---
# System prepended metadata

title: Sensors and Sensor Circuit design.

---

# Sensors and Sensor Circuit design. 
---
# Lesson 1 

    
## What is sensing ? 


* ***Transducer*** - a device that converts primary form of enery into a signal of different energy form, take form of a sensor or actuator.
 ``` Primary energy forms - mechanical, theral electromagnetic optical chemical , etc.```
 
 * ***Sensor*** - it provides a usable output in response to a specified measurand.
* ***Actuator*** - device that generates stimulus 

![](https://i.imgur.com/i857o7c.png)

![](https://i.imgur.com/7ZECGJY.png)

## Sensor Systems 

***Electronic sensor*** - converts a real world parameter into an elecrically measurable signal.
``` 
Primary transducer - changes real life to electric signal
Secondary Transducer - electric signal into analog or digital values
```


![](https://i.imgur.com/u6n7Flp.png)

--

Conventional transducer (large reliable)- 
1. thermocouple (temp)- temperature difference 
2. compass (magnetic) - direction


![](https://i.imgur.com/RZCCQSq.png)

---

![](https://i.imgur.com/LliPFO6.png)

---

## Categorization of Sensor


![](https://i.imgur.com/lkNfNHS.png)

### Classification can be done in power requirement 

***Active Sensors(Self generating type)***

* do not require external power source for functioning 
*  Generate power within themselves to operate and hence called self generating type. 
*  energy of cuntioning is derived from the quantity being measured. 
* piezoelectric crystals generate electrical output(charge) when subjected to acceleration

***Passive Sensors*** - require external source of energy for their functioning 

## Sensor Environment in industry

![](https://i.imgur.com/2epbJRu.png)

## Sensor Applications 

* monitoring of processes 
* control of processes 

## Calibration 

Due to wear and tear the charecteristics of the instrument will have drifted from standard specification by an unacceptable amount then it has to be recalibrated back to standard specifications. 

## Sensor Characteristics 

* Static Characteristics - Static accuracy of a sensor indicates how much the sensor signal correctly represents the measured quantity after it stabilizes. 
* Dynamic Characteristics - time response of the sensor system.

### Static Characteristics of Sensors 

* ***Range*** - Minimum and maximum value of physical variable it can measure 
* ***Span*** - Difference b/w max and min values of input. 
* ***Full scale Output(FSO)*** - THe Algebraic difference between the electrical output signals measured with maximum input stimulus and the lowest input stimulus applied.
The upper limit of output over the measurand range is called the full scale .
* ***Resolution*** - The amount of change in input that can be accurately detected by the sensor.

![](https://i.imgur.com/KihuwfW.png)

* ***Accuracy*** - Difference b/w the measured value and the true value . 
``` Absolute error = |Measured value - True Value |```
``` Xt is calculated by taking the mean of infinite number of measurements.```

* ***Linearity*** - The maximum gap between the measured curve(output curve) and the desirable curve(expected) is called linearity. 


![](https://i.imgur.com/qIiJxMA.png)

* ***Sensitivity*** - Ratio to the change in output to change of the sensor related to the unit change of input.

Y - o/p quantity 
x - I/P sensitivtiy 

```S = dY/dX```

![](https://i.imgur.com/k00076D.png)

 
 
* ***Summary*** - ![](https://i.imgur.com/rxKZU4c.png)

* ***Drift*** - deviation from specific reading of the sensor when the sensor is kept at the value for a prolonged period of time.
Effect of disturbance - (a) zero drift (b)sensitivity drift (c) zero drift plus sensitivity drift 

**zero drift** - shifting due to slippage , permanent set 

**sensitivity/span drift** - proportional change in a upward scale 
reasons - changes in ambient pressure, humidity, temperature etc 

![](https://i.imgur.com/ZeOqYGQ.png)

* ***Hysteresis*** - difference between the output of a sensor for a given input value x , when x is increased and decreased or vice versa.

``` Difference between the two output values that corresponds to the same input depending on the trajectory followed by the sensor```

* ***Dead Space*** - The region of input values where there is no change in output value.

* ***Precision*** - Clossness among the set of values 

![](https://i.imgur.com/XCmo9o2.png)

* ***Repeatability & Reproducibility*** - Closeness of output readings if the measurement **conditions are constant** and as reproducibilty if the measurement conditions vary. 

### Dynamic Characteristics 

* Dynamic Characteristics of a sensor represent the time response of the sensor system to variable input. 
* Reason for dynamic characteristics is the presense of energy storing is the presence of energy storing elements 

![](https://i.imgur.com/jHrffwD.png)

### Review Questions 


![](https://i.imgur.com/hEWB7Ec.png)


# Hall Effect


    
*If an electric current flows through a conductor in a magnetic field, the magnetic field exerts a transverse force on the moving charge carriers which tends to push them to one side of the conductor. This is most evident in a thin flat conductor as illustrated. A buildup of charge at the sides of the conductors will balance this magnetic influence, producing a measurable voltage between the two sides of the conductor. The presence of this measurable transverse voltage is called the ***Hall effect.****

Due to the distortion in the magnetic field of the charge carriers, the negatively charged electrons will be deflected to one side of the plate and positively charged holes to the other side. A potential difference b/w the both sides of the plate called the **Hall Voltage**. 


Vh = IB/qnd 

![](https://i.imgur.com/KeZg1eR.png)

## Mathematical expression of hall effect

In equilibrium the electric force will balance out the magnetic force exerted on charge carriers due to magnetic field. 

![](https://i.imgur.com/SsoRUq7.png)

## Hall Co-efficient 

The hall coefficient can be defined as the hall's field per unit current density per magnetic field. 

![](https://i.imgur.com/DxLWI2U.png)

## Application of Hall Effect Sensor 

- Hall effect sensors are used to time the speed of the wheels and shafts 
- To detect the position of permanent magnets in brushless electric DC motors 
- used in smart phone to check whether the flip cover is closed.



# Current Sensor

    
## Types of Current Sensing 

### Indirect current sensing
- based on ampere's law and faraday's law of induction
- typically is used for load currents in the 100A-1000A range
- requires expensive sensors 
- example: hall effect , rogowski coils 

### Direct current sensing 
- based on ohm's law 
- by placing the shunt resistor in series with the system load, the voltage is generated across the shunt resistor that is proportional to the system load current 
- for load currents less than 100A

## Current sensing Principles 

### Current Transformer(series transformer)

[Current Transformer](https://www.electronics-tutorials.ws/transformer/current-transformer.html)

Current Transformers produce an output in proportion to the current flowing through the primary winding as a result of a constant potential on the primary​.

#### Working principle 




#### CT Ratio

* A current transformer must satisfy ***amp turn equation***.
* Ratio of primary current input to secondary current at full load. For ex a CT of ratio 300:5 is rated for 300 primary amps at full load and will produce 5 amps of secondary current. 


![](https://i.imgur.com/KjrS3YJ.png)

#### (viva question) So can a current transformer should never be left open circuited or operated with no load attached when the primary current is flowing ?  

- ***NO*** , it produces an abnormally large secondary voltage. 
- The high secondary voltage could damage the insulation or cause electric shock if the CT's terminals are accidently touched. 
* This is because when the secondary is in OC the iron core of the transformer operates at high degree of saturation and with nothing to stop it.

#### Burden of CT 

***Burden*** is the product of voltage and the current on the secondary when the CT supplies the instrument or relay with is maximum rated value of the current. (output expressed in terms of volt-ampere [VA])


## Hall Element Current Sensor
+ measures the AC and DC currents.
+ indirect or non-intrusive type.
+ Concentrator  focuses magnetic flux lines, which are generated by electrical current flowing through a conductor, at the center of the air gap where the Hall-effect magnetic sensor IC is placed.
+ The efficiency of the concentrator and thus the current sensing system depends on the following factors:
1. core material
2. core dimensions
3. Air gap
4. core geometry

### 1. Open Loop Hall Element Current Sensors

##### Characteristics
+ measure DC to AC(less than 10kHz)
+ Affordable
+ lacks precision due to the linearity of the Hall element and the B-H characteristics of the magnetic core.
+ Hall elemnt causes drifting due to humidity and chang over time so, it is not suited for long-term measurement.

##### Measurement Principle
When the measured current(principle current) passes through the magnetic core’s aperture, a magnetic flux is induced in the core. As this magnetic flux flows through the Hall element, a voltage generates in proportion to the magnetic flux.This voltage induction is known as the Hall effect.

+ Since the voltage induced by the Hall effect is small, it is boosted with an amplifier before being output.
+ The output voltage which is proportional to the measured current allows for current measurement.

### 2. Hall effect closed loop current Sensor/Transducer

Also called Hall effect 'compensated' or zero flux transducers.

- Current under measurement I1, is applied to the primary winding.
- 𝐼1 current attempts to magnetize the core but in doing so generates a  potential difference 𝑉𝐻 across the thin sheet of semi conductor material.
- VH, amplified, drives the secondary winding and compensates the core flux level.
- the secondary current, I2, creates a flux equal in amplitude, but opposite in direction, to the flux created by the primary current.(zero flux condition)
- Voltage drop across the resistor 𝑅2, produced by secondary curr ent 𝐼2, represents the transducer output.

 #### Benefits of zero flux condition
 
 + eliminates the drift of gain with temperature.
 + secondary winding acts as current transformer at higher frequencies, significantly extending the bandwidth and reducing the response time of the transducer.
 + At zeri flux conditon, the magnetic potential of the two coils are identical. 
 `N1*I1 = N2*I2`
       
### Applications of Hall Effect Current Sensor 
![](https://i.imgur.com/zmLUFlm.png) ![](https://i.imgur.com/orxSy4X.png)

## Rogowski Coil Method
+ Dedicated to AC only.
+ Use special coreless, non-magnetic helical coils, they can measure high currents as they do not saturate.
##### Characteristics
+ Large currents can be measured as coreless structure eliminates any magnetic saturation.
+ No heat generation
+ No saturation
+ No hysteresis
+ high susceptibility to noise so high precision measurement not recommended.
##### Applications
+ Used mainly for research and development in power semiconductors, in very high voltage monitoring such as for arc furnaces, for monitoring bearing and shaft currents in large motors, in high-end welding systems, and for short-circuit testing.
+ Can be used to find small AC currents in the presence of large DC currents, Since they are immune to direct current.
#### Operating Principle

+ Difference between Rogowski coils and CT is in the core. In Rogowski coils, an air core is used and in CT,s- steel core is used.
