CASCADED MIRROR ARRAY AND SCANNING SYSTEM THEREOF
1. A cascaded mirror array scanner system for scanning a light beam, the system comprising:
- a plurality of reflector units, each of the reflector units comprising;
an array reflector movable between a finite number of predetermined angular positions;
a support structure supporting the array reflector while allowing the array reflector to change angular position of the array reflector; and
a position limiting structure that limits movement of the array reflector to the finite number of the predetermined angular positions;
a control and driving device electrically connected to the plurality of the reflector units for generating a digital switching signal to each of the reflector units respectively to cause change of the angular positions of corresponding ones of the array reflector;
wherein the light beam is directed to pass by and be reflected by each of the array reflector in a sequential order.
A Cascaded Mirror Array optical scanning system applies an array of reflectors, with each reflector (called array reflector for convenience) movable between discretized angular positions and independently switched by digitized electrical signals, to process a light beam by cascaded reflections to generate desired deflection angle of the beam. The precision of the discretized angular positions of each array reflector is maintained by a support structure, which supports the reflector while allowing it to rotate or tilt, and a set of positioning limiting structure, which limits the allowable angular position of the array reflector between a finite number of angular positions corresponding to the discretized angular positions.
- 1. A cascaded mirror array scanner system for scanning a light beam, the system comprising:
a plurality of reflector units, each of the reflector units comprising; an array reflector movable between a finite number of predetermined angular positions; a support structure supporting the array reflector while allowing the array reflector to change angular position of the array reflector; and a position limiting structure that limits movement of the array reflector to the finite number of the predetermined angular positions; a control and driving device electrically connected to the plurality of the reflector units for generating a digital switching signal to each of the reflector units respectively to cause change of the angular positions of corresponding ones of the array reflector; wherein the light beam is directed to pass by and be reflected by each of the array reflector in a sequential order.
- View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
- 11. A reflector steering and scanning system comprising a first uneven-leg tilting mechanism for generating small angular changes of the reflector, the uneven-leg tilting mechanism comprises a base;
a first leg having an end jointed to one end of the base; a second leg having an end jointed to the other end of the base, the second leg and the first leg having a predetermined difference in length; and a top member having two ends jointed to the other end of the first leg and the other end of the second leg respectively; the reflector being disposed on the top member, thereby tilting the legs with respect to the base member resulting in small angular position changes of the top member and the reflector.
- View Dependent Claims (12)
This application claims the priority benefit of Taiwan Patent Application No. 107138562, filed on Oct. 31, 2018, in the Taiwan Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to a light beam scanning system, more specifically, a precision light beam scanning system of simple construction, fast response, high impact resistance and low cost based on non MEMS fabrication processes.
Vector scanning technology, also called point-to-point scanning or random-access scanning, has applications in vector graphics, laser marking, laser machining, laser additive manufacturing, LIDAR, and optical routing. Another emerging market is laser tracking/communication for UAVs (Unmanned Aerial Vehicles), automobiles, aircrafts and satellites, because the exponential rise of UAVs and increasing numbers of vehicles and satellites is creating overwhelming demand for data communication and straining radio frequency (RF) bandwidth capacity.
Galvano mirrors are the main commercial scanners and often used in industrial vector scanning. In a galvano mirror, a mirror is driven by a servo motor for fast and precise positioning. A galvano mirror has a wide scanning angle and can reach the scanning frequency of several kHz. An acousto-optic deflector is another type of scanner, which mainly uses the acousto-optic effect of special crystals to operate. This type of scanner can achieve an extremely high scanning frequency up to 100 MHz, but is confined to a small scanning angle. The acousto-optic deflector having an extremely high scanning frequency is suitable for material processing using a pulsed laser. However, these traditional scanners are quite expensive and huge, and not suitable for use in UAVs. Particularly, in a consideration of load and energy consumption, these traditional scanners are not suitable for small-sized UAVs.
