US 3804397 A
A pneumatically powered fixture for engaging and holding workpieces while inspection operations are performed on them includes a vacuum cup supported on the end of a cylinder rod. An air passage through the rod connects to the cylinder area behind the piston. When air is evacuated from a pair of passages through the cylinder, disposed on opposite sides of the piston, the cylinder rod moves outwardly since the rear chamber is connected to atmosphere through the open vacuum cup. When the vacuum cup engages a workpiece and locks against it, the area to the rear of the piston is no longer vented to atmosphere and the pressure on both sides of the piston equalize, terminating motion of the rod. Brake means operating on the rear end of the piston rod then lock the rod against motion.
Description (OCR text may contain errors)
United States Patent 11 1 1111 3,804,397 Neumann Apr.'l6, 1974 AUTOMATIC POSITIONING VACUUM CUP Primary Examiner-Harold D. Whitehead  Inventor: Don B. Neumann, Ann Arbor, Mich. Attorney Agent or Krass Young &
. Gerhardt  Assignee: GCO, Inc., Ann Arbor, M1ch.  Filed: Aug.'19, 1971  ABSTRACT  Appl. No.: 173,273 A pneumatically powered fixture for engaging and holding workpieces while inspection operations are performed on them includes a vacuum cup supported 152 US. (:1. 269/21, 294/64 R on the end of a cylinder An air Passage thmugh 51 1111.131 B25b 11/00 the rod connects to the cylinder area behind the P ; Field 61 Search 269/21; 248/363; 214/1 13s, when airfis evachated from a P i Passages 214 BT; 271/26 294/64 R through the cylinder, dlsposed on opposite sides of the piston, the cylinder rod moves outwardly since the rear chamber is connected to atmosphere through the References Cited open vacuum cup. When the vacuum cup engages a 1 UNITED STATES PATENTS workpiece and locks against it, the area to the rear of 1 715 35s 6/1929 Harrold 271 26 R the Piswn is no kmger vented m atmosphere and the 2247.787 7/1941 Schmidt pressure on both sides of the piston equalize, terminat- 2,34|,521 2/|944 Baker et alw ing motion of the rod. Brake means operating on the 3,311,400 3/1967 Rowekamp rear end of the piston rod then lock the rod against 3,460,822 8/1969 Link 269/21 motion.
9 Claims, 3 Drawing Figures PATENTEMPR 16 m4 3304397 I N VEN TOR.
B 90 5. JVeamcmn AUTOMATIC POSITIONING VACUUM CUP BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to vacuum holding systems for supporting workpieces with respect to operating machines, and more particularly to such systems which automatically engage and support workpieces despite variations in the workpiece configuration and the position of the workpiece with respect to the machine.
2. Description of the Prior Art In many situations encountered in manufacturing and inspection of parts it is desired to retain a workpiece fixed relative to a base member while some operations are performed on the workpiece such as material removal or inspection. In such situations it is common to grip the workpiece with a vacuum holding device which includes a cup having a flexible rim adapted to be urged against the workpiece surface. The volume between the concave surface of the cup and the abutting workpiece surface is then evacuated, either by the tendency of the resilient cup to pull away from the surface or by external vacuum means so as to firmly couple the cup to the surface. Such vacuum holding devices are often supported by actuators so that the workpiece may be moved to a predetermined position with respect to a reference surface after it has been gripped. In a class of such devices the actuator takes the form of a pneumatic cylinder through which a piston is moved by the same pneumatic source that evacuates the vacuum cup. The cup and cylinder are interconnected so as to ensure that the positioning force exerted on the vacuum cup does not cause it to separate from the workpiece. A system of this type is disclosed in US. Pat. No. 3.460.822.
