Friday, January 15, 2016

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Basics of PID controller (Proportional - Integral - Derivative)

  • Friday, January 15, 2016
  • denizen robo

  • PID (Proportional - Integral - Derivative ) Controller
    How PID works - Animation

    Proportional

    The proportional term makes the current error signal multiplied with a gain (Kp).The result will be output signal.

    So output signal = Kp * Error_Signal


    Integral

    The integral term makes the current error signal value and duration multiplied by with a gain (Ki).
    The result will be the output signal.

    So output signal = Integral term output signal

    Where Ki - is the integral gain
    t is the instantaneous time
    e(t) - is the error signal

    The integral of a signal is the sum of all the instantaneous values that the the signal has been,from whenever you started counting until you stop counting.

    The integral term (when added to the proportional term) accelerates the movement of process towards setpoint and eliminates the residual steady-state error that occurs with a proportional only controller.

    Derivative

    The derivative term makes the rate of change the error signal multiplied with a gain (Kp).The result will be the output signal value.

    So output signal  =  Derivative control equation

    -where Kd is the derivative gain
    -e(t) is the error signal .

    The derivative term slows the rate of change of the controller output and this effect is most noticeable close to the controller setpoint.

    PID Control System

    Here is block diagram of a PID control system.
    Includes the :
    -PID Controller
    -Process/Plant
    -Feedback
    -Setpoint

    PID Control System

    The setpoint is the value that we want the process to be.
    The output must be equal to the setpoint,else error signal will not be zero.
    The error signal will be the [Setpoint - Measured]
    The 3 gains [P,I & D] will be summed together to output 1 signal that will get the output equal to the setpoint 
    The process is the plant plant/model of the system. Example - Motor
    A disturbance is added to system. Example - Friction to shaft of the motor.


    PID Controller Block Diagram

    The setpoint is subtracted from the measured to create the error.
    The error is simply multiplied by one,two or all of the calculated P,I and D actions.
    Then the resulting "error x control action" are added together and sent to the controller output.

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    Override Control

  • denizen robo
  • Override

    It is sometimes necessary to limit a variable in order to maintain safe operation or protect process equipment. If this variable is a function of the system’s primary controlled variable, the two variables can be interlocked in an override system. The primary variable maintains control as long as the second variable does not exceed its safe limit, at which point the second variable assumes control.
         
    In the compressor control system shown below, discharge pressure is controlled by PC1 manipulating the bypass valve so long as suction pressure remains above its low limit. However,  when suction pressure reaches the limit value, PC2 takes over control and holds it until normal conditions are re-established. The take-over in either direction is through a low selector and is both bumbles and automatic. Both controllers are proportional plus reset and must have external feedback from the selector output (not shown in the figure) for smooth take over.

    In this example, the suction controller is direct acting , the discharge controller is reverse acting and the valve is air-to-close. Other examples include reactor temperature control with pressure override and compressor discharge flow control with discharge pressure override.

    Override Control

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    Monday, January 11, 2016

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    Feedforward Control System

  • Monday, January 11, 2016
  • denizen robo
  • Feedforward


    A Feedforward control system measures a disturbance in variable, predicts its effect on the process and applies corrective action.

    Given an exact model of the process, the feedforward controller will adjust the manipulated variable (m) so that the controlled variable (c) is unaffected by the disturbance. In fact, the controlled variable has no influence over the control, corrective action is totally in response to their disturbance u1.

    Feedforward_Control_System


    This system has three major drawbacks :
    1.  The model must be exact (including dynamics and nonlinearities).
    2.  All instruments in the loop must be perfectly calibrated.
    3.  Disturbances other than the feedforward variable are not controlled.

    Thus feedforward by itself is insufficient control. However, combined with conventional feedback, it can be a powerful control tool. If a load change in a process occurs so frequently that the controller cannot keep up, or if the disturbance is so large that the controlled variable cannot be held within tolerable limits, and if the disturbance variable cannot be controlled, consider adding feedforward control to the system.

    The feedback controller does the same job and has the same responses and settings as if it were acting alone. It does not have much work to do. The feedforward control cancels the major effect of the measured disturbance. Since feedback acts as the system’s watchdog,  the process model need not be exact. In fact, simple gain and lead-lag element will usually suffice.

    The effect of load changes other than the measured disturbance will be corrected by the feedback system.

