LECTURE#08 PROCESS CONTROL STRATEGIES AUTOMATION & ROBOTICS LECTURE#08 PROCESS CONTROL STRATEGIES By: Engr. Irfan Ahmed Halepoto Assistant Professor 1

PROCESS CONTROL STRATEGIES Manual Process control Feedforward control (open-loop control) Automatic Process control Feedback control (closed-loop control) Cascade Control Ratio Control PID Control

PROCESS CONTROL STRATEGIES Feedforward Control: Reacting to the disturbance before the error occurs Feedback Control: Control action after an error exists Cascade Control: used to improve the response of a feedback loop, based on feedforward control strategy. Ratio Control: keep the ratio of two variables (mostly as a percentage) at a specified value. PID Control: Based on three different control mechanism, as Proportional, Integral and Derivative to compensate the process errors.

Open loop control An open-loop control (non-feedback) system utilizes a controller or control actuator in order to obtain the desired response. Example: electric heater The plant is the thermodynamic behavior of the room. our desired process response is a comfortable temperature in the room. Controller is the person in the room who switches the heater on when he feels chilly. The actuator is the heater itself. If there is no intervention once the fire has been switched on, then the room temperature will eventually settle at a constant temperature, which is dependent on two things: The amount of heat generated by the fire The heat losses from the room

Closed loop control Closed-loop control system utilizes an additional measure of the actual output, to compare the actual output with the desired output response. Closed-loop control system tends to maintain a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control. In the case of the driver steering an automobile, driver uses his/her sight to visually measure and compare the actual location of the car with the desired location. The driver then serves as the controller, turning the steering wheel. The process represents the dynamics of the steering mechanism and the automobile response.

TYPES OF CONTROL SYSTEM Two types of control systems: Regulatory control: In some processes the controlled variable deviated from the set point because of disturbances, regulatory control refers to systems designed to compensate for these disturbances. Servo control: In some processes, the most important disturbance is the set point itself. That is, the set point may be changed as a function of time. Servo control refers to control systems designed for this purpose In the process industries, regulatory control is far more common that servo control.

MANUAL CONTROL SYSTEM To begin with the shower is cold. To start the heating process, the valve in the hot water line is opened. The operator can then determine the effectiveness of the control process by standing in the shower. If the water is too hot, the valve should be closed a little or even turned off. If the water is not hot enough then the valve is left open or opened wider.

Functions of a Manual Control System Manual control system, completed by the operator, possesses the following functions: Measurement: This is essentially an estimate or evaluation of the process being controlled by the system. This is achieved by the right hand of the operator. Comparison: This is an examination of the likeness of the measured values and the desired values. This is carried out in the brain of the operator. Computation: This is a calculated judgment that indicates how much the measured value and the desired values differ and what action and how much should be taken. Here, operator will calculate the difference between the desired temperature and the actual one. Correction: This is ultimately the materialization of the order for the adjustment. The left hand of the operator takes the necessary actions following the order from brain.

Manual Process Control System Purpose: To heat the process fluid from inlet temperature, Ti(t), up to a desired outlet temperature, T(t). Here many variables can change, causing outlet temperature to deviate from its desired value. So some action must be taken to correct for this deviation. If, outlet temperature T(t) is below its desired value, steam valve could be opened more to increase the steam flow to the heat exchanger. If, outlet temperature T(t) is above its desired value, steam valve can be throttled back to cut the steam flow(energy) to the heat exchanger; Heat exchanger

Disadvantages of manual process control Operator should look at the temperature frequently to take corrective action whenever it deviates its desired value. Different operators would make different decisions as to how to move the steam valve, resulting in inconsistent operation. This corrective procedure would require a large number of operators. So, We would like to accomplish this control automatically. Without requiring intervention from the operator. Alternatively , a suitable control mechanism or technique may be used to compensate error.

Control Strategies : Feedforward control The feedforward control technique is a very common control strategy in the process industries. It is the simplicity that accounts for its popularity. The objective of feedforward control is to measure the disturbances and compensate for them before the controlled variable deviates from the set point. If applied correctly, the controlled variable deviation would be minimum.

Control Strategies : Feedforward control In Feedforward control, a sensor or measuring device is used to directly measure the disturbance as it enters the process and the sensor transmits this information to the feedforward controller. Feedforward controller determines the required change in the manipulated variable (MV), so that, when the effect of the disturbance is combined with the effect of the change in the MV, there will be no change in controlled variable at all. Controlled variable is always kept at its setpoint and hence disturbances have no effect on the process.

