US3433061A - Movable crosshead loading device - Google Patents

Description

March 18, 1969 G. s. BURR 3,433,061

MOVABLE CROSSHEAD LOADING DEVICE Filed June 2'7, 1966 AMPLIFIER i2 70 ea COMMAND SIGNAL COMPARATOR SOURCE AMPLIFIER I v AMPLIFIER AMPLIDYNE l United States Patent 3 Claims ABSTRACT OF THE DISCLOSURE Materials testing machine having a hydraulic ram between its movable crosshead and one of two grips, a control system for actuating the ram to adjust a condition of the specimen, and a control system to adjust the crosshead including a sensor for directly sensing the relative positions of the ram and the crosshead.

This invention relates to control systems for materials testing machines having movable crossheads.

It is a primary object of this invention to provide, in such a machine having its crosshead coupled to the movable grip through an hydraulic ram, for a highly accurate and reliable, yet simple system for coordinating ram and crosshead position.

The invention features, in a materials testing machine, a movable crosshead, a pair of grips for holding a specimen to be tested, a drive mechanism for moving the crosshead and a hydraulic ram. The hydraulic ram has a first portion mounted for movement with the crosshead and a second portion coupled to one of the grips and hydraulically coupled to the first portion for movement relative thereto parallel to the crosshead. The ram has a stroke substantially shorter than the overall length of travel of the crosshead. A control system actuates the ram in response to a deviation between the actual and desired position of the grip coupled to the second portion of the ram A, and controls the drive mechanism and ram to restore, after actuation, the relative positioning of the crosshead and second portion of the ram. The control system includes a sensor mounted to directly sense the relative positioning of the crosshead and second portion of the ram.

Other objects, features, and advantages will appear from the following description of a preferred embodiment of the invention, taken together with the attached drawing thereof which is a diagram of a materials testing machine with a hydraulic ram and the control system of this invention.

There is shown in the drawing a portion of a materials testing machine 10 having a frame 12; crosshead 14 driven by motor 26 through screws 16 and 18 and gears 20, 22, and 24; and specimen grips 28 and 3t Mounted on crosshead 14 is a hydraulic ram including a hydraulic cylinder 32 and piston 34. Connecting rod 36, in fluid-tight, slidable engagement with aperture 38, conects grip 30 to piston 34. Rod 40, in fluid-tight, slidable engagement with aperture 42, connects to piston 34 and extends through sleeve 44 in crosshead 14.

Chamber 46 has compartments 48 and 59 on opposite sides of piston 34. Passages 52 and 54 connect compartments 48 and 50, respectively, to electrically controlled valve 56 which selectively applies a source of hydraulic pressure to the hydraulic fluid 58 in said compartments. Tube 60 connects valve 56 to a source of hydraulic pressure (not shown).

A load cell 62 senses the load being applied to the specimen 64 by the machine 10 and produces a negative voltage which is introduced through amplifier 66 to an input of comparator 68. Command signal source 70,

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which may be a manually operated potentiometer circuit, provides a positive voltage indicative of the desired load to be applied to comparator 68. The output of comparator 68 is delivered through amplifier 72 to valve 56.

Fastened to crosshead 14 is a linear variable differential transformer (LVDT) 74 which senses the axial move ment of rod 40 in relation to crosshead 14. The output of LVDT 74 is a positive voltage when piston 34 is displaced from its centered position in cylinder 32 toward crosshead 14; a negative voltage results when piston 34 moves from its centered position towards grip 30. In either case the output of LVDT 74 controls, through amplifier 76, the output of amplidyne 78, which in turn controls motor 26. A positive output voltage from ILVDT 74 causes motor 26 to move crosshead 14 downwardly; a negative voltage to move it upwardly.

In operation, with the machine in a stable state, i.e., with zero output from comparator 68, a call for an increased load by manipulation of command signal source 70 instantaneously causes a positive voltage to appear at the output of comparator 68. After being amplified in amplifier 72, this positive voltage operates valve 56 connecting channel 52 to the source of hydraulic pressure through tube 60, thereby increasing the pressure in compartment 48 and driving piston 34 downwardly.

The downward movement of piston 34 is sensed by LVDT 74 and causes it to transmit a positive signal representative of the displacement of piston 34 to amplifier 76 and then to amplidyne 78 where it controls the polarity and magnitude of the power to motor 26. Since the voltage from LVDT 74 is now positive, motor 26 is driven to move crosshead 14 downwardly until piston 34 is once again centered in cylinder 32.

In a similar manner, a call for a decreased load causes upward movement of piston 34 and corresponding coordination of the crosshead position.

The subject matter discloses above and relating to the use of a hydraulic ram with a crosshead in a materials testing machine is the joint invention of John Linnartz Preston and George Sterling Burr and is the subject of a separate patent application filed the same day as this application.

Other embodiments (e.g. in position control operation) will occur to those skilled in the art and are within the following claims.

I claim:

1. A materials testing machine comprising:

a movable crosshead,

a pair of grips for holding a specimen to be tested,

a drive mechanism for moving said crosshead,

an hydraulic ram having a first portion mounted for movement with said crosshead and a second portion coupled to one of said grips and hydraulically coupled to said first portion for movement relative thereto parallel to said crosshead,

a support for the other of said grips,

a source of hydraulic fluid for said ram,

a sensor for sensing a condition of said specimen,

a first control system responsive to said sensor for actuating said ram in response to a deviation between the actual and desired value of said condition, and

a second control system for actuating said drive mechanism to restore, subsequent to said actuation of said ram, the relative positioning of said crosshead and said second portion of said ram, said control system including a second sensor mounted to directly sense the relative positioning of said crosshead and said second portion of said ram.

2. The machine of claim 1 wherein said second sensor commands said drive mechanism for said crosshead.

3 4- 3. The machine of claim 2 in which said second sensor Structural & Materials Testing Systems, pub. MTS is an LVDT, and said first control system further com- Division, Research Incorporated, dated August 1964. prises: Tinius Talks, vol. 17, no. 1, published by Tinius Olsen an amplifier, responsive to the output of said LVDT, Testing Mach. Co., November 1965, pages 1, 6, and 7.

and 5 an amplidyne, responsive to the output of said am- JAMES J, GILL, Primary Examiner.

plifier, for controlling said drive mechanism. JERRY W MYRACLE Assistant Examiner References Cited US Cl XR.

A New System of Weighing Machinery, published by the Yale & Towne Mfg. Co., pages 225 and 253-255, published June 1884.

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