Patent Publication Number: US-7587971-B2

Title: Pneumatic actuator for precision servo type applications

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/554,441, filed Mar. 19, 2004 entitled “Rodless Pneumatic Actuator for Precision Servo Type Applications” which is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to pneumatic cylinders and, more particularly, to pneumatic cylinders with reduced acoustical vibration. 
     BACKGROUND 
     Conventional pneumatic cylinders provide a conduit for airflow into and out of two working volumes by means of ports machined into the respective end caps. These ports serve as anchor points for plumbing that then communicates airflow to a control valve or valve network. While such an arrangement has a certain level of operability, it typically creates a poor dynamic relationship between airflow and differential pressure. More specifically, such arrangements typically produce excess noise (i.e., acoustical vibrations) in the air column used to move the piston. This noise affects the precise movement of the piston of the pneumatic cylinder. Consequently, attempts to apply such devices in precision applications have met with limited success. 
     An inherent disadvantage of this construction lies in the fact that the distance between each piston face and its respective air port changes as the piston slews within the cylinder bore. Therefore, the time required for a shock wave emanating from an air port to effect a force change on the piston is dependant on the position of the piston in the bore. Furthermore, shock waves that propagate longitudinally along the cylinder bore may be reflected off either end cap or either piston face. This phenomenon has the potential to create undesirable acoustical characteristics. 
     SUMMARY 
     The pneumatic cylinder disclosed herein provides a unique way to communicate airflow between a control valve and the working volumes of the pneumatic cylinder. A piston assembly, sealed at both ends by caps, contains and guides the motion of a piston assembly. Pressure forces on the piston assembly are transmitted via a mechanical structure that distends via a slot that runs the length of the piston assembly. A flexible steel band, which passes through the piston assembly, seals the slot to reduce air leakage. 
     An air control device, such as a servo valve, is operatively coupled to the piston assembly and travels with the piston assembly when a differential pressure is produced on the piston assembly. This arrangement results in a dynamic relationship between airflow and differential pressure that is conducive to precision force and motion control. In addition, the end caps may include snubbers to diffuse sound waves associated with air moving in the piston assembly. These improvements to the cylinder&#39;s acoustics allow for greater controllability in precision servo type applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a pneumatic cylinder designed to convert compressed air into mechanical output. 
         FIG. 2  is a cross-sectional view of an example pneumatic cylinder with absorptive type acoustic snubbers in the end caps. 
         FIG. 3  is a cross-sectional view of an example pneumatic cylinder with dispersion type acoustic snubbers in the end caps. 
         FIG. 4  is an orthogonal view of an example piston insert. 
         FIG. 5  is an orthogonal view of an example piston shell. 
         FIG. 6  is a cross-sectional view of an example piston shell including a piston insert and piston plugs. 
         FIG. 7  is a cross-sectional view of an example piston shell showing the path of airflow during an extension of the piston assembly. 
         FIG. 8  is a cross-sectional view of an example cylinder body showing a servo valve coupled to the piston assembly. 
         FIG. 9  is a side view of an example pneumatic cylinder showing a mounting bracket coupled to the neck of the piston assembly. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A pneumatic cylinder  100  designed to convert compressed air into mechanical output is illustrated in  FIGS. 1–9 . Although a rodless pneumatic actuator is illustrated, any suitable type of pneumatic actuator may be used (e.g., with a rod connected to the piston). Differential pressure across a piston assembly  102  produces a force that can extend (e.g., left on the page) the piston assembly  102 , or cause the piston assembly  102  to retract (e.g., right on the page). The differential pressure is the difference in air pressure between a first working volume  104  and a second working volume  106 . 
     The first working volume  104  is the cylindrical chamber created by the piston assembly  102 , a cylinder bore  108 , and a first end cap  110 . The second working volume  106  is cylindrical chamber created by the piston assembly  102 , the cylinder bore  108 , and a second end cap  112 . The cylinder bore  108  also serves to guide the piston assembly  102 . It should be noted that the air pressure in each working volume  104  and  106  is not necessarily uniform, and that variations over space for any specific point in time are to be expected. In addition, although cylindrical shapes are discussed in the exemplary embodiment herein, it will be readily recognized that any suitable shape(s) may be used. 
     Air pressure in each working volume  104  and  106  can be altered in any suitable manner. For example, the mass of air contained within a working volume  104  and/or  106  can be changed by allowing air to flow into or out of the working volume  104  and/or  106 . During an extension, air flows into the first working volume  104 , thus increasing pressure in the first working volume  104 . Also during an extension, air flows out of the second working volume  106 , thus decreasing pressure in the second working volume  106 . Preferably, a pneumatic control valve  114  is used to control the communication of airflow into and out of the working volumes  104  and  106 . The pneumatic control valve  114  is capable of directing compressed air into one of the working volumes  104  or  106 , and conversely, discharging compressed air out of the other working volume  106  or  104  (e.g., to atmosphere). 
