GB1124195A - Materials testing - Google Patents

Abstract

1,124,195. Correspondence control systems. INSTRON CORP. Jan.27, 1967 [June 27, 1966], No.4218/67. Heading G1N. In a material testing machine a specimen 64 to be tested is held between grips 28, 30. The grip 30 is fixed to a piston rod 36 of a hydraulic ram controlled by an electrically actuated valve 56. A load cell 62 produces a negative signal in response to load applied to the specimen which signal is compared with a positive desired load signal, from a source 70, in a comparator 68 the amplified output from which operates valve 56 to selectively apply hydraulic pressure to one or other side of a piston 34. In operation, with the machine in a stable state i.e. zero output from the comparator a call for increased load (e.g. by manually adjusting a potentiometer in the source 70) produces a signal from the comparator and piston 34 is caused to move downwards. A linearly variable differential transformer senses the downward movement of the piston rod and transmits a positive signal to an amplidyne 78 to control the magnitude and polarity of a voltage to a motor 26 which through gears 20, 22, 24 and via screws 16, 18, moves a crosshead 14 supporting the ram cylinder 32 downwards until the piston is again centred in the cylinder, at which position the output from the differential transformer is zero. Similarly a call for decreased load causes upward movement of the piston which is followed by an upward movement of the cross arm to centre the piston again.