DCS Control Strategies: PID, Cascade, Ratio and Feedforward Explained

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DCS and Automation

DCS Control Strategies: PID, Cascade, Ratio and Feedforward

A DCS can run several control strategies in parallel. PID handles most single-loop regulatory control. Cascade improves disturbance rejection by nesting two loops. Ratio maintains a proportional relationship between two flows. Feedforward corrects for a known disturbance before it reaches the controlled variable.

This guide covers all four with block diagrams, worked examples, and an interactive DCS control strategy selector.

PID Control Cascade Control Ratio Control Feedforward Control

These control strategies are not interchangeable. PID reacts after the process deviates. Feedforward acts before. Cascade tightens control against secondary disturbances. Ratio locks the relationship between two process streams. Each strategy solves a different problem.

DCS Control Strategies: Why One Loop Is Often Not Enough

Hello everyone! Today we are going to explore the four most important control strategies used in process plants -- PID, Cascade, Ratio, and Feedforward. A single PID loop handles many situations very well, but some processes have disturbances that arrive faster than a PID can react, or relationships between streams that must be kept in proportion. That is where the other strategies come in. Let us go through each one with real plant examples.

A basic PID loop measures one variable, compares it to a setpoint, and adjusts one output. That covers most regulatory control needs in a plant.

But some processes need more. A reactor with slow temperature response needs cascade control. A combustion air system tracking fuel flow uses ratio control.

A heat exchanger with variable inlet temperature needs feedforward control. Click any term to expand.

Cascade Control: Two PID controllers nested together. The outer (primary) controller measures the main process variable and sends a setpoint to the inner (secondary) controller. The inner controller measures a faster responding variable and manipulates the final element. Disturbances in the inner loop are corrected before they reach the outer loop.

Ratio Control: One flow (the wild flow) is measured but not controlled. A ratio station multiplies the wild flow by a set ratio value to produce the setpoint for a second flow controller. The second flow tracks the first in proportion. Used for fuel to air ratio in combustion, reactant blending, and dilution control.

Feedforward Control: A disturbance variable is measured directly. The DCS applies a correction to the manipulated variable based on the disturbance measurement, before the disturbance can affect the controlled variable. Feedforward does not close a feedback loop -- it is always combined with a PID feedback controller to handle unmeasured disturbances and model errors.

Core DCS control strategies covered: PID, Cascade, Ratio, Feedforward

Primary / Secondary

Cascade loop names -- secondary loop must be 3 to 10 times faster than primary

Wild Flow

The uncontrolled stream in ratio control that the controlled stream must track

Before

Feedforward acts before the disturbance reaches the controlled variable -- PID acts after

The 4 DCS Control Strategies Explained

1. PID Control -- The Foundation of DCS Regulatory Control

Feedback only Single loop, single output Default strategy for most loops

The PID controller compares the process variable (PV) to the setpoint (SP). The difference is the error (E). The three PID terms -- Proportional, Integral, and Derivative -- each react to the error and together produce the output to the final element (FE).

PID Output Equation

Output = Kp × E + Ki × ∫E dt + Kd × dE/dt

Kp: Proportional gain -- reacts to current error
Ki: Integral gain -- eliminates steady-state offset over time
Kd: Derivative gain -- reacts to rate of change, reduces overshoot
E: Error = Setpoint minus Process Variable

PID is correct for most single-input, single-output loops where the process responds predictably to the manipulated variable. It handles slow disturbances well.

Its limitation is response speed. PID can only react after the PV has already deviated from setpoint.

When a fast disturbance arrives -- for example, sudden feed composition change -- PID allows a large transient before correcting. See the PID controller types guide and the PID tuning guide.

2. Cascade Control -- Faster Disturbance Rejection

Two loops nested Primary sets SP for secondary Best for: slow primary + fast secondary

Cascade control nests two PID loops. The primary (outer) controller measures the main variable -- for example, reactor temperature. Its output becomes the setpoint for the secondary (inner) controller.

The secondary controller corrects disturbances in the inner loop before they reach the outer loop.

The key design rule: the secondary loop must respond 3 to 10 times faster than the primary loop. If both loops have similar response speeds, cascade adds complexity without benefit. See the cascade control guide for a full worked example.

Loop	Measures	Controls	Example
Primary (outer)	Reactor temperature (slow)	Setpoint of secondary	Temperature controller output = jacket flow SP
Secondary (inner)	Jacket flow (fast)	Flow control valve	Flow controller maintains jacket flow at the SP from primary

A disturbance in jacket supply pressure changes the flow immediately. The secondary loop corrects it in seconds. The primary temperature loop never sees the disturbance -- it has been corrected before it could affect the reactor temperature.

When commissioning cascade, always tune the secondary loop first with the primary in manual. Once the secondary is stable, switch the primary to auto. Never tune both loops simultaneously -- the interaction makes tuning impossible.

3. Ratio Control -- Proportional Stream Tracking

Controlled flow tracks wild flow Ratio station multiplies wild flow by R Best for: combustion, blending, dilution

Ratio control keeps two flows in a fixed proportion. One flow (the wild flow) is measured but not directly controlled.

The DCS multiplies the wild flow by the target ratio R to compute the setpoint for the controlled flow controller.

Ratio Control Calculation

Controlled Flow SP = Wild Flow × Ratio (R)

Wild Flow: measured stream that is not directly controlled (e.g. fuel gas)
Ratio R: operator-entered target ratio (e.g. 2.5 kg air per kg fuel)
Controlled Flow SP: setpoint fed into the second flow controller (e.g. combustion air)

The most common application is combustion air control. Fuel gas is the wild flow.