+ When current passes through the conductor, it will create a magnetic field. Due to an intersection with a magnetic field, a voltage is induced between the terminals of the Rogowski coil. 
+ Rogowski coil rise to an induced voltage within the coil which is proportional to the rate of change of the current being measured.
+ To obtain an output voltage proportional to I it is necessary to integrate the coil voltage E; hence an electronic integrator is used.

## Shunt Resistor Current Sensing Method
- Low-value resistor in series is used to sense the current.
- When the current flow through a low-value resistor, it produces a voltage difference across the resistor.


## Resistor current sensing classification


### High Side Current Sensing:
* current sensing connects the sensing resistor between the power supply and load.
* + Directly monitors the current delivered by the supply, which considers the identification of load shorts.
* + Does not create ground disturbances.
* Applications such as battery chargers or overcurrent protection
### Low side current sensing

- current sensing connects the sensing resistor b/w the load and the ground 
- + low input common-mode voltage 
- common-mode voltage is near ground , which takes into consideration the utilisations of single supply , rail to rail / outputs op-amps 
- disturbances to the system load's ground potential and the inability to detect load shorts
- **It has reduced accuracy for *low value*  sense resistors**
- **Delivers a higher level of accuracy, especially whenhigher currents are sensed, because any voltagedrops from the resistor to the PCB ground are removed**

### Ground Disturbance for Low-Side and High-Side Current Sensing


![](https://i.imgur.com/S6jpTMD.png)

![](https://i.imgur.com/v8IQqdG.png)


• Ground disturbances are problematic when other circuits in a system are required to interface with the
load.
• Placing the shunt resistor above the load, as in high-side current sensing, eliminates ground disturbances
because the shunt resistor is no longer connected directly to ground.
• In fig. the voltage potential difference, Vground, between the grounds of the system load and MCU in
low-side current sensing are different, whereas in high-side current sensing, the ground potentials are
equal.

### Load Short to Ground Condition for Low Side and High-Side Current Sensing

![](https://i.imgur.com/A91A1CR.png)

* High-side current sensing is that it can detect a load short to ground condition.
* Notice that with high-side current sensing, the shunt resistor remains in the circuit and is able to detect a surge in current from a short to ground condition .
* Low-side current sensing, the shunt resistor is removed from the circuit

![](https://i.imgur.com/TgAKprV.png)


# Thermal sensors

    
#### Temperature Sensors 

Temp Dependence of Resistance 

* Resistivity of metals 

Resistivity (ρ) of metals is dependent on the relaxation time(τ) of the free electrons in themetal as
                     ρ∝1/τ

* Resistivity of Semiconductors
Temperature dependance is usully exponential which makes semiconductors very useful in electronic circuits detecting small changes in temperature. 


## RTD
+ Resistance thermometers employing metallic conductors for temperature measurement are called Resistance Temperature Detector (RTD)
+ Resistance Temperature Devices (RTDs) are accurate, but require excitation current and are generally used in bridge circuits.
+ Resistance is proportional to the temperature.
+ sensor is made from platinum, nickel, and copper metals.
+ To measure temperature in the range between -270oC to +850oC.
+ RTD is a passive sensor -requires an external current source to function properly. However, the current produces heat in a resistive element (self-heating ) causing an error in the temperature measurements

## Errors in RTD
+ Self heat
+ Lead wire errors - resistance of the lead wires also changes with temperature, and these effects can add to errors in the sensing circuit since these resistances are not negligible.

## RTD Connection styles
+ ![](https://i.imgur.com/E9mszYE.png)
+ (a) Two-wire (uncompensated). (b) Three-wire. (c) Four-wire. (d) Two-wire with compensation loop. b,c,d - configurations is to facilitate compensation for the lead wires.

## RTD Characteristic
+ ![](https://i.imgur.com/YMvJij2.png)

## RTD Characteristic(Terms Of Conductivity)
+ ![](https://i.imgur.com/65phHE1.png)

## Conductivity and Temperature Coefficients
+ ![](https://i.imgur.com/f7Gdnl3.png)

## Silicon Resistive Sensor
+ ![](https://i.imgur.com/MKa11sk.png)



## Thermistor
* DEFINITION- Thermistor (or thermal resistor) is defined as a type of
resistor whose electrical resistance varies with changes in
temperature.
* The relationship between a thermistor’s temperature and resistance is non-linear.
### CONSTRUCTION
* Made with the sintered mixture of metallic oxides like 
manganese, cobalt, nickel, cobalt, copper, iron, uranium, etc.
### TYPES
1. Negative Temperature Coefficient (NTC) Thermistor
![](https://i.imgur.com/yVOHlyG.png)
2. Positive Temperature Coefficient (PTC) Thermistor
![](https://i.imgur.com/fpjSFvC.png)
#### COMPARISON OF NTC AND PTC
![](https://i.imgur.com/kTcEItP.png)
### CHARACTERISTICS OF NTC
![](https://i.imgur.com/JaCGe89.png)
#### HOW THERMISTORS CAN BE USED AS THERMOSTATS??
![](https://i.imgur.com/D7ZDcfe.png)
### USES OF THERMISTORS
* Digital thermometers (thermostats).
• Automotive applications (to measure oil and coolant temperaturesin cars & trucks).
• Household appliances (like microwaves, fridges, and ovens).
• Circuit protection (i.e. surge protection).- PTC
• Rechargeable batteries (ensure the correct battery temperature is maintained).
• To measure the thermal conductivity of electrical materials.
• Useful in many basic electronic circuits (e.g. as part of a beginner Arduino starter kit).
• Temperature compensation (i.e. maintain resistance to compensate for effects caused by changes in temperature in another part of the circuit).
• Used in wheatstone bridge circuits
## DIFFERENCE BETWEEN THERMISTOR AND RTD
• The main difference is that the electrical resistance of the resistor
used in a thermistor varies in a non-linear manner with respect to
temperature.
• Sensing element used in the thermistor is made up of either a
ceramic or polymer, while RTD uses pure metals as its sensing
element.
• Due to high sensitivity, thermistors are used in narrow span
measurements and low temperature ranges from -20 degree Celsius
to +120 degree Celsius. But RTD’s are used over wide and larger
temperature ranges.
## SEEBECK AND PELTIER EFFECT
* Seebeck effect is a direct conversion of thermal energy into electric energy.
* Electrons in hot region have higher velocities than electrons in cold region,
resulting in net diffusion of electrons from hot region to cold region.
Varying temperature along the bar is a source of electromotive force
(voltage) and current will flow. This is the principle behind the
thermocouple.
* For small changes in temperature, the Seebeck voltage is linearly
proportional to temperature, Vab = a(T1-T2).



## Thermocouple

+ small, rugged, relatively inexpensive, and operate over the widest range of all temperature sensors.
+ useful at extremely high temperatures (up to +2300°C) 
+ Require cold-junction-compensation (CJC) techniques .
+ More linear than many other sensors.
+ Most common metals used are Iron, Platinum, Rhodium, Rhenium, Tungsten, Copper, Alumel (composed of Nickel and Aluminum), Chromel(composed of Nickel and Chromium) and Constantan (composed of Copper and Nickel).

![](https://i.imgur.com/I4hgxBE.png)

+ Type E thermocoulpes are the most sensitive, producing the largest output voltage for a given temperature change.
+ Type S thermocouples are the least sensitive.

### EMF generated in thermocouple
A thermoelectric current flows in a circuit comprised of two junctions at different temperatures so, an emf is developed across the open circuit.

![](https://i.imgur.com/TN9hrEo.png)


### Thermocouple Interfacing

The thermocouples generate voltage in microvolts so we need to ampilfy it using instrumentation amplifier specially made for thermocouple metal types available in IC form. The cold junction temperature is compensated by producing a voltage equal to the thermocouple voltage at 0°C.

#### Interfacing thermocouple with Arduino UNO

Thermocouple consists of two different conductors forming an electrical junction at different temperatures, through which a voltage is produced.
ADC output of this voltage can be processed by a microcontroller to give the temperature.

## Temperature Sensor Circuit

![](https://i.imgur.com/Coxm8FU.png)


## SemiConductor Temperature Sensors 

Semiconductor Temperature Sensors
The main types of ***semiconductor temperature sensors*** include:
• Voltage Output Temperature Sensors
• Current Output Temperature Sensors
• Digital Output Temperature Sensors
• Resistance Output Temperature Sensors
• Diode Temperature Sensors

### p-n junction temperature sensors 


* Semiconductor sensors are thedevices that come in the form of ICs.

* **Semiconductor Temperature Sensor** is based on the fact that the junction voltage across a p-n combination ofsemiconductors, like a diode junctionor “base-emitter” junction of regulartransistors, is a function oftemperature


AD590 and LM35 temperaturesensors are the most populartemperature sensors.

![](https://i.imgur.com/4s10a6w.png)

* Eg is the bandgap energy (in joules) of the material, C is a temperature-independent constant for the diode, and I is the current through the junction ,k is Boltzmann’s constant, and T is the absolute temperature (K)


## Summary 
![](https://i.imgur.com/I9VPGbh.png)



# Voltage Sensor/ Transducer

    
*Current and voltage sensors are used for current and voltage monitoring*
#### Voltage Sensor Classification
**From the measurement principle**
   * Hall effect voltage sensor
   * Photoelectric isolation voltage sensor
   * Voltage transformer (electromagnetic induction principle) and so on.
**According to the voltage polarity**
   * DC voltage sensor
   * AC voltage sensor
#### Applications
- electrical installations and automatic control in power,
- post and telecommunications, petroleum, coal, metallurgy, railway, municipal and other departments And scheduling system.
- DC voltage sensor is also widely used in power, remote monitoring, instrumentation, medical equipment, industrial automatic control, that require power isolation measurement and control industry.
## Hall Effect Closed Loop Voltage Transducers/Sensor
* Hall sensor is placed in the gap of a flux concentrating magnetic core.
* Voltage under measurement 𝑉1, applied to the primary winding, produces the current 𝐼1.
* 𝐼1 current attempts to magnetize the core but in doing so generates a potential difference 𝑉𝐻 across the thin sheet of semiconductor material.
* 𝑉𝐻, amplified, drives the secondary winding and compensates the core flux level. I2 is thus produced.
* Voltage drop across the resistor 𝑅2, produced by secondary current 𝐼2, represents the transducer output.

## Optocouplers
*Also known as optoisolator, and photo coupler.*

#### Photoelectric Isolation For Voltage Sensor
Transfers electrical signals between two isolated circuits by using light, especially for low voltage or highly noise sensitive circuits.

*Octocouplers are used to block high voltages and voltage transients, so that a surge in one part of the system will not disrupt or destroy the other parts.

Transformers and opto-isolators are the only two classes of electronic devices that offer reinforced protection — they protect both the equipment and the human user operating this equipment.

### Sensing AC voltage for microcontroller using Optocoupler Voltage Sensor
![](https://i.imgur.com/sgmCYHR.png)
![](https://i.imgur.com/0zWV1Zn.png)

### Design of Optocoupler Voltage Sensor
![](https://i.imgur.com/3kUnhG5.png)
![](https://i.imgur.com/p7zBoSH.png)


## Potential Transformers
+ Voltage transformers (VTs)/ Potential Transformers - step-down transformers with highly accurate turns ratio.

#### Functions Of Voltage Transformer
+ To convert the high voltage proportionally into the standard secondary voltage of 100V or a lower grade for the use of protective and measuring devices or instruments. That means the primary winding of a potential transformer is connected to the high voltage circuit and the secondary winding of a transformer is connected to a voltmeter. Due to the mutual induction, the two windings are magnetically coupled to each other and work on the principle of electromagnetic induction.
+ Due to the high impedance in the potential transformer, the small current flows through the secondary winding and operates similarly to the ordinary transformer with no or low load. Hence these types of transformers operated at a voltage range of 50 to 200VA.
+ To isolate the high voltage from electricians using the potential transformer.

## Aplications Of Potential Transformers
+ Used in relay and metering circuits
+ Uses in power line carrier communication circuits
+ Used in protection systems electrically
+ Used for protecting feeders
+ Used for the protection of impedance in the generators
+ Used in synchronization of generators and feeders.
+ Used as protection voltage transformers

## Types Of Voltage or Potential Transformers
#### Based On the construction
+ Electromagnetic Potential Transformers
+ Capacitive Potential Transformers

#### Electromagnetic Potential Transformers
+ ![](https://i.imgur.com/uWVvBpW.png)

#### Capacitive Potential Transformers
+ ![](https://i.imgur.com/JcFPIRR.png)
+ ![](https://i.imgur.com/RuSfb0m.png)

#### Capacitive Voltage Transformer Working
+ ![](https://i.imgur.com/I2iwtz1.png)

## Turns ratio expression for CVT
+ ![](https://i.imgur.com/yUvPxb0.png)
+ ![](https://i.imgur.com/YAmcovt.png)

## Compare CT and PT
![](https://i.imgur.com/Or445TZ.png)



# Bridge Circuits

    
## Bridge Circuits
+ If the electrical components are arranged in the form a bridge or ring structure, then that electrical circuit is called a bridge.
+ n general, bridge forms a loop with a set of four arms or branches.
+ Bridge circuit works as a pair of two-component voltage dividers connected across the same source voltage, with a null detector meter movement connected between them to indicate a condition of “balance” at zero volts.

## Classification
*Based on the voltage signal with which those can be operated, it is classified as*

### DC Bridge
![](https://i.imgur.com/Cd1PbJS.png)

#### Wheatstone bridge and balance condition
+ ![](https://i.imgur.com/vnbRUOE.png)

### AC Bridges
* excited with an AC voltage source 
* Used to measure thevalueof unknown inductance, capacitance and frequency.
![](https://i.imgur.com/NjkBUeH.png)

# Displacement , position and Proximity sensor 

## Definitions 

![](https://i.imgur.com/tKYgl3w.png)

## Postion Sensors 

![](https://i.imgur.com/bTOE5cf.png)

### Types of positon sensors 

![](https://i.imgur.com/b1cTaBq.png)

## Proximity Sensors 

A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact.

A proximity sensor often emits an electromagnetic field or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal. The object being sensed is often referred to as the proximity sensor's target. Different proximity sensor targets demand different sensors. For example, a capacitive proximity sensor or photoelectric sensor might be suitable for a plastic target; an inductive proximity sensor always requires a metal target.

### Types of Proximtiy sensors 
![](https://i.imgur.com/a7zZxCv.png)

### Where it is used ? (Applications)

![](https://i.imgur.com/9lFdW0g.png)

## Inductive Type proximity sensors 
![](https://i.imgur.com/SK4PZ9t.png)

## Capacitive Proximity Sensors 

![](https://i.imgur.com/CTKKiHx.png)

### Factors for Capacity proximity sensors 

![](https://i.imgur.com/MfwHVdv.png)

### Advantages and Disadvantages proximity sensors 

![](https://i.imgur.com/u7XNdcG.png)






## Potentiometer 


A potentiometer is a type of position sensor. They are used to measure displacement in any direction. Linear potentiometers linearly measure displacement and rotary potentiometers measure rotational displacement.

The mechanical construction of rotary and linear potentiometers is very similar, each type consists of a contact wiper and a conductive element or track. In Linear Potentiometers the track is straight and in Rotary potentiometers the track is circular. The wiper moves along the track to measure the displacement through proportionally dividing the input voltage.

## Inductance 

![](https://i.imgur.com/5qT2qeD.png)

Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of the current, and follows any changes in current. From Faraday's law of induction, any change in magnetic field through a circuit induces an electromotive force (EMF) (voltage) in the conductors, a process known as electromagnetic induction. This induced voltage created by the changing current has the effect of opposing the change in current. This is stated by Lenz's law, and the voltage is called back EMF. 

