Process control is the automatic control of an output variable by the sensing of the amplitude of the output parameter from the process and comparing it to the desired or set level and feeding an error signal back to control an input variable. The processor uses the error signal to generate a correction signal to control the actuator and the input variable.
A feedback loop is the signal path from the output back to the input for any correction by comparing it to the desired or set level.
A controlled or measured variable is the monitored output variable from a process.
A manipulated variable is the input variable or parameter to a process that is varied by a control signal from the processor to an actuator.
A set point is the desired value of the output parameter or variable being monitored by a sensor. Any deviation from this value will generate an error signal.
An instrument is a device for measuring physical quantities.
A sensor is a device that can detect physical variables.
A transducer is a device that can change one form of energy to another form of energy.
A converter is a device that can be used to change the format of a signal without changing the energy form.
An actuator is a device that is used to control an input variable in response to a signal from a controller.
A controller is a device that monitor signals from transducers and take the necessary action to keep the process within specified limits according to a pre defined program by activating and controlling the necessary actuators.
Ladder networks are normally used to program the controllers.
Programmable Logic Controllers are microprocessor based systems, that have the ability to monitor several variables and control several actuators.
An error signal is the difference between the set point and the measured variable's amplitude.
A correction signal is the signal used to control power to the actuator to set the level of the input variable.
A transmitter is a device used to amplify and format signals so that they are suitable for transmission over long distances with zero or minimal loss of information. The transmitted signal can be in pneumatic, digital or analog.
Reproductibility is the ability of an instrument to repeatedly read the signal over time and give the same output under the same conditions.
Sensitivity is a measure of the change in the output of an instrument for a change in the measured variable (transfer function).
Offset is the reading of an instrument with zero input.
Drift is the change in the reading of an instrument of a fixed variable with time.
Hysteresis is the difference in readings obtained when an instrument approaches a signal from the opposite direction.
Resolution is the smallest change in a variable to which the instrument will respond.
Linearity is a measure of the proportionality between the actual value of a variable being measured and the output of the instrument over its operating range.
Rules for Ladder Logic
1. Vertical lines indicate power supply ===> Power flow is from left to right.
2. Ladder diagram is read top to bottom and left to right.
3. Electrical devices are normally shown in their de energized condition.
4. Devices that indicate a start operation for a particular item are normally wired in parallel.
5. The contacts associated with coils, timers, counters and other instructions have the same numbering convention as their control device.
6. Devices that indicate a stop operation for a particular item are normally wired in series.
7. The operation of latching is used where a momentary start input signal latches the start signal into the ON condition so that when the start input goes into the OFF condition, the start signal is energized ON.
8. An output address status is readily available to rungs or branches which follow its generation.
The Different Ladder Logic Instructions
1. Standard Relay Logic Type
2. Timers and Counters
3. Arithmetic and Logical
4. Move
5. Comparison
6. File Manipulation
7. Sequence Instruction
8. Specialized Analog (PID)
9. Communication Instructions
10. Diagnostics
11. Miscellaneous (Sub routines)
A Ladder Logic Network
The standard relay logic type are of two types which are:
1. Normally Open Contact: This instruction examines its memory address location for an ON condition. If ON instruction is ON. If OFF instruction is OFF.
2. Normally Closed Contact: This instruction examines its memory address location for an OFF condition. If ON instruction is OFF. If OFF instruction is ON.
Timers are of three types which are:
1. Timer ON Delay
2. Timer OFF Delay
3. Retentive Timer
The three parameters associated with each timer are:
1. Preset Value: This is the constant number of units of time that the timer 'times to' before being energized or de energized.
2. Accumulated Value: This is the number of units of time recording how long the timer has been actively timing.
3. Time Base: This indicates the unit of time in which the timer operates .
There are basically two types of counters which are:
1. Count Up Counters: The counter increments the accumulator value by 1 for every transition of the input contact from false to true.
2. Count Down Counters: The counter decrements the accumulator value by 1 for every transition of the input contact from false to true.
The move instruction moves the source value at the defined address to the destination address every time it is executed.
A sub routine file is a stand alone module of ladder logic code which is used repeatedly by the main program.
There are two ways of transferring control of laddrr logic program from the standard sequential path in which it is normally executed are:
1. Jump to a part of the program when a rung condition becomes true
2. Jump to a separate block of ladder logic called a sub routine.
Restrictions in the use of Ladder Logic Diagrams
1. Number of coils and contacts per rung (or network)
2. Vertical Contacts
3. Nesting of Contact
4. Preset Value Ranges
5. Direction of Power Flow
A typical data acquisition system consists of a host computer, operating system program, data acquisition hardware, field wiring and control devices and transducers.
The main objective of data acquisition is to digitize an analog signal without any loss of information and without introducing invalid information.
The sampling theorem states that it is important to sample a signal with a maximum frequency component of f Hz and a minimum sampling frequency of 2F Hz.
An A/D board consists of the input multiplexer, input signal amplifiers, sample and hold circuit, analog to digital converter, bus interface and thus timing system.
There are two methods of connecting signal sources to the data acquisition board which are:
1. Single Ended
2. Differential
Factors to consider when selecting a data acquisition board are:
1. Board Throughput
a. A/D Converter Speed
b. Rated Maximum Throughput
c. Typical Maximum Throughput
2. Analog Inputs
a. Simultaneous Sampling
b. Resolution
c. Unipolar/Bipolar Inputs
3. Number of Channels
a. Strain Gauge Input
b. Overload Protection
c. Caliberation
4. On Board Features For A/D Section
a. Pacer Check
b. Channel Gain Array
c. Burst Scan Triggering
5. Analog Outputs
a. Number of Channels
b. Conversion Speed
c. Noise Level
In a control system, the variable one wants to control is called the process variable (PV). The variable to be manipulated in order to control the process variable is known as the manipulated variable.
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.
Note the following:
1. The common industry standard pneumatic signal is 3 - 15psig.
2. Digital signals are discrete levels or values that are combined in specific ways to represent process variables and aalso carry out information. Manufacturers can
either use an open or proprietary digital protocol.
3. There are local, pneumatic and programmable controllers.
4. The correcting of final control element is the part of the control system that acts to physically change the manipulated variable.
5. An actuator is a part of a final control device that causes a physical change in the final control device when signalled to do so.
6. Discrete controllers does not actually hold the variable at setpoint but keeps the variable within proximity of setpoint in what is known as DEAD ZONE.
7. Multistep controllers are controllers that have at least one other possible position in addition to on and off.
8. Effective process control is required to maintain safe operations, quality products and business viability.
9. Process is a method of transferring inputs to outputs.
10. The controller output is the signal from the controller to the final control element.
11. Disturbances are uncontrolled changes in the process inputs or resources.
12. Closed loop can be manual, On-Off, PID or Advanced PID.
13. To modulate is to vary the amplitude of a signal or a position between two fixed points.
14. Time proportion control is a variant of PID control that modulates the On-Off time of a final control element that only has two command positions.
15. Controlling a process requires knowledge of four basic elements which are process, sensor, final control element and controller.
16. Finally, aliasing is the process whereby anything less from the application of sampling theorem on a signal results to incorrect information (or aliases) being introduced into the sampled data.
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