PRS Alpha JERK/BANG

[jim_vv]

Greetings,


A few days ago I was running an area clear offset strategy toolpath with a .375” round nosed end mill at a 12% SO at 6 IPS in an oval approximately 22” x 15". Approximately a third of the way through the part file the machine made a sudden JERK/BANG and then continued on its way. There was only a small mark in my part, but the sudden noise certainly got my attention! I have run this same part file many times and have never noticed this. I have occasionally had the same experience when surfacing my table when using an area clear offset strategy with a 2.5” surfacing cutter at 8-10 IPS. If I use an area clear raster strategy, I do not have any problems. Today I had some time so I air cut the oval area clear part file (at 6 IPS) without my dust collector running thinking that maybe my dust collector was causing interference and I got the same JERK/BANG about a third of way through the part file. I ran the part file again at 5 IPS and hadn’t any problems.


Suspecting a communications issue, I ran the SB Speed Test Application which reports that I am getting 85.7% transmission efficiency. My control computer is running Windows XP Pro at Black Viper’s bare-bones settings on a 3 GHz Pentium processor with 1GB RAM. No other programs are running, no network connections, no antivirus, and use a powered USB hub for the ShopBot only. My machine is an ‘07 96x48 PRS Alpha with 2.2 Spindle.


Does anyone have any ideas as to what may be causing the JERK/BANG?


Kind regards,


JIM

[abentley]

Jim
I had the same problem when I first set up the shopbot. Mine was a bad limit switch and electric interference. Try turning off your limit switches and see if that helps.

[dana_swift]

That is very likely a bad connection or a bad connector to the stepper motor, check the joint where the heat shrink tubing covers the two connectors. If it is not tight, that is a red flag for that kind of problem. If the heat shrink is on tight, get some spare, cut it open and check all the wires. Flex motion may have broken a wire at the connector (the most likely place for it to occur.) Give each wire a gentle tug, it should feel solid.

Replace the heatshrink when the inspection is done.

With the alphas, if the feedback wires get disconnected the machine can move unexpectedly at a very high rate of speed. Always be mindful of that possibility when working around moving equipment.

Hope that helps-

D

[jim_vv]

Greetings,

Thank you for your responses.

Alan, I didn't have the limit switches on when it happened. After your response I went out and air cut the file with the limit switches on and off and had the same problem in both cases.

Dana, Thank you. I have a Cabinet maker friend three hours south of me. His Alpha does the same thing occasionally. I have inspected all of the motor connections although I did not remove the heat shrink. I ran an offset area clear of a 2' x 4' rectangle at 8IPS today with no issues. If I run an offset area clear of an oval above 5IPS I get the JERK/BANG. It happens in different areas of the file so I think the file is O.K.

I will do some more testing on Monday.

Kind regards,

JIM

[dana_swift]

Jim, have the shopbot run a file in the air while you wiggle the wires, you will find out real quick which one is the trouble.

If the wires are making electrical contact, then it should run perfectly.

D

[garyc]

Jim...
A couple things to test:
1) Check for loose USB cable
2) At what time into the file does this happen?
3) Double check with Wdws TskMgr for unknown background operations
4) Do a test with dust collector hose(s) and skirt detached from machine

[richards]

I had the same problem when I had a wireless card attached to my control computer. Windows stopped processing the SB file when it serviced the interrupts for the wireless card. Even though you wrote that you had no network connections on that computer, other services might be generating interrupts that cause the SB software to take second place.

I agree with Gary. Check the task manager's list of processes. Something might have a higher priority than the SB software.

[jim_vv]

Greetings,

Gary, thank you for the checklist. Mike, thank you for your input, but I have no wireless network adapter or connections set up. I spent most of the day yesterday testing different things to see if I could narrow things down.

1) I checked all cables and then wiggled them while the file was running. . . All connections were secure and nothing strange happened while the machine was moving.
2) The first BANG/JERK varies, but usually occurred after five to six minutes into the file.
3) The WdwsTskMgr showed no other background operations. With the WdwsTskMgr open while the file is running I get a lot of JERK/BANGS.
4) I completely removed the dust collector hose and skirt from the machine and ran the part file without the spindle or anything else running in the shop.

