Abstract:
Laser power control arrangements interrupt power to a laser used in electro-optical readers upon detection of operating conditions not conforming to preestablished standards, and adjust power to the laser to enhance reader performance without violating prevalent safety standards. Dual monitors are used to independently monitor the output power of the laser.

Description:
BACKGROUND OF THE INVENTION 
   Various electro-optical systems or readers have been developed for reading indicia such as bar code symbols appearing on a label or on a surface of an article. The bar code symbol itself is a coded pattern of graphic indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers function by electro-optically transforming the pattern of the graphic indicia into a time-varying electrical signal, which is digitized and decoded into data relating to the symbol being read. 
   Typically, a laser beam from a laser is directed along a light path toward a target that includes the bar code symbol on a target surface. A moving-beam scanner operates by repetitively sweeping the laser beam in a scan line or a series of scan lines across the symbol by means of motion of a scanning component, such as the laser itself or a scan mirror disposed in the path of the laser beam. Optics focus the laser beam into a beam spot on the target surface, and the motion of the scanning component sweeps the beam spot across the symbol to trace a scan line across the symbol. Motion of the scanning component is typically effected by an electrical drive motor. 
   The readers also include a sensor or photodetector which detects light along the scan line that is reflected or scattered from the symbol. The photodetector or sensor is positioned such that it has a field of view which ensures the capture of the reflected or scattered light, and converts the latter into an electrical analog signal. 
   In retroreflective light collection, a single optical component, e.g., a reciprocally oscillatory mirror, such as described in U.S. Pat. No. 4,816,661 or U.S. Pat. No. 4,409,470, both herein incorporated by reference, sweeps the beam across the target surface and directs the collected light to the sensor. In non-retroreflective light collection, the reflected laser light is not collected by the same optical component used for scanning. Instead, the sensor is independent of the scanning beam, and has a large field of view so that the reflected laser light traces across the sensor. 
   Electronic control circuitry and software decode the electrical analog signal from the sensor into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal generated by the photodetector may be converted by a digitizer into a pulse width modulated digitized signal, with the widths corresponding to the physical widths of the bars and spaces. Alternatively, the analog electrical signal may be processed directly by a software decoder. See, for example, U.S. Pat. No. 5,504,318. 
   The decoding process usually works by applying the digitized signal to a microprocessor running a software algorithm, which attempts to decode the signal. If a symbol is decoded successfully and completely, the decoding terminates, and an indicator of a successful read (such as a green light and/or audible beep) is provided to a user. Otherwise, the microprocessor receives the next scan, and performs another decoding into a binary representation of the data encoded in the symbol, and to the alphanumeric characters so represented. Once a successful read is obtained, the binary data is communicated to a host computer for further processing, for example, information retrieval from a look-up table. 
   Reading performance is a function of many factors, one of which is power output of the laser. Reading performance is enhanced when the laser power output is increased. Yet, stringent safety standards dictate the maximum power output of the laser. Also, reader malfunction must be reliably monitored. 
   SUMMARY OF THE INVENTION 
   One feature of the present invention resides, briefly stated, in laser power control arrangements in electro-optical readers for reading indicia, such as bar code symbols, by generating a laser beam having an output power upon energizing a laser, by monitoring various operating conditions of the reader, and by controlling the output power of the laser beam as a function of each monitored operating condition. 
   In one arrangement, the electrical current passing through the laser and/or the output power level of the laser is directly monitored and, if preestablished settings are not met, the laser is deenergized. In another arrangement, the laser includes a laser diode, and a monitor photodiode for monitoring the output power of the laser. If preestablished settings for the monitor photodiode are not met, the laser is deenergized. In yet another arrangement, a redundant auxiliary photodiode, independently of the monitor photodiode, is operative for monitoring the output power of the laser. 
   Whenever the laser is deenergized, this signifies that a reader malfunction has occurred, or may be imminent, in which case, removal of the laser from its source of power discontinues the generation of the laser beam and serves as a safety measure. Whenever the laser power is increased, this boosts reader performance. 
