder  10  but also to positioning control of the rod.