Abstract:
A dual cylinder pivotal work piece holding device includes a fixture pivotally mounted on a frame. A crank arm drives the fixture about a pivot axis. First and second cylinders have rods pivotally connected to the crank arm at a drive point offset from the fixture pivot axis. The rods form an oblique angle with each other and a controller is provided for extending and retracting the rods in a coordinated cycle so that at least one of the rods is moving at any instant during rotation of the fixture.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority based upon U.S. Provisional Application Serial No. 60/221,951 filed Jul. 31, 2000 by Express Mail (Label # EL515382260US). 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a positioning device, and specifically to a positioning table or fixture used for robotic automated systems. Rotary tables or fixtures carrying work objects are often used in robotic systems. An operator places the work object on one side in one workstation while a robot processes another work object on the opposing side of the fixture. The fixture then rotates in a reciprocating motion to reverse the two parts. 
     In order to accomplish the high-speed station exchange, the positioning device must be able to accelerate the table or fixture quickly and decelerate it quickly. If the fixture is traveling at full speed near the end of the exchange, it will have difficulty stopping and may cause damage to the system and the need for frequent repair or replacement of parts. 
     Other previous designs have used a flow torque control device that includes a rack and pinion driven by a singular air hydraulic cylinder. The pinion in these designs is small and does not produce much torque. 
     Another prior art design involves high-speed tables that use an electric motor. The problem with this type of table, however, is that the motor must be oversized to accommodate the fast acceleration and deceleration required for the table. 
     Additionally some prior art devices, such as that disclosed in U.S. Pat. No. 5,704,601 to Mangelsen, et al., have solved the above stated problems through the use of a singular cylinder assembly including a cylinder, a piston, and a rod carried by the piston. While the system of the &#39;601 patent is remarkable in its solution to the above stated problems, the singular cylinder design required inertia be relied upon in order to effectuate the proper rotation of the position device. The positioning device contained a no power point, or a point at which the cylinder was incapable of exerting force upon the positioning table or fixture. It is therefore desirable to provide a positioning device that is capable of exerting force, and thereby control, over the positioning table or fixture at all points of its rotation. 
     There is therefore a need in the art for a reciprocating high speed positioning device which can quickly accelerate and decelerate without putting stress on the parts of the system. 
     It is therefore a primary objective of the present invention to provide a reciprocating high speed positioning device which can overcome the problems found in the prior art. 
     Another objective of the present invention is to provide a reciprocating high-speed positioning device that is powerful, yet not oversized. 
     A further objective of the present invention is to provide a reciprocating high-speed positioning device, which can accelerate and decelerate quickly without damaging the parts of the system. 
     A still further objective of the present invention is to provide a reciprocating high speed positioning device which is easy to use and economical to manufacture. 
     The means and method of accomplishing these and other objects will become apparent from the following description of the invention. 
     SUMMARY OF THE INVENTION 
     The forgoing objects may be achieved by reciprocating positioning device comprising a frame, a table or fixture, and a bearing assembly rotatably mounting the table or fixture to the frame for rotation about a table or fixture axis. The table or fixture axis can be horizontal, vertical, or at any particular angle that is convenient for the human or robot operator. 
     Dual cylinder assemblies are connected to the frame at an oblique angle with respect to each other. Each cylinder assembly includes a cylinder, a piston, and a rod carried by the piston. Each piston and rod are moveable within the respective cylinder between an extended and retracted position. Each rod is pivotally connected to the table or fixture at a pivot point spaced from the table or fixture axis. The table or fixture is rotatable from a first position wherein the first rod is in its extended position and the second rod in a partially retracted position to a second position wherein the second rod is in its extended position and the first rod in a partially retracted position. 
     The dual cylinder assembly allows for precise rotation of the table or fixture to any position between the first and second positions. The exact position of the table or fixture may be monitored through the use of a sensor, such as a digital encoder. The encoder provides an electrical pulse train that can be used to determine speed and direction of rotation. This allows for precise controlling of the exact table or fixture position and prevents any damage to parts of the system. Of course, it is to be understood that the procedure for rotating the table or fixture easily may be reversed and therefore the table or fixture may be rotated a full 180° in the opposite direction. Therefore, the table or fixture has a complete 180° range of motion. 
