Hi Rick;
Another useful article in less than a week, working over time I see.
I just got to this article so I've read most of the thread up until now. First in car you are wondering the spread sheet opened fine in Numbers, the Mac spread sheet. It did complain about missing fonts but that is normal. This should mean that the formal would be usable on my iPhone!!
It is a great piece but it would be helpful to see how you derived the formula. There are a couple of reasons one is verification, it is always nice to check that the numbers we are getting are good and the ranges for which they are good. You noted the limit at the lower range of angles but I have to wonder what happens past 90 degrees. This probably isn't an issue with a sin plate but your arrangement of parts would allow mechanically for angles beyond 90 degrees. Sometimes + 90 degree angles can make for far easier setups.
I noticed you are having issues with embedding the spread sheet within the PDF. Unfortunately PDF is kinda of limited here and the use of Word doesn't help any. Thus I'm wondering if you have considered coding in HTML/Javascript. Yeah I know in some respects more work but it does allow you to integrate interactive code with the content. Of course HTML files are less than ideal in their own right as they have to be distributed as a collection of loose files. Modern web browser address this to some extent also. Unfortunately there is no perfect solution here for distribution of complex documents that are open and freely available for use.
You pointed out the need to keep your caliper square to the stud and showed using a washer to do that. I might suggest that you get rid of the washer and simply cut a shoulder on the stud to support the blade. It will of course impact the ability to close up the angle but as you note accuracy suffers anyway.
The thread then teeters off course a bit with discussion about how to better measure the angle. First it isn't likely that a DIY approach will be anywhere nears as compact nor robust as a factory engineered job. In any event in industry we use a lot of linear transducers, everything from encoders, to resistive to Temposonic with each and everyone having its own issues. A resistive solution might be fairly easy to implement but you would need a fairly large radius to wrap the element on to get suitable resolution. Or you could drive a linear sensor with a rack and pinion of some sort. This would give you a linear voltage representing the angle. The big problem with instrumentation resistors is that they aren't cheap and are surprisingly bulky for what they are.
In the end I would thing the best effort would be had with a rotary encoder of some sort. For example a 4000 count rotary encoder would give you a resolution of 0.09 degrees out of 360 degrees. Of course 4000 count encoders aren't cheap but you can find stuff under a $100. example: http://usdigital.com/products/encoders/incremental/rotary/kit/e5, another example: http://www.avagotech.com/pages/home/
As a side note I found this http://usdigital.com/products/interfaces/pc/usb/qsb#description looking for encoder info. This is an incredibly cheap encoder and stepper motor interface and should make for an low cost interface to a stepper driven indexer.
Honestly I think it is a waste of time to spend excessively on building a digital protractor. Such time could be put to better use building a digitally controlled rotary table or other shop apparatus. Maybe that is use me.
In any event thanks for the great article.
Dave
On Sep 5, 2011, at 7:01 PM000, RG Sparber wrote:
> This article presents a simple variation on a sine bar that uses my digital
> caliper to set the angle. Making this instrument is simple. Dealing with the
> equation needed to set it to the right angle may be more than some can
> tolerate even though the heavy lifting is all done in an Excel spreadsheet.
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> If you are interested, please see
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> http://rick.sparber.org/dag.pdf
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> Questions and comments are always welcome. All of us are smarter than any
> one of us.
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> Rick
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> [Non-text portions of this message have been removed]
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