On the other hand, microelectromechanical technology has potential in mass production of small and lightweight scanners at low cost, and the devices capable of performing vector scanning have also been commercialized. A microelectromechanical scanner typically includes a single mirror attached to a rotatable structure. However, in order to maintain precision, these devices still require complex circuits, built-in sensors, and driving control scheme for specific devices. Due to the complexity of the device structure, the manufacturing process and the need to use MEMS plants, a complete MEMS system including devices, driver electronics, and control software still costs a few thousand US dollars. (For examples, see product information of Mirrorcle Technologies, Inc. of Richmond, Calif., USA, company website and product brochure, http://mirrorcletech.com/pdf/MirrorcleTech_Device_Prices.pdf and of Sercalo Microtechnology Ltd., Switzerland, company website and product information, http://www.sercalo.com/product.php?idsubcat=9#product-23)
Still another type of beam scanning approach under research applies cascaded stages of liquid crystal (LC) cell and prism pairs to achieve digital beam scanning. The switchable LC cell acts as a polarization switch to control the state of polarization of a light while a passive birefringent material prism is used to steer the beam into one of the two scanning destinations. By cascading several polarization switch-prism pairs, multiple scan angles can be obtained. For examples, see Khan S. A. and Riza N. A. (2004), “Demonstration of 3-dimensional wide angle laser beam scanner using liquid crystal,” Opt. Express 12, 868, 2004 and McRuer et al. (1990), “Ferroelectric liquid-crystal digital scanner,” Optics Letters, 15, 1415, both papers are incorporated herein in entirety by reference. This approach differs from the MEMS approaches in two ways. First, MEMS fabs are generally not needed so the manufacturing process can potentially be less complicated. Second, each stage is controlled digitally, i.e. having only two switchable states, so that control can be significantly simplified. However, precision prisms are still needed in each stage and they are not cheap to make. Another issue is that polarization switching efficiency and light transmission through a LC cell are both limited. Therefore, light beam quality after passing many stages of LC cells and prisms could deteriorate, especially in later stages corresponding to wide angle deflections.
A different liquid crystal beam scanner approach applies a so called liquid crystal clad waveguide. Light is introduced into and propagating along a thin waveguide core with one surface of the core cladded with a surface layer of voltage-controlled liquid crystal that can change the phase delay of the light propagating in the waveguide. By patterning an electrode into the shape of a prism in the plane of the waveguide, the controlled liquid crystal can effectively tune the refraction index of the prism and achieve the function of deflecting the direction of the passing light. See Davis et al. (2010), “Liquid crystal waveguides: new devices enabled by >1000 waves of optical phase control,” Emerging Liquid Crystal Technologies V, ed. by Liang-Chy Chien, Proc. of SPIE Vol. 7618, 76180E-1. However, the limitation is that the light has to propagate within the thin, planar wave guide, which makes scanning in a perpendicular second direction difficult. To connect two devices for 2D scanning could also be difficult.
In addition, environmental and field operation issues are also important factors to consider. For example, moisture and temperature variations may have larger effect on liquid crystal devices than on mechanical devices. And noises from impact or external magnetic effect or statics could affect MEMS devices more than traditional scanners of heavier build.
Therefore, existing products or technologies still cannot quite satisfy the low-cost and light-weight requirements in commercial or consumer UAV applications. A low-cost, light weight scanner can have many new application areas. For example, multiple devices can be mounted onto a UAV, a car or in a robotic system for monitoring situations in different directions, not just for communications. This present invention provides a new digital scanning approach based on cascaded reflectors that can be made by simple, low-cost non-MEMS processes and has the potential of high speed, precision and light-weight.
In view of the above-mentioned conventional problems, an objective of the present invention is to provide a cascaded mirror array and a scanning system to solve the problem of inability to meet the requirements in low cost and light weight for applying in consumer products.
The basic principle of digital scanning by the Cascaded Mirror Array (CMA) is to apply an array of reflectors, with each reflector (called array reflector for convenience) movable between discretized angular positions and independently switched by digitized electrical signals, to process a light beam by cascaded reflections to generate desired deflection angle of the beam. The precision of the discretized angular positions of each array reflector is maintained by a support structure, which supports the reflector while allowing it to rotate or tilt, and a set of positioning limiting structure, which limits the allowable angular position of the array reflector between a finite number of angular positions corresponding to the discretized angular positions. The digitized switching electrical signal does not steer the reflector or conduct precision control. The signal merely deflects the reflector and the support and position limiting structure determine the discretized angular positions. In this way, the cost of electrical control can be minimized and the structures and precisions can be made and maintained by traditional manufacturing methods.
