FIELD OF THE INVENTION

5 The present invention relates to the field of measurement systems and more specifically concerns an optical positioning system to determine the position of an object along X, Y and Z axes.

BACKGROUND OF THE INVENTION

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Positioning systems are used in various industries to measure the distance between an object and a reference point.

Typical optical systems include a light source emitting a laser beam onto a reflective i s surface and a detector reading the reflected laser beam. The delay or time difference, also known as "time of flight", between the emission of the laser beam and the detection of the reflected beam is used to measure the distance of the object from the source, the time of flight being directly proportional to the distance of the object from the source. In other words, the farther away the object, the longer it will take for the 0 laser to hit the object and reflect back.

Such systems are used, for example, in the construction industry, where the distance between two walls or the width of a window is routinely measured. Already known to 5 the Applicant are the following documents: US 6,963,409; US 7,199,883; US 7,238,932; US 7,365,854; US 6,017,125; US 2005/0007579; US 2010/027171 ; US 201 1/0001985 and US 201 1/0188055 While some systems are able to measure the lateral and transversal deviation (left- right and up and down movement) of a distant object, they are often complex and expensive. A simpler system that can determine the lateral, transversal and longitudinal position of a remote object is desirable, since such position can be use for controlling or stabilizing that object.

There is therefore a need for a contactless position measuring system capable of determining the position of an object in all three dimensions, that is along the X, Y and Z axes. Such a system should be robust, capable of withstanding tough industrial environments, vibrations, temperature variations and dust. It would also be desirable for such a system to be simple, cost effective and fast.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical positioning system and a method that satisfies at least one of the above-mentioned needs.

According to a first aspect of the invention, an optical positioning system is provided for determining an XYZ position of an object relative to a reference position. The system comprises:

- multiple reflective bars laterally aligned and spaced from each other on a planar, non-reflective surface. The bars have predetermined parameters and are located on the object, at least one of the bars having an angled lateral side;

- a scanning light source for periodically projecting a light beam towards said reflective bars. The light beam is projected with a scan angle on the surface and transversally across the reflective bars at a given angular speed. The system also comprises a trigger generator for generating a trigger signal including a trigger pulse;

- a detector for detecting a reflected light beam reflected from the bars and for generating a target signal, the target signal including target pulses, each corresponding to a corresponding one of the reflective bars, each pulse having a width; and a) a processing assembly including at least one port for receiving said trigger and target signals, a memory for storing the scan angle, angular speed and predetermined parameters ; and a processor for computing, based on the angular speed, the scan angle and the predetermined parameters an X coordinate using a time delay between the trigger pulse and one of the target pulses; a Y coordinate using a duty cycle of at least two pulses of said signal; and a Z coordinate using the width of at least two target pulses of said signal, thereby determining the XYZ position of the object.

Preferably, the reflective bars have a trapezoidal shape.

Preferably, the reflective bars are made of retroreflectors, for allowing the reflected light beam to be reflected back towards said light source and said detector.

Preferably, the scanning light source and the detector are located in proximate vicinity in a common enclosure.

Preferably, the light beam is projected normal to the reflective bars. Preferably, the system also comprises a central reflective bar laterally aligned and centered between the reflective bars, for providing a central pulse reference in the target signal. Preferably, the number of reflective bars is pair, half of the reflective bars being located on a right side of the central bar, the other half being located on a left side of the central bar. The processor computes the X, Y and Z coordinates of the object based on the target pulses associated with the two outermost reflective bars. Preferably, the processor corrects and verifies the computed XYZ position based on the target pulses of the other inner bars. Preferably, the trigger generator includes one photo-detector for generating the trigger pulse when hit by the laser beam. Still preferably, the trigger generator comprises an other photo-detector, for generating an other trigger pulse when hit by the laser beam, the processor computing the angular speed of the scanning light source based on the two trigger pulses.

Preferably, the detector is a narrow-angle photo-detector adapted to detect the reflected light beam from a Z distance varying between 0.5 and 15 meters.