In a related variety of situations it is not necessary to position the part with respect to a reference surface but the part must be rigidly supported with respect to a machine base or surface. For example, if the dimensions of the workpiece surface are to be gauged with a high degree of accuracy it is necessary to ensure that the workpiece is rigidly mounted with respect to a reference surface on the base. In inspection devices which gauge workpiece surfaces to millions of an inch, such as interferometric, or holographic interferometric gauging devices, any minute displacement of the workpiece relative to the reference surface as may be caused by vibrations would destroy the accuracy of the device. It is desirable in such systems that means be provided for engaging the workpiece and rigidly retaining it with respect to the machine base.
In certain such systems it is undesirable, or difficult, toposition the workpiece with respect to the machine base in addition to rigidly retaining it. For example, when the workpiece constitutes a large, heavy casting, it may be extremely difficult to moveit relative to the inspection stand utilizing pneumatic devices of the type previously described. It is. however, still desirable to provide some device which will automatically engage the workpiece in order to rigidly support it.
SUMMARY OF THE INVENTION The present invention is therefore addressed to a vacuum holding device for engaging and retaining workpieces with respect to a machine base and more particularly tosuch devices which will automatically move the vacuum cups into engaging contact with the workpiece as long as the surface to be engaged is disposed within the near vicinity of the vacuum holders.
In a preferred embodiment of the invention, which will subsequently be described in detail, the automatic workpiece engaging device is use-d in connection with an inspection machine. In that embodiment the work engaging device takes the form of a vacuum cup supported on the end of a piston rod. The piston is adapted to move within a cylinder which is fixed on the machine base. The piston may be energized so as to move the vacuum cup outwardly in the direction of a workpiece, or to retract it from a workpiece.
The actuating cylinder includes fluid passages at its forward and rear ends which communicate with the cylinder volumes in front of and behind the piston. The volume behind the piston also connects with a central passage formed through the piston rod and terminating on the interior side of the center of the vacuum cup. This passage serves two purposes: first, it allows the volume on the interior side of the work engaging cup to be evacuated; second, it acts to connect the cylinder volume behind the piston to atmosphere until such time as a workpiece surface is firmly engaged by the vacuum cup. I
In order to bring the vacuum cup into contact with the workpiece a pneumatic power source is connected to both the forward and rear passages on the cylinder so as to draw air out from both of them. A flow control valve is preferably disposed in the circuit of the rear passage so that the flow through that passage may be adjusted relative to the flow through the forward passage. Since the cylinder volume to the rear of the piston is connected to the atmosphere through the passage in the piston rod and the opening in the vacuum cup, the pressure in the volume to the rear of the piston remains substantially that of atmosphere while the volume of the cylinder forward of the piston is evacuated. This causes the piston to move in a forward direction extending the vacuum cup toward the workpiece. This motion continues until the vacuum cup firmly contacts the workpiece. At this point the vacuum drawn through the rod causes the cup to firmly lock onto the workpiece and the pressure in the cylinder volume to the rear of the piston is then reduced to the same pressure that exists forward of the piston. This equalization of pressure causes the piston motion to terminate. A pres-.
sure sensitive switch in the air line to the rear cylinder passage is then actuated providing fluid pressure to a brake which surrounds a rearward. extension of the piston rod. This locks the rod in place to firmly couple the workpiece to the machine base.
In the preferred embodiment of the invention the vacuum cup carries a workpiece contacting plate which is attached to the piston rod by a universal joint. The air passage communicating with the cup includes the volume around the joint so that when a vacuum is drawn through the line the universal joint locks into place, fixing the plate to the rod at an angle which accommodates to the position of the workpiece surface being engaged.
Other objectives, advantages and applications of the invention will be made apparent by reference to the following detailed description of the'preferred embodiment of the invention. The description makes reference to the accompanying drawings in which:
FIG. 1 is a perspective view of an inspection machine incorporating a pair of workpiece engaging and positioning devices formed in accordance with the present invention;
FIG. 2 is a sectional view through one of the workpiece engaging devices of FIG. 1 with associated pneumatic power equipment illustrated in schematic form; and
FIG. 3 is a detailed sectional view through a vacuum cup of the positioning device of FIG. 1 as it engages a workpiece surface at an angle.
