Application Cases

Experiment Name: Ultrasonic Ranging Measurement Based on Piezoelectric Micro-Machined Ultrasonic Transducer (PMUT)

Research Directions: Medical Imaging, Parking Sensors

Experimental Objective:
This study investigates ultrasonic ranging measurement based on piezoelectric micro-machined ultrasonic transducers (PMUTs), exploring close-range measurements in a higher frequency band of 400 kHz. The aim is to achieve higher resolution and smaller measurement errors.

Test Equipment:
Signal generator, ATA-1372A broadband amplifier, oscilloscope, PMUT specimens, etc.

Experimental Procedure:
Various excitation waveforms for PMUTs were tested, including triangular waves (primarily used in scanning circuits). The peak-to-peak voltage for all waveforms was set to 10 V, with 100 pulses applied to achieve full amplitude saturation. As shown in Figure 2, the full-wave square wave produced the highest excitation amplitude, while the full-wave sine wave produced the smallest. This is because, under the same voltage amplitude, the square wave has the highest root mean square (RMS) voltage, resulting in greater driving capability. The figure also indicates that different waveforms have little effect on the steepness of the rising and falling phases of PMUT vibration, primarily influencing only the amplitude of steady-state vibration. Therefore, the full-wave square wave was selected as the excitation signal for the PMUT.

For this ranging experiment, two PMUTs were used as the transmitting and receiving transducers, respectively. The signal generator provided 50 pulses to the power amplifier with a measurement interval of 100 ms. The power amplifier drove one PMUT to emit ultrasonic waves, which were reflected and received by the second PMUT. The received signal was observed on an oscilloscope.

Figure 1: Physical Setup for Ultrasonic Ranging

Figure 2: Transient Responses Under Different Waveform Excitations

The experimental setup consisted of a signal generator, ATA-1372 broadband amplifier, oscilloscope, and PMUT specimens. The PMUT specimens in this experiment were primarily composed of aluminum nitride (AlN), with a structural layer thickness of 10 μm, a piezoelectric layer thickness of 0.5 μm, and an electrode thickness of 1 μm. Considering the residual stress introduced during AlN deposition, the radius was set to 300 μm. The measurement frequency band was controlled around 400 kHz, with the wavelength kept below 1 mm for high-precision close-range ultrasonic measurements.

Figure 3: 3D View and Cross-Sectional Structure of the PMUT Design

The overall design features a circular diaphragm with fixed circumferential boundaries and rotational symmetry about the central axis. The neutral axis, which experiences zero stress during bending vibration, bends without elongation. The PMUT consists of a three-layer stacked structure: the bottom gray layer is the structural layer, the middle green layer is the piezoelectric layer, and the top yellow layer is the electrode layer. Thickness is denoted by $t$, radius by $r$, and distance from the central axis by $z$.

Experimental Results:
The designed PMUT was tested as described, and the results are shown in Figure 4. The tests revealed that the PMUT could detect weak amplified echo signals within a range of 2 cm. Beyond 2 cm, no significant echoes were detected, primarily due to the low acoustic pressure emitted by the PMUT. The thicker structural layer (10 μm) and thinner piezoelectric layer (0.5 μm) of the PMUT limited its emission capability. To achieve higher emission acoustic pressure and a larger measurement range, PMUT arrays with thinner structural layers and thicker piezoelectric layers were fabricated.

Figure 4: Pulse Echo Signal for Ranging with a Single PMUT

Recommended Power Amplifier: ATA-1372A

Figure: Specifications of the ATA-1372A Broadband Amplifier