A light beam to be processed travels on a plane (the scan plane) through the CMA, passes by and is reflected by each of the reflector units in a sequential order. The overall deflection angle of the light beam on the plane by the CMA is the combined beam deflections at all the reflector units, provided that the discretized angular position changes of all the array reflectors are also on that plane. Because each array reflector can independently form a reflection angle, the combined deflections of all array reflectors can result in an overall beam deflection angle that is variable and adjustable within a set range.
Depending on the tilting angle of an individual reflector unit, two different constructions of deflection mechanism can be used to generate the required discretized, finite angular positions of the array reflector. For array reflectors of large tilting angles, a simple lever mechanism with a pivot that supports the array reflector and allows the array reflector to rotate or tilt about a fixed axis, e.g. mirror on a torsion beam or a pair of torsion hinges, with landing points of predetermined dimensions as position limiting structure can be applied. And electromagnetic driving similar to a MEMS galvano mirror can be used.
For small tilting angles of an individual reflector unit, a new type of micro-deflection mechanism, called Uneven-Leg Tilting (ULT) mechanism, is devised. The basic configuration of a ULT mechanism comprises 3 thin slab bar members and a base member jointed ends to ends to form a closed-loop. A top thin slab bar member is supported and jointed at two ends by two other thin slab bar members (called legs for convenience), which are attached to the two ends of the base member respectively, thereby forming a closed-loop structure. The two legs have a predetermined difference in length and are arranged basically in parallel to each other. Tilting the legs with respect to the base member results in slightly different rotation angles of the two legs, due to the length difference of the two legs, and the slightly different rotation angles of the two legs results in an even smaller rotation of the top member. By placing a mirror on top of the top member, a tilting of the ULT mechanism results in, in the transverse direction of the mirror'"'"'s lateral displacement, a differential displacement between the two ends of the top member that rotates the mirror by a small angle.
The joints between the thin slab bar members and the base member can be hinge-like flexible joints. In this case, the ULT mechanism is in principle similar to a 4-bar linkage. Alternatively, the ULT mechanism can also apply a construction similar to a clamped-flat-spring type flexural bearing. That is, the 4 corners of the basic 4-member structure can be made rigid but the two legs are made flexural to facilitate lateral displacement and reflector tilting.
Both the simple lever deflection mechanism and the ULT mechanism can be made to generate two finite angular positions for each reflector unit by positioning two landing points at two ends of the mechanism or the mirror to restrict the deflection range of the mechanism. Therefore, each reflector unit can deflect a light bean at two finite deflection angles. When values of the finite rotational angle corresponding to different reflector units in a CMA device are arranged as a geometric progression with a common ratio of 2, then the combinations of all possible reflection angles by the reflector units can result in different overall beam scanning angles as an arithmetic progression.
The ULT mechanisms can use an electromagnetic actuation arrangement to provide magnetic fields across the legs by either permanent or electro magnets. An electric current can be passed through the loop of a ULT mechanism, in perpendicular directions relative to the directions of magnetic field, by entering from one leg and flowing out of the other, to induce a lateral force to tilt the mechanism. Reversing the current direction reverses the directions of the force and the tilting.
When a light beam travels through a CMA device on a plane, the CMA can be a 1D scanner that deflects and scans the light beam on the plane. 2D scanning can be achieved by adding into the internal beam reflection path of a 1D scanning CMA configuration a second set of reflector units with their finite angular position changes oriented to deflect the light beam off the plane.
The ULT mechanism can also be used in an analog, i.e. not digital, optical deflector, especially for small angle deflection. A mirror or a prism can be attached at the top of the top member to deflect a reflected or transmitted beam. Lateral displacement of the mirror can be actuated manually, such as using a fine screw, or by electrical signals, such as controlling the current passing through the legs to regulate electromagnetic forces. A 2D deflector can be made by using two ULT mechanisms in cascaded arrangement with each scanning on a different plane.
