Abstract:
A method for measuring a distance between an object ( 256 ) and a light source ( 230 ) and sensing an object orientation includes applying light from a plurality of sources ( 230, 240  or  810 ) on the object; detecting a reflected energy ( 270 ) level from an object; measuring the reflected energy level from the object; computing a distance calibration function; determining at least one measuring range ( 301 ) indicated by a minimum value ( 302 ) and a maximum value ( 304 ) within the distance calibration function; computing an angle calibration function indicating energy level relation sampled at predetermined time slots within periods of a modulation function; and modulating each of the plurality of light sources with the modulation function ( 435, 445 ) such that a total energy applied from the plurality of light sources on the object during the time of the light emission is represented by a light emission predetermined function.

Description:
FIELD OF THE INVENTION 
   This invention relates to an optical sensor and methods for non-contact measuring of a distance and orientation of a surface and more particularly for measurement using a fiber optics displacement sensor. 
   BACKGROUND OF THE INVENTION 
   Contactless distance measurements are widely used in the industry. Different technologies are deployed, dependant on the specific application needs. Technologies such as laser triangulation, confocal, fiber based, interferometer, and chromatic are common in the field of distance or displacement measurement, and are implemented by using optical methods. Each of the technologies are chosen to fit specific application requirements. For example, some of the computer-to-plate (CTP) imaging machines use the laser triangulation principle in order to focus the imaging head. One of the drawbacks of such a sensing device is the relatively high cost and form factor aspects, which impose substantial imaging head design constraints. 
   Another method of non-contact displacement measurement allowing small sensor dimensions is disclosed in U.S. Pat. Nos. 7,071,460 (Rush); 4,739,161 (Moriyama et al.); 5,017,772 (Hafle); and 4,801,799 (Tromborg et al.). All the disclosed patents use two or more optical fibers for measuring the distance to the media. Each of the patents disclosed are based on a predetermined media orientation in respect to the sensor. For applications where media orientation is not predetermined, for example, computer-to-plate (CTP) head calibration, such an assumption is not valid and it is impossible to accurately measure the distance to arbitrary oriented media. 
     FIG. 1A  shows the functionality of a sensor according to U.S. Pat. No. 4,801,799, with media  130  oriented at a 90 degree angle to the sensor optical axis.  FIG. 1B  shows media  130  oriented at an angle other than 90 degrees. In both cases, the distance between media  130  and the outlet of the optical fiber  110  is identical. 
   It is apparent from  FIG. 1A  that the light coming from the light source  100  through fiber  110  and lens  120  is reflected back from media  130 , which is oriented perpendicular with respect to the fiber light emission axis, and returns through lens  120  and fiber  140  to the light sensor circuitry  150 . In this case the quantity of the reflected light energy detected by the light sensor  150  is a function of media-to-fiber outlet distance. (See FIG. 4 of U.S. Pat. No. 4,801,799.) 
     FIG. 1B  shows the sensor functionality with a tilted orientation of media  130 . In this case the light sensor  150  will receive smaller amounts of the reflected light energy or none at all. As it can be seen from  FIG. 1B  even a slight change in the media  130  orientation angle might lead to the deviation of light sensor  150  output signal even though the distance between the media and the fiber outlet does not change. In other words the media  130  orientation angle may significantly change the light sensor  150  output signals thus increasing the measuring errors or making measurements impossible. 
   In head-to-media distance measurement applications, the orientation or shape of the surface, or head to sleeve distance measurement, often varies. In those cases the fiber sensor errors caused by media orientation or shape variances becomes a substantial disadvantage of the measurement sensor. Additionally, imaging head alignment deviations or sleeve or drum eccentricity changes are also affect media-to-sensor distance measurements. 
   SUMMARY OF THE INVENTION 
   An object of this invention is to provide a new and improved optical fiber displacement sensor device capable of providing measurements between sensor and non-flat shape surfaces or flat surfaces positioned within the range of angles between the surface and sensor axis. 
   Briefly, according to one aspect of the present invention a method for measuring a distance between an object and a light source and sensing an orientation of the object includes applying light from a plurality of light sources on the object; detecting a reflected energy level from an object; measuring the reflected energy level from the object; computing a distance calibration function indicating the relation to the distance between the object and the light source; determining at least one measuring range indicated by a minimum value and a maximum value within the distance calibration function; computing an angle calibration function indicating energy level relation sampled at predetermined time slots within periods of a modulation function; and modulating each of the plurality of light sources with the modulation function such that a total energy applied from the plurality of light sources on the object during the time of the light emission is represented by a light emission predetermined function. 
