VU (value units) problem

[davidallen]

I'm setting up my system after a move and I'm having a problem getting the units set right.

this is a PR system that was upgraded to the PRT control box and running version 3.1.1 control software. after connecting the wires and getting the motors running, I checked the accuracy by measuring the distance traveled and it was way off. I'd move it by keyboard 50" but it'd only register 39". (measured 50, displayed 39) the units value was 183.347. I changed it to 733.386 and it was way off the other way. by trial and error, I found the setting of 462 gave an accurate move across the Y axis (accurate to the width of the line on the tape). I put that value into both the x and y cells and measured the x-axis. an 'mx 50' command resulted in a move of about 53".

any suggestions?

[davidallen]

update:

Gary sent me a table of values (thanks). the 458.366 value seems to work OK for the x-axis but when I tell the y axis to move 60", I have to move it 0.06" to hit the mark.

is this typical?

[garyb]

David, not sure if this is typical as the numbers have been spot on for me, I would try tweaking by the 0.06 and see if that resolves your problem
gary.

[billp]

David,
Since you seem to be exactly 1000th off percentage wise, would an equivalent change in unit values get you there? Seems like we had to do some sort of figuring like this when setting up a machine on a trade show floor one time, and it took a little bit of math to get to the correct number...

[gerald_d]

I would trust the 458.366 more than the measured error of 0.06" over 60". The precision of machined rack gears (number of teeth per inch) is generally extremely good, and there is nothing else in a SB that can go out of tune to force a unit value different from the calculated one.

Is your 0.06" error constant whether you approach from either side? ie. Does it always overshoot or undershoot? (Do you have a loose gear?)

What is the error over 30"? If it is still 0.06" then you certainly don't have a scale value (unit value) problem.

Does your tape measure lie flat for measuring the 0.00 and the 60.00? Hooked into exactly the same place for both measurements? Is your eye parallel to the pointer every time?

I wouldn't jump to the automatic conclusion that fiddling the unit value is the way to go - there are plenty of other things that can cause inaccuracy, perceived or real.

[davidallen]

Gerald
to measure it I glued down a tape to the top of a rail (unistrut version) and used a small piece of the same tape as a marker. the marker is held to the carriage with a magnet and the edge is just above the glued tape (it barely rubs the tape in one location).

I moved the carriage until the marker was exactly over an inch mark (the edge of the marker just occluded the inch mark) then zeroed the SB.

I gave it the command to move 60" and noticed that the marker was off the inch mark. to get it to the inch mark, I had to move the carriage back 0.06".

giving the command to move to 0 put the marker back on the original mark with no variance (SB in move absolute mode). to me, that implies no slip in the system and that the number of steps is off.

I'm' still setting up after a move so maybe there's still some 'run in' that needs to be done and I guess that 60 thousandths over 60 inches really isn't that far off.

[davidallen]

final update:
after running several CR commands with pocketing, it now seems to be working OK.

the only difference between being off and not was running over a couple of hours.

now to get it square....

[gerald_d]

David, what happens if you give a command to first go to 61" and then to 60"? (approaching the mark from the other side)

[bleeth]

I just had an odd discovery today. Like David I recently moved and having finally gotten the final stage (regrounding) of the bot finished it was time to take her out for a little test run. After resetting home zeros I did a couple of test moves and found z moving around 1.25 inches instead of my requested .5!! Imagine my surprise when I discovered unit value for z had gone to 2000. After resetting it all was fine but the real question is has anyone else experienced this and has anyone come up with a way to prevent it from happening?
Other than this and David's previous post are there any other unexpected things I might need to look out for? I have a couple of very pricey cabinet end panels to carve quick and am trying to avoid uh ohs.

Dave

[davidallen]

my speed also seems to have been affected. it may be a 1/2, 1/4, 1/8 stepping issue.

before the move, I could move at 4 ips and set the jog speed at 6, now it has problems sometimes when moving at jog speed.

all of a sudden it stops moving like it ran into something. I stop, then move, and everything's OK. sort of like it's overclocking. it's happened when jogged from the keyboard and from the command line.