LabView Controlled Scale

Description 

• Abstract

The type of design chosen to be carried out is the three-point bending. The three point bending test is when a beam, with its two ends on two pin supports and a force applied at its center. The force P can be calculated by multiplying the mass of the object by the gravitational acceleration 9.81m/s2, The force then results in bending in the beam, and can be used to find the moment and moment of inertia of the four strain gauges. The moment is found by dividing the beam into different sections. For the beam used in this project, the bending beam is divided into two different sections. One section is from one end to an arbitrary point somewhere between the midpoint where the force is applied; the other section is taken by adding to the first section the length of beam that include the point of application of the force plus an additional arbitrary length beyond that point. Force and moment analysis is done on each of the two sections until the moment diagram is found.

A total of four strain gauges will be mounted on this beam: two on top and two on bottom. For this project, strain gauges numbered 2 and 3 are on top, and 1 and 4 are on the bottom. The strain gauges will be placed so they are equally apart. For this project, the strain gauges are placed so their centers are 0.38 inches horizontally from the center of the beam, and also is vertically positioned at the center. When the beam is bent, two of the strain gauges mounted on top will change due to compression, and the two on the bottom will be changed due to tension. Two of the resistance change will be positive, and two will be negative. This change in resistance then causes an imbalance in the wheatstone bridge, and this in turn produces a differential output that corresponds to the force placed, and thus the weight.

The change in voltage is calculated from the data gathered above. It is then amplified to about 10 V through the amplifier, which makes it easier for the LabView program to produce an accurate result. The output of the amplifier, which is a voltage, is then connected the analog input of the DAQ, and the DAQ is in turn connected to the computer. Therefore, the weight of the object is first turned to a force pressing down on the three point bending beam, which is then used to calculate the strain. The voltage change is then calculated by using the strain, and amplified with the amplifier. The mass of the object to be weighted is kept between 0 and 200 grams, and therefore the change V produced will be very small, which is hard for LabView program to read and process. This is why the change in voltage is ran through the amplifier and brought up to around 10V. In doing so, it will become easier for the LabView program to process, and also be more precise since the original small changes will now be enlarged also. It is finally converted to an analog signal when it is connected to the analog input of the DAQ.