Regulating the lateral position of the web material in a roll to roll machine is critical for a wide variety of converting applications. Variations in lateral disturbances can result in substandard finished product, or in extreme cases may result in material breakage which leads to web guide downtime. The lateral variations are primarily caused due to roller misalignment, web splices, web thickness variations, roller thickness variations, poor quality input roll, etc. Even with a perfectly aligned web guiding system with well machined rollers, poor quality raw material (cambered web or webs with thickness variations or with wound roll edge variations) will result in lateral variations. Hence it is necessary to control these variations actively using a closed-loop feedback control system.
Active control of lateral position of the material in manufacturing machines is carried out using intermediate guide or a terminal guide. Intermediate web guiding systems, as the name suggests, are used in the middle of the roll to roll machine at a location where accurate lateral position control is desired. While terminal web guiding systems are used to regulate the position of the raw material fed into the machine or the finished product produced by the machine.
Several types of intermediate web guides are used to control the lateral position of the web materials. These include: displacement guide or offset-pivot guide, steering guide, center pivoted guide and end pivoted guide; the latter two guides are rarely used in contemporary machines. The most common type of a lateral guide is a displacement guide. Displacement guides are simpler to install and use. With proper installation the web guide can truly displace the web without introducing bending stresses in the web. The performance of a web guiding system is a function of several parameters such as the precision and accuracy of the edge sensor, the precision and accuracy of the guide mechanism, and the speed or the dynamics response of the guide mechanism. ARIS, Roll-2-Roll Technologies LLC’s web guiding system (Figure 1) is equipped with high performance stepper motor based linear actuator driven by high efficiency constant current chopper drive.
The specifications of ARIS are provide in Table 1.
Guiding Performance Quantification
Web guide manufacturers typically provide specification of the web guiding system without much data about performance quantification. This is mainly because the guiding performance is subjective and depends on various process parameters such as web speed, tension, traction, web sensor resolution and accuracy, dynamic response of the guide mechanism, etc. Moreover, most of the currently available web guide sensors provide an indirect/inferred measurement which is dependent on the physical properties of the material; quantification based on an inferred measurement is also subjective. However, ARIS is equipped with a patented material agnostic fiber optic web edge sensor which provides true, absolute measurement of the web position. Hence a true quantification of the guiding performance can be made easily without the need for calibrating each and every material used.
Experimental Roll to Roll Machine
In order to quantify the guiding performance we test the web guides in an in-house closed-loop roll to roll experimental platform (see Figure 2). The length of web material threaded in the machine is about 20 meters and the web can be transported at a speed of up to 10 meters/sec. The web tension in the machine is maintained using a pneumatic dancer system. Webs with 100 mm and up to 450 mm in width can be transported in the machine, which is equipped with two web guides. Two spans with a total length of 1200 mm separate the two web guides. For experimental purposes the upstream web guide is used to create disturbances while the downstream web guide is used to correct for the disturbances.
Two additional web edge sensors, with a sensing window of 48 mm (see Figure 3), are used to measure the lateral position before and after the downstream web guide. These web edge sensors provide a clear quantification of the magnitude of lateral disturbance entering the web guide and the corrective response of the web guide.
Sinusoidal Disturbance Rejection
Sinusoidal disturbances are common lateral errors seen in industrial converting machines. Hence the performance of the ARIS web guiding system is evaluated for sinusoidal disturbances. In the experiments reported in this white paper, sinusoidal lateral disturbances with a magnitude of 8 mm and frequencies of 0.25 Hz, 0.5 Hz, 1 Hz, 2 Hz and 5 Hz were introduced with the upstream web guide while the downstream web guide is used to correct for the disturbances. Kraft paper with a width of 152 mm was transported in the machine with varying speeds up to about 5 m/sec; tension was maintained in the machine using a passive dancer.
A summary of the time domain results are shown in Figure 4. All the plots show the disturbance entering the web guide's roller (measured using the upstream sensor) and the guiding performance of ARIS (measured in the downstream span using a sensor).
A FFT of the lateral disturbance and the guiding performance is shown in Figure 5 to further summarize the results.
Step Disturbance Rejection
Response to step disturbance is also a performance metric that can be used to quantify the response time of the web guide and the magnitude of attenuation. These types of disturbances are common with web splices. Experiments were conducted to evaluate the performance of the ARIS web guiding system for these types of disturbances and a summary of results is shown in Figure 6. The disturbance entering the guide roller is captured by the upstream sensor while the actual lateral position downstream of the guide roller is measured using the downstream sensor.
Summary and Conclusions
From the experimental results it is evident that the ARIS web guiding system provides good web guiding performance in the presence of both sinusoidal and step disturbances. Significant reduction in sinusoidal disturbances were observed with low frequency disturbances and rapid response to step disturbances were also observed. Further increase in performance in the guiding performance can be obtained with a high dynamics response actuator.