Centroid Problems

[Engine Tech]

I have a SAJO knee mill with Centroid M400 controls. The servos are SEM DC, brushed units. I had a problem with the encoder on the y-axis. I ordered another and installed it. After the installation, the machine will not home. It starts attempting to home on the z-axis, which now says full power without motion, in other words it has lost position indication on that axis. It will not move the x, y, or z axis with the jog commands. Any ideas?

[cncsnw]

An encoder problem on one axis may prevent homing, but it will not prevent you from jogging the other axes. There is something else going on.

The status message window in the upper right corner of the screen is your friend.

Choose Slow, Incremental jog mode and press variously the X, Y and Z jog buttons. What messages appear in the status window? Do any of the axes move in either direction? Switch to Slow, Continuous jog mode and try each direction again. What messages appear in the status window this time?

Before you try to home, make sure all three axes are well clear of their limit switches and hard stops. Power off and turn the pulleys by hand if you need to.

When you try to home, and it says "411 Z axis(3) full power without motion", does the Z axis actually move (e.g. run away rapidly)? Or does it sit still? Or does it move under control at the usual slow jog speed until it reaches the switch or the end of travel, then give you the stall message?

Assuming that none of the axes move at all when you try to jog them or home them, then you probably have a servo drive power problem. Perhaps you are not getting DC power to the VM terminals of your servo drive. Perhaps the limit switch cable between the RTK2 and the servo drive is loose (assuming this is a pre-2002 M400, not a newer M400S model).

Is your control a pre-2002 model with a SERVO1 drive; a 2002-2004 M400S with a SERVO3IO drive; or a post-2004 M400S with a DC3IO drive? See Centroid Servo Drives and Field Guide to Centroid Controls, 1994-2010 - CNC Services Northwest for pictures if necessary.

What software version do you have? The software version number is displayed on the F1/Setup screen.

[Engine Tech]

Here is what I have:

SERVO1 3-3T on the servo drive
Software Version CNC10 Mill V1.51

On the servo drive, the three green and three red leds labeled USV, UV, DSF, Z, Y, and X are on, not sure what that means. I can hear the servos begin to move (clicking) and sometimes they do move very slightly before I get the "axis full power without motion" error.

I see no loose cables anywhere. Is there a way to check for DC power to the servo board, or does the fact that the leds are lit mean it is getting power?

Update: I checked the voltage on the VM terminal. It appears to be around 3.7 volts, although it starts higher and then settles down to 3.7 volts.

[cncsnw]

It sounds like you had your meter on AC volts.

Voltage between the VM terminal and the adjacent "GND" terminal (actually the minus side of Vm, not grounded at all) should be around 110VDC. You need the meter on DC volts and a suitable range to check that.

Having the green UV and USV LEDs lit is a strong indication that there is adequate voltage at the VM terminals.

Locate the gray limit switch cable which runs from your RTK2 PLC unit to the limit switch header of the SERVO1 drive. Unplug it from the SERVO1 drive. Locate the eight DIP switches on the SERVO1 drive, just below where the limit switch cable was plugged in. Make note of their current positions (very likely they should all be toggled down towards the board). Switch them so they are all up away from the board. This will defeat the limit switch inputs to the servo drive, telling it that all switches are okay. Power up and see if you can jog all axes in both directions.

If you can jog okay with the limit switches defeated, then you have a problem with the gray cable connection between the RTK2 PLC and the servo drive. Inspect for damage, and try cleaning the connections at both ends with electronic contact cleaner. Plug the cable back in and return the DIP switches to their original positions.

If you have adequate power at the VM terminals; have no drive fault indications; and cannot jog even with the limits defeated, then you probably have a servo drive failure of some kind and will need to have the board repaired or swapped.

For a guide to the cable connections, see: http://www.cncsnw.com/ServoLogicPM.htm.

[Engine Tech]

No, I tried two different meters and both did the same. They were on VDC.

Either way, today when I started up the unit prior to trying your suggestions, the machine homed on the x and z axis, which is what it would not do since changing the y axis encoder. It still moves as if it is getting no position feedback. The y axis begins motion at full speed and then stops, with the same error as before. Maybe I did not install the new encoder correctly. I just removed the old encoder and unplugged the cable. Bolted up the new encoder and plugged in the cable. Maybe there is more to it than that. Do you have a procedure for encoder replacement?

[cncsnw]

Go to the PID configuration screen: F1/Setup, F3/Config, "137", F4/PID.

In the lower center of the screen there is a table of live status information about the axes. Look for a column labeled "Abs Pos". This is the position of each axis, in encoder counts.

Press Emergency Stop in and turn your motor by hand. You should see Y counting up and down by 8000 counts per turn of the motor. If it is not counting at all, or counting inconsistently, then you have a problem with your encoder, cable, or connections.

If it counts consistently 8000 counts per turn, but when you try to move the axis under power you get a brief jump opposite the commanded direction and a "410 Y axis(2) position error" message, then you have the encoder polarity reversed. Swap the channel A wires with the channel B wires, or just exchange one pair (e.g. swap A with /A).

[Engine Tech]

Thanks for the help. It appears that I do have a consistent 8000 counts per revolution on the y axis and I'm getting the "410 Y axis(2) position error" , as you suspected.

Now, how to fix it. Tell me more on how to reverse the polarity. My encoder connection is a flat, thin connector that just plugs into the other half from the servo motor. I'm not sure what you mean by swap A with /A or do I have to disassemble the connector?

[cncsnw]

It is fairly easy to swap the encoder polarity if you have the right tools and supplies. It is possible to do it without the right tools and supplies if you are intimately familiar with crimp-on inserted-pin type connectors.

I would guess that your new encoder is a Dynapar F18, like that pictured here: Encoders Used on Centroid Controls. If you pull the BLU and BLU/BLK wires out of positions 1 and 2 of the C-Grid connector, exchange them, and put them back, then the encoder will count the opposite direction.

The crimp pins have a tab on one side that clicks into a cutout in the black plastic housing when the pin is fully inserted. If you press the tab down (e.g. with the tip of a jeweler's screwdriver or a bent paperclip) then you can pull the pin out.

If you do not then replace the pin with a new one (see "right tools and supplies" above), then you need to carefully bend the tab back into shape so that it latches properly when you reinsert it. Otherwise the pin may push back when you try to plug it into the mating connector.


An alternative solution is to reverse the polarity of the motor power (plus and minus armature leads) coming out of terminals 3 and 4 at the bottom of the servo drive.

You can do that as a quick test: if you leave the encoder as it is and reverse the polarity of the power, you should find that the axis moves under control without errors (but moves in the wrong direction). If you still have problems that way, then reversing the encoder won't help either.

If you decide to swap the power polarity as the permanent fix, then you also need to swap the plus and minus limit switch wiring connections; swap the limit switch and home switch numbers for that axis on the Machine Configuration -> Motor Parameters menu; and toggle "Direction Reversal" for that axis on the Machine Configuration -> Motor Parameters menu.

[Engine Tech]

OK, I swapped the polarity on the encoder. Yeah! The y-axis is alive!

Problem is that the y-axis moves in the wrong direction and never finds the home switch. My guess is that the encoder polarity needs to be put back the way it was and reverse polarity to the servo motor by swapping terminals 3 and 4 as suggested earlier.

[Engine Tech]

I switched the encoder polarity back as it originally was and swapped the leads on terminals 3 and 4 of the servo drive. The y-axis now moves as it should and the machine did home, although it took a few attempts as it kept tripping the limit switch 2-3 times, but I would jog back and it eventually homed. Now I think I have a similar issue with the x-axis, in that it moves opposite what I think it should. X+ should be moving the table to the left, but the machine calls that X-. Is the procedure the same? Will or could there be limit switch issues?

[cncsnw]

When was the last time you saw this machine working correctly (including correct directions) on all axes? Is this machine new to you? Has it seen some undocumented modifications or repairs?

Motors and limit switches generally don't rewire themselves spontaneously. Software configurations (e.g. the Direction Reversal setting on the Machine Configuration screen) don't generally change themselves spontaneously either.

It sounds like you are starting from a completely unknown condition. Do you have a local Centroid dealer and service technician who can help you through the process of checking axis directions, limit switch wiring, limit switch number assignments, and anything else that might have gotten changed at the same time? If so, you should hire him to come in and check it over thoroughly.

[Engine Tech]

I purchased the machine from a guy in New Jersey. It worked fine when I got it, but within a couple of days, the encoder in the y-axis went out. I thought it was a bad servo motor and sent it out to a repair shop. They said the motor was fine and it needed an encoder, which would cost $1000. I got to checking around and found I could purchase and encoder for around $150, so I did and installed it myself. When I talked to centroid, they closest guy around was in Alabama. I'm in SE Georgia and didn't want to pay some guy travel time both ways to install an encoder. The machine has sat for some time waiting for other projects to get caught up on. Maybe Centroid has someone closer than 8-10 hours away now. Anyway, does it really matter if the x-axis works in the opposite direction as long as I know that it does?

Any thoughts on why the y-axis goes to the end of travel limit switch several times before finding home? Perhaps the home limit and the end of travel limit are too close to each other?