     The piston assembly  102  includes a piston shell  116 , a piston insert  118 , and a pair of piston plugs  120 . Airflow communication between each working volume  104  and  106  and its respective air port  122  and  124  is directed through the piston assembly  102  by way of dual channels formed by the piston shell  116  and the piston insert  118 . The piston insert  118  divides the bore  146  of the piston shell  116  into a first piston chamber  126  exposed to the first working volume  104  and a second piston chamber  128  exposed to the second working volume  106 . 
     The annular area created by the inner diameter and the outer diameter of the piston shell  116  defines a first piston face  130  and a second piston face  132 . Pressure in the first working volume  104  is integrated over the surfaces in the first piston chamber  126  and over the first piston face  130  to create a force that extends the piston assembly  102 . Pressure in the second working volume  106  is integrated over the surfaces in second piston chamber  128  and over the second piston face  132  to create a force that retracts the piston assembly  102 . 
     The piston insert  118  and the piston plugs  120  create a channel  134  nested within the piston shell  116 . The channel  134  allows a flexible steel band  136  to pass between the first working volume  104  and the second working volume  106  while keeping both working volumes  104  and  106  isolated from one another. The flexible steel band  136  seals a slot  138  that runs the length of the cylinder  100 . A neck  140  of the piston shell  116  extends through the slot  138 . The piston shell  116  and piston insert  118  are preferably bonded together by a process such as brazing before being integrated into the pneumatic cylinder  100 . 
     During an extension of the piston assembly  102 , the air control device  114  (e.g., servo valve) directs air from a compressed air source through the first air port  122  into the first piston chamber  126 . The air then moves out through an opening in the piston shell  116  into the first working volume  104 . Conversely, air flows from the second working volume  106  into the second piston chamber  128  and then out through the second air port  124  before being discharged via the air control device  114  to atmosphere. 
     An example of an air control device  114  is shown mechanically coupled to the neck  140  of the piston shell  116  in  FIGS. 8 and 9 . In one embodiment, the air control device  114  is mounted to the neck  140  of the piston shell  116  via a shock absorbing material such as rubber or foam. The air ports  122  and  124  of the piston assembly  102  engage similar air ports featured in the air control device  114  and seal thereupon. A mounting bracket  142  is mechanically coupled to the neck  140  of the piston shell  116  to transmit the force on the piston assembly  102  to an external load. A pressure sensor and/or an accelerometer may be mounted within the air control device  114 . Such a disposition provides for an efficient integration of the sensors with the electronics required to drive the air control device  114  while minimizing delay and distortion. 
     The air control device  114  travels with the piston assembly  102  when a differential pressure is produced on the piston assembly  102 . This arrangement shortens the length a shock wave must travel between each air port  122  and  124  and the corresponding piston faces  130  and  132 . By shortening this length, the time required for a shock wave generated by the air control device  114  to effect a force change on the piston assembly  102  is reduced. In addition, this arrangement keeps the length the shock wave must travel between each air port  122  and  124  and the corresponding piston faces  130  and  132  constant regardless of the position of the piston assembly  102  relative to the cylinder end caps  110  and  112 . 
     To further improve the dynamic relationship between airflow and differential pressure, acoustical snubbers  144  may be incorporated into the first end cap  110  and/or the second end cap  112 . During operation of the pneumatic cylinder  100 , pressure waves may emanate from the piston assembly  102  and travel longitudinally along the length of the cylinder bore  108 . In the case of the first working volume  104 , the shock waves travel between the piston assembly  102  and the first end cap  110 . In the case of the second working volume  106 , the shock waves travel between the piston assembly  102  and the second end cap  112 . The acoustical snubbers  144  reduce the magnitude of the reflected shock wave. The acoustical snubbers  144  may accomplish this task by dispersing the shock wave, deflecting the shock wave, and/or absorbing the shock wave. Similarly, any chamber and/or channel within the pneumatic cylinder  100  may be lined with any suitable material that absorbs noise. 
     While the specification and the corresponding drawings reference preferred examples, it should be appreciated that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present invention as set forth in the following appended claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention, as set forth in the appended claims, without departing from the essential scope thereof. Therefore, it is intended that the present invention not be limited to the particular examples illustrated by the drawings and described in the specification as the best modes presently contemplated for carrying out the present invention, but that the present invention will include any embodiments falling within the description of the appended claims and equivalents thereof.