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Dec
18

Simulation Using Foundation Fieldbus Function Blocks

Training an operator on a new control system often includes hands on experience with a training system that supports dynamic process and control system simulation. The hardware for such a training system may be constructed using spare parts from the new control system. A simple simulation of the process can often be implemented using the control system tools provided to configure calculations and logic. The plant control strategies and operator interface should be used without change when create the training system. However, one of the barriers in doing this is the fact that the IO configuration used for measurement, calculations, and control strategy may need to be modified to work with a process simulation. As the Foundation Fieldbus team worked on the function block specifications, one of our objectives was to provide an easy means of integrating process simulation into measurement and control applications. Also, the ability to override IO values was something we felt that an instrument technician or control engineer would find helpful in checkout of a control strategy or a display configuration.

After some investigation, the function block team proposed that a SIMULATE parameter

be included in all IO blocks. This parameter was defined to have the following attributes:

 Simulate Enable/Disable

 Simulate Value

 Simulate Status

 Field Value

 Field Status

The actual measurement value and status of the IO block are reflected in the Field Value and Status. When the Simulate Enable/Disable attribute is changed to Enable, then the IO function block uses the Simulate Value and Status in place of the Field Value and Status. Thus, an instrument technician that is checking out a control strategy before startup can simply Enable simulate and then write to the Simulate value and status attribute. In the IO blocks, the simulated value and status are processed the same as the field element signal. Thus, when a process simulation writes calculated measurement values and status to the Simulate Value and Status attributes of IO blocks then values based on these simulated measurements will appears in the control strategy and operator screen.

When the function block team initially presented the Simulate parameter to the Fieldbus Foundation Technical Advisory team, there was much discussion about whether we should including this capability in field devices. The concern was that, in an on-line system, the operator would not know if the measurement he sees at his interface station is simulated or the true measurement value. To address this concern, the function block team added to the specification the requirement that all Foundation fieldbus devices support a physical jumper that can be used to disable the simulation capability in an on-line system. Also, an explicit alarm was added to BLK_ERR that indicates when simulation is enabled. By taking these steps, the function block team was able to include SIMULATE as a standard parameter in Fieldbus Foundation IO blocks. This capability has proven to be very valuable in system check and in enabling the development of operator training systems.