Laser imaging system with adjustable optical element mounting fixture and related methods

A laser imaging system may include a laser source, a laser receiver, a rotatable base defining a rotation axis, and an optical element (OE) carried by the rotatable base in an optical path between the laser source and laser receiver. An adjustable OE mounting fixture may mount the OE to be adjustably movable with respect to the rotatable base in a plane transverse to the rotation axis. A controller may be configured to adjust the adjustable OE mounting fixture to provide scan angle compensation.

TECHNICAL FIELD

The present invention relates to the field of remote sensing devices, and more particularly, to laser based 3D imaging systems.

BACKGROUND

Remote sensing systems are used to acquire information about an object or target without making physical contact with the object. For example, optical sensing systems, such as Light Detecting and Ranging (LIDAR) systems, use reflected light to determine ranges to targets, as well as to perform imaging or mapping of the target terrain. LIDAR systems may be carried by an airborne platform (e.g., airplanes, satellites, etc.) to capture optical image data from overhead (e.g., nadir looking) views of a geographical area of interest. The raw image data captured using LIDAR, etc., may be processed into a desired format, such as a digital elevation model (DEM), for example.

Some LIDAR systems include an active illumination system which includes components to scan a laser (or other light source) beam over a target area. Such a configuration may be desirable in that increasing the speed of a scanner increases the area coverage rate of a sensor, thereby reducing operating costs. One example of a LIDAR system employing one or more rotating optical elements (e.g., mirrors, glass wedge (Risley) prisms or holographic optical elements (HOEs)) is the High Resolution Quantum LIDAR System (HRQLS) from Sigma Space Corporation of Lanham, Md. This system employs a pair of monolithic rotating glass Risley prisms. Another example is the Georgia Tech Research Institute (GTRI) bathometric LIDAR. This system employs a single rotating HOE with a fixed glass optical element for the transmit channel.

Despite the existence of such configurations, further enhancements for optical and laser imaging systems may be desirable in some applications.

SUMMARY

An imaging system may include a light (e.g., laser) source, a light (e.g., laser) receiver, a rotatable base defining a rotation axis, and an optical element (OE) carried by the rotatable base in an optical path between the light source and light receiver. An adjustable OE mounting fixture may mount the OE to be adjustably movable with respect to the rotatable base in a plane transverse to the rotation axis. A controller may be configured to adjust the adjustable OE mounting fixture to provide scan angle compensation.

More particularly, the OE may be aligned along an optical path segment from the light source. Furthermore, the light source and the light receiver may be in a coaxial arrangement.

The rotatable base may have an opening therein, and the adjustable OE mounting fixture may include a collar carried by the rotatable base within the opening thereof and configured to hold the OE therein, and a flexure mount carried by the rotatable base adjacent the opening and coupled to the collar. Furthermore, the flexure mount may include a solid monolithic body having a proximal end coupled to the rotatable base, a distal end coupled to the collar, and an intermediate portion having a reduced thickness providing flexibility for the flexure mount.

In addition, the adjustable OE mounting fixture may further include an arm coupled to the collar opposite the flexure mount, and an actuator carried by the rotatable base and coupled to the arm. The imaging system may also include a position sensor system carried by the rotatable base and configured to sense a position of the collar. More particularly, the position sensor system may include a main reflector carried by the collar, a reference laser carried by the rotatable base for directing a reference laser beam at the main reflector, and a linear detector array carried by the rotatable base for receiving reflected laser light from the main reflector and coupled to the controller. Additionally, at least one secondary reflector may also be carried by the rotatable base and in an optical path between the reference laser and the linear detector. The additional reflectors may be used to increase the optical path length between the reference laser and the linear detector, thus increasing the linear motion of the laser reference on the linear detector for a given angular offset at the main reflector mounted to the flexure mount. The imaging system may also include an inductive power interface carried by the rotatable base.

