Sensors & Transducers Study Notes for Instrumentation Engineering

In this topic we will deal with the concept Sensors & Transducers also along with we cover the topic such as Resistive, capacitive, inductive, piezoelectric Transducers & Hall effect sensors and associated signal conditioning circuits.

1. SENSOR 

It is defined as an element which produces signal relating to the quantity being measured. Here, the output is usually an ‘electrical quantity’ and measurand is a ‘physical quantity, property or condition which is to be measured’. Thus in the case of, say, a variable inductance displacement element, the quantity being measured is displacement and the sensor transforms an input of displacement into a change in inductance.

Typical Sensor Applications: 

2. TRANSDUCERS 

A device that converts a primary form of energy into a corresponding signal with a different energy form Primary Energy Forms: mechanical, thermal, electromagnetic, optical, chemical, etc.

• Typically sensor system: convert the desired parameter into an electrically measurable signal
• General Sensor system:
  • Sensor/ transducer: sense “real world” parameter and converted into a suitable signal
  • Signal conditioning: converts the sensed signal into an analog or digital electrical value.
• The desirable features of sensors are Range /span Errors, accuracy, Non linearity, Hysteresis, Dead band and Saturation, Output impedance, Repeatability, Reliability, Sensitivity, Resolution, Frequency Response, Response time calibration.

1. Resistance Temperature Detector (RTD):  A commonly used conductor is platinum, which is stable over a wide temperature range.This sensor has a thin platinum wire is wound in a coil on a ceramic core. Sometimes platinum metal film is used to make sensors, which are very
small and economical, but it is not as stable as pure platinum wire. RDTs can be obtained in resistance values from 10 Ω to several KΩs, but by far the most commonly used value is 100 Ω. The temperature coefficient for platinum (Pt) is .00385/°C, and is called a (alpha).

R = resistance at new temperature
R_O = resistance at reference temperature (0°C)
α= temperature coefficient of wire (0.00385/°C for pt)
ΔT = change in temperature

2. Strain Gage Sensor: A strain gauge is a device used to measure strain on an object. The most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. A strain gauge takes advantage of the physical property of electrical conductance and its dependence on the conductor's geometry. When an electrical conductor stretched within the limits of its elasticity such that it does not break or permanently deform, it will become narrower and longer, changes that increase its electrical resistance end-to-end.

Gage Factor: Gage Factor, G, used to compare strain-gate materials                

      &

• μ- Poisson Ratio
• G- gauge factor
• ΔL/L is the strain ε.

3. Capacitive element based sensor

• It comprises of three plates, with the upper pair forming one capacitor and the lower pair another. The linear displacement might take in two forms:

• one of the plates is moved by the displacement so that the plate separation changes
• area of overlap changes due to the displacement.
• Capacitance, C =  εA/x
   ε = dielectric constant
   A = area of capacitor plate A = area of capacitor plate
   x = plate separation distance Generally, displacement sensors rely on changes in x
• If the central plate moves near to top plate or bottom one due to the movement of the element/workpiece of which displacement is to be measured, separation in between the plate changes. This can be given as

• When C₁ and C₂ are connected to a Wheatstone bridge, then the resulting out of balance voltage would be in proportional to displacement x.

4. Linear variable differential transformer (LVDT)

• Linear variable differential transformer (LVDT) is a primary transducer used for measurement of linear displacement with an input range of about ± 2 to ± 400 mm in general. It has non linearity error ± 0.25% of full range. It has three coils symmetrically spaced along an insulated tube. The central coil is primary coil and the other two are secondary coils. Secondary coils are connected in series in such a way that their output suppose each other. A magnetic core attached to the element of which displacement is to be monitored is placed inside the insulated tube.

• Due to an alternating voltage input to the primary coil, alternating electromagnetic forces (emfs) are generated in secondary coils. When the magnetic core is centrally placed with its half portion in each of the secondary coil regions then the resultant voltage is zero.
• If the core is displaced from the central position more in secondary coil 1 than in coil 2, then more emf is generated in one coil i.e. coil 1 than the other, and there is a resultant voltage from the coils. If the magnetic core is further displaced, then the value of resultant voltage increases in proportion with the displacement. With the help of signal processing devices such as low pass filters and demodulators, precise displacement can be measured by using LVDT sensors.
• A rotary variable differential transformer (RVDT) can be used for the measurement of rotation. Readers are suggested to prepare a report on principle of working and construction of RVDT sensor.

