For professional coater, the tension is isolated into 3 zones: unwinding zone, internal zone, rewinding zone. Tension=torque/roll radius. During coating, the diameters of unwinder and rewinder are constantly change; therefore, to keep tension constant, torque should change according to roll diameter change.
In principle, tension in internal zone or coating zone should be stable since there is no roll diameter change. However, there are tools which have no tension isolation, e.g. no isolation between internal zone and unwinding zone. The tension fluctuation in unwinding zone can affect coating zone.
The following are notes from CAC‘s presentation in Youtube.
Determination of proper tension value
Three possible ways to determine tension:
- Experience: For coating on plastic film, a typical tension is about 100N/m while 150-200N/m for metal foil.
- Rule of thumb: tension should be about 10% to 20% of the carrier’s tensile strength.
- Trial and error based on the process
Tension control methods
Manual
-No control
Potentiometer adjust a drive or electric brake or clutch to adjust torque according to roll diameter change
Pressure regulator adjusts an air brake or clutch
Radius measurement
-open loop control
Roll radius can be measure by
- Follower arm
- Ultrasonic
- Radius calculator
Tension measurement-closed loop control
There are two type of tension control through tension measurement methods: Dancer and load cell.
Dancer
- Pivot arm
- Linear
- Rotational
All the three dancer methods have the same common principle as the following:
- Loaded in one direction
- Web tends to move them in opposite direction
- Sensor detects dancer position
- If tension fluctuates, dancer will slight move and sensor detects the position change-> feedback to control –> Control tells drive to add or substrate materials from the dancer to keep position constant–> tension in downstream (after dancer) remains constant
Load cell
Combination
Combination of open and close loop tension control is necessary because:
- Open is very stable but not accurate (no direct measurement of tension)
- Close loop: unstable; don’t measure distance; don’t know roll build ; but it is very accurate in measuring tension and adjusting drives relative to set point
Comparison between different tension control methods
| Tension Control | Advantages | Disadvantages | |
| Manual | Inexpensive | Inaccurate | |
| Radius measurement (open loop) | Follower arm | Simple Good replacement of manual control Inexpensive Easy to install | Could oscillate from out-of-round rolls Contacting coating materials Don’t know true tension Only controls unwind and rewind |
| Ultrasonic | Very simple Excellent replacement of manual control Inexpensive Easy to install True linear output relative to roll radius No contact with coating materials | Open loop-don’t know true tension Only controls unwind and rewind Foreign objects could be sensed | |
| Radius calculator | Very simple Excellent replacement of manual control InexpensiveEasy to install True linear output relative to roll radius Nothing in the way od unwind and rewind No material contact | Open loop-don’t know true tension Only controls unwind and rewind Control logic is more complicated Calibration is needed | |
| Tension measurement (Close loop) | Dancer-pivot arm | Simplest design Least expensive Can be designed with small or large amount of web storage | Gravity needs to be considered Bulky Momentum and inertia |
| Dancer-linear dancer | Compact A lot of web storage Multiple pass-accumulator Small package | Most expensive Gravity Complex design Momentum and inertia Rodless cylinders are not low friction | |
| Dancer-rotational dancer | Not affected by gravity Momentum and inertia reduced Generous web storage Small package | Expensive Not easy to increase web storage | |
| Load cell | Accurate tension measurement I/O and PLC integration Simplifies calibration Easy to install at idler roll position | No mechanical web storage Web tension can be unstable in unwind and rewind zones | |
| Combination of open/close loop | Accuracy of close loop Stability of open loop I/O and PLC interfaces | Expensive complicated |