High Dynamic Resolution Measurements

Fotonic sensors are ideal for making dynamic measurement of very small oscillatory motions. It’s possible to get down to the angstrom level when using the MTI-2100 in conjunction with a spectrum analyzer.

We measures the sinusoidal motion of a small Piezo electric ceramic transducer being driven by an AC signal generator. By controlling the amplitude of the applied signal we can control the magnitude of the displacement.

The piezo electric stack is placed in a fixture to anchor the base of the piezo stack and with a mounting arm to hold the fotonic probe over the top of the piezo stack see fig 3 . We gap the probe at max signal in cal mode (optical peak) and press the calibrate switch on the MTI -2100. This scales the signal up to 10 Volts DC. Then we switch to displacement and reposition the probe towards the stack such that we are in the middle of the range 1 slope anywhere in 2-7 volts region. In Displacement mode we chose -2.7 um . Next we increase the resolution of the signal by pushing the range 2 (10X) switch and adjust the offset pot such that the output signal is not saturated . I also used the modules built in high pass filter set to 20Hz to help reduce seismic motion pickup (bench vibration) and set the modules low pass filter to 1kHz to help reduce noise as we are only looking at a 100hz signal.

Next a signal generator is connected to the piezo stack and we set the amplitude to control the peak to peak motion of the piezo device. To start we set the generator’s amplitude to 26 mV RMS . This makes the amplitude of the piezo stack 4.5 nm p-p . We see this as about 8 mV p-p on the scope Fig 5 and about 4-5 nm on the display.

Spectrum analyzer showing the 100 Hz signal at an amplitude of 1.7 mV RMS which is about 8mV p-p .

Converting this to engineering units (.00057 um /mV from module’s sensitivity factor) we are observing 4.5 nm p-p motion. What is important to note here is that the signal to noise ratio on the spectrum analyzer is quite large. With the bandwidth shown here we could get down to about 200uV rms ~ 0.5 nm resolution ! That is 5 angstroms resolution. So using a spectrum analyzer in the frequency domain in conjunction with the MTI-2100 and MTI-2032RX high resolution Fotonic probe allows us to resolve down to angstroms of mechanical motion. The MTI -2100 is able to resolve down to about 1-2 nm with its built in peak to peak display. The module is capable of measuring up to about 100kHz motion.