A reflector unit in the CMA device can also have more than two finite deflection angles. For small deflection angles, a multiple-angle ULT mechanism can be constructed by combining and coupling two ULT mechanisms to provide 4 angular positions for a reflector unit. One approach is to apply a first ULT mechanism as an adjustable position-limiting structure, which can generate two sets of positional limits. A second ULT mechanism carrying the reflector mirror then uses the first ULT mechanism as positional limits. By actuating the two ULT mechanisms independently, four angular positions on the second ULT mechanism can be generated. Another approach is to use a first ULT mechanism as an adjustable base structure and build a second ULT mechanism, which carries the reflector mirror, on top of the first ULT mechanism. By actuating the two ULT mechanisms independently, this combined structure will also have four angular positions.
A CMA device can be further cascaded (staged) with other beam scanning or deflecting devices to increase scan angle or increase resolution. For example, a CMA device can be arranged after (i.e., downstream of) an analog fine-angle scanner to increase the latter'"'"'s angle of scan while still keeping its fine resolution. Two CMA devices of different resolution ranges can also be cascaded for the same purpose.
According to above contents, the cascaded mirror array and the scanning system of the present invention can have at least one of the following advantages.
First, the cascaded mirror array and the scanning system can use simple mechanism members to create the change in the deflection angle of the laser light beam, instead of using a micro-electromechanical process so that the precision required for the deflection angle can be achieved and the device manufacturing cost can be effectively reduced.
Secondly, the cascaded mirror array and the scanning system can use a structure in which the legs can be tilted to the landing points, and the electromagnetic driving manner of controlling the direction of the current to change different angle of the deflection mechanism, thereby improving operational convenience and response efficiency.
Furthermore, the cascaded mirror array and the scanning system of the present invention does not need to use complicated control circuits, so the overall volume and weight of the scanning device can be reduced, and weight reduction of the scanning device can be achieved.
Thirdly, the cascaded mirror array and the scanning system can form different scanning angles by connecting multiple cascaded arrays, or increase the scanning angle range by using the multi-angle deflection mechanism, thereby increasing the variety of the change range of deflection angles, and make the cascaded mirror array compatible with various scanning devices.
The structure, operating principle and effects of the present invention will be described in detail by way of various embodiments which are illustrated in the accompanying drawings.
The following embodiments of the present invention are herein described in detail with reference to the accompanying drawings. These drawings show specific examples of the embodiments of the present invention. It is to be acknowledged that these embodiments are exemplary implementations and are not to be construed as limiting the scope of the present invention in any way. Further modifications to the disclosed embodiments, as well as other embodiments, are also included within the scope of the appended claims. These embodiments are provided so that this disclosure is thorough and complete, and fully conveys the inventive concept to those skilled in the art. Regarding the drawings, the relative proportions and ratios of elements in the drawings may be exaggerated or diminished in size for the sake of clarity and convenience. Such arbitrary proportions are only illustrative and not limiting in any way. The same reference numbers are used in the drawings and description to refer to the same or like parts.
It is to be acknowledged that although the terms ‘first’, ‘second’, ‘third’, and so on, may be used herein to describe various elements, these elements should not be limited by these terms. These terms are used only for the purpose of distinguishing one component from another component. Thus, a first element discussed herein could be termed a second element without altering the description of the present disclosure. As used herein, the term “or” includes any and all combinations of one or more of the associated listed items.
It will be acknowledged that when an element or layer is referred to as being “on” or “connected to” another element or layer, it can be directly on or connected to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on” or “directly connected to” another element or layer, there are no intervening elements or layers present.
In addition, unless explicitly described to the contrary, the word “comprise/include” and variations such as “comprises/includes” or “comprising/including”, will be acknowledged to imply the inclusion of stated elements but not the exclusion of any other elements.
