Experiment No.: 06
Name of Experiment: –
Measurement of Linear Displacement using LVDT.
Apparatus Required: –
| Sl. No. | Name | Specification | Quantity |
|---|---|---|---|
| 1 | Displacement Indicator | 19.99 mm range, 200mv output, maximum indication 1999. | 1 no. |
| 2 | LVDT with screw gauge | +-1 mm | 1 no. |
| 3 | Mili Voltmeter | (0-100) mV | 1 no. |
| 4 | Connecting Wire | PVC Insulated copper | As per required. |
Circuit Diagram/Block Diagram: –
Theory: –
Displacement due to pressure, force change in liquid level etc. can be measured using different types of analog or digital transducer as discussed below: –
a. Potentiometric resistance type transducer: –
This is a wire wound potentiometric which may be used as transducer for converting mechanical displacement to an electrical output. This may be linear or angular type. The motion of the object changes the effective resistance and hence the output voltage between common and sliding contact or resistive transducer is directly proportional to the displacement of the object. If displacement with respect to some references (initial) position has to be measured then arrangements are made to set initial zero by two potentiometer are used in parallel; one as initial position transducer and other as displacement transducer. The potential difference between two variable points on the potentiometric resistance transducer in proportional to displacement with respect to initial position.
b. Inductive type transducer:
In this type of transducer magnetic characteristics of an electrical circuit changes due to motion of the object is used to generate an analog electrical output. This can be classified into two types:
(i) Self generating type:
In which a voltage is generated in the transducer, because of relative motion of a conductor and magnetic field. Electrodynamic, electromagnetic and eddy current types of transducer belong to this category.
(ii) Non-self generating or external power source type transducer-:
In this an external source is needed to energize a coil/coils, the inductance of which would change due to motion of the object. The following type of transducer belong to this category: Attachment type inductance transducer, air gap type, Linear voltage differential transformer (LVDT) type and magneto strictive type transducer.
c. Capacitive type transducer:
This is a displacement sensitive transducer. Due to motion, there is a change in the capacitive between two parallel plates; one of them is stationary and other movable. Suitable high sensitive op-amp amplifier circuit is use to measure change in capacitance with respect to a reference capacitor. These transducers are highly sensitive, and can be used to measure displacement in microns.
LINEAR VARIABLE DIFFERENCIAL TRANSFORMER (LVDT)
An LVDT is non self generating type inductive transducer often used to measure force, pressure, or position figure M2-1 shows the basic structure of an LVDT. It consists of three wires wound coils on same former with a movable iron core. An ac excitation signal of 1 to 20 KHz is applied to the primary. The two secondary’s connected in phase opposite usually positioned on either side of primary gives a zero output voltage if movable core has been positioned in middle. If the core is centered then the induced voltages in secondary are equal and they cancel, so there is no output voltage. If the core is moved off center, coupling will be stronger to one secondary coil so that coil will produce a greater output voltage. The result will be a output voltage. The phase relationship between the output signal and input signal is an indication of the direction. of the core movement from center position. The amplitude of the output is linearly proportional to the core displacement from the center or zero position.
An LVDT can be used directly in this form to measure displacement and position. If we add a spring so that a force is required to move the core, then the voltage output of the LVDT is proportional to the force applied to the core. In this form LVDT can be used in load cell for an electronic scale. Likewise, if an spring and core is attached to a diaphragm in a threaded housing, the output from LVDT will be proportional to the pressure exerted on the diaphragm. Like this can think of many applications of LVDT in different situations.
DIGITAL DISPLACEMENT INDICATOR
The digital displacement indicator uses spring loaded LVDT as transducer, which can be connected to the socket provided at the back of the instrument. The specific range of different LVDT such as +/-1mm, +/-5mm, +/-10mm & +/-20mm, can be selected with help of the RANGE SELECT SWITCH. The 3.5 digit/digital output displays the displacement in mm; but it has to be calibrated for a particular LVDT. For this purpose two variable resistor provided in front panel marked as ZERO ADJUST & CALIBRATE. The zero adjust potentiometer makes the display zero when LVDT core is in center position (both secondary have equal induced emf) and calibrate potentiometer is adjusted after giving full scale displacement to the LVDT core. There is an analog output available at back of the displacement indicator for X-Y recorder. The circuit configuration is shown in fig. M2-1.
Procedures: –
- Connect the mili-volt meter and LVDT to the displacement indicator.
- Adjust and calibrate the displacement indicator (In LVDT keep both the scale of screw gauge at 5mm, consider it as zero output as both the secondary coil of LVDT are producing same voltage).
- Calculate the screw gauge displacement for calibration of displacement indicator as per below formula:
Total displacement= [Horizontal scale distance + (Least count*Vertical scale distance)]
Least Count = (Pitch factor/Total division in circular scale)
Pitch factor= (1mm displacement in horizontal scale/ Number of revolution taken in circular scale for 1mm displacement in horizontal scale).
4. Take the readings as per observation table.
Observation Table: –
| Sl. No. | Screw gauge reading in mm | Digital Display of displacement in mm | Analog Output Voltage in mili-volt |
|---|---|---|---|
| 1 | -5 | ||
| 2 | -4 | ||
| 3 | -3 | ||
| 4 | -2 | ||
| 5 | -1 | ||
| 6 | 0 | ||
| 7 | 1 | ||
| 8 | 2 | ||
| 9 | 3 | ||
| 10 | 4 | ||
| 11 | 5 |
Plot the analog output and digital output with respect to the screw gauge reading assuming it as standard displacement.
Conclusion: –
To be written by student.