Abstract:
A spark plug gap adjustment system is provided which is designed to adjust a spark gap between a ground electrode and a center electrode installed in a spark plug to a target value. The system determines the position of the ground electrode in a given coordinate system to calculate a target distance required to move an end of the ground electrode toward an end of the center electrode for bringing the spark gap into agreement with the target value and moves the ground electrode under pressure by the target distance to define a desired spark gap. This achieves the spark gap adjustment precisely at high speeds.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   The present invention relates generally to an apparatus and a method for adjusting or defining a desired spark gap of spark plugs precisely at high speeds. 
   2. Background Art 
   Typical spark plugs consist of a center electrode, a metal housing or shell retaining the periphery of the center electrode through a porcelain insulator, and a ground electrode secured in the metal shell so as to face the center electrode. The interval between the center electrode and the ground electrode is adjusted to, for example, 1 mm to define a suitable spark gap. 
   Japanese Patent No. 2636814 discloses a spark gap adjustment method. This method includes steps of holding a spark plug, capturing an image of the spark plug around ends of a center electrode and a ground electrode through a CCD camera to produce an image signal, processing the image signal to measure the interval between the ground electrode and the center electrode, and pressing an outer wall of the ground electrode toward the center electrode using a press unit to adjust the ground-to-center electrode interval to a desired value. 
   In order to improve the productivity of spark plugs, quick adjustment of the spark gap is sought. The above method, however, performs the step of pressing the ground electrode while measuring the ground-to-center electrode interval and thus encounters a problem that the speed at which the press unit is moved to push the ground electrode depends upon the time required to measure the ground-to-center electrode interval, and too fast pushing the ground electrode will make it difficult to measure the ground-to-center electrode interval precisely. 
   For instance, if the time required to scan one frame of an image captured by the CCD camera is 0.2 sec., a typical tolerance of a 1 mm spark gap is 0.1 mm, therefore, an allowable maximum speed of the press unit will be 0.5 mm/sec. Increasing the accuracy of measuring the ground-to-center electrode interval requires decreasing the speed of the press unit. 
   SUMMARY OF THE INVENTION 
   It is therefore a principal object of the invention to avoid the disadvantages of the prior art. 
   It is another object of the invention to provide an apparatus or a method for defining a desired spark gap of a spark plug between a ground electrode and a center electrode precisely at high speeds. 
   According to one aspect of the invention, there is provided a method of adjusting a spark gap of a spark plug precisely at high speeds. The method comprises the steps of: (a) holding the spark plug having installed therein a center electrode and a ground electrode which have ends opposed to each other; (b) measuring a location of the end of the center electrode to determine a target distance required to move the end of the ground electrode toward the center electrode for bringing a gap between the ground electrode and the center electrode into agreement with a target value as a function of the measured location; and (c) pressing the end of the ground electrode toward the center electrode using a press device to move the end of the ground electrode the target distance to bring the gap between the ground electrode and the center electrode into agreement with the target value. 
   In the preferred mode of the invention, the press device includes a press head which has a contact surface which comes in contact with an outer surface of the end of the ground electrode when the ground electrode is pressed and a stepped surface which continues from the first surface and faces the end of the ground electrode through a given gap when the ground electrode is pressed. 
   The method further comprises the step of measuring an interval between the end of the ground electrode and the end of the center electrode to determine whether the measured interval is greater than the target value of the gap between the ground electrode and the center electrode or not after the end of the ground electrode is pressed toward the center electrode through the press device and the step of correcting the target distance the ground electrode is to be moved toward the center electrode so as to compensate for a difference between the target value and the measured interval. 
   According to another aspect of the invention, there is provided a spark plug gap adjustment apparatus. The apparatus comprises: (a) a holder which holds a spark plug having installed therein a ground electrode and a center electrode which are opposed to each other; (b) a press unit which works to press an end of the ground electrode toward the center electrode so as to decrease an interval between the end of the ground electrode and an end of the center electrode; and (c) a controller which measures a location of the end of the ground electrode held by the holder to calculate a target distance required to move the end of the ground electrode toward the end of the center electrode for bringing a gap between the ground electrode and the center electrode into agreement with a target value as a function of the measured location. The controller controls an operation of the press unit to move the end of the ground electrode the target distance. 
