Fanuc 6t dc motors "jitter" then says servo alarm?

[blakemachine]

Ok, So I have a fanuc 6t on a miyano 6bc lathe. It has always worked fine till now. I can run parts for a couple of minutes then the Z axis starts jittering back and forth maybe 10 thou or so, but the position page doesnt change. Then it throws the 401 alarm. THe weird thing is that sometimes it does it in the Z and others in the X. I have tried turning the lathe off and on to see if that would correct it, it doesnt. I opened the panels to see if it had any lit alarms when it does this and I dont see any either. It will do this in all modes as well. I checked the pulse encoder to make sure they were not loose on the motors, and they are not. Weird thing is that it does it to both drives, whenever it wants. Any Ideas?

[tc429]

have you had the machine running a while? if its sat a long time or you just got it, check the lube system- stiction on old analog drives can cause this if the servo gain is too low and the slides are tight. when it does it, you can see the movement at dgn 800-801 (machine position doesnt display following error unless configured to do so).

if its a new problem and slides move freely, Id throw a meter on the power supply and watch for erratic 5v. the odd thing is its two axes- each has its own LSI chip...I dont recall if the little 455 op amps (8 pin DIP next to axis LSI chips) for driving the +/-10 analog out are single or double output on a 6- possible they are shared for 2 axes...radio shack has them cheap, they do fail on occasion. If I get a chance tonight I'll look in the drawings and see for sure if its a single or double chip. most of the ones Ive had go bad caused erratic slow movement, but were ok at higher rates. if the 455's are single axis output, again, look for a noisy 5v.

[blakemachine]

Here is a video that shows the two different axis, and the 800 diag. I havent ran the machine in about a month but before that I have had it for 5 years with no problems.

[blakemachine]

Fanuc 6t jittering - YouTube

Fanuc 6t jittering - YouTube

[tc429]

by the 4 digit dgn, I think thats a 6T-A series...possibly with 6045 or older scr drives? what model servo drives do you have, and was the machine relocated- the old scr drives were rotation sensitive... those old drives had brown plastic fuses near the contactor, and would kinda run with one missing, or single phase (I think if ran out of phase they blew a fuse or two- but its been a couple decades since Ive even seen one)...

if you have 6047 DC or 6050 AC drives (could have either- but rare to see on a 'A' control) the phasing dont matter.

[blakemachine]

They have the plastic brown fuses. Phasing has not changed. And the weird thing is that it would run for about 15 minutes before it would start jittering. But now it starts to jitter after only a second or two. Phasing has not changed in the last few years that I have had it running.

[Dan Fritz]

Well, let's try a little deductive reasoning here ...

1) The CNC control itself is probably not at fault because the servos are taking off on their own. The CNC is shutting down the servo when it detects that a servo is running away.

2) The likelyhood that you have TWO defective servo boards is very remote, so I would rule out a bad X and Z servo board

3) Since either (or both) servos misbehave, it must be something that is common to both axes ... but it's not in the CNC control. Hmmm

4) There are only two things I can think of that are common to both servos. One is the phasing or a missing phase, but you say that phasing of the AC has not changed. So ...
have a close look at the power supply unit on the X axis servo board. It's got a black aluminum heat sink, and it's output is shared by both servo boards via a ribbon cable. The Z axis gets power from the X axis power supply, so that is the only component that might cause either (or both) servos to go whacky. Pull the fuses on the back of both servo units ( 3 black plastic 15A indicator fuses for smaller motors, and 3 cartridge fuses with 3 1.3 A indicator fuses for larger motors) With all fuses removed, you should be able to power up the control and get a "ready", but the servos are dead. If you have an axis that can drift down, be sure to block it up with some wood, otherwise the axis will drift far enough to trigger the same servo alarm. Now check the voltages on both the servo boards. Use a voltmeter on the DC scale, and use check pin CH3 for a ground reference with on probe of your meter. Then check for +24v, +15v and -15v on both boards. There should be check pins marked for those voltages. Let the power supply warm up for a while, then see if one of those voltages becomes unstable or goes away. A bad regulator chip in the power supply on the X board is my guess.

[blakemachine]

I am not sure we are all talking about the same drives. I could be totally wrong as I am far from any expert. But I am not seeing the power supply for the drives, that is attached to the drives. The Z drive is on the Left, and the X is on the right I belive. Here are some photos:

[Dan Fritz]

In your photo IMAG0094.JPG, you are showing the backsides of the X and Z servos. If you remove 2 screws at the very top of that chassis and hinge it down so the servos are horizontal. You will then be able to see the servo boards that I was talking about.

Your Z servo is on the left of the photo because it has 3 cartridge fuses and 3 black "indicator" fuses. The X servo is on the right, and it has only 3 brown 15 A indicator fuses.

These are 3-phase full wave SCR type drives, which were very common on Fanuc's back then.

[blakemachine]

Ok, got it. Measured across CH3 and the pins and this is what I found Out:

On the +24v I get 21.89 across both boards.

On the +15v I get 14.85 across both of them.

On the -15v I get -15.21 on both of them.


So just for seeing what would happen with the machine, I left the drives hanging horizontaly and fired it up. Didnt get any jittering at all, ran about a 10 minute long program, ran fine no jittering.


But now what it does, which it also did in its jittering stages as well, when It wasnt jittering, was:

Anytime you do a G28 command or a G0 command, and it rapids, it will alarm out and say x servo or y servo, like the motors are drawing too much current from the drives, and it alarms out with a servo alarm. The axis themselves are not sticky as I can move the lead screws by hand with no power on with the machine. Any Ideas?

[blakemachine]

Also, when you go to zero return, and if you just bump the axis button, to move in that direction, it will not alarm out, but if you hold the zero return button for a second or more, and give it time to get up to speed, it will alarm out on either axis. For a quick fix, is there a parameter to slow down the rapid speed? ON this control there is no rapid overide percentage switch, although I wish it had one.

[Dan Fritz]

Please set your voltmeter on the AC scale with a range large enough to show about 200vac. Then, with the cnc power turned on, check the voltages on the back side of those servos on the terminal strip at the bottom. You will see wires on screw terminals labeled "A", "1", "2", "3", "4", "5" and "6"

Check the AC voltage between terminal "A" and terminal "1"
Check the voltage between terminal "A" and terminal "2"
Check the voltage between terminal "1" and terminal "2"

They should all be about the same voltage. Check the X servo and the Z servo.