The construction of the CMA applies direct reflection by simple reflectors on simple mechanical structure so that light beam quality is affected to the minimum, control can be simplified and manufacturing can be done without the need of MEMS fabs. In comparison to an existing MEMS galvano mirror based on single reflector, the CMA concept applies simple deflection mechanisms based on easy to make position limiting structures to provide repeatable precision angular positions driven by digitized electrical signals from simple circuits.
As illustrated in
In general, the precision of the discretized angular positions of each array reflector is maintained by a support structure (rotatable member B0 and pivot P0 of reflector unit M0, or rotatable member B1 and pivot P1 of reflector unit M1), which supports the reflector while allowing it to rotate or tilt, and a set of positioning limiting structure (the two landing points), which limits the allowable angular position of the array reflector between the two angular positions. The mirror can be a discrete mirror or can be a coating on the rotatable member (B0, B1). The digitized switching electrical signal does not steer the reflector or conduct precision control. The signal merely deflects the reflector and the support and position limiting structure determine the discretized angular positions. In this way, the cost of electrical control can be minimized and the structures and precisions can be made and maintained by traditional manufacturing methods.
Depending on the tilting angle of an individual reflector unit, two different constructions of deflection mechanism can be used. For mirrors of large tilting angles, a simple lever deflection mechanism, e.g. mirror on a torsion beam or a pair of torsion hinges, and electromagnetic driving similar to a MEMS galvano mirror can be applied, for example, referring to Urey H. (2002), “Torsional MEMS scanner design for high-resolution display systems,” Optical Scanning II, Proc. SPIE Vol. 4773, pp. 27-23, Seattle, Wash. For mirrors of small tilting angles, using the simple lever mechanism will need very small height differences between the landing points, as shown in the example in Tab. 1, which shows landing points height differences of different digital positions in a CMA of N=12 mirrors and φ=30°. The height differences in the lower digital positions (i=0-6) go down below a few micrometers to sub-micrometers. The associated manufacturing cost will be high because tolerances have to be very tight to avoid significant angular errors, even if applying traditional manufacturing processes other than MEMS fabs. Traditional manufacturing processes such as regular machining and polymer injection molding have dimensional tolerances roughly between 10 to 100 μm, unless expensive grinding and polishing are involved, referring to Kalpakjian and Schmid 2010, Manufacturing Engineering and Technology, 6th ed., Chap. 40, pp. 1150-1151.
Therefore, to facilitate small tilting angles, a new type of micro-deflection mechanism, called Uneven-Leg Tilting (ULT) mechanism, is devised. Referring to
The ULT mechanism can have a very significant mechanical reduction effect on the rotation of the mirror on the top linkage member. Geometric analysis of
Wherein h1 is the length of leg L1, w is the length of the top member, Δw is the lateral displacement of the top member, θ1 is rotational angle of the first leg L1 and θ2 is the rotational angle of the second leg L2.
The ULT mechanisms can use an electromagnetic actuation arrangement as shown in
A 4-bit CMA scanner prototype was built as an example of implementation details, to demonstrate the feasibility of the proposed concept and to show that the proposed system can be made from common materials in a regular lab without using microfabrication processes.
The configuration of the prototype scanner is depicted in
The prototype was set up to deflect a diode laser beam onto a gridded paper target screen. A digital camera recorded position changes of the projected laser spot on the screen.
Other than the 4-bar linkage design of
It is desirable to have large electro-magnetic driving force in order to increase high frequency amplitude, scanning speed and frequency. Increasing electro-magnetic driving force can be achieved by using multiple current loops in the ULT mechanism, while keeping the supplied current and power unchanged.