Workpiece engaging and supporting devices formed in accordance with the present invention may be used in a wide variety of situations but are preferably employed in connection with inspection machines to rigidly retain a part to be inspected with respect to the machine. One type of inspection machine which particularly requires a high degree of stability of the workpiece with respect to the inspection apparatus, because of its high degree of measurement precision, is a holographic interferometric analysis unit. The preferred embodiment of the invention takes the form of an inspection stand for performing such holographic interferometric analysis and FIG. 1 is a perspective view of a system utilizing a pair of positioning devices formed in accordance with the present invention in connection with such interferometric analysis.
Referring to FIG. 1, a workpiece to be inspected, generally indicated at 10, is supported on a base plate 12 along with optical apparatus, generally indicated at 14, of the type employed to perform holographic interferometric analysis, and a pair of positioning devices, generally indicated at 16 and 18, which are formed in accordance with the present invention. The workpiece may take the form ofa relatively large structural element suchas a weldment or casting, a surface of which is to be analyzed by the optical apparatus 14. The positioning devices 16 and 18 are fixed with respect to the base 12 and function to automatically engage the workpiece 10 so as to rigidly retain it relative to the base during the inspection operation.
The optical apparatus 14 may be of the type disclosed in U.S. Pat. No. 3,506,327 and employs a laser 20 which acts as a source of a coherent light beam 22. The beam is split into two parts by a half-silvered mirror 24 and one of the beam sections is reflected by a mirror 26, to a spatial filter 28 which causes the beam to diverge so as to illuminate a section on one surface of the workpiece 10. A photographic plate 30 is positioned so as to receive light reflected from the illuminated panel section. The photographic plate also receives a beam of reference light which is derived from the half-silvered mirror 24 and passes through a spatial filter 32 before it is reflected to the photographic plate 30 by a mirror 34. This apparatus may be used in the manner set forth in U.S. Pat. No. 3,506,327. It should be recognized that other forms of holographic interferometric inspection apparatus as well as totally different inspection systems might be used in connection with the present invention.
The positioning devices 16 and 18 are disposed on the base 12 so as to engage surfaces of the workpiece 10 which are not then being inspected by the optical apparatus 14. The units 16 and 18 are identical except for their position on the base 12 and the drawings of FIG. 2 and 3 refer equally well to both of the units.
The units 16 and 18 are rigidly retained on the base 12 by supports 38. Referring to FIG. 2 the positioning unit 16 includes a cylinder 40 formed of a length of circular tubing. The forward end of the cylinder 40 at the left in FIGS. 1 and 2, is closed off by a cylindrical front cap 42 having a smaller diameter extension 44 which fits within the front end of the cylinder 40 so that the end of the tube abuts the shoulder formed between the sections 42 and 44. Similarly, the rear end of the cylinder 40 is closed off by a rear cap member 46.
A piston rod 48 formed of steel tubing is journalled in a central hole 50 formed through the front cap 42 and a central hole 54 formed in the rear cap 46. O-ring seals 56 and 58 are disposed in the front and rear caps 42 and 46 respectively and act to provide a fluid seal for the interior cylinder area.
An annular piston 60 is affixed to the rod 48 and is disposed within the cylinder 40 between the end caps 42 and 46. The piston 60 is retained with respect to the rod by a pair of snap rings 62 and 64 which flt into grooves formed in the rod adjacent to the forward and rear edges of the piston. The outer cylindrical edge of the piston 60 is grooved so as to receive a pair of o-ring type piston rings 66 and 68 which bear against the internal surface of the cylinder 40 so as to form a fluid seal between the volume within the cylinder forward of the piston and the volume within the cylinder to the rear of the piston.
A hole 72 is formed in the piston rod 48 behind the piston 60 so as to communicate the cylinder volume to the rear of the piston with the central passage in the piston rod 48.
