Use of Mode in Process Control

One of the foundation pieces of measurement and control as utilized by the process industry is the concept of mode. The mode of a measurement inputs to a control system may be used to indicate if the associated device is in or out-of-service. For a control or output function in a control system, the plant operator typically uses mode to select the source of the setpoint or output. In some cases, mode may also be used to indicate if a calculation function is in or out-of-service. Thus, the IEC61804 international standard, Function Blocks for Process Control, specifies that all measurement, control and output function blocks must contain a mode parameter.

Mode has traditionally been defined in different ways by manufacturers of control systems and field devices. One of the things that the ISA SP50 User Layer Committee realized was that a consistent definition of mode is required to achieve control system interoperability with field devices. Therefore, the technical report produced by this committee defined the mode parameter. The mode parameter structure proposed by the SP50 committee was adopted with minor changes by the Fieldbus Foundation’s function block team. As an integral part of the interoperability test performed by the Fieldbus Foundation, the mode parameter implementation is verified to be consistent with this Function Block specification.

The mode parameter support by Foundation fieldbus function blocks consists of four attributes rather than a single target attribute found in some traditional control systems.

 Target mode attribute

 Actual mode attribute

 Permitted mode attribute

 Normal mode attribute

The plant operator uses the target mode attribute to select the desired mode of operation. The target mode selections defined by the specification are;Out-of-Service (O/S), Automatic (Auto), Manual (Man), Cascade (Cas), Remote Cascade (Rcas), and Remote Output (Rout). In the past, different terms have been used by manufacturers for some of the target mode enumerations. For example, Cascade mode is the equivalent to Remote Setpoint (RSP) in some traditional systems. Remote Setpoint and Remote Output are referred to as Supervisory and DDC mode respectively.

Based on the status of inputs to a function block and other conditions that impact block operation, it may not be possible for the block to operate in the requested mode. For example, if the output track input to a control block is active, then the block will not continue to operate as request e.g. Automatic mode. The actual mode attribute is used to reflect the mode of operation that can be achieved. Thus, the actual mode attribute is calculation by the block each execution. Two actual modes are defined that may not be selected as the target mode.

 Local Override (LO) mode – the block track input is active.

 Initialization Manual (IMAN) mode – the downstream path to the process is broken.

Since a control application may only require a few of the target modes supported by a device, the user may configure what operation modes are appropriate for his application through the permitted mode attribute. When this is done, the function block limits the target modes to those that are permitted. Similarly, the mode the operator should choose during normal plant operation is configured in the block using the Normal mode attribute. Even though this parameter is not utilized by the function block, it may be useful to other applications, such as an operator station to flag loops that are not running in the normal mode of operation.

One the challenges that the Fieldbus Foundation function block specification team addressed was how to define target mode to support both single knob (Man, Auto, Cas, RCas, Rout) vs. dual knob interfaces (Auto/Man + Cas/Rcas/Rout). By defining the Target mode attribute (bitstring) to use multiple bits for each target mode selection (including bits to indicate previous mode) it is possible to support both type of interfaces. Because of this capability, it is easier for legacy systems that use a dual knob interface to support the installation of fieldbus devices. Some modern control systems have adapted the Fieldbus Foundation’s definition of mode. In these systems, the mode parameter is used in a consistent manner independent of whether the associated function block resides in a field device or in the controller.