In Industry, To increase the efficiency, quantity & quality in production line; Sensors play a very crucial role. It is quite difficult without sensors in this modern world.
” Sensors are machines, devices or gadgets which monitors physical quantities like temperature, pressure, flow, light, motion, weight, vibration & then; produces an output in the form of electrical signal. A signal from a sensor; been given to a controller. Than controller; either displays the physical parameters & produces an output.”
Industrial Sensors
A good sensor Obeys the following Rules :
It is sensitive to measure quantity & insensitively to any other property likely to be encountered in its application.
It doesn’t influence the measured quantity.
There are many type of sensors which are being widely used in many type of Industries. In this post, There is explanation of 10, Common 10Sensors which are usual in industry.
Before proceeding further, We should know about the methods how these sensors provides electrical signals. It will be much easier to understand these sensors by knowing these methods of signals. Sensors provides signal through multiple methods; either through ‘Electromechanical switching‘ or ‘Electronic switching‘.
Electromechanical switching
Symbols of ‘NO & NC’ Contacts
Electromechanical switching is a combination of ‘electrical’ & ‘mechanical’ elements.
These switches contains moving contacts Which either work through electromagnet or through mechanical arrangement.
Electromechanical switching contains two contacts :
Each of these contacts has two states, 1st is Normal state/default state while 2nd is Forced state/pressed state.
Let’s understand these electrical contact switching with an Example below.
” Bell switch-
In unpressed position, the contact of bell switch remains open while when we press the bell switch, contacts get closed & bell starts ringing until we release the bell switch.
When we release the bell switch, bell switch contact get open again & bell stops ringing. Bell switch is the best example of NO contact, At normal position/Not pressed position contact is open while at forced/pressed position contact is closed.
Refrigerator’s door switch–
When refrigerator door open, the refrigerator light get on until we closed the door. When door get closed, the door ‘s light get OFF. Refrigerator door switch is particularly aNC switch. At normal/not pressed position contact is closed while at forced position/pressed position contacts is open. “
Changeover relay(Type of Electromechanical Switch)
Changeover relay is mechanical arrangement which contains an electromagnet with spring mechanism. Both the contacts NO & NC along with one common contact are available in this relay.
At normal/default state, When electromagnet is not energized common pin is connected with NC pin with the help of spring mechanism.
At forces state, When electromagnet get energized the common pin get connected toNO pin until electromagnet remains energized.
A Schematic diagram of changeover relay is shown below :
Electromagnetic changeover relay
A Changeover relay printed on temperature controller (TC344AX) by Selec.
Changeover relay used in SELEC Temperature controller (TC344AX)
Electronic switching
Electronic switching does not contain any moving part like in electromagnetic relay. Rather it uses electronics switch element.
Generally, A 3 terminals electronic element is used in electronics switching. That’s why electronic switching is fast & having a long life.
A ‘BJT’ or ‘FET’ is used in electronic switching.
BJT (Bipolar junction transistor) as a switch
” In an image above, NPN transistor is used in switching circuit which contains three terminals collector, emitter & base. LED is connected at collector terminal & emitter is connected with negative. When no base current is applied no collector current flows. But applied voltage on base terminal triggers the collector current which results LED starts glowing.”
NOTE: Every day is the new day which come up with advance version of old devices Or machines. In this post, We will the explain the working of sensors which are been available to us. There is complete possibility that advance version of explained sensor can be available in the market.
10 Common Industrial Sensors
1.Temperature Sensors — Types, Working & Uses
Digital temperature devices can’t measure the temperature directly instead these devices observe electrical signal changes produced by temperature sensors. These sensors produces signal in proportion to the temperature.
For digital Temperature meter, There are two types of temperature sensors are available- ‘Thermocouples’ & ‘RTD’. Both the sensors work on different working principles. These sensors provide signal to temperature controller as per change in temperature & in return controller shows temperature As per stored information in it.
Thermocouplesare junction of two dissimilar metals which produces milli-volt change as per change in temperature. Thermocouples are two wired sensors with wide temperature range from -199°C to +1700°C. Thermocouples further have many types J,K,T,R&S & these types have different temperature ranges.
A table of input ranges of different thermocouples is shown below :
Input ranges of thermocouples
K- Type Temperature device with K-type thermocouple shown below :
K- Type Temperature device with K-type thermocouple
RTD stands for ‘resistance temperature detector’. RTD sensors are available in 2,3&4 terminals.
The Resistance value of these sensors changes as per change in temperature around it. RTD has temperature range from -150°C to +850°C.
RTD PT100 3 terminals; Length 12″ with 6mm thickness
The 2 wires RTD are used where sensor’s leads are short because longer leads increases line resistance which increases error.
While 3 & 4 wires sensors have not such impact because longer leads resistance is eliminated in these sensors which is better explained below.
Circuit diagram of 2 wires & 3 wires RTD
” As shown in an image above,
2 wires RTD total resistance is XA+R+YB. IF both the lead resistances are same than XA=YB Which means total resistance is R+2XA. This extra 2XA resistance produces error value; Larger the leads Larger the error.