A cylindrical brake housing 76 is affixed to the rear end of the rear cap 46. The housing 76 has a central chamber 78 with a diameter slightly larger than that of the piston rod 48. The chamber 78 is disposed coaxially with the chamber 40 so as to form a rearward extension thereof and is retained on the rear cap 46 by bolts 80. The housing 76 has an opening 82 at its rear end through which the rearward extension of the piston rod 48 passes. An O-ring seal 84 is affixed within the housing and wipes the rod so as to effectively form a fluid seal between the opening 82 and the volume within the housing chamber 78.
A cylindrical sleeve formed of a plastic material such as Mylar is affixed within the chamber 78 and surrounds substantially the entire length of the piston rod 48 which is disposed within the housing 76. The sleeve 90 is formed with an annular cap 92 at one end which is supported with its forward edge abutting the rear edge of the rear cap 46. A tubular volume 94 is thus formed between the outer wall of the chamber 78 and the outer diameter of the sleeve 90 to the rear of the cap 92. A fluid passage 96 in the wall of the housing 76 communicates with the volume 94. When the volume 94 is pressurized through the passage 96, as will subsequently be discussed, the sleeve 90 tightens about the piston rod 48 and thereby brakes the rod against motion relative to the cylinder 40.
The rear end of the piston rod 48 projects out through the opening 82 in the rear end of the housing 76. The extreme end of the rod is closed by a plug 100.
The forward end of the piston rod 48 projects through the hole 50 in the forward cap 42 and a vacuum cup 102 is supported on the extreme forward end of the rod. The vacuum cup 102 constitutes a dish of flexible rubber having a truncated edge 104 which bounds the interior Concave surface. The cup is fixed, at its center, to a retaining plate 105 which is in turn fixed to the extreme forward end of the rod 48. A pair of air passages 106 connect the central passage and the rod 48 with the interior of the cup.
A ball socket member 110 is fixed interiorly of the cup 102 on the retaining plate 105. A support plate 112 having a flat outer surface is affixed to a ball member 114 which moves within the socket so that the plate 112 may be angularly positioned with respect to the cup.
The pneumatic power for the units 16 and 18 is derived from a power unit schematically illustrated at 116 which is capable of pumping air in both directions. This unit connects to a forward port 118 which passes through the forward cup 42 and communicates with the volume within the cylinder 40 forward of the piston 60. The unit 116 is connected to the port 118 through an air line 120. Similarly, a rear port 122 formed in the rear cap 46 provides communication with the volume within the cylinder 40 to the rear of the piston 60. The pneumatic power unit connects to the rear port 122 through a line 124. A flow adjustment valve 126 and a pressure sensitive switch .128 are both disposed in the line 24. The pressure sensitive switch 128 controls a three way solenoid valve 130 disposed in a fluid line 132 which connects the pneumatic power source 116 with the fluid port 96 in the brake housing 76.
In operation, first consider the unit to be positioned with the piston 60 toward its rear position. With the solenoid valve 130 in a position wherein the volume 94 is vented to atmosphere, vacuum is first applied to lines 120 and 124 to'draw air through the ports 118 and 122, respectively. This tends to evacuate the volume within the cylinder both forward and to the rear of the piston Since the cylinder volume to the rear of the piston 60 is in communication with the atmosphere through the passage 72, the interior of the piston rod 48, and the passage 106, the pressure in this volume is not reduced substantially below atmosphere. The restrictor valve 126 allows the volume forward of the piston to be substantially fully evacuated despite the venting ofline 120 from. the volume to the rear piston. Accordingly, a pressure differential is created between the two sides of the piston, with the higher pressure to the rear of the piston, and the piston begins to move in the forward direction. This motion continues until the vacuum cup 102 comes in contact with the surface of the workpiece 10.
The resilient material of the vacuum cup deforms to conform to the contour of the contacted surface until edge 104 is in full abutment with the surface of the workpiece so as to close off the interior of the vacuum cup 102 from the atmosphere.
