A compact two-axis scanning-mirror assembly (see figure) has been developed for use as part of a laser rangefinder on a robotic vehicle. The assembly is designed to enable scanning of the laser beam throughout the space surrounding the vehicle for three-dimensional mapping of objects in the vicinity.
Two-axis scanning-mirror assemblies of prior design exhibit two notable weaknesses, one being actuator backlash caused by gear-train slop. The present assembly contains direct-drive actuators that exhibit little or no backlash; as a result, the precision and resolution of beam positioning is limited only by the resolution of a feedback subsystem that is part of a control system operated in conjunction with the assembly. Moreover, because no gear trains are included, the number of moving parts is reduced and thus reliability is increased, relative to assemblies of prior design. The other notable weakness of prior designs is inability to pan continuously; the present assembly includes sliprings, which enable continuous panning.
The assembly includes a base that consists of a housing for the electronic drive circuitry and for the laser system of the rangefinder. Built into the top of the housing is the stator of a pan motor. An integral part of the stator mount is a domelike structure that supports the upper end of a pan rotor subassembly. An upper support bearing and the stator of a slipring subassembly lie within the domelike structure near its top.
The pan rotor subassembly consists of the rotor of the pan motor and an arch that is part of a tilt subassembly. The arch is tipped by the rotor of the slipring subassembly. The arch spans the open bore of the rotor of the pan motor. Between the sides of the arch is the mirror, supported by a bearing on each end of a tilt axle. The mirror is driven in tilt by an ultrasonic motor (USM) on one end of the tilt axle.
The overall design was driven by the desire to build the smallest unit possible for a given mirror size [≈1.4 in. (≈3.6 cm) high by ≈2 in. (≈5.1 cm) wide] while making it possible to orient the mirror as precisely as practicable. To this end, a through-bore motor was chosen as the pan actuator. The open bore makes it possible to place part of the laser circuitry within the motor envelope, saving volume. The direct-drive, through-bore pan motor is capable of continuous pan motion and of angular positioning with relatively high resolution. Also, the direct-drive design eliminates mass and volume that would otherwise be associated with a gear train.
One important advantage of a USM is passive braking; when power is not applied, it holds its position. Hence, the use of the USM as the direct driver on the tilt axis also eliminates mass and volume that would otherwise have been associated with an external brake that would have to be put on a gear train. In addition, a USM weighs less than does a conventional electric motor.
This work was done by Brett Kennedy of Caltech for NASA's Jet Propulsion Laboratory.