A Return to Single-Loop-Integrity

Control in the field is becoming mainstream once again – just as the ISA SP50 fieldbus committee predicted it would be. CONTROL's regular contributor Dave Harrold looks at the integrity of the control loop.

By Dave Harrold

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By Dave Harrold, co-founder of the AFAB Group

The ANSI/ISA S50.2 fieldbus standard recognizes eight different fieldbus technologies, but only Foundation fieldbus (FF) incorporates the entire S50.2 standard including the inclusion of function blocks that are specifically designed for deploying control strategies into field-devices – a capability the SP50 committee strongly supported.

Dick Caro, a past chairman of the ISA SP50 fieldbus committee, believes that about 80% of today’s process control loops are candidates for taking advantage of FF function blocks to achieve control in the field (CIF) and restore the once-popular ‘best practice’ known as single-loop integrity.

Single-loop integrity (sometimes referred to as single-control strategy integrity) is a simple concept – design control loops so that if any of the loops devices fail only that loop is affected. The overwhelming benefit of a single-loop integrity design philosophy is that loss of one control loop (or strategy) rarely results in a complete or even partial unit shutdown of the process. When CIF is strategically deployed to include field-devices and the process automation system (PAS), a field-centric control architecture is created that produces a simpler, more reliable, less costly implementation.

A robust implementation of CIF begins with Fieldbus Foundations’ specification that defines standard, flexible, and safety system function blocks (See Figure 1).

Each FF function block is an encapsulation of inputs, outputs, parameters, variables, and processing algorithms appropriate to achieve a defined functionality.

The standard and safety system function blocks are each pre-configured and include the number and type of inputs, outputs, parameters, and variables and algorithm.

Two types of flexible function blocks are defined, free form and pre-configured.

As the name implies, free form blocks are fully user configurable.

Pre-configured flexible function blocks, such as multiple analog input, are similar to standard function blocks in as much as each has a defined number and type of inputs and outputs. The blocks flexibility is achieved by allowing the user to configure the blocks embedded algorithm.

Once created, all flexible function blocks are instantiated and connected to standard and/or other flexible function blocks to form a robust, deployable control solution. (See “Instantiation Basics” sidebar below.)

The question on many minds about now is, “If FF devices already support the CIF concept the S50 committee members felt so strongly about, why are we just beginning to hear about CIF’s use?”

Pioneering Has Ended
Actually, there are a significant number of CIF deployments, but a mere handful of end-users have openly shared their achievements.

Recently joining CIF pioneers from Bowater’s Kraft paper mill in Gatineau, Quebec, Canada; and International Specialty Products BDO plant in Lima, Ohio; is the Shanghai SECCO Petrochemical facility in the Shanghai Chemical Industrial Park located in the Shanghai Province of China.

Following a quantified examination of the risks and benefits SECCO determined that CIF provided significant benefits over the more traditional PAS implementation practices typically associated with DCS implementations. Among the benefits SECCO identified was that CIF reduced the impact on production of a control strategy failure; reduced fieldbus network communications; reduced PAS controller loading thus reducing the number of PAS controllers required; reduced system investment and startup costs; and provided the necessary device and process health information to implement effective predictive maintenance practices.

What makes the SECCO decision to adopt a field-centric architecture significant is that SECCO has about 47,000 control loops deployed, about a third of which reside in field devices. SECCO’s commitment to a field-centric architecture sends a clear indication that pioneering efforts have ended and that CIF is now a mainstream solution.

What’s also noteworthy is that as the number of CIF deployments has increased so too has the innovation and sophistication of these deployments.

Simple Elegance
The name of each standard function block is generally sufficient to understand the blocks functionality (e.g., Analog input, Integrator, PID control, Ratio control, Timer, etc.).

While several function blocks clearly indicate “control” capabilities, such as Discrete control, PID control, etc. Other blocks provide control strategy enhancements, such as Bias and gain, Lead/lag, and Setpoint ramp generator, while others support placing both simple and complex calculations in field devices. When appropriately combined, the standard function blocks offer users a rich set of control strategy development tools.

Using standard function blocks to develop a control strategy is as easy as selecting the appropriate blocks and then connecting inputs and outputs together. For example, Figure 2 below illustrates that to create a simple PID Control solution requires using three function blocks – analog input, PID control, and analog output.

Enhancing a control loop is equally as easy. For example, adding feedforward to a control loop requires another analog input block for the feedforward value and then connecting the blocks output to the PID control block’s FF_VAL connection.

Similarly, modifying figure 2 to form a cascade control strategy requires adding an analog input block, a PID control block, and the appropriate connection of the inputs and outputs.

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