The use of a hydraulic actuator is a traditional way of applying load to test a structure or structural element. The actuator is typically pressurised by a hydraulic power pack. Although the pump within the power pack provides a constant pressure the control of pressure to the actuator is performed by a servo valve. The servo valve operation is controlled by applying an analog voltage to it. Traditionally this voltage has been provided by a custom built analog control panel. Such a panel suffers from many disadvantages including complicated operating procedure, inflexible operation and high cost of maintenance and modification.

With this in mind the Department of Civil Engineering, Dundee College of Technology commissioned a micro- computer controlled hydraulic system for its heavy structures laboratory. The system was developed by two companies Craig Hydraulic Engineering Co. and Crest Developments (Fife), to provide a wide range of facilities including manual, automatic and cyclic loading while still being easy and very safe to operate.

The system developed is based on a NASCOM II micro-computer with 16k ROM and 32k RAM. The control program is in FORTH but reverts to machine language for the parts requiring fast operation. 14 BIT analog to digital and digital to analog converters control the servo valve by a software "feedback" system. The user selects all of the operating options from screen menus and is prevented from starting any test until reasonable selections have been made of all the operating options. Once ram cross sectional area and displacement transducer calibration have been entered, all other parameters (e.g. loading/displacement rates, maximum allowable displacement etc.) are selected in S.I. units.

Trials of the system, with a twenty tonne actuator, have proved it to be exceptionally easy to use and very controllable. For example, if a specimen being tested under load control reaches a peak load the system automatically changes to deflection control and continues straining the sample into the plastic zone. It has thus been possible to load cores of rock and concrete over the peak of their stress-strain graphs without the need for either a "stiff" frame or a double acting actuator.

Modifications to the system should also be very straightforward as all that would be required is modification of the software.

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