    Feedforward and Cascade are often confused because of their similarities : two measured variables, one manipulated variable, one independent set-point. But cascade systems control both measured variables, with the master determining the set-point of the slave. In contrast, feedforward and feedback corrections independently adjust the control valve and there is no control applied to the feedforward variable.
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    Sunday, January 10, 2016

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    Cascade Control System

  • Sunday, January 10, 2016
  • denizen robo

  • Cascade

    In cascade control system two controllers are cascaded  to each other as shown in the diagram below.  Primary controllers output is fed to secondary controller as a Set point. Both the controllers has their own sensing elements but only secondary controller gives output to the control valve. Primary controller receives set point from operator 

    Cascade Control Loop


    For example in Batch reactor temperature is controlled using Cascade control strategy. The output of temperature controller TC-1 adjusts the set point of TC-2 in the batch reactor control system. The primary variable batch temperature is controlled by the master controller, but not by directly adjusting the cooling water valve. Instead, it manipulates the set point of the slave or secondary controller, which in turn supplies jacket water at the asked for temperature. There are two controlled variables, one manipulated and one independent set point.


    example of cascade control system


    Cascade controller tuning -

    1.First,tune the secondary controller (with the primary controller in manual mode)
    2.Then,tune the primary controller (with the secondary controller in cascade mode)

    Cascade control has two functions : 
    1. Reduce the effect on the total control system of the dynamic elements in the secondary loop.
    2. Correct for  disturbances which occur within the secondary loop before they affect the master loop (such as cooling water temperature changes).

    The slave controller need have only proportional response. The major time constant must not be in the slave loop.

    Cascade control is normally not applied in fast control loops - flow, pressure, etc. It is more useful in temperature or composition control systems.

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    Saturday, January 9, 2016

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    Ratio Control

  • Saturday, January 9, 2016
  • denizen robo
  • Ratio Control

    In Ratio Control, the controlled variable is the ratio of two measured variables. Control is effected by adjusting one of the variables -  the controlled variable, to follow in proportion to a second - the wild variable. The proportionality constant is the ratio.

    Ratio relays are nothing more than manually adjustable gain devices and the ratio setting is the true set-point of the system. If the flow measurements are with differential pressure transmitters, the actual linear ratio setting must be the square of the required ratio. Most ratio relays have optionally calibrated scales which allow direct setting of the required ratio.

    Ratio Control


    Ratio Control often monitors processes that require mixing one material in proportion to another.

    For example, the product may require one part of material A for every two parts of material B.
    With ratio control one of the materials flows at the rate required by other parts of the process.This is known as the wild flow. The flow rate of the second,called the controlled flow could be adjusted by a ratio controller.

    The ratio control loop uses the rate of the wild flow to determine the rate of controlled flow so that the materials are mixed in the appropriate ratio. 

    Ratio control loop has one sensor and transmitter in the wild flow and another set in the controlled flow.Both sensors transmit signals representing the respective flow rates to the ratio controller.
    The controller maintains the flow rate of the the controlled flow at a ratio to the wild flow.
    The desired ratio is set by adjusting ratio the setpoint. Ratio control then monitors the value of one variable and sets the value of the second variable at a specific ratio to the first.
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    Thursday, January 7, 2016

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    Common Control Loops

  • Thursday, January 7, 2016
  • denizen robo

  • Common Control Loop 

    Instrumentation engineering Control loop


    1.Flow

    Liquid flow processes are fast,  typically 0.5 sec response or less. Gas flow is slightly slower because of the compressibility of the gas. Control components - transmitter, valve and transmission lines -  are the main dynamic elements.  The most common measuring means is the orifice plate and differential pressure transmitter ; flow is proportional to the square root of differential pressure.
    If a centrifugal pump is the flow energy source, the throttling valve can be placed in the pump discharge line. If a positive displacement pump is used , then the valve must be in a bypass line; alternately, pump speed or stroke can be controlled instead of bypass flow.
    A linear valve should be used in preference to a percentage valve, especially where the pressure drop across the valve varies. This helps counteract the non linearity of the differential measurement. If the measurement is linear    (magnetic flow-meter, turbine meter etc.) an equal percentage valve is probably the better choice.
    Flow processes are determined by noise caused by fluid turbulence and equipment vibration. Controller gain is invariably low usually less than 1.0. Reset must be used to overcome offset,  derivative cannot be used.