Preferably, the processing assembly comprises an amplifying and conditioning module to amplify and condition the signal prior being processed by the processor.

Preferably, the processor includes a Digital Signal Processor.

According to another aspect of the invention, there is also provided a method for determining an XYZ position of an object relative to a reference position. The method comprises the steps of:

a) providing multiple reflective bars laterally aligned and spaced from each other on a planar, non-reflective surface. The bars have predetermined parameters and are located on the object, at least one of the bars having an angled lateral side; b) periodically projecting a light beam towards the reflective bars, the light beam being projected with a scan angle on said surface and transversally across the reflective bars at a given angular speed;

c) generating a trigger signal including a trigger pulse;

d) detecting a reflected light beam reflected from the bars;

e) generating a target signal based on the reflected light beam, the target signal including target pulses, each corresponding to one of the reflective bars, each pulse having a width; and

f) computing, based on the angular speed, the scan angle and the predetermined an X coordinate using on a time delay between the trigger pulse and at least one of the target pulses; a Y coordinate using on a duty cycle of at least two pulses of the target signal; and Z coordinate using the width of at least two target pulses of the target signal;

thereby determining the XYZ position of the object.

Preferably, in step b), the light beam is projected normal to the reflective bars.

Preferably, step c) comprises a step of generating an other trigger pulse. Preferably, the method comprises a step of computing the angular speed of the scanning light source based on the two trigger pulses.

Preferably, in step f). for each target pulses, a centroid value is used to compute the X, Y and Z coordinates.

Preferably, the method comprises a step of transmitting the XYZ position of the object computed in step f).

Preferably, the method comprises a step of computing an angular deviation of the object relative to the reference position based two of the target pulses. It should be noted that a reflective bar with angled lateral sides has a width which varies from the top to the bottom of the bar. Preferably, the light source is a rotating laser light source or an oscillating light source.

Preferably, the target comprises three reflective bars, the two external bars having an asymmetrical shape and the center bar having a rectangular shape.

Still preferably, the target comprises seven bars, including six asymmetrical bars and a rectangular center bar. Preferably, the system comprises means for generating a fixed trigger point. Preferably, the reference point is located at the light source.

Preferably, the position Z (equivalent to the displacement along the Z-axis) between the target and the source is determined by measuring the pulse width of the reflected beam, said pulse width being inversely proportional to the distance Z.

Still preferably, the position X of the target (equivalent to the displacement along the X-axis) is determined by calculating the delay between the time at which the light beam hits the center bar and the time at which the laser beam hits a trigger point located inside the light source enclosure.

Still preferably, the position Y of the target (equivalent to the displacement along the Y-axis) is determined using the duty cycle, (or ratio of high levels versus low levels of the signal), of the reflected light beam. The optical positioning system of the invention is advantageous in that it allows one to determine the 3D position of an object, that is, the position along X, Y and Z coordinates with a minimum of two reflective bars. The system does not need to be calibrated prior to being used, it is simple to implement and inexpensive. Another advantage of the positioning system is that it is relatively insensitive to dust accumulation on the light source or on the reflective bars. Provided there is a minimum signal amplitude, degradation of the laser intensity or dust accumulation will have little effect on the system. Other features and advantages of the present invention will be better understood upon a reading of the preferred embodiments thereof, with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic top perspective view of the optical positioning system according to a preferred embodiment of the invention. Figure 1A is a front view of a target with reflective bars, according to a preferred embodiment. Figure 2 is a schematic perspective view of the optical positioning system according to another preferred embodiment of the invention.

Figure 3 is a schematic perspective view of a light source and of a detector, according to a preferred embodiment.

Figure 4 is a front view of two reflective bars, according to a preferred embodiment. W

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Figures 5A, 5B and 5C are graphs representing a trigger signal, a target signal and the combined trigger and target signals, respectively, according to a preferred embodiment. Figures 6A is a schematic view of a set of reflective bars positioned at a first X coordinate. Figure 6B is a graph representing a target signal generated from a beam reflected from the bars of Figure 6A.

