CONTROL ENGINEERING INSTRUMENTATION FOCUS 
 

How can standard S88/IEC 1512 be used for shorter design and safer operation in batch process control? 
 

In the field of process control for batch workshops the model proposed by the ISA S88 committee and used in standard IEC1512 (see fig. 1) is now a reference for most suppliers of control equipment.  The top-down approach adopted by the S88 group is unambiguous and rigorously formal for the recipe, operation and phase layers.  A lack of formalism in the lower "equipment-oriented" layers leaves the designer completely free to implement the actuator (element) control mechanisms from the upper layers as he sees fit. 
 

The quality of the solution, the respect of basic safety rules and the interlocking of actuators depend entirely on the technical knowledge of the designer.  Thus there is a potential for poor quality and a lack of security. 
 

In a move to increase intrinsic quality and safety, and to make them less dependent on design teams, Rhône Poulenc has defined a process model with four classes of hierarchical objects.  This model, known as ASTRID^TM, enriches the S88/IEC1512 model. 
 

GENERAL PRINCIPLES: FOUR CLASSES OF HIERARCHICAL OBJECTS (see fig. 1) 
 

We can show that a batch process operating mode can be described completely by referring to only four classes of objects.  In a bottom-up approach we find: 
 
 

• Two classes of equipment-oriented objects 
 

• Two classes of function-oriented objects:

Commands are forwarded from the upper layers to the actuators through links established dynamically (according to recipe progress) between recipes, functions and resources or permanently (during a manufacturing campaign) between resources and actuators.  Dialogue between objects in adjacent layers takes place through finished status charts that are specific to each object class.

ASTRID ENRICHES THE S88/IEC1512 MODEL 
 

The resource (equipment) layer is part of the S88/IEC1512 functional pyramid (see fig. 1).  This layer provides a framework for the description of automatic control and behavior directly linked to field equipment. 
 

The S88/IEC1512 model manages the recipes object class and dialogue with ASTRID functions or S88 phases. 
 

THE RESOURCE: A SAFETY FILTER FOR ACTUATOR COMMANDS (see fig. 2) 
 

Guaranteeing the safety of commands for actuators and particularly cut-off valves has never been easy.  Every instrument designer encounters two problems: 
 

1. How can a complex process be broken down into simple parts in view of mastering its complexity and in the absence of formal rules for such an operation? 
 
 

2. Cut-off valves are located between two process elements and thus belong to both resources.  How can actuator control be mastered and made unique no matter which control mode is selected: local, remote, semi-automatic (function by function) or automatic (controlled by the recipe)? 
 

The ASTRID model responds to these two requirements with rules for decomposing processes into resources, and with generic mechanisms for controlling commands that functions send to actuators. 
 

The resource is a P and ID schema element between two cut-off actuators.  Commands from upper layers, the function or operator remote control must go through the resource.  Thus during a manufacturing campaign actuators are permanently linked to one and only one resource.  It is recommended that each cut-off actuator be linked to the resource upstream. 
 

If basic resources are always used in the same way, they can be grouped together to form a larger resource whose complexity is managed by the resource itself.  In this case it is viewed by the environment as an object in the resource class.  The higher the degree of polyvalence, the finer the decomposition into resources.  
 

When a function is activated it occupies the necessary hardware resources if such resources are available.  A mailbox mechanism, for example, associated with each actuator and managed by the resource, can be used to guarantee the integrity of the physical path made up of all the resources controlled by the function.  Locks (see fig. 2) are placed on actuators on the periphery of this physical path; they prevent unwanted activation by another function or a remote operator command.  All actuators that are neither busy nor locked are available through remote control. 
 

The model manages both activities that generate flows between a source and a destination (transfer from A to B) and activities associated with a device. 
 

INCORPORATING THE ASTRID MODEL IN OPERATING CONTROL SYSTEMS 
 

The ASTRID model, which is in the public domain, has been analyzed by an EXERA working group and presented to two other European associations of users of instruments and automation equipment (WIB and SIREP), in view of encouraging its use by equipment manufacturers.  It has been presented to the current leaders in the field of  digital control systems. 
 

Over 30 workshops at Rhone Poulenc and other pharmaceutical and agro-chemical companies now use ASTRID.  Control systems involved at present are Schneider Automation, Rockwell Automation, Siemens and ABB Mod 300. 
 
 

For more information, contact Dr Francois Lebourgeois ; Fax :33 472 935 210 ; Email : francois.lebourgeois@rhone-poulenc.com