Displacement Transducer : Circuit, Types, Working & Its Applications

The position sensor is a type of device used to monitor & measure a change within the position of an object in a device/ machine or in certain vicinity & changes into signals which are appropriate for transmission, processing, or control. There are different kinds of position sensors available where displacement transducer is one specific kind of position sensor. Generally, normal sensors sense the existence of the object whereas displacement sensors simply detect the displacement once any object moves from one location to another. So, the amount of displacement detection simply allows you to determine the object’s thickness & height. This article discusses an overview of a displacement transducer – working with applications.

What is Displacement Transducer?

A displacement transducer is an electromechanical device used to convert the motion of an object into electrostatic, electromagnetic, or magnetoelectric signals which are read & interpreted into data. There is a wide range of displacement transducers like linear & rotary. These transducers are also helpful in measuring the physical distance between the sensor & a target. Most displacement transducers measure static & dynamic displacements, so they are frequently used for measuring the vibration of an object. The measured displacements range from micro inches to a few feet.

The displacement transducer working principle is based on the extremely reliable inductive measurement principle. These transducers are rugged, very easy to use & can attain high precision. Displacement transducers give reliable measurement results in different areas of production, research & development.

Displacement Transducer Circuit Diagram

The displacement transducer used in the below circuit is an inductive transducer. This circuit is used to measure displacement with an Inductive transducer.

In the above circuit, the transformer includes a primary winding & two secondary windings. The two secondary winding’s endpoints are connected together thus, we can declare that these two windings are simply connected within series opposition.

The ‘VP’ voltage is applied at the transformer’s primary winding, let the voltage developed across every secondary winding be 𝑉𝑆1 & 𝑉𝑆2. So, the ‘V0’ output voltage is received across the first points of secondary windings. So the output voltage can be written as V0 = VS1 – VS2. The transformer used in the above circuit is the differential transformer because it generates an o/p voltage, which is the dissimilarity between VS1 & VS2.

If the core is positioned at the central point, then the induced voltages across two windings S1 & S2 are equivalent. So, the output voltage V0=0. In this condition, we say that there is no displacement.

If the core is displaced above the central position, then the emf generated within coil S1 is more i.e, V1>V2.

In the same way, if the core is displaced below the central position, then the emf generated within the S2 coil is more i.e, V2>V1.

So in these two cases, we have two displacements upward & downward. In these two cases, the magnitude of output voltage ‘V0’ will be proportional to the core position relative to the center.

Thus, if we want to measure the displacement of the body then we must connect the body to the central core. Therefore, once the body shifts in a straight line, then the middle point of the core changes, so, the o/p voltage like ‘V0’ also varies accordingly. In this condition, we can get the displacement by simply measuring the o/p voltage. So, the phase and magnitude of the output voltage signify the displacement and direction of the body correspondingly.

Displacement Transducer Calibration

Generally, transducer calibration is an essential requirement to maintain the accuracy, repeatability & reliability of the results attained from a measurement system. These transducers are common in academic & industrial applications. So, calibrating them is usually a time-consuming procedure, however, by using calibration apparatus, it is very easy by twisting a knob & pushing a button.

The calibration system of the displacement transducer is a complete solution to calibrate these transducers with up to 50.8mm displacements with 13 microns resolution. It is used with any system, although it comes with custom software to let fast and easy calibration once used with NI systems.

Displacement Transducer Types

There are various types of displacement transducers available which are also known as displacement sensors like a potentiometer, strain gauge, capacitive, and LVDT. So each type is discussed below.

Resistive Transducer

A resistive transducer is also called a variable resistance transducer because it works on the principle of variable resistance transduction. This transducer is one of the most frequently used displacement transducer, used to measure different physical quantities like pressure, displacement, force, temperature, and vibrations and converts them into an electrical signal.

Capacitive Transducer

A capacitive transducer is a passive transducer that works by using external power. This transducer is mainly used to measure pressure, displacement, movement, force, velocity & other parameters. This transducer works on the variable capacitance principle, so the capacitance of this transducer changes due to many reasons like dielectric constant, overlapping of plates, and change within distance in between the plates. This is a passive type where equal & opposite charges are generated on the plates because of the applied voltage across the capacitor plate which is separated through the dielectric material.

Linear Variable Differential Transformer

The LVDT or linear variable differential transformer is one kind of displacement transducer. This transducer includes three symmetrically spaced coils where the primary coil is the center coil & remaining two coils are secondary coils. These are mainly connected in series & positioned equally with respect to the main coil. Please refer to this link to know more about – LVDT.

Inductive Transducer

An inductive transducer is one kind of displacement transducer that works on the transduction or electromagnetic induction principle. To measure the necessary physical quantities such as force, displacement, velocity, pressure, acceleration, torque, s mutual or self-inductance is varied. The best example of this transducer is LVDT. Please refer to this to this link to know more about inductive transducers.

Strain Gauge

A strain gauge displacement transducer is used to change physical quantities like pressure, displacement, or load into mechanical strain, and this mechanical strain is changed into electrical o/p with mounted strain gauges on the elastic body. A strain gauge displacement transducer is mainly used for measuring the displacement in the 0 to 10 mm range. This transducer has a short body length as compared to LVDT & is free from electromagnetic effects. These strain gauge transducers have highly stable & reliable performance. Please refer to this to this link to know more about the strain gauge.

Advantages and Disadvantages

Displacement transducer advantages are discussed below.

• Displacement transducers have excellent linearity.
• They have extremely high accuracy.
• They have up to 0.01 µm outstanding resolution.
• These are tolerant to high magnetic fields, radio-active environments & wide temperature ranges.
• They have a strong design and excellent stability.
• These transducers can be mounted in any direction.
• The power consumption of LVDTs is low.
• These are very sensitive & very easy to align & maintain.
• These transducers have low hysteresis loss.
• The measurement range is higher.
• This transducer is a frictionless device.

Displacement transducer disadvantages are discussed below.

• The displacement transducer needs very high displacement to produce high voltage.
• It needs shielding because it is very responsive to the magnetic field.
• The transducer performance can be affected by vibrations as well as changes in temperature.
• It needs an external demodulator to get DC output.
• The dynamic response of this transducer is limited.

Applications

The applications of displacement transducers include the following.

• Displacement transducers are used to measure the relative movement between the sensor tip & the rotating shaft.
• It is used in rigid machinery wherever extremely small vibration from the shaft to the machine casing is transmitted.
• These are used in the industrial sector & even in the public sector like machine automation, aerospace & aircraft, power turbines, hydraulics, etc.
• LVDTs are used for measuring displacements that range from mm to cms.
• These are used in CNC machines for the measurement of displacement.
• These are used for the thickness & measurement of rolled metal sheets.
• These are used within channels for tension measurement.
• RVDT displacement transducers are used within flight control systems.
• Potentiometer types are used for measuring force, acceleration & pressure.

Thus, this is an overview of a displacement transducer – working with applications. If a body shifts from one position to another within a straight line, afterward the length between those two positions is known as called displacement. The displacement is a physical quantity like velocity, temperature, force, etc.

So a displacement transducer is used for converting mechanical vibration/ motion, particularly rectilinear motion into electric signals, a changeable electrical current, or voltage. Examples of displacement transducers are; displacement & bending strains measuring measurements of normal displacement, measuring cracks within the concrete, and girder bend. Here is a question for you, what is the function of a transducer?