## Thursday, March 9th, 2017

Daily Archive

Daily Archive

Thursday 09 Mar 2017
##
So:

As Sigray has mentioned, why do we need processing power? Well, a couple reasons.

First, let’s look at the numbers we returned in yesterday’s exercise.

This is the raw data, carried out to the last available decimal place, and this is the plot of those line segments.

Looks pretty curvy, doesn’t it? It’s not. It’s nine line segments. And that is at the resolution of the actual calculations. Most machine tools don’t have the ability to deal with anything past three digits, so you get these values instead. (As always, click to embiggenate)

Those values plot the red line. At this resolution it almost appears to be one line.

But it isn’t. Truncating the numbers to three digits past the decimal introduces an error. And that error is a nightmare to someone who needs a critical dimension on a part.

As we can see from the images below, the very first section shows the difference between the path of the black (accurate) line, and the red (Rounded off) line. Yes, it is fractions of a thousandth of an inch, but it matters. Also, the next photo shows the same relationship just three arc segments in, and the red and black lines have changed positions. The part cut in this manner would fail any stringent quality control test. it would be like asking for a golf ball and getting an egg.

In order to not automatically generate these errors, even machines that only have S3 or S4 digits (The number of digits past the decimal) use a different system for display and cutting, called “Floating point”. This allows the machine to cut far more accurately than the actual resolution of the machine, but it costs processing power. And that’s not all.

Let’s go back to the etch a sketch for a moment. If you could very carefully turn both knobs at exactly the same speed, the line you drew would be 45 degrees. If you turned one handle 3 times for every time you turned the other handle four, you would have an angle of about 36.87 degrees. And that is another thing the control has to do. It doesn’t just have to figure out where it has to go, it has to use the servos to get it there. The ratio of the distance travelled in X to the distance travelled in Y, the servos have to move in that ratio, regardless of speed, and they have to also constantly monitor the location so they know when to stop.