The digital micromirrors that make possible solid-state scanning lidar may soon give way to more capable MEMS devices able to do much more than just beam a laser across a scene.
Leland Teschler | Executive Editor
The Digital Micromirror Devices (DMD) now used for video projection also play an important role in autonomous vehicle technology. These microelectromechanical systems (MEMS) can quickly reflect a laser across a scene and make it possible to field solid-state lidar devices.
Indications are that new generations of MEMS-based micromirror arrays resembling DMDs are on the verge of ushering in dramatically more capable lidar. The primary reason is that the new arrays have more degrees of freedom. DMDs have only two states, on or off. In the on state, the DMD reflects light onto a target – onto a screen in the case of video projection, onto a field of view in the case of lidar. In the off state, the light is directed elsewhere (usually onto a heatsink). In contrast, the MEMS-based mirror arrays now on the drawing boards have three control directions: tip, tilt, and piston (basically moving the mirror forward and back). These movements potentially make feasible such ideas as focusable lidar and sensing techniques that depend on steerable lasers.
Today, research groups at 49 companies and 23 academic institutions have come up with 2,631 research papers and patents on micromirror array (MMA) technologies. So says Bob Panas, an engineer at Lawrence Livermore National Lab’s Materials Engineering Div. Panas was part of a group that cataloged work taking place in MMA worldwide. LLNL has its own MMA project that it is pursuing in conjunction with UCLA and MEMS consulting group AMFitzgerald. Panas says lidar is just one potential application for the LLNL devices. Others include optical switches, precision optical alignment, imaging, 3D displays, and new approaches to micro-additive manufacturing.
The LLNL MMA design is indicative of what researchers are striving for in the technology. It will eventually contain 10,000 hexagonal-element arrays, each measuring 1 mm2. To give each mirror a large range of motion in tip, tilt, and piston directions, LLNL researchers came up with a hybrid manufacturing technique consisting of additive manufacturing and conventional integrated circuit lithography.
Additive techniques are used to print structures on the back of the mirrors that become part of the actuators for mirror positioning. But the additively manufactured components are only temporary. “Because we are in the R&D phase, we’ve designed the mirrored actuators as assemblies, so we can adjust the flexure designs. But we think we can roll this design into a standard microfabrication process where all the features are turned into masks for lithography,” says Panas.
Surprisingly, the MMA devices could be economical to produce once they are translated into monolithic chips. “A conventional DMD device has 20 layers. Our design has fewer than that,” says Panas. “We will be able to build all the mirrors on a single wafer simultaneously. We expect to get 20,000 to 30,000 mirrors per wafer.”
The micromirror array is composed of hexagonal unit cells, each of which contains three bipolar electrostatic comb drive actuator paddles, three decoupling flexure linkages, and a hexagonal mirror. Currently, the mirrors are fabricated in a batch process from an SOI wafer, where the device layer is cut into hexagonal patterns, gold coated, then released to form free mirrors that can be placed into the array.
Panas aims to eventually construct 10,000-element mirror arrays covering about a 10 cm2 surface. Moving in unison, the mirrors will be able to move as a single reflective device if desired. Each mirror can also be controlled independently to create sophisticated effects.
The performance of MMAs is characterized in terms of a speed-range product. Panas says the commercially available devices he’s examined have speed-range products about 100 times lower than that of LLNL’s device. The LLNL micromirrors can realize ±10° rotation and translations exceeding ±30 µm while working at about 40 kHz.
For comparison, DMD chips developed by Texas Instruments exhibit about a ±12° tilt, says Panas. But TI DMD mirrors measure only about 10 µm across where LLNL mirrors are about 1,000 µm. “The inertia difference between the two kinds of actuators goes to about the fourth or fifth power,” Panas explains. “We have something like a million time more inertia to drive. The tradeoff is that smaller devices can move much faster, but they lose the possibility of precise control – you are not exactly sure where your mirrors are pointing. But that works just fine in a digital device that is just on/off.”
Interdigitated electrostatic combs serve as combination actuator/sensors for torquing the mirrors. Attached to platforms holding the mirrors, these voltage-controlled actuators double as capacitive feedback position sensors. Measured capacitance is a function of the mirror angle and is sampled at a rate of about 100 kHz.
Of course, each mirror in an array would have three feedback control loops, one for each degree of freedom. That means a single 10,000-element array would have 30,000 feedback loops. Panas says the ancillary electronics necessary for all that motion control sets a lower limit on the size of an array that is feasible.
“It turns out that the way to solve the feedback control problem is with a hierarchical control scheme,” says Panas. “Each mirror has an area of about one-millimeter-square behind it. We fit a low-level feedback controller in that space which takes care of low-level high-speed control. We are using a distributed technique that eliminates the need to do everything from a single high-level controller.”
Panas thinks lidar is the most immediate application for the Lab’s mirror arrays. The arrays are capable of running up to 1,000 times faster and hitting much higher resolutions than conventional systems, performance that could lead to locating objects quickly and at greater distances than are practical today. The super-high controllability of the arrays could also let lidar systems focus on specific areas of interest in their field-of-view rather than scanning areas indiscriminately regardless of whether they contain data of interest.
Panas also says the arrays would be small enough to be integrated into vehicle taillights and other unobtrusive locations. Additionally, they could provide a lidar capable of a 360° field-of-view without requiring any mechanical moving parts, a feat difficult to realize today without resorting to motorized scanning. Panas says he has had discussions with lidar makers interested in licensing the LLNL technology.