Applications of LVDT sensors:

• Measurement of spool position in a wide range of servo valve applications
• To provide displacement feedback for hydraulic cylinders
• To control weight and thickness of medicinal products viz. tablets or pills
• For automatic inspection of final dimensions of products being packed for dispatch
• To measure distance between the approaching metals during Friction welding process
• To continuously monitor fluid level as part of leak detection system
• To detect the number of currency bills dispensed by an ATM

5. Hall Effect Sensor

• The principle of working of Hall effect sensor. Hall effect sensors work on the principle that when a beam of charge particles passes through a magnetic field, forces act on the particles and the current beam is deflected from its straight line path. Thus one side of the disc will become negatively charged and the other side will be of positive charge. This charge separation generates a potential difference which is the measure of distance of magnetic field from the disc carrying current.
• The typical application of Hall effect sensor is the measurement of fluid level in a container.An electric circuit with a current carrying disc is mounted in the casing. When the fluid level increases, the magnet will come close to the disc and a potential difference generates. This voltage triggers a switch to stop the fluid to come inside the container.

• Where: V_H is the Hall Voltage in volts, R_H is the Hall Effect co-efficient, I is the current flow through the sensor in amps, t is the thickness of the sensor in mm, B is the Magnetic Flux density in Tesla.

Application of Hall effect:

• In instrumentation, the Hall Voltage is used as Voltage multiplier.
• In Semiconductor Physics the Hall Effect is used to determine the type of semiconductor.
• In Semiconductor Physics it is also used to determine the type of charge carrier present in the semiconductor.

INDUSTRIAL INSTRUMENTATION: 

Instrumentation and process control can be traced back many millennia. Some of the early examples are the process of making fire and instruments using the sun and stars, such as Stonehenge. The evolution of instrumentation and process control has undergone several industrial revolutions leading to the complexities of modern day microprocessor-controlled processing. Today’s technological evolution has made it possible to measure parameters deemed impossible only a few years ago. Improvements in accuracy, tighter control, and waste reduction have also been achieved.

Process Control: Process control is the automatic control of an output variable by sensing the amplitude of the output parameter from the process and comparing it to the desired or set level and feeding an error signal back to control an input variable—in this case steam.

A temperature sensor attached to the outlet pipe senses the temperature of the water flowing. As the demand for hot water increases or decreases, a change in the water temperature is sensed and converted to an electrical signal, amplified, and sent to a controller that evaluates the signal and sends a correction signal to an actuator. The actuator adjusts the flow of steam to the heat exchanger to keep the temperature of the water at its predetermined value.

Definitions of the Elements in a Control Loop

Figure below breaks down the individual elements of the blocks in a process-control loop. The measuring element consists of a sensor, a transducer, and a transmitter with its own regulated power supply. The control element has an actuator, a power control circuit, and its own power supply. The controller has a processor with a memory and a summing circuit to compare the set point to the sensed signal so that it can generate an error signal. The processor then uses the error signal to generate a correction signal to control the actuator and the input variable.

  The definition of these blocks is given as follows:

• Feedback loop is the signal path from the output back to the input to correct for any variation between the output level from the set level. In other words,the output of a process is being continually monitored, the error between the set point and the output parameter is determined, and a correction signal is then sent back to one of the process inputs to correct for changes in the measured output parameter.
• Controlled or measured variable is the monitored output variable from a process. The value of the monitored output parameter is         normally held within tight given limits.
• Manipulated variable is the input variable or parameter to a process that is varied by a control signal from the processor to an actuator. By changing the input variable the value of the measured variable can be controlled.
• Set point is the desired value of the output parameter or variable being monitored by a sensor. Any deviation from this value will generate an error signal.
• Instrument is the name of any of the various device types for indicating or measuring physical quantities or conditions, performance, position, direction,and the like.
• Sensors are devices that can detect physical variables, such as temperature,light intensity, or motion, and have the ability to give a measurable output that varies in relation to the amplitude of the physical variable. The human body has sensors in the fingers that can detect surface roughness, temperature,and force.       
• Transducers are devices that can change one form of energy to another, e.g.,a resistance thermometer converts temperature into electrical resistance, or a thermocouple converts temperature into voltage.
• Converters are devices that are used to change the format of a signal without changing the energy form, i.e., a change from a voltage to a current signal.
• Actuators are devices that are used to control an input variable in response to a signal from a controller. A typical actuator will be a flow-control valve that can control the rate of flow of a fluid in proportion to the amplitude of an electrical signal from the controller.
• Controllers are devices that monitor signals from transducers and take the necessary action to keep the process within specified limits according to a predefined program by activating and controlling the necessary actuators.
• Programmable logic controllers (PLC) are used in process-control applications,and are microprocessor-based systems. Small systems have the ability to monitor several variables and control several actuators, with the capability of being expanded to monitor 60 or 70 variables and control a corresponding number of actuators, as may be required in a petrochemical refinery.
• An error signal is the difference between the set point and the amplitude of the measured variable.
• A correction signal is the signal used to control power to the actuator to set the level of the input variable.
• Transmitters are devices used to amplify and format signals so that they are suitable for transmission over long distances with zero or minimal loss of information.

All the Best.

Team Gradeup!