The only way to prevent the JERK/BANG was to run the file at 5IPS instead of 6IPS.

Finally, I moved the powered Hub's USB connector to all of the ports on the back of the computer and ran a speed test each time. I chose the fastest one which provided 86.1% Transmission Efficiency. After doing this, I ran the part file a couple of times without any JERK/BANGS.

It is pretty obvious that the problem is my computer. After reading the post regarding USB hubs (http://www.talkshopbot.com/forum/showthread.php?t=15547&highlight=hubs) I realized that my computer setup may be light. When I purchased the ShopBot and control computer new in July ’07 the computer was well over ShopBot's recommendations at the time and all has worked well. I think that I am now noticing communication issues because I am now running faster feed rates than I had in the past because of my recently improved vacuum hold down system (http://www.talkshopbot.com/forum/showthread.php?t=14802).

It appears that some computer upgrades are in order if I want to take advantage of my Alpha’s capabilities.
My current control computer is built as follows:
Dell Dimension E520 (http://support.dell.com/support/edocs/systems/dime520/en/SM_EN/specs.htm#wp1052310)
Intel Pentium D 3GHz Processor
1 Gb RAM
8 USB 2.0 ports. Two on front, six on back, but not a separate PCI to USB adapter
Shared video (not on separate adapter)

I can add memory, add a PCI to USB adapter, and a video card rather inexpensively. I did find a Core 2 Duo Processor that will fit my mother board as well. Is it worth spending another $130 to upgrade the processor?

Any other suggestions?

To all, thank you for your help.

Kind regards,

JIM

[shilala]

Jim, the speed and amount of ram in your control computer is your bottleneck.
Going to a dual core processor isn't going to help, but adding ram should, and it's cheap.

[dana_swift]

So far the source has not been identified.. can you video the jerk/bang and post a video in a private area on youtube and post a link here or in a private message so we can see what is happening? That may assist get the correct solution.

D

[jim_vv]

Greetings, Dana

I am pretty sure that my control computer is the problem. I have ordered a USB card, a video card and some more memory to shore up my computer. I will report back in a few days. If I still have issues after the computer upgrade I will figure out how to put up a video.

Kind regards,

JIM

[richards]

Jim,

The Windows Task Manager actually told you that the problem was interrupts running in the background. You noted that the problem significantly increased when Task Manager was running. Task Manager takes priority over other tasks, meaning that the SB3 program had to wait until Task Manager serviced itself.

The jerk/bang is caused because a motor does not get a "step pulse" on time. The pulse is delayed so that when it comes, it is "out of step".

Be sure to disable any hardware in your BIOS that you don't absolutely have to use on that computer. (Serial, parallel, sound, etc.) Even if your programs do not use those devices, interrupts may be running. By turning them off in the BIOS, they can't generate interrupts.

Any process, however small, that requires use of a timer or an interrupt can cause grief when you're trying to send out a steady stream of pulses. If you attach an oscilloscope to the step pulse train, you can see how the speed varies even when no other major processes are running. Windows OS is not a real-time operating system, meaning that strict timing parameters cannot be guaranteed. In most cases Windows works just fine, but when any program corrupts the timing, things stop working properly.

I've had great success with process control computers that use an Intel DH77EB motherboard, an i5 Intel processor and 4GB of RAM (not all of the RAM is recognized with Windows XP) Those components usually cost just under $400. You would have to add a case, a power supply and a hard disk to complete the unit. (Those same components with 8GB of RAM easily run ten virtual servers (Web servers, FTP servers, mail servers, MySQL servers) on a Linux CentOS system running KVM (virtualization software).

[jim_vv]

Greetings,


I have installed a new USB card, 4Gb of memory, and a video card in my control computer and I can now run my oval offset area clear file at 6IPS without any JERK/BANGS.

A few notes:

- I ran several speed tests. My transmission efficiency actually went down from 86.1% to 84.5% after installing the new USB card.

- Although I would never run this fast, I can run my oval offset area clear part file as fast as 8IPSwithout any issues. I ran it at 9IPS with some JERK/BANGS and at 10IPS with a lot of JERK/BANGS.

- I see all 4Gb of memory in the Bios, but XP only shows 3Gb of memory.

Thanks to all for your help.

Kind regards,

JIM