of a plunger, and a small oil pump that supplies a constant oil pressure to the actuator.

  In this machine, a strain-gage-type load cell is usedfor detection of the load.  The upper and the lowerpeak values of the load are detected from its output.The amplitude and the mean value of the load arecalculated from these peak values through an opera-tional circuit.  Then, the load is controlled with inde-pendent  feedback  of  the amplitude  and  the meanvalue of load. Figure 1 shows the whole view of thismachine.  The fundamentals of the control system ofthis machine are shown in Fig. 2 and are explainedwith it. The oil pressures for the upper and the lowercylinders of the actuator which is constructed on the

machine frame are independently supplied.  A speci-men is loaded by the differential pressure of oil ofthe upper cylinder from the lower one.  Namely, the load exerted on the specimen is equivalent to theproduct  of  the  differential  pressure  of  the  uppercylinder from the lower one and the cross sectionarea of the cylinders.  When fatigue tests are con-ducted, the oil pressure of the upper cylinder is main-tamed at a constant pressure P}, and that of the lowercylinder alternates conforming to the formula Pt+P} sin wt. here,尸。is the mean pressure of the lower

cylinder; and PQ sin }t is the alternating pressure ofit. Thus, the load P ,+Pa sin wt corresponding to thedifferential pressure of the upper cylinder from thelower one is exerted on the specimen. 

  An accumulator in which high-pressure nitrogengas is pressurized, is connected with the pipe line tothe upper cylinder.  Thus, the alternation of the oil pressure of this cylinder, accompanied by the change in volume with the elongation or the shrinkage of aspecimen, becomes negligibly small and the pressure

is regarded to be constant for a short period of time.

  For the compensation of a decrease in pressure ofthis cfinder which follows from a leakage of oil witha lapse of time, a servo-valve is actuated by the com-mand of a signal from a servo-amplifier, which isequivalent to the difference of the output signal of apressure transducer inserted in the upper cylinder

from the value P} set at the beginning of test in theconstant-pressure setting unit.

  The alternating component of pressure, Pa sin wt, inthe lower cylinder is excited by a mechanical hy-draulic  pulsator.  This  is  composed  of  a  pulsatorcylinder on a rocking-arm  which rocks with a rota-tion of a crankshaft.  The pulsator ram bobs up anddown with the movement of the rocking arm, makesthe volume of cylinder alter, and excites the alter-nation of oil pressure.Thealternating oil pressureexcited in this manner reaches the lower cylinderthrough the pipeline and applies the alternating loadto the specimen.  The amplitude of this alternatingoil pressure, Pa, is determined by the position of theplunger on the rocking arm.  The closer the positiontakes to the fulcrum of the rocking arm, the smallerthe amplitude becomes.  Thus, when Pa has to vary,the pulsator cylinder is moved on the rocking arm bythe run of a servo-motor.  The value of amplitudedetected through peak detecting circuits is comparedwith the value which has been previously set in anamplitude-value-setting unit at the beginning of test.Then the load amplitude is kept constant by the ac-tion of the servo-motor run by the amplified signal ofthe difference in this comparison

  As for the mean-load control, the output signal ofthe mean value, Pm, is compared with the value thathas been previously set in a mean-value-setting unit.

The servo-valve supplies the oil pressure, Pt, in thelower cylinder which corresponds to the differencesignal in this comparison only of the mean value.

 Flow rates of the servo-valves that are used for the