Really Real Simulations
The difference between standalone and connected systems also can get a little hazy because its sometimes possible to connect the standalone to the process control system. Many simulators run on standard PCs under Windows, so you can connect them any way you like to nearly any kind of hardware, including a control system.
Nearly all DCS manufacturers offer virtual stimulation controller software (See Figure 3 below) that simulates one or multiple controllers running on a single PC, explains McKim. This virtual system then connects to a simulator process model via a cross-reference table that ties model points to control system I/O points. The advantage of the virtual control system is that it runs the exact same control code that a real controller does, reduces the amount of hardware, and most importantly integrates seamlessly with DCS HMI stations. This is the most effective simulation of the DCS because the system behaves just as real controllers would, and the operator interface is identical to that in the control room.
|FIGURE 3: STIMULATING SIMULATIONS|
The Plant Simulator uses a Virtual Stimulated representation of a control system. A standard PC is running controls simulation, HMI, and the Simulator Process Model. Source:Invensys
ngineers can review these runs in an accelerated mode the next day, and review the performance of the control logic. Weve had virtual cells run connected to control systems for two to three weeks without stopping. We think truly testing a control system prior to installation is one of the best benefits of this technology.
Siemens can do this too, but Marsh says they dont see it very often. With the SIMBApro and SIMIT simulation packages from Siemens, a hardware simulator in a PCI card can be used in conjunction with an engineering station or a dedicated simulation station to test the actual response of the control system. One large food and beverage manufacturer uses this package with its PCS 7 system to test new recipes. The cards are connected via Profibus to an actual hardware controller. The card then simulates the actual electrical signals and dynamic response that would be generated by their remote I/O and smart instrumentation.
Some external simulators can force variable changes in the plant historian, thereby simulating inputs to the process control system. Some DCS vendors supply simulators that can run in either mode. In some cases, you can buy a PC-based software version of a large-scale DCS, so you can simulate your entire control system in a PC. In short, if you have a standard DCS, you can probably purchase several different simulators, ranging from a simple OTS to a full-bore dynamic process simulator.
Bringing up the Simulator
Though simulation software is widely available and 10 times more capable than it was a few years ago, that doesnt mean its easy to set up. Tom Badura, electronics project engineer at a Midwestern plastics manufacturer, says hed like to use simulation, but doesnt have the resources. Although the chemical processes in our plant would be complex to simulate accurately, much process and control system training simulation could be developed without truly simulating the process, he says. This can be done using the functionality of the control system and HMI packages. Unfortunately, we dont have the manpower and time to develop even this limited simulation. Weve used vendor-developed simulation for operator training when installing a new control system. This training had limited success partly because we werent able to dedicate technical or management resources to the training program.
Thats certainly a problem, but you have to give vendors credit for trying to simplify. For example, FEED techniques allow users to establish one database for the control system and simulator. Its even possible to import P&IDs into a simulator. Protocols exist for exchanging this data between software applications like simulations and drawing packages through a common database, explains Henderson. Control system vendors are providing soft-control environments that can be integrated with simulators. This allows actual plant control and logic configurations to be downloaded from the plant directly into control components integrated with the simulator. These data exchange methods eliminate effort in data manipulations and entry during the initial model development and subsequent model updates as the plant design changes. Many steady-state simulators also have the ability to generate the process flow diagrams (PFDs).
Siemers adds, Mimic simulates the process input and output parameters of the DeltaV control modules using DeltaV Simulate. We use the actual configuration by importing the DeltaV.fhx files. The simulator simulates the process equipment mathematically, reads the outputs from DeltaV, applies the dynamics and writes the inputs to the system.
McKim says, Most of SimSci-Esscor's simulators use virtual stimulation controls with DCS database files, graphics, and other data exactly from the real control system, without translation or emulation. In addition, they offer connections for nearly all major DCSes, including Foxboro, Triconex, Emerson Ovation and Delta V, ABB Industrial IT, GE MkVI, Honeywell, Metso, GE Fanuc, and many other DCS and PLC control systems.
As with all systems, theres an administrative effort required to configure the system and support the simulation environment, says systems integrator Lane. The effort and skills required are different from those needed to configure and support a DCS. Training from the vendor will usually be required. Our experience at Weyerhaeuser was that about one effort-month was required to configure the test system. However, during times of peak software testing, administering the system and assisting users conducting tag loads and testing was nearly a full-time job.
In process control, nothing is easy or inexpensive. Operator training simulators certainly appear to be worth all the trouble, especially when they can save time and money during commissioning and startup.