SPLATT-Laboratory

Test Plan

The laboratory activity is intended to assess the rejection of external vibrations thanks to contactless actuators. When the mirror is operating, the thin shell floats at 100-200 um from the reference body thanks to the magnetic force produced by the actuators. Such distance is kept constant, as the actuator are controlled in close loop, fed by co-located capacitive position sensors. Since the actuator control bandwidth is approximately 1 s, a faster displacement of the reference body cannot be tracked by the local close loop, thus realizing an embedded mechanical insulation. During the test we inject a controlled vibration on the mechanical support stand (the white horizontal flange in the figure), then we measure the optical tilt when:

a) the shell is "glued" to the reference body with the actuators pulling at maximum force and

b) when the shell floats at a given (100 um, e.g.) gap from the reference body.

The measurement in a) is a reading of the disturbance injected in the system; then the ratio of the measurements b/a is the vibration attenuation (vs frequency).

Laboratory setup

The interferometer and folding mirrors are on an insulated optical bench, while the LATT prototype is installed on a self-standing test tower above the bench (and not in contact with it). A piezo actuator is mounted on the LATT elevation arm to "shake" the support. The piezo excitation is sinusoidal (single frequency) or frequency sweep, in the range 1 to 120 Hz. The interferometer is a dynamic, Twyman-Green with a frame rate of 350 Hz. The exposure time is < 100 us, so that the fringes are frozen during the integration time. Each phase map is analyzed to compute the instantaneous tilt amplitude.

Test results

In the figure the ratio Tilt_F/Titl_L (respectively, with thin shell floating and thin shell lifted -glued on the reference body). The blue line is the reference (1). In the region 1-20 Hz, the disturbance is amplified because of the control bandwidth of the actuators loop.

As a very first step, we measured the SPLATT attenuation in the frequency range 1 Hz to 120 Hz. The result is shown in the figure, for different proportional gains of the system. The picture shows the (expected) overshoot at the low frequencies, corresponding to the settling time of the internal loop. The resonance frequency and amplitude is in facts depending on the loop gain, with a lower gain resulting in a lower frequency (kP=250, freq=5 Hz). From the plot we observe that the system shows in general an effective attenuation, with significantly poorer performances in the 60 Hz and 100 Hz bands. We investigated such regions with dedicated frequency sweep measurements.

We repeated the test with different loop gains and the result is shown in the following picture, where we we plot the tilt values instead of the attenuation, so that they shall be compared with the black plot which is the tilt measured with the TS attached on the RB. The three plots are well superimposed, meaning that the attenuation, in this frequency band, does not depend from the loop parameters. The resonance at 100 Hz, in particular, is identical for the three plots, thus suggesting it cannot be originating from the internal loop.

We then repeated the sweep measurement at different gaps, i.e. with the TS floating at different distances from the RB. The result is shown in the last picture. Under the assumption the behaviour is due to the air coupling, we expected to observe a larger attenuation at larger gaps. The experimental data confirmed the hypothesis and we observed a progressive increase in attenuation changing the gap from 20 um to 200 um. We also observed that the resonance peak is higher al larger gaps. This point is consistent with the following hypothesis: in the band 60 Hz to 90 Hz, a large air gap transmits less efficiently the external excitation; at 100 Hz a resonance of the TS is excited and a larger gap is less efficient in damping it. Such scheme is just a qualitative description.

A test in a vacuum chamber is required to fully understand the behaviour.

Page updated

Report abuse