Figures 6C is a schematic view of a set of reflective bars positioned at a second X coordinate. Figure 6D is a graph representing a target signal generated from a beam reflected from the bars of Figure 6C.

Figures 6E and 6F also represents target signal generated from the reflective bars of Figures 6A and 6C respectively.

Figures 7A is a schematic view of a set of reflective bars positioned at a first Y coordinate. Figure 7C is a graph representing a target signal generated from a beam reflected from the bars of Figure 7A. Figures 7B is a schematic view of a set of reflective bars positioned at a second Y coordinate. Figure 7D is a graph representing a target signal generated from a beam reflected from the bars of Figure 7B.

Figures 8A is a schematic view of a set of reflective bars positioned at a first Z coordinate. Figure 8C is a graph representing a target signal generated from a beam reflected from the bars of Figure 8A.

Figures 8B is a schematic view of a set of reflective bars positioned at a second Z coordinate. Figure 8D is a graph representing a target signal generated from a beam reflected from the bars of Figure 8B. Figure 8E is a schematic top view of the target of Figure 8A and 8B, relative to the light source.

Figure 9A is a schematic view of a set of reflective bars rotated along the Z axis. Figure 9B is a graph representing a target signal generated from a beam reflected from the bars of Figure 9A.

Figure 10 is a block diagram of the optical positioning system, according to an embodiment of the invention.

Figure 1 1 is a side view of a lift truck, provided with an optical positioning system according to a preferred embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Optical position system

Referring to Figure 1 , an optical positioning system 10 according to a preferred embodiment of the invention is shown. The system includes multiple reflective bars 12, which are preferably part of a target, a scanning light source 14, a trigger generator 16, a detector 18, and a processing assembly 20. By multiple reflective bars, it is meant that the object must be provided with at least two reflective bars. The bars are positioned on the object for which the position is to be determined. The light source 14 projects a moving light beam 22 towards the reflective bars 12 and the detector 18 detects a reflected light beam 24. This reflected light beam 24 is then converted into a target signal which can be processed by the processing assembly 20, in order to determine the position of the bars 12, and thus of the object 11 , relative to a reference point 15. Throughout the description, it should be noted that the terms "lateral", "horizontal", "transversal" , "vertical" and "longitudinal" are used in relation to the bars of the target, that is, they refer to the orientation of the bars rather than to the position of the target with reference to the ground.

As best shown in Figure 1A, the system 10 includes multiple reflective bars 12, which are laterally aligned and spaced from each other on a planar, non-reflective surface 26. At least one of the bars is be provided with an angled lateral side 28. Preferably, both lateral sides of the bars are angled. While in the present case the bars have a trapezoidal shape, other shapes can be considered, as long as there is a variation in the slope of one of the lateral sides. The angled lateral side does not necessarily need to be straight, it can also be curved. For example, it can be convex or concave. As can be appreciated, the width of the bars 12 varies from top to bottom, and there is no symmetry axis about the X-axis. Each bar is defined by predetermined parameters, relative to the shape and dimension of the bars, and to the distance between the bars 12.

Preferably, the target includes more than two bars 2. Still preferably, the bars 12 are located on each side of a central reflective bar 30, said central bar 30 being also laterally aligned and centered between the reflective bars 12. The central bar 30 has perpendicular, non-angled lateral sides. In the present case, the central bar 30 has a rectangular shape.

The bars 12 are located on the object 1 1. If the object 1 1 already has a planar and non-reflective surface 26, the bars 12 can be simply affixed on the object 11. Otherwise, the reflective bars 12 can be affixed on a non-reflective material, for example, on a sheet or plate, forming with the reflective bars a target which can be affixed on the object 1 1 for which the position is to be measured. Preferably, the non- reflective surface 26 is black, and the reflective bars are white. The reflective bars 12 are preferably made of retroreflectors, allowing the reflected light beam to be reflected back towards the light source 14 and the detector 18, reducing the need for a perfect optical alignment.

