Proximity Switch

What is a proximity switch and how it works?

It is the switch that detects the closeness/ proximity of an object. The switches are non-contact type sensors that use magnetic, electric, or optical means to sense the proximity of objects.

The below schematic diagram symbol for a proximity switch with mechanical contacts is the same as for a mechanical limit switch, except the switch symbol is enclosed by a diamond shape, indicating a powered or active device.

The proximity switch will be in its “normal” condition/status when it is distant from any actuating object that we have to detect. Due to the non-contact type in nature, it is often used instead of direct-contact limit switches for the same purpose of detecting the position of a machine part, with the advantage of never wearing out over a long time due to repeated physical contact. However, the greater complexity and cost of a proximity switch over a mechanical limit switch shows their use in applications where lack of physical contact yields tangible benefits.

What are different types of proximity switches?

Most of the proximity, switches are active in design. They have a powered electronic circuit inside them to detect the proximity of an object.

• Inductive proximity switches detect the presence of metallic objects with the help of a high-frequency magnetic field.
• Capacitive proximity switches detect the presence of non-metallic objects with the help of a high-frequency electric field.
• Optical proximity switches detect the interruption of a light beam by an object when it passes through the beam.
• Ultrasonic proximity switches detect the presence of dense matter by the reflection of sound waves by sending the sample waves.

Most of the proximity, switches have no “dry contact” outputs. Instead of the contacts output elements are transistors configured as either source current or sink current. Both “sourcing” and “sinking” will be understood by visualizing electric current in the direction of electron flow instead of conventional flow. The below schematic diagrams show the direction of current (conventional flow notation). The load being driven by each proximity switch is a light-emitting diode (LED) in both the examples:

What is sinking and sourcing? or How the switch connected as “NPN” type and “PNP type?

An NPN switch uses a transistor in its output referred to as an electronic switch designed to sink current through its signal wire. Similarly, a PNP switch is referred to as an electronic switch designed to source current through its signal wire. To understand this, recognize the emitter terminal of the output transistor is always the one connected to the power supply rail. The emitter must connect to the negative rail, necessitating an NPN transistor to do the switching, for a sinking switch. And the emitter connects to the positive rail for a PNP transistor will suffice known as a sourcing switch.

Sometimes sourcing and sinking transistor switches are referred to as high-side switches and low-side switches, respectively. The emitter terminal is attached directly to the “high” rail (+) of the DC power supply known as sourcing transistor (PNP). And the emitter terminal is attached directly to the “low” rail (-) of the DC power supply known as the sinking transistor (NPN).

There are two different styles of electronic proximity switch shown below:

Most of the industries that use proximity switches have built-in LED indicator lamps to help technicians diagnose circuit problems by directly indicating the switch status.

The below photograph shows a proximity switch detecting the passing of teeth on a chain sprocket, generating a slow square-wave electrical signal as the sprocket rotates. Those switches may be used as a rotational speed sensor (sprocket speed proportional to signal frequency) or as a broken chain sensor (when sensing the rotation of the driven sprocket instead of the drive sprocket):

Proximity switches come in both “normally open” (NO) and “normally closed” (NC) types. Normally-open proximity switches that are sinking (NPN) as well as normally-open proximity switches that are sourcing (PNP), and normally-closed proximity switches in either sinking or sourcing designs as well. The proximity switch detection range is usually a fixed parameter rather than being adjustable.

List of Prominent Manufacturers: Adtek, Aeco, Aignep, Airmar, Autonics, Azbil, Balluff, Baumer, Bernstein, Broadcom, Comus, Crouzet, Datalogic, Di-soric, DropsA, Electro Sensors, Elen, Etatron D.S., Fargo Controls Inc, Festo, Fipa, Fiama, Gimatic, Hasco, Honeywell, IPF Electronic, JPC Connectivity, Klaschka, Littlefuse, MaxBotix, Melexis, Microsonic, Micro Detectors, Migatron Corp, Monarch, Nidec, Nivelco, Norelem, Omron, Optex, Parker, Pepperl+Fuchs, Proxitron, RS Pro, Schmersal, Sensopart, Soway, Stahl, Telemecanique, Univer, Waycon, Weber, Wick Sensors, Yaskawa, Zimmer

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Limit Switch

What is a limit switch?

