of the servo-controlled electro-hydraulic machine andis equivalent to that of usual mechanical hydraulic-pulsator machine. Thus, the present machine is suit-able for fatigue tests with sinusoidal waveform forhigh load and long-stroke fatigue tests which can berealized alone by the use of the hydraulic-type machine.

  Thus, the present system, which controls the ampli-tude and the mean value independently, could be op-plied also to the servo-controlled electro-hydraulicmachine.  Furthermore,  in the high-frequency test,where the control-loop becomes open and where theamplitude and the mean values change gradually asthe record OA in Fig. 4 due to the change in the tom-pliance of the specimen, that in the temperature ofoil, and insufficient gain of the system, the servo-controlled electro-hydraulic machines are impossibleto compensate the error from the set value.  On theother hand, the present control system could tompensate for it.

Conclusions

  An hydraulic fatigue-testing machine combining amechanical hydraulic pulsator and a closed-loop con-trol system was constructed.  Its performance was asfollows:

  1. The consumption of electric power and coolingwater of this machine is about one third of the servo-controlled  electro-hydraulic machine and is equiv-alent to that of usual mechanical hydraulic-pulsator machine.

  2. The stability of the control system of the presentmachine is 0.5 percent of the range in a whole periodof fatigue testing or in a week. This is superior to thatof  the  servo-controlled  electro-hydraulic  machine,and  much  better  than  the  mechanical  hydraulic-pulsator machine, by using the peak-value controllingcircuits.

                           Acknowledgment

  The authors wish to thank the late Tamotsu Fuku-moto, who was formerly head of Material StrengthResearch Division of National Research Institute forMetals, for his useful suggestions on the onset of thisplanning.  Wealso take this opportunity to thank M.Kosuge, who assisted in the construction of the peak-detecting circuits.  We are also grateful to S. Yoshidafor his reading of the manuscript.  In addition, wewish to express our thanks to the men of Mori TestingMachine Co. Ltd., in particular to Y. Nakata, S. Ka-gaya, K. Takayama, S. Kobayashi and Y. Imayoshi,for their construction work on this machine.  We areindebted also to H. Kubota for the preparation ofdrawings and to Miss H. Iwanami for the typing of

the manuscript.

                            References

1. Forrest, P. G, Fatigue of Metals, 24-34, Pergamon Press, Oxford (1962).

2. Cazaud, R., Pomey: C., Rabbe, P. and ]anssen, Ch., The Fatigue of Metals, 123-130, Dunod, Paris, in French (1969).

3. Kagaya, S, "The Analysis of Power of Hydraulic Fatigue Testing Machine under Pulsating Tension,". of the Soc. of Mater. Sci, 15 (151), 236-243, japan, in apanese (April 1966).

4. Tobey G. E, Graeme, J. G. and Huelsman. L. P, Operational AmpTifers-Design and Applications, 353-358, McGraw-Hill Book Co, New York (1971).

Plc control system design elements

Originally, the PLC was represented by the acronym PC. There was some confusion with using this acronym as it is commonly accepted to represent personal computer. Therefore, PLC is now commonly accepted to mean programmable logic contoller.

A  PLC is a user-friendly, microprocessor-based specialized computer that carries out control functions of many types and leves of complexity. Its purpose is to monitor crucial process parameters and adjust process operations accordingly. It can be programmed, controlled, and operated by a person unskilled in operating computer. Essentially, a PLC’s operator draws the lines and devices of ladder diagrams with a keyboard onto a display screen. The resulting drawing is converted into computer machine language and run as a user program.