While Figure 1 shows the light source 14, the detector 18 and the processing assembly 20 in a single enclosure, they can be distributed in distinct enclosures. In the case where the reflective bars are made of retroreflectors, the emitter 14 and detector 18 must be very close to each other.

As shown in Figures 1 and 2, the light source, or scanner, 14 and the detector 18 are preferably contained in a single casing. The scanning light source 14 periodically projects a light beam 22 towards the reflective bars 12 with a given scan angle Θ. The light beam is projected transversally across the reflective bars, at a given angular speed. The angular speed can be already known to the system 10, for example by using the speed of the motor rotating the light source, or it can be measured more accurately, using photo-detectors, as it will be explained more in detail below.

The light source 14 is preferably a rotating laser light source, but another moving light source could be used, such as an oscillating light source. In industrial applications, the light source is preferably a scanner emitting a laser light beam. Best shown in Figure 1 , the light beam is preferably projected normal, or perpendicularly, to the reflective bars. Referring to Figures 1 and 3, the trigger generator 16 is for generating a trigger signal, including a trigger pulse. Preferably, the trigger generator 16 includes a photo- detector 32 which generates the trigger pulse when hit by the moving laser beam 22. In the case where the scanning light source 14 is a rotating light source, the photo- detector 32a will generate a trigger pulse at each rotation of the laser light source. 1 1 reflective surface 26 is black, and the reflective bars are white. The reflective bars 12 are preferably made of retroreflectors, allowing the reflected light beam to be reflected back towards the light source 14 and the detector 18, reducing the need for a perfect optical alignment.

While Figure 1 shows the light source 14, the detector 18 and the processing assembly 20 in a single enclosure, they can be distributed in distinct enclosures. In the case where the reflective bars are made of retroreflectors, the emitter 14 and detector 18 must be very close to each other.

As shown in Figure 2, the light source, or scanner, 14 and the detector 18 are preferably contained in a single casing. The scanning light source 14 periodically projects a light beam 22 towards the reflective bars 12 with a given scan angle Θ. The light beam is projected transversally across the reflective bars, at a given angular speed. The angular speed can be already known to the system 10, for example by using the speed of the motor rotating the light source, or it can be measured more accurately, using photo-detectors, as it will be explained more in detail below.

The light source 14 is preferably a rotating laser light source, but another moving light source could be used, such as an oscillating light source. In industrial applications, the light source is preferably a scanner emitting a laser light beam. Best shown in Figure 1 , the light beam is preferably projected normal, or perpendicularly, to the reflective bars. Referring to Figures 1 and 3, the trigger generator 16 is for generating a trigger signal, including a trigger pulse. Preferably, the trigger generator 16 includes a photo- detector 32 which generates the trigger pulse when hit by the moving laser beam 22. In the case where the scanning light source 14 is a rotating light source, the photo- detector 32a will generate a trigger pulse at each rotation of the laser light source. 12

This trigger pulse will serve as a reference to compute the X position, or the lateral displacement, of the object, as it will be explained more in detail below. This photo- detector 32a is preferably located near the periphery of the window or aperture by which the laser beam exits the casing of the scanner 14. Preferably, the trigger generator 16 includes another photo-detector 32b, for generating another trigger pulse when hit by the laser beam 22. This second photo-detector 32b advantageously allows the processing assembly 20 to compute the angular speed of the scanning light source 14 based on the two trigger pulses. This second photo-detector 32b is preferably located at the periphery of the window or aperture of the casing of the scanner 14, opposite the first photo-detector 32a, along the light source trajectory. The distance between the two photo-detectors 32a, 32b in the casing being known, the time elapsed between the two trigger pulses allows to determine precisely the angular speed of the scanning light source 14. This is particularly advantageous in applications requiring that the position of the object be determined with a high degree of precision.

