Digital servo control improves web process measurement

Digital servo control improves web process measurement.

To effectively control web processes (e.g., coating, laminating, papermaking), you must be able to measure the amount of material being applied to the web while the web is moving. Barber-Colman Indev's 6880 Microcomputer System was developed for just such a measurement. Problems with its initial design, however, made it difficult for the system's multiple scanners to each measure the same spot on the web during line speed changes. A solution to this problem was found in Multibus I motion control cards. The system specifics

The Indev 6880 uses multiple scanning sensors to measure thickness or "basis weight" at different locations along the web (Fig. 1). By calculating the difference in material thickness before and after the process, the amount of material used can be determined. This information can then be used to regulate the process so that the desired amount of material is applied.

In operation, a Betamike source emits a safe, controlled beam of beta radiation that passes through the web, decreases in magnitude, and enters the detector. The detector's output voltage is proportional to the basis weight or thickness of the web material. The signal is calibrated for display in industry standard units in the control station. The difference in weight readings between sensor one and two indicates the amount of material added by process one, while the difference between sensor two and three provides the same indication for process two. Up to four sensors may be used with the system.

Originally, each scanning head moved at a fixed speed across the web. By carefully adjusting the dwell time at the end of each scan, multiple scanners would scan the same spot on the web as it moved between the sensors.

The problem was that if the line speed changed, the scanners were no longer synchronized and did not scan the same area of the web each time, affecting the accuracy of the

measurement. This required constant readjustment of the dwell times to maintain acceptable performance.

Two Creonics Multibus I motion control cards (MCCs) provided a solution. By replacing the analog speed controller on each scanner with an all-digital servo positioning system, precise synchronization between the scanning heads and the web can now be maintained over a wide range of web speeds. An electronic gearing feature of the MCCs allows the position of each scanning head to be precisely controlled relative to the position of the web. Because each MCC controls two frames, a maximum of four servo-controlled frames can be handled.

Electronic gearing, a digital, entirely electronic replacement for mechanical gears, allows a closed-loop servo axis to precisely follow the motion of a master encoder. The gear ratio is specified as a 24-bit digital word rather than by the number of gear teeth. Being electronic, it can be changed at any time without disrupting the motion in-progress. In addition, changing the sign of the gear ratio changes the direction of the servo-controlled axis.

The scan angle can also be directly controlled by programming the appropriate gear ratio. Larger gear ratios produce faster sensor speeds relative to the web speed and thus a smaller scan angle. Smaller gear ratios produce larger scan angles. Reversing the scan direction after each scan is easily accomplished by changing the gear ratio's sign.

By synchronizing positions rather than time, line speed can be varied as required by the process without affecting the accuracy of the measuring system. At a web speed of 150 ft/min, successive scans are within 0.01 inch of the original scan. Measuring the same spot increases measurement

Fig. 1 (left): Weight readings from three different sensors provide information Co determine whether or not the proper amount of material has been applied to the web.

Fig. 2 (below): Sensors travel back and forth along a frame to assure that every spot on the web is scanned.

Цифровое сервоуправление улучшает измерение процесса сети.

Чтобы эффективно управлять процессы сети (например, покрытие, расщепление, бумажное производство), Вы должны быть квалифицированы измерить количество материала,  применяемого к сети, в то время как сеть перемещается. 6880 Микрокомпьютерных Системы Парикмахера - Colman Индева была развита для только такого измерения. Задачи с его начальным проектом, однако, сделали это трудным для множественных развертывающих устройств системы к, каждый измеряет ту же самую отметку на сети в течение изменений(замен) линейной скорости. Решение этой задачи было найдено в Мультиавтобусе(мультишине) мной платами управления движения. Специфические особенности системы