Discrete adaptive neural network disturbance feedforward compensation for non-linear disturbances in servo-control applications

Abstract

This article introduces a novel adaptive neural network compensator for feedforward compensation of external disturbances affecting a closed-loop system. The neural network scheme is posed so that a non-linear disturbance model estimate for a measurable disturbance can be adapted for rejection of the disturbance affecting a closed-loop system. The non-linear neural network approach has been particularly developed for ‘mobile’ applications where the adaptation algorithm has to remain simple. For that reason, the theoretical framework justifies a very simple least-mean-square approach suggested in a mobile hard disk drive context. This approach is generalised to a non-linear adaptive neural network (NN) compensation scheme. In addition, usual assumptions are relaxed, so that it is sufficient to model the disturbance model as a stable non-linear system avoiding strictly positive real assumptions. The output of the estimated disturbance model is assumed to be matched to the compensation signal for effectiveness, although for stability this is not necessary. Practical and simulation examples show different features of the adaptation algorithm. In a realistic hard disk drive simulation and a practical application, it is shown that a non-linear adaptive compensation scheme is required for non-linear disturbance compensation providing better performance at similar computational effort in comparison to well-established schemes.

Keywords:

• feedforward compensation
• adaptive neural network (NN) control
• discrete control
• servo control
• hard disk drive

Acknowledgements

The authors would like to thank Dr Venkatakrishnan Venkataramanan for practical advice. This work was supported by NSF Grant No. ECS-0501451, ARO Grant No. W91NF-05-1-0314 and A*Star SERC Grant No. 052 101 0097.