# LECTURE#09 PROCESS CONTROL STRATEGIES

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LECTURE#09 PROCESS CONTROL STRATEGIES
AUTOMATION & ROBOTICS LECTURE#09 PROCESS CONTROL STRATEGIES By: Engr. Irfan Ahmed Halepoto Assistant Professor 1

PROCESS CONTROL STRATEGIES
Two position control (On-Off Controller) Ratio control Multiple position control Cascade Control Feed Forward Plus Feedback Control (Hybrid)

ON /OFF CONTROL SYSTEM On/off control activates an output until the measured value reaches the reference value. A common example is the household thermostat. No control action takes place until the measured value deviates from the setpoint by a minimum amount (dead band). The output then goes from full off to full on, turning off again when the setpoint is reached.

Typical ON/OFF Response
The input to output characteristic waveform for a two position controller is a step function. The controller switches from its "OFF" state to its "ON" state when the error signal (set point - measured variable) becomes positive.

ON/OFF Control Action: Heater Control
If the process value is lower than the set point, output will be turned ON and power will be supplied to the heater. If the process value is higher than the set point, output will be turned OFF and power to the heater will be shut off. This control method, in which the output is turned ON and OFF based on the set point in order to keep the temperature constant, known as ON/OFF control action. With this action, the temperature is controlled using two values (i.e., 0% and 100% of the set point), therefore, the operation is also called two-position control action.

Heater ON/OFF Control Action Mechanism

On-Off Control: Tank Level control Example
Level control in a water tank can be as simple as on-off. Water level is the measured and regard as controlled variable. Inlet water flow rate is the manipulated variable.

On/off temperature control of water in a tank
thermostat On/off switching action On/off temperature control of water in a tank Tank temperature versus time

Ratio Control Systems Ratio control systems are installed to maintain the relationship b/w two variables to control a third variable. Ratio control systems are the elementary form of feedforward control. Ratio control is applied almost exclusively to flows, known as wild flow. Wild flow can be uncontrolled, controlled independently, or controlled by another controller that responds to variables of pressure, level, etc.. Objective of a ratio control scheme is to keep the ratio of two variables at a specified value, so ratio (R) of two variables (A & B), R=A/B is controlled rather than controlling the individual variables. Note: A (disturbance) and B (manipulated) are physical variables, not deviation variables.

Ratio Control Architecture
Ratio control reduce the effects of variations in the feed flow rates. Flow rate for stream A is a disturbance, referred to as the “wild” stream. The controller takes measurements of the disturbance, stream A, and then applies ratio control to immediately bring about the appropriate change in the flow rate of stream B, the manipulated stream. The output and the manipulated variable of the controller is, therefore, a ratio, as opposed to a single variable. The controller takes a measurement of the flow rate of “A”, enacts ratio control, and immediately sets a new flow rate for “B. Block diagram of a ratio control loop

Ratio Control: Method I
The flow rate of the two streams is measured and their ratio calculated using a 'divider' (just a piece of extra electronics). The output of the divider is sent to the ratio controller (which is actually a standard PI controller). The controller compares the actual ratio with that of the desired ratio and computes any necessary change in the manipulated variable

Ratio Control: Method II
one stream is under standard feedback control. The flow of the second stream is measured and sent to a 'multiplier' (again just a piece of extra electronics) which multiplies the signal by the desired ratio yielding the setpoint for the feedback control law.

Ratio Control Conceptual Diagram
Conceptual diagram shows that the flow rate of one of the streams feeding the mixed flow, designated as the wild feed, can change freely based on maintenance options, product demand,energy availability, the actions of another controller in the plant. The other stream shown feeding the mixed flow is designated as the controlled feed. A final control element (FCE) in the controlled feed stream receives and reacts to the controller output signal, COc, from the ratio control architecture. Note: other flow manipulation devices such as variable speed pumps or compressors may also be used in ratio control implementations. Ratio Control Conceptual Diagram

Relays in the Ratio Architecture
As the conceptual diagram illustrates, we measure the flow rate of the wild feed and pass the signal to a relay, designated as RY in the diagram. The relay is typically one of two types: Ratio relay: where the mix ratio is entered once during configuration and is not accessible for change during normal operation. Multiplying relay: where the mix ratio is presented as an adjustable parameter on the operations display and is thus readily accessible for change.  In either case, the relay multiplies the measured flow rate of the wild feed stream (PVw), by the entered mix ratio to arrive at a desired or set point value (SPc),for the controlled feed stream. A flow controller then regulates the controlled feed flow rate to this set point value (SPc), resulting in a mixed flow stream of specified proportions between the controlled and wild streams.

Ratio controllers: Linear Flow Signals Required
A ratio controller architecture requires that the signal from each flow sensor/transmitter change linearly with flow rate. Thus, the signals from the wild stream process variable, (PVw), and the controlled stream process variable (PVc), should increase and decrease in a straight-line fashion as the individual flow rates increase and decrease. In case of any abnormality, additional computations (function blocks) must then be included between the sensor and the ratio relay to transform the nonlinear signal into the required linear flow-to-signal relationship. Turbine flow meters and certain other sensors can provide a signal that changes linearly with flow rate.

