Mechanical hydraulic-pulsator machine with three control loops has been constructed. Stability of control is 0.5 percent of the range. Consumption of electric power is about one third of electro-hydraulic machine57942

ABSTRACT   An hydraulic fatigue-testing machine combin-ing a mechanical hydraulic pulsator and a closed-loop controlsystem has been constructed as a trial. The dynamic capaci-ties for load and stroke ranges are 1 MN superposed by ameanload ranging from 0 to 1 MN and 5 ram, respectively,over a frequency range from 3.3 to 10 Hz. The consumptionof electric power and tooting water for this machine is aboutone thirdof the servo-controlled electro-hydraulic machine.The stability of this control system is 0.5 percent of therange. Some experimental results are presented to demon-strate the excellent stability of load during fatigue testing.IntroductionHydraulic and resonant-type fatigue-testing machinesare used for high-load fatigue tests.l, 2 The resonant-type machine can be run with a small amount ofelectric power; however it cannot be run in a high-damping condition of specimens, or long stroke and

high-load fatigue test. The hydraulic-type machinescan be run in these conditions. Thus, the hydraulicfatigue-testing machines are used for high-load andlong-stroke fatigue tests.This type of machine isusually pided into two categories:oneis the servo-controlled electro-hydraulic machine2; and the other is the mechanical hydraulic pulsator. 1-3 The servo-controlled electro-hydraulic fatigue-testing machine 2has a closed-loop cont"0! system, and this continu-ously corrects the differer/ce of the load or elongationsignal from the          command        signal.Therefore,theservo-controlled electro-hydraulic machine can be operated in an arbitrary waveform of the load orelongation under limited-frequency conditions. Thatis, it can produce not only a sinusoidal waveform butalso a random waveform in accordance with a com-

mand signal generator. The machine of this type,however, needs large amounts of electric power andwater for cooling of oil, so it is expensive both in construction and in operation.In case a laboratorydoes not have a sufficiently large electric supply sys-tem, this kind of machine cannot be equipped. On theother hand, the mechanical hydraulic pulsator ma-chine 3 needs less electric power and less water forcooling than the servo-controlled electro-hydraulicmachine does. However, this machine has an open-loop control system. That is, once the amplitudevalue of load has been set at the beginning of thetest, this value is not controlled automatically. More-over, the mean value of load is kept only within 5

percent of load range by an action of two oil-pressureswitches which are connected through the mechani-cal-pulsator unit by means of a rotary valve con-nected to the working cylinders. This on-off controlsystem cannot maintain the set value at the beginningof the test because of the change in the compliance of he specimen and the change in the temperature of

he oil.

   Therefore, an operator has to correct manually theoad that changes every minute with the advance ofatigue testing. Further, this machine could only beused for constant-load fatigue tests of sinusoidalwave.

   Thus, in order to obtain a low consumption of elec-ric power as well as good stability in control, anhydraulic fatigue-testing machine equipped with amechanical hydraulic pulsator and a closed-loop con-

rol system was constructed as a trial.This paper eports the principle of the system and the excellentperformance of this machine.

Control System

  As mentioned above, this machine was constructedbased on the load-generating mechanism of the me-chanical hydraulic pulsator. 1-3 This mechanism gen-erates the alternating load and the mean load inde-

pendently.   Namely, it consists of two parts: a me-chanical hydraulic pulsator which has a smaller cross ection and a longer stroke than the actuator andgenerates alternating oil pressure by the reciprocation