### Factors affecting inductance 
![](https://i.imgur.com/UugXkbj.png)




## Mutual Inductance 


![](https://i.imgur.com/QxGXrSg.png)

# LVDT (Linear Variable Diffrential Transformer)

https://www.te.com/usa-en/industries/sensor-solutions/insights/lvdt-tutorial.html


It is a common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT linear position sensors are readily available that can measure movements as small as a few millionths of an inch up to several inches, but are also capable of measuring positions up to ±30 inches (±0.762 meter)

## Construction of LVDT 
![](https://i.imgur.com/aXn3zeN.png)

## Working of LVDT 

LVDT stands for linear variable differential transformer. It works on the principle of mutual induction LVDT illustrated in figure consist of three symmetrically spaced coils bound out and illustrated bobbin. A magnetic core , which moves through the bobbin, provides a path for magnetic flux linkage between coils. The position of the magnetic core control the mutual inductance between the primary coils and two secondary coils. When a carrier excitation is applied to the primary coil, voltage is induced in the two secondary coils that are wired in series opposing circuit.

An LVDT measures displacement by associating a specific signal value for any given position of the core. This association of a signal value to a position occurs through electromagnetic coupling of an AC excitation signal on the primary winding to the core and back to the secondary windings

## What happend when the LVDT's core is in diffrent axial positions 


![](https://i.imgur.com/jO3ZO1O.png)

## Proportionally Linear LVDT Response to Core displacement 

![](https://i.imgur.com/0h6660n.png)

## LVDT as a Level Sensor 

![](https://i.imgur.com/HJRe0xn.png)

## Applications of LVDT sensors 

![](https://i.imgur.com/2gZOqkJ.png)

## Advantages of LVDT 
![](https://i.imgur.com/DLbiWwh.png)

## Limitations of LVDT 

![](https://i.imgur.com/UEv3z9w.png)

## RVDT 


![](https://i.imgur.com/IiVAZ2l.png)



























# Pressure Sensor
* Sensing in gases and fluids, direct measurement of force is not an attractive option—only pressure can be measured and related to properties of these substances, including the forces they exert.
* When pressure is applied to a piezo resistor, depending on the material, its resistance varies. When pressure or stress changes, resistance across the piezo material changes, and not a charge or voltage.
* Sensing of pressure, which is force per unit area, follows the same principle as the sensing of force—that of measuring the displacement of an appropriate member of the sensor in response to pressure.
* Any device that will respond to pressure either by direct displacement or equivalent quantities (such as strain) is an appropriate means of sensing pressure. Thus the range of methods is quite large and includes thermal, mechanical, as well as magnetic and electrical principles
## Unit of Pressure
![](https://i.imgur.com/A3NOg0v.png)

## Force Sensor
* main tool in sensing force is the strain gauge.
* Although strain gauges, as their name implies, measure strain, the strain can be related to stress, force, torque, and a host of other stimuli, including displacement, acceleration, or position.
* With proper application of transduction methods, it can even be used to measure temperature, level, and many other related quantities.
* Metal changes its electrical resistance as it deforms. Strain gauges take advantage of this property.
### Strain
* Strain (dimensionless) is defined as the change in length per unit length of a sample.
### Stress
* Stress is pressure(Force/Area) [N/m^2] in a material.
### Modulus Of Elasticity
* Modulus of elasticity is the ratio of stress to strain.

## Principle of strain gauge
![](https://i.imgur.com/XfyjsLz.png)
![](https://i.imgur.com/8ofIuBv.png)
![](https://i.imgur.com/l9QJT87.png)

## Gauge Factor(GF)
* Gauge factor is defined as the ratio of fractional change in electrical resistance to the fractional change in length (strain).
![](https://i.imgur.com/zrxVMLe.png)
* Gauge Factor for metallic strain gauges is typically around 2.

## Bridge Circuit of Strain Gauge
![](https://i.imgur.com/AeOhimq.png)

## Semiconductor Strain Gauges
![](https://i.imgur.com/EyDXjxm.png)

## Difference between conductor and semiconductor strain gauges
![](https://i.imgur.com/iWrp6pB.png)

## Piezoresistive pressure sensor
![](https://i.imgur.com/pEYO0LQ.png)
![](https://i.imgur.com/zFL4pd4.png)

### Working
![](https://i.imgur.com/c140jmm.png)

## Piezoresistive pressure sensing
![](https://i.imgur.com/GNsP9Uf.png)
![](https://i.imgur.com/otDqyI4.png)
### Quarter Bridge
![](https://i.imgur.com/mLNlSy5.png)
### Half Bridge
![](https://i.imgur.com/qgzZkd5.png)
![](https://i.imgur.com/wrlarKG.png)
### Full Bridge
![](https://i.imgur.com/tJVAbdg.png)
## Strain Gauge Applications
![](https://i.imgur.com/248Gjp9.png)
### Strain Gauge Accelometer
![](https://i.imgur.com/4GsjEY7.png)
### Aerospace
* Strain fixedgauges are to the structural load-bearing components to measure stresses along load paths for wing deflection or deformation in an aero plane.
