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General Mach Discussion / Height Control for Router Table

« on: August 13, 2011, 09:14:12 AM »

I want to use an ultrasonic sensor for height control as my sheets are not always perfectly flat. Blowing compressed air will clear the chip so I get accurate readings from the sensor. The ultrasonic sensor is ideal for this application as it can be used on transparent material and doesn't contact/damage the material.

The sensor output is 0-10V. I can use an op-amp and adjust the gain to condition the signal for THC or similar device. Or I could use it to compare to a set-point to generate THC UP or THC DOWN, but that doesn't solve the problem of no PID loop. I need PID to smooth out the signal so it's not constantly moving up and down. Any recommendations that don't involve electronic design, or at least very simple with minimal development time?

Has anyone developed a PID that works well for this application? If not, can anyone recommend a THC for this application?

Any thoughts?

Someone must have done something like this already!

Thanks,
Chris

2

General Mach Discussion / Bug in formulas

« on: November 27, 2010, 04:03:27 PM »

I am having a problem with the formulas. Due to slight mechanical misalignment my router table is slightly out of square and with only a single drive on the y axis and a 48" gantry it also cuts longer along the y axis as x increases. Obviously the ultimate would be to correct mechanically, but short of changing to a slaved drives there will always be some practical error.

I have spent many hours on this problem and here is what I've found:

When you press "Cycle Start" it seems that the axis calculations are run upon the first move for all axis. This is normally not a problem UNLESS you have referenced the axis itself in the formula like f(y) = y - 0.001 * x * y. For my programs, Z is always moved to clearance height before moving X and Y to start location. When the Z starts moving and it does the formula calculation it IMMEDIATELY changes the DRO's to match the formulas.

For example, take a fictitious, exaggerated formula set like:
f(x) = x
f(y) = y - 0.001 * x * y
f(z) = z

Now, if my machine is at co-ordinate (X,Y,Z) (10,10,3) when I press "Cycle Start" then when the first movement code is executed (eg. G01 Z0.2500) the trajectory planner sends the Z axis to 0.25 at the feedrate specified like normal. However, at the same time, it also IMMEDIATELY sends the Y axis to 9.9 as per the formula, at infinite speed and acceleration. The result is an abrupt jerk and worst case scenario lost steps.

Why does this happen? Even if it was intended to apply the formula before the first move why isn't the correction move obeying the acceleration parameter? Also, I don't think it should even calculate the start position. If I have a part blank I want to add features to and I carefully set it all up, and then it does an initial correction move then the features will be offset. It should only calculate the formula for actial moves.

I may have a workaround using some careful scripting, or I could write a plugin, but I would prefer if it worked right in the first place. The correction in the end works well as it is (my parts are now square and y cuts equal lengths no matter where I cut the part on the table) but it is very hard on the stepper motors (they clunk pretty good).

Thanks for the software, Art. It is very versatile and works quite well.