Still referring to Figures 1 and 3, the detector 18 is for detecting the reflected light beam 24 reflected from the bars 12, and for generating a target signal. The target signal will thus include target pulses, each corresponding to one of the reflective bars 12. Each target pulse will have a given width, which will vary according to the height at which the light beam 22 has crossed the bar 12.

The detector 18 includes a photosensitive element capable of reading back the reflected light beam 24 coming from the reflective bars 12, without being influenced by ambient or off-axis lights. The moving light reflected on bars 12 of the target produces a series of target pulses with a signature, or characteristics, unique to the position of the bars 12 along the three axes X, Y and Z. These characteristics include the width, or duration, of the pulses, or the time elapsed between two given pulses. The reflected beam 24 is detected and converted into a target signal which can be 13 analyzed by the processing assembly 20. Preferably, the scanning light source 14 and the detector 8 are located in proximate vicinity in a common enclosure 34. The detector 18 is preferably a narrow-angle photo-detector adapted to detect the reflected light beam from a Z distance varying between 0.5 and 15 meters. This interval can vary, by increasing or decreasing the power of the light source or the sensitivity of the detector, for example. It is provided with a 1 pixel camera having a narrow field of view. Preferably, the detector does not operate using direct reflections, since it would not be desirable in such an industrial environment. Of course, other types of detectors 18 can be considered, according to the requirements of the application in which the system 10 is to be implemented.

Referring to Figures 1 , 2 and 9, the processing assembly 20 includes at least one input port 36, a memory 38 and a processor 40. The input port(s) 36 is for receiving the trigger signal and the target signal. The memory 38 is for storing the scan angle Θ and the angular speed ω of the light source 14, and the predetermined parameters characterizing the reflective bars. These parameters can be for example, the slope of the lateral sides of the bars, the width at the base and summit of the bars and the distance between said bars at their base. As explained earlier, the angular speed ω can be pre-programmed based on the motor speed provided by the manufacturer of the motor, or measured during each oscillating or rotating period, using the photo- detectors 32a, 32b. The processor 40 computes the X, Y and Z coordinate of the object based on the angular speed w, the scan angle Θ and the predetermined parameters of the bars. Of course, the memory 38 can be included within the processor 40. Preferably, the processor includes a Digital Signal Processor, commonly referred to as DSP. The processing assembly 20 may also include an amplifying and conditioning module 42, in order to amplify and condition the target signal prior to being processed by the processor 40. The assembly 20 can also include a communication module 44 for transmitting the XYZ position of the object. This communication module 44 can also be used to exchange other types of information, for example relative to the motor of the light beam. The trigger and target signals can go through various algorithms to extract the three positions (Z, X, Y) at a high speed rate, such as 10 to 100 times per second for example. The processing assembly 20 can include modules for filtering the signal and averaging the measurements. The microprocessor 40 can also monitor the strength of the signals and send alarm signals if either the reflective bars 12, or the detector 18, requires maintenance. The microprocessor 40 can control the scanning light source 14 and the communication with an outside host. The measured position of the object can be communicated to a host computer via the communication module 44, which can include a serial link such as RS-232, RS-422, LIN-Bus, CAN-Bus, USB, or the like. The position can be sent on-demand or continuously.

Referring to Figure 4, an example of a target 13 provided with two reflective bars 12 is shown. Figures 5A represents a graph in time of the trigger signal generated by the trigger generator 16. Assuming the light source 14 is a rotating light source, it completes a rotation during a period T. In this graph, the first trigger pulse 46a of a given period T corresponds to the one generated by the photo-detector 32a, and the second trigger pulse 46b corresponds to the one generated by the photo-detector 32b.

