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zhu fei

question about real time testing using OpenFresco

I read the file,and I had some questions. 1: Because this real time test is a distributed test, so the OpenFrecso is used. In other words, if I just do a local real time test, maybe I doesn’t need to use OpenFresco. Does this opinion is right or not? 2:This real time test is based on an soft real time system, so what’s the key measurement for the system? Thanks advance.

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    Stephen Alan Mahin

    The examples in the report by Schellenberg are meant to demonstrate the flexibility and adaptability of OpenFresco. They are not intended to indicate preferred or limiting applications. OpenFresco can do both local and distributed hybrid tests, either in real time, slower than real time or faster than real time implementations. If you have another application to run hybrid simulations, you can use it locally, but if you have openFresco, you can use it for a variety of applications, local, distant, fast or slow. No need to change your set up, just its settings. The ability of doing real time testing depends on a number of factors. For example, the time needed to carry out the computations to determine the position of the test specimen at the end of the current step, the maximum velocity capacity of the actuator, the precision of the actuator/controller system at high loading rates, etc. If these are not a limiting factor, in an experiment, one uses openFresco and the actuator control system to set the time step over which displacements are being imposed to the numerical integration step size. That is, if the integration step size is 0.005 seconds, the computed displacements are imposed in the same time. This gives one real time. This is done by parameter settings within openFresco and the controller. Soft real time is generally used for most applications since the computations might not complete soon enough (for nonlinear systems that require iterations, there is not fixed time needed for the calculations), or the actuators may not move fast enough to move to the required location within the time step. Thus, in these cases, OpenFresco allows you to slow the motion of the actuators temporarily (rather than suddenly stopping) while waiting for the computational results or feedback from the structure. The tests then resume the normal rate until another slow down is required. This is not exact, but provides a great deal of flexibility in dealing with non-ideal conditions in tests. OpenFresco does not preclude hard real time tests. This simply requires that OpenFresco be installed in a computer having a real time operating system. This is beyond what can be discussed here, but it is straight forward. The issue however is what the system will do when a data point from a computation or feedback signal is not available when needed. For real time testing, generally one cannot pick an arbitrary integration time step size. This is because we are dealing with digital electronics. Controllers have a given frequency at which the provide output. This would normally be related to the number two raised to a power. For example, a common controller frequency is 1024 Hz. Thus, to keep the test and integration in synch, it is typically necessary to pick an integration step size that is a multiple of 1/1024 in such cases. If you are new to hybrid testing, there is a new application called OpenFresco Express that will be released quite soon that can get you up and running is short order to explore these topics.

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