Modelling and Control of a Vacuum Air Bearing Linear Drive in the Nanometer Range

The limits for the positioning accuracy of a direct linear drive based on a vacuum air bearing with a voice coil for high precision positioning are investigated using different control techniques.

The original contributions of this research are as follows:
- The slider of a vacuum air bearing linear drive has been positioned in the nanometer range using the special control techniques of Forced Dynamic Control (FDC) and Observer
Based Robust Control (OBRC) for the first time. The results have been compared with those obtained with Sliding Mode Control (SMC) and conventional cascade control. The
Matlab/Simulink simulations show good agreement with the experiments implemented using dSPACE.
- A rotational vibration mode due to the compliance of the air cushion has been discovered.
It has been concluded that it is observable due to the displacement between the position measurement encoder location and the centre of mass of the moving part. The impact of this on the performance of the aforementioned controllers has been investigated by simulation using a specially derived dynamic model and the validity of the results confirmed experimentally. Each controller has been shown to give varying degrees of active damping of the vibration mode and the two robust control techniques, OBRC and SMC, have been shown to achieve the highest positioning accuracy.

At the outset, it was found necessary to mount the vacuum air bearing on a large seismic block to eliminate vibration modes that would impair the positioning accuracy. The aforementioned rotational vibration mode, however, remained. An initial attempt was made to alleviate this
problem by raising the natural frequency of vibration through increasing the prestress pressure differential of the vacuum air bearing and therefore the amount of energy required to cause a given amplitude of vibration of the slider. This, however, was found to excite higher and
audible vibration modes in the aluminium structure of the moving part and therefore abandoned in favour of obtaining active vibration damping from the controllers. In order to achieve this using the FDC method, a modification had to be made in the form of an artificial measurement variable from an observer. The overall accuracy obtained from the robust SMC and OBRC approaches were found to be the best and were improved further by fixing an additional seismic
block to the stator of the voice coil, thereby permitting higher gains in these controllers that would otherwise cause oscillatory instability.

In summary, the research has led to control techniques that can be recommended for motion control systems based on vacuum air bearings whose positioning accuracy has to be in the nanometer range.

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