Figure 5B represents a graph in time of the target signal generated by the detector 18, from reflections of the light beam 22 over the bars 12 of Figure 4. In this graph, the first target pulse 48 of a given period T corresponds to the one generated by the detector 18 on a first reflective bars 12, and the second target pulse 48b corresponds to the one from the other reflective bar 12. The object, and thus the target, has moved from one period to another, and this is why the width of the pulses varies. From the first period to the second, the target has moved in Z toward the light source 14, since the width of the pulses is larger. From the second to the third period, the target was 15 tilted in the X-Y plane, or in other words, about the Z axis, since the width of the pulse 48a is larger than the pulse 48b. Figure 5C represents a graph in time combining the trigger and the target pulses. With reference to Figures 6A to 8D, the computation of the X, Y and Z coordinate will be explained. Figures 6A to 6F illustrate how the X coordinate is computed using a time delay between the trigger pulse and one of the target pulses. Figures 7A to 7E illustrate how the Y coordinate is computed using a duty cycle of at least two pulses of the trigger signal. Figures 8A to 8E illustrate how the Z coordinate is computed using the width of at least two target pulses.

Referring to Figures 6A, 6C, 7A, 7B, 8A and 8B, another example of a target 13 is shown. Using multiple bars 12 will generate multiple pulses which can be used for redundancy, thus providing a more accurate measure by averaging readings of the pulse widths. In the present case, a pair number of reflective bars 12 are used, each with at least one angled side. Half of the reflective bars 12 are located on a right side of the central bar 30, the other half being located on a left side of the central bar 30. The processor can compute the X, Y and Z coordinates of the object based on the target pulses associated with the two reflective bars 12, and correct the position based on the target pulses of the other bars 12. In the present case, the target 13 includes six bars 12 having a trapezoidal shape, the center bar 30 being rectangular. The reflective bars are preferably white and 3000 times brighter than a perfect white diffusive surface. In this particular configuration, the target 13 is 300 mm wide and 150 mm high.

Figures 8A and 8B represents the target 13 at two different Z positions, the target 13b of Figure 8B being closer to light source 14 than the one of Figure 8A. Figure 8C and 8D represent graphs of the target signal detected and generated from the reflected light beam over the target of Figure 8A, and Figure 8B, respectively. When the light 16 beam 22 hits one reflective bar 12 of the target 13, the photo-detector detects a reflected light beam and generates a corresponding pulse 48. The pulse width w is inversely proportional to the distance Z of the target 13 from the light source 14. In other words, when the target 13 is closer to the light source 14, the pulse width w, or pulse duration, will be greater than when the target 13 is far away from the source 14. As it can be appreciated, the system 10 uses the principle according to which a bar 12 will appear smaller to the detector when it's farther away, in order to estimate the position Z of the target 13 from the emitting-detecting device 14, 18. Since the scan angle Θ, the angular speed co, the actual dimensions and the shape of the bars are known, the Z distance of the object from the reference point can be obtained based on the width, or duration, of at least two of the target pulses, using known trigonometric equations. Using the respective widths of more than two bars 12 advantageously provides redundancy in the distance measurement. Providing the object with a reflective central bar 30 between the reflective bars 12 advantageously allows the system 10 to determine the center of the target, even if some of the bars are out of the scanning zone, or "field of view", of the light source 14. Since the central bar 30 has a width smaller than the bars 12 with angled lateral sides, the system will detect a shorter pulse 50 from the reflection onto said central bar. It will therefore be able to determine which target pulse corresponds to which bar 12, based on this center pulse 50 associated with the center bar 30.

Determination of the X coordinate of the object will be explained with reference to Figures 6A to 6F. Figures 6A and 6C represent the target 13 at two different X positions, with reference to light source 14. Figures 6B and 6D represent graphs of the target signal detected and generated from the reflected light beam over the target of Figure 6A, and Figure 6C, respectively. With reference to Figure 6A, reference number 32 is a schematic representation of the photo-detector 32a used to generate 17 a fixed trigger point, or trigger pulse 46a. The photo-detector is preferably located near the light source 14, which, when hit by the rotating laser beam 22, generates a trigger pulse 46a. With reference to Figure 6B and 6C, it can be considered to use a center bar with a distinct shape (such as a rectangle, for example) allows the system 10 to detect the X coordinate, or lateral movement along the X-axis, of the target 13, by measuring the time delay from the trigger pulse 46A to the center pulse 50 regardless of any displacement along the Z-axis. At each rotation of the laser beam 22, a time delay can be measured, by calculating the difference between the time to at which the beam is reflected on the central bar and the time t-i at which the beam hits the photo- detector 32a. The X coordinate of the target can be computed, based on the speed of the light source and the time delay. In order to compute the X coordinate with more accuracy, another alternative is to use the mid-point between the centroids of the two pulses on each side of the central pulse. Referring to Figures 6E and 6F, the X coordinate of the target is obtained based on only two reflective bars 12a, 2b, without relying on the central bar 30. In this case, the centroid, or center values, of the two pulses 48a, 48b are used to determine the X coordinate of the object. The centroid of the other pulses can also be used to verify and validate the measured X position. Of course, it can be considered to use only one target pulse 48a or 48b, to determine the X coordinate of the object.

