MARINE AUTOMATION AND CONTROL (TOPIC 5)

Controllers  The devices that receive the measured value signals and compare them with the desired value signals (set points) and process their differences if any, to produce correction signals in response to any system disturbances.  However, there are various time lags or delays that the controller must compensate for and ensure a steady output as near to the desired value as possible.

Controller Actions – Time Lags or Delays  Measurement Lag - The actual value in time that it takes a measuring unit to indicate or transmit a signal equal in value to the variable being measured after a disturbance has occurred.  Process Lag - The value in time that it takes the process being controlled to change by the required amount after the correcting (regulating) unit has been adjusted to effect the change, i.e. if water is being heated in a tank by a steam coil, the process lag is the time taken for the water to attain a new value after the regulating valve has been adjusted.  Transfer Lag - A process will contain a transfer lag if, for example, the process liquid is heated indirectly by causing steam to heat a transfer medium in an inner tank.  Distance Velocity Lag - The time delay between a change in process condition at the correcting element and the arrival of this changed condition at the detecting element.

Effects of Process Lags on the Response of the Controlled Condition The function of the controller (a) Distance velocity lag Position of correcting is to overcome and (b) (L) element compensate for the effects (c) of the system lag. (d) (L) Perfect process with no (e) lags Change in controlled (f) condition Process with ideal distance velocity lag Figure showing the effects of process lags on controlled Process with single condition response. capacity lag Process with single capacity and distance velocity lag Process with two capacity and transfer lags Time

Controller Action – Two-Step or On/Off Control In this, the simplest of controller actions, two extreme positions of the controller are possible, either on or off. If the controller were, for example, a valve it would be either open or closed. A heating system is considered with the control valve regulating the supply of heating steam. The controller action and system response is shown in the figure below. As the measured value rises above its desired value the valve will close. System lags will result in a continuing temperature rise which eventually peaks and then falls below the desired value. The valve will then open again and the temperature will cease to fall and will rise again. This form of control is acceptable where a considerable deviation from the desired value is allowed.

Two-Step or On/Off Control

Controller Action – Proportional Action This is a form of continuous control where any change in controller output is proportional to the deviation between the controlled condition and the desired value. The proportional band is the amount by which the input signal value must change to move the correcting unit between its extreme positions. The desired value is usually located at the centre of the proportional band. Offset is a sustained deviation as a result of a load change in the process. It is an inherent characteristic of proportional control action. Consider, for example, a proportional controller operating a feedwater valve supplying a boiler drum. If the steam demand, i.e. load, increases then the drum level will fall. When the level has dropped the feedwater valve will open. An equilibrium position will be reached when the feedwater valve has opened enough to match the new steam demand. The drum level, however, will have fallen to a new value below the desired value, i.e. offset.

System response to proportional controller action

Proportional Band and Gain  Proportional Band = the input change required to change the output 100%.

Wide, Narrow, Low & High

Percentage Valve Opening Open 100% Proportional 20% Proportional 100 Band Band 80 20% Proportional • Band 60 20 40 60 80 100 40 Percentage of Scale Range 20 0 Closed

Proportional Band  Is the amount the measured value of a controlled condition must change in order that the control valve may be moved from fully closed to fully open position.  The proportional band setting required for any given application will depend on plant characteristics and the various lags in the control loop.  There is an optimum value which will give stable control. If the p.b. is made too narrow (high controller sensitivity), the process will become unstable, while if it is made too wide ( low controller sensitivity), the process will be sluggish.

Response Versus P.B., Proportional Control only.

Controller Action – Integral Action This type of controller action is used in conjunction with proportional control in order to remove offset. Integral or reset action occurs when the controller output varies at a rate proportional to the deviation between the desired value and the measured value. The integral action of a controller can usually be varied to achieve the required response in a particular system.

Controller Action – Derivative Action Where a plant or system has long time delays between changes in the measured value and their correction, derivative action may be applied. This will be in addition to proportional and integral action. Derivative or rate action is where the output signal change is proportional to the rate of change of deviation. A considerable corrective action can therefore take place for a small deviation which occurs suddenly. Derivative action can also be adjusted within the controller.

Controller Action – Multiple Term Controller Action The various controller actions in response to a process change are shown in the figure below. The improvement in response associated with the addition of integral and derivative action can clearly be seen. Reference is often made to the number of terms of a controller. This means the various actions: proportional (P), integral (I), and derivative (D). A three-term controller would therefore mean P+I+D, and two-term usually P+I.

System Recovery Time Response to Controller Actions

Controllers – Basic Construction Flapper Nozzle Air Supply Output, p Comparator Set Point Measured bellows Value Signal Correcting Unit

Description of the Basic Construction The diagram shows the same flapper nozzle arrangement seen earlier. One end is connected to comparator bellows. The movement of this end of the flapper is thus dependent upon the relative values of the measured value and set point signals. Any difference between the two values brings about movement of the flapper which, in turn, changes the separation between the flapper and the nozzle thereby changing the value of the generated pneumatic pressure signal. It should be noted, however, that the total pressure change of 0.2 to 1.0 bar is brought about by change in separation of 40 um. That means, in this form, this device is a very high gain device and as such of not much practical use.

Proportional Controller Flapper Nozzle Comparator Set Point Air Supply Measured bellows Relay Valve Value Signal Correcting Unit

Proportional Controller Description. The diagram shows addition of a negative feedback bellows or proportional bellows being supplied with the output signal and actuating the other end of the flapper. The separation between the flapper and nozzle is thus under the combined influence of the opposite movements brought by the two ends of the flapper. By a suitable selection of relative bellows coefficients, it is possible to arrange to have any desired change in separation between flapper and nozzle for a specified change in the value of the measured variable, and thus control the gain or sensitivity of the device. This then forms the basis of a pure proportional controller.

Summary  (P) Proportional control: action of a controller whose output signal is proportional to the deviation. i.e. Correction signal ∞ deviation  (I) Integral control: action of a controller whose output signal changes at a rate which is proportional to the deviation. i.e. Velocity of correction signal ∞ deviation Objective: To reduce offset to zero.  (D) Derivative control: action of a controller whose output signal is proportional to the rate at which the deviation is changing. i.e. Correction signal ∞ velocity of deviation Objective: Gives quicker response and better damping.