    VARIABLE
    PROCESS
     CONTROL  SYSTEM
        FLOW
    ·         Very fast.
    ·         Most lags are in the control system.
    ·         Non-linear (square) measurement common.
    ·         Noisy.
    ·         Proportional plus reset  controllers.
    ·         Low gain, fast reset.
    ·         Derivative hurts.
    ·         Linear valves for differential pressure measurement.
    ·         Equal percentage valves for linear measurement.
    ·         Valve is the major dynamic element.

    2.Pressure 

    Liquid - Liquid pressure control is very similar to flow control. The system is non-linear : pressure varies as flow squared. Noise is ordinarily present. 


    Gas - Gas pressure processes are single capacity systems and do not normally present much of a control problem. Self-acting or simple pilot-operated controllers can often be used with good results. (Line pressure is connected to the valve top either directly or through a simple pilot valve.) Gains of these proportional-only controllers are typically 20 to 50 or higher. A pneumatic reducing valve is a simple example of a self-acting pressure regulator.

    The important secondary dynamic element is the valve - the process itself is usually a large single capacity element and measurements are fast. Distance-velocity lag is not present.

    Proportional control is usually adequate ; reset can be added to completely remove offset. High gains are generally achievable or moderate gains with fast reset if small offsets are a problem.


    Vapour - The most important vapour pressure applications involve heat transfer - distillation columns, evaporators etc., where the control system is basically heat balance control. Vapour pressure control loops act like temperature control loops


            VARIABLE
             PROCESS
          CONTROL  SYSTEM


    PRESSURE    Liquid

    ·         Fast
    ·         Most lags are in the control system.
    ·         Non-linear (square).
    ·         Noisy.

    ·         Proportional plus reset controllers.
    ·         Gain near 1,fast reset rate.
    ·         Derivative of no value.
    ·         Linear valve.


    PRESSURE    Gas

    ·         Single capacity.
    ·         No dead time.
    ·         Linear, no noise.
    ·         Simple process.

    ·         Self-acting or high gain proportional controllers.
    ·         Reset seldom necessary.
    ·         Derivative unnecessary.
    ·         Valve characteristic relatively unimportant.


    PRESSURE    Vapour

    ·         Dynamics vary.
    ·         Dead time possible.
    ·         Slow compared to other pressure processes.
    ·         Linear,  no noise.


    ·         Three-response controllers.
    ·         Settings vary.
    ·         Equal percentage valves.


    3.Level 

    Level is a single capacity, integrating system. The tank, capacitance is directly proportional to the vessel diameter. Large diameter tanks with low  thruput present no problem, small diameter, high thruput systems are more difficult to control but not as common. The tank lag is volume thruput.

    Level control systems fill into two distinctly different categories : precise control and averaging control.

    For precise control, proportional controllers will provide adequate regulation on large capacity systems. As capacity decreases, controller gain must be decreased and reset becomes necessary.

    Averaging level systems sacrifice tight control in order to keep output flow rate constant. Typical applications are where the vessels under control are surge capacities between sections of a multi-stage process, their purpose being to absorb changes between stages. Level is allowed to wander between wide limits, with corrective action applied gradually so long as the level remains safely between the limits. Low gain, reset controllers do the job. Special dual-gain or dual-reset controllers are sometimes used. They have low gain or reset in the safe mid-level area, switch to higher gain or fast reset if the level strays too fat.

        VARIABLE
              PROCESS
            CONTROL  SYSTEM

               
    LEVEL

    ·         Single Capacity (integrating)
    ·         No dead time.
    ·         Linear.
    ·         Infrequent noise.

    ·         Precise control:
    High gain or proportional plus reset controllers.
    ·         Averaging control:
           Low gain proportional                plus reset or specialised controllers.
    ·         Valve characteristic unimportant.


    4.Temperature 

    Temperature Control  systems vary  from simple to very difficult and there is no such thing as a typical temperature application. Almost all temperature control problems 
    are heat transfer problems and are characterised by long time constants and slow reaction rates. Distance-velocity lag is common . The measurement lag can pose a serious problem, especially if the thermal system is protected with a well. The measurement time constant depends on the mass and surface area of the bulb (or the well), the fluid being measured and its velocity past the bulb. Special care should be taken in locating the bulb to maximise heat transfer.

    Temperature control problems are complicated by nonlinearities. Heat transfer processes have parameters which vary with flow, so that time constants and distance-velocity lag vary with load or operating point.

