New low-cost technique for accurate surface velocity measurement of a rotating drum

The accurate determination of the velocity profile of an almost uniformly rotating drum is considered. The quality of estimation of the surface velocity obtained through usage of a measuring wheel placed in direct contact with the drum, and rigidly coupled to an encoder of high line count, is usually poor, due to wheel eccentricities and misalignment of the wheel and drum. A new algorithm is described which achieves the nonlinear filtering of the encoder output in such a manner as to yield a very close approximation to the actual drum velocity in most circumstances. A very good estimate of the effective variable gearing ratio between the drum and measuring wheel is achieved, yielding excellent discrimination between drum-based and wheel-based variations. The compensation method is justified theoretically. Computer simulation and experimental results obtained from a web processing plant illustrate the utility of the method. The latter results are shown to compare well with data obtained from a costly laser Doppler-based measurement system.

I. Introduction

This PAPER focuses on the application of encoders to digital velocity measurement in the processing industry, with particular emphasis on the quality control requirements of high precision web manufacturing, which is intrinsic to the production of profile-critical plastic film. Due to the tension transfer mechanism inherent in the transportation of film via rollers [1], the effects of velocity variation propagate through the process, so that the precise control of surface velocity of the casting drum, which is where the molten extrudate forms a solid web, is crucial to the final profile variation of the film. Because the steady-state velocity variation of the casting drum will be small in a well regulated system [certainly less than 1% peak-to-peak (pk-pk), and ideally approaching 0.1% pk-pk], any velocity measurement device needs to exhibit a high accuracy.