     A control means is connected to the dual cylinder assembly and to any sensors used so as to cause the table or fixture to move from its first position to any other desired position. The control means will extend or retract the appropriate rods as required to properly place the table or fixture in correct position. 
     Another feature of the invention includes an optional safety brake, which provides emergency stopping to the system. The emergency brake is automatically activated when the power system is off. The control means that powers the reciprocating turntable or fixture also provides the power to inactivate or activate the safety brake. The brake is mounted between the bearing assembly and the turntable or fixture and applies pressure, which prevents rotation of the reciprocating table or fixture assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the pivotal work piece holding and positioning device of the present invention. 
     FIG. 2 is a top plan view of the device of FIG.  1 . 
     FIG. 3 is partial sectional view taken along line  3 — 3  in FIG. 2, which shows the fixture configuration of the device from another angle with most of the frame components omitted for simplicity and clarity. 
     FIG. 4 is a sectional view taken along line  4 — 4  in FIG.  2  and shows the dual cylinders and crank arm of this invention in a first position. 
     FIG. 5 is a sectional view similar to FIG. 4, but shows the dual cylinders and crank arm of this invention in a second position. 
     FIG. 6 is a view similar to FIGS. 4 and 5 and shows the movement of the crank arm between the first and second positions by the dual cylinders of this invention. 
     FIG. 7 is a schematic diagram of the control means used to operate the dual cylinder work piece positioner of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The work piece holding and positioning device of the present invention is generally shown in the figures. It can be appreciated from these drawings how a control means may operatively select a desired position and then move the positioning device so that the table or fixture will arrive at the desired position. 
     Referring to FIGS. 1 and 2, the positioning device  10  of the present invention includes a frame  20  having one or more supports  22 ,  24  capable of supporting a pivotally mounted positioning table or work piece holding fixture  30  along the rotary axis  46  of the fixture or positioning table. The support  22  is generally referred to as the tailstock, and the support  24  is generally referred to as the headstock. The rotary axis  46  is shown to be horizontal, but could be vertical or at any other angle with respect to a horizontal plane without detracting from the invention. 
     The fixture  30  rotatably supports a pair of work piece mounting drums  12 A,  12 B on which work pieces (not shown) to be processed can be mounted. A pair of spaced tubular arms  14 ,  16  on the fixture  30  support the drums  12 A,  12 B. A servo motor and gearbox drive mechanism  18  mounts to each arm  16  near the headstock  24  to rotate the drums  12 A,  12 B about their respective longitudinal axes. The fixture  30  has a framework that includes a pair of longitudinal tubular members  32 ,  34  connected together by a pair of tubular end members  36 ,  38  so as to define a space  39  therebetween. 
     Referring to FIGS. 4-6, a crank arm  40  secures to a shaft  44  that pivotally mounts the positioning fixture  30  to the frame  20  at one of the end members  38 . Thus, the positioning fixture  30  secures to the shaft  44  so as to rotate therewith when the crank arm  40  is rotated. The central longitudinal axis of the shaft  44  defines the pivot axis  46  of the fixture. The movement of the shaft  44  in FIGS. 4-6 illustrates the movement of the fixture  30 , which is hidden by the inside wall of the headstock  24 . 
     A pair of cylinder assemblies  50 ,  52  includes cylinders  51 ,  53  with one end pivotally connected to the frame  20  and a rod  54 ,  56  that pivotally connects to the crank arm  40  at a drive point  58  to define a pivot axis  59  offset from and parallel to the pivot axis  46 . The cylinder assemblies  50 ,  52  and their rods  54 ,  56  are mounted at an oblique angle with respect to each other. In the embodiment shown, the crank arm  40  and the fixture  30  attached thereto can be rotated 180° around the axis  46 . 
     Precise control of the pivotal movement of the fixture  30  is achieved through the use of a speed and position sensor  100 . The sensor  100  is mounted on the frame  20  and directed toward the crank arm  40 . Although other types of sensors will suffice, the sensor  100  is preferably a digital encoder with a rotatable wheel  102  that contacts an arcuate surface  60  on the crank arm  40  to generate an electrical signal indicative of the speed and position of the fixture  30 . The arcuate surface  60  is disposed a constant radial distance from and parallel to the fixture pivot axis  46 . For ease of mounting the sensor  100 , the preferred location of the arcuate surface  60  is offset from the pivot axis  46  in a direction opposite from the drive point  58 . 
