Close or Esc Key

Arduino Projects   |   Raspberry Pi   |   Electronic Circuits   |   AVR   |   PIC   |   8051   |   Electronic Projects

Position Sensor : Types of Position Sensor

Written By: 

Preeti Jain

Sensors are very important organs of any measurement system.  They collect data from the surroundings/ physical parameter and provide electrical signal as the input to the systems. Amongst wide variety of sensors operating on different sensing principles and used in different applications, position sensors play an important role in different systems. Whether it is fly-by-wire aircraft systems, drive-by-wire cars, bullet trains taking round curves, injection molding machines, packaging machines, medical equipments, and so on, position sensors finds their applications, of course in different ways.

 
What are Position Sensors?
 
Most common way of classifying the wide spectrum of sensors is based on the specific application of the sensor. Sensor used for measuring humidity is termed as humidity sensor, the one used for measurement of pressure is called pressure sensor, sensor used for measurement of liquid level is called level sensor and so on though all of them may be using the same sensing principle. In a similar fashion, the sensor used for measurement of position is called a position sensor.
Position sensors are basically sensors for measuring the distance travelled by the body starting from its reference position. How far the body has moved from its reference or initial position is sensed by the position sensors and often the output is given as a fed back to the control system which takes the appropriate action. Motion of the body can be rectilinear or curvilinear; accordingly, position sensors are called linear position sensors or angular position sensors.
 
Types of Position Sensor
 
Position sensors use different sensing principles to sense the displacement of a body.  Depending upon the different sensing principles used for position sensors, they can be classified as follows:
1.      Resistance-based or Potentiometric Position sensors
2.      Capacitive position sensors
3.      Linear Voltage Differential Transformers
4.      Magnetostrictive Linear Position Sensor
5.      Eddy Current based position Sensor
6.      Hall Effect based Magnetic Position Sensors
7.      Fiber-Optic Position Sensor
8.      Optical  Position Sensors
 
Potentiometric Position sensors
Potentiometric position sensor use resistive effect as the sensing principle. The sensing element is simply a resistive (or conductive) track. A wiper is attached to the body or part of the body whose displacement is to be measured. The wiper is in contact with the track. As the wiper(with the body or its part) moves, the resistance between one end of the track and the wiper changes. Thus, the resistance becomes a function of the wiper position. The change in resistance per unit change in wiper position is linear. 
 
Resistance, proportional to wiper position, is measured using voltage divider arrangement. A constant voltage is applied across the ends of the track and the voltage across the resistance between the wiper and one end of the track is measured. Thus, voltage output across the wiper and one end of the track is proportional to the wiper position.
 
The conductive track can be made linear or angular depending upon the requirements. The tracks are made from carbon , resistance wire or piezo resistive material.
 
POTENTIOMETRIC-POSITION-SENSORS.jpg
 
Three types of potentiometers are used.
a)      Wirewound
Wiper slides along coil of Ni-chrome wire
Wire tends tofail, temperature variations
b)      Cermet
Wiper slides on conductive ceramic track
Better than wire inmost respects
c)      Plastic film
High resolution.
 
Advantages of these sensors are their ease of use.
 

Capacitive Position sensors

 
Capacitance between any two plates depends upon the permittivity of the dielectric between the plates, overlapping area between the plates and the distance between the two plates. Any of these three parameters can be varied in order to design a capacitive sensor.
 
Capacitive position sensors can use following two configurations:
1.      By changing dielectric constant
In this configuration, the body or its part whose displacement is to be measured is connected to the dielectric material between the plates. As the body moves, the effective dielectric constant between the plates is the resultant of the dielectric constant due to air and dielectric constant due to the dielectric material. The changing dielectric constant leads to change in capacitance between the plates. Thus, capacitance becomes a function of the body position.
 
By-changing-dielectric-constant.jpg
 
This principle is commonly used in level position sensors wherein two concentric tubes are used and fluid acts as the dielectric. The variation in capacitance with the fluid level is linear.
 
2.      By changing overlapping area
In this configuration, the body or its part whose displacement is to be measured is connected to one of the plates, the other plate remains fixed. With the movement of the body, overlapping area between the plates changes. The changing overlapping area between the plates leads to change in capacitance between the plates. Thus, capacitance becomes a function of the body position.
This principle can be employed for both linear as well as angular motions.
 
By-changing-overlapping-area.jpg
 
LINEAR VOLTAGE DIFFERENTIAL TRANSFORMER
 
Linear Variable Differential Transformer commonly known by its acronym, LVDT is an electromechanical transducer which converts rectilinear motion of an object into a corresponding electrical signal. It is used for measuring movements ranging from microns upto several inches.
 
LVDT  consists of a primary winding and a pair secondary windings. Primary winding is sandwiched between the secondary windings. Secondary windings are symmetrically spaced about the primary and are identically wound. The coils are wound on a hollow form of glass reinforced polymer and then secured in a cylindrical stainless steel housing.  The windings form the stationary part of the sensor.
The moving element of an LVDT is called the core made of highly permeable magnetic material; the core moves freely axially in the coil’s hollow bore. The core is mechanically coupled to the object whose displacement is to be measured.
 
