Cell Voltage Noise Removal and Cell Voltage (or Resistance) Slope Calculation
M. Dupuis GéniSim Inc. 3111 Alger St.
Jonquière, Québec, Canada G7S 2M9
ABSTRACT
Cell voltage noise removal and cell voltage (or resistance) slope calculation are
critical aspects of modern high amperage cell controller operations, yet, those operations are not covered much in the literature. In the present work, the efficiency of several algorithms is compared.
INTRODUCTION
Aluminium reduction cell process control development have been fairly recently
summarized by Bearne [1]. Modern high amperage cell control strategies are all based on continuous tracking algorithms [2,3,4,5]. The aim of those continuous tracking algorithms is to operate the cell at the lowest possible concentration of dissolved alumina in the bath because this maximizes current efficiency [1,6]. Those alumina concentration control algorithms are based on alternating between an underfeeding and an overfeeding regime because as explained in [7], using a constant nominal feeding regime is impossible over a long period of time because the negative correlation between dissolved alumina concentration in the bath and current efficiency makes the system unstable.
All those continuous tracking control algorithms are using the slope of the cell
voltage over time or more accurately the cell pseudo-resistance as an indicator in order to decide when it is time to shift between the underfeeding and the overfeeding regime [1]. However, it is not so easy to accurately evaluate the low frequency slope as the cell voltage (or resistance) also has a high frequency component.
The low frequency component is due to the slow change of the alumina
concentration, average anode to cathode distance (ACD), bath temperature etc, while the high frequency component is due to magneto-hydro-dynamic (MHD) generated bath- metal interface long traveling waves and gas driven short bath-metal interface waves. Together those two types of waves are responsible for the high frequency component variation of the cell voltage (resistance) called cell noise. That high frequency noise
