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Overview of Solenoid as an Example of a Final Control Element

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A final control element is a device that physically changes a process as a result of a change in a control system setpoint. This is typically done with the use of an actuator and a control valve. The actuator accepts a signal from the control system and in response, the control valve position is moved to a different position in order to create a change in the control system. Final control elements are a vital inclusion to control systems, as they allow the operator to achieve a desired output by controlling a variable system setpoint.

A solenoid is an example of a final control element that is created from a coiled length of metal wire, which is looped around a metallic core. When a voltage is applied to the solenoid, a current is passed through the wire and a magnetic field is produced. Electricity is converted to magnetism and magnetism is converted to electricity, which results in these two forces being combined into one mechanical force. This resultant force can be incredibly powerful, capable of achieving large amounts of work.

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With the metal wire being coiled around a core, when an electrical current is passed through, the core of the solenoid is magnetically attracted towards the centre of the coil. This is turn compresses a small spring which is attached to one end of the core. When the electrical current is turned off, the solenoid is ‘de-energises’, and the electromagnetic field that was generated dissipates and the energy that was stored in the compressed spring forces the core back from its position, settling back into its original position.

The strength and magnitude of the resultant magnetic field is determined by the number of coils that are present in the solenoid. For this reason, a solenoid is classed as a type of electromagnet. Electromagnets can be switched on and off by increasing or decreasing the flow of current through them. Electromagnets are incredibly useful in circuits because of their ability to be controlled using electricity, which makes them advantageous to use as automated valves and switches.

There are multiple types of solenoids, differing in materials, design and function, however the same electrical principles are present in each type. As a result, solenoids have many different applications. Solenoids are commonly used in locking mechanisms, and the range of applications in locking applications is involved in many industries. A basic yet incredibly common type of locking solenoids is in the use of door locks. This can range from hotel doors, to the doors on safes.Using solenoids in locking applications takes advantage of the metallic core that is present in the centre of the solenoid. This core is driven forward or backward with force by the change in magnetism that results from a change in electric current. This core either becomes that latch that is socketed into the frame to retain the door or it becomes the driving force to move levers which lock the doors.The solenoid is an important component to a great number of industrial devices. Its application can be found in devices that require holding, rotating, diverting, positioning, locking, pinching, valve operations, and more. One example of this is in sprinkler and irrigation systems. In irrigation systems, solenoids are coupled with valves to control water pressure. The metallic core of the solenoid is spring loaded to act as a piston, so that when the valve is closed, the inlet port hole of the irrigation system is covered. When water in the upper chamber of the system is released, it triggers a change in system input for the solenoid, which results in the valve opening which allows the water to travel through the port hole. This solenoid change consequently outputs a force which pushes the water through the system into a separate area (into another chamber or out of the sprinkler outlet).

A distinct advantage of the solenoid over other electrical components is that, once electricity is applied to the device, the resultant output reaction of the core is instantaneous. This immediate response is particularly valuable in applications where fast reactions to state changes are crucial. One area where this is of importance is in medical devices. Medical devices are required to act fast when needed, while still being very precise and accurate. One device that makes use of the solenoid is in dosing machines.A solenoid dosing pump uses both a solenoid arrangement and a diaphragm, and makes use of positive displacement theory to displace fluid into a output discharge line. The solenoid arrangement consists of the solenoid core attached to a spring. When the solenoid is activated with an input of electric current, the spring core pushes the attached diaphragm which displaces the fluid. When the solenoid is deactivated, the spring core returns into the centre of the solenoid releasing the force placed on the diaphragm, which allows the fluid from the input suction line to enter the chamber where it sits ready for delivery, awaiting a change in solenoid state again.

There are a number of different types of solenoid arrangements that vary in both design and their function. Even though there are so many different types of solenoids, they all operate on the core principle of electromagnetism. A linear solenoid is the most common type of solenoid arrangement. This device converts electrical energy into a linear mechanical ‘push’ or ‘pull’ force, which acts on the core of the solenoid, forcing it against a neighbouring mechanical device. When the solenoid is de-energised the core is forced back into its original rest position. This type of solenoid can be used as a holding device (continually energised), as a latching type (On-Off Pulses), or as a proportional motion control where the core position is set to be proportional to the power input.

An incredibly common example of a linear solenoid comes from the automotive industry – the starter motor. In a solenoid starter motor, when an electrical current is applied to the solenoid, the solenoid moves in a linear fashion, bringing two contacts together. When these two contacts are brought together, they allow the power to flow from the battery of the vehicle to various other components which are required to start the car, thus allowing the vehicle to run.


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