   In the preferred mode of the invention, the controller captures an image of a portion of the spark plug around the end of the ground electrode to determine a position of the end of the ground electrode in a given coordinate system and calculates the target distance as a function of the determined position. 
   The controller calculates a difference between the target value of the gap between the ground electrode and the center electrode and an interval between the ground electrode and the center electrode after the press unit moves the ground electrode the target distance and corrects the target distance so as to compensate for the difference between the target value and the interval. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
     In the drawings: 
       FIG. 1  is a plan view which shows a spark plug gap adjustment apparatus according to the present invention; 
       FIG. 2  is a partial view which shows a press head working to press a ground electrode toward a center electrode to bring a ground-to-center electrode interval into agreement with a target gap; and 
       FIG. 3  is a flowchart of a program performed by the spark plug gap adjustment apparatus of  FIG. 1  to adjust a ground-to-center electrode interval to a target gap. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the drawings, particularly to  FIG. 1 , there is shown a spark plug gap adjustment apparatus according to the invention which is designed to define a desired interval between a ground electrode and a center electrode of a spark plug, which will also be called a spark gap below. 
     FIG. 2  shows a typical spark plug  10  for use in internal combustion engine, for example, which consists of a center electrode  11  retained in a porcelain insulator (not shown) and a ground electrode  12  installed in a metal shell (not shown). The ground electrode  12  is bent at an end thereof to define a desired spark gap  13  between itself and an end of the center electrode  11 . A broken line in  FIG. 1  indicates the ground electrode  12  bent temporarily before final adjustment of the ground-to-center electrode interval. The center electrode  11  and the ground electrode  12  are made of a known material. The ground-to-center electrode interval is adjusted by the spark plug gap adjustment apparatus of this embodiment to, for example, 1 mm. 
   Referring back to  FIG. 1 , the spark plug gap adjustment apparatus generally includes a plug holder  20 , a press unit  30 , a lighting unit  40 , a camera  50 , an image processing unit  60 , and a press controller  70 . The plug holder  20 , as shown in the drawing, holds the spark plug  10  upright with the spark gap  13  oriented upward. The press unit  30  works to push the ground electrode  12  for decreasing the interval between the ground electrode  12  and the center electrode  11 . The lighting unit  40  works to emit light toward the ground-to-center electrode interval of the spark plug  10 . The camera  50  captures an image of a portion of the spark plug  10  around ends of the center and ground electrodes  11  and  12  and provides a signal indicative thereof to the image processing unit  60 . The press unit controller  70  controls an operation of the press unit  30 . The spark plug  10  in  FIG. 1  is the one as viewed from the left side in FIG.  2 . 
   The press unit  30  consists of a press head  31 , a servomotor  32 , an output shaft  33 , and a base frame  34 . The servomotor  32  is connected to the press head  31  through the output shaft  33  and works to move the press head  31  up and down. The base frame  34  is secured on a stationary base (not shown) and holds the output shaft  33  rotatably. 
   The output shaft  33  consists of a rotary shaft  33   a  and a joint  33   b  connecting the rotary shaft  33   a  and the servomotor  32 . The rotary shaft  33   a  is inserted through a ball screw unit  34   a  installed on the base frame  34  and has the press head  31  installed on an end thereof projecting from the ball screw unit  34   a.    
   The press unit  30  is activated by the press controller  70  to move the press head  31  through the output shaft  33  downward to press an outer wall of the ground electrode  12  to adjust the ground-to-center electrode interval of the spark plug  10  and to lift the press head  31  upward after the desired spark gap  13  is established. 