Please refer to
The combination of the eccentric rod calibration and the ULT mechanism with reflector can be applied to reflector unit of mirror deflection angle up to about 10 milli-radians, based on analysis. For reflector units of larger deflection angles, simple lever mechanisms need to be used to carry the reflectors. The challenge is how to set/calibrate these large deflection angles precisely. A preferred design for setting and calibrating large deflection angles of mirrors carried on simple lever mechanisms is shown in
The precision of a cascaded mirror array scanner can be determined by a relationship between a position of a position limiting structure (such as the landing point) and the deflection mechanism (such as flexible joint or leg) carrying reflector. The repeatability is determined by a resting post of the deflection mechanism at the landing point. The reliability of the entire system can be determined by the fatigue limit of the deflection mechanism. The lateral displacement can be adjusted to achieve the required precision, and the adjustable tolerance mechanism can reduce the requirements in the machining tolerance. In order to achieve high repeatability, the mirror deflection mechanism must be repeatedly operated and the landing point and calibration position should be disposed stably. The deflection mechanism operates within the elastic range and the fatigue limit of the flexible material of the joint. In order to achieve high reliability, the deflection mechanism must operate within the fatigue limit of the flexible material of the joint. For the reflector unit, the estimated maximum stress is 7.5 MPa when the mirror lateral displacement is 80 μm. Generally, the fatigue limit of thermoplastic polyester elastomers is between 5 MPa and 11 MPa, the fatigue limit of the Du Pont Hytrel 7246 material is 11 MPa. For the reflector unit with large angle deflection, the deflection angle of the first leg L1 is in about the same magnitude. However, the deflection angle of the second leg L2 becomes several times larger so that material having higher fatigue limit should be used for the second leg L2. For example, the fatigue limit of ABS is as high as 22 MPa, the fatigue limit of polycarbonate is up to 39 MPa, and these high fatigue limits indicate the feasibility of reliability. Since the fatigue limit is related to the sample size, temperature, and the amount of the driving force affecting the bending torque, the design and material of the cascaded mirror array should consider the overall design of the reflector unit, and the actual fatigue limit of the used material should also be determined experimentally.
The above-mentioned embodiment of the cascaded mirror array is a one-dimensional scanning system. Two-dimensional scanning system can be achieved by placing reflector units for x-direction scanning on one side of the reflection path and units for y-direction scanning on the opposite side.
The ULT mechanism can be further modified to provide more than two discretized positions. One basic approach is to combine multiple 2-position mechanisms that can be controlled independently and then use the combined position/pose variations to generate multiple positions.
In broad sense, the second ULT mechanism (ULT22) in
To facilitate independent control of the two ULT mechanisms, the conductor loops in the two ULT mechanisms need to be separated. Conductor loops in the ULT mechanisms and magnets arrangement are basically same as previously mentioned embodiments. For the second example design, as shown in
The advantage of applying 4-position reflector units in a CMA scanning system is that for a required scanning resolution fewer reflector units will be needed. For example, to have a 1D CMA scanner of 1024 scanning spots, 10 array reflectors (210=1024) are needed if using 2-position reflector units. If using 4-position reflector units, then only 5 units (45=1024) are needed. This also reduce total system size of the CMA scanner.
The present invention disclosed herein has been described by means of specific embodiments. However, numerous modifications, variations and enhancements can be made thereto by those skilled in the art without departing from the spirit and scope of the disclosure set forth in the claims.
For example, although the embodiments of 1D CMA systems described above use a relay mirror at the opposing side of the array reflectors, the array reflectors can also be distributed and placed on two sides of the beam path and the relay mirror can be removed. In fact, the array reflectors do not even need to be distributed in a linear layout. The 1D reflector array can be distributed with turns, much like a polygon shape. These all help to reduce overall size or form factor.
For another example, the preferred embodiments of the ULT mechanism use flexural joints or legs to form an integral structure, to avoid relative movement between disjointed parts and poor repeatability, and the legs and members are of slab shape, for structure stability. However, the ULT mechanism can also be constructed using disjointed parts as long as good joints or bearing are applied.
For another example, the CMA scanning system can not only be applied to scan an outputting light beam or laser beam, it can also be applied to scan inputting light rays from a narrow field of view at different angles in imaging applications. Therefore, in the broadest sense, the meaning of the light beam includes light rays in a scenario of imaging application as well. A light sensor behind a CMA scanner can collect color or light intensity data coming from different distant spots at different angles relative to the CMA scanner. These color or intensity data can then be combined with the angular data to reconstruct a 1D or 2D image at a distance.