A limit switch detects the physical motion of an object by direct contact with that object. It will be in its “normal” status when it is not in contact with anything (i.e. not touching the switch actuator).

An example of a limit switch is a switch detecting the open position of a door or an elevator, automatically energizing the cabin light when the door opens.

Limit switches find many uses in industry, generally in robotic control and CNC (Computer Numerical Control) machine tool systems. In many motion-control systems, the moving elements have “normal” positions where the computer assigns a position value of zero. These normal positions are detected by means of limit switches. The computer gives the command to each servo motor to travel fully in one direction until a limit switch on each axis trips. The position counter for each axis resets to zero as soon as the respective limit switch detects that the normal position has been reached.

A typical limit switch design uses a roller-tipped lever to contact the moving part. Screw terminals on the switch provide connection points with the NC and NO contacts inside the switch. Most limit switches of this design share a “common” terminal between the NC and NO contacts like shown below:

This switch contact arrangement is referred to as a form-C contact set since it incorporates both a form-A contact (normally-open) as well as a Form-B contact (normally-closed) inside it.

A close-up view of several limit switches shows the arrangement of connection terminals for form-C contacts. As shown above, each limit switch has its own “NO” (normally-open), “NC” (normally-closed), and “C” (common) screw terminal for wires.

As shown in the photograph above a limit switch assembly attached to the stem of a rotary valve used to detect the fully-closed and fully-open positions of the valve.

List of Prominent Manufacturers:  ABB, Allen Bradely, Ametek, Auspicious, Azbil, B-Command, Bartec, Berstein, Craig & Derricott, Crouzet, Divatec, Eaton, Elen, Elfin, Emass, Euchner, Festo, Gessmann, Greegoo, HASCO, Honeywell, IMO, Mayr, Metso, Micronor, Omron, Otto, Panasonic, Prisma, Schmersal, Telemechanique, Topworx, Vatroc,

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Hand Switch

What is a hand switch?

An electrical switch actuated manually by a person’s hand motion is known as a hand switch and that is exactly what the name implies. It may be the form of toggle, pushbutton, rotary, pull-chain, etc. A common industrial pushbutton switch looks likes as shown below:

The threaded neck inserts through a hole cut into a metal or plastic panel, with a matching nut to hold it in place. Thus, the button faces the operator(s) while the switch contacts reside on the other side of the panel. When pressed, the downward motion of the actuator breaks the electrical bridge between the two NC contacts, forming a new bridge between the NO contacts.

The schematic diagram symbol for this type of switch looks much like the real thing, with the normally-closed contact set on top and the normally-open contact set below:

Connecting two of the terminals together makes this form of switch electrically identical to a Form C as seen below:

This switch contact arrangement is sometimes referred to as a form-C contact set, since it incorporates both a form-A contact (normally-open) as well as a form-B contact (normally-closed).

Most industrial hand switches or push buttons are available in modular form, where sets of switch contact blocks may be “stacked” together to be actuated by the same pushbutton or rotary knob.

List of Prominent Manufacturers: ABB, Aignep, Allen Bradelley, Auspicious, Autonics, BACO, Banner, Benedict, Bernstein, Camdenboss, Captron, Eao, Eaton, Elektra, Elfin, Emass, Gessmann, Gewiss, IDEC, Johnson Electric, Lovato Electric, Marchel Electric, Omega, OMRON, Pepperl + Fuchs, RS Pro, Schaltbau, Schmersal, Siemens, Stahl, Telemechanique, Wenglor, Weg

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Normal status of a Switch (open or closed)

Electrical switch contacts are named as either normally open or normally closed, referring to the open or closed status of the contacts under “normal” conditions.

