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COMSATS INSITUTE OF INFORMATION

TECHNOLOGY, LAHORE

Assignment No. 03

Instrumentation and Process Control

Cascade Control System

Ratio Control System

Prepared By:

Umer Farooq

BEC-FA11-093

Dated:

01-06-2014

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Cascade Control System:

Introduction

In the previous chapters, only single input, single output (SISO) systems are discussed. SISO

involves a single loop control that uses only one measured signal (input). This signal is then

compared to a set point of the control variable (output) before being sent to an actuator (i.e.

pump or valve) that adjusts accordingly to meet the set point. Cascade controls, in contrast, make

use of multiple control loops that involve multiple signals for one manipulated variable. Utilizing

cascade controls can allow a system to be more responsive to disturbances.

Before venturing further into the topic of cascade controls, the terms 'manipulated variables',

'measured variables' and 'control variables' should be clarified. The definitions of these terms

commonly found in literature are often interchangeable; but, they typically refer to either the

input or output signal. For the purpose of this article, 'control variables' will refer to inputs like

flow rates, pressure readings, and temperature readings. 'Manipulated variables' and 'measured

variables' will refer to the output signals which are sent to the actuator.

The simplest cascade control scheme involves two control loops that use two measurement

signals to control one primary variable. In such a control system, the output of the primary

controller determines the set point for the secondary controller. The output of the secondary

controller is used to adjust the control variable. Generally, the secondary controller changes

quickly while the primary controller changes slowly. Once cascade control is implemented,

disturbances from rapid changes of the secondary controller will not affect the primary

controller.

Cascade Control

To illustrate how cascade control works and why it is used, a typical control system will be

analyzed. This control system is one that is used to adjust the amount of steam used to heat up a

fluid stream in a heat exchanger. Then an alternative cascade control system for the same process

will be developed and compared to the typical single loop control. The figure below shows the

performance of cascade control vs. single-loop control in CST heater

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Cascade control gives a much better performance because the disturbance in the flow is quickly

corrected

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Example of Cascade Control

Figure 1. Single loop control for a heat exchanger

In the above process, the fluid is to be heated up to a certain temperature by the steam. This

process is controlled by a temperature controller (TC1) which measures the temperature of the

exiting fluid and then adjusts the valve (V1) to correct the amount of steam needed by the heat

exchanger to maintain the specified temperature. Figure 2 shows the flow of information to and

from the temperature controller.

Figure 2. Flow of information when single loop feedback control is used for a heat exchanger

Initially, this process seems sufficient. However, the above control system works on the

assumption that a constant flow of steam is available and that the steam to the heat exchanger is

solely dependent on opening the valve to varying degrees. If the flow rate of the steam supply

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changes (i.e. pipeline leakage, clogging, drop in boiler power), the controller will not be aware of

it. The controller opens the valve to the same degree expecting to get a certain flow rate of steam

but will in fact be getting less than expected. The single loop control system will be unable to

effectively maintain the fluid at the required temperature.

Implementing cascade control will allow us to correct for fluctuations in the flow rate of the

steam going into the heat exchanger as an inner part of a grander scheme to control the

temperature of the process fluid coming out of the heat exchanger. A basic cascade control uses

two control loops; in the case presented below (see Figure 3), one loop (the outer loop, or master

loop, or primary loop) consists of TC1 reading the fluid out temperature, comparing it to

TC1set (which will not change in this example) and changing FC1set accordingly. The other loop

(the inner loop, or slave loop, or secondary loop) consists of FC1 reading the steam flow,

comparing it to FC1set (which is controlled by the outer loop as explained above), and changing

the valve opening as necessary.

Figure3. Cascade control for a heat exchanger

The main reason to use cascade control in this system is that the temperature has to be

maintained at a specific value. The valve position does not directly affect the temperature

(consider an upset in the stream input; the flow rate will be lower at the same valve setting).

Thus, the steam flow rate is the variable that is required to maintain the process temperature.

The inner loop is chosen to be the inner loop because it is prone to higher frequency variation.

