Control System Instrumentation

What are the types of instrumentation switches?

What is a switch

A switch is used to open or close an electric circuit, a switch can break an electric circuit. The switches are called binary devices because they are either completely open or completely closed. We can see different types of switches used to do an industrial process. These switches are the major part of the industrial control system. The important function of an instrumentation switch is to turn on and off according to the different process conditions.

What is the normally open and normally closed configuration in a switch?

The normally open mode or NO mode, in a switch, means that the switch does not allow the power supply to reach the circuit so there is no electric power flow through the load. During this condition, the load and the contact will be deactivated.

The NC-mode is considered as a deactivation condition it would allow the power to reach the load and thus there would be a current flow through the load. In this mode, the contact is deactivated and the load will be activated.

Push button switch

A push-button switch is a press and release button and these switches have a spring mechanism so after pressing the switch it would reach back to its original position. A push to make the switch to switch on If we push the switch then electricity flows between its contacts and if the button is released then the circuit is broken and this type of switch is called as normally open switch an example for this will be computer keys and calculator buttons.

Push to break switch

The push-to-break switch is the exact opposite of this switch. So the electricity would flow if the button is not pressed but if we press the button then the electricity flow would stop. This is called a normally closed switch an example of this will be the alarm switches in fail-safe circuits.

Emergency stop switch

It is a general-purpose push button switch that can be used to control a process it can start the operation of a machine or switch modes. The emergency stop push button switch can be considered as a shutdown switch it can be used to stop hazardous loads.

Limit switch

These switches can detect the physical motion of an object by direct contact with the object. This switch would operate as an automatic control to control a process in case if the process goes beyond a determined limit. So the limit switch can be described as a human operator. A limit switch has a lever that would be roller tipped and it would make contact with the moving part. The switch body has a screw terminal and it is connected with the NO and NC contact with the switch.

Level switch

A level switch is an electromechanical switch and it is used to determine the level of liquids and solids. These switches are placed in tanks, hoppers, or bins, these switches would give their measurement to the control system.

Toggle switch

These switches are used for switching circuits and they are hand-operated, these switches are actuated by a lever and the lever will be angled in one or two numbers. The switch we use in our home is an example of the toggle switch, the electrical circuit in a toggle switch is opened and closed by moving the lever back and forth. We could determine the status of the circuit according to the lever position.

Speed switch

By using this switch we can determine the rotary speed of the shaft and this could be determined by using a centrifugal weight mechanism mounted on the shaft or it would also do a noncontact detection of the shaft and it would do it by optical or magnetic means.

Selector switch

These switches are used to select the position, actuator, or mode and these switches are used in control and low power systems. By using a rotary knobe these switches are actuated and thus it can select two or more positions. So mostly we can use a selector switch to select the right or left position, and the selector switch is of two types and they are spring return and stay put. Its switching sequence would be three or two ways.

Rocker switch

These switches can be considered electromechanical devices and they are hand-operated which can be used to switch circuits. We could determine the status of the circuit by visualizing the switch like if it is on or off. The working of this switch is similar to that of the rocker switch but the major difference between a rocker switch and a selector switch is their compactness. The Rocker switch is compact than the selector switch and because of this, it can be used for low-power appliances.  

Rotary/cam switch

These switches are similar to the selector switches and the major difference between these switches are it has three or more positions which can be used to select the position, mode or actuator. These switches are widely used in control systems and also in control circuits.

Pressure switch

This switch would close or open an electrical contact if a certain level of fluid pressure has been reached. These switches are designed in a way that it could make or break the contact in case if there is any rise in pressure or fall in pressure. These switches are widely used in a process it can control or protect the system from low discharge pressure or from a high discharge pressure. The switching mechanism is done by the gas or liquid pressure, if this pressure is applied to piston, diaphragm, or bellows then this pressure would be converted to mechanical pressure. Pressure switches are used in manufacturing machinery, utility plants, and also in public buildings. The pressure switch would use diaphragms or bellows as the pressure sensing element.

Proximity switches

These switches are used to detect the proximity of an object and these switches are non-contact switches. It would use magnetic or electrical properties to detect the proximity of the object. Most of the proximity switches would have an electronic circuit to detect the object. There are three types of proximity switches and they are inductive, capacitive, and optical proximity switches. The metallic objects can be detected by the inductive proximity switches and the non-metallic objects can be detected by the capacitive switches and the optical switches can detect the object by using a light beam.

The above image shows the proximity switch which is used in a chain sprocket to check the passing of teeth on a chain. So during the rotation of the sprocket, it would generate an electrical signal. So this switch can be used as a rotational speed sensor.

Mercury tilt switch

A Mercury tilt switch is a sensor that uses a lump of mercury each time when it is tilted. This type of switch doesn’t have a fast response, plumes of mercury are used to determine contact in an open or closed switch. The major advantage of the mercury tilt switch is that it is immune to switch contact degradation from the dirty or dusty atmosphere and also from explosive atmosphere.

Flow switch

These switches are used to detect the flow of the fluid, these switches are used to determine if the required flow is available or not. So if the required flow is not available then it could damage the system. There are different types of flow switches and we can choose the flow switches according to the line size. 

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