The DCS multiplies the fuel flow by the air to fuel ratio to compute the air flow setpoint. The air flow controller tracks it. See the ratio control guide.

Ratio control is also used for reactant blending (maintaining stoichiometric feed ratios), dilution water control (tracking product flow to maintain concentration), and mixing two streams to a target composition.

4. Feedforward Control -- Acting Before the Disturbance Arrives

Measures disturbance directly Corrects before PV deviates Always combined with PID feedback

Feedforward measures a disturbance variable (DV) and applies a correction to the manipulated variable before the disturbance affects the controlled variable.

It does not close a feedback loop -- it is an open loop correction added on top of a PID controller.

A heat exchanger outlet temperature is controlled by steam flow. The main disturbance is variation in the inlet flow.

A feedforward element measures the inlet flow and corrects the steam valve position when the inlet changes. The feedback PID handles the remaining error.

Feedforward plus Feedback Output

Valve Output = PID Output + Feedforward Correction

PID Output: feedback correction based on current temperature error
Feedforward Correction: open loop correction based on measured disturbance (e.g. inlet flow change)
Both signals sum to give the final valve position

Feedforward requires a model of how the disturbance affects the process. The simplest model is a gain (ratio): if inlet flow increases by 10%, increase steam by 8%. More complete models include dead time and lead lag compensation to match the process dynamics.

Feedforward never works perfectly -- process models are never exact. That is why the PID feedback loop is always kept in place to correct the residual error. The combination of feedforward plus feedback is the tightest achievable control with conventional DCS control strategies.

Feedforward adds complexity. Only implement it when the disturbance is measurable, significant, and faster than the PID feedback loop can handle. A poorly tuned feedforward element can make control worse than a well-tuned PID alone.

DCS Control Strategy Selector

Which DCS Control Strategy Is Right for Your Loop?

Select your process situation for a recommendation

Primary problem

Secondary constraint

Comparing All 4 Control Strategies

Strategy	How it works	Acts when	Best for	Limitation
PID	Reacts to error (SP minus PV)	After PV deviates	Most single loops, slow disturbances	Cannot anticipate fast disturbances
Cascade	Outer loop sets SP for inner loop	After secondary PV deviates	Slow primary with fast secondary variable	Secondary must be 3 to 10 times faster than primary
Ratio	Controlled flow = Wild flow times R	Every scan, tracking wild flow	Combustion, blending, dilution	Does not correct for composition -- flow ratio only
Feedforward	Corrects MV based on measured disturbance	Before PV is affected	Fast, measurable, significant disturbances	Needs a process model; never standalone

Real Plant Applications of Each Strategy

Boiler Drum Level (Three Element Control)

A classic combination of control strategies. Feedforward measures steam demand. Cascade controls feedwater flow as the inner loop, with drum level PID as the outer loop.

Together they handle the shrink and swell effect. See DCS function block programming for implementation.

Reactor Temperature Control

Outer loop: reactor temperature. Inner loop: cooling water flow or jacket temperature.

Cascade is ideal here -- the jacket responds in seconds while the reactor changes over minutes. A cooling water pressure disturbance is corrected by the inner loop without the reactor ever seeing it.

Combustion Air Control

Ratio control tracks the fuel gas flow and generates the combustion air setpoint. The ratio R is set by the operator. See the ratio control guide.

Heat Exchanger Outlet Temperature

Feedforward measures the inlet flow. When flow increases, the DCS immediately increases steam flow before temperature drops.

Without feedforward, PID alone allows a temperature dip on every rate change. The DCS historian records transients to validate the feedforward benefit.

Watch: Cascade Control: DCS Loop Nesting Explained

DCS Control Strategies Questions

When should I use cascade instead of a single PID loop?

Use cascade when a slow primary variable has a faster secondary variable. The secondary loop must respond at least 3 times faster than the primary for cascade to help.

Can feedforward control work without a PID feedback loop?

No. Feedforward cannot correct for model errors or unmeasured disturbances. It is always combined with PID feedback -- feedforward handles the measured disturbance, PID handles the rest.

What is the wild flow in ratio control?

The wild flow is measured but not controlled. The controlled stream tracks it in proportion. In combustion, fuel gas is the wild flow and combustion air is the controlled stream.

How are DCS control strategies implemented in a DCS?

Using Function Block Diagram (FBD) programming. Each strategy is a wired arrangement of standard function blocks -- PID blocks, ratio stations, lead lag blocks, and summing junctions. See the DCS function block programming guide.

Which DCS control strategy is used most often?

PID is the most widely used DCS control strategy. Most plant loops run as simple PID feedback. Cascade, ratio, and feedforward are used selectively where the process demands tighter control.

External References

What We Learn Today

PID is the foundation strategy -- reacts to error after the PV deviates from setpoint
Cascade nests two loops -- inner loop must be 3 to 10 times faster than outer; tune inner first
Ratio control makes the controlled flow track the wild flow in proportion: SP = Wild Flow times R
Feedforward corrects for a measured disturbance before the PV is affected -- always combined with PID
Three element boiler drum level control combines all three advanced strategies at once
Selecting the right DCS control strategy depends on the disturbance type, speed, and measurability

“PID waits for the process to tell it something is wrong. Feedforward acts before the process knows anything has changed. The best control uses both.”

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