### Cable Bridges
![](https://i.imgur.com/MOyONFM.png)
### Rail Monitoring
![](https://i.imgur.com/adfZG1w.png)
# Piezoelectric Sensor
![](https://i.imgur.com/Nmaxu6o.png)
## Piezoelectric sensor applications
![](https://i.imgur.com/ADRU7Pt.png)
## Advantages and Disadvantages of Piezoelectric sensors
![](https://i.imgur.com/Bat41mY.png)
## WHAT IS STRAIN GAUGE AND ITS PRINCIPLE ??
A strain gauge is a resistor used to measure strain on an object. When an external force is applied on an object, due to which there is a deformation occurs in the shape of the object. 
![](https://i.imgur.com/9ndl5Aw.png)
## WHAT IS PIEZO RESISTIVE SENSOR AND ITS WORKING PRINCIPLE ??
Piezoresistive sensors rely on the changes in resistance that occurs when a mechanical load is imposed on a conductor or semiconductor.


A Piezoresistive Pressure Sensor contains several thin wafers of silicon embedded between protective surfaces. ... When the resistance changes, less current passes through the pressure sensor. The Wheatstone bridge detects this change and reports a change in pressure.
## WHAT IS PIEZO ELECTRIC SENSOR AND ITS WORKING PRINCIPLE ??
A piezoelectric sensor is a device that uses the piezoelectric effect to measure changes in pressure, acceleration, temperature, strain, or force by converting them to an electrical charge

When a force is applied to a piezoelectric material, an electric charge is generated across the faces of the crystal. This can be measured as a voltage proportional to the pressure (see diagram to the right). ... A given static force results in a corresponding charge across the sensor.
## DIFFERENCE BETWEEN PIEZO ELECTRIC AND STRAIN GAUGE ??
Strain gauge sensors have very low and insignificant drift in output when subjected to a load for a long period of time; piezoelectric sensors have very large drift in output value, resulting in measurement errors for prolonged loading

## APPLICATIONS OF STRAIN GAUGE AND PIEZO RESISTIVE SENSOR ??
Strain gauges are used for many applications. They are often used within other sensors to measure the strain or stress. ... This means they can measure changes in force, pressure, weight and tension by giving a change in electrical resistance.


The piezoresistive sensor is used in a wide variety of applications involving mechanical stress measurement. The automotive industry employs them as vacuum and pressure sensors or to give indication of oil and gas levels. They are also used in the medical field in devices such as blood pressure measurement equipment.

## HOW STRAIN CAN BE MEASURED ??
The most commonly used instruments to measure strain are electrical strain gauges. These are known as conventional strain gauges or foil strain gauges. ... Strain gauges are usually used in Experimental Stress Analysis (ESA), durability testing, and transducer manufacturing.

# END SEMS 

## Noise Signal 

![](https://i.imgur.com/hFSQVkZ.png)

## Lock-in Amplifier 

![](https://i.imgur.com/OVBDLvQ.png)
![](https://i.imgur.com/YDTXRjy.png)
## Lock-in consists of : 
![](https://i.imgur.com/PRKNW5F.png)

## Lock-in Amplifier Stages 

![](https://i.imgur.com/kCasHjv.png)

## Phase-Sensitive Detection(noise free) - Multiplier 
![](https://i.imgur.com/s4xVOoF.png)
![](https://i.imgur.com/j0iHJAa.png)



### Demodulator 
![](https://i.imgur.com/SMq1VCm.png)
![](https://i.imgur.com/pt6UxUM.png)

## V signal 
![](https://i.imgur.com/RO9Gpyz.png)
## V ref 
![](https://i.imgur.com/be9pRQh.png)
## V psd 
![](https://i.imgur.com/HSnBMOb.png)
## Phase Sensitive output 
![](https://i.imgur.com/mckt8Ld.png)
## low pass filter 
![](https://i.imgur.com/pJUGvbP.png)
## What is the importance of lock in amplifier in sensor circuit ?
Lock-in amplifiers are used to detect and measure very small AC signalsall the way down to a few nanovolts. Accurate measurements may be made even when the small signal is obscured by noise sources many thousands of times larger.

## Do we need lock in amplifier for DC sensor output signals ? 

> ## Circuit elements
> 
> * Common mode voltage
> 
> The suggested circuitry has a single power supply (VDD) of 3V. This can be created using two standard 1.5V batteries connected in series. In order to make a full span of the output voltage, the common mode voltage must be 1.5V, so resistors RB1 and RB2 must have the same large values, in this case 1MΩ. Capacitor CB1, which filters the noise, must also have a large value and it is suggested that CB1 is realised by connecting two capacitors with values of 100μF and 220nF in parallel.
> 
> * Quadrature oscillator
> 
> This generates two pulsed, phase shifted voltage signals for gating the corresponding op amps. In order to reduce power consumption, resistors RI, RT1, RT2 and RT4 must have large values. In the suggested design, RT1, RT2 and RT4 are set at 1MΩ, while RI = 700kΩ. To keep the oscillating frequency near to 100Hz, CI should be 2.2nF.
> 
> In order to reduce the hysteresis of the Schmitt trigger and the additional phase shift of the in quadrature signal, RT3 should be 1kΩ.
> 
> * Differential preamplifier
> 
> As power consumption is a major concern, the values of the resistors RF and RL have been chosen to be 1MΩ.
> 
> The values of resistors RS – the sensor – and RB must be chosen such that the differential preamplifier can amplify a pulsed signal with a frequency of more than 100Hz without distorting it. Therefore, the optimal values of resistors RS and RB are approximately 100kΩ. This design results in a bandwidth of approximately 1kHz.
