Adaptive Feedforward Cancellation Research Articles

Rejection of periodic disturbances of unknown and time‐varying frequency

AbstractThe paper reviews available methods for the rejection of periodic disturbances. Such disturbances are often encountered in active noise and vibration control, due to rotating machinery. The emphasis of the paper is on feedback control problems where reference sensors are not available. The case where the frequency of the disturbance is known is considered first. Two sets of algorithms are discussed: one based on the internal model principle of feedback control theory, and the second based on adaptive feedforward cancellation. An interesting observation is that algorithms originating from both approaches can be shown to be equivalent under certain conditions. When the frequency of the disturbance is unknown, an intuitive approach consists in combining a method for the rejection of disturbances of known frequency with a frequency estimator. Alternatively, one may seek to develop a stable adaptation mechanism so that the disturbance is cancelled asymptotically. While algorithms can be designed to adapt to plant and disturbance parameters, the most successful approaches use some limited plant information to adapt the magnitude, frequency, and phase parameters of the control signal. Applications are discussed throughout the paper. Copyright © 2004 John Wiley & Sons, Ltd.

Disk Wobble Control in Optical Disk Drives

Focusing errors in optical disk drives comprise significant periodic components and an eventful dc content. The dc component is not explicitly taken into account in many periodic compensation methods. This paper investigates the design of adaptive feedforward cancellation (AFC) and frequency adaptive control technique (FACT) algorithms for periodic components as well as dc disturbance compensation. Both algorithms are applied as a plug-in module to an optical disk drive for demonstrating their facility in the reduction of focusing errors. By making good use of frequency sampling filter (FSF), the real-time harmonic identification can be fulfilled in FACT method for various playing speeds. Analysis and experimental results show that FACT has better properties than AFC in terms of dc cancellation and harmonic independence. It is also shown that AFC fails when both the dc content and harmonics are compensated simultaneously.

A loop shaping perspective for tuning controllers with adaptive feedforward cancellation

This paper presents a loop-shaping perspective on the tuning of systems using adaptive feedforward cancellation (AFC). The AFC technique is very useful for reducing/eliminating errors in a controlled system at a selected set of harmonics. Such a technique has applicability for hard disk and optical disk track following, vibration rejection in spindle systems and helicopters, camshaft and piston machining, as well as in fast tool servos for diamond turning. This last application provides the motivation for the work described herein. The paper shows how an AFC loop with N resonators results in 2 N parameters, a gain and phase advance parameter for each resonator, which must be adjusted to maximize closed-loop performance. This paper details a frequency shaping method of selecting the AFC phase advance parameters and resonator gains in multiple AFC resonator systems. We have applied these techniques to our prototype diamond turning machine with a fast tool servo for cutting ophthalmic lenses. Experimental results comparing the performance of this machine with and without AFC control are presented.

Implementation of adaptive feedforward cancellation algorithms for pre-embossed rigid magnetic (PERM) disks

Pre-embossed rigid magnetic (PERM) disks provide the potential to improve storage capacity, simplify the manufacturing process, and reduce overall manufacturing costs. Since the tracks including servo sectors are pre-embossed and assembled into the drive, there is runout mainly due to the inherent eccentricity. In order to compensate repeatable runout, an adaptive feedforward cancellation (AFC) scheme is implemented. It is shown through experiments that the AFC is effective in selectively canceling the desired frequency. Cancellation of the first three harmonic components is demonstrated, and experimental results are presented. Without track following servo, the AFC is able to reduce the position error signal (PES) by 83% of the track pitch.

Efficient implementation of adaptive feedforward runout cancellation in a disk drive

This paper presents an improved adaptive feedforward compensation (AFC) technique for periodic disturbance cancellation. It has a better rate of convergence than AFC. It can also compensate for a particular harmonic without amplifying the adjacent uncompensated harmonics. A frequency domain design strategy is proposed. The results are verified both through simulations and by an experiment on a disk drive with a dedicated servo channel.

Advanced methods for repeatable runout compensation [disc drives

This paper presents and compares experimental results from two types of periodic disturbance compensation methods. The repeatable runout (RRO) cancellation techniques studied in this paper are adaptive feedforward cancellation (AFC) and repetitive control. Two modifications (phase advance and a feedthrough term) to the basic AFC structure are also studied experimentally. Of the AFC methods, the feedthrough technique is superior, but the repetitive controller provides better RRO rejection. Overall it is found that the removal of repeatable runout improved the tracking precision by as much as 53%. >

Design of adaptive feedforward algorithms using internal model equivalence

AbstractThe paper investigates the design of adaptive feedforward cancellation (AFC) algorithms with sinusoidal regressors for repetitive control. Such adaptive algorithms are equivalent to linear controllers based on the internal model principle (IMP). Using this equivalence and root locus rules, the phase advance of the regressor of the adaptive algorithm can be chosen to maximize the phase margin at low gains. A surprising result is that selecting the optimal phase advance is equivalent to placing a zero in the open right half‐plane in certain cases. A complete design and analysis for the compensation of a single‐frequency periodic disturbance is done. A new variation of the AFC algorithm is also developed in which the adaptive portion acts in parallel with a feedthrough term. the IMP equivalent of this algorithm has two zeros instead of one. Analysis and simulation show this method to have superior convergence and robustness properties when compared with the method having no feedthrough term. Discrete time versions of the algorithms are briefly considered.

Harmonic generation in adaptive feedforward cancellation schemes

The paper investigates the generation of harmonics in adaptive feedforward cancellation schemes. Specifically, it is shown that an algorithm designed to reject a certain number of frequency components may in fact be capable of reducing higher-order harmonics as well. This effect originates in the time-variation of the adaptive parameters. A consequence is that the adaptive system may perform better than the algorithm with the fixed, ideal parameters. Surprisingly, the response to higher-order harmonics can be calculated precisely, using a Laplace transform analysis. The origin of the numerical procedure is in the equivalence between the adaptive feedforward cancellation scheme and a control scheme based on the internal model principle. The implication of the results on design and implementation issues is discussed. >