Testing

After the initial prototype was constructed, it was required to test the section with all the electronics embedded onto the section. The test involved testing how accurately the system could measure the weight of an object placed on the section, and whether the centre of pressure could be calculated correctly. Listed below are the resources required to conduct the test:

- Arduino Uno R3

- Prototype Breakout Board with AD7193 - 4x ASB1000 Load Cells

- Prototype Section - Point Load Stand - 20kg Weight

- Computer running MATLAB

Figure 119: 20kg Weight Placed on Top of Point Load Stand (Left), Centre lines drawn on sides to help align weight with grid (Right).

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System Configuration

- The AD7193 was configured to have an output rate of 300Hz. Because four channels are enabled, the effective output rate per load cell is only 75Hz.

- The AD7193 was also configured to have a gain of 128, therefore the RMS noise is 85nV [32].

- The load cells were adjusted to be as close to the corners as possible in order to give the maximum surface area.

- A grid of 425mm x 600mm with a spacing of 25mm was drawn onto the platform. This was done to ensure the point load stand would be placed on known locations. The centre of the point load stand was also found, and this was aligned with the grid to ensure accuracy during the experiment.

Figure 120: Platform Test Grid (Left), 20kg Point Load Placed on Prototype Section (Right)

From Figure 121, it can be seen that all four load cells interface with the AD7193. The AD7193 and the Arduino Uno communicate using the SPI bus. The Arduino then simply forwards the results to the computer for further processing. The computer runs MATLAB to do the number crunching.

Results

Note: The total weight shown on Figure 122 to Figure 126 is not correct as the weight of the point load stand was not deducted when the graph was being plotted. The results on the left hand side of the graph however are correct.

Figure 122: Load Placed on Bottom Left Corner (0, 0)

In Figure 122: Load Placed on Bottom Left Corner (0, 0), it can be seen that the bottom left load cell measured a weight of about 20kg and the X-coordinate was approximately 1mm, and the y-coordinate was approximately 0.5mm. This was expected as the weight was placed in the bottom left corner.

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In Figure 123: Load Placed on Bottom Right Corner (425, 0) it can be seen that the bottom right load cell measured a weight of about 20kg and the X-coordinate was approximately 422mm, and the y-coordinate approximately 0.5mm. This was expected as the weight was placed in the bottom right corner.

Figure 124: Load Placed on Top Left Corner (0, 600)

In Figure 124: Load Placed on Top Left Corner (0, 600) it can be seen that the top left load cell measured a weight of about 20kg and the X-coordinate was approximately 1.8mm, and the y-coordinate approximately 600mm. This was expected as the weight was placed in the top left corner.

In Figure 125: Load Placed on Top Right (425, 600) it can be seen that the top right load cell measured a weight of about 20kg and the X-coordinate was approximately 422mm, and the y- coordinate approximately 598mm. This was expected as the weight was placed in the top right corner.

At this point of time, it was evident that the section was measuring the weight and position very accurately. The final point of interest was to place the weight in the centre of the section to determine how accurate the system measures the location, and how well the load cells are sharing the load. The load cells should share the weight equally; 5kg per load cell.

Figure 126: Load Placed in the Centre of the Section

In Figure 126, it can be seen that by having the load placed in the centre of the section, the total load still measured a weight of about 20kg. The X-coordinate was approximately 212mm, and the y-coordinate approximately 301mm. This was expected as the weight was placed in the centre of the section.

The noise present as seen in Figure 126 is due to the AD7193 having a set gain of 128 which caused the AD7193 to have a RMS noise of 85nV [32]. Other contributors to the noise are the load cells which has a signal output of 2mV/V ± 0.1% [29], and the voltage reference which has a signal output 4.096V ± 0.05% [30].

Please see Appendix 2 for all the results that were recorded while conducting the experiment. The statistical results of the centre of pressure and weight accuracies can be seen in Table 8: Mean and Standard Deviation of X & Y and 0.814 ± 1.788 mm.

Table 9: Mean and Standard Deviation of Weight respectively.

Table 8: Mean and Standard Deviation of X & Y Position

X-Position

_Y-Position

Mean Deviation

_1.005mm

Standard Deviation 2.172mm Standard Deviation 1.788mm

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Maximum 5.377mm Maximum 3.512mm

It can be seen from Table 8: Mean and Standard Deviation of X & Y Position that the X- position accuracy was calculated to be 1.005 ± 2.172 mm, and the Y-position accuracy was calculated to be 0.814 ± 1.788 mm.

Table 9: Mean and Standard Deviation of Weight

Weight

Mean Deviation 20.087kg

Standard Deviation 0.034kg

Minimum 0.024kg

Maximum 0.180kg

It can be seen from 0.814 ± 1.788 mm.

Table 9: Mean and Standard Deviation of Weight that the weight accuracy was calculated to be 20.087 ± 0.034 kg.

The mean weight error was calculated and found to be 0.44% using a 20kg weight.

Conclusion

It was concluded that the accuracy of the system overall was excellent. The x-position however had a larger standard deviation. This might have been due to human error when aligning the point load stand, vivid marks being too thick or grid pattern not being 100% straight.

Overall the results were very good, and it was decided to move onto the final prototype design of the project.