Flow Fraction (Ratio) Controller
Instead of using a relay, an alternative ratio control architecture based on a flow fraction controller (FFC) can also be used. The FFC is essentially a "pure" ratio controller in that it receives the wild feed and controlled feed signals directly as inputs. Ratio set point value is entered into the FCC, along with tuning parameters and other values required for any controller implementation.

MULTI-POSITION CONTROLLERS
Two popular control strategies for improved disturbance rejection performance are cascade control and feed forward with feedback trim. Improved performance comes at a price. Both strategies require that additional instrumentation be purchased, installed and maintained. Both also require additional engineering time for strategy design, tuning and implementation. The cascade architecture offers attractive additional benefits such as the ability to address multiple disturbances to our process and to improve set point response performance. In contrast, feed forward with feedback trim architecture is designed to address a single measured disturbance and does not impact set point response performance in any fashion.

Cascade Control Cascade control is widely used within the process industries. Cascade control is used to improve the response of a single feedback strategy. The idea is similar to that of feedforward control: to take corrective action in response to disturbance variable (DV) before the CV deviates from setpoint. Cascade control schemes have two distinct features: There are two nested feedback control loops. There is a secondary control loop located inside a primary control loop. The primary loop controller is used to calculate the setpoint for the inner (secondary) control loop. The secondary control loop is located so that it recognises the upset condition sooner than the primary loop.

The Inner Secondary Loop The dashed line in the block diagram, circles a feedback control loop. Here "inner secondary" have been added to the block descriptions. Variable labels also have a "2" (secondary) after them.

To construct a cascade architecture, we nest the secondary control loop inside a primary loop as shown in the block diagram. Note that outer primary PV1 is our process variable of interest in this implementation. PV1 is the variable we would be measuring and controlling if we had chosen a traditional single loop architecture instead of a cascade.

Measurement and Control of an "early warning" process variable is essential element for success in a cascade design.

In the cascade architecture, inner secondary PV2 serves as a early warning process variable. Essential design characteristics for selecting PV2 include that: it be measurable with a sensor, same FCE (valve) used to manipulate PV1 also manipulates PV2,  the same disturbances that are of concern for PV1 also disrupt PV2, PV2 responds before PV1 to disturbances of concern and to FCE manipulations. Since PV2 sees the disruption first, it provides our "early warning" that a disturbance has occurred and is heading toward PV1. The inner secondary controller can begin corrective action immediately. Since PV2 responds first to final control element (e.g., valve) manipulations, disturbance rejection can be well underway even before primary variable PV1 has been substantially impacted by the disturbance. With such a cascade architecture, the control of the outer primary process variable PV1 benefits from the corrective actions applied to the upstream early warning measurement PV2.

Outer Disturbance must impact Early Warning Variable PV2
With a cascade structure, there will likely be disturbances that impact PV1 but do not impact early warning variable PV2. The inner secondary controller offers no "early action" benefit for these outer disturbances. They are ultimately addressed by the outer primary controller as the disturbance moves PV1 from set point. So, a proper cascade can improve rejection performance for any of a host of disturbances that directly impact PV2 before disrupting PV1.

A level-to-flow cascade structure includes: Two controllers: the outer primary level controller (LC) and inner secondary feed flow controller (FC) Two measured process variable sensors: the outer primary liquid level (PV1) and inner secondary feed flow rate (PV2) One final control element (FCE): the valve in the liquid feed stream. As required for a successful design, the inner secondary flow control loop is nested inside the primary outer level control loop. That is: The feed flow rate (PV2) responds before the tank level (PV1) when header pressure disturbs the process or when the feed valve moves. The output of the primary controller, CO1, is wired such that it becomes the set point of the secondary controller, SP2. Ultimately, level measurement, PV1, is our process variable of primary concern. Protecting PV1 from header pressure disturbances is the goal of the cascade.

Feed Forward & Feedback Control
The feed forward with feedback architecture is constructed by coupling a feed-forward-only controller to a traditional feedback controller. The feed forward controller seeks to reject the impact of one specific disturbance (D), that is measured before it reaches our primary process variable, PV, and starts its disruption to stable operation. Typically, this disturbance is one that has been identified as causing repeated and costly upsets, thus justifying the expense of both installing a sensor to measure it, and developing and implementing the feed forward computation element to counteract it.

Feed Forward with Feedback Trim Architecture

Combined Feed forward & Feedback Controllers
Combinations of feedback and feedforward control give us : Benefits of feedback control: controlling unknown disturbances and not having to know exactly how a system will respond Benefits of feedforward control: responding to disturbances before they can affect the system

Comparison of Feedback & Feedforward Control
Feedback (FB) Control Advantages Corrective action occurs regardless of the source and type of disturbances. Requires little knowledge about the process (process model is not necessary). Versatile and robust (Conditions change? May have to re-tune controller). Disadvantages FB control takes no corrective action until a deviation in the controlled variable occurs. FB control is incapable of correcting a deviation from set point at the time of its detection. Theoretically not capable of achieving “perfect control.” For frequent and severe disturbances, process may not settle out.

Comparison of Feedback & Feedforward Control
Feedforward (FF) Control Advantages: Takes corrective action before the process is upset. Theoretically capable of "perfect control“ Does not affect system stability Disadvantages: Disturbance must be measured (capital, operating costs) Requires more knowledge of the process to be controlled (process model) Ideal controllers that result in "perfect control”: may be physically unrealizable. Use practical controllers such as lead-lag units