Control Element.ppt

Final Control Elements

Quick Introduction• A final control element is the device manipulated by a

control loop to affect the process, principally by means of changing a flow.

• Final control elements are an essential part of nearly every process control system.

• Without final control elements, there is no way of controlling the process. We could not change operating points or correct for disturbances. There may be several layers of control loops, but it is usually a flow that a final control element ends up changing in a process. The most notable exceptions are heater or electrode current and mixer speed.

Quick Introduction• By far, the most common final control element is the

control valve, with its attendant positioner, actuator, and other components. Variable speed peristaltic pumps are used for the exceptionally small flows of bench top and pilot plant operations. Variable speed positive displacement pumps are used for small additive and reagent flows in production. For large flows in plants and powerhouses, variable frequency drives and dampers are sometimes used instead of control valves to reduce capital and operating costs.

Quick Introduction• Axial and centrifugal blowers, fans, and pumps are used

for the flow ranges normally associated with gas and liquid streams in industrial plants. A variable frequency drive (VFD), particularly in large utility flow applications, can save energy by the elimination of a control valve and its pressure drop. However, the energy savings is usually overestimated for process streams by not taking into account the service time and efficiency at low flow and the loss in turndown due to static head.

Quick Introduction• A damper can reduce the cost of the final element or fit in

a non-circular duct. Dampers are commonly used in HVAC systems, boilers, furnaces, and scrubbers to manipulate air and vent gas flows. Dampers have a lower pressure drop than a control valve, but generally the performance (e.g., rangeability, resolution, sensitivity, speed, and seal) of a damper is not as good as a control valve. The leakage and limited dynamic response and materials/ruggedness of construction of dampers relegate their application to mostly utility and vent systems.

Today’s Discussion

Suppose you have visited a process industry. Now identify at least three real elements of final control operations.

Do you think that different types of sensors used in a process industry are final control elements?

Process control

CONTROLLERoSwitching elementsoActuatorso control valvesoHeatersoElectric motors

oSensorsoSignal conditioners

Final Control Element

Steps necessary to convert the control signal into proportional action on the process itself.

E.g. 4 – 20 mA control signal to vary flow of 10m3/min – 50 m3/min.

For a typical process-control application, the conversion of a process-controller signal to a control function can be represented by the three parts which are signal conversions, actuators, and control element.

FIGURE 7.1 Element of the final control operation

Signal ConversionThis step refers to the modifications that must be made to the control signal to properly interface with the next stage of control - that is, the actuator. The devices that perform such signal conversions are often called transducers because they convert control signals from one form to another, such as current to pressure, current to voltage, and the like.

ActuatorsThe results of signal conversions provide an amplified and/or converted signal designed to operate (actuate) a mechanism that changes a controlling variable in the process. The direct effect is usually implemented by something in the process, such as a valve or heater that must be operated by some device. The actuator is a translation of the (converted) control signal into action on the control element. Thus if a valve is operated, then the actuator is a device that convert the control signal into the physical action of opening or closing the valve.

Final Control ElementControl element is the last element of the final control operations. This device has direct influence on the process dynamic variable and is designed as an integral part of the process. Thus, if flow is to be controlled, then the control element, a valve, must be built directly into the flow system. Similarly, if temperature is be controlled, then some mechanism or control element that has a direct influence on temperature must be involved in the process. This could be heater/cooler combination that is electrically actuated by relays or a pneumatic valve to control influx of reactants.

FIGURE 7.2 A process-control system showing the final control operations.

Final Control Element (cont)The controller compares the measurement to a set point and outputs a 4-20 mA signal that regulates the conveyer belt feed-motor speed to adjust baking time. The final control element operation is then represented by a signal conversion that transforms the 4-20 mA signal into a 50-100 V signal as required for motor speed control. The motor itself is the actuator, and the conveyer belt assembly is the control element.

Pneumatic signals• Application in gas pressure.

e.g. provide a force by the gas pressure’s acting on a piston or diaphragm.

• Pneumatics as a means of propagating information, a signal carrier.

Pneumatic principles

• Information is carried by the pressure of gas in a pipe.

• Pressure signal travels down the pipe at a speed sound of gas 330m/s.

• Hence, if a transducer varies gas pressure at one end of 330 m pipe in response to some controlled variable, then the same pressure occurs at the other end after a delay of 1s.

• Safer than electrical signal.

FIGURE 7.4 A pneumatic amplifier or booster converts the signal pressure to a higher pressure or the same pressure but with greater gas volume.

FIGURE 7.5 Principles of the nozzle/flapper system.

FIGURE 7.6 Using a nozzle/flapper for a current-to-pressure converter.

Different Actuators

If a valve is used to control fluid flow, some mechanism must physically open or close the valve. If a heater is to warm a system, some device must turn the heater on or off or vary its excitation. These are examples of the requirements for an actuator in the process control loop.

Electrical Actuators

SolenoidA solenoid is an elementary device that converts an electrical signal into mechanical motion, usually in a straight line. As shown in fig. 7.25, the solenoid consists of a coil and plunger. The plunger may be freestanding or spring loaded. The coil will have some voltage or current rating and may be ac or dc. Solenoid specifications include the electrical rating and the plunger pull or push force when excited by the specified voltage. Solenoids are used when a large and sudden force must be applied to perform some job.

