Abstract:
An improved laser marking focal length setting device allows an operator of a laser marking apparatus to easily and properly set the distance between a movable laser source output and an item to be marked. The device includes a sensor which is mounted at the same distance from the item to be marked as the laser source output. The device further includes an array of light emitting diodes, each of which corresponds to a predetermined focal length. In operation, the laser source output and sensor are moved together along the axis of the laser beam. The sensor supplies a signal indicative of the distance between the sensor and item. Because the laser source output and sensor are located at the same distance from the item to be marked, this signal is also indicative of the distance from the laser source output to the item. When the distance sensed is a focal length corresponding to one of the LED&#39;s, that LED lights. When the lit LED corresponds to the focal length of the current output lens, the operator may fix the output in its current position. Alternatively, the output is drivably connected to a motor, which is responsive to the sensor to drive the output to one of a plurality of predetermined distances.

Description:
TECHNICAL FIELD 
     This invention relates generally to laser marking of products and, more particularly, relates to detection and indication of the distance between a laser marking apparatus and a product to be marked. 
     BACKGROUND OF THE INVENTION 
     There exist many different ways to place printed matter on a product package, including ink printing on the product, laser marking on the product, and ink or laser preprinting of a label to be placed upon the product. Of these, laser marking the product is often the most desirable alternative because of its low cost and easy adaptation to different marking jobs. It is especially useful in putting final markings, such as expiration dates and the like, on completed products. The technique of laser marking entails directing a focussed beam of light generated by a laser at a product. The focussed laser light marks the surface by burning caused by absorption of the light and a transformation of that light, or photon, energy into heat energy. Alternatively, the photons of the focussed laser light may interact directly with the material of the target without an intermediate transformation into heat energy. 
     In either case, the degree of visual change in the target material will be a function to some extent of the energy density in the focussed laser light, which is partly a function of the beam focus. Additionally, the sharpness of the mark may also be affected by the degree of focus of the laser beam. Accordingly, it is important that the distance between the product to be marked and the laser light focussing lens closely match the focal length of the lens. Additionally, the laser light focussing lens is often changed in order to facilitate the making of different sized markings. It is desirable in such cases that an operator be able to easily and quickly adjust the distance between lens and product to match the desired distance. 
     Many lasers used for marking applications emit light that falls outside of the human visible spectrum, and thus visual optimization of the focus is impractical. Even where the laser light is visible, it is often difficult to visually optimize the focus. Accordingly, in the past, operators of industrial laser marking systems have manually measured the distance between the lens and product, using a ruler, and have adjusted the distance to match the known focal length of the lens. Alternatively, or in addition, the distance is sometimes varied as several trial pieces of product are marked, and the distance is fixed when the product appears to be properly marked. Neither of these methods is fast or easy, and the second method involves wasting the improperly marked trial pieces. 
     A method and apparatus are needed whereby a laser marking equipment operator may receive a human-perceivable indication of the distance from lens to product to be marked, so that such distance may be set equal to the focal length of the lens, or to another desired distance. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, a distance measuring sensor is mounted proximal to and at the same vertical height as a laser light focussing lens, in an industrial laser marking apparatus. Indicators actuated by the distance measuring sensor give a human-perceivable indication of the distance between the sensor and a target. Due to the proximal placement of the distance measuring sensor to the laser light focussing lens, the human-perceivable indication serves to indicate the distance between the laser light focussing lens and the target as well. The human-perceivable indication may be comprised of light emitting diodes (LED&#39;s) which are activated selectively to indicate the measured distance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the appended claims set forth the features of the present invention with particularity, the invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a schematic diagram generally illustrating an operating environment and mode of operation for a sensor system according to an embodiment of the invention; 
     FIG. 2 is a flow chart of operations according to an embodiment of the invention; 
     FIG. 3 is a schematic diagram of a circuit usable in an embodiment of the invention to activate a distance-indicating LED; and 
     FIG. 4 is a block diagram illustrating a mode of operation of an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Turning to the drawings, wherein like reference numerals refer to like elements, FIG. 1 illustrates a preferred operating environment and mode of operation for a sensor system embodying the invention. An industrial laser marking apparatus has a movable laser head  1  for directing the output of a laser to a product  2  to be marked. A lens mount  3  is disposed to alternately receive different output lenses having different focal lengths. Typically, the necessary output lens focal length is a function of the size of the features to be marked. Accordingly, if the size of the marked features is to be changed, for example from a smaller to a larger font size, then the output lens focal length is also changed. 
