What is Linear Displacement? 

The term "displacement" refers to when something shifts away from its original "location," and "linear" refers to a straight line. As a result, “Linear Displacement” can be described as the movement of an object in a straight line along a single axis, for example, from side to side or up and down. Non-contact sensors such as LVDTs and other linear location sensors can calculate linear displacement. Non-contact sensors such as LVDTs and other linear location sensors can calculate linear displacement. Linear displacement is usually measured in millimeters or inches and may be positive or negative. 

Concept 

The movement along a single axis in one direction is called linear displacement. The distance an object has travelled from a particular reference point can be measured; this is done using a position/linear displacement sensor. From this, even the direction of motion can be attained. 

Linear Variable Differential Transformers (LVDTs)

One of the devices which is used to measure displacement is LVDT (Linear variable differential transformers). The principle of operation is that of a transformer. The basic construction of LVDTs includes a coil assembly and a core. The coil assembly is made stationary and the core is attached to something whose displacement is being measured. Three coils of wire are wound on a hollow form. A soft iron is used as the core which can move freely through the centre of this hollow. The AC source is connected to the inner coil and this is known as the primary. Thus the magntic flux induced here links with the secondary also; this induces an voltage in the secondary. The LVDT transducer, is highly robust, since the sensing element has no physical contact with other things, there are no chances of wear and tear. Also, it has infinite resolution. Thus it is very effective to measure even the smallest amount of displacement.  

LVDT Measurement 

A particular position of the core is given a particular signal value. This signal value is given through electromagnetic coupling of the AC signal to the primary of the coil and back to the secondary of the coil. The core’s position tells if the signal of the primary coil is coupled to each of the secondary coil tightly. There are two secondary coils which are wound in opposite directions in series (series- opposed). So the two signals would be 180 º out of phase to each other. So we get the direction from the phase of the output signal and its amplitude gives the magnitude. 

Due to the core, the magnetic flux linked to the primary, is coupled to the secondary. When the core is perfectly between both the secondaries and the primary, the output would be zero. This is because the voltage induced in each secondary would have equal magnitude and opposite phase. This case is shown here. 

When the core is displaced to the left, the first secondary coil couples more strongly to the primary coil (than the second secondary). Thus we have higher voltage from the first secondary coil. This causes the output to be in phase with the primary voltage. Similarly if the core is displaced to the right, the second secondary coil is more coupled to the primary coil. This greater voltage from the second secondary coil gives an out of phase output voltage with respect to the primary voltage. 

This is how a LVDT functions. IT is an ideal zeroth-order displacement sensor at low frequency, where the output is a direct and linear function of the input. Both the direction and displacement can be measured from the amplitude and phase of the voltage. There is linearity only within the range of core displacement. The output is non linear near the boundaries, since less magnetic flux gets coupled. This non linearity near the boundaries can be predicted using a polynomial curve fitting. Thus we have a good range for this device. 

Signal Conditioning for LVDTs 

The output is an AC waveform itself so it has no polarity. There is an increase in the magnitude of the output of LVDT, which is independent of the direction of movement from zero position. 

To find out the location of the center of the core is in which half of the device, the phase and magnitude of output needs to be considered. The output can be in phase or out of phase with the AC excitation voltage of the primary coil. This gives the position of center of coil.  

The signal conditioning electronics converges the data of the amplitude as well as the phase of the output so that a person gets both the direction and magnitude of the displacement from the reference zero position.  

These signal conditioners produce a sine wave signal with frequency between 50 Hz and 25 Hz, as an excitation source for the primary. The carrier frequency is selected such that it is at least 10 times greater than the highest expected frequency of the core motion. Also the demodulation of the secondary output signal from the primary voltage is carried out by the signal conditioning circuit. The resulting DC voltage would be proportional to core displacement. The polarity of the DC voltage indicates whether the displacement is left or right. 

Advantages of PIPS Linear Displacement Sensors 

Linear displacement sensors have the following advantages: 

• High reliability 
• Have improved stroke to length ratio when compared to other inductive position sensors 
• Suitable for use where space is restricted 
• Extremely robust 
• Highly accurate 

Other types of linear displacement sensors 

There are other types of linear displacement sensors : 

• Potentiometers 
• Eddy Current Sensors 
• LVDTs 
• Cable Extension Transducers 
• Ultrasonic 
• Laser position sensors 

Linear Displacement is something that requires measuring in almost every industry, some of these areas are; 

• Robotics 
• Automation 
• Instrumentation 
• Machine Tools 
• Food processing 
• Mobile Vehicle 
• Autosport 
• Motorsport 
• Agricultural Machinery 
• Suspension Measurement 
• Instrumentation and Test 

Context and Applications    

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for  

• Bachelors in Science in Physics  
• Masters in Science in Physics