Technical Notes - ControlSoft https://www.controlsoftinc.com Reliable Process Control Optimization Tue, 16 Sep 2025 20:21:16 +0000 en-US hourly 1 https://wordpress.org/?v=7.1.2 Tuning Cascade Loops https://www.controlsoftinc.com/tuning-cascade-loops-2/ Thu, 11 Sep 2025 19:41:57 +0000 https://www.controlsoftinc.com/?p=14203

Tuning Cascade Loops

Cascade control is a widely used strategy for managing systems affected by both fast and slow disturbances. While a standard PID controller can be tuned to handle slower disturbances effectively, it often struggles to respond adequately to rapid changes. By implementing a cascade control structure—where one controller manages a secondary, faster loop nested within a primary, slower loop—the system can respond more efficiently to both types of disturbances, improving overall performance and stability.

One of the most common questions we get asked is, “I have a cascade loop; how do I tune it?” Typically, it is not the tuning of the PID loop that is the issue, but rather a lack of understanding of how the cascade structure is designed to control a loop. The tuning of a cascade loop is quite simple once you understand the basic concepts. To illustrate the basics, let’s look at a common cascade loop application.

Figure 1 shows a tank that is heated via a steam valve. The goal is to control the outlet temperature [T1] via the steam valve. We can attach a standard PID controller to this loop to control the temperature, as shown.

Figure 1. Standard PID Control of Tank Outlet Temperature

PID Control of Tank Outlet Temperature

Handling Disturbances

Two of the many types of disturbances that can affect this process are:

  • Inlet Disturbances – Fluctuations in the infeed flow or infeed temperature can cause disturbances to T1.
  • Supply Flow Disturbances – Fluctuations in the steam supply pressure (detected more quickly in F2 and T2) cause flow to fluctuate and thereby cause a disturbance to T1. The PID can only be tuned as fast as the process will allow. There is a significant amount of time (both deadtime and lag time) in the process.

In this process, the changes in steam pressure cause the steam flow to fluctuate, and the controller will not detect such pressure variation until it affects the temperature. The overall process is very slow. A common way to resolve this problem is to use a cascaded PID configuration as shown in Figure 2.

Figure 2. Cascaded PID Control of Tank Outlet Temperature

Cascaded PID Control of Tank Outlet Temperature

PID 1 can be referred to as the Primary or Outer Loop. PID 2 can be referred to as the Secondary or Inner Loop. Essentially, what we did was to take our process response and split it into two pieces – a fast piece [G2(s)] and a slower piece [G1(s)].

  • Note that we still only have one control Instead of manipulating the steam valve directly for temperature control, the valve is now controlling the flow of the steam to the process from PID 2.
  • The temperature controller (PID 1) now determines the desired amount (the setpoint) of flow to control the temperature.

Cascade Control Considerations

  • The inner loop (PID 2) must be faster than the outer loop (PID 1) for cascade to be of benefit. If the inner loop is not faster than the outer loop, then the cascade will not offer any significant improvement in the process.
  • PID 2 can be tuned relatively fast and can respond to flow disturbances.
  • PID 2 will minimize any fluctuation disturbances in the steam supply.
  • PID 1 still controls temperature, and is tuned relative to the temperature process, which is slower than the flow process.
  • Any disturbances within the inner (fast) loop can be corrected (by PID 2) without waiting for the disturbance to show up in the temperature loop (PID 1) for better (tighter and faster) temperature control.
  • The cascade configuration requires tuning of two PIDs instead of the one PID as in our original configuration.

Figure 3. Outline of PID Control compared to Cascaded PID Control

Cascade Configuration Considerations

  • In looking at Figure 3 we notice that the CO output of PID 1 is fed directly into the setpoint of PID PID 1 is dependent on PID 2 being in control to control the temperature.
  • In our control logic, we should ensure that PID 1 is in track mode if PID 2 is in manual control.
  • Also, when PID 2 saturates, it is important to inhibit PID 1 (that is, to stop PID 1 from increasing or decreasing). If PID 1 is not inhibited, controller windup will occur. Therefore, it requires proper engineering to implement cascade control correctly.
  • Some controller manufacturers have a Cascade PID block that is designed to handle the coordination logic required to run the cascade correctly.

Summary

The simple answer to our question “I have a cascade loop; how do I tune it?” is:

  1. Tune PID 2 before tuning PID 1. Often, a simple proportional plus integral (PI) control is sufficient.
  2. PID 2 needs to be in control (typically AUTO mode) when tuning PID 1.