   The light sensor response signal measured at any fixed time within the modulation period T is a function of media-to-fiber distance and the present invention provides for measuring distance between the media and sensor even at the case where media is oriented at other than a 90 degree angle with respect to the optical fiber sensor light emission axis, or the media having a non-flat shape. The distance is determined by the light sensor response, measured at a fixed and predetermined point of modulation period, while angle direction and value is determined by a phase and amplitude of the signal. 
   In the case wherein the media is oriented perpendicular with respect to the light emission axis, then the sum of the emitted light measures will stay constant, regardless of the type of modulated or un-modulated light sensor response, which is a function of the media-to-fiber distance. In the case wherein the media is positioned non-perpendicular in respect to the light emission axis or has a non-flat shape, then the measured light sensor output signal will become modulated, and the phase and amplitude of this signal will point to the direction and value of the deflection angle respectively. Other advantages of the invention will become apparent from the following description and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are schematics of prior art illustrating sensor functionality; 
       FIGS. 2A-2C  are schematics illustrating sensor functionality of the invention; 
       FIG. 3  is a schematic illustrating a media to fiber distance response calibration function; 
       FIGS. 4A-4C  are timing diagrams illustrating various types of modulations; 
       FIG. 5  is a schematic illustrating response of a light sensor when media is orientation perpendicularly to the light emission axis; 
       FIGS. 6A and 6B  are schematics illustrating light sensor response while media orientation is non-perpendicular in respect to light emission axis; 
       FIGS. 7A and 7B  are schematics illustrating light sensor response while media orientation is non-perpendicular in respect to light emission axis; and 
       FIG. 8  is a schematic illustrating a sensor with plurality of light sources. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 2A-2C  are a schematic representations of optical fiber displacement apparatus according to the present invention, referred to in general by numeral  10 . Optical fiber displacement apparatus  10  consists of signal generator  205 , which generates control signals  215  and  220  for providing light modulation of light source  230  and light source  240 . Optical fibers  235  and  245  conduct light from the light sources  230  and  240  respectively. Lens for focus light beams comes from optical fibers  235  and  245  on media  256 , and light sensor  270  measures the reflected light from media  256  coming through the lens  250  and the optical fiber  260 . The reflected light through light sensor  270  is acquired by the acquisition circuitry  210  and analyzed. Sensor configuration might comprise more than two emitting fibers and the single light sensor  270 . 
   The signal generator  205  issues control signals  215  and  220  for light sources  230  and  240  respectively. For example, the light sources can utilize laser diodes. The control signals deliver electrical current which stimulates the energy emitted for each of the light sources. The emitted light energy related to the linear range of the light source characteristic is proportional to the light source current. To summarize, the emitted light from each of the light sources will change according to the respective light source current modulation function. 
   Let us assume that light energy E 1  of the light source  230  is pulse modulated as it is shown in  FIG. 4A  represented by function  445  (dashed). The light energy E 2  of the light source  240  is modulated by the similar pulses of the same amplitude and same duty cycle of 50%, but shifted by the length of one pulse (half a period) as is shown in  FIG. 4A  by function  435 . 
   The total light energy EΣ emitted from both light sources  230  and  240  for each modulation period may be written as follows: 
                   E   ∑     =           ∫   0     0.5   ⁢   T       ⁢         E   1     ⁡     (   t   )       ⁢           ⁢     ⅆ   t         +       ∫     0.5   ⁢   T     T     ⁢         E   2     ⁡     (   t   )       ⁢     ⅆ   t           =       E   1     +     E   2                 (   1   )               
And the light emitted power respectively will be:
 
                   P   ∑     =         E   ∑     T     =           E   1     +     E   2       T     =         E   1       0.5   ⁢   T       =         E   2         0.5   ⁢   T     ⁢               =   const                   (   2   )               
Equation (2) shows that the modulation signal described above provides a constant light power when it reaches the surface of the media.
 