Feedforward control :Example Suppose that “major” disturbance is the inlet temperature Ti(t). To implement feedforward control, this disturbance must first be measured and then a decision made as to manipulate the steam valve to compensate for them.

Feedforward control Model Feedforward element is constructed from models of process and disturbance. CO= controller output signal D= measured disturbance variable e(t) = controller error, SP – PV FCE = final control element (e.g., valve, variable speed pump or compressor) PV = measured process variable SP = set point

Advantage of feedforward control It has the characteristic of forward control So, if we use this strategy correctly, the controlled variable will not deviate set point.

Disadvantage of feedforward control Feedforward control cannot compensate for all disturbances that enter the process The feedforward control system can compensate only one of disturbances. If any of the other disturbances enter the process, this strategy will not compensate for it, and the result will be a permanent deviation from set point of the controlled variable. In pure feedforward control, there is no monitoring on the controlled variable. If the controlled variable strays from its setpoint there is no corrective action to eliminate the error. This makes pure feedforward control somewhat impractical and a rarity in typical process application

Feedforward control system :Applications Washing machine Oven Microwave oven Air conditioner

AUTOMATIC CONTROL SYSTEM A temperature measurement device used to measure the water temperature, which replaces right hand of the operator, resulting in improved accuracy. Instead of manual valves, special kind of control valve can be used, which is driven by compressed air or electricity. This will replace the left hand of the operator. A controller (temperature controller) used to replace the brain of the operator. functions of comparison & computation and can give orders to the control valve. Signal and order connections between the measurement device, control valve & controller are transferred through cables & wires, which replace nerve system in the operator.

Automatic Process Control Measure the temperature of the process stream by a sensor (thermocouple, resistance temperature device, thermisters,etc) Transmitter transmits the signal to the controller Controller compare the signal to the desired value, and decides what to do to maintain the temperature at its desired value. Controller sends a signal to the final control element to manipulate the steam flow.

Three basic operations Components of Automatic process control Three basic operations Three components Sensor/transmitter: Primary and secondary elements. Controller: brain of the control system. Final control element: control valve, but not always. variable-speed pumps, conveyors and electric motors Three Operations Measurement (M): Measuring the variable to be controlled Decision (D): Based on the measurement, the controller decides what to do to maintain the variable at its desired value. Action (A): As a controller’s decision, the system must take an action. This is usually accomplished by the final control element.

Control Strategies : feedback control system If inlet process temperature decreases, this will create a disturbance, its effect must propagate through the heat exchanger before the outlet temperature decreases. Once, outlet temperature changes, signal from the transmitter to the controller also changes. It is then that the controller becomes aware that a deviation from set point has occurred and it must compensate for the disturbance by manipulating the steam valve. The controller then signals the valve to increase its opening and thus increase the steam flow. Heat exchanger control loop

Control Strategies: feedback control system Block diagrams of closed-loop control systems

Control Strategies: Feedback Loop

Closed-loop control: Response Initially, outlet temperature decreases because of the decrease in inlet temperature, but then it increases, even above the set point and continuous to oscillate until it finally stabilizes. This oscillatory response is typical of feedback control and shows that it is essentially a trial and error operation. That is, when the controller notices that the outlet temperature has decreased below the SET POINT, it signals the valve to open. But the opening is more than required. Therefore, outlet temperature increases above the SET POINT. Noticing this, the controller signals the valve to close again somewhat to bring the temperature back down. This trial & error continued until the temperature reached and stayed at SET POINT. INLET TEMPERATURE OUTLET TEMPERATURE CONTROLLER OUTPUT Response of feedback control

Advantages of feedback control Compensate for all disturbances The result of any disturbance entering the process is to make the controlled variable deviate from the SET POINT. Once the controlled variable deviates from the set point, the controller changes its output to return the controlled variable to SET POINT(its desired value). The feedback control loop does not know, nor does it care, which disturbance enters the process. It only tries to maintain the controlled variable at set point, and in this way compensates for all disturbances. The feedback controller works with minimum knowledge of the process. Actually, the only information it needs is in which direction to move, and how much to move is usually adjusted by trial and error.

Disadvantage of feedback control Can compensate for a disturbance only AFTER the controlled variable has deviated from the set point because of the disturbance. Can not give the controlled variable a timely control