The rationale behind this example is that the steam in flow can fluctuate, and if this happens, the

flow measured by FC1 will change faster than the temperature measured by TC1, since it will

take a finite amount of time for heat transfer to occur through the heat exchanger. Since the

steam flow measured by FC1 changes at higher frequency, we chose this to be the inner loop.

This way, FC1 can control the fluctuations in flow by opening and closing the valve, and TC1

can control the fluctuations in temperature by increasing or decreasing FC1set .

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Thus, the cascade control uses two inputs to control the valve and allows the system to adjust to

both variable fluid flow and steam flow rates. The flow of information is shown in figure 4.

Figure 4. Flow of information when cascade control is used for a heat exchanger

In order to accomplish this, relationships between the primary and secondary loops (see

definitions below) must be defined. Generally, the primary loop is a function of the secondary

loop. A possible example of such relations is:

TC1 = f (FC1)

FC1 = f (V1)

Primary and Secondary Loops:

In Figure 3, there are two separate loops. Loop 1 is known as the primary loop, outer loop, or the

master, whereas loop 2 is known as the secondary loop, inner loop, or the slave. To identify the

primary and secondary loops, one must identify the control variable and the manipulated

variable. In this case, the control variable is the temperature and the reference variable is the

steam flow rate. Hence, the primary loop (loop 1) involves the control variable and the secondary

loop (loop 2) involves the reference variable. The information flow for a two loop cascade

control system will typically be as shown in Figure 5. Please note that the user sets the set point

for loop 1 while the primary controller sets the set point for loop 2.

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Figure 5: Information flow of a two loop cascade control

In addition to this common architecture, cascade control can have multiple secondary loops;

however, there is still one primary loop and a main controlled variable. Unfortunately, with

multiple inner loops, tuning the PID becomes even more challenging, making this type of

cascade less common. The secondary loops can be either independent of each other, or

dependent on each other, in which case each secondary loop affects the set point of the other

secondary loop. When tuning such controller, the inner most loop should be tuned first. The loop

that manipulates the set point of the inner-most loop should be tuned next and so for. The figure

below shows an example of using two secondary loops, independent of each other, in a fuel

combustion plant. In this combustion furnace, the master controller controls the temperature in

the furnace by changing the set point for the flow of fuels A and B. The secondary loops

correspond to the change in the set point for the flow, by opening or closing the valves for each

fuel.

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Cascade control is generally useful when

A system error affects the primary control variable only after a long period of time as it

propagates through dead time and lag time.

A system has long dead times and long lag times.

Multiple measurements with only one control variable are required for better response to a

disturbance of a system.

Variance occurs in multiple streams

Ratio Control system:

For many processes, such as blending and boiler combustion, a key objective is to maintain the

flow rates of two process streams in some proportion to one another. In such cases, ratio control

may be applied. When ratio control is applied, one process input, the dependent input, is

proportioned to the other process input, known as the independent input. The independent input

may be a process measurement or its setpoint. The proportion that is to be maintained between

the inputs is known as the ratio. For example, a ratio of 1:1 would specify that the two inputs are

to be maintained in the same proportion. As the value of the independent input changes, through

ratio control the other process input is changed to maintain the proportion of the inputs specified

by the ratio setpoint. In nearly all ratio control applications, the ratio controller sets the setpoints

of the flow controllers rather than the valve positions, as illustrated below. Thus, any non-

linearity installed characteristics associated with valves is addressed by the flow controllers and

has no impact on the ratio controller being able to maintain the ratio setpoint.

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In most cases, the independent input measurement, not the setpoint, is used as the input to the

RATIO block. The reason is that for some operating conditions, the independent loop output may

become saturated.

The ratio block is used to implement ratio control. The input to the ratio block is the

measurement or setpoint of the independent flow input to the process. This independent

measurement is also known as the process “wild flow” input. In the RATIO block, this input is

multiplied by the ratio to determine the setpoint of the dependent flow input to the process. A

typical implementation of ratio control is illustrated below.

Under normal operating conditions, the RATIO block multiplies the independent input flow

provided on IN_1 by the RATIO block setpoint to provide an output to the downstream block.

The actual ratio is calculated by the block using the IN and IN_1 inputs.