Then air isevacuated from the interior of the cup through the passage 106, the piston rod 48, the passage 72, and the fluid port 122. This reduces the pressure within the vacuum cup so as to cause the cup edges to firmly engage the workpiece, and it additionally reduces the pressure in the cylinder volume to the rear of the piston. This tends to decrease the pressure differential between the two sides of the piston and slows the forward movement of the piston rod. The motion of the piston rod continues until the plate 112 abuts the adjacent surface of the workpiece and inclines to the angle force the plates 112 against the adjacent surface of the workpiece 10, thereby locking the ball 114 against the socket 110. This adds to the stability of the system.
The pressure sensitive switch 128 in the line 124 senses this reduced pressure and energizes the solenoid valve 130. This connects the line 132 to supply a positive fluid pressure to the volume 94 within the housing 76, through the passage 96. This pressure forces the sleeve firmly against the adjacent section of the piston rod 48 increasing the friction between the two to the point where motion of the piston rod is effectively prevented.
At this point the workpiece 10 is firmly engaged and retained with respect to the base 12. At such time as both of the locators 16 and 18 have coupled to the workpiece, the unit is ready for the performance of holographic inspection. After the inspection process is completed, the workpiece is released by shifting the valve 130 so as to vent the volume 94 to atmosphere and applying positive pressure to lines and 124 to return the cup to its retracted position.
Having thus described my invention, I claim:
1. A device for engaging and supporting a workpiece relative to a base, comprising: a workpiece engaging member having a flexible, closed, work engaging lip formed about a concave cup; a fluid passage formed through the cup adapted to communicate with the volume between the cup and a workpiece when the lip engages the workpiece; an actuator member fixed with respect to said base and connected to said workpiece engaging member so as to move the workpiece engaging member toward the workpiece; fluid pressure means connected to said passage and adapted to evacuate the area interior of said concave cup; and means for terminating motion of said actuator toward the workpiece at such time as the pressure within said cup decreases to a predetermined level indicative of firm engagement between the cup and the workpiece.
2. The device of claim 1 wherein said means for terminating motion of said actuator at such time as the pressure within said cup decreases to a predetermined level includes a fluid passage connecting the interior of said concave cup with said actuator so as to vent said actuator member until such time as said cup is engaging a workpiece.
3. The device of claim 2 wherein the actuator member consists of a piston movable within a cylinder, and said device further includes a connection between said fluid pressure means and said cylinder.
4. The device of claim 1 wherein said actuator consists of a piston movable with respect to a cylinder, said device further including fluid passages between said fluid pressure means and said cylinder, and said means for terminating motion of said actuator at such time as the pressure within said cup decreases to a predetermined level constitutes a passage venting the cylinder, on one side of the piston, through the concave cup, so that the vent is terminated at such time as the concave cup engages a workpiece so that the pressure within said cup decreases to said predetermined level.
5. The device of claim 4 where in said fluid pressure means connects to said cylinder at two points, one disposed on each side of said piston.
6. A device for engaging and supporting workpieces with respect to a base, comprising: a cylinder fixed with respect to the base; a piston movable within said cylinder; a rod connected to said piston and passing through a hole in said cylinder; a vacuum cup supported on said rod externally of said cylinder; brake means associated with said piston rod; fluid pressure means operative to cause said piston to move with respect to said cylinder; and means for causing the fluid pressure means to terminate motion of the piston with respect to said cylinder and to energize said brake means at such time as said vacuum cup engages a workpiece.'
7. The device of claim 6 wherein said rod includes sections disposed on opposite sides of the piston, both of which pass through holes in the cylinder, and said brake means is disposed about the end of said rod opposite to the end on which said vacuum cup is supported.
8. The device of claim 6 wherein said brake means consists of a flexible cylindrical section surrounding a portion of said rod and means for imposing fluid pressure on said section.
9. The device of claim 6 wherein a passage is formed through said rod communicating at one end to said vacuum cup and at the other end with said cylinder and said passage acts to vent a portion of the volume of said cylinder to atmospheric pressure until such time as the vacuum cup engages a workpiece.