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Considerations For The Design Of An Equipment Model
According to ISA/S88 guidelines, a manufacturing process can be represented in terms of two models, a Procedural Model and an Equipment Model.  The Equipment Model includes a functional group of equipment that can perform a finite number of specific, minor, processing activities.  It is important that the Equipment Model be well designed, providing flexibility and modularity as well as impacting the overall performance of the equipment.  One approach to the design of the Equipment Model is based upon the use of phases, where the phase is regarded as a building block for the process or as a specific activity.  This first, in a series of papers, is focused upon the activities performed by the phases and discusses:

  • How does the phase interact with operators and automation control system?
  • What information is recorded and made available to an operator and journals?
  • How does a phase respond to failure of a component in the equipment?

The second paper will present seven steps that are recommended to be followed in the design of the Equipment Model.

In considering the design of the phases it may be useful, as an illustration, to equate a phase to an ideal operator, who approaches a unit, selects a module of interest, e.g. water addition, or temperature control, and using the HMI pop-up inserts the values required for the process.  Those actions are some of the function of the phase.  In addition, it is important that before any actions are taken, the phase should establish the status of the equipment to be used and inform the operator of conditions of exceptions.  Any abnormal conditions will be logged, and the operator can obtain specific information about the equipment.  Is any component faulted?  What and where is the fault?  Are any components in manual mode?

Phases should NOT automatically change the mode of their subordinate equipment but rather should prompt an operator if such changes are desired.  For example, if it is logged that a component of the equipment is in manual mode the phase should only inform the operator of this condition and not change it to auto in order to run the equipment.  All changes in parameter set points, changes that may be carried out by an operator or code, must be logged by the phase.  If for any reason phase failure occurs (typically an equipment failure), the phase should be commanded to go into HOLD and specific information regarding the cause of the failure should be logged.  It is preferable that to complete a task a phase does not depend upon a parallel phase be required unless required by the functionality.  In such instances, the recipe author must know the logic involved, a situation that is not desirable.  Furthermore, the absence of such dependence can simplify the design.

It should be understood there are situations where the coordination of phases is necessary.  An example is the Transfer IN and Transfer OUT activities that occur between units. In a material addition activity, the phase should identify if a dosing error occurred, informing the operator of the error showing it as “out of tolerance high” or “out of tolerance low” and allow to adjust to that condition.  Another situation that requires the coordination of phases is the use of multiple phases requiring common resources.  For example, providing water addition to multiple locations when totalized with a common flow meter, this will require arbitration, i.e. the coordination control that becomes necessary to determine how a resource should be allocated when the demands for that resource are many at one time.  The use of a common phase can minimize arbitration.  It is preferred that one phase be shared by multiple units.  However, it may be necessary to use multiple phases where different units and their phases are controlled by different controllers (PLCs).  In such cases, a resource is created that is arbitrated among the multiple phases.

In the design of the Equipment-model advantage should be taken of the availability of Material Manager, a component of FactoryTalkBatch.  The Material Manager has the capability

  1. to identify the location of the resources, i.e. are they stored in tanks, silos, supersacks, barrels, drums, or on pallets,
  2. to define and control the type of material allowed in each location e.g. rice rather than corn, fine grain material not coarse material,
  3. to prioritize the usage of the materials when found in various storage locations,
  4. to define material properties such as density, moisture content, concentration, potency, humidity, pH, etc.  This allows the phase to compensate for variations in material properties, such as moisture content, potency, etc.

It is strongly recommended that individual documents are developed for each phase or Phase class in which all relevant design information is captured.  i.e. phase logic, Phase failures that may occur, interactions with operators and the equipment model design.  The creation of documents for each phase reduces the time spent waiting for all documents to be completed before programming activities can be initiated.  Some phases such as agitation, or prompting the operator, are well established so that programming of those phases can be undertaken before all documents are complete.  This allows an earlier completion of the project as well as staggering of tasks. The terms used in naming Parameter and Reports (Tags) must be meaningful and not rather obscure engineering terms that make little or no sense to the recipe authors or the audience of this data.

Phases can take advantage of the availability of high order functions on today’s controllers, mathematical derivatives and integrals, or possibly the use of fuzzy logic or advanced process control.  This capability may allow the determination of the efficiency of a single phase, which may be designated as the phase OEE.  Information is collected for individual phases and these calculations to calculate a flow rate in the absence of a flowmeter e.g., the change in weight of a tank with time during the addition of material, defines the flow rate which in turn allows prediction of the total time that will be required to complete the dosing activity.  Integration of temperature-time profiles during a heating process will indicate the amount of heat impinged on the product. This can give an indication of the amount of cooking as well as the sterilization of the equipment during the CIP & SIP tasks.  Integrals also allow the determination of the total power used in agitation activity.  Generic timer phases can be used to determine recipe activities OEE, by placing count-up timers in parallel with the recipe activities, allows us to utilize this data and information to determine the OEE of each activity of the recipe and not just the overall time it took to complete a recipe

With all the above requirements in mind, we can proceed to considering the topic of the second paper – The Seven Steps to the design of an Equipment Model.



Posted In: White Papers