Open Control Systems

There has been a move toward so-called "open systems" and more specifically Open Control Systems (OCSs) since the early 1990s. There are many different definitions of "open", but members of the control and information technology (IT) industry would broadly define "openness" as the ease with which different systems and sets of data can be integrated to provide a cohesive and encompassing solution. This combination of systems and data can broadly be defined as "open applications", with no inherent implication on where the specific application is executed. 

To fulfill the requirements of the IEEE definition and these criteria of openness, an open control system must be (Pritschow et al. 2001) ... 

Vendor-neutral open control system (according to OSACA)... 

Open Architecture, Fig. 4 General architecture of an open control system... 

Open Control System The topology of the control kernel depends on the process. It offers interchangeability, scalability, portability, and interoperability. 

From an architectural point of view, an Open control system is divided into two major parts (Fig. 4) ... 

Novel multiphase technology has been coupled with dynamic process simulation, open control system interfaces and a custom user interface to provide a necessary tool for process operators. We hope to be able to report interesting experience and observations from the commissioning of the system. 

Users need to integrate software themselves into control systems and want to profit from cost saving effects like reuse of software and use of Standard-hardware, reduced machine downtime, easy integration of custom technologies, and improved response to changing customer demands. To achieve this, modular control systems have to be transformed into open control systems. 

Many control vendors claim very different products to be open systems. This requires a clear definition of open control systems. In the ISA dictionary open system is defined as one that complies with the requirements of the OSI reference model in its communication with other open system. Intuitively open can be defined as a combination of data connectivity and hardware and software portability. 

Open control system definitions presented by Chrysler, Ford GM in the original 1994 white paper, Requirements for Open, Modular Architecture Controllers for Applications in the Automotive Industry specify that open control systems must be ... 

Five capabilities of modules in open control systems... 

An open-loop system positions the manipulated variable either manually or on a programmed basis, without using any process measurements. This operation is acceptable for well-defined processes without disturbances. An automanual transfer switch is provided to allow manual adjustment of the manipulated variable in case the process or the control system is not performing satisfac torily. 

Open-Loop versus Closed-Loop Dynamics It is common in industry to manipulate coolant in a jacketed reacdor in order to control conditions in the reacdor itself. A simplified schematic diagram of such a reactor control system is shown in Fig. 8-2. Assume that the reacdor temperature is adjusted by a controller that increases the coolant flow in proportion to the difference between the desired reactor temperature and the temperature that is measured. The proportionality constant is K. If a small change in the temperature of the inlet stream occurs, then depending on the value or K, one might observe the reactor temperature responses shown in Fig. 8-3. The top plot shows the case for no control (K = 0), which is called the open loop, or the normal dynamic response of the process by itself. As increases, several effects can be noted. First, the reactor temperature responds faster and faster. Second, for the initial increases in K, the maximum deviation in the reactor temperature becomes smaller. Both of these effects are desirable so that disturbances from normal operation have... 

Up to + 0.01% in open-loop and 0.001% in closed-loop control systems... 

The form of compensation is the 2-pole-2-zero method of compensation. This is to compensate for the effect of the double pole caused by the output filter inductor and capacitor. One starts by determining the control-to-output characteristic of the open-loop system. 

Fig. 4.1 Block diagram of a closed-loop control system. R s) = Laplace transform of reference input r(t) C(s) = Laplace transform of controlled output c(t) B s) = Primary feedback signal, of value H(s)C(s) E s) = Actuating or error signal, of value R s) - B s), G s) = Product of all transfer functions along the forward path H s) = Product of all transfer functions along the feedback path G s)H s) = Open-loop transfer function = summing point symbol, used to denote algebraic summation = Signal take-off point Direction of information flow.

The open-loop transfer function for a control system is... 

A control system has the open-loop transfer function given in Example 5.8, i.e. 

Closed-loop control systems are classified according to the number of pure integrations in the open-loop transfer function. If... 

A unity feedback computer control system, has an open-loop pulse transfer function... 

The elements of a PM plan include periodic inspection, cleaning, and service as warranted, adjustment and calibration of control system components, maintenance equipment and replacement parts that are of good quality and properly selected for the intended function. Critical HVAC system components that require PM in order to maintain comfort and deliver adequate ventilation air include a outdoor air intake opening, damper controls, air filters, drip pans, cooling and heating coils, fan belts, humidification equipment and controls, distribution systems, exhaust fans. 

---