For additional assistance with tuning cascade loops, consider using the One-Shot Cascade tuning feature in INTUNE PID Tuning Tools, available from ControlSoft, Inc.

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[This Tuning Cascade Loops technical note has been revised for comprehensiveness.]

Need assistance fine-tuning your cascade loops? Reach out to us for expert guidance.

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]]> Top 6 Questions to Answer to Reduce Process Variability eBook https://www.controlsoftinc.com/top-6-questions-to-answer-to-reduce-process-variability-ebook/ Mon, 28 Oct 2024 20:40:18 +0000 https://www.controlsoftinc.com/?p=13772 Top 6 Questions to Answer to Reduce Process Variability There are times when a process variable (PV) shows an unacceptable level of variability around its setpoint (SP) while being regulated […]

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]]> Top 6 Questions to Answer to Reduce Process Variability

There are times when a process variable (PV) shows an unacceptable level of variability around its setpoint (SP) while being regulated by a PID controller. In order to improve the quality of regulation of the process variable, there are six important questions to consider.

In this eBook we provide the six questions along with subsequent observations to help you in answering the questions.

Download the eBook
Need more help with process control optimization?
We offer in-person and remote Engineering Services for additional guidance. Learn more >

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]]> Tuning Rules for PI and PID https://www.controlsoftinc.com/tuning-rules-for-pi-and-pid/ Wed, 28 Aug 2024 15:48:24 +0000 https://www.controlsoftinc.com/?p=13741 Tuning Rules for PI and PID Determining Which Controller to Use A key challenge in setting up a control system is determining what type of controller to use. The PID […]

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]]> Tuning Rules for PI and PID

Determining Which Controller to Use

A key challenge in setting up a control system is determining what type of controller to use. The PID controller is generally accepted as the standard for process control, but the PI controller is sometimes a suitable alternative. A PI controller is the equivalent of a PID controller with its D (derivative) term set to zero.

It is important to understand how controllers interact with each different type of process. For details, see Table 1.

Table 1 – How PI and PID Controllers Interact with Different Kinds of Processes

Controller Pl Controller PID Controller
Effective for These Processes Fast processes, such as flow, pressure, and some temperature loops. Slower processes, such as level and insulated temperature.
Tuning Parameters P = Proportional
I= IntegralPl controller is the equivalent of a PID controller with its D (derivative) term set to zero.		 P = Proportional
I= Integral
D = DerivativeThe derivative term is particularly important for integrating processes, such as level, position, & well-insulated temperature.In general, using a derivative term can significantly increase the speed of the response of a non-integrating process and suppresses overshoot.
Response Speed Response is slower, thus enabling a smooth and accurate PV change. Response is faster, thus enabling setpoint to be reached more quickly.
Overshoot Overshoot will likely occur. Reduced or no overshoot.
Download the data sheet

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]]> Derivative Tuning for PID Control https://www.controlsoftinc.com/derivative-tuning-for-pid-control/ Thu, 01 Jun 2023 18:18:17 +0000 https://www.controlsoftinc.com/?p=11439

Derivative Tuning for PID Control

Of the three letters in PID the D, Derivative Tuning, is probably the most misunderstood and certainly the least used! It is well-known that Derivative Tuning is largely unnecessary for fast loops (such as flows and pressures) due to those loops’ naturally quick response time. However, derivative tuning is extremely useful for particularly slow loops (such as temperature) and is an absolute must-have for integrating processes (such as level and insulated temperature loops).

Why is Derivative Tuning Important?

The answer is in the predictive action D provides. When you drive a car and approach a stop sign, you need to apply the brake. But how much brake should you apply? Among other things, the answer depends on how fast you are going and how close you are to the stop sign. The closer you are and the faster you are going, the more brake you need in order to reach the stop sign safely. Applying too much brake or applying it too early means you will not reach the stop sign. Applying too little brake or applying it late means you will travel past the stop sign.

It’s the same in process control. Derivative action is a predictive brake for the controller. We want to use the right amount of D (the brake) so that we get to setpoint quickly (the stop sign) without overshooting (traveling past the goal) or undershooting (stopping short of the goal). And as anyone performing loop tuning can testify, getting the right amount of D is often a difficult task.

To help understand derivative controller action, let’s examine two common misuses of the D term. To aid our study, we’ll examine the effects of the D term on a 3rd order integrating process with a gain rate of 0.34, a deadtime of 40 seconds, a lagtime of approximately 90 seconds, and some higher order dynamics as well. In the following examples, the PID controller uses a non-interacting equation when performing its calculations.