   In the event media  256  is oriented perpendicularly in respect to the emitted light axis as is depicted in  FIG. 2A , the portions of the reflected light returning to the light sensor  270  through the lens  250  and the optical fiber  260  are equal due to the system symmetry. The reflection angle and power losses of each laser beam are equal, and the total reflected light power which is acquired by sensor  270  will stay constant as well. This is represented in Equation (3) below:
 
P S =kP 1 =kP 2 =const  (3)
 
wherein k represents the reflection coefficient. In other words, despite the fact that each light source power is modulated, the reflected power measured by sensor  270  will show a constant value during the light source modulation described above.
 
   It is well known in the industry that the constant power which is emitted from the light source and passed through the optical fiber being reflected from a perpendicularly oriented media and returned through the other optical fiber to the light sensor is a function of media to light source distance. The graphical representation of such a function is shown in  FIG. 3 , wherein the light sensor response, for example, is introduced by voltage signal V and the distance to media is introduced by X. Practically the linear range  301  of V=f(X) characteristic should be used. As an example three points; point a  302 , point b  303 , and point c  304 , are related to the linear range  301 . As it can be seen the distances Xa, Xb, and Xc are represented by respective sensor response Va, Vb, and Vc, wherein the Xa is the minimum distance detectable within the linear range and the Xc is the maximum distance respectively. 
     FIG. 5  shows a timing diagram for light sensor  270  response to signal V related to the distances Xa, Xb, and Xc when the total emitted power is constant and media is oriented perpendicular in respect to the light emission axis  254  ( FIG. 2A ). The solid line  510  shows the light sensor response time dependence Vb when media is located in the middle of the distance measuring range Xb. The lines  520  (dotted line) and  530  (dashed line) show the light sensor  270  responses Va and Vc when media is located at Xa and Xc respectively. It can be seen as well that for any moment of time tn the media to optical fiber sensor distance may be determined by the light sensor  270  response signal measured at that time tn. 
   When the media  256  is oriented at the angle θ to the vertical axis  255  as is depicted in  FIG. 2B  and in  FIG. 2C , the reflected light from media  256  will change the direction of reflection by the same angle θ (dashed lines).  FIG. 2B  shows media deflection in a positive direction (+θ) and  FIG. 2C  shows media deflection in a negative direction (−θ). The deflected media orientation will cause a misbalance of reflected portions of the light received by the light sensor  270  from light source  230  and light source  240 . As shown in  FIG. 2B , a portion of light from the light source  230  is reflected to the light sensor and is greater than the portion of reflected light from light source  240 . Consequently the reflections of the light source  230  will affect the light sensor  270  more than reflections from the light source  240 , thus resulting in a light sensor response signal modulated with frequency and phase of control signal  215 . 
     FIGS. 6A and 6B  shows the light sensor  270  response behavior while light control signals  215  and  220  are modulated as described and shown by  FIG. 4A , and media  256  is deflected from the vertical position  255  by the angle of (+θ). The acquisition circuitry  210  measures the light sensor  270  output signal synchronously with the modulation frequency. If media is oriented according to  FIG. 2B  and is aligned within the distance Xb, the measurements are performed at designated time slots denoted by t 1 , t 2  . . . In as illustrated in  FIGS. 6A and 6B  by solid line  510 . The measurements read at time slots t 1 , . . . t 2   n +1 (the odd time slots) will relate to the light source  230  reflections measurements, whereas the measurements read at time slots t 2 , . . . t 2   n  (even time slots) will relate to the light source  240  reflections respectively thus showing the different readings due to unequal reflected portions of light coming into the light sensor  270 . 
   If the media is shifted (moved with the same angle) to the position Xa, then due to the fact that the media deflection angle +θ is not changed, the portion of reflected from media light coming to the light sensor  270  from the light source  230  is still dominated upon the reflected light from the light source  240 . But because the Xa position is closest to the optical fiber sensor  202 , both of reflected portions will grow proportionally and the light sensor  270  response (dotted line  520 ) will be higher. Respectively the light sensor  270  response measured while media is shifted to the Xc position (dashed line  530 ) will be lower than measured in the middle range Xb (line  510 ). 
   If the deflection angle of media, shown on  FIG. 2C , is (−θ) a portion of light from the light source  240  is reflected to the light sensor  270 , and is greater than the portion of reflected light from light source  230 . Consequently, the reflections of the light source  240  will affect the light sensor  270  more than reflections from the light source  230 . Thus, the light sensor receives a higher portion of the signal modulated with the frequency and phase of control signal  220 . The solid line  510  of  FIG. 6B  shows the light sensor response measured while deflected by angle (−θ) when media is located in the middle range Xb distance from the optical fiber sensor  202 . As shown, the angle direction change from +θ to −θ leads to a phase change of the response signal. (The odd time slots are bigger than even ones while media is under the deflected angle +θ and the odd time slots are smaller than even ones while media is under the deflected angle −ƒ). 
   As described above, the deflection of media by −θ degrees to positions Xa and Xc will lead to respective rising ( FIG. 7B , line  520 ) or falling ( FIG. 6B , line  530 ) of the response curves relative to the middle range distance response curve ( FIG. 6B , line  510 ). This means that the same synchronous time slot measurement of light sensor  270  may be used for distance determination in spite of the media deflection. Additionally the difference between the odd and even time slot measurements or their respective relation determines the value of the deflection angle. 
     FIG. 4A  shows also the other modifications of modulation signal; triangular modulation depicted on  FIG. 4B  and curved modulation on  FIG. 4C  respectively. The solid lines  435  relate to the modulation of the light source  230  and the dashed lines  445  to the modulator light source  240 . The common feature of all of the modulation signals depicted in  FIGS. 4A-4C  can be represented by the following mathematical equation: 
                     ∑     i   =   1     n     ⁢       f   i     ⁡     (     ω   ⁢           ⁢   t     )         =   const           (   4   )               
Where i represents the index of a specific light source, and n the total number of light sources. Parameter ω is the angular frequency which indicates that modulation function is periodical.
 