FIGURE 7.25 A solenoid converts an electrical signal to a physical displacement.

Curtis JohnsonProcess Control Instrumentation Technology, 8e]

Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

FIGURE 7.26 A solenoid used to change gears.




Electrical Motors

Electrical motors are devices that accept electrical input and produce a continuous rotation. Motor styles and sizes vary as demands for rotational speed, starting torque, rotational torque etc. There are numerous cases where motors are employed as actuators in process control.

Different Actuators

If a valve is used to control fluid flow, some mechanism must physically open or close the valve. If a heater is to warm a system, some device must turn the heater on or off or vary its excitation. These are examples of the requirements for an actuator in the process control loop.

Electrical Actuators

SolenoidA solenoid is an elementary device that converts an electrical signal into mechanical motion, usually in a straight line. As shown in fig. 7.25, the solenoid consists of a coil and plunger. The plunger may be freestanding or spring loaded. The coil will have some voltage or current rating and may be ac or dc. Solenoid specifications include the electrical rating and the plunger pull or push force when excited by the specified voltage. Solenoids are used when a large and sudden force must be applied to perform some job.

FIGURE 7.25 A solenoid converts an electrical signal to a physical displacement.




FIGURE 7.26 A solenoid used to change gears.




Electrical Motors

Electrical motors are devices that accept electrical input and produce a continuous rotation. Motor styles and sizes vary as demands for rotational speed, starting torque, rotational torque etc. There are numerous cases where motors are employed as actuators in process control.

Type of Motor

Three most common varieties of motor dc motor ac motor stepping motor

dc MotorThe rotation of a dc motor is produced by the interaction of two constant magnetic fields. Fig. 7.27a shows one type of dc motor that employs a permanent magnet to form one of the magnetic fields. The second magnetic field is formed by passing a current through a coil of wire contained within the PM field. This coil of wire, called the armature, is free to rotate. The coil is connected to the current source through slip rings and brushes (called a commutator) but the slip rings are split so that the current reverses direction as the armature rotates.The use of dc motors in control systems ranges from low energy, delicate control applications, to heavy-duty control operations in elevators and vehicles.

FIGURE 7.27 Permanent magnet dc motor.




Ac Motors

The rotation of ac motor is also produced by the interaction between two magnetic fields. In this case. Both fields are varying in time in consonance with the ac excitation voltage.2 basic types of ac motors, synchronous and induction

Synchronous motor

In a synchronous motor the ac voltage is applied to the field coils, called stator in an ac motor. This means the magnetic field is changing in time in phase with impressed ac voltage. The armature called the rotor for ac motors, is either a permanent magnet or a dc electromagnet, and possesses a fixed magnetic field.

FIGURE 7.30 Simple ac synchronous motor with a permanent magnet rotor.




ac induction motor

Induction ac motors are characterized by a rotor which is neither a PM not a dc excited electromagnet. Instead current induced in a coil wound on the rotor generates the interacting magnetic field of the rotor. This current is induced from the stator coils.

FIGURE 7.31 The induction motor depends on a rotor field from current induced by ac field coils, as shown in FIGURE 7.30.




Stepping motor

A stepping motor is a rotating machine that actually completes a full rotation by sequencing through a series of discrete rotational steps. It can be interfaced with digital circuits.

FIGURE 7.33 An elementary stepping motor.




Pneumatic actuators

Pneumatic actuator converts energy (in the form of compresses air, typically) into motion. The motion can be rotary or linear, depending on the type of actuator.

If we imagine that a net pressure difference is applied across a diaphragm of surface area A, then a net force acts on the diaphragm.

F=A(p1-p2)

FIGURE 7.36 A direct pneumatic actuator for converting pressure signals into mechanical shaft motion.




FIGURE 7.37 A reverse-acting pneumatic actuator.




∆x=A∆p/k

Hydraulic Actuators

Hydraulic actuators get their power from pressurized hydraulic fluid, which is typically oil. The actuator consists of a cylinder barrel, in which a piston connected to a piston rod is moving. The barrel is closed by the cylinder bottom and by the cylinder head where the piston rod comes out of the cylinder. The piston has sliding rings and seals. The piston divides the inside of the cylinder in two chambers, the bottom chamber and the piston rod side chamber. The hydraulic pressure acts on the piston to do linear work. A hydraulic cylinder is the actuator or "motor" side of this system. The "generator" side of the hydraulic system is the hydraulic pump, that brings a fixed or regulated flow of oil into the system

The hydraulic pressure is given by

pH =F1/A1

Where pH=hydraulic pressure

F1=applied piston force

A1=forcing piston area

This pressure is transferred equally throughout the liquid, so the resulting force on the working piston is

Fw=pHA2

so working force Fw=A2F1/A1

FIGURE 7.38 A hydraulic actuator converts a small force, F1 , into an amplified force, FW .




FIGURE 7.39 A hydraulic servo system. The process-control system provides input of the setpoint to the hydraulic servo.




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Control Element.ppt