PHOTOGRAMMETRY FOR DISPLACEMENT MEASUREMENT

Experiment : PHOTOGRAMMETRY FOR DISPLACEMENT MEASUREMENT

ABSTRACT:

Among the various serviceability criterias, deflection is one of the major criteria of a structure. Measurement of the deflection with the help of dial gauge has been a common technique. In this experiment, a new technique called as Photogrammetry was introduced for measurement of displacement of a simply supported beam. Photogrammetry is the practice of determining the geometric properties of objects from photographic images. It relies on image processing to derive meaningful real life information. In this experiment, a simply supported beam was subjected to 4 different loads and displacement under the loads was calculated by both dial gauge and photogrammetry method and the results were compared.

OBJECTIVES:

To determine structural deflections of a simply supported beam using photogrammetry technique and to compare those deflection readings with those of dial gauge and to give a conclusion on the accuracy of the photogrammetry technique. 

EXPERIMENTAL  SETUP:

The setup consisted of a simply supported beam which acted as the structure undergoing deflections. A stationary reference frame with two marks A and B 100 mm apart was fixed above the beam. The reference attached to structure was at the midpoint of the beam and marked as C. The beam was loaded with 4 subsequent loads (with some time gap). Dial gauge readings were noted for each displacement and simultaneously the photographs of the deflected positions of each loading were taken online using the camera. The photographs were then analysed for deflections using MS paint. The pixel reading of the normal drawn from C to A can be converted into deflections of C in mm, making use of the fact that the real distance between points A and B is 10cm and that do not change.

RESULTS:

Initial Dial Gauge reading = 0 mm

Tables:

Table- 1 : Comparison of deflection values from dial gauge and photogrammetry.

Loads on simply supported beam (grams)	Deflection  from dial gauge (mm)	Deflection  from    photogrammetry (mm)
49.3	1.46	1.72
95.7	2.73	3.44
138.29	4.23	5.16
182.29	5.63	6.88

 Figure 1:Experimental set up

Figure 2:Load vs Deflection plot

CONCLUSIONS

It can be inferred that the observed values of displacement by photogrammetry was nearly equal to that of actual displacement measured by the dial gauge and hence photogrammetry can be used to monitor the deflections of structure in remote areas.

As we know that the load vs deflection curve is linear which can be seen in the results obtained using photogrammetry. Thus, photogrammetry technique is more precise than dial gauge.

REFERENCES

1.        Lab Presentation Notes,CEP726, Dr. Suresh Bhalla , Civil Engineering Department , IIT Delhi.

2.    Manual of Experiment No.4 of Virtual Smart Structures and Dynamics Lab.