Define “normal” for a switch?

The “normal” status for a switch is referred to as its electrical contacts are under a condition of no physical stimulation or without energization. Another way to think of the “normal” status is in terms of the switch being at rest position. For a momentary-contact pushbutton switch, this would be the status of the switch contact when it is not being pressed or stimulated. The “normal” status of any switch is the way it is drawn in an electrical schematic as described below.

Normally Open Switch (NO)

The below switch is a normally-open (NO) switch because it is drawn in an open position. The lamp will energize only if someone presses the switch, holding its normally-open contacts into the “closed” position. Normally-open switch contacts are referred to as form-A contacts in the electrical industry.

(normally-open pushbutton switch controlling a lamp)

Normally Close Switch (NC)

If we had used a normally-closed pushbutton switch instead of normally-open pushbutton, the behavior would be exactly the opposite. The lamp would energize if the switch was left alone or in normal condition, but it would turn off if anyone pressed the switch. Normally-closed switch contacts are referred to as form-B contacts in the electrical industry.

(normally-closed pushbutton switch controlling a lamp)

A flow switch is built to detect fluid flow conditions through a pipe. In the below schematic diagram, the switch symbol appears to be a toggle switch with a “flag” hanging below. The schematic diagram, of course, only shows the circuitry and not the pipe where the switch is physically mounted.

This particular flow switch is used to trigger an alarm light if coolant flow through the pipe ever falls to a critical level, and the contacts are normally-closed as evidenced by the closed status in the diagram. Even though this switch is designated as “normally-closed,” it will spend most of its lifetime being held in the open status. Only when there is enough flow through the pipe the switch return to its “normal” status and conduct electrical power to the lamp. In other words, the “normal” status of this switch (closed) is an abnormal status for the process it is sensing (low flow).

The manufacturer of the switch has no idea whatsoever as to your intended use and does not know whether you intend to use their flow switch as a low-flow alarm or as a high-flow alarm. In other words, the manufacturer cannot predict what is the typical status of your process will be, and so the definition of “normal” status for the switch must be founded on some common criteria unrelated to your application. That common criteria are the resting status when the sensor is exposed to the least (or no) amount of stimulation from the process it senses.

Listing of “normal” definitions for various discrete sensor types:

• Hand switch refers to no one pressing the switch
• Limit switch refers to target not contacting the switch
• Proximity switch refers to target far away
• Pressure switch refers to low pressure (or even a vacuum)
• Level switch refers to low level (empty)
• Temperature switch refers to low temperature (cold)
• Flow switch refers to low flow rate (fluid stopped)

These are the normal conditions represented by the switch statuses shown in a schematic diagram as per the manufacturers.

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Indicator, Recorder and Pressure Switch & Alarm

Generally, we use instruments like transmitters, controllers, and control valves to sense, control, and influence the process variables to run the industry's process smoothly. Apart from that, some other instruments play important functions, such as indicators, recorders, and process switches and alarms.

Indicators

The purpose of the indicator is to provide a human-readable indication of an instrument signal proportional to the process value. Some transmitters are not equipped with readouts for whatever variable they measure, they just transmit a standard instrument signal to another device or control panel.

An indicator gives a convenient way of what the output of the transmitter is without having to connect test equipment. Indicators may be located far from their respective transmitters, providing readouts in the locations more convenient than the transmitter's location. It may also be used in “field or process” areas to directly indicate measured variables if the transmitter device lacks a human-readable indicator of its own.

List of Prominent Manufacturers: Ametek, Ascon Technologic, Autonics, Barksdale, BD Sensors, Dwyer Instruments, Endress + Hauser, Eurotherm, Flowline, Futek, Gerfan, GIVI Misure, Motrona, OHM Group, Omega, Pepperl + Fuchs, Simex, Yokogawa

Recorders

Recorder (generally a chart recorder or a trend recorder), is used to draw a graph of the process variable(s) over time. It usually has indications built into them for showing the instantaneous value of the instrument signal(s) simultaneously with the historical values, and for this reason, are usually designated as indicating recorders. A circular chart recorder uses a round sheet of paper, rotated beneath a pen moved side-to-side by a servomechanism driven by the instrument signal.

It is extremely helpful for troubleshooting process control problems. The recorder is configured to record the process variable as well as the controller’s setpoint and output variables. It became a powerful diagnostic tool when coupled with the controller’s manual control mode. By placing a controller in “manual” mode and allowing direct human control over the final control element (valve, motor, heater), can show a lot about a process.

Check When the output signal is “stepped” to a new value and when the process variable responds to the change, the time delay between. This sort of time delay is generally not good for a control system. The main causes of these problems are transport delay and mechanical problems in the final control element. This problem is usually caused by mechanical friction in the final control element (for example, sticky valve stem packing in a pneumatically actuated control valve) and is analogous to ““lose”” steering in an automobile, where the driver must turn the steering wheel a little bit extra after reversing the steering direction.

List of Prominent Manufacturers: Ascon Technologic, Chino, Eurotherm, Omega, PCI Instruments, Sensia, Stiko, Yokogawa

Process switches and alarms

The purpose of a pressure switch is to turn on and off with varying process conditions. Mainly, switches are used to activate alarms to alert human operators to take special action. In some other situations, switches are directly used as control devices.

The “PSH” (pressure switch, high) activates when the air pressure reaches its high control point. The “PSL” (pressure switch, low) activates when the air pressure drops down to its low control point. Both switches feed discrete (on/off) electrical signals to a logic control device which then controls the starting and stopping of the device (ex: motor, fan, etc).

In addition to providing alarm capability, the SPA module provides a digital display to show the analog signal value for operational or diagnostic purposes. Some of the alarm types it shows are high process, low process, out-of-range, and high rate-of-change. We may add pressure-actuated process alarm switches to pneumatic signal lines coming from pneumatic transmitters to add alarm capability for continuous measurement. Two-alarm switches (LSL and LSH) indirectly sense water level by monitoring the pneumatic signal pressure which is output by the level transmitter (LT).

Process alarm switches are sometimes used to trigger a special type of indicator device known as an annunciator. An annunciator has an array of indicator lights and associated circuitry designed to secure a human operator’s attention by blinking and also sounding an audible buzzer when a process switch actuates into an abnormal state. The alarm state may be then “acknowledged” by an operator pushing a button, causing the alarm light to remain on rather than a blink, and silencing the buzzer. The indicator light does not turn off until the actual alarm condition (process switch) has returned to its normal state.

Panel-mounted annunciators are used in the past and computer-based alarm displays replace them with advanced capabilities such as time logging, first-event recording, and multiple layers of acknowledgment/access. Time logging is an important factor in the process industries, as the sequence of events is often extremely important in investigations following an abnormal operating condition.

List of Prominent Manufacturers: Afriso, Barksdale, Danfoss, Dwyer Instrument, Festo, Lefoo, PCI Instrument, Toscano, UEC

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Instrumentation Terminology

Both the measurement device and the final control device connected to the process and controlled by the controller as per the requirement.

The first step is always measurement and if we can’t measure, then there is no meaning of control the process as everything depends upon the measuring value.

For example Pressure, temperature, flow, volume, position, motion, acceleration, concentration, voltage, current and resistance.

When we get the measured value, we transmit a signal representing this quantity to an indicating or computing device where either human or automated action then takes place. Where the controlling action is automated, the computer sends a signal to a final controlling device to act.

For example Control valve (for throttling the flow rate of a fluid), Electric motor, Electric heater.

What are the instrumentation terms that we used in industries regularly?

Process: The physical system where we measure and attempt to control. For example steam boiler, oil refinery unit, power generation unit, dm plant, iron and aluminum industry, etc.

Process Variable (PV): The specific quantity we measure in the process. For example pressure, level, temperature, flow, electrical conductivity, pH, position, speed, vibration.

Setpoint (SP): The specific value/quantity which we have to maintain the process, otherwise named as the target value.

Primary Sensing Element (PSE): A device that direct exposure to the process and senses the process variable, then convert it to an analog signal for use. For example thermocouple, thermistor, bourdon tube, microphone, potentiometer, electrochemical cell, accelerometer.

Transducer: A device that converts the signal getting from the primary sensing element into another standardized instrumentation signal, and/or performs some sort of processing on that signal. For examples: I/P converter (converts 4-20 mA electric signal into 3-15 PSI pneumatic signal), P/I converter (converts 3-15 PSI pneumatic signal into 4-20 mA electric signal), square-root extractor (calculates the square root of the input signal).

Transmitter: A device that contains a transducer which translates the signal produced by a primary sensing element into a standardized instrumentation signal and then conveyed to an indicating the device, a controlling device, or both. For example,  4-20 mA DC electric current, Fieldbus digital signal packet, etc.

Lower- and Upper-range values (LRV and URV): The values of process measurement deemed to be 0% and 100% of a transmitter’s calibrated range. For example, if a temperature transmitter is calibrated to measure a range of temperature starting at 50 degrees Celsius and ending at 200 degrees Celsius, then 50 degrees would be the LRV and 200 degrees would be the URV.

Zero and Span: Alternative descriptions to LRV and URV for the 0% and 100% points of an instrument’s calibrated range. “Zero” is the beginning point of an instrument’s range (equivalent to LRV), while “span” refers to the width of its range (URV − LRV). For example, if a temperature transmitter is calibrated to measure a range of temperatures starting at 50 degrees Celsius and ending at 200 degrees Celsius, its zero would be 50 degrees and its span would be 150 degrees.

Controller: A device that receives a process variable (PV) signal from a transmitter, then compares that signal to the desired value for that process variable (called the setpoint) and calculates an appropriate output signal value to be sent to a final control element (FCE). For example an electric motor or control valve.

Manipulated Variable (MV): The output signal generated by a controller to achieve the set point. This is the signal manipulating the final control element to influence /react to the process.

Final Control Element (FCE): A device that receives the signal from a controller to react directly or influence the process. Examples: control valve, variable-speed electric motor, electric heater, etc.

Automatic Mode: When output gathered by controller, signal based on the relationship of the process variable (PV) to the setpoint (SP).

Manual Mode: When the controller bypassed to take a decision and a human operator directly determine the output signal sent to the final control element.

A home thermostat is a common example of a measurement and control system, with the home’s internal air temperature being the “process” under control. Here, the thermostat usually serves two functions: sensing and control, while the home’s heater adds heat to the home to increase temperature, and/or the home’s air conditioner extracts heat from the home to decrease temperature. The job of this control system is to maintain air temperature at some comfortable level, with the heater or air conditioner taking action to correct temperature if it is too far from the desired value (called the setpoint).

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Intro to Industrial Instrumentation

Instrumentation is the science of measuring, indicating, recording the process parameters and control the process.

We will cover industrial instrumentation basics such as the measurement of process variables, control, design, and implementation of systems that incorporate them.

Process: The physical system/Field where we are attempting to control or measure.

Examples: Power plant, Oil & Gas refinery, Water filtration system like DM plant, Metal industries, etc.

Process Variable (PV): The specific quantity we are measuring in a process.

Examples: Pressure, temperature, level, flow, position, speed, vibration, etc.

Instruments that sense, indicate, control, and influence process variables are indicators, Transmitters, controllers, recorders, switches, and control valves.

From the next session onwards, we will elaborately discuss this.

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