By-changing-overlapping-area-2.jpg
 
When the primary winding of LVDT is energized by alternating current of suitable amplitude and frequency, AC voltage is induced in the secondary.  The output of the LVDT is the differential voltage between the two secondary windings; the differential voltage varies with the position of the core. Often, differential AC output voltage is converted into DC voltage for use in measurement systems.
 
When primary winding is excited, the voltage induced in the secondary depends upon the coupling of the magnetic flux by the core to the secondary windings.  When the core is at the centre, equal flux is coupled to the two secondary windings and hence, the differential voltage output is zero. However, when the core is at off-centre, unequal flux is induced in the secondary windings  and the amount of flux in the two windings and hence the differential voltage between the two wingings depend upon the position of the core.
 
By-changing-overlapping-area-3.jpg
LVDTs offer various advantages like Friction-Free Operation, very high resolution, unlimited mechanical life, high reliability, no cross sensitivity, environmentally rugged, and so on.
 
For measuring angular motions, a variant of LVDT, i.e, Rotary Voltage Differential Transformer is used. RVDT is exactly similar to LVDT in terms of operation; difference is in their construction.
 

Magnetostrictive Linear Position Sensors

MAGNETOSTRICTIVE LINEAR POSITION SENSORS
 
Magnetostriction refers to the effect wherein a material changes its size or shape in the presence of the magnetic field the material due to the alignment of the magnetic domains, within the material, with the applied magnetic field. Materials having such properties are ferromagnetic materials such as iron, nickel and cobalt. Reverse effect, i.e. property of changing magnetic properties due to applied stress, is called Villari effect.
 
MAGNETOSTRICTIVE-LINEAR-POSITION-SENSORS.jpg
 
Primarily comprising of five components, i.e, the position magnet, waveguide, pickup, damp, and electronics module,  a magnetostrictive position sensor measures the distance between a position magnet and the head end of the sensing rod. The sensing rod is mounted along the motion axis to be measured.  The position magnet is a  ring shaped permanent magnet attached to the member that will be moving and it travels along the sensing rod.
 
MAGNETOSTRICTIVE-LINEAR-POSITION-SENSORS-2.jpg
 
An interrogation(or current) pulse is sent down the waveguide from the electronics module. At the location of position magnet, magnetic field generated by the current pulse interacts with the magnetic field from the position magnet. The result is the generation of sonic wave or torsional strain wave in the waveguide. The strain wave travels towards the head end where the pickup device senses its arrival. Strain wave  travelling away from the head end is removed by the damping module.
 
Position Sensor
 
Time difference between the generation of the interrogation pulse and the arrival of the return pulse(strain wave) indicates the location of the position magnet(or the body connected to it)
 

Eddy Current based position Sensor

Eddy Current based position Sensor
 
Eddy Currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux. They normally travel parallel to the coil's winding and the flow is limited to the area of the inducing magnetic field.
 
EDDY-CURRENT-BASED-POSITION-SENSOR.jpg
 
Principle of operation of eddy current sensors is as follows:

EDDY-CURRENT-BASED-POSITION-SENSOR--2.jpg

 
Applied alternating current fed to the coil induces a primary magnetic field. Primary magnetic field induces eddy currents in the electrical conducting material (in vicinity of the coil). Eddy currents, in turn, induce secondary field. This secondary magnetic field has an effect on the coil impedance. Presence or absence of the conducting material alters the secondary field and in turn, the coil impedance. Change in the coil impedance can be used measure the distance of the electrical conducting body.
 

EDDY-CURRENT-BASED-POSITION-SENSOR--3.jpg

For a defined measuring target the change of coil impedance is a function of the distance. Therefore, the distance can be derived by measuring impedance change.

HEBM & Fiber-Optic Position Sensor

Hall Effect based Magnetic Position Sensors
 

Hall Effect Based Magnetic Position Sensor

The Hall Effect principle states that when a current carrying conductor is placed in a magnetic field, a voltage will be generated perpendicular to the direction of the field and the flow of current.
When a constant current is passed through a thin sheet of semiconducting material,  there is no potential difference at the output contacts if the magnetic field is zero. However, when a perpendicular magnetic field is present, the current flow is distorted. The uneven distribution of electron density creates a potential difference across the output terminals. This voltage is called the Hall voltage. If the input current is held constant the Hall voltage will be directly proportional to the strength of the magnetic field.
 

HALL-EFFECT-BASED-MAGNETIC-POSITION-SENSORS-2.jpg

In position sensors which use hall efffect, the moving part is connected to a magnet.Thus, the sensor consists of a Hall element and a magnet housed within the sensor shaft. With the movement of the body or its part the magnet also moves and therefore, the magnetic field across the Hall element and so the Hall voltage. Thus Hall voltage becomes a function of the position of the moving part.
Commercially available Hall elements are made of Bulk Indium Arsenide (InAs), Thin Film InAs, Gallium  Arsenide (GaAs), Indium Antimonide (InSb).
 
