Abstract:
A system and method for controlling an amplifier in a bar code reader are disclosed, wherein the method may include receiving light at a photodiode within the bar code reader from a bar code being scanned by a scan mirror powered by a scan motor; converting the received light into an initial electrical signal; determining whether the scan motor is undergoing a change in direction; establishing a gain value for the amplifier based on an outcome of the determining step; and amplifying the initial electrical signal into an amplified signal using the established gain value.

Description:
RELATED APPLICATIONS 
     This application is related to U.S. application Ser. No. 13/363,894, filed Feb. 1, 2012, entitled “System and Method for Noise Reduction in a Bar Code Signal” which is hereby incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     A transition from fluorescent and other forms of traditional lighting to Light Emission Diode (LED) illumination is occurring in various environments including retail outlets, office buildings, warehouses, hospitals, and private homes. LED illumination may provide the benefits of low power consumption, low running cost, long life, and high color rendering effect among other desirable features. Some nations are now moving to ban further manufacture of conventional light bulbs for environmental reasons. 
     Bar code readers are commonly used in retail environments, including convenience stores, supermarkets and the like. Generally, a laser-scanned barcode reader operates by sweeping a laser beam, commonly having a 650 nm wavelength, over a bar code and receiving light energy reflected from the bar code, which is processed to generate a bar code signal. In a typical application, a laser beam using a 100 Hz scan rate will produce a signal having a frequency range of 30 kHz (kilohertz) to 200 kHz, depending on the resolution of the bar code and the read distance (the distance from the bar code to the bar-code reader). 
     To suppress power consumption, LED bulbs are generally driven at a frequency within a range of about 30 kHz to 100 kHz, a range which overlaps with the frequencies of many bar code signals. It would be hard for a bar code reader to distinguish light energy from ambient light from light energy from a bar code signal if the frequency ranges of the two signal types overlap. To eliminate interference of the ambient light with bar code readers, U.S. Pat. No. 6,811,087, which is incorporated by reference herein, discloses a technique to scan a bar code using a pulsed laser at a frequency of 2 MHz (megahertz) and using a synchronous detector to detect this frequency and preferably no other frequencies. This technique significantly removes ambient light having a constant intensity (such as sunlight) and light energy from high frequency L.E.D. illumination. However, where there are ambient light frequency components in common with a bar code signal, the decoder within the bar code reader could misread ambient light as being part of a bar code signal, which could lead to a signal reading failure. 
     Moreover, other possible sources of noise may be present in bar code reading environments as discussed in the following. Laser-scanned bar code readers commonly have exit windows made of glass or plastic (i.e., polycarbonate, Polymethyl methacrylate material) to protect the sensitive parts inside the reader housing. Although coated with an anti-reflective film, dirt or a finger-print on the exit window would present an optical obstruction resulting in significant back-scatter light being directed toward the photo sensor. The back-scattering of light would be more severe in a retro-reflective type barcode reader, in which the outgoing laser beam and the collected light beam received by the reader share the same optical path. Whereas the signal intensity from a bar code at a distance of 300 to 500 mm has a magnitude of about 0.1 uW (microwatts), the back scatter light could reach a magnitude of 1 uW, which is ten times the magnitude of the bar code signal. The above-described situation may thus lead to an inability of the bar code reader to accurately read a bar code. Accordingly, suitable amplification of the bar code signal is desirable. 
     Existing preamplifier circuits have amplifier controllers that provide feedback resistance that is controllable based on the magnitude of the output signal. When the output magnitude causes the amplified signal to reach the saturation point of the circuit, the resistance used as part of the amplification circuit is decreased to a smaller value by adding resistor in parallel to the feedback resistance, to lower the output to a level below the saturation level. However, the resistance remains at a larger value if the output is safely below the circuit saturation level, in order to maintain a high signal-to-noise ratio. The feedback resistance can be altered such that the output always reaches a predetermined maximum value. 
     The existing art also discloses a low pass filter coupled to an amplifier as described above for transmitting therethrough a low-frequency component of the voltage signal amplified by the preamplifier. A thresholding function may be implemented such that if the output signal of the low pass filter is lower than a predetermined level, the output is brought to a zero level. A function may be implemented to linearly increase the magnitude of the output signal when the output signal is equal to or higher than a selected predetermined level. 
     Using an existing approach, two or more different feedback resistance values are switched depending on scanned signal levels. When the feedback resistance is switched, the signal level variation becomes larger, and if the switching occurs during bar code scanning, the switching action may lead to ambiguity in determining whether a received bar code signal corresponds to a “bar” or “no-bar” condition within the bar code being scanned. 
     A low-pass filter (LPF) may be used to resolve the signal interpretation issue discussed above. However, it is difficult to ensure that the LPF is tuned with sufficient precision to ensure accurate interpretation of signals that are received as the resistance values are in the midst of being altered. Accordingly, there is a need in the art to ensure that ambiguity in interpreting signal values from a bar code reader is not generated by altering the gain of an amplification circuit while reading a bar code. 
     SUMMARY OF THE INVENTION 
     According to one aspect, the invention is directed to a method for controlling an amplifier in a bar code that may include receiving light at a photodiode within the bar code reader from a bar code being scanned by a scan mirror powered by a scan motor; converting the received light into an initial electrical signal; determining whether the scan motor is undergoing a change in direction; establishing a gain value for the amplifier based on an outcome of the determining step; and amplifying the initial electrical signal into an amplified signal using the established gain value. 
     Other aspects, features, advantages, etc. will become apparent to one skilled in the art when the description of the preferred embodiments of the invention herein is taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purposes of illustrating the various aspects of the invention, there are shown in the drawings forms that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
         FIG. 1  is a schematic diagram of a bar code reader incorporating an amplifier control circuit in accordance with one or more embodiments of the present invention; 
         FIG. 1A  is a schematic diagram of a bar code reader in which a polygon mirror has been substituted for the reciprocating mirror of  FIG. 1 , in accordance with an embodiment of the present invention; 
         FIG. 2A  is a schematic diagram of circuit for implementing variable feedback resistance in an amplifier, in accordance with an embodiment of the present invention; 
         FIG. 2B  is a schematic diagram of circuit for implementing variable feedback resistance in an amplifier, in accordance with another embodiment of the present invention; 
         FIG. 3  is a block diagram of a method for controlling the operation of an amplifier in accordance with an embodiment of the present invention; 
         FIG. 4  is a chart showing the interactions between various conditions within the bar code reader of  FIG. 1 , in accordance with an embodiment of the present invention; and 
         FIG. 5  is a block diagram of a computer system adaptable for use with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, for purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one having ordinary skill in the art that the invention may be practiced without these specific details. In some instances, well-known features may be omitted or simplified so as not to obscure the present invention. Furthermore, reference in the specification to phrases such as “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of phrases such as “in one embodiment” or “in an embodiment” in various places in the specification do not necessarily all refer to the same embodiment. 
     An embodiment of the present invention is directed to adjusting the gain of an amplifier within a bar code reader based on one or more of the following inputs: (a) a voltage level of the signal emerging from the subject amplifier being at or exceeding a low-level threshold or a high-level threshold; and/or (b) an indication that the direction of motion of a scan mirror of the bar code reader has changed. The amplifier gain may be changed by adjusting the feedback resistance connected in parallel with an operational amplifier (op-amp) used in an amplifier, which may be a pre-amplifier in the electrical signal processing circuitry of the bar code reader. 
       FIG. 1  is a schematic diagram of a bar code reader  10  incorporating an amplifier control circuit  130  (also referred to herein as an amplifier controller) in accordance with one or more embodiments of the present invention. Bar code reader  10  may include laser diode  106 , pulsed laser driver  104 , oscillator  102 , scan mirror  112 , scan motor  140 , photodiode  116 , pre-amplifier  118 , amplifier control circuit  130 , high-pass filter  120 , automatic gain control circuit  122  and/or synchronous detector  124 . Laser diode  106  may emit outgoing laser beam  108 , and light beam  114  may be received within bar code reader  10  from bar code  110 . 
       FIG. 1A  is a schematic diagram of a bar code reader  10  in which a polygon mirror  212  has been substituted for the reciprocating mirror  112  of  FIG. 1 , in accordance with an embodiment of the present invention. In the view provided by  FIG. 1A , polygon mirror  212  may be rotated counterclockwise to cause the beam reflected from laser diode  106  to scan over the length of bar code  110 . Once any given scan reaches its conclusion, the location of the reflected beam from laser diode  106  may be reset as a subsequent flat surface of the polygon rotates into position and starts scanning from the rightmost part of bar code  110 , and once again scans over the entire bar code  110 , as polygon mirror  212  rotates counterclockwise. 
     Returning to  FIG. 1 , amplifier control circuit  130  may include motor controller  132 , amplitude controller  134 , high-level detector  136  (for detecting an upper limit for the magnitude of the amplified signal  152  (FIG.  2 )), and/or low-level detector  138  (for detecting a lower limit for the magnitude of the amplified signal  152  ( FIG. 2 )). A method that may be implemented within amplitude controller  134  is discussed in connection with  FIG. 3 , later herein. In one embodiment, low level detector  138  may use a detection threshold of about 500 millivolts, and high level detector  136  may use a detection threshold of about 1.52 volts. However, the present invention is not limited to the above-disclosed thresholds. Threshold values, for low level detection and/or for high level detection, lower than, or higher than, the above-disclosed thresholds may be used, and all such variations are intended to be included within the scope of the present invention. 
     Amplitude controller  134  may be implemented with a programmable computer having communication interfaces with detectors  136  and  138 , motor controller  132 , and/or amplifier  118 . Alternatively, amplitude controller  134  may be implemented with logic incorporated into digital circuitry, instead of with software running on a programmable computer. 
       FIG. 2A  is a schematic diagram of an amplifier  118  in accordance with an embodiment of the present invention. When only R1  254  is connected in parallel with op-amp  252 , the resistance in at a high level (equal to the resistance value of R1 itself). When switch  258  is activated so as to close the connection running through R2  256 , the effective resistance connected in parallel with op-amp  252  is the parallel equivalent resistance of R1 and R2 which may be determined according to mathematical formulae that are known in the art. In one embodiment, the high-amplitude resistance may be about 3 mega-Ohms (i.e. three million ohms), which may be achieved by employing an R1 value of 3 mega-ohms. In this same embodiment, the low-amplitude resistance value may be 200 kilo-ohms (i.e. two hundred thousand ohms). A low-amplitude resistance value of 200 kilo-ohms may be obtained by employing a value of R2 equal to about 200 kilo-ohms. However, the present invention is not limited to the use of the above-specified resistance values for R1  254  and R2  256 . 
       FIG. 2B  shows an alternative way of implementing amplifier  118 . In the embodiment of  FIG. 2B , variable resistor  262  is used in place of the switchable resistor arrangement shown in  FIG. 2A . Variable resistor  262  may be implemented using an active solid-state electronic circuit. Alternatively, variable resistor  262  may be implemented using a passive resistance element and a mechanism for adjusting the point along the passive resistance element at which electrical contact is made with an current path joining a left-side node of op-amp  252  with a right-side node of op-amp  252 . Moreover, the present invention is not limited to the above-discussed ways of implementing amplifier  118 . 
       FIG. 3  is a block diagram of a method  300  for controlling the operation of an amplifier  118  in accordance with an embodiment of the present invention. 
     Amplitude controller  134  may be activated (step  302 ), may set pre-amplifier  118  to a low resistance mode of operation (step  304 ), and may reset detectors  136  and  138  to initial conditions such that high-level detector  136  is set to “0” and low-level detector  138  is set to “1”. At step  308 , the method  300  preferably determines whether the direction in which scan motor  140  has changed. If there is no change in scan direction, step  308  is repeated. If there has been a change in the direction of scan motor  140 , method  300  preferably continues at step  310 . An indication of a change in scan direction may by scan motor  140 , or by a sensor configured to sense the direction of rotation of a shaft coupled to scan motor  140 . Prior to discussing step  310 , various mechanisms for notifying amplitude controller  134  of the occurrence or non-occurrence of a change in scan direction are discussed below. 
     In an alternative embodiment step  308  may include determining that there has been transition from one scan operation over bar code  110  to a subsequent scan operation. However, the transition may be detected differently depending on whether the flat mirror  112  of  FIG. 1  or the polygon mirror  212  of  FIG. 2  is used. In the embodiment of  FIG. 1 , detection of the transition from one scanning process to the next may be identified by detecting a change in the scan direction of mirror  112 , as discussed above. In the embodiment of  FIG. 1A , which employs polygon mirror  212 , a transition from one complete scan to a subsequent scan may be detected using a rotary position encoder coupled to polygon mirror  212 . 
     The indication of a change in the direction of scan mirror  112  may be provided by having motor controller  132  transmit a signal to amplitude controller  134  indicating a change in the direction of motion of scan motor  140  substantially simultaneously with the transmission of control signaling from motor controller  132  to motor  140  that actually changes the direction of motor  140 . Alternatively, a sensor could be built into scan mirror  112  which emits a signal indicative of a change in direction whenever scan mirror transitions from one scanning direction to another. Moreover, the present invention is not limited to the above means of informing amplitude controller  134  of a change in scan direction. 
     At step  310 , the method determines whether or not the pre-amp  118  is operating in a low-resistance mode, as previously discussed in connection with  FIGS. 2A and 2B . 
     If pre-amp  118  is in the low-resistance mode, (i.e. the “yes” condition output from block  310 ) the method determines, at step  312 , whether the output signal  152  ( FIG. 2 ) has reached a low enough value to cause the low-level detection signal to be set high by low-level detector  138 . If the low-level detection signal is low, meaning that the output signal  152  is above the low-level detection threshold, no further action is taken, and the execution of method  300  may resume at step  306 . If the low-level detection signal is high (the “yes” output from block  312 ), the pre-amp  118  is preferably shifted into the high-resistance mode (discussed in connection with  FIG. 2 ) in step  314 . Thereafter, the method  300  may resume at step  306 . 
     We now return to the “no” branch output from block  310 . If pre-amp  118  is in low-resistance mode, the method  300  preferably checks to see whether the high-level detector  136  indicates that signal  152  has reached or surpassed the high level voltage threshold. If the high-level detection threshold has not been reached, the method  300  preferably resumes at step  306 . If the high-level detection threshold has been reached, the method  300  preferably causes pre-amp  118  to operate in the low-resistance mode, at step  318 . Thereafter, the method  300  preferably resumes operation at step  306 . 
       FIG. 4  is a chart showing the relationship between changes in scan direction and switching of the feedback resistance in pre-amp  118  in accordance with one embodiment of the present invention. 
       FIG. 5  is a block diagram of a computing system  500  adaptable for use with one or more embodiments of the present invention. Central processing unit (CPU)  502  may be coupled to bus  504 . In addition, bus  504  may be coupled to random access memory (RAM)  506 , read only memory (ROM)  508 , input/output (I/O) adapter  510 , communications adapter  522 , user interface adapter  506 , and display adapter  518 . 
     In an embodiment, RAM  506  and/or ROM  508  may hold user data, system data, and/or programs. I/O adapter  510  may connect storage devices, such as hard drive  512 , a CD-ROM (not shown), or other mass storage device to computing system  500 . Communications adapter  522  may couple computing system  500  to a local, wide-area, or global network  524 . User interface adapter  516  may couple user input devices, such as keyboard  526 , scanner  528  and/or pointing device  514 , to computing system  500 . Moreover, display adapter  518  may be driven by CPU  502  to control the display on display device  520 . CPU  502  may be any general purpose CPU. 
     It is noted that the methods and apparatus described thus far and/or described later in this document may be achieved utilizing any of the known technologies, such as standard digital circuitry, analog circuitry, any of the known processors that are operable to execute software and/or firmware programs, programmable digital devices or systems, programmable array logic devices, or any combination of the above. One or more embodiments of the invention may also be embodied in a software program for storage in a suitable storage medium and execution by a processing unit. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.