PRS Alpha Missing Steps

[chubb]

Hi everyone,

I've been running into a constant issue with my PRS Alpha table for the past couple days.

When doing a complex cut job i've noticed the machine will seem to 'nudge' work in either the X or Y axis someway through the job. Once it nudges the work (usually by about 5-10mm) it continues cutting with that offset the rest of the job. So one part is in the right place (initial cuts) then the rest of the work is offset.

It is unpredictable and happens at different times in the same cut job in either axes.

Does anyone have any idea why this would happen? or have any suggestions as to how to overcome this issue?

Currently my only solution is to just repeat the cut on new material hoping that it works.

I've attached some photos of work that shows correctly set work on the left and work thats been nudged on the right (you can see in the bottom right the pattern has cutover the border line when it shouldn't.

Any comments, thought or opinions are welcome and appreciated

Thanks,


Mark

P.S.
I know it's not the machine missing steps since PRS has a feedback loop with the motors.

[bleeth]

This is almost always a grounding issue, particularly when it happens intermittently on different axis' and not in a constant spot.
Unfortunately, that can cause a hiccup, even in an Alpha.
Check all of your ground connections, etc. They do need maintenance periodically, just like the wires on a car battery (although you likely wouldn't see the buildup).

[Brady Watson]

Mark,
Please post a screenshot of your VR settings from SB3.

Also, are you running ASM911AA or ASM98AA-SG7.2 motors on your Alpha?

-B

[chubb]

This is almost always a grounding issue, particularly when it happens intermittently on different axis' and not in a constant spot.
Unfortunately, that can cause a hiccup, even in an Alpha.
Check all of your ground connections, etc. They do need maintenance periodically, just like the wires on a car battery (although you likely wouldn't see the buildup).

Hi Dave,

Admittedly, I have neglected my grounding cables. Having knocked a couple of them loose. I had my doubts about it being static though because of just how frequently it would occur. I didn't think the machine could be so susceptible. Nonetheless, I shall listen to the wiser and fix up all my grounding wires and give it another go

Mark,
Please post a screenshot of your VR settings from SB3.

Also, are you running ASM911AA or ASM98AA-SG7.2 motors on your Alpha?

-B

Hi Brady,

I've attached a clipping of my VR Settings and also a snapshot of one of the motor caps. I don't know if it makes a difference to the numbers, but the machine is set in millimetres (mm). I have ASM98AC-T7.2 motors on the Alpha.

Mark

[Brady Watson]

Mark,
Your VR looks fine @ default settings. What speeds were you cutting your parts? (MS values)

Also, what version of SB3 are you running? How long have you been running this version?

-B

[chubb]

Mark,
Your VR looks fine @ default settings. What speeds were you cutting your parts? (MS values)

Also, what version of SB3 are you running? How long have you been running this version?

-B

Hi Brady,

I was being pretty aggressive with my cut speeds.

I set the feed rate at 10 ips (plunge - 1.5 ips) and was relying (hoping) that the ramping would control the fury.

too much?

I'm running the latest controller software. Updated around the week it came out.


Mark

[Brady Watson]

I set the feed rate at 10 ips (plunge - 1.5 ips) and was relying (hoping) that the ramping would control the fury.

too much?

Yes.

Ramping will take care of many situations, but you cannot defy the laws of physics. This is like trying to make your car go 0-100 to 0 in the grocery store parking lot. Just ugly.

You may find this article helpful.

-B

[bleeth]

Looks like MDF and fairly shallow cuts. Your speed is high for that but if it is a good sharp bit it may work. Backing off to 6 is a good thing to do as well as lowering your plunge to .5 to 1.
In the meantime-The machine can be very susceptible. Get and keep it grounded correctly and then let us know how it goes.

[chubb]

Hi all,

So, it was a long day today trying all sorts of things to rectify the problem. To absolutely no avail. I've had to completely stop work on the machine because it's costing more to rectify mistakes than to have it running.

Here's what I tried doing:

1) Completely ground the machine.

2) Changed feedrate to 5ips and plunge to 0.5ips.

3) Split the cut pattern into 4 separate cut files.

4) Switched out the computer running the machine.

5) Gave the machine a complete service and check over.

No change.......well no good change;

I started getting "Unexpected Output" Faults and a weird new error I've never seen before: 'Shopbot No Longer Being Recognized", that forced the entire program to shut down.

Also, it messed up one of the jobs so bad I just let it run through to see how bad it would get. Nudged the work 3 times.

The slightly weird thing is that when it completed 'cutting' the work and jogged back home, it didn't jog back to 0,0 as it's meant to. it jogged back instead to the total amount it had gone off course by and it showed in the axes readings (not just by physically looking at the position of the spindle on the table).

I've attached a screen clip of the axes window showing the readings when it finished cutting. They should obviously read 0,0,25 but don't. The only thing that's accurate is the Z reading (haven't had any errors on that axis).

Does this help in sussing out what the problem might be?

I'm stumped.



Mark

[richards]

I believe that Brady is very politely telling you to slow down. The "offset" is a classic indication that the closed loop system cannot keep up with the physical forces expected. There is no cross-channel communication between axes when a motor stalls. In other words, the Y and Z axes do not slow down when the X-axis is overloaded. That can lead to an "offset".

I frequently had similar problems when I first used my PRT-Alpha in 2006. I ruined a lot of material before I slowed things down about 25%. When I slowed things down, the machine stopped wasting material.

Depending on the motor driver settings, the motor may stall for up to about 1/2-second before an alarm is sent and the system shuts down. If you're running 10-ips, that means that an axis can be as much as 5-inches "off" before the machine stops. If the driver is able to recapture those steps and recovers without sending an alarm, the other axes may may moved significantly while the problem was occurring. Nothing in the system recaptures the proper settings for all axes.