   The press head  31 , although not always necessary in this invention, has, as clearly shown in  FIG. 2 , a flat surface  31   a  and a recessed surface  31   b . The flat surface  31   a  works to press the ground electrode  12  in direct contact therewith. The recessed surface  31   b  is kept at a given interval away from the ground electrode  12  while the ground electrode  12  is being pressed by the flat surface  31   a   1 . This establishes a constant engagement of the press head  31  with the ground electrode  12 , thus resulting in increased accuracy of adjustment of the ground-to-center electrode interval. Additionally, the formation of the recessed surface  31   b  on the press head  31  allows a sectional area of the press head  31  to be increased, thus resulting in increases in strength and wear resistance of the press head  31 . 
   The lighting unit  40  consists of an optical fiber  41  and a diffusion plate  42 . The optical fiber  41  is connected to a light source (not shown) and emits light. The diffusion plate  42  diffuses the light from the optical fiber  41  uniformly over the ground-to-center electrode interval of the spark plug  10 . The optical fiber  41  and the diffusion plate  42  are retained by the base frame  34 . 
   The camera  50  may be implemented by a CCD camera and installed on a mount base  51  secured on the base frame  34  so that an optical axis of the camera  50  may be aligned with an optical axis of the optical fiber  41 . The camera  50  captures an image of the ground electrode  12  and the center electrode  11  of the spark plug  10  and provides an image signal to the image processing unit  60 . 
   The image processing unit  60  has disposed therein a known image processing processor which processes the image signal inputted from the camera  50  according to a given algorithm to locate the center electrode  11  and the ground electrode  12  in a coordinate system. The image processing unit  60  can be of any known structure, which is not essential part of this invention, and explanation thereof in detail will be omitted here. 
   The press controller  70  may be implemented by a programmable logic controller (PLC) and works to actuate the servomotor  32  for controlling the movement of the press head  31  based on a positional relation between the center electrode  11  and the ground electrode  12  derived by the image processing unit  60 . 
     FIG. 3  shows a flowchart of a program or logical steps performed by the spark plug gap adjustment apparatus of this embodiment to define the desired spark gap  13  of the spark plug  10 . 
   Prior to start of the program, the spark plug  10  is mounted manually or using an automatic handling unit in the plug holder  20  with the tip thereof oriented upward. The ground electrode  12  is, as indicated by the broken line in  FIG. 2 , bent temporarily before installed in the plug holder  20  so that the end thereof may face the end of the center electrode  11  through a gap greater than the desired spark gap  13 . 
   Upon pushing a start switch of the spark plug gap apparatus, the routine proceeds to step  1  wherein the location of the tip of the center electrode  11  is measured. Specifically, the lighting unit  40  is activated. The camera  50  captures an image around the tip of the center electrode  11  and provides a signal indicative thereof to the image processing unit  60 . The image processing unit  60  processes the inputted image to determine the position of the tip of the center electrode  11  in the coordinate system. 
   For instance, the camera  50  produces an image signal indicating an image within a rectangular range, as indicated by a broken line K in FIG.  2 . The image processing unit  60  defines a lower left corner of the rectangular range as an origin of an xy coordinate system and determines a y coordinate of the tip of the center electrode  11  or the distance y 1 , as shown in  FIG. 2 , which will also referred to as a center electrode tip coordinate distance y 1  below. 
   The routine proceeds to step  2  wherein the target distance L required to move the press head  31  downward for adjusting the ground-to-center electrode interval to a target value is determined. Specifically, the location (i.e., a y coordinate b) of the press head  31 , the thickness c of the ground electrode, and a target interval d between the center electrode  11  and the ground electrode  12  are fixed and known in advance. The image processing unit  60  stores these values and determines the target distance L by subtracting the sum of the values c, d, and y 1  from the value b. 
   The y coordinate of the tip of the center electrode  11  as measured by the image processing unit  60  usually changes between spark plugs because the distance the tip of the center electrode  11  projects from the porcelain insulator is different between the spark plugs, however, the measurement of the center electrode tip coordinate distance y 1  ensures the accuracy of determination of the target distance L at all times. 
