Experiment 4: Measurement of linear displacement using LDR,Strain gauge and Potentiometer.

Displacement measurement using LDR

Setup:

The complete setup consists of a sensor module and its signal conditioning circuit. In the sensor module there is an LDR and a light source enclosed inside a metallic enclosure. There is a 20 mm screw gauge to provide known displacement. A rod is connected to the LDR inside the enclosure and it is attached to the screw gauge outside. Hence we can provide known displacement to the LDR by the screw gauge. Three terminals are attached to the sensor module, Vcc and GND for the light source and one terminal of LDR w.r.t ground. The output of the sensor module is fed to the signal conditioning and processing unit. There is a IC regulated power supply source that provides constant voltage to all the circuits for their functioning. A zero adjustment and gain adjustment pot is there for calibration purpose. And there is a four 7 segment LED display which shows the output directly in mm.

Working:

LDR stands for Light Dependent Resistor, it is basically a resistor whose resistance depends upon light intensity. And the relation is inversely proportional means when the light intensity is more the resistance is less and vice versa. The LDR is attached to the rod at one end inside the enclosure. The light source has been kept fix. When the screw gauge is rotated it gives a linear displacement to the rod attached to it. This displacement changes the distance between light source and LDR. When the distance is less the resistance of LDR is less and when the distance is more the resistance will be more. This change of resistance is given to the signal conditioning circuit where it is calibrated to give the output directly in mm.

Procedure:

Connect the LDR sensor leads with the trainer kit terminals according to their colors. Switch ON the power. Keep the DPM switch at mm position. Connect a patch cord between output terminal and DPM terminal. Keep the micrometer at zero position and adjust zero pot to set zero reading on the display, set the micrometer at 15mm and adjust the adder pot to show 15mm on the display. Now increase the micrometer in steps of 1 mm and note down the in following table in increasing and decreasing mode. Plot the graph between micrometer reading and DPM reading in mm.

Observation:

Sl no	Reading in increasing mode		Reading in decreasing mode
	Micrometer reading (mm)	DPM reading (mm)	Micrometer reading (mm)	DPM reading (mm)

Displacement measurement using linear pot

Setup:

The setup has a linear pot and a screw gauge arrangement module and its signal conditioning circuit. The linear pot is a sliding type potentiometer enclosed inside a metallic enclosure. A rod is attached to the sliding arm of the potentiometer at one end and to the screw gauge at the other end. Three terminals are taken out from the sensor (pot), one for Vcc one for GND and another for output from the wiper of the pot. These three terminals are connected to the signal conditioning where the output is processed and calibrated in terms of displacement. In the signal conditioning unit there is an IC regulated power supply source that provides constant voltage to all the circuitry inside the unit and to the pot also. There is a zero and span adjustment pot for calibration. And a four 7 segment LED display screen.

Working:

The pot is generally a variable resistor. It has three pin/ terminal, two are fixed and one is sliding. Between two end terminals the resistance is fixed/constant, and between the sliding terminal and the fixed terminal the resistance is variable according to the position of the sliding arm. The resistance between the sliding arm and any one of the fixed terminal is depends upon the physical distance between them. If the distance is more the resistance will more and vice versa. If a constant voltage is applied between the two fixed terminals the pot creates a voltage divider circuit itself. The output voltage between sliding terminal w.r.t GND depends upon the resistance which eventually depends upon the position of the slider. The rod connected between the slider and the screw gauge provides the known displacement to the slider of the pot. The output voltage is further processed and calibrated in terms of displacement.

Procedure:

Connect the linear potentiometer leads with the trainer kit terminals according to their colors. Keep the DPM switch at mm position. Connect patch cord between output terminal and DPM terminal. Switch ON the trainer kit. Keep the micrometer at zero position and adjust zero pot to set zero reading on the display, set the micrometer at 20mm and adjust the adder pot to show 20mm on the display. Now increase the micrometer in steps of 1 mm and note down the in following table in increasing and decreasing mode. Plot the graph between micrometer reading and DPM reading in mm.

Observation:

Sl no	Reading in increasing mode		Reading in decreasing mode
	Micrometer reading (mm)	DPM reading (mm)	Micrometer reading (mm)	DPM reading (mm)

Displacement measurement using strain gauge transducer:

Setup:

In the setup the transducer used for displacement measurement is strain gauge. The transducer module consists of four strain gauges with some mechanical arrangement to measure linear displacement provided by a screw gauge. In the mechanical arrangement a steel plate (measuring scale) is used, four strain gauges are fixed over it, two on each side. One end of the scale is fixed, but the other is exposed to linear force. A conical shaped plastic object having 3cm length and 1.5cm dia is fixed at one end of the rod, and the other end of the rod is attached with the screw gauge. The four strain gauges form a Wheatstone bridge so four leads are taken out of the transducer for electrical connection. These four leads are connected to the signal conditioning unit. In the signal conditioning unit there is an excitation source, some electronics circuitry, a zero adjustment and span adjustment pot for calibration and a four 7 segment display unit.

Working:

The linear displacement provided by the screw gauge is converted in to the linear force by the conical plastic object, which is then converted in to change in resistance of the strain gauge by the steel plate. This resistance change causes voltage change in Wheatstone bridge output which is further processed and calibrated in terms of linear displacement. And is directly shown in the display unit in mm.

Procedure:

Connect the transducer leads with the trainer kit terminals. Keep DPM switch at mm position. Connect patch cord between output terminal and DPM terminal. Switch ON the unit. Keep the micrometer at zero position and adjust zero pot to set zero reading on the display, set the micrometer at 20mm and adjust the adder pot to show 20mm on the display. Now increase the micrometer in steps of 1 mm and note down the in following table in increasing and decreasing mode. Plot the graph between micrometer reading and DPM reading in mm.

Observation:

Sl no	Reading in increasing mode		Reading in decreasing mode
	Micrometer reading (mm)	DPM reading (mm)	Micrometer reading (mm)	DPM reading (mm)