APPARATUS AND METHOD FOR CONTROLLING A RESONANT TESTING MACHINE

A machine for testing the strength of material specimens by applying a mean and a cyclical load thereto includes a hydraulic actuator connected to the specimen, a servo valve for controlling the flow of hydraulic fluid to and from the actuator and a sensor connected to the specimen. The servo valve is controlled by a train of pulses directed to its actuating coil. The use of pulses as the forcing function allows the system to attain higher operating efficiency. A feedback signal from the sensor is utilized to correct the repetition rate of the pulses to the resonant frequency of the specimen system under test and, after the resonant frequency is attained, to maintain the rate at the resonant frequency. The pulses are area controlled to supply more or less energy to the system as required to maintain the system in resonance at the desired load. A mean load on the specimen is also maintained at a selected value. The machine may be used for standard, non-resonant testing by changing the position of a few switches.