Fiber-Optic Position Sensor
 
Optical fibers offer distinct advantages of their immunity to EMI, inability to generate sparks in potentially explosive environment. Position sensors based on optical fibers can be used for measurement ranging from few centimeters to few meters where very high resolution is not of paramount importance.
Fluorescence followed by absorption is at the heart of this sensor. Pump source is connected to the body or its part whose motion is to be sensed. The fiber is fluorescent, and at the ends of the fiber are placed two photo-detectors.
 
FIBER-OPTIC-POSITION-SENSOR.jpg
 
The logarithm of the ratio of the two signals S1 and S2 is linear in x and independent of the strength of the pump source.
 

Optical Position Sensor

Optical Position Sensor
 
Optical sensors are based one of the two mechanisms. In first type, light is transmitted from one end and received at the other. Change in one of the characteristics- intensity, wavelength, polarization or phase- by the physical parameter is monitored. In second type, transmitted light is reflected from the object and light returned towards the source is monitored.
 
First type of optical sensors are used in optical encoders commonly  used to provide feedback to provide position feedback for actuators. Optical encoders consists of a glass or plastic disc that rotates between a light source(LED) and light receiver(photodetector). The disc is encoded with alternate light and dark sectors so that pulses are generated as the disc rotates.  Based on the count of the pulses, speed of the disc and hence the angular position is computed. To identify the direction of movement, two photodetectors are used. Absolute optical encoders have a unique code that can be detected for every angular position.
An example of second types of sensors is found on machine tools measure the position of the work table is measured and displayed.
 
OPTICAL-POSITION-SENSOR.jpg
 
The strip or disc has very fine lines engraved on it which interrupt the beam. The number of interruptions is counted electronically and this represents the position or angle.
 

Sensor Selection

SENSOR SELECTION
 
When the choices are many, choosing one often becomes an uphill task. As with other types of sensors, position sensors primarily are selected to suit the application requirement. Parameters which needs to be taken into account while selecting a position sensors are
·         Contact/Non Contact type
·         Motion- Linear/Rotary
·         Measurement Range
·         Constraints- Dimensions /Weight
·         Environment
·         Accuracy
·         Resolution
·         Response Time

·         Cost

·         Output

 

Potentiometers are often the cheapest option for position sensing, but needs physical contact with the moving target. Hall sensors are also cheap but are used in ON/OFF type of applications. It is effective only for applications where detailed position information is not required. Optical sensors have very fast response as they are non-contact type, light in weight and don’t need to counter friction. Accuracy is governed by the number of counts. More the counts, better is the accuracy. However, proper alignment and protection from harshy or dust environment is necessary. They are relatively costly. Eddy current based position sensors are moderately priced but are not preferred in applications requiring highly detailed positioning information or where large gaps exist between the sensor and the target. These can tolerate dirty environments and are good when mounted on stationary mechanical structure. LVDTs or RVDTs are priced highly but can tolerate dirty or harsh environments. They offer high accuracy, high precision as well as high sensitivity. They find applications in industrial and aerospace applications.

 

 

Comments

Hello Sir,

I want to control the position of dc gear motor position with a 10k potentiometer interfacing with ATMEGA16.

I am using L293D motor driver for motors.

I have connected PC0 to input1 of motor driver and PC1 to input2 , so that when PC0 is high the motor has to ratate in one direction and PC1 it rotates in  opposite direction. I have connected 10k POT to ADC channel 1.

when I rotate the potentiometer in one direction and if the voltage across is increasing then PC0 should be high 

if the voltage is decreasing when we rotatae pot then PC1 should be high.

 

CAN YOU PLEASE GIVE ME CODE  

we are looking for possition sensor digital type

application: to be used for check  garbage position

U can check magnetometer I thik that will help u

the recomendation of ligal networrk has beren pass the necesarry iof alwatyes as like for grabe position

web page  address used for this chake nasnd position  9507762271 thart all over the contry at thios time for this jhob

there arethree type of liegal netork are at this time of the network area ass rhe suporitendent that the suprior of like manager rthis position as well as three idiot as 

 

I want to make position sensor. So please guide us from where we can get the linear carbon film resistor PCB.
I want to. Measure desplacment. By using angular potentiometer interfacing with pic micro controller so how to write the code with c language ( i need the code)

We are using this type of sensor to measure the linear movement positions in heavy vibration area. values are meaured in profibus DP mode. intermittenly values from LPT are Zero & profibus indications become red. we have cross checked almost most of the possible causes of the profibus diagonistic. we have some doubts on LPT inside materials. can any one suggest the maximum vib limit a LPT can withstand & what need to be checked 

<p><span style="font-size: 12.8px;">We are using this type of sensor to measure the linear movement positions in heavy vibration area. values are meaured in profibus DP mode. intermittenly values from LPT are Zero &amp; profibus indications become red. we have cross checked almost most of the possible causes of the profibus diagonistic. we have some doubts on LPT inside materials. can any one suggest the maximum vib limit a LPT can withstand &amp; what need to be checked&nbsp;</span></p>

please send me the working of optical position sensor of QNET MECHKIT