Application Cases

Experiment Name: Application of Power Amplifiers in Modal Testing Research of MEMS Microstructures

Research Direction: Component Testing

Test Objective:
With the widespread application of MEMS devices in various fields, obtaining their dynamic characteristic parameters through modal testing of microstructures is of great significance for their design, simulation, fabrication, quality control, and evaluation. Considering that microstructures possess characteristics such as small size, high natural frequency, and low amplitude, and are used in different environments, this paper conducts modal tests on rectangular cross-section micro-cantilever beams made of single-crystal silicon and 304 stainless steel under high temperatures ranging from 20°C to 300°C. A method based on air-coupled ultrasonic excitation is proposed, and the relationship between the natural frequency of the micro-cantilever beam and temperature is investigated through theoretical analysis, simulation, and experimental testing.

Testing Equipment: ATA-2042 Power Amplifier, Ultrasonic Probe, Analog Output Board, Industrial Control Computer, Controller, Data Acquisition Card, Platinum Resistance Temperature Sensor, Digital Temperature Controller, etc.

Experimental Procedure:
This research conducts modal tests on MEMS microstructures from room temperature (20°C) to high temperature (300°C) based on the air-coupled ultrasonic excitation method. By acquiring vibration signals of the measured  microstructure at different temperatures, the relationship between its natural frequency and temperature is analyzed. Before building the experimental setup, three key issues must be understood and addressed: exciting the microstructure, measuring the vibration signal of the microstructure, and creating a high-temperature environment.

Figure: Overall Block Diagram of the Test System

Based on the analysis of the three pre-setup issues, the technical approach  for modal testing of MEMS microstructures using air-coupled ultrasonic excitation is as follows: Ultrasonic waves emitted by an air-coupled ultrasonic focused transducer  device are used to excite the microstructure. A laser Doppler vibrometer (LDV) system is employed to detect the vibration response signal of the microstructure at high temperatures. Resistance heating is used to create the high-temperature environment. The overall block diagram of the test system is shown above, mainly comprising a temperature control unit, an air-coupled ultrasonic excitation unit, and a laser Doppler vibrometry unit.

The air-coupled ultrasonic excitation unit mainly includes a point-focus air-coupled ultrasonic probe. This probe has a bandwidth of 140 kHz, a focal length of 38 mm, a focal spot diameter of 2 mm, a rated power of 350 kHz, and an effective diameter of 13 mm. The point-focus design provides a smaller beam profile and longer focal length, offering greater resolution and higher energy intensity. It also includes the Aigtek ATA-2042 power amplifier, a dual-channel high-voltage signal amplifier capable of amplifying AC/DC signals. It features a maximum output power of 400 Vp-p (±200 Vp), a maximum output current of 100 mAp, a bandwidth (-3 dB) of DC~500 kHz, and a slew rate ≥ 445 V/μs, enabling it to drive high-voltage loads. The voltage gain is digitally controlled and adjustable, and common settings can be saved with one key, providing convenient operation. It can be used with  mainstream  signal generators to achieve perfect signal amplification. The unit also includes an analog output board card and an industrial control computer. When excitation of the microstructure is required, an arbitrary waveform generator is first programmed using LabVIEW in the industrial control computer to generate a double-sideband suppressed-carrier amplitude modulated (DSB-SC-AM) signal with a carrier frequency of fc = 350 kHz. The modulation frequency Δf can be arbitrarily selected within the range of 0 to 140 kHz. This signal is input into the PCI-1721-AE analog output board card. The analog signal is output through an analog channel into the ATA-2042 power amplifier. After amplification to a peak-to-peak value of 400 V, the driving signal causes the point-focus air-coupled ultrasonic probe to emit two ultrasonic beams with a frequency difference of Δf. These two beams are focused onto the microstructure surface, creating interference that causes the microstructure to vibrate at the difference frequency Δf. By sequentially changing the modulation frequency Δf within the 0~140 kHz range to excite the microstructure, the laser Doppler vibrometer can acquire the vibration response signal at each difference frequency Δf. When the difference frequency Δf coincides with the microstructure's natural frequency, resonance occurs.

Figure: MEMS Modal Test Experimental Setup Based on Air-Coupled Ultrasonic Excitation

The laser Doppler vibrometry unit primarily consists of an OFV-3001 controller, an OFV-512 optical head, a PCI-1712UL-AE data acquisition card, and an IPC-610L industrial control computer. The principle of vibration signal detection is as follows: a laser beam emitted by the OFV-512 optical head is focused onto the measured  object. The frequency and phase of the reflected light change. This reflected light is coupled back into the interferometer within the sensor head. By comparing its phase and frequency with an internal reference beam, the velocity and position of the  measured  object can be calculated, thereby obtaining the dynamic characteristic parameters of the microstructure. As shown in the figure above, the vibration testing system can achieve a test bandwidth  of up to 1.5 MHz, with a minimum achievable laser spot diameter of 15 μm. The vibration response signal of the microstructure is picked up by the OFV-512 optical head, transmitted to the velocity decoder within the OFV-3001 controller for decoding, then sent via the PCI-1712UL-AE data acquisition card to the IPC-610L industrial control computer. Finally, a data acquisition program written in LabVIEW analyzes and stores the vibration response signals.

The temperature control unit comprises a Pt100 platinum resistance temperature sensor, an ADAM3013 signal conditioning module, an E5AZ-C3MT digital temperature controller, a JKH-D1 thyristor trigger, a BTA06 bidirectional triode thyristor (TRIAC), and cartridge resistance heaters. To test the microstructure in a high-temperature environment up to 300°C, four cartridge resistance heaters are uniformly installed in mounting holes on the four sidewalls of a brass mounting plate. The heat generated by the heaters reaches the microstructure layer by layer through thermal conduction, thereby heating it. The Pt100 platinum resistance temperature sensor, with a measurement range of -200°C to +850°C, detects the current temperature of the microstructure. The collected temperature signal is sent via the ADAM3013 signal conditioning module to the E5AZ-C3MT digital temperature controller. After comparing it with the set target temperature, the JKH-D1 thyristor trigger controls the conduction time of the BTA06 TRIAC based on the comparison result. This achieves intelligent control of the heating power of the cartridge resistance heaters, consequently enabling accurate temperature control of the microstructure.

Experimental Results:
A high-temperature environment microstructure modal testing system was established. Ultrasonic waves emitted by an air-coupled ultrasonic focused transducer  device are used to excite the microstructure. A laser Doppler vibrometer system is employed to detect the vibration response signal of the microstructure at high temperatures. Resistance heating is used to create the high-temperature environment.

Figure: ATA-2042 High-Voltage Amplifier Specifications and Parameters