problem with servo- kinetix 7000

I am bothering by a problem involving kinetix 7000 drive along with motor & controlled by 1756 mose3 module.

The problem is that the servo is configured to run in dual position servo mode & sometimes fall short of the commanded position.Although the readings suggest that the servo has perfectly achieved the commanded position , physically it has not.

The auxillary feed back device is working fine.The problem is abrupt & final physical position varies from time to time.

pls help.- what could be reason?

What is the position tolerance?

If the Position Error is very close to zero eg 0.0001 of a unit then the only thing that I can think of is that the Auxillary feedback encoder has slipped or "Jumped" its position

the position error tolerance is 1112.20.But system was working for over a year since its installation & commissioning.

note: its sheet cutting application which uses rolls(controlled by servo) to feed the sheet.

If it has been working for a year and it still works most of the time, then I would conclude that something is mechanically slipping - motor to gearbox, gearbox to rolls, product to rolls. Depending on the mechanical setup, a loose belt or chain can sometimes cause similar problems.

Could this be a motor encoder/ cable problem or slippage between the encoder & motor shaft (some clamping getting loosened or broken)?

A cable is possible, but I doubt it. A cable problem will tend to cause a fault - not wrong positioning.

If you are asking about a servo motor encoder, typically, the encoder is part of the servo motor and isn't likely to slip like an encoder that is strapped onto the back of a standard motor with a coupling that is held on with setscrews.

You mention an aux feedback device. I don't know how it is mounted, but I would certainly suspect it for slippage.

Its more perplexing because the variation is on positive & negative sides.It overshoots & undershoots.In case of a slippage it would overshoot!

rejoe.koshy said:

Its more perplexing because the variation is on positive & negative sides.It overshoots & undershoots.In case of a slippage it would overshoot!

Click to expand...

Not always true
How about this: Low acceleration to top speed (No slipping) then a High deceleration to stop (Slipping) compared with the opposite {High accel and low decel}

Have you checked the coupling of the auxiliary encoder? I had a machine that was giving over / under shoot - the broken coupling was engaging for forward but was disengaging during stopping and slipping during reverse.

i have checked it found it ok. Has the tuning of the servo axis been disturbed?

Which CPU are you using L62? L73
What is the version of RS logix 5000 you are using?

Post a trend with "Positionfeedback" and "AuxPositionFeedback" and ActualPosition during a move

To get the values to update change the realtime axis information as shown below and make sure the Motion group auto update is enabled




Please provide 4 tab images

when I issue a MSO command to the servo drive , the motor's rotor shaft gets locked. Does the encoder of the motor have any role to play in the locking mechanism?

pls find the attached images of tabs.

kekrahulik said:

If it has been working for a year and it still works most of the time, then I would conclude that something is mechanically slipping - motor to gearbox, gearbox to rolls, product to rolls. Depending on the mechanical setup, a loose belt or chain can sometimes cause similar problems.

Click to expand...

+1 - Look at the mechanics first - the electronics does not change unless the physical environment has changed.

MSO causes the Servo to maintain position - you will not be able to turn the motor as the servo is keeping the motor from turning to keep position

Servo tuning only changes if the physical system has changed - Eg More weight added or higher friction (bearing failing or alignment), other examples exist.



Get a trend - The trend is your friend it will show you what is going on

After seeing your screen shots, I might alter my previous statement that it wouldn't be an encoder cable issue. Now that I see your aux feedback is TTL (which for dual position servo mode acts as THE position feedback for the motor), a bad encoder signal could create these problems. I always use sin/cos with servos to provide an absolute position signal - I assume TTL is just pulses for the servo to count. If that is the case, then a bad connection or cable could create noise to throw things off.

You have Position Error set as Attribute1. Try setting up a trend of Position Error. I don't know how the program is setup, but typically I would expect the position error graph to be very similar for every index. With a servo, position error is normally very, very small. Look for any obvious sudden changes in the trend.

On the Limits tab, what is the position error tolerance and the position lock tolerance?