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Flow Control

Introduction

When comparing the 4 major process variables: temperature, pressure, flow, and level, flow is probably the least difficult to control but has the largest number of loops because a continuous process is about moving material flows from the beginning of the process to the end.

Typically, the flow loop is controlled by manipulating a valve, a variable frequency drive (VFD), or a pump.

Why Flow Control Can Be Difficult

It is relatively easy to control a flow loop because it is naturally a first-order process with a small delay time. Since the flow loops are the foundation of a process control system (for instance, it is typically used as the inner loop of a cascade control system), their control performance is critical. Namely, how fast the PV reaches SP, whether there is an overshoot, how fast the overshoot damps out, and how fast PV can settle within a desired range can all be the quality criteria for the flow loop.

In flow control, the commonly used actuators are all nonlinear components by nature.

1. A control valve is almost never a linear component. It can have a concave, convex, or S-shaped nonlinear relationship between its input and output. Some of them even have hysterisis behavior that makes the problem much worse.

2. A variable frequency drive (VFD) saves energy but is naturally a nonlinear device.

3. A flow pump driven by a pulse-width modulator (PWM) based on the duty cycles does not necessarily have a linear relationship with the flow.

In addition, the inevitable wear and tear of the actuator can make the nonlinear behavior worse. Therefore, the challenge for flow control is mainly how to control a nonlinear process with stringent control performance requirements.

MFA Control Solution

The Nonlinear MFA controller is well suited to control flow for the following reasons:

1. It is a general-purpose controller that provides a more uniform solution to nonlinear control problems.
2. It requires neither process models nor nonlinear characterization.
3. It can be easily configured with the parameters listed in Table 1.

Table 1. Nonlinear MFA as a Flow Controller

Nonlinear MFA Parameter Description Default Value Fast Flow
Ts Sample interval (seconds) 0.3 seconds 0.03 seconds
Kc MFA controller gain 1.0 1.0
Tc Process time constant (seconds) 3 seconds 0.3 seconds
Linearity Factor Defines how nonlinear the process is
(0 – linear, 10 – extremely nonlinear)
5 5
SP Range (%) Defines the range for nonlinear control
(0% to 100%)
5 5

As illustrated in the following configuration menu, when using Nonlinear MFA, the specific characteristics of the nonlinear curve are not a concern. The curve can be concave, convex, or S-shaped. Simply advise the controller whether the process is extremely nonlinear (enter a 9 or 10), quite nonlinear (enter a 5 or 6), or somewhat nonlinear (enter an 1 or 2). The Nonlinear MFA is smart enough to handle the rest.

Simulations and real applications show that Nonlinear MFA can tightly control various nonlinear flow loops in the full control range, even when the process gain changes hundreds of times.

Summary

Based on the core MFA control method, various MFA controllers have been developed to solve specific control problems. This applies to flow control applications as well. The Nonlinear MFA is simply the most effective and user-friendly flow controller on the market that is the best candidate to become the next generation mainstream flow controller.

Case Studies

To read more about implementations of CyboSoft’s MFA flow control solutions, click on the following case studies:

Model-Free Adaptive Control of Steam Drum Level

MFA Control and Optimization of Gas Mixing Process

 

 

 
     
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