> 
> * High pass filter
> 
> The differential preamplifier output consists of a slow varying part, proportional to the common mode voltage VB, and a fast varying part proportional to in phase signal P. The high-pass filter removes the slow varying signal, the offset voltage and low frequency flicker noise from the differential amplifier.
> 
> In order to achieve a cut-off frequency of about 1Hz and a low output current, CHPF is set at 220nF and RHPF at 1MΩ.
> 
> * Amplifier and multiplier
> 
> The amplifier and multiplier stage amplifies the signal from the high pass filter and performs digital multiplication by gating the op amps. In order to achieve relatively high gain without distorting the signal, while minimising power consumption, the following resistors value have been chosen – RG1=100kΩ and RG2=1MΩ.
> 
> * Adder, filter, and amplifier
> 
> This stage has a triple role – to sum the output signals of the amplifier and multiplier, to perform low-pass filtration and, additionally, to amplify the signal. The sum of the signals occurs at the common node of the resistors RF1 and RF2, which also serve as the low-pass filter components.
> 
> In order to keep low power consumption, sufficient amplification and filtration with the second order Butterworth filter, which has a cut off frequency of approximately 1Hz, the following values should be used: RA1=100kΩ, RA2=1MΩ, RF1=2MΩ, RF2=1MΩ, CF1=56nF and CF2=390nF.

## 4 - 20 mA current loop
![](https://i.imgur.com/USYpX0y.png)
## Components of 4-20 mA current loop
![](https://i.imgur.com/myhsdz0.png)
![](https://i.imgur.com/IZm1d0t.png)
## Power Supply 
![](https://i.imgur.com/yaS9ATi.png)
## Transmitter 
![](https://i.imgur.com/BLbhzuJ.png)
## Receiver resistor 
![](https://i.imgur.com/VowA7SB.png)
## 4-20 mA sensor configs 
![](https://i.imgur.com/RBf5OPX.png)
## 2-wire 4-20 mA sensors 
![](https://i.imgur.com/eD2Zhel.png)
![](https://i.imgur.com/7JDzxou.png)
![](https://i.imgur.com/mfpKmff.png)
## 3-wire 4-20 mA sensors 
![](https://i.imgur.com/my8inWl.png)
## 4 wire 4-20 mA sensors
![](https://i.imgur.com/PT2o5Fw.png)
## Advantages and Disadvantages of every phases.
![](https://i.imgur.com/ebzEV3N.png)



 







## Why Amplifiers for Sensors
![](https://i.imgur.com/Fx193n3.png)

## Ideal v/s Pactical Characteristicss of IC741
![](https://i.imgur.com/aQV1yZA.png)
## Basic Opamp Configuration
### Voltage Comparator
![](https://i.imgur.com/JqsfXjg.png)
### Voltage Follower 
![](https://i.imgur.com/CGRI2pK.png)
### Non-Inverting Amp
![](https://i.imgur.com/r8NE7sO.png)
A non-inverting amplifier produces an output signal that is in phase with the input signal,

An inverting Amplifier produces an output which is out of phase with respect to its input by 180o.

Differential amplifiers are used mainly to suppress nois
### Inverting Amp
![](https://i.imgur.com/IssuMr5.png)
### Differential Amp
![](https://i.imgur.com/jcBajYY.png)
### Current-to-Voltage
![](https://i.imgur.com/m2PnLKn.png)

## Op-amp Characteristics - DC Characters & AC Characters
![](https://i.imgur.com/mbLqqmJ.png)

## Output Offset Voltage
![](https://i.imgur.com/pxopwso.png)
![](https://i.imgur.com/bB1wYcZ.png)
In the case of the ideal op-amp, the DC voltage of the VIN(+) and VIN(-) terminals match exactly when the input voltage (Vi) is 0 V. In reality, however, there are differences in input impedance and input bias current between the VIN(+) and VIN(-) terminals, causing a slight difference in their voltages. This difference called input offset voltage is multiplied by a gain, appearing as an output voltage deviation from the ideal value.
When used in amplifiers of sensors, etc., the input offset voltage of an op-amp results in an error of sensor detection sensitivity. To keep sensing errors below a specified tolerance level, it is necessary to select an op-amp with low input offset voltage.
## Input Offset Voltage
![](https://i.imgur.com/XJ7XHSF.png)
![](https://i.imgur.com/Ayh8zWx.png)

## Input Bias Current
![](https://i.imgur.com/V7Q0mrm.png)

## Input Offset Current
![](https://i.imgur.com/xY6nq0i.png)

## Thermal Voltage Drift
![](https://i.imgur.com/WKrxE2m.png)
![](https://i.imgur.com/Dgg5hAM.png)

## Input Bias Current Drift
![](https://i.imgur.com/18wBvdw.png)

## Input Offset Current Drift
![](https://i.imgur.com/eAKZt8I.png)

## Sensor Circuit to Differential Amplifier
![](https://i.imgur.com/Ow5orT2.png)

## Integrated Instrumentation Amplifiers
![](https://i.imgur.com/yPAPg2K.png)

## Applications of Instrumentation Amplifier
![](https://i.imgur.com/nXQG1T5.png)

## Triangular Oscillator using Op-AMP
![](https://i.imgur.com/B9tJc1F.png)
![](https://i.imgur.com/Nm3Z0Yk.png)

