Conclusion
Timing Chart
Fig 23 - Pie Chart of Scan Time
Stepping the Motors
This takes 55 ms for a movement of 100 mm. The speed is restricted by the
specifications of the motors.
Thinking Time
This is 5 ms and has already been reduced from 40 ms. It could only be
reduced significantly more by getting a faster computer, but this could only
speed it up by a maximum of 5 ms.
Lowering and Raising the LVDT
This uses compressed air to lower and a return spring to raise. Lowering takes
50 ms and raising takes 55 ms. Increasing the pressure on both of these to
speed things up is likely to damage the sample.
Bedding In
This takes 300 ms. A lot more investigations would have to be performed to find
out the cause of the delay in the moving down the last 20 mm of travel. It is
unlikely that the time spent investigating this effect would ever pay for itself in
terms of increased speed alone
Confirming Steady Reading
This takes 125 ms for the PC to take 100 readings to confirm that the reading
from the LVDT has stabilised. This is dependant on the speed of the PC so if the
number of readings taken was reduced, this could result in less accurate
readings if the PC was changed for a faster one at a later stage.
Displaying Roughness Visually
To add a routine to the program to enable a cross section, or even a series of
cross sections, on the screen would have been fairly easy and would have
looked really impressive. However to show the data in a useful format so that it
could be rotated about any axis etc. to check effects such as the successful
marrying up of scans using the merge facility, or to view the effect of changing
from a blunt to a sharp probe, was a completely different matter. Unless a
package can be found that can do this with little or no modification, then adding
a routine to visually display the roughness is a project in itself.
Purge Routine
The feeling at Nuclear Electric was that this routine should be expanded to
check the points on the periphery of the scan as well as those that have all 4
neighbours, and also, if possible, give the facility to automatically re-check those
points that are inconsistent. Both these modifications are possible but as the root
cause of the problem that the purge routine was created to fix has gone, it
seems unlikely that the purge routine will be needed in future so such
modifications would be a waste of time.
The software meets all the new requirements for the program and has fixed all
the existing problems such as the parallelogram effect and probe sticking effect.
Many more improvements have been added because of the availability of
additional input lines such as checking the limit switches and the remote keypad.
The scan is also faster as a result of using a variable speed and faster ADC
conversion rate. The DMA (direct memory access) file structure has resulted in a
far more flexible merge routine as well as many by-products such as Point Edit.
Small refinements such as 'Estimated Time to Finish' have been added where
ever I have seen a need.
The result is a product vastly superior to the old BBC system. This was readily
acknowledged by the users of the systen when it was recently demonstrated to
them.