Dynamic Measurement of Small Oscillatory Motions


This application note describes the measurement of a very small oscillatory motions (nanometers) produced by a piezo electric device with the MTI-2100 and the MTI-2032RX high-resolution module.

Fotonic™ sensors from MTI Instruments are ideal for making dynamic measurements of very small oscillatory motions. Using the MTI-2100 Fotonic Sensor with a spectrum analyzer, angstrom level measurements are possible.

Would you like to know more? Okay, let’s set up some basic equipment and get started.

Photo of the Equipment used with call-outs on the Test Bench.

Piezo electric crystals expand and contract when voltage is applied across the stack. By controlling the amplitude of the applied signal, we control the magnitude of the displacement.

Piezo electric crystals expand and contract when voltage is applied across the stack.

Here we will measure the sinusoidal motion of a small piezo electric ceramic transducer being driven by an AC signal generator.

Figure 3 showing the piezo electric stack 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.

Fotonic Probe on Piezo Stack

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 volts as an operating position that’s within the R1 linear range. 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. The sensitivity is increased by 10X because the lamp intensity increases by 10X.

The module’s built-in high pass filter is set to 20 Hz to help reduce seismic motion pickup (bench vibration) and the module’s low pass filter is set to 1 kHz to help reduce noise as we are only looking at a 100 Hz signal.

Figure 4 showing the Signal Generator used in the test.

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 nanometers p-p.

Figures 5 and 6 showing an Oscilloscope and the MTI-2100 Fotonic Sensor.

We see this as about 8 mV p-p on the scope (Fig. 5) and about 4-5 nm pk-pk on the display (Fig 6).

Spectrum Analyzer showing 100Hz Peak and 1.723 mV rms

The spectrum analyzer (Fig. 7) is showing the 100 Hz signal at an amplitude of 2.8 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 100 kHz motion.

Conclusion: Dynamic Measurement of Small Oscillatory Motions

The MTI-2100 Fotonic sensor with MTI-2032RX probe can resolve down to about 1-2 nm with its built in pk-pk display. The MTI-2100 when used in conjunction with a Spectrum analyzer is able to further get down to < 5 Angstroms resolution provided the motion being observed is sinusoidal and also if there is a lot of other noise being observed at the same time.