    Processes dominated by one large capacity - as large temperature baths or air heating systems -can be controlled with on-off controllers. Some cycling results, but is in the order of 1% of the span.

    Proportional plus reset control is used in smaller capacity systems where load changes are large and where distance-velocity or measurement lags are important. Most shell and tube heat exchangers fall into this category.

    Derivative is helpful, provided the distance-velocity lag is not the dominant secondary dynamic element. Shell and tube heat exchangers or plate heaters have large effective dead time so that derivative is of limited value. But other temperature systems such as batch reactors are dominated by linear lags and derivative is very helpful. 

          VARIABLE
              PROCESS
       CONTROL  SYSTEM


      TEMPERATURE

    ·         Multiple capacity system.
    ·         Dead time possible. (especially heat exchangers).
    ·         Non-linear.
    ·         No noise.

    ·         Three-response controllers.
    ·         Settings vary but gain usually above 1.
    ·         Derivative of limited value if dead time is large.
    ·         Equal percentage valves.
    Measurement dynamics are important.

    5.Composition

    Composition Control can be simple mixing problem (blending of lubricating oils to a desired viscosity), a separation problem (product quality control in a distillation column) or a reaction problem (neutralization with pH control). Generalizing on typical dynamics on-line  control is difficult.  

    On-line analyzer promote simpler control. They are relatively fast and do not require sampling systems. however, they are often noisy. Most analyzers are linear throughout their operating range.  pH is the notable exception. Sampling systems introduce distance-velocity lag into the control loop and the longer the dead time the tougher the control problem. Sampling systems also require careful design to insure that a sample representative of the total process stream is analysed.

    Analyzers are normally sensitive devices having narrow spans. The high gain element in the loop forces the controller to have a low gain. Reset is an essential control mode. Derivative is sometimes useful. 

      
        VARIABLE
              PROCESS
       CONTROL  SYSTEM
     COMPOSITION
    ·         Dynamics vary.
    ·         Dead time usually present.
    ·         Usually linear.
    ·         Sometimes noisy due to poor mixing.
    ·         Proportional plus reset controllers.
    ·         Low gain, variable reset rate.
    ·         Derivative sometimes useful.
    ·         On-line analysers fast, often noisy.
    ·         Sampling systems complicate both measurement and control, add dead time.
    ·         Linear  valves.


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    Friday, January 1, 2016

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    What is pH meter ? How pH meter works?

  • Friday, January 1, 2016
  • denizen robo
  • pH meter

    A pH meter is an electronic device used for measuring pH(acidity or alkalinity) of a liquid.

    A typical pH meter consist of a special measuring probe connected to an electronic meter that measures and displays the pH reading.

    pH meter Probe


    The probe is a key part of a pH meter,it is a rod like structure usually made up of glass.

    At the bottom of the probe there is a bulb,the bulb is sensitive part of a probe that contains the sensor.

    Never touch the bulb by hand and clean it with the help of an absorbent tissue paper with very soft hand,being careful not to rub the tissue against the glass bulb in order to avoid creating static.

    To measure the pH of a solution the probe is dipped into the solution

    The probe is fitted in an arm known as the probe arm.

    pH Scale

    pH of different liquids

    How does a pH meter works?


    A pH meters measures the concentration of the hydrogen ions [H+] in a solution

    An acidic solution has far more positively charged hydrogen ions in it than an alkaline solution,so it has greater potential to produce an electric current under certain conditions

    It is like a battery that can produce a greater voltage

    How pH meter works

    A pH meter takes advantage of this and work like a typical voltmeter.

    It consist of a pair of electrodes connected to a meter capable of measuring small voltages,on the order of mili volts.

    It measures the voltage (electrical potential) produced by the solution whose acidity we are interested in compares it with the voltage of a known standard solution and uses the difference in voltage (the potential difference) between them to calculate the difference in pH.

    pH Meter Calibration and Use


    For very precise work the pH meter should be calibrated before each measurement.

    Calibration should be performed with at least two standard buffer solutions that span the range of pH values to be measured.

    For general purpose buffers at pH 4.01 and pH 10..0 are acceptable 

    For more precise measurements,a three buffer solution calibration is preferred

    The calibration process correlates the voltage produced by the probe (approximately 0.06 volts per pH unit) with the pH scale.


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