   The routine proceeds to step  3  wherein the image processing unit  60  outputs a signal indicative of the target distance L to the press unit controller  70  to move the press head  31  downward by the target distance L. Specifically, the press unit controller  70  determines a target number of turns of the servomotor  32  as a function of the target distance L and provides a control signal to the servomotor  32 . The servomotor  32  then rotates in a normal direction to move the press head  31  downward to push the ground electrode  12  toward the center electrode  11  until the ground-to-center electrode interval reaches the target value d. The speed at which the press head  31  is moved downward is controlled or adjusted to a desired value by the press unit controller  70 . The speed at which the press head  31  moves before coming into contact with the ground electrode  12  may be set equal to that at which the press head  31  moves to press the ground electrode  12  downward. 
   The routine proceeds to step  4  wherein after the servomotor  32  has moved the press head  11  by the target distance L, the press unit controller  70  reverses the servomotor  32  to lift the press head  31  away from the ground electrode  12 . 
   The routine proceeds to step  5  wherein the image processing unit  60  captures an image around the tips of the ground electrode  12  and the center electrode  11  to determine the ground-to-center electrode interval. 
   The routine proceeds to step  6  wherein it is determined whether the ground-to-center electrode interval measured in step  5  is greater than the target value d or not. If a YES answer is obtained, then the routine returns back to step  1 . Alternatively, if a NO answer is obtained meaning that the ground-to-center electrode interval has matched up with the target value d, then the routine terminates. 
   As apparent from the above discussion, the spark plug gap adjustment apparatus of this embodiment is designed to determine the target distance L required to move the press head  31  for adjusting the spark gap  13  to the target value. Specifically, the desired spark gap  13  is defined only by moving the press head  31  the target distance L without measuring the interval between the center electrode  11  and the ground electrode  12  during the pressing of the ground electrode  12 . Therefore, as compared with a case where the ground electrode  12  is pressed, as in the conventional apparatus, while the interval between the center electrode  11  and the ground electrode  12  is being monitored, the speed at which the ground electrode  12  is pressed may be increased greatly (e.g., up to 3 mm/sec.). The time required for steps  1  and  2  in the flowchart of  FIG. 3  is very short (e.g., 0.1 msec. or less) and hardly reflects on the total time of the gap adjustment. The increase in press speed, thus, results in improvement of the productivity of spark plugs. 
   In the conventional spark gap adjustment method, the ground electrode is pressed, as discussed above, while the interval between the center electrode and the ground electrode is being monitored. Thus, if a stop signal is provided to the press unit at the instant the ground-to-center electrode interval has reached a desired value, the delay will occur between the input of the stop signal to the press unit and a time when the press unit is stopped actually, thereby causing the ground-to-center electrode interval to be decreased below the target value. In order to avoid this problem, the stop signal must be outputted to the press unit before the ground-to-center electrode interval reaches the desired value, which, however, results in an undesirable variation in ground-to-center electrode interval. The spark plug gap adjustment apparatus of this embodiment works to move the press unit only the target distance L, thus eliminating the above problem to minimize the variation in ground-to-center electrode interval. The inventor of this application has performed tests and found that the time required for the gap adjustment in the spark plug gap adjustment apparatus of this embodiment may be decreased to about one-third of that required in the conventional spark gap adjustment method, and the variation in ground-to-center electrode interval is reduced by half. 
   While the invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims. 
   For example, if a NO answer is obtained in step  6  of  FIG. 3  meaning that the ground-to-center electrode interval d 1  as measured in step  5  is smaller than the target value d, the target distance L the press head  31  is to be moved downward may be corrected based on a difference between the measured interval d 1  and the target value d in a subsequent spark gap adjustment. Specifically, after step  6 , a correction value is determined according to an equation of (d 1 −d)/n where n is a preselected constant that is selected in a range of 2 to 6. In step  2 , the target distance L is determined in the same manner as described above. The correction value is added to the target distance L. This avoids the production of spark plugs in which the spark plug gap  13  is smaller than the target value d and which have difficulty in adjusting the spark plug gap  13  again. 
   The image processing unit  60  defines a lower left corner of the rectangular range K in  FIG. 2  as an origin of the xy coordinate system, but however, may determine a lower right, an upper left, an upper right corner of, or any point in the rectangular range K as the origin of the xy coordinate system.