Synchronisation issue for Hoists of a Gantry crane

smartatuls

Member
Join Date
May 2013
Location
India
Posts
30
Hi,

We are trying Hoist synchronization for 4 hoists which has same motor, gearbox and drive.
In PLC(Schneider) there is a FB for synchronization in which our hoist2 is master and hoist 1,3, 4 are slaves.
It was not synchronising properly initially, later we found there is speed difference in them which causes to increase GAp between them.
To comensate such difference we have a Gear ratio variable where we try to reduce gear ratio for hoists which are running faster.
Since Hoist2 is master, Hoist1,3,4 are following him and the gear ratio we modify applies to Hoist1,3,4.
For Hoist3 and 4 it is working ok as per new gear ratio but not working for Hoist1.
We tried alot but Hoist1 comes down (15-25 mm) in one cycle then keep on increasing/accumulating this difference. Some times Hoist1 does not create such difference or creates less tahn 5 mm. Its behaviour is not consistent.
Can anyone please help?
 
Last edited:
Is this a new machine or has it been running and had this problem just now?

Are the encoders working correctly? What you describe on hoist 1 seems to be an encoder issue.
 
Is this a new machine or has it been running and had this problem just now?

Are the encoders working correctly? What you describe on hoist 1 seems to be an encoder issue.

This is new machine.
Encoders are working ok. I took traces of all 4 encoder counts they are linearly increasing from 0 to 65535. I dont see any problem there.
But not sure about such erratic behaviour of Hoist1.
I am not sure but think it could be mechanical coupling/gearbox issue.
 
One more info i forgot to add is, while hoisting down Hoist1 comes more down(15-25 mm) but when hoisting up it goes properly compared to other 3 hoists and this extra distance (15-25 mm) is not shown as encoder difference, I mean as per encoder pulses all pulses are almost equal. Also the Hoist1 sometime works ok but most of the time it travels more while hoisting down.
 
Please tell us more about your system's host controllers.

Is it a variable-frequency drive ? A servo controller ?

What kind of position and velocity feedback do you have from the motor ?

What sort of mechanism is the hoist; a cable-on-drum ? A screw jack ?

What model of Schneider PLC are you using ?

In general, crane controls should not be attempted by novice engineers because of the hazards involved in the hoisting of overhead loads.
 
Have you had hoist1 inspected? That sounds a lot like something is loose. Also, are those 15-25mm "seen" by the encoders or are you measuring it?

Have you looked at the hoist when moving to see if the discrepancy happens as soon as it starts lowering or if it gradually increases?

I do agree with Ken and you should not touch the software. In case of failure, even if unrelated having a modified piece of software on the crane can be an excuse for the company to get out of it.
 
Please tell us more about your system's host controllers.

Is it a variable-frequency drive ? A servo controller ?

What kind of position and velocity feedback do you have from the motor ?

What sort of mechanism is the hoist; a cable-on-drum ? A screw jack ?

What model of Schneider PLC are you using ?

In general, crane controls should not be attempted by novice engineers because of the hazards involved in the hoisting of overhead loads.

It is VFD (ATV71). I am reading encoder pulses and the Synchronization function block monitors if there is any difference in encoder pulses and corrects them by varying frequency.

There is gearbox after motor and connected to Hoist drum which has ropes. There are 2 spreaders Hoist1,2 has one sprader and Hoist3,4 has one. But all 4 should operate simultaneously.

Schneider PLC is ATV-IMC card which is a controller fixed inside ATV71 drive. Communication is on CANopen.
 

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