Determination of the Y coordinate of the object will now be explained with reference to Figures 7A to 7E. Figures 7A and 7B represent the target 13 at two different Y positions, the target 13 of Figure 7B being "higher", than the one of Figure 7A. Figure 7C and 7D represent graphs of the target signal detected and generated from the reflected light beam over the target of Figure 7A, and Figure 7B, respectively. 18

As introduced earlier, providing the reflective bars 12 with a lateral angled side, allows the Y coordinate, or transverse deviation (Y), to be measured. Since the ratio of dark to bright varies according to where the beam hits the target transversally, or in other words along the direction of the bars, as best shown in Figures 7A and 7B, this information can be used to measure the Y coordinate. In other words, movement of the target along the Y-axis can be obtained by measuring the duty cycle of at least two target pulses, regardless of the displacement along the Z-axis.

As shown in Figure 7D, if the laser beam 22 hits a lower zone of the target 13, each of the target pulses will have a "high level" portion, W _H L2, that is wider than the "low level" portion W _L L2, the "high level" part of the pulse corresponds to the pulse hitting the reflective bar. As shown in Figure 7C, if the laser beam hits a higher zone on the target 13, the duty cycle of the target pulses will be different, with narrower "high levels" (W _H u) and wider "low levels" (W _L i_i). Since the characteristics, or parameters, defining the bars are known, and the angular speed of the light beam 22 is also known, the Y coordinate of the target can be determined, by using the ratio of "high levels" versus "low levels", i.e. the duty cycle, of the target pulses.

In order to compute the Y coordinate with more accuracy, it is preferable to compute the duty cycle based on the widths of the low and high levels of more than two pulses. In this case, the ratio is obtained by dividing the sum of the widths of the high levels of the pulses, by the sum of the widths of the low levels.

With reference to Figure 9A, a target which is tilted in the X-Y plane is shown. Figure 9B represents the target signal generated from the reflected beam over the target of Figure 9A. As it can be appreciated, the duty cycle of the target pulses vary from the rightmost to the leftmost pulse. The variation of the duty cycles can be used to compute the angular deviation Ω of the object based on the respective widths of two of the target pulses. It can also be used to cancel the effect of the rotation of the 19 target on the Z axis (twisting of the load) by comparing the duty cycles on the left side of the central bar 50 versus the duty cycles of the right side and by applying a compensation factor. Referring to Figure 10, components of the system 10 according to a preferred embodiment are illustrated. The system 10 comprises a target 13 formed by multiple reflective bars 12 aligned on a non-reflective surface 26. A scanning light source 14 emits a light beam 22 towards the target 13 using a laser source 52 and rotating mirrors 56, thanks to a motor 54 controlled by the processor 40. The reflected light beam 24 is detected through a lens 58. A module 42 to amplify and condition the signal detected is provided, and the target signal indicative of the position of the target 13 relative to a reference point is generated. Preferably, the reference point corresponds to the rotating axis of the light source 14. The system preferably includes a communication module 44 to communicate with external devices.

