1 General Information Film tension Calculation[edit | edit source]
Let's have a general look at film tension over a roll:
If we hang a weight of 100kg (=981N) according to the film run, the loadcells of the roll see twice the force, because both ropes carry a load of 100kg. The Siwarex will deliver us a value corresponding to 200kg. The film tension - by definition - is the force in one rope. We want to display the actual force of 981N (100kg) in WinCC.
Therefore the SEN_ACT object must be parametrized with
FacMul = 9.81
FacDiv of 2.0.
These values are valid in above picture with 180° wrap angle and with a ideal non-existent friction.
Now we usually do not have 180° wrap angles.
2 Specific Film Angles and Data Sources[edit | edit source]
2.1 PET[edit | edit source]
In documents 654523 (PET MDO Inlet) and 654505 (PET MDO Outlet) we have wrap angles of e.g. 96°at the inlet and 154° at the outlet.
What does that mean for the measured force?
The load cell does not see the full force of the 100kg, but only a part.
Therefore the WinCC parameters must be adjusted slightly according to the real wrap angle.
Let's make sure we understand the drawing MDO Outlet DokNr: 654505:
If the angle "b" is given with 13° this translates into a wrap angle as we are used to of 2*(180°-90°-13°)=154°.
If the angle "b" was "zero" we would have a full 180° wrap angle.
So according to the drawing we need to mulitply the measured force with 0.51 to get the correct resulting film tension.
Because we have decided to use FacDiv for the geometry, we must set 1/0.51. >> FacDiv of 1.96 (this is slightly less than 2.0 which we would have with 180°).
In document 654523 Mdo INLET we have given angles of 44° = 92° Wrapangle, 42° = 96° Wrapangle, 39° = 102° Wrapangle.
So the load cell see less force than at the outlet. Therefore the FacDiv must be set according to the drawing to approx. 1/0.7=1.42, 1/0.67=1.49 or 1/0.64=1.56.
The correct values for all wrap angles will be written in the drive schemes from now on.
EESA uses those angle values and calculates the effective factors in the s7 software.
There should be no need to adjust those values anymore because the geometry is known and considered.
Also - when hanging a weight with a rope, it makes no sense to modify the FacMul and FacDiv slightly to exactly display the hangweight.
The error that we are making - is the rope in the middle, how about the rope friction, is the granulate sack really 25.0kg.... are much higher.
The hangweight should nevertheless tell us if we are in the proper range for measurement and not off by factor 2.
2.2 PP[edit | edit source]
In documents 685976(Mdrive scheme MDO BOPP vmax=600m/min) we have:
File:MdoFlmTenPPDrivescheme.png
| location | incoming angle | outgoing angle | wrap angle | facDIV |
|---|---|---|---|---|
| Inlet | 4.1° | 8° | 167.9° | 1.988 (=1/0.503) |
| Outlet | -12.4 | 10.3 | 182.1° | 1.960 (= 1/0.51) |
3 Used Types of Load Cells[edit | edit source]
3.1 Adjustable Load Cell (ring design) ATB-PBK300:[edit | edit source]
Used for PET MDOs
Load cell mechanical alignment The mechanical alignment is done based on the measuring direction. The measuring direction is depending on the film wrap angle. Check the MDO mechanical drawings, because wrap angle is different between inlet and outlet. The alignment of the load cells will be done by the mechanical colleagues. Basically the load cell is turned in a way so that we get the maximum value.
On the load cell you will find one red orientation mark which will help you to make the proper alignment.
To be able to move the load cell for alignment, just release the screws from the back side of the frame.
Mechanically adjusting the loadcells makes life easier for us.
They are mechanically turned in a way that the primary measuring direction of the load cell matches with the resulting film force.
Nominal Data are:
Load = 10000N Characteristic = 1.8mV/V
Parametrize the SIWAREX to 20000N (two Loadcells on one roll) and in Units NEWTON - no caluclation of kg > N anymore.
WinCC Parameters:
File:Siwarex WP321 WinCC Spezific MDO.jpg
Negative influence from threading chain bearing rings
In this design it has been found that the tension of the threading chain directly influences the load cell's measurement.
That means that after zeroing the LoadCells you will see an Offset when you start and stop the threading chain. This offset can be several 100N.
If you pull the chain a little by hand it will significantly change the value again.
Having discovered this design flaw, there will be a modification to suppress this influence.
Note that this is not something we can improve electrically. It does not make sense to zero-set the load cell many times.
3.2 Straight bar load cells (HBM Z7A)[edit | edit source]
One load cell per roll.
Typical data: Load = 500kg Characteristic = 2mV/V
WinCC Parameters:
