Achtergrond van Bruco

Laser Scanning Module MEMS mirror controller

A Laser Scanning Module (LSM) consists out of one or two moving MEMS mirrors and an electronic circuit for driving and controlling the mirror. By combining a fast and slow resonating MEMS mirror the LSM can be used for 2D scanning applications.

Application areas of the LSM include optical measurement, laser printing, and RGB hand held projector.
Artist's impression of LSM

Challenge

MEMS mirrorThe mirror is driven in resonant mode; at the resonance frequency the maximum amplitude is reached. The controller should be able to activate the MEMS mirror, change the activation frequency and lock on the MEMS' resonance frequency.
For reaching the maximum amplitude it is essential to know the exact tilting angel of the mirror, since activation should be applied at the optimum moment. The system can be compared by activating a rocking chair; pushing a chair at the wrong moment results in less amplitude.
On average the activation voltage required is relative high (up to 80V). Sensing the mirror tilting angle in a "hostile" environment of HV pulses is challenging.

Solution

A dedicated ASIC is designed in a 0.35µm process with HV (up to 80V) options. This ASIC is capable of sensing and activating 1D MEMS mirrors using the same capacitance on the mirror. Capacitive sensing is used to determine the tilting angle of the MEMS.
Layout of Laser Scanning Module MEMS mirror controller

During startup the mirror is activated till certain amplitude, subsequently the ASIC switches over to normal mode and keeps the MEMS in resonance. The control loop is a PLL based system that compares the actual mirror position with the actual actuation signal.
The controller ASIC supports a wide range of mirror frequencies and capacitances.

A discrete MEMS emulator circuit is developed so it is possible to test the ASIC without connecting the MEMS mirror