The system 10 will preferably go through an alignment process at the factory to ensure that the detector 18 is able to detect a laser reflection at the desired distance. Such an alignment can be made by aligning a view cone of the detector 18 over the target area. When installed, on a customer application or product, the enclosure containing the source 14 and detector 18 should be aligned such that the laser beam hits within a predetermined scan zone band. Both alignments are approximate and not critical, as long as they remain stable.

Container handler trucks must often stack loads of up to 30 metric tons at heights of around 12 meters. Precise positioning of the lift elevator is critical, so that the operator can evaluate if he has the necessary clearance to unload the containers.

Referring to Figure 1 1 , an application of the optical positioning system is shown. A lift truck 60 is provided with the optical positioning system 10. The reflective bars 12, 20 which are preferably part of a target 13, are positioned on one of the forks 62 of the lift truck 60. The light source 14, trigger generator 16 and the detector 18 are positioned on the truck 60, near the ground, at a predetermined offset distance 64. The processing assembly is in a separate enclosure, and can be linked to the control 5 system of the truck 60.

In summary, the system 0 can determine the position along any or all of the X, Y and Z axes of an object relative to a reference point (such as the detector for example) using the width, time delay and duty cycle of target pulses resulting from reflection of l o the moving light beam 22 on the reflective bars 12, as described above.

Method for determining the XYZ position of an object

According to another aspect of the invention a method for determining the XYZ 15 position of an object is also provided. The method will be explained with reference to Figures 1 to .

The first step of the method is to provide the object with at least two reflective bars 12. The reflective bars 12 must be placed on a planar and non-reflective surface. 20 Each bar 12 has at least one angled lateral side, and preferably both. The shape and dimension parameters of the bars 12 are known and predetermined.

The next step is to project a light beam 22, and preferably a laser light beam, towards the reflective bars 12. The light beam 22 is projected periodically, at a known scan 5 angle Θ and at a given angular speed ω. The light beam 22 is projected transversally across the reflective bars 12, preferably normally to the surface 26. 21

The method also includes a step for generating a trigger signal including a trigger pulse 46a. This trigger pulse 46a is generated during each period, or in other words, each time the light beam is projected towards the reflective bars 12. Preferably, two trigger pulses 46a, 46b are generated during a period, for computing precisely the 5 angular speed ω of the scanning light source 14.

The reflected light beam 24, reflected from the reflective bars 12, is then detected. A target signal is then generated from this reflected light beam 24. The target signal includes target pulses 48a, 48b, each corresponding to one of the reflective bars 12. 10 Each pulse is defined by a width w, which corresponds to a duration during which a reflection is detected.

Based on the angular speed ω, the scan angle Θ and the predetermined parameters of the reflective bars, the X, Y and Z coordinates of the object are computed. For a i s given period, the X coordinate is computed based on the time delay between the trigger pulse and one of the target pulses. For the same period, the Y coordinate is computed based on the duty cycle, or ratio of high versus low level, of the target signal, using two target pulses. The Z coordinate is computed using the width of at least two of the target pulses. Once the X, Y and Z coordinates have been computed, 0 the XYZ position is determined.

Preferably, the angular deviation of the object can be computed based on the widths of two of the target pulses. This deviation can be cancelled out by applying a compensation factor in the computations, this step being performed prior the 5 computation of the X, Y and Z coordinated. 22

Of course, it is preferable to use several reflective bars, in order to have several target pulses on which the computations can be based. Preferably, a centroid value of the target pulses is used for the computations. It will be appreciated that the reflective bars 12 have no characters encoded within them, such as know bar codes, but that the information is embedded in the shape, width and spacing of the bars 12. Advantageously, the reflective material and the shape, or pattern, of the reflective bars is designed so that the system 10 is insensitive to rotation of the target along the three axes. Indeed, when the light beam hits the reflective bars 12, the light beam 24 is reflected back towards the source 14, regardless of the angle of the target 13. In order to cancel the effect or rotation of the target on the Z axis the duty cycles of the left side of the central bar of the target can be compared to the duty cycle on the right side. Advantageously, the target is passive, has no power source and emits no light.