   The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an electro-optical reader in accordance with the prior art; 
       FIG. 2  is a circuit schematic depicting laser power control arrangements in accordance with the present invention especially useful in the reader of  FIG. 1 ; 
       FIG. 3  is a diagrammatic view of additional laser power control arrangements in accordance with the present invention; 
       FIGS. 4A-4E  are diagrammatic views of details of still more components for additional laser power control arrangements in accordance with the present invention; and 
       FIG. 5  is a diagrammatic view of yet more components for an additional laser power control arrangement in accordance with the present invention 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As used herein, the term “symbol” broadly encompasses not only symbol patterns composed of alternating bars and spaces of various widths as commonly referred to as bar code symbols, but also other one- or two-dimensional graphic patterns, as well as alphanumeric characters. In general, the term “symbol” may apply to any type of pattern or indicia which may be recognized or identified either by scanning a light beam and detecting reflected or scattered light as a representation of variations in light reflectivity at various points of the pattern or indicia.  FIG. 1  shows an indicia  15  as one example of a “symbol” to be read. 
     FIG. 1  depicts a handheld laser scanner device  10  for reading symbols. The laser scanner device  10  includes a housing having a barrel portion  11  and a handle  12 . Although the drawing depicts a handheld pistol-shaped housing, the invention may also be implemented in other types of housings such as a desk-top workstation or a stationary scanner. In the illustrated embodiment, the barrel portion  11  of the housing includes an exit port or window  13  through which an outgoing laser light beam  14  passes to impinge on, and scan across, the bar code symbol  15  located at some distance from the housing. 
   The laser beam  14  moves across the symbol  15  to create a scan pattern. Typically, the scanning pattern is one-dimensional or linear, as shown by line  16 . This linear scanning movement of the laser beam  14  is generated by an oscillating scan mirror  17  driven by an oscillating motor  18 . If desired, means may be provided to scan the beam  14  through a two-dimensional scanning pattern, to permit reading of two-dimensional optically encoded symbols. A manually-actuated trigger  19  or similar means permit an operator to initiate the scanning operation when the operator holds and aims the device  10  at the symbol  15 . 
   The scanner device  10  includes a laser source  20  mounted within the housing. The laser source  20  generates the laser beam  14 . A photodetector  21  is positioned within the housing to collect at least a portion of the light reflected and scattered from the bar code symbol  15 . The photodetector  21 , as shown, faces toward the window  13  and has a static, wide field of view characteristic of the non-retro-reflective readers described above. Alternatively, in a retro-reflective reader, a convex portion of the scan mirror  17  may focus collected light on the photodetector  21 , in which case the photodetector faces toward the scan mirror. As the beam  14  sweeps the symbol  15 , the photodetector  21  detects the light reflected and scattered from the symbol  15  and creates an analog electrical signal proportional to the intensity of the collected light. 
   A digitizer (not shown) typically converts the analog signal into a pulse width modulated digital signal, with the pulse widths and/or spacings corresponding to the physical widths of the bars and spaces of the scanned symbol  15 . A decoder (not shown), typically comprising a programmed microprocessor with associated RAM and ROM, decodes the pulse width modulated digital signal according to the specific symbology to derive a binary representation of the data encoded in the symbol, and the alphanumeric characters represented by the symbol. 
   The laser source  20  directs the laser beam through an optical assembly comprising a focusing lens  22  and an aperture stop  23 , to modify and direct the laser beam onto the scan mirror  17 . The mirror  17 , mounted on a vertical shaft and oscillated by the motor drive  18  about a vertical axis, reflects the beam and directs it through the exit port  13  to the symbol  15 . 
   To operate the scanner device  10 , the operator depresses trigger  19  which activates the laser source  20  and the motor  18 . The laser source  20  generates the laser beam which passes through the element  22  and aperture  23  combination. The element  22  and aperture  23  modify the beam to create an intense beam spot of a given size which extends continuously and does not vary substantially over a range  24  of working distances. The element and aperture combination directs the beam onto the rotary mirror  17 , which directs the modified laser beam outwardly from the scanner housing  11  and toward the bar code symbol  15  in a sweeping pattern, i.e., along scan line  16 . The bar code symbol  15 , placed at any point within the working distance  24  and substantially normal to the laser beam  14 , reflects and scatters a portion of the laser light. The photodetector  21 , shown mounted in the scanner housing  11  in a non-retro-reflective position, detects the reflected and scattered light and converts the received light into an analog electrical signal. The photodetector could also be mounted in a retro-reflective position facing the scan mirror  17 . The system circuitry then converts the analog signal to a pulse width modulated digital signal which a microprocessor-based decoder decodes according to the characteristics of the bar code symbology rules. 
   As shown in  FIG. 2 , the laser source  20  includes a laser diode  25  and a monitor photodiode  26  operative for monitoring the output power of the diode  25 . The monitor photodiode  26  is part of a feedback circuit operative for maintaining the laser output power constant. The feedback circuit includes a comparator  27  having a reference voltage applied to a positive input of the comparator through a voltage divider comprised of resistors  28 ,  29 . The monitor photodiode  26  is connected to a negative input of the comparator via a resistive network including resistors  30 ,  31 . The output of the comparator  27  is conducted through a resistor  32  and capacitor  34  to a gate G of a field effect transistor (FET)  33 . The drain output of the FET  33  is connected to the laser diode  25 . The source output of the device  33  is connected to ground through a current sense resistor  35 . 
   The circuit of  FIG. 2  is conventional in that the monitor photodiode  26  detects changes in output power of the laser beam emitted by laser diode  25  and sends a feedback signal to the comparator  27  for driving the FET  33  to allow more or less current to pass through the current sense resistor  35  and, in turn, through the laser diode  25 . The greater this current, the greater the laser output power, and vice versa. 
   A current sense comparator  36  has one input connected to the current sense resistor  35  to monitor the current flowing therethrough, and another input connected to a reference voltage that corresponds to the maximum current allowable through the resistor  35 . The output of the comparator  36  is connected to an OR gate  37  which, in turn, is connected to a latch  38  and a switch  39 , which is connected between a power supply  40  and the laser diode  25 . If the comparator  36  senses that the current passing through the resistor  35  exceeds a maximum preestablished value, then an output control signal is conducted to the gate  37  and, in turn, to the latch  38  for opening the switch  39  to remove the power source  40  from energizing the laser diode  25 . 
   In further accordance with  FIG. 2 , a window comparator  41  is connected to the resistor  32  and monitors the voltage being applied to the gate G of the FET  33 . A maximum gate voltage and a minimum gate voltage are also applied to the window comparator  41 . The comparator  41  is, in turn, connected to the OR gate  37 . If the comparator  41  senses that the gate voltage being applied to the gate G is greater than the preestablished maximum gate voltage, or is less than the preestablished minimum gate voltage, then a signal is sent to the OR gate  37  to operate the latch  38  and open the switch  39 , thereby deenergizing the laser diode. Thus, power is removed from the laser diode  25  in the event of malfunction or failure of the monitor photodiode  26 , the FET  33 , the comparator  27 , the laser diode  25 , or any circuit connection. 
   More specifically, the  FIG. 2  circuit removes the power source  40  from the laser  20  after detecting an out-of-range condition in the error amplifier  27  that controls the output power of the laser. This circuit will remove power from the laser under the following conditions: 
   A failure of the device  33  in the output of the laser drive causes excess current to flow through the laser, thereby causing the laser output to exceed the factory set limit. 
   The monitor diode  26  connection is lost due to a device  33  failure or a circuit connection failure. 
   The laser fails and the laser drive current significantly increases as resistor  35  is used to sense a high current drive condition. 
   Advantageously, a timer could be added to the  FIG. 2  circuit to remove power only when a malfunction persists for a predetermined time. 
   As shown in the arrangement of  FIG. 3 , the laser source  20  is connected to the power source  40  by the switch  39  under control of a microcontroller  44 , preferably the same component that decodes the symbol and controls overall reader operation. A temperature sensor  43  is connected to the microcontroller  44  for monitoring the ambient temperature of the reader, preferably in the vicinity of the laser source  20 . If the monitored temperature exceeds a preset value, the microcontroller  44  opens the switch  39  to protect the laser source. A laser regulator  45  is connected to the laser source  20  and enables the microcontroller  44  to monitor the laser current and/or the laser output power and, if those values or other laser settings are outside preestablished values for these parameters, then the microcontroller  44  also opens the switch  39  to protect the laser source. 
   The aforementioned motor  18  for oscillating the scan mirror  17  in alternate circumferential directions denoted by the double-headed arrow  46  is under the control of a motor regulator  42  and the microcontroller  44 . The microcontroller itself monitors the amplitude of scan angle A and the frequency of oscillation at which the mirror  17  is oscillated. If these values or other motor settings are outside preestablished values for these parameters, then the microcontroller  44  opens the switch  39 . The microcontroller  44  can store the preestablished values, or it can communicate with a remote host  47  to retrieve the preestablished values, or updated values, or communicate to the host the presence of a fault condition, such as a laser or motor fault, or, at the request of the host, communicate operating parameters of the system such as motor frequency, temperature, and/or laser power, or the host can attempt to resolve such problems by initiating and controlling a system calibration episode to correct such faults, i.e., reduce laser power, or increase scan amplitude. 
   Other arrangements for removing power from the laser source  20  are depicted in  FIGS. 4A-E . In each figure, the laser diode  25  emits the laser beam toward the focusing lens  22 , and the monitor photodiode  26  is positioned behind a rear facet of the laser diode  25  to monitor the output power level. 
   In  FIG. 4A , an auxiliary photodiode  48  is positioned behind a beam splitter  49  to monitor the outgoing laser beam. In  FIG. 4B , a diffractive structure is located on the focusing lens  22  to direct a part of the outgoing beam to the auxiliary photodiode  48 . In  FIG. 4C , the auxiliary photodiode  48  itself is shaped as an annulus to serve as the aperture stop  23 , and the walls bounding the aperture receive the outgoing beam for detection by the auxiliary photodiode. The auxiliary photodiode  48  is redundant to the monitor photodiode  26  and is especially useful when the monitor photodiode  26  fails or loses sensitivity. In  FIG. 4D , an auxiliary photodiode is not used, but a light scattering surface  50  is provided inside the laser source to reflect a part of the outgoing beam back to the monitor photodiode  26 . In  FIG. 4E , again an auxiliary photodiode is not used, but an incident surface of the focusing lens  22  is used to backscatter a part of the outgoing beam back to the monitor photodiode. 
   In  FIG. 5 , the redundant auxiliary photodiode  48  is surface-mounted on a printed circuit board (PCB)  52  spaced away from a chassis  54  having a hole  56 . The laser diode  25  and the focusing lens  22  are mounted on the chassis  54 . The monitor photodiode is not shown in  FIG. 5 . The laser diode  25  emits the laser beam through the focusing lens, and a fraction of the laser beam is reflected by the focusing lens toward a casing  58  that houses the laser diode. The fraction of the laser beam is also reflected from the casing  58 , as well as from a holder  60  for the focusing lens  22 . A portion of the fraction of the laser beam passes through the hole  56  located in the region between the laser diode and the focusing lens  22  and is detected by the redundant auxiliary photodiode  48  and serves as a measure of the output power of the laser beam. A rubber boot  62  surrounds the redundant auxiliary photodiode  48  and serves as an enclosure to prevent stray ambient light from being detected by the photodiode  48 . 
   In the arrangements of  FIGS. 4A-E  and  FIG. 5 , the additional light detected by either the auxiliary photodiode  48 , or the monitor photodiode  26 , is monitored and converted by the microcontroller  44  to generate a control signal used to open the switch  39  when the monitored operating condition does not meet a preestablished value. This feature promotes safety in the use of a reader in which a laser beam is generated. 
   It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types described above. 
   While the invention has been illustrated and described as embodied in laser power control arrangements in electro-optical readers, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
   Although described in connection with moving-beam readers, the laser control arrangements of this invention can equally well be applied to imaging readers, laser projection displays and, in general, any system in which a light source is used for illumination of, and for aiming at, a target. 
   Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims. 
   What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.