Open Loop is a control system whereby the values to be controlled (PV) is not used to determine the control action. The principles of feed forward control is to manipulate a variable of the process in such a way that it compensates for the impact of process disturbances. An open loop has no feedback.
The control loop measures the process variable and compares the process variable with the desired or target value, the set point and determines a control action.
Error can be defined as the difference between the process variable and the set point.
Most control loop controllers are capable of controlling with three control modes which can be used separately or together, they are:
1. Proportional Control: The control action is proportional to the error and cannot eliminate error completely.
2. Integral Control: This control action eliminates error completely and reduces stability in the control action.
3. Derivative Control: This control action adds dynamic stability to the control loop.
The stages of Closed Loop Tuning (Continuous Cycling Method) are:
1. Put controller in P - Control only
2. P - Control on error = Set Point - Process Variable
3. Put the controller into automatic mode
4. Step change to the set point
Process Control refers to the methods that are used to control process variables when manufacturing a product.
Manufacturers control the production process for three reasons which are:
1. Reduce Variability
2. Increase Efficiency
3. Ensure Safety
The three tasks necessary for process control to occur are:
1. Measure
2. Compare
3. Adjust
The objective of any control scheme is to minimize or eliminate error.
The three major components of error are:
1. Magnitude
2. Duration
3. Rate of Change
Duration is the length of time that an error condition has existed.
Note the following:
1. A load disturbance is an undesired change in one of the factors that can affect the process variable.
2. A control algorithm is a mathematical expression of a control function.
3. A closed control loop exists where a process variable is measured, compared to a set point and action is taken to correct any deviation from set point.
4. An open control loop exists where the process variable is not compared and action is taken not in response to feedback on the condition of the process variable,
but is instead taken without regard to process variable conditions.
5. A transmitter is a device that converts a reading from a sensor or transducer into a standard signal and transmits that signal to a monitor or controller.
There are three kinds of signals which are:
1. Pneumatic Signal: These are signals that are produced by changing the air pressure in a signal pipe in proportion to the measured change in a process variable.
2. Analog Signal: These are signals that are represented in their continuous (or varying) form.
3. Digital Signal: These are signals that are represented in their discrete form.
Protocol is a methodology that is used to combine digital signals. Recorders that create charts or graphs are called CHART RECORDERS.
Distributed Control Systems are controllers that in addition to performing control functions provide readings of the status of the process maintain databases and advanced man machine interface.
Final control elements are valves, pump motors etc that are used to increase or decrease fluid flow.
The three types of controllers are:
1. Discrete
2. Continuous
3. Multistep
Gain can be defined as the change in the output divided by the change in the input.
Fast processes may require less gain to achieve stability
Slow processes may require high gain to achieve responsiveness
The proportional mode is used to set the basic gain value of the controller. The settings for the proportional mode may be expressed as either proportional gain or proportional band.
Limits of Proportional Action are:
1. Responds only to a change in error.
2. Does not return process variable to set point
Proportional band is expressed in terms of the percentage change in error that will cause 100% change in the controller output.
Note the following:
1. Small PB(%) results to Minimize Offset
2. Large PB(%) results to Large Offset
3. High Gain(%) results to Possible Cycling
4. Low Gain(%) results to Stable Loop
Tuning reduces PB (increases gain) until the process cycles following a disturbance then double the PB (reduce gain by 50%).
The controller output from the integral or reset mode is a function of the duration of the error.
Integral or reset action may be expressed in terms of:
1. Repeats per Minute: How many times the proportional action is repeated each minute.
2. Minutes per Repeat: How many minutes are requires for one repeat to occur.
Advantage of Integral Mode
1. It eliminates error.
Disadvantage of Integral Mode
1. Reset windup and possible overshoot.
Fast reset leads to high gain, possible cycling and fast return to set point.
slow reset leads to low gain, stable loop ad slow return to set point.
Trailing and Error Tuning increases repeats per minute until the process variable cycles following a disturbance, then slow the reset action to a value that is one - third of the initial setting.
Relationship between Control and Error
1. Proportionate deals with the magnitude of the error.
2. Integral deals with the duration of the error.
3. Derivative deals with the rate of change of the error.
The derivative action is initiated whenever there is a change in the rate of change of the error or the slope of the process variable and its setting is expressed in terms of minutes.
The actions is to apply an immediate response that is equal to the proportional plus reset action that would have occurred some number of minutes in the future.
Merit of Derivative Mode
1. Rapid output reduces the time that is required to return the process variable to the set point in slow process.
Demerit of Derivative Mode
1. It dramatically amplifies noisy signals and can cause cycling in fast processes.
Large minutes results to high gain, large output change and possible cycling.
Small minutes results to low gain, stable loop and small output change.
Trial and Error Tuning: This increases the rate setting until the process cycles following a disturbance, then reduces the rate setting to one - third of the initial value.
By using all three control algorithms together ie PID control, process operators can:
1. Achieve rapid response to major disturbances with derivative control
2. Hold the process near set point without major fluctuations with proportional control.
3. Eliminate offset with integral control.
PI control is used in an application where noise is present but where no offset can be tolerated.
PID control is used in an application where no offset can be tolerated, no noise is present and where dead time is an issue.
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