NOTE: Given the integrating nature of this process, PID control in these examples was used with an extremely weak integral term. This essentially created a PD-only tuning situation since the integrating (I) action is naturally built into the process dynamics.

Misuse Example #1: Derivative Tuning Action Too Strong

Figure 1 shows the reaction of our process to a PID controller with a Derivative tuning setting that is too strong.

Figure 1. P = 0.6, D = 100. Non-interacting (ISA) equation.

Derivative Tuning is Too Strong

As the trend indicates, so much braking force is applied to the process that our derivative action actually hinders the PV from reaching setpoint. Not only that, but because the derivative action is so aggressive, it is preemptively acting on what it predicts will soon be a negative SP-PV error (though the actual error remains positive). This preemptive action on a predicted negative error is what causes the saw-tooth pattern in the PV and the CO.

Misuse Example #2: Derivative Tuning Action Too Weak

Figure 2 shows the reaction of our process to a PID controller with a Derivative tuning setting that is too weak.

Figure 2. P = 0.6, D = 5. Non-interacting (ISA) equation.

Derivative Tuning is Too Weak

As the trend indicates, so little brake is applied to the process that our PV far overshoots its goal. For this process, product quality is significantly affected due to poor derivative tuning, and this particular company spends far more on natural gas to heat this loop in startup than is necessary. Multiplied out over similar loops in the rest of the plant, this is a significant energy and product resource waste—all from a poor use of derivative action in PID control.

Correct Use Example #3: Derivative Tuning Action Just Right

Figure 3 shows the reaction of our process to a PID controller with a Derivative tuning setting that is just right.

Figure 3. P = 0.6, D = 50. Non-interacting (ISA) equation.

Derivative Tuning is Well Tuned

Improving Process Control Performance with PID Loop Tuning Software

In an ideal world, we could easily estimate the right amount of derivative action for a process. Of course, we don’t live in an ideal world and trying to understand the proper amount of D to use can be a long and complicated trial-and-error process, especially if you are tuning slower temperature and level loops.

What if you could have a PID loop professional monitor your process 24 hours a day, 7 days a week and continuously make recommendations about how to adjust your P, I, and D for increasingly better control?

ControlSoft’s Advisory AdaptTune technology, available in INTUNE PID Loop Tuning Tools, provides that exact service.

In a recent test, the INTUNE AdaptTune function was allowed to monitor a difficult-to-tune, slow-responding temperature loop that was taking days to tune by hand. Even though it was understood that strong PD control was the proper approach to tuning this loop, the operator could not find the right combination of P and D.

Over the course of just several hours (not days), AdaptTune made several minor recommendations on the P term and several suggested adjustments for the D term to bring the loop under increasingly better control. While in Auto mode, the setpoint was stepped up and down at half-hour intervals to test the response of the new tuning recommendations. The steps were within a healthy operating range for the process.

Figure 4. AdaptTune first and second recommendations, indicated by red arrows.

AdaptTune First and Second Recommendation

Figure 5. AdaptTune third recommendation, indicated by red arrow.

AdaptTune Third Recommendation

Figure 6. AdaptTune final recommendation, time indicated by red arrow.

AdaptTune Final Tuning Recommendation

Table 1 shows the recommendations AdaptTune used to bring the loop under control.

Table 1. INTUNE AdaptTune Recommendations.

Initial1st Iteration 2nd Iteration3rd Iteration4th Iteration
P0.76 0.680.450.360.32
I3000 3000 3000 3000 3000
D1012182733

By gradually reducing the Proportional action and strongly increasing the Derivative action, AdaptTune was able to tame one of the operator’s toughest, slowest, and most problematic control loops.

So, Don’t Deny the D. If properly used, derivative control can enhance your controller response for integrating processes and particularly slow loops. Use it with confidence.

Final Word of Caution: Derivative Action Applied to a Noisy Process

As useful as the D term is, and as liberally as it can be applied in the case of slow, integrating processes, Figures 7a and 7b show why it is better to avoid a lot of D if you are dealing with an inherently noisy process.

Figure 7a. Response of PV to a noisy process with D term included in the tuning.

Noisy Process with Derivative

Figure 7b. Response of PV to a noisy process without D term included in the tuning.

Noisy Process without Derivative

To help alleviate this problem, a maintenance team should check the sensors, valves, and other hardware to make sure you are receiving the clearest, most accurate information possible.

You also want to make sure that what appears to be noise is not really a disturbance issue coming from some other aspect of the process upstream.

[This article was first published in 2007 and has been revised for comprehensiveness.]

Stilll Unsure How to Use the D? Contact Us For a Quick Consult.

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