Synchronisation issue for trolleys on a Overhead crane

Hi,

We have 3 Trolley crab on a over head crane. Each trolley has 2 motors connected to a Single drive(Drive is ATV71 Schneider make). The two motors of a crab are driving their respective wheels and are not mechanically coupled. On one motor we have Encoder which gives pulses to PLC(ATV IMC card) and using synchronization function block we are synchronising 2 trolleys and also 3 trolleys together.

The synchronization logic works well, I mean there is not much difference between the gap of adjacent trolleys if i check in PLC but physically there is difference between them which increases with every back and forth move. We are not able to control/reduce this distance.

First and third trolley motors are identical. Second trolley motor is little bigger as its trolley wheels are bigger than first and third trolley.

We tried changing parameters like Motor control type (Closed loop, open loop) Slip compensation but got no success .

Need help of your expertise.

Are these just wheels on rails or is there some kind of positive movement(ie. rack and pinion).

These are wheels on rails.

Wheels on rails slip. I did one job and found that I had to rehome on a sensor on one end of travel after every move. Didn't need to do it every move, but it was easiest.

The first thing you should do is get rid of the encoders that will do you no good. In fact they could add to your problems.
Let try to explain cranes have steel wheels rolling on steel rails steel on steel have very low friction so you have a large slip. That slip will be larger on acceleration and deceleration.
Also the steel wheel will wear down over time ( diameter get smaller circumference get shorter while the pulse per revolution remains the same the pulse per meter will change)
Another thing crane wheels have a flange to keep them on the rail but they do at time run up on the flange a little until they slip off an back on the face of the wheel this again will change the pulse per meter that you read.
The motor configuration you have 2 motor on a single common VFD is used quite often on cranes and works very well. Any difference in loading on the motor will be compensated for with the natural motor slip.
I did a 6 crane system years ago, each crane could be used independently , in side by side pares or all 6 cranes at the same time for a single lift. It that case all c cranes were on a 1,200 ft rail the bridges were 80’ wide. All the crane movement have to be together to work with a common load or bad thing happen. With no encoder no PLC and at all. It’s been running for over 10 years and has been copied by the same company of there crane sin other buildings.
What is reason for the PLC in this application ?
You have 3 cab operator cranes by cab operator I assume the each crane has an operator in the cab operating it if that is the case then you task is more difficult because you must ensure that all the commands are done at the same exact time. And delay of one operator will make that crane go out of position for the one next to it.
If you are trying to do positioning with the PLC then take the individual operator out and just use one to control all three cranes at the same time. I prefer to use radio controls on all crane controls. For a number of reasons.
If all you want is to have all the cranes move at the same speed then the key is to use a good VFD.
Set each up so that the motion on each crane is the same. In your case you have 2 cranes that are the same they will be easy to setup the 3rd will require a bit of tuning as the gearing is different. If as you said the wheels are larger than the speed of that VFD will be slower than the others .
Typically we use 3 preset speeds on cranes although I had set cranes with 6 or more speeds the same way without any problems. Also the ACC and DCC times must be set the same on all VFD’s .
The biggest problem you will have is to get all the operators to work together and give the commands at the exact same time.
If you require positioning then consider a laser measuring system for each crane Bridge and trolley.
This also would be a very complex program to bring it all together.
Another problem, you need to think about when using multiple cranes to do a common lift is what happens if one VFD in fails or faults out while running. This could cause the load to drop with possible injury to personal on the area of damage to the items you are lifting.
Some VFD manufactures market there VFD’s to be used o cranes most are just relabeling a standard VFD
http://www.magnetekmh.com/
Magneteck designs then for cranes with additions safety features built in
http://www.cattron.com/dnn/
Cattron has been making radio controls for cranes for many years and I can highly recommend them

Good Luck let us know how you make out

FWIW, I believe the Magneteks are just rebranded Yaskawa drives. This goes back to before Yaskawa Electric America, when Magnetek was the US Yaskawa importer. Not overly relevant, just some trivia.

Automax
Magentek are using Yaskawa hardware but they are defiantly not rebranded. the rewrite the firmware just for cranes some of the features they *** is a load check on the load break before a and after a lift, slack cable check and many others just to name a few.
in fact some of the major crane manufactures rebrand Magnetek with their own private label and sell them on new cranes as there own.
Anybody that works with cranes should check them out. they adding features all the time always safety first.

You will need an extra sensor to see the gap, as the encoder does not check position, only the movement of the wheel

Thanks all. We figured out the issue finally. We have used Slave synchronization FB in schneider PLC which controls the speed reference for all 3 CT drives(CT2 is master and CT1,3 are slave). It was not working properly because the gear ratio(ratio of Master speed to slave speed in MPM) was incorrect in Slave synchronization logic.
I mean theoretically calculated value of gear ratio was different and practically it was something different. We keep on modifying this value on trial and error basis and got success.