US3290999A

US3290999A - Pneumatic servo especially useful in extensometers

Info

Publication number: US3290999A
Application number: US404683A
Authority: US
Inventors: James M Denker
Original assignee: Instron Corp
Current assignee: Instron Corp
Priority date: 1964-10-19
Filing date: 1964-10-19
Publication date: 1966-12-13
Anticipated expiration: 1983-12-13
Also published as: GB1078147A

Description

J. M. DENKER Dec. 13, 1966 2 Sheets-Sheet 1 Filed Oct. 19, 1964 T I I 1 l 1 Dec. 13, 1966 J. M. DENKER 3,290,999

PNEUMATIC SERVO ESPECIALLY USEFUL IN EXTENSOMETERS Filed Oct. 19, 1964 2 Sheets-Sheet 2 United States Patent chusetts Filed Oct. 19, 1964, Ser. No. 404,683 2 Claims. (Cl. 91-47) This invention relates to servos, and more particularly to such devices especially useful in extensometers.

It is the object of this invention to provide a pneumatic servo capable of moving an element thereof in step with a specimen clamp gripping transversely a specimen to be tested by longitudinal extension, and causing this movement not only with a considerable force amplification so that almost no longitudinal force is exerted on the specimen by the clamp, but at the same time with a very fast response and freedom from oscillation or hunting, and with expenditures of very small quantities of fluid, all without any need to maintain the source of pressure at any very constant level.

Generally speaking, the invention features a piston mounted for longitudinal reciprocal movement in a housing, the piston having each of its two ends exposed to gas from a common source, the gas exerting pressure against different areas at the different piston ends and by virtue of a restriction between the source and the piston end of greater effective area being at a lower pressure thereagainst. In preferred embodiments said restriction is made adjustable for response control, and an adjustable valve for dumping control is placed between the source of pressure and the other end of the piston. In the most preferred embodiments, the larger piston end elfective area is divided into two portions, one of which is separated from the other by a restriction causing a lag in the latters reaching the pressure over the former, to give especially effective stability.

Other objects, advantages, and features will appear from the following description of the presently preferred embodiment of the invention, taken with the attached drawings, in which:

FIG. 1 is a vertical sectional view, partially broken away, of said embodiment;

FIG. 2 is a side elevation of an extensometer incorporating two of said embodiments; and

FIG. 3 is a plan view of said extensometer.

Referring now to the drawings, there is shown in FIG. 1 a cylinder indicated generally at (fabricated of elements, 12, 14, and :16) in which is seated for reciprocal movement piston 18. Mating cylinder and piston surfaces are finished :to any accuracy such that no gaskets are required, and at the same time leakage therebetween is practically insignificant, by well-known techniques.

Housing caps

20 and 22 complete the housing.

Secured on the lower (as shown in FIG. 1) end of piston 18 is valve block 24 in which is slide valve 26, downwardly biased by spring 28, and with an adjustable downward stop screw 30.

Secured beneath valve block 24 is extensometer clamp arm assembly 3 2, which is pivotally mounted relative to valve block 24 through a flexible leaf spring clamped between elements 34 and 36, and between element 38 and base 40. Block 42 secured to base 40 includes blind hole 44 cooperating with the head of screw 46 to limit travel of the clamp arm assembly 32 about its axis of flexure. Teflon element 48 carrying spring 50 yieldably urges specimen clamp base 52 against the mating seat of element 48.

Mounted on valve block 24 is LVDT (linear variable differential transformer) 56, leads to which extend up ice through (tgastightly) the servo. The core 58 of LVDT 56 engages screw 60 secured in bushing 6.2 mounted on the valve block of another servo of the embodiment, the latter being however upside down relative to the servo thus far described, as indicated in partial dotted outline in FIG. 1, and shown in FIG. 2.

FIGS. 2 and 3 show my preferred embodiment of extensometer incorporating my above-described servos. Each servo has a pair of

valve bodies

64 and 66 which are seated respectively in

bronze bushings

68 and 70 for rotation about a single vertical axis. The

bushings

68 and 70 are carried in servo supports 72 which extend from mounting base '74, which carries a pair of clamps 76 for mounting the entire extensometer on a testing machine. The specimen clamps 78 and '80 are also best shown in FIGS. 2 and 3.

Other elements of the preferred embodiment may most effectively be mentioned in the following description of operation of each servo and the extensometer.

Referring to FIGS. 1 and 2, air under pressure enters through conduit 100, dividing then :to pass through

conduits

102 and 104 tothe upper and lower supports 72. Within upper support 72, the stream divides again to flow upwards and downwards in conduit 106.

The air moving upwards through conduit 106 is adjustably valved by valve 108 into conduit 110, through which it is in communication with pressure zone 112, which overlies half the effective upper end area of piston 18. The other half of the effective (to gas pressure, in terms of net downward longitudinal forces) pressure on the upper end of the piston 18 is exerted through cond-uit 114, adjustably valved by valve *116, in zone 117.

Air moving downwardly through conduit 106 passes through adjustable valve 118 and conduit 120 into pressure zone 122, which has the same effective area as pressure zone 112.

The servo functions by maintaining the gap between spool valve 26 and exhaust hole 124 at such an amount as to keep the forces upward and downward on the piston in balance, and since the piston upper end area is greater, the pressure in zones 112 and 1 17 must at equilibrium be lower than that in zone 122. Thus, when specimen clamp 78 (or moves for example downwardly, the spool valve 26 drops, decreasing its gap with the exhaust hole 124. This increases the pressure above the piston, forcing the piston to follow the specimen clamp down until (when it has tracked it down the identical amount) equilibrium is again re-establis-hed. The servo operates on the same balancing way in the reverse direction, of course.

By using a greater effective area on one end of the piston, I make possible operation with only a single exhaust gap, thus conserving gas. Furthermore, in conjunction with this approach, I have divided the upper effective area between two pressure zones, with a restriction therebetween; while they come to the same pressure, this restriction causes a lag which acts as a sort of cushion, both increasing smoothness of operation and very effectively combating oscillation or hunting. Stability is further aided by valve 118, which permits adjustably tuning desirable damping here. Adjustable valve 108 permits varying the speed of response. I have provided thus a servo that will with great accuracy track specimen clamp movement at 30 cycles per second or greater, at the same time amplifying a force there in fractions of grams to one at the cylinder in hundreds of pounds.

Furthermore, since each piston is rotatable in its piston about a vertical axis, while each cylinder is in turn rotatable about a different (but common to the cylinders) vertical axis, not only is mounting a specimen between the specimen clamps facilitated, but even more importantly undesired components of force on the specimen are avoided Specimen extension is, of course, the differential movement of

specimen clamps

78 and 80, which in the manner indicated produces an electrical signal through the LVDT.

I claim:

1. A pneumatic servo comprising a piston with a pair of oppositely directed ends and a housing supporting said piston for reciprocal movement, defining with said ends at least one pressure zone at each of said ends, and including conduits from a common conduit separately to each of at least one pressure zone .at each of said ends, one of the separate conduits including a response restriction, and the total effective area of the piston end at least part of which is at the zone with which the last-mentioned conduit communicates being greater than the total effective area of the other piston end, the piston end of larger eifective area being divided into two pressure zones in communication with each other through a stability restriction.

2. The servo of claim 1 in which said stability restriction is adjustable.

References Cited by the Examiner UNITED STATES PATENTS 1,512,804 1 0/1924 Roucka 91476 3,048,151 8/1962 Kudlaty 9l--4l7 FOREIGN PATENTS 1,169,890 1/1959 France. 1,191,239 10/1959 France.

15 -MARTIN P. SCHWADRON, Primary Examiner.

EDGAR W. GEOGHEGAN, Examiner.

P. T. COBRIN, Assistant Examiner.

Claims

1. A PNEUMATIC SERVO COMPRISING A PISTON WITH A PAIR OF OPPOSITELY DIRECTED ENDS AND A HOUSING SUPPORTING SAID PISTON FOR RECIPROCAL MOVEMENT, DEFINING WITH SAID ENDS AT LEAST ONE PRESSURE ZONE AT EACH OF SAID ENDS, AND INCLUDING CONDUITS FROM A COMMON CONDUIT SEPARATELY TO EACH OF AT LEAST ONE PRESSURE ZONE AT EACH OF SAID ENDS, ONE OF THE SEPARATE CONDUITS INCLUDING A RESPONSIVE RESTRICTION, AND THE TOTAL EFFECTIVE AREA OF THE PISTON END AT LEAST PART OF WHICH IS AT THE ZONE WITH WHICH THE LAST-MENTIONED CONDUIT COMMUNICATES BEING GREATER THAN THE TOTAL EFFECTIVE AREA OF THE OTHER PISTON END, THE PISTON END OF LARGER EFFECTIVE AREA BEING DIVIDED INTOW TWO PRESSURE ZONES IN COMMUNICATION WITH EACH OTHER THROUGH A STABILITY RESTRICTION.