     FIGS. 7A and 7B  show the response curves of light sensor  270  measured while media deflected by the angle of +θ located in positions Xa, Xb, and Xc and the light source control signals  215  and  220  are generated according to triangular  FIG. 4B  and  FIG. 4C  modulation modifications. Both of these types of modulation are complied with the Equation (4). 
     FIG. 8  shows the front end view of another sensor embodiment of the invention. This embodiment depicts more than two fibers used for modulation. This example describes three optic fiber pairs  810 ,  820  and  830  conducting light from the light sources and emitting light in three planes respectively, additionally a single optic fiber  840  is configured to conduct media reflections of reflected light to the light sensor  270 . The signal generator applies control signals to each of the light sources thus providing light modulation for each fiber of the pairs  810 ,  820 , and  830 . At the same time, the control signals provide modulation such that the total emitted light energy from all optical fiber pairs will be constant. See Equation 4. In this case the acquisition circuitry measures each pair as described above, thus providing distance and angle of media. The media orientation is finally defined by superposition of each plane measurements. 
   The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
   PARTS LIST 
   
       
         10  optical fiber displacement apparatus 
         100  light source 
         110  fiber 
         120  lens 
         130  media (object) 
         140  fiber 
         150  light sensor 
         202  optical fiber sensor 
         205  signal generator 
         210  acquisition circuitry 
         215  control signal 
         220  control signal 
         230  light source 
         235  optical fiber 
         240  light source 
         245  optical fiber 
         250  lens 
         254  light emission axis 
         255  vertical axis 
         256  media (object) 
         260  optical fiber 
         270  light sensor 
         301  linear range 
         302  point a (minimal point of the linear range) 
         303  point b (within the linear range) 
         304  point c (maximum point of the linear range) 
         435  modulated function for second light source 
         445  modulated function for first light source 
         510  light response for mid distance point 
         520  light response for maximal distance point 
         530  light response for minimal distance point 
         810  optical fiber pair 
         820  optical fiber pair 
         830  optical fiber pair 
         840  single optical fiber