US 3854563 A
A printer is disclosed in which a character forming print head is disposed on a carriage which pivots about a central point to describe an arcuate path, adjacent to which is situated an arcuate platen. The media to be marked upon conforms to the arcuate shape of the platen. The print head oscillates or reciprocates from one end of the arc to the other, and back. The arcuate path length may be varied to encompass printing in one or more coplanar stations on an arc in the plane of the print head and pivot.
Claims available in
Description (OCR text may contain errors)
[451 Dec. 17,1974
United States Patent [191 Cowardin et al.
mo em Adm Cu RK Colombo Wales Delbert C. Thomas, Jr.; William D. Thorne, both of Raleigh, all of NC.
Primary Examiner Edgar S. Burr Assistant Examiner-R. T. Rader Attorney, Agent, or Firm-Edward H. Duffield s e .m my MN k %n no Ur. BA l um .wfi r.D. er. m0 IC e n .m s S A M 7 ABSTRACT  Filed: June 25, 1973 A printer is disclosed in which a character forming print pivots  Appl. No.: 373,057
head is disposed on a carriage which about a central point to describe an arcuate path, adjacent to which is situated an arcuate platen. The media to be marked upon conforms to the arcuate 41 42 shape of the platen. The print head oscillates or recip- 43; 346/104 rocates from one end of the arc to the other, and
back. The arcuate path length may be varied to en- Refe en e Cit d compass printing in one or more coplanar stations on UNITED STATES PATENTS an arc in the plane of the print head and pivot.
1,947,450 Ansley 197/10 5 Claims, 2 Drawing Figures PATENTEL $581 71974 BACKGROUND OF THE INVENTION This invention relates to impