Suppressing the influence of external disturbances, Optimizing the performance, Increasing the productivity, Cost effective.
Any system comprised of dynamic variables usually involved in manufacturing and production operations. It is defines as a series of operations during which some materials are placed in more useful state.
It is a variable which is altered by the automatic control equipment so as to change the variable under and make it conform with the desired value.
It is the quantity of control system which is directly measured and controlled.
The output will move from one steady state to another for the sustained change in input. This means that for change in some input variable the output variable will rise until it reaches a steady state (inflow = outflow).
It is the tendency of the process to adopt a specific value of controlled variable for nominal load with no control operations.
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The physical equipment of the chemical process we want to control have not been constructed. Consequently we cannot experiment to determine how the process reacts to various inputs and therefore we cannot design the appropriate control system. If the process equipment needs to be available for experimentation the procedure is costly.
Therefore we need a simple description of how the process reacts to various inputs, and this is what the mathematical models can provide to the control designer.
Step, Ramp, Impulse, Sinusoidal, Pulse inputs
Drum boiler system, in which the flow rate of the cold feed water is increased by a step the total volume of the boiling water and consequently the liquid level will decreased for a short period and then it will start increasing.
Load changes in first process affects the second process and vise versa when both are connected in series nature is called interacting system. Eg. Two level tanks are connected in series.
T=R/A, R=H/Q=10/(5X5.885X10-4)
A system that grows without limit for a sustained change in input (constant outflow or outflow independent of inflow condition).
The smaller the value of time constant the steeper the initial response of the system. A first order lag proce ss is self regulating the ultimate value of the response equal to Kp (steady state gain of the process) for a unit step change in the input.
A process in which the materials or work flows more or less continuously through a plant apparatus while being treated is termed as continuous process. The problem of continuous process is due to load changes. (e.g.) storage vessel control.
A process in which the materials or work are stationary at one physical location while being treated is termed as batch process. (e.g.) furnace.
The element in a process control loop that evaluated error of the controlled variable and initiates corrective action by a signal to the controlling variable.
Components of a control system (such as valve) is used to directly regulates the flow of energy or materials to the process. It directly determines the value of manipulated variable.
It is the scheme that describes how much the manipulated variable should change inorder to bring the controlled variable back to the setpoint.
– Controlled variable directly indicates the performance of the process Eg. Water heater system
the performance of the process.
– Controlled variable indirectly indicates Eg. Annealing
If the purpose of the control system is to make the process follow the changes in setpoint as quick as possible, then it is servo operation.
Set of equations that characterize the process is termed as Mathematical Modelling.
The dynamic behaviour one tank is affected by the other, but the reverse is not true, then it is non-interacting system. Here the liquid heads are independent of each other.
The dynamic behavior one tank is affected by the other, but the reverse is also true, then it is non-interacting system. Here the liquid heads are dependent of each other.
(i) The output of controller is zero at constant error condition.
(ii)It will amplify the noise present in the error signal.
Define appropriate performance criterion (ISE, IAE, ITATE). Compute the value of the performance criterion using a P, PI, or PID controller with the best setting for the adjusted parameters Kp, Ti, Td. Select controller which give the best value for the performance criterion.
A controller mode in which the controller output is directly proportional to the error signal P=Kpep+p0 P-controller output Kp= Propotional gain, ep=error in percent of variable range, P0-Bias.
Proportional band is def ined as the change in input of proportional controller mode required to produce a full-scale change in output
The reciprocal of gain expressed as a percentage is called proportional band. Kp=100/PB
It is the steady state deviation (error) resulting from a change in value of load variable.
It is the difference at any instant between the value of controlled variable and the set point. E=S.P-P.V
Refer Curtis Johnson, Page No.418, and Fig. 10.17.
Electronic signals operate over great distance without time lags. Electronic signals can be made compatible with digital controllers. Electronic devices can be designed to be essentially maintenance free. Intrinsic safety techniques eliminate electrical hazards. Less expensive to install. More energy efficient. Due to the above said properties electronic controllers are preferred to pneumatic controller.
The element in process control loop that evaluates error of the controlled variable and initiates corrective action by a signal to the controlling variable.
The present output depends on the time history of errors and such history is not known, the actual value of controller output floats at an undetermined value.
If the deviation persists controller saturates at either 100% or 0% and remain there until an error drives it towards opposite extreme.
Refer Curtis Johnson, Page No. 368, and Fig.9.7.
The series capacitor in the derivative controller will amplify the noise in the error signal.
The over charging in the presence of a continuous error of the integral capacitor which must discharge through a long time constant discharge path and which prevents a quick return to the desired control point.
Discontinuous and continuous mode are the two modes of controller.
If for only two values of error, control action is taken, it is Discontinuous mode of controller.
If for every value of error, control action is taken, it is Discontinuous mode of controller.
Discontinuous-ON-OFF controller.
Oscillations of error about zero is called cycling.
Deciding what values to be used for the adjusted parameters of the controller is called controller turning.
In process controller, the reaction curve is obtained by applying a step change (either in load or in set point) and plotting the response of the controlled variable with respect to time.
Keep the maximum error as small as possible. Minimize the integral of the errors until the process has settled set Point.
The maximum gain of the proportional controller at which the sustained oscillations occur is called ultimate gain (Ku).
ITAE means Integral Time Absolute Error. To suppress the errors that persist for long time, the ITAE criterion will tune the controllers better because the presence of large t amplifies the effect of even small errors in the value if integral.
Process Parameters (K,ô), Controller parameters (Kp,Ti, Td),performance creation (ISE, IAE, IATE)
It constitutes a measure of how far the system is the brink of instability. Higher the gain margin (above the value of one), the higher the safety factor we use controller turning.
Typically, a control designer synthesizes a feedback system with gain margin larger than 1800.
The gain margin and Phase margin are the safety factors which is used for the design of a feedback system. Beyond the phase margin and gain margin the system goes to unstable position.
It is the procedure in which adjusting the proportional gain of controller upto ¼ th decay ratio waveform is obtained.
Integral Square Error (ISE), Integral of absolute value of error (IAE), Integral of time weighted absolute error.
If we want to suppress large errors, ISE is better than IAE Because errors are squared and contribute more to the value of integral.
If we want to suppers small errors, IAE is better than ISE Because when we square small numbers, they even become smaller.
To suppress errors that persist for long times, ITAE criterion will tune the controllers better because the presence of large t amplifies the effect of even small errors in value of integral.
It is reasonable trade off between fast rise time and reasonable setting time.
Proportional Control is the satisfactory control for liquid level process.
Proportional Control is the satisfactory control for liquid level process.
PI Control is the satisfactory control for vapour pressure process having fast response.
PID Control is the satisfactory control for temperature process.
PID Control is the satisfactory control for composition process.
Ratio control is a special type of feed forward control where two disturbances are measured and held in a ratio to each other.
Cascade control is defined as a control system composed of two loops where the set point of one loop (the inner loop) is the output of the control ler of the other loop (the outer loop)
In some process the secondary variables in it introduce disturbance throughout the system is measured and controlled by a separate loop.
In heat exchangers, the control objective is to keep the exit temperature of stream. But the flow rate of the stream creates the low disturbance throughout of its a function. The secondary loop is used to compensate the flow rate of the stream.
Such control configurations select among several measurements the one with the highest value and feed it to the controller. Thus it is a selective controller which possesses several measured outputs and only one manipulated input.
Override control for the protection of process equipment, auctioneering control.
In some cases it is necessary to change from the normal control action and attempt to prevent a process variable from exceeding an allowable upper or lower limit. This can be achieved b y the use of special t ype switches called limit switches.
High Select Switch (HSS), Low Select Switch (LSS).
High Select Switch (HSS) is a limit switch which is used whenever a variable should not exceed an upper limit.
Low Select Switch (LSS) is a limit switch which is used whenever a variable should not exceed an lower limit.
During the operation of the plant, it is possible that some of the process variables exceed the limit. In such cases it is necessary to change from the normal control action and attempt to prevent a process variable from exceeding an allowable an allowable upper or lower limit.
This can be achieved by the use of special type switches called limit switches called limit switches (HSS and LSS). This type of protective control is called override control.
To control A single process output can be controlled by co-coordinating the actions of several manipulated variables all of which have same ef f ect on controlled output. Such systems are called split-range control systems.
Split-range control system involves one measurement and more than one manipulated variables but sele ctive control system involves one manipulated variables and several controlled outputs.
To obtain the most efficient combustion.
The special element introduced in a system with two strongly interacting loops to cancel the interaction effect between the two loops and thus render two non-interacting control loops is called decoupler.
It is used in some cases where the output of the process and the influence of the disturbance cannot be measured.
Acts before the disturbance is felt by the process. It is good for slow systems.
Requires identification of all possible disturbances and their direct impact. Cannot cope with unmeasured disturbances.
It does not require identification and measurement of disturbance.
It is unsatisfactory for slow processes with significant dead time.
When a liquids enters a valve and the static pressure at the vena contracta less than the fluid vapour pressure and the valve outlet pressure is also less the fluid vapour pressure the condition called flashing exists.
If the diaphragm actuator does not supply sufficient force to position the valve accurately and overcome any opposition that flowing conditions create a positioner may be required.
Motor, Solenoids.
In some process loop the controller is electronic and the fin al control element is electronic one.
To interconnect these two we need a device that should linearly converts electric current in to gas pressure (4-20mA-315 psi). such device is called I/P converter.
Inherent characteristics is which the valve exhibits in the laboratory condition where the pressure drop is held constant.
Installed or resultant characteristics is the relationship between flow and stroke when the valve is subjected to pressure conditions of the process.
Used to transmit the input signals into standard instrumentation pneumatic output signals (3 to 15 psi or 20 to 100 KPa).
Components of a control system (such as valve) is used to directly regulates the flow of energy or materials to the process. It directly determines the value of manipulated variable.
When a liquid enters a valve and the static pressure at the vena contracta drops to less than the fluid vapor pressure and the recovering to above fluid vapour pressure, this pressure recovery causes an implosion or collapse of the vapour bubbles formed at the vena contracta. This condition is called cavitation.
For equal increment of stem travel at constant pressure drop an equal percentage change in existing flow occurs.
Inherent characteristics, Installed characteristics.
Inherent characteristics is which the valve exhibits in the laboratory condition where the pressure drop is held constant. Installed or resultant characteristics is the relationship between flow and stroke when the valve is subjected to pressure conditions of the process.
For smaller movement of the stem, there is maximum flow rate.
If stem position varies linearly with flow rate, then it is linear.
Q=Cv.sqrt(P/Sg) Q-Flow rate
Cv-Valve coefficient
P-pressure difference across valve. Sg-Specific gravity of liquid.
Control Valve.
The function of control valve in flow control system is to regulate the flow.
Solenoid coil used to change gears.
Single-seated and double-seated plug type control valves.
It is the ratio of maximum controllable flow to minimum controllable flow.
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