Research Activities

Development of piezo hardmounts for precision equipment

Research group: H.M.J.R. Soemers, J.B. Jonker, J. van Dijk, ir. D. Tjepkema

Project term:
August 2008 – July 2012

mechatronic design principles, control, adaptive, feedforward, feedback, identification, vibration-sensors

Project background

The increasing demand of miniaturization in precision technology leads to higher accuracy requirements of precision equipment. Examples are: wafer scanners, microscopes, atomic force measurement equipment and laser surfacing machines. Disturbances, caused by external excitation due to floor-vibrations, cables, environmental sound and air currents and internal excitations due to forces stemming from accelerating machine components (stages) or vibrating machine components (vacuum pumps), are issues to deal with in order to meet the future demands for these accuracy requirements.

State of the art

State of the art is that solutions are found by passive and hybrid (passive and active) mounting techniques for problems stemming from a particular source of excitation. For instance, soft active mounts are commonly applied with success for disturbance rejection caused by floor-vibrations. Until now the general approach for reduction of the transfer of floor-vibrations in high precision equipment is based on the application of active air mounts. In this case the sensitive equipment is mounted in a heavy frame. To benefit from good passive isolation the air mounts provide little mounting stiffness in the dominant directions resulting in low frequent suspension mode resonance (1-3 Hz.) and compliant connections between table and base-frames or the floor. The latter endangers machine stability.

Approach in related project

A more promising method is the concept of using active hard mounts for machine support. The hard mount's larger stiffness results in less compliance to direct disturbances and increased machine stability. But on the other hand, the influence of floor vibrations is increased, because the suspension mode resonance now becomes higher (15-20 Hz.). Therefore, an active vibration isolation control (AVIC) system must be used, which compensates for floor vibrations in the lower frequency range. This hybrid hard mount concept is research in the project “Design of a smart-mount for application of vibration isolation in precision machinery”.

Approach in this project

In this project hybrid hard mounts will be designed which have per mount only two piezo actuators and the number of mounts for precision equipment is intended to be three. As a consequence the mounts will be less costly compared to the hard mounts on the market (15 actuators). Exactly constrained design is used to provide the passive isolation in parasitic paths of the mount. Feedforward and feedback combined control strategies will provide optimal adjustment of damping of internal modes next to isolation from external disturbances.

Research objectives

  • A design procedure to adequately design a system based on hybrid isolation techniques.
  • A piezo based mount will be designed and realized with a wide application scope for active isolation and damping purposes in precision machines. The mount consists of two piezo actuators with high stiffness in actuation direction and must be compliant in non-actuation direction.
  • An adequate scale model of a precision machine for testing and demonstrating purposes of the mounts.
  • An important issue is the measurement of low frequent floor-vibrations via either accelerometers or geophones (velocity measurement) combined with their specific signal processing hardware suffer from bad signal to noise ratio for frequencies below 1 Hz. In order to have appropriate vibration reduction the wish is to have good signal to noise ratio properties from to the frequency region>0.1 Hz. Possible solutions can be found in the application of MEMS based sensors. There are a few on the market, but also the opportunities offered by other research in the consortium SmartPie (thin film piezo sensoring) should be exploited.

The most promising experimental results on the SISO setup have been obtained by using a combination of (fixed gain) feedback control and adaptive feedforward control. The system's floor mass is excited by a shaker, resulting in a 45 mm/s2 rms floor vibration level. Figure 1 shows the power spectra of the measured acceleration signal on payload mass 1, for both the uncontrolled system and controlled system (after convergence of the adaptive feedforward controller). The residual vibration level was 8 mm/s2 rms, which corresponds to an average reduction of 21 dB over the 0 – 1 kHz frequency range.

Figure 1: Error signal power spectra; for both the uncontrolled and controlled system (after convergence of the adaptive feedforward controller)

Figure 1:
Error signal power spectra; for both the uncontrolled and controlled system (after convergence of the adaptive feedforward controller)

Future research topics

  • Development of a laboratory setup with 6 degrees of freedom (DOFs).
  • Design of a hard mount prototype with 2 actuated DOFs and low stiffness in the remaining DOFs.