1. Field of the Invention
The present invention relates to laser scanning systems for reading bar code symbols, and more particularly, pertains to a novel digitizer processing circuit for detecting the times at which a scanning light spot moves from a space onto a bar code symbol or moves from a bar code signal onto a space.
2. Description of the Prior Art
The increased use of bar code symbols to identify products, particularly in retail businesses, has resulted in the development of various bar code reading systems. Many users of bar code readers require portable hand-held scanners which place a premium on small size, lightweight, and low power consumption requirements for the devices. One such system is a laser scanning bar code reading system as described in U.S. Pat. No. 4,496,831, commonly assigned to the same assignee as the present patent application.
The laser scanning system disclosed in U.S. Pat. No. 4,496,831 includes a portable hand-held scanning head which may be embodied in various shapes but preferably has a gun-shaped housing made of lightweight plastic. A handle and barrel portion are provided to house the various components of the scanning head therein. Within the barrel portion are mounted a miniature light source, a miniature optic train including focusing lenses and a scanning system for directing light from the light source across a bar code symbol, and miniature sensing means for detecting reflected light from the bar code symbol being scanned.
The miniature light source can comprise a laser tube such as a coaxial helium neon laser tube, or preferably a semiconductor laser diode which is considerably smaller and lighter than a laser tube, thus reducing the required size and weight of the scanning head and making the scanning head easier to handle and more maneuverable. Light generated by the light source passes through the optic train which directs the beam onto the scanning system which is mounted within the barrel portion of the scanning head. The scanning system sweeps the laser beam across the bar code symbol and comprises at least one scanning motor for sweeping the beam lengthwise across the symbol, and may comprise two motors wherein the second motor sweeps the beam widthwise across the symbol. Light reflecting means such as mirrors are mounted on the motor shafts to direct the beam through the outlet port to the symbol. A sensing circuit then detects and processes the light reflected from the symbol and generally comprises a photodetector element such as a semiconductor photodiode. The user positions the hand-held unit so the scan pattern traverses the symbol to be read, and the photodetector element produces serial electrical signals to be processed for identifying the bar code. A circuit such as that of the present invention for a digitizer signal processing circuit for a bar code produces a signal which is directed to a bar pattern decoder circuit for decoding the bar pattern. The reader unit can have a movable trigger employed to allow the user to activate the light beam and detector circuitry when pointed at the symbol to be read, thereby conserving battery life if the unit is self-powered. The lightweight plastic housing contains the laser light source, the detector, the optics, signal processing circuitry, a CPU, and a battery. The reader is designed to be aimed at a bar code symbol by the user from a position where the reader is spaced from the symbol, i.e., not touching the symbol or moving across the symbol. Typically, this type of hand-held bar code reader is specified to operate in the range of perhaps several inches.
Although the present invention is described with respect to linear or single line bar codes, it is not limited to such embodiments, and may also be applicable to more complex scanning patterns and to stacked or two dimensional bar codes such as Code 49 and similar symbologies. The present invention may also find application for use with various machine vision or optical character recognition applications in which information is derived from other types of indicia such as characters or from the surface characteristics of the article being scanned.
It is well known that it is desirable to locate in time the moment when the center of the moving scanning laser spot is located on the edge of a bar as it moves onto the bar and again when it moves off the bar. It is also well known that there is an inflection point in the analog signal that corresponds to the above-stated moments in time. Various forms of signal processing have been used in bar code scanners to locate these inflection points.
The most common technical approach to accomplish this is to use a zero crossing detector to locate when the second derivative of the analog signal crosses through zero, as this zero crossing corresponds in time to the inflection points of the analog signal. One problem with this approach is that there also are zero crossings of the second derivative that do not correspond to the inflection point of the analog signal. Various technical approaches have, therefore, been developed to distinguish between the desired zero crossings and false ones. Most of these approaches depend upon information obtained from either the analog signal or the first derivative of the analog signal. Examples of these technical approaches are disclosed in Spectra Physics Patent Nos. 4,000,397 and 4,749,879. Another known method is to offset the analog signal by half of its amplitude so that it is centered around zero and then to compare it directly to the second derivative with a voltage comparator. Another common feature is a circuit to ensure noise-free margins the white area on either side of the bar code) by setting a noise threshold based upon the averaging of noise over a period of time. Signals below this average noise level will not be digitized.