A method for using an imaging system, such as the one described briefly above, is also provided. The method may include operating the light source and light receiver, and adjusting the adjustable OE mounting fixture to adjustably move the OE with respect to the rotatable base in the plane transverse to the rotation axis to thereby provide scan angle compensation between the light source and light receiver.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring initially toFIG. 1, a laser imaging system30in accordance with an example embodiment is first described. In the illustrated example, the laser imaging system30is carried by an airborne platform, here an airplane31, although other platforms (e.g., satellites, helicopters, UAV's, etc.) may also be used. With respect to typical active imaging systems including a single or multiple optical elements with a common rotational axis, such as the above-described LIDAR systems, problems may arise with increasing scan rates. More particular, as the rotational speed of the element(s) increases and/or the slant range of the common line of sight (LOS) increases, as seen inFIG. 1, at some point the round trip time of a imaging pulse (i.e., from the time of transmission (t0) to the time of reception (t2)) will increase to the point that the optical receiver will no longer be pointed at the area illuminated by the transmitted pulse when the returning transmitted pulse reaches the sensor. As a result, despite the existence of a rotating scanner to increase coverage area, systems such as the above-noted LIDAR systems may still either have to operate at a relatively low altitude or a relatively low scan rate to keep the transmit and receive channels in synchronization.

Referring additionally toFIGS. 2-6, the system30may advantageously provide for real-time adjustment of transmit and/or receive optics to account for LOS differences in the transmit and receive channels of a laser scanner32. In the illustrated example, the laser imaging system30includes a laser source33, a laser receiver34, the laser scanner32, and a controller35coupled with the laser source, laser receiver, and laser scanner. The controller35may be implemented using appropriate hardware (e.g., a microprocessor, etc.) and associated non-transitory computer-readable medium having computer-executable instructions for causing the controller to perform the operations noted herein.

The laser scanner32illustratively includes a spider scanner mount36, a scanner stator assembly37coupled to the spider scanner mount, and a hub assembly38rotationally mounted on the scanner stator assembly. A laser receiving optical element (OE)39(here, a holographic beam steering optical element) and a clocking mechanism40are coupled to the hub assembly38with a series of components41including washers, retaining rings, and a spring, as well as with bolts42(although other coupling configurations may be used in different embodiments). A cover43is also provided for the clocking mechanism40. The clocking mechanism40includes an adjustable OE mounting fixture45which is carried by a housing46, as will be described further below.

The rotatable hub38defines a rotation axis44, and a transmit OE60(seeFIG. 6) is carried by the adjustable OE mounting fixture45so that the transmit OE is in the optical path between the laser source33and laser receiver34, along with the receive OE39. Moreover, the laser source33and laser receiver34may be in a coaxial arrangement, as in the illustrated embodiment. While the transmit OE60is illustratively shown as a single refractive wedge prism optical element herein, in some embodiments a single holographic wedge element or multiple holographic or wedge elements may used in series, for example.

The adjustable OE mounting fixture45is positioned on a base47(here a circular substrate) carried within the housing46. The adjustable OE mounting fixture45further illustratively includes a collar48carried by the base47and extending through an opening in the base which is aligned with an opening50in the rotating hub assembly38. The collar48is configured to hold the transmit OE60therein, and a flexure mount51is carried by the base47adjacent the opening50and coupled to the collar48. The flexure mount51illustratively includes a solid monolithic body52having a proximal end coupled to the base47(by bolts53in the illustrated example), a distal end coupled to the collar48, and an intermediate portion54(also referred to as a “flexure hinge” herein) having a reduced thickness providing flexibility for the flexure mount. It is the thin section of the intermediate portion54(e.g. circular contour flexure or perforated hinge flexure) that determines the center of rotation for the flexure mount.

The adjustable OE mounting fixture45further illustratively includes an arm56coupled to the collar48opposite the flexure mount51, and an actuator assembly carried by the base47and coupled to the arm. More particularly, the actuator assembly in the present example illustratively includes a gear motor58, a motor mount62for the gear motor carried by the base47, a shaft coupler64which is driven by the gear motor, and a preload block66carried by the base on an opposite side of the arm56. The collar48and arm56are also formed as an integral body (i.e., a single piece) with the monolithic body52and intermediate portion54in the illustrated example, although this is not required in all configurations).

The drive motor and gear box, or gear motor,58and preload block66cooperate to move the arm56up and down (as indicated by the double-headed dashed arrow), which in turn moves the collar48(and thus the transmit optical element60) relative to the rotating base47(base rotation is indicated by the arrow68inFIG. 4) in a plane transverse to the rotation axis44. In the illustrated example, the base47is a separate component that is fixedly mounted to the hub38and accordingly rotates at the same speed as the hub, but in other embodiments the base and hub may be integrally formed or a single unit. In the illustrated example, the collar48is orthogonal to the rotation axis44, and the plane through which the transmit optical element60rotates is therefore perpendicular to the rotation axis44, but in other embodiments the collar could be canted with respect to the rotation axis (i.e., non-orthogonal), if desired. The adjustable OE mounting fixture45further illustratively includes a flexure hard stop70, which may optionally be included to limit the amount of travel the arm56can move. By way of example, the present configuration is configured to provide for ±6 miliradians from a central starting position of the arm56, but smaller or greater ranges may also be used. Furthermore, the gear motor58configuration may allow for adjustments with a granularity on the order of several microradians (e.g., 10 microradians), although different adjustment increments may also be used.

A circuit board72and associated circuitry for controlling the gear motor58are carried by the base47on an opposite (back) side thereof. Along with the circuit board72, a position sensor system is also carried on the back side of the base47, which is configured to sense a position of the collar48. More particularly, the position sensor system illustratively includes a main reflector74(e.g., a mirror) carried by the collar48, a reference laser76carried by the base for directing a reference laser beam78at the main reflector, and a linear detector array80carried by the base for receiving reflected laser light from the main reflector.

The linear detector array80provides feedback to the controller35, which is used in adjusting the position of the collar48(and, accordingly, the transmit OE60). In the present example, a plurality of secondary reflectors77are also carried by the base47and in the optical path between the reference laser76and the linear detector80, which provides a greater optical path length, as will be appreciated by those skilled in the art. However, different numbers of secondary reflectors (or none at all) may be used in other embodiments. Furthermore, other types of position feedback sensor arrangements may used in different embodiments as well, as will also be appreciated by those skilled in the art.

By way of example, the controller35may communicate with the circuitry on the circuit board72via wireless (e.g., RF) communication to obtain the feedback data from the position sensor system, and to provide control commands to the gear motor58for adjusting the position of the collar48to thereby provide scan angle compensation. Power for the circuitry on the circuit board72, the position sensor system, and the gear motor58may be provided by an inductive power interface82carried by the scanner stator assembly37, and a corresponding power interface carried within the rotatable hub38(not shown).

Assembly of the transmit OE60within the collar48is shown inFIG. 6. In the illustrated example, a spacer84is positioned within the collar48, followed by the transmit OE60(here a refractive prism). In this embodiment the transmit OE60is bonded into the collar48, although other attachment arrangements may also be used.

As a result of the above-described configuration, the clocking mechanism40may advantageously be used to adjust the angular alignment between receive and transmit channels due to changes in angular offset between each optical channel and target, i.e., by rotating the transmit OE in a plane transverse to the rotational axis44of the hub38to make the adjustment. Moreover, the flexure system advantageously provides frictionless rotation adjustment via the flexure hinge54without the need for bearings, etc. Another advantage of the above-described approach is that it allows for dynamic alignment of transmit and receive channels, as well as greater operational flexibility to use faster scan rates and/or at higher altitudes. By way of example, the hub38may be rotated at speeds of 1500 RPMs or more. This accordingly allows for an increase in the area collection rate of the system30with respect to approaches such as those described above, which in turn may contribute to lower collection costs.

Furthermore, inductively powering the clocking mechanism40allows the clocking mechanism to operate on the spinning rotatable hub38without the need for slip-rings or cable connections, as will be appreciated by those skilled in the art. As noted above, real-time control of the clocking mechanism40may be provided via a low power RF link, thus avoiding the need to install control electronics on a rotating portion of scanner. The system30accordingly provides for relatively fast, high precision line-of-sight (LOS) positioning which may be used for various applications such as mapping and visualization.

It should be noted that, while the time of flight (TOF) compensation operations have been described herein with respect to the transmit path (i.e., by adjustment of the position of the transmit OE60), the compensation operations could instead (or in addition to) be accomplished in the receive path (i.e., through adjustment of a receive OE), as will be appreciated by those skilled in the art. Moreover, since the present approach uses independent transmit/receive apertures, the laser source33and laser receiver34may be arranged separately, instead of coaxially as discussed above.

A related laser imaging method using the laser imaging system30is also provided. The method may include operating the laser source33and laser receiver34, and adjusting the adjustable OE mounting fixture to adjustably move the transmit OE60with respect to the rotatable hub38in the plane transverse to the rotation axis44to thereby provide scan angle compensation between the laser source and laser receiver, as discussed further above.