3 wires RTD RTD has 1 extra lead which has resistance is YC. IF all the 3 wires have same resistance than XA=YB=YC. Temperature controller produces known current through CYB. So that an average lead resistance can be known, By subtracting the lead resistance, we can get actual temperature resistance. As, R=(XA+R+YB)-(YB+YC) “
// A Temperature controller TC344 by Selec is discussed below. These controller has provision for both the temperature sensors thermocouples & RTD.
Temperature controller TC344 by SELEC
Temperature Sensor selection on TC344 by SELEC
2. PROXIMITY SENSORS
” Proximity sensors” produces an electrical output Whenever they sense an object near around their periphery without any type of physical contact. There are 2 type of proximity sensors such as ‘Inductive’ & ‘capacitive’ sensor.
Capacitive type proximity sensor works on the principle of capacitance. Any type of material appear across its sensory periphery, It calculates the change in capacitance value & accordingly it produces an electrical output.
Inductive type proximity sensors sense ferrous material without any physical contact. This sensor works on electromagnetic induction principle in which it continuously emits electromagnetic field around its periphery.
Whenever a ferrous material come in its vicinity, eddy current get induced on ferrous material. Which results, Sensor impedance get changes which is being detected by the microprocessor. After that, microprocessor produces an output signal.
There are different types of proximity sensors are available Which varies as per their :
Shape & size
Sensing length
Working voltage
Output Signal type
Number of signal output
These different variations are explained through PNP NO 24VDC proximity inductive sensor below:
PNP NO 24VDC proximity inductive sensor; sensing length 4mm; dimension are 12mm*60mm(Diameter*Length)
Sensor shown in an image having a
Shape & Size- Sensor is cylindrical in shape with 12mm Diameter & 60mm Length.
Sensing length- Sensor’s Sensing length is 4mm.
Working voltage- Sensor’s working voltage is 24Volt DC.
Output signal- For output type there are two types of sensors PNP & NPN. PNP sensor produces positive signal while NPN sensor produces negative signal. In an image, Sensor is PNP NO which means output signal is positive with NO contact.
Number of signal output- Sensor has 3 wires in which 2 wire supply wires are positive & negative. While third wire is signal wire which is PNP NO. There are sensors available with 2 output both NO & NC.
3. Reed sensors
Reed sensor contains contacts with ferromagnetic materials which act as a magnetic switch. These contacts changes its state in the presence of magnetic field. While moving away from magnetic field, The electrical contact of reed sensor get back to its normal state.
These sensors are generally fitted around pneumatic cylinders; Whose piston head contains a magnet. This sensor sense the positioning of cylinder head & give command to the PLC.
In an image below, 3 wired 24VDC reed sensor is shown in which 2 wires are positive & negative while 3rd wire is signal wire. For signal indication an LED bulb is being present on the sensor.
Magnetic Reed Sensor by JANATICS INDIA
4. Limit switches
Limit switches are the most common industrial sensor which are most widely used.
Limit switch is the mechanical arrangement which contains a lever. When mechanical movement pushes the lever, The lever with the help of connected mechanical arrangement, pushes the relay. Which results relay’s electrical contacts get changed parallelly.
When that mechanical movement get away from the lever, Lever get back to its previous normal state with the help of spring connected to it.
Meanwhile, electrical contacts also get back to its normal state due to the springs attach to it.
Both the states of the limit switch; pressed & unpressed are clearly explained in an images below. Contacts 13&14 are NO(Normally open) & Contacts 21&22 are NC(Normally closed)
Roller lever Limit switch by SIEMENS
Limit switch in unpressed position (Electrical contacts in normal state)
Limit switch in pressed position (Electrical contacts in forced state)
5. Pressure sensors
” Pressure sensors produces an electrical signal when an particular level pressure is crossed ! “
The pressure sensor explained below is fully mechanical which is being fitted on air cylinder. Air pressure inside the cylinder produces the same pressure inside the sensor.
Pressure sensor here helps to maintain the air pressure inside the cylinder while operating the compressor under the set air pressure.
The mechanical arrangement inside this sensor produces a movement when a particular defined pressure get applied on the sensor. This movement is used to trigger the electrical relay which further results electrical contacts get changed. When pressure decreases below the defined pressure the electrical contacts get back to its previous normal state.
Relay contacts are NC contacts. At normal state, Relay’s contacts are closed while at forced state contacts are opened.
500 Liter Air Compressor With Pressure Switch range 7-15 Kg/Cm²
Alfa air Pressure Sensor with range 7-15 kg/Cm²
Pressure relay shown in an image above contains three electrical contacts which all are NC contacts. This Alfa pressure sensor contains a Auto/Manual switch along with a pressure adjustable nut. By moving this NUT we can adjust the pressure as per our need, Tightening the nut leads to increase in set pressure while loosing the nut leads to decrease in set pressure.
6. Level sensors
There are many types of level sensors with different working principle. Next, Micro-processor based vibrating fork tine levelsensor is explained !
This sensor contains fork tines with special shape & structure. It continuously vibrates due to piezo-electric phenomenon at standard frequency & amplitude in the absence of product.
When these forks get surrounded with product, There is change occurs in frequency & amplitude. Which is been observed by the micro processor, than a signal is being triggered to the relay.
The level sensor explained here below is made by SAPCON. It works on both the supplies AC(90-265) & DC(24-55). It can detect all the industrial fluid up to the temperature of 200°C. Small switches are provided for calibration & time adjustment for both covered & uncovered position. For output signal there are 2 changeover relays with contact rating of 6Amps,230VAC(Resistive).
Vibrating fork tine level Sensor by SAPCON
Vibrating fork tine level Sensor by SAPCON
7. Flow sensors
Flow sensors are mechanical arrangement with changeover relay. Flow sensors are needed to be fitted on liquid pipe line so that metal sheets of flow sensor get inside the pipe line.
When liquid flows through pipe line, Pressure get available in the line. This pressure pushes metal sheet of the sensor; metal sheets are being connected with mechanical arrangement which in turn pushes the changeover relay. Which results electrical contacts get changed.
Flow switch is explained in collage images below :
Flow Sensor
Flow Sensor Working
8. Photo electric sensors
Photoelectric sensors can sense the object without getting in touch with the object. These sensor has two chambers; One is emitter & Second one is receiver.
It’s emitter continuously emits lights. One falling on solid objects like plastic or wood; The light reflect back to the sensor.
On getting the lights back at receiver end; sensor produces an Electrical Signal.
There are three types of photoelectric sensors in which;
Type1 has emitter & receiver are two different units which are being placed in front to each other.
Type2 has emitter & receiver are fitted on single unit but this sensor needs a reflector to reflect the light beam.
Type3 has emitter & receiver are fitted on single unit; no reflector needed; object work as a reflector. These sensors are also known as energetic diffuse sensor.
Next, We are going to explain energetic diffuse sensor made by LEUZE Which is basically a Type3 photoelectric sensor. This 18mm*46mm(Diameter*Length) compact sensor has 4 wires in which 2 wires are supply wires (10-30VDC) & Rest 2 wires are signal wires. Both signals wires are light switching & dark switching which is nothing just a NO & NC contacts. Since sensor is PNP, signal supply is positive.
Photoelectric sensor by LEUZE
9. Current transformers working principle
Voltage has concern with wire’s insulation; High voltage demands high insulation While Current has a concern with Conductor thickness; High current demands thick conductor. Parameter measurement; Voltage & current; is very important from every aspect. If we measure higher values of these parameters directly than our measuring instrument will be large & Bulky. That’s why Indirect measurement methods are being introduced.
To measure higher value of current, Current transformer is being used. This current transformer lower the measured current proportionally which is being fed to energy meter. Current transformer is a form of inductive coil with two terminals which is hollow from inside. Through this hollow space, the conductor must be placed whose current value has to be measured. This whole arrangement act as a transformer where Current transformer act as secondary winding while conductor which is passing through the current transformer act as primary.
When current flows through the conductor, magnetic field get created around the conductor; This magnetic field links with current transformer which results current starts flowing through the current transformer which is being proportional to current flowing through measuring conductor.
Current transformer with ratio 200/5 by ESSMA; Working voltage <600Volts
Current transformers placed on conductors
10. Load Cells — How They Measure Weight
Digital weighing scale contains a load cell in it. This load cell provides electrical signal to the microcontroller as per according to the weight present on the load cell, than microcontroller provides value on the display as per stored information.
Load cell works on a method of Wheatstone bridge with 4 strain gauges. When weight get placed on the load cell; strain gauge shape get altered, This alteration produces change in the resistance of strain gauge as Resistance=ρ*(length/Area),Where ρ is resistivity
Since in balanced condition, the output of Wheatstone bridge is zero while in loaded condition output has some voltage proportional to the weight placed on the load cell; this generated milli volts is used as signal to microcontroller.
1.1 TON load cell by METTLER TOLEDO
SUMMARY
Sensors are machines, devices & gadgets which monitors physical quantities like temperature, pressure, flow, light, motion, weight, vibration & then produces output in the form of electrical signal.
Sensors produces electrical signal through two methods either through electromechanical switching or either through electronic switching.
Electromechanical switching has moving part while electronic switching has no moving parts that’s why electronic switching is fast & having a long life.
There are many types of Industrial sensors but this post contains explanation for 10 sensors, 10 common Industrial sensors. 10 sensors are Temperature sensors, proximity inductive sensors, proximity reed sensors, limit switches, pressure sensors, level sensors, Flow sensors, Photoelectric Sensors, Current Transformers & Load cells.
Hope, Post is helpful. If you have any doubt, suggestion or query please do comments!