     As shown in the schematic diagram of FIG. 7, the sensor  100  is electrically connected to a control means or assembly  104 , which controls the cylinder assemblies  50 ,  52  based upon speed and position signals from the sensor  100 . The control assembly  104  includes a programmable computer or microprocessor  72  that is also connected to an index button  73  and a cycle start button  75 . Index button  73  signals the computer  72  to move the fixture  30  a single time by a fixed amount, such as  180  degrees. Cycle button  75  signals the computer  72  to initiate an automatic reciprocation of the fixture  30  each time the robotic equipment completes its work on the work piece. The connection of the computer  72  to the robotic equipment is not shown, but is well known. 
     The control assembly is preferably an electronically controlled pneumatic power means. The computer  72  electrically connects to the solenoids  82  of a pair of three-position four-port valves  74 A,  74 B so as to independently control the respective cylinder assemblies  50 ,  52  through air lines  81  and quick exhaust valves (QEV)  88 . An air supply source  86  provides air to the supply port of the valves  74 A,  74 B through an air line  81 , a filter  84  (F), a regulator  85  (R), and a lubricator  87  (L). A vent  119 , a two-position brake valve  114 , and a brake  120  are also provided. The brake  120  can take a variety of forms, including but not limited to an air brake with pads  112  that grip a disc  122  that is secure to the fixture  30  for rotation therewith. 
     FIG. 4 illustrates how, in operation, the fixture starts in a first position. Actuation of either button  73  or button  75  causes the computer  72  to activate the appropriate solenoid  82  on the valve  74 B so as to move the valve  74 B into its third position  80 . In response, the rods  54 ,  56  of both cylinders  50 ,  52  retract. Rod  56  is powered and valve  74 A is vented in its second position  78  so that rod  54  merely follows and retracts, too. The fixture  30  attached to the crank arm  40  swings downwardly in a counterclockwise direction, as shown in FIG.  6 . 
     The wheel  102  of the sensor  100  rotates as the crank arm  40  swings and sends a signal in the form of a pulse train of signals over time to the computer  72 , which indicates the position and speed of the fixture  30 . Based upon the signal from the sensor  100 , the computer  72  initiates phases of the cylinder power cycle at the appropriate positions or times. During this second phase, which is initiated before the second rod  56  is fully retracted, the computer  72  activates the appropriate solenoids  82  so that the valve  74 A moves to its first position  76  wherein the rod  54  is powered to extend. The computer  72  commands the valve  74 B into its vented second position  78  wherein the rod  56  is carried by the rod  54  through further retraction and then extension. 
     One skilled in the art will appreciate that the command signals from the computer  72  to the valves  74 A and  74 B are preferably overlapped in time. This prevents the occurrence of a null point in the cycle where neither of the cylinders  50 ,  52  are powered. During the normal null point for the individual cylinders  50  or  52 , the other cylinder  52  or  50  is always powered. Thus, the overall effect is to eliminate the null point or no power point, thereby providing more direct and reliable control of fixture movement. The computer  72  could also be programmed to power both cylinders  50 ,  52  for concurrent retraction or extension. 
     As the crank arm  40  approaches the second position, which is shown in the example of FIG. 5 to be 180 degrees from the first position, the computer signals the brake valve  114  to move from the second position  118  to the first position  116 . This activates the brake  120  to hold the crank arm  40  in place. The computer  72  signals the valves  74 A,  74 B to the second (vented) position  78 . The brake  120  is released and the cylinder power cycle repeated in reverse to return the fixture  30  to the first position if desired, of course, the brake  120  can also be used as an emergency brake to stop the fixture at any time. 
     Thus, the present invention at least satisfies its stated objectives. 
     The preferred embodiment of the present invention has been set forth in the drawings and specification, and although specific terms are employed, these are used in a generic or descriptive sense only and are not used for purposes of limitation. Changes in the form and proportion of parts as well as in the substitution of equivalents are contemplated as circumstances may suggest or render expedient without departing from the spirit and scope of the invention as defined in the following claims.