SPEKTRA develops exciters for vibration tests up to megahertz frequencies
For testing and characterizing sensors based on MEMS technologies, sensor manufacturers and their customers require increasingly high-frequency mechanical exciters that can generate both translational and rotational vibrations. Whereas 10 years ago, frequencies between 20 kHz and 50 kHz were still sufficient, tests are now already being carried out at frequencies of several hundred kHz – and the requirements for the near future are moving into the megahertz range.
The topic of excitation and testing of MEMS sensors with vibration-like oscillations has occupied SPEKTRA since the company was founded. With the rapid development of MEMS sensors, the testing requirements have become more stringent in recent years. This is especially true for tests where the immunity of the MEMS to external vibration sources is to be tested. These sensors are used in a wide variety of applications. In applications such as mobile phones, they are, for instance, mounted on increasingly densely packed circuit boards, which often places them in close proximity to sources of vibration interference such as DC-DC converters or fingerprint sensors that rely on ultrasonic excitation. Exciters for increasingly high-frequency translational as well as rotational oscillations are therefore required for the tests.
"We realized that with the development of MEMS sensors the requirements for tests would become increasingly high and more demanding."
In 2008, SPEKTRA received their first request with higher requirements. A manufacturer of angular rate sensors for installation into car control units had discovered during testing that high-frequency excitations, such as those caused by short impacts or stone impacts, resulted in an unexpected signal from the rotation rate sensor. He wanted to test the susceptibility of MEMS to interference by means of vibration tests in the frequency range above 20 kHz, which was still considered very high at the time. We first solved this challenge with our calibration vibration exciter SE-09 , which is designed for operation up to 50 kHz. "It worked quite well at the time for the tests it was intended for. But it was still a work in progress”, says product specialist Michael Mende in his presentation at the German Specialized Press Days 2021.
"We realized at that time that with the development of MEMS sensors and applications, the requirements for tests and corresponding exciters would become increasingly high and more demanding. Therefore, our developers set out to create special vibration exciters for these particular applications."
In 2012, the SE-11 with its special armature was launched. However, this is still a standard electro dynamic exciter designed for the frequency range up to 50 kHz, while allowing for improved mounting of the DUTs in various orientations to the vibration axis. But the requirements have increased further to higher frequencies – 100 to even 200 kHz.
These frequencies were achieved by the vibration exciter SE-16, which is a “scaled-down” version of the SE-11 that shifts the resonance frequencies of the vibration head into a higher frequency range due to the lower moving masses. A further challenge in the development of the SE-16 was the question of a suitable reference sensor with which the vibration of the armature can be measured during operation and whose signal can be used for control if necessary. While a suitable acceleration sensor could still be found for the SE-11, the SE-16 only allows for the use of a laser vibrometer as a reference sensor.
Increasing demands and the emergence of the megahertz exciter
But 200 kHz was not the final destination. When a customer request in 2018 called for tests up to 400 kHz, it quickly became clear that this would only be possible with the SE-16 with limited performance parameters and some trickery. In addition to a special power amplifier, optimized processing of the laser vibrometer signal was now required, since the maximum possible vibration velocity of the SE-16 in this range decreases with frequency and the signal-to-noise ratio of the available laser vibrometer became worse and worse with increasing frequency.
“At that time, we already thought that a limit had been reached with the 400 kHz for the controlled excitation of mechanical vibrations for MEMS tests,” says Michael Mende. But customer requests went further – beyond 400 kilohertz toward excitation in the megahertz range. SPEKTRA has now pioneered a piezoelectric vibration exciter that enables the controlled excitation of MEMS sensors or other small components up to a frequency range of 2 MHz with translational and, for the first time, also rotational excitation (tilting vibration). For translational excitation, piezoelectric vibration exciters already existed in the past, but due to pronounced eigenmodes in many frequency ranges, they no longer allowed for well-controlled vibration at such high frequencies. By segmenting the exciter, we were able to suppress unwanted eigenmodes to a large extent. In addition, by controlling the segments differently, this exciter could for the first time also generate tilt oscillations in this frequency range, which can be used for MEMS tests.
The excitation modes could be measured using 3D scanning vibrometers, and an approach for a software-controlled vibration control system was found which can specifically control the vibration modes and amplitudes at very high frequencies.