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Old June 13th, 2007, 12:25 PM   #1
United States

kpizzolatto is offline
Join Date: Jun 2007
Location: NEO
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Position Feedback with Mitsubishi Servos

Does anybody have experience with Mitsubishi servos and time to answer a question?

Most parameters, except target positions, are set by GX-Configurator QP.

All the equipment in question is from Mitsubishi.
I've got a QD75M4 motion control module and four MR-J2S servo amplifiers. They are controlling four HC-KFS43K motors.
The PLC involved is a Q12H, not a dedicated motion control PLC.

I'm using simple single-step (end) or two step (continue, then end) motions to send servos to PLC-specified positions.
I'm using a couple of bits to generate a "move complete" status in my PLC:
X14 (positioning complete--the step is complete)
NOT XC (busy--the servo is executing a move)
And I'm monitoring position feedback using Md.20.

The problem is that in certain moves when the servo is mechanically prevented from achieving its target position, I still see
And my position feedback (via Md.20) shows that I have reached my target position.

I have tried to find another method using this equipment to achieve the goal, but I'm having no luck.

I want to send the servo to a position, get feedback indicating "at position" when it's there, and monitor the actual position whether the servo reachse setpoint or not.
Has anybody successfully done this?
How might this be done with the equipment above (using the general-purpose Q12H PLC, not a dedicated mitsubishi motion control PLC)?

A more detailed version of the above question is restated below.

Thanks for your help!

The manual I reference below is QD75M Position Module Software Manual V2-D.pdf, identified as
Type QD75M Positioning Module
User's Manual

The servos drive ball screws with "sleds" mounted on the nut to carry workpieces
That's not really important as the encoders are attached to the motors, not the ball nut.

All axes are independant and all moves are simple "go-to-position" commands.
The moves all use CTRL Method 1: ABS line 1 (Axis #1 Linear control (ABS)) move, which controls speed to a set position.
Each move can consist of either
a 0: END (End command) segment
a 1: CONT (continue positioning control) segment followed by an 0: END (End command) segment.
The second type of move is for high speed approach followed by creep speed final positioning.

Two of the axes are pick-and-place, and reliably reach their commanded position every time.

The other two axes bring two workpieces together, and due to lack of precision in prior measurement sometimes cannot reach their final position.

For all axes, I create a "move complete" bit in the PLC using X14 (positioning complete) and NOT(XC) BUSY per p6.3 (Fig. 6.3 ON/OFF timing of each signal at start of positioning).

Also, for all axes, I read current feed value via Md.20

For axes which do not achieve target position, we expect to see either the "move complete" (generated in the PLC as above), or an error.
We also monitor the current feed value from Md.20 as above.
This is so that we can adjust the position of the next equipment in the process.

It seems that the QD75M4 reports move complete and current feed value Md.20 = target event though position has not been achieved.

To test this, the Engineer onsite installed an oversized workpiece so that the ball screw could not achieve final position, then executed the move to go to that position.

1. We get "move complete" (X14 TRUE and XC FALSE with no faults, per sec. 6.3 above) even though the sled doesn't make it to the specified position. The command in-position width is .00100"

We would expect the servo to fault on a following error if it never made position, but it just stops with no fault and maintains force against the workpiece. We monitor axis faults at Md.23 (Active Error Code).

2. The current feed value Md.20 shows that actual position = target position, even though we can see it has not occurred.

Previously, the Engineer scribed a line on the ballscrew carrier while the sled was at the target position.
During this test he observed the sled is around an inch short of the line, even though the current feed value Md.20 reported target position had been achieved.

I would expect the feedback to report the actual position, not the target position. If not at Md.20, then I would expect some other parameter to report the actual position.

To check this, the Engineer grabbed the sled and moved it back about 6" against the force of the motor. After releasing the sled, the motor returns it to the position against the workpiece where he grabbed it.
Current Feed value at Md.20 and Machine Feed Value Md.21 never changes.

If the Engineer drops 240V power, but not control power to the servos, (just opens the safety gate) current feed value from Md.20 abruptly changes without the sled observably moving. Reported value in Md.20 is roughly consistent with actual observed positon.
At that point, moving the sled produces the expected change in current feed value Md.20.

We have tried:

Monitoring Md.21 instead of Md.20 Both behave similarly, except that Md.20 reflects the offset applied during setup via OPR.
Changing Pr.21 (Current feed value during speed control) to 1 (The current feed value will be updated.) produces no change in the problem. Ref 5.34, Pr.21.

I've read multiple Mitsubishi motor, PLC, servo, and motion controller manuals, reviewed the parameters the software GX-Configurator-QP, looking for some parameter I have set wrong, or the correct parameter I should be monitoring. I'm finding nothing.

I want to send the servo to a position and get feedback to the PLC that tells me:
1. I've reached position within tolerance.
2. The actual position of the encoder at all times

This seems like a fairly straightforward application of a servo (a very basic one, actually), but I just don't see how to do it with these products.

Has anybody successfully done this?
How did you do it?

Thank you very much for your kind assistance!
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Old June 13th, 2007, 01:48 PM   #2
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kamenges is offline
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I haven't dealt with the Mitsi stuff. I took a quick cruise through the QD75M4 manual to try and get some clues. That is not a very friendly publication. However, from what I can see, MD.20 and MD.21 are not feedback values but instantaneous position command values. They are basically the position output of the position profiler. This is why the MD.20 value seems to jump to the right spot when you disable the move. The position profiler will want to start a move from the axis actual position so the axis doesn't take a jump when you initiate a move.

From what I could tell there is no actual position value that you can get at. You might want to look at the troubleshooting section in the manual. Toward the end of that mess they talked about determining when the axis is in position. That may make more sense to you than it does to me.

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