     Product  2  to be marked traverses a path  4  which typically intersects and is locally perpendicular to the axis of the output laser beam, at a product position in front of the beam output. The position of the laser head  1  is typically movable at least along the axis of the output laser beam, while the position of the path  4  of the product  2  is typically not changeable along this axis. The distance between the output lens  5  and the path  4  of the product  2  to be marked is temporarily fixed during marking in accordance with the focal length of the current output lens  5 . 
     Preferably, a sensor unit  6  is fixedly mounted to a movable output of the laser apparatus via a clamp  7 , and an extension arm  8 . The sensor unit  6  is mounted such that a sensor element  9  is aligned with the laser output lens  5 . In such an embodiment, the distance from the laser output lens  5  to the products on the path  4  will correspond to the distance from the sensor element  9  in the sensor unit  6  to the products on the path  4 . Accordingly, any subsequent adjustment of the distance from the output lens  5  to the product path  4  will cause the distance from the sensor element  9  to the products on the path  4  to change by the same amount. 
     In a preferred embodiment, the sensor element  9  is an ultrasonic sensor such as the Hyde Park SM606A-BOB-00. This sensor comprises an ultrasonic transmitter and ultrasonic sensor. The sensor converts sensed distance into an analog output signal. Thus, as the product passes along the product path  4  in front of the sensor element  9 , the ultrasonic sensor generates an analog signal indicative of the distance  11  from the laser output lens  5  and the sensor element  9  to the products on the path  4 . 
     The sensor unit  6  has mounted thereon a human-perceivable indicator of the distance  11 . In a preferred embodiment, the human-perceivable indicator is an array of five LED&#39;s  10  which are sequentially disposed to light according to the distance  11 . Thus, in this embodiment, there exists in the array  10  an LED for each potential focal length. These focal lengths may be 2.5″, 3.75″, 5″, 7.5″, and 10″. Alternatively, there may be more or different desired focal lengths, each of which would preferably have a corresponding LED in the array  10 . The LED&#39;s may be of the same or different color, and are preferably disposed adjacent to a key  12  which explains the meaning of each. For example, the distance to which a given LED corresponds may be written on the key next to the appropriate LED. 
     The operation of this embodiment is shown in the flow chart of FIG. 2; with reference to the elements of FIG.  1 . Initially, in Step  1  a human operator selects an output lens  5  having a known focal length, based on desired laser marking font size or other appropriate criteria. This lens will be inserted into the lens mount  3  at any point in time prior to the commencement of laser marking. The human operator turns the printer on in Step  2 , supplying power and turning on the sensor unit  6 . Having a sample of the product to be marked stationarily or periodically disposed along the product path  4  at a location in front of the sensor unit  6 , the operator in Step  3  varies the distance  11  between the output lens  5  (and sensor element  9 ) and the products on the path  4 . While the operator varies the distance  11 , he or she also visually monitors the LED array  10 . Thus, in Step  4  when an LED corresponding to the known focal length becomes activated, indicating that the distance  11  is equal to that focal length, the operator ceases moving the laser head  1 , and fixes it in position. Depending upon whether the product is stationarily or periodically placed during this process, the appropriate LED will light in either a constant or periodic manner. After performing the process depicted in FIG. 2, the laser output lens  5  will be properly located at a distance equal to its focal length from products  2  which pass along the product path  4 . 
     The schematic of FIG. 3 depicts a calibrated window comparator circuit usable in an embodiment of the invention to activate a given LED in the array  10  when the distance  11  corresponds to the distance indicated by that LED. In this embodiment, one circuit according to FIG. 3 is used for each LED to be lit. The circuit depicted in FIG. 3 is calibrated to indicate a distance of 2.5″. One skilled in the art will appreciate how different resistor values may be used in the circuit to calibrate the circuit to respond to the same or different sensed voltages and thus the same or different desired distances. The circuit may be independently powered, or may be powered by the laser marking apparatus via an auxiliary port. The example in FIG. 3 receives power at an input  200  from the laser marking apparatus. 
     Although the particular selection and arrangement of elements is not critical, the sensor processor of FIG. 3 is constructed as follows: Resistors  250 ,  252 ,  254 ,  256 ,  258 , and  226  have values respectively of 6.34 k, 316, 8.25 k, 316, 1.8 k, and 1.8 k ohms. Capacitor  220  has a value of 0.1 microfarads. These values are not critical. For example, a variance of the values of resistors  250  and  252  without changing their ratios will not affect the operation of the circuit. Similarly, a variance of the values of resistors  254  and  256  without changing their ratios will not substantially affect the operation of the circuit, although it may affect the power consumption of the system. The values of resistors  258  and  226  may also be varied without substantially affecting the operation of the circuit. Capacitor  220  may be replaced by a slightly smaller or larger capacitor without substantially affecting the operation of the circuit. 
     A positive voltage, in this case 15V, is applied at input  200  via line  260  to the supply voltage inputs on comparators  202  and  204 , and to a first terminal of resistor  254 , and to a first terminal of resistor  250 , and to a first terminal of resistor  258 , and to a first terminal of capacitor  220 . The comparators may be built from standard op-amps such as those contained in the LM324A low power quad operational amplifier IC produced by SGS-Thomson Microelectronics, or may be built from separate transistors, other op-amps, or specialized comparator circuits. A second terminal of resistor  254  is connected via line  268  to a first terminal of resistor  256  and to the positive input of comparator  202 . A second terminal of resistor  250  is connected via line  264  to a first terminal of resistor  252  and to the negative input of comparator  204 . A second terminal of resistor  258  is connected via line  262  to the collector of transistor  212  and to the anode of LED  214 . LED  214  may be any LED, such as the Radio Shack catalog number 276-0004 LED, or any other device which lights when a voltage is applied across it. Transistor  212  may be any transistor, such as the Motorola MMBT2222L, which acts in substantially the same way, or any other device which provides the same function within the circuit. Ground is connected via line  266  to a second terminal of resistor  256 , to a second terminal of resistor  252 , to the emitter of transistor  212 , and to the cathode of LED  214 , and via line  270  to the ground supply inputs of comparators  202  and  204 , and via line  272  to a second terminal of capacitor  220 . An analog input signal from the sensor  9  of FIG. 1 is applied via line  206  to the positive input of comparator  204  and to the negative input of comparator  202 . The output of comparator  204  is connected via line  276  to the anode of diode  224 . The output of comparator  202  is connected via line  274  to the anode of diode  222 . The cathode of diode  224  is connected via line  278  to a first terminal of resistor  226 . The cathode of diode  222  is connected via line  278  to the first terminal of resistor  226 . A second terminal of resistor  226  is connected via line  280  to the base of transistor  212 . Diodes  222  and  224  may be the BAV70 high capacitance diode sold by Fairchild Semiconductor, or any other element which operates in substantially the same manner, by allowing current to flow substantially in one direction only. 
     The comparators  202  and  204  have threshold values set by the resistors  250 ,  252 ,  254 , and  256 . Using resistors of 6.34 k, 316, 8.25 k, and  316  ohms respectively for  250 ,  252 ,  254 , and  256  results in thresholds of about 0.55V and 0.7V for comparators  202  and  204 . This is dependent on the supply voltage, which is 15V in this embodiment. For the Hyde Park SM606A-BOB-00, the analog output for a sensed distance of 2.5″ falls between 0.55V and 0.7V. Accordingly, when the input  206  receives this voltage, the outputs of comparators  202  and  204  on lines  274  and  276  respectively, are low. This results in transistor  212  acting essentially as an open circuit, allowing LED  214  to be energized. 
     When the analog output of the sensor element  9  falls outside of this voltage window, the output of comparator  202  or comparator  204  will be high. This will cause transistor  212  to act essentially as a short circuit, pulling the potential on line  262  down to approximately zero volts. Accordingly, LED  214  will not be lit. It can be seen that several such circuits tuned to different voltage windows, and connected in parallel to the analog output of the sensor element  9 , will allow several LED&#39;s to be selectively activated depending upon the distance  11  measured by the sensor element  9 . Thus, a given distance  11  in FIG. 1 will result in a given analog output of the sensor element  9 , causing the LED whose comparator circuit window includes that particular voltage to be lit. 
     In an alternative embodiment, the top comparator  204  may be switchably or permanently disabled (output low) or eliminated entirely. This would result in each LED remaining lit as long as the sensor output was higher than its threshold setting. Thus, instead of one LED being lit at a given distance  11 , all of the LED&#39;s up to the most recently triggered LED would be lit. 
     In an alternative embodiment, the sensor processor circuits are instead connected to provide a control signal to an actuator  418  which is drivably connected to the movable laser output  402  as shown in FIG. 4 to facilitate adjusting the distance between the laser output  402  and a product to be marked  422 . In particular, similarly to the previous embodiment, the distance sensor  400  is preferably stationarily mounted with respect to the movable laser output  402 , such that the distance sensed by the sensor corresponds to the distance between the laser output  402  and the product  422  positioned along the product path  404  in front of the laser output  402 . The sensor processors  406 ,  408 ,  410 ,  412 ,  420  receive a signal output from the sensor  400  and preferably each supplies a sensor processor control output responsive to a different turn-on signal value corresponding to a particular distance between the laser output  402  and the product  422 . 
     A signal sensor circuit  424  contains the sensor processors  406 ,  408 ,  410 ,  412 ,  420 , and a mechanism such as multiplexer  414  communicatively connected via a control input  426  to selection mechanism  416 . The sensor processor control outputs of the sensor processors  406 ,  408 ,  410 ,  412 ,  420  may be multiplexed by the multiplexer  414  responsive to the selection mechanism  416 , or otherwise, such that the actuator  418  receives from the signal sensor circuit  424  a distance control output corresponding to a selected sensor processor circuit. In response to the received distance control output, the actuator  418  causes the movable laser output  402  to move relative to the product path  404 , and to stop when the control output indicates that the signal output of the sensor  400  has reached the turn-on signal value of the selected sensor processor circuit. In such an embodiment, the sensor processor circuits  406 ,  408 ,  410 ,  412 ,  420  may still, but need not, provide a human-perceivable indication of the sensed distance. The selection mechanism  416  may be manually operable by an operator. Alternatively, the selection mechanism  416  may be automatically operated responsive to the detection and identification of a particular output lens being utilized, via coded contacts or other measures well known to those of skill in the art. 
     Those skilled in the art will readily appreciate that the above-described circuits may be modified by substituting other circuit elements for those specifically identified, or by constructing different circuits to serve the same purpose, and to obtain a substantial equivalents. In view of the many possible embodiments to which the principles of this invention may be applied in view of the disclosed embodiments, it should be recognized that the embodiments described herein with respect to the drawing figures are meant to be illustrative only and should not be taken as limiting the scope of invention. For example, those of skill in the art will recognize that human-perceivable indicators other than LED&#39;s or the particular configuration of LED&#39;s disclosed may be used to implement the invention. An audio signal may be provided in lieu of or in addition to the visual indicator. Such an audio signal could be for example a synthesized or pre-recorded voice, or a tone. Furthermore, the circuitry shown is not critical to the invention. For example, a delay circuit may be included so that in operation the LED corresponding to the current sensed distance stays lit, without flashing on and off, until a product distance that differs by a substantial amount is detected for a certain period of time. The illustrated embodiment can be further modified in arrangement and detail by those skilled in the art in view of the present teachings without departing from the spirit of the invention. Therefore, the scope of the invention as described herein contemplates all such embodiments as may come within the scope of the following claims and equivalents thereof to the fullest extent permitted by law.