Siemens PCS 7 Tools: Sequential Function Chart (SFC) Types

When creating or expanding your Siemens PCS 7 project, there are several bulk systems engineering tools available to assist in the process to optimize efficiency, organization, and future flexibility. Tools that will be covered in this blog series include Process Tag Types, Process Object View, and Sequential Function Chart Types (SFC Types). These Siemens tools allow you to create templates used to generate several instances in a short amount of time and to efficiently edit large pieces of programming from a central interface.

You may wonder how exactly these tools will benefit your application. By utilizing these tools, you are allowing yourself the following advantages:

  • Consistency among control modules or sequences of the same type
  • Efficient, bulk creation
  • One-time approval of a type template
  • Bulk modifications of several instances at once

The alternative to using these systems engineering tools is to manually create each and every control module and sequence. This method would undoubtedly require more engineering time while creating more room for error and a lack of consistency. While using these tools will take more upfront planning and consideration, it saves time in creating, testing, and modifying individual instances. The more familiar you become with these tools, the more you can recognize how to utilize them to their full potential and how the upfront planning and decisions result in optimal results with minimal engineering.

Read the blogs below to learn more about how and why Cross Company uses these tools with PCS 7 projects.

What are Sequential Function Chart Types?

Sequential Function Chart Types (SFC Types) are like Process Tag Types for sequences. You create a template sequence that contains logic for a generic function that is repeated throughout a plant.

The SFC Type is stored in your controller charts folder, like the continuous function charts (CFC charts), but are edited using the SFC Editor.

Siemens PCS 7 Tools: SFC Editor | Cross Company

View of an SFC Type in the SFC Editor tool.

This template sequence is the SFC Type and the multiple areas, equipment, or process specific instances are deployed as blocks of code in your project. Each block instance is connected to its specific logic charts (devices, external code, etc.) to perform the sequence logic on its specific equipment or area.

Siemens PCS 7 Tools: SFC Types and Instances | Cross Company

Overview of SFC Types and Instances.

How To Identify Sequence Templates For a Plant

First, you must be aware of the standards that are available to take the guesswork out of designing your project. S88 Standards are the most widely adopted process control system configuration standards for manufacturing in the U.S. and Europe. Following these standards when planning your project allow you to design a flexible control system, and help you avoid common pitfalls or irregularities. These two blogs (linked above) provide a great detailed explanation of the background knowledge needed to design your Siemens PCS 7 project. 

Second, once you understand how to follow the S88 Standards for your project design, you need to be able to recognize where you can apply these standards to allow for the bulk creation of sequence instances.

When designing and configuring sequences for your PCS 7 project, you have some flexibility and control over whether two process sequences can follow the same template. As long as two process sequences have the same pieces of equipment being controlled and follow the same general steps, a sequence template, or SFC Type, can be designed and maintained to be utilized in multiple instances.

It is important to keep things simple and general when designing SFC Types, to take advantage of several instances using the same sequence template. Common examples include:

  • Agitator control
  • Material loading/unloading
  • Reactor temperature control

Configuration

SFC Types have a set of characteristics that are used when programming the sequential logic steps and transitions (actions and verifications).

Siemens PCS 7 Tools: SFC Characteristics | Cross Company

List of characteristics for the SFC Type in the editor tool.

These must be named in general terms to be applicable to every instance of the type. Some examples:

  • Control strategy: FILL TANK.
  • Setpoint: FILL_SP for setpoint to stop filling.
  • Process value: TK_LEVEL to read tank level.
  • Control value: TOT_STRT to start totalizer.
  • Block contact: BTM_VALVE for a bottom valve.

An example logic step would command open BTM_VALVE and send a signal to TOT_STRT. The transition step would then wait for TK_LEVEL > FILL_SP until the next step would command the valve closed and stop the totalizer.

Siemens PCS 7 Tools: SFC Sample Logic | Cross Company

Sample logic in an SFC Type sequencer.

Cross Company has developed and maintains internal documentation to define standards to follow in configuring SFC Types. This allows for efficient and consistent engineering throughout projects.

Instances

SFC Types are organized in libraries and are handled as normal function blocks, i.e. laid in a CFC chart, parameterized, and with interconnections linked.

Siemens PCS 7 Tools: SFC Type in an Instance Chart | Cross Company

SFC Type used in an instance chart.

Each general characteristic that was defined in the SFC Type is now connected to its instance-specific devices for monitoring and control. The diagram below shows the logical connections made for the “Fill Tank” example described previously.

Siemens PCS 7 Tools: SFC Logic Connections | Cross Company

Logical connections of an SFC Type instance.

Defined parameters which are evaluated in the sequence can have their values interconnected at the instance to use a unique value-- denoted “Instance value” in the block properties.

Each instance generates its own HMI block icon for monitoring and control of that specific instance. A faceplate for each instance icon allows operators to control each sequence (Start, Hold, Resume, Abort, etc.) and to view the status of a running sequence, down to the specific step or transition being evaluated.

Editing SFC Types

Making a change to the SFC Type in the SFC editor applies this change to all instances of that type. Common examples include adding another control strategy for additional equipment functions or adding a valve for redundancy in sequence operation.

Also, just like Process Tag Types, since SFC Types are deployed as blocks, they can have certain attributes edited using Process Object View.

What’s Next?

This completes the blog series on PCS 7 Engineering Tools. If you would like additional information or would like to talk to an engineer at Cross Company about how these tools can be used on your next PCS 7 project or applied to your current system, please feel free to contact us.

Contact Cross Company

Lindsey is a Systems Integration Engineer with Cross Company Integrated Systems Group. She has been working in System Integration since 2009, after graduating Summa Cum Laude from Virginia Tech with a degree in Chemical Engineering. She has gained experience among multiple controls platforms and HMI packages, but specialize in Siemens PCS7. Trained in PCS7 System Engineering I & II and PCS7 SIMATIC Batch, she has been certified in PCS7 Best Practices methods.

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