Many production processes utilise roll nip treatments. In paper industry processes like calendering, coating and printing are the most typical ones. However, nips are very intolerant systems from a viewpoint of vibration. Rolling contact is a complex process, where the system response depends on the fixed design parameters of the rolls, roll covers and loading mechanism as well as on the adjustable running parameters including the control gains. In order to drive a nip in controlled conditions, a pilot roll press (Figure 7, Kivinen 2001) has been down-scaled to half size from the ones used in industry exhibiting unstable response behavior by means of delay-resonance vibration.
Early start-ups showed, that similar resonance phenomenon was successfully incorporated to the laboratory unit providing an excellent environment for the experimental analysis as well as for the control of the resonance states.
Figure 7. The pilot roll press at TUT machine dynamics laboratory.
Roll drives can be roughly divided to geared and direct drive systems, where the electric motor is connected to the roll with a coupling or by integrating it to the roll end. Control demands have an influence on the selection of drive type. Control performance can be measured for example by control accuracy of static speed regulation. The most effective, so called tight drives can hold speed to within 0.01 % from the line speed
value (Roisum 1998). This is important because web tensions must be held within given tolerances to avoid web loosen or break.
Two types of drives were installed consecutively into the test nip system. The first drive type was a traditional AC motor drive with vector control including gear transmission and the second drive was a new type permanent magnet AC direct drive motor. First one is a general solution in paper machinery, latter one is a more sophisticated and modern drive, whose large operating range eliminates the need of gearboxes. Also, because of the reduced mass of the permanent magnet motors, they can be installed directly to roll ends instead of using separate bed for motors and gearboxes. The geared and direct drive assemblies of the pilot roll press are shown in Figure 8. The advantages of the direct drive solutions is to decrease the sensitivity of the roll system to torsional oscillations, to reduce the number of components in the power train and save floor space when customized beds are not needed for a complete roll drive unit as found out from TEKES project Smartroll - Dynamics and control of direct drive rolls.
a b
Figure 8. Geared (a) and gearless (b) drive units of the pilot roll press used in this investigation.
In direct drives the control strategy is called Direct Torque Control (DTC) (ABB 2010).
With DTC technology the orientation of the magnetic field is achieved without encoder feedback from the rotational frequency of the motor. The DTC control algorithm calculates the motor state e.g. torque and magnetic flux by 40 kHz frequency (ABB
2010). The controlling variables are motor magnetizing flux and motor torque. With DTC there is no requirement for a tachometer or position encoder to feed back the speed or position of the motor shaft. Because torque and flux are motor parameters that are being directly controlled, there is no need for a modulator, as used in conventional drives, to control the frequency and voltage. This, in effect, speeds up the response of the drive to changes in required torque. DTC uses digital signal processing and advanced mathematical description of motor functions. The result is a drive with a torque response that is typically 10 times faster than with conventional AC or permanent magnet AC drives. The resonance vibration avoidance methods are tested with the direct drive design.
The power input of motors is consumed by elastic deformations in soft roll cover, roll accelerations, thermal losses caused by dissipation in soft roll cover, bearing frictions, damping of loading actuator and frictions of other interfaces. Experiences from industrial units and from laboratory size sheet calenders show that web corrugation can be avoided by eliminating the tangential traction between the rolls. This is: the rolls are not allowed to be driven over the nip friction. This situation can be reached by driving the rolls with almost similar torques. Hard roll rotational frequency is usually controlled with speed control, so hard roll is the master and the soft roll is controlled with torque control, so soft roll is the slave. This principle helps to synchronize the surface speed of the rolls with the line speed, because the effective radius for hard roll is known.
Paper web entering the nip needs certain amount of time for proper processing between the rolls. Because machines tend to reach ever higher production speeds, the nip manipulation time might get too short with solid cast iron rolls. To increase the nip contact time, soft rolls are nowadays widely used. The soft roll cover distributes pressure in the nip more uniformly and produced paper’s density is more constant (Jokio 1999).
In film size presses or in calendering units, the nip load is one of the main control parameters. Nip load is conventionally produced by hydraulic actuators at both ends of the roll and is usually expressed as nip force between the rolls per unit width of the roll or width of the web between the rolls. A typical nip loading system is schematically presented in Figure 9. Cylinder pressure creates force to loading arm counteracting to
the weight of the lower roll and the arm. Nip force can be calculated from geometry of the design. A fixed bottom roll would be easier to design, but loaded bottom roll design provides more safety because loading and unloading the nip is quicker and easier just by closing or opening the nip by moving the lower roll with the hydraulic cylinder.
Figure 9.Schematic example of a typical nip loading system.
The nip is very sensitive to disturbances, and main problems are controlling the load as well as keeping the run free of vibrations (Lehtinen 2000). The nip load in machine cross direction (CD) should be an even line load to confirm uniform product quality.
There are typically two ways to ensure even CD line load. The methods are crowning of rolls and zone-controlled rolls. In crowning, the roll is ground into a barrel-shaped form to compensate the deflection of the tubular roll structure mainly arising from beam bending and shell flattening effects. Crowning is performed for a certain nominal nip load. If nip load is varied, also the nip CD load profile varies and loses its uniform shape. In zone-controlled rolls this compensation has been done by using a set of hydraulic shoes mounted on equidistant locations at roll span and pressing the counter roll to produce a constant load over the whole nip line.