Part 5. Principles of Testing Machine Control Feedback.

INTRODUCTION: The control of machines in the soil mechanics laboratory can vary from the simple “on-off” variety that are operated manually to the more modern machines that are controlled by computer.

The control can be “open loop” whereby the computer takes in measurements from a logger connected to transducers, carries out computations, and makes some decision such as to start or stop a test. The control can more likely be “closed loop”, however, whereby the machine is controlled from a transducer on the test specimen (e.g. a radial strain transducer controlling a  K0 test), or from a transducer such as a load cell in a force actuator or loading frame. The transducer is providing “feedback” i.e. a direct measurement that can be compared to some set or target value. The microprocessor in the machine can then make the machine operate in such a way as to minimise the gap between the measured and target value so that the target value (which may be moving) can be achieved. This enables a digital solution to utilising feedback that, in the pre-digital age, made use of (analogue) process controllers to, for example, control  K0 consolidation and  K0 swelling in the triaxial apparatus (Menzies et al.,1977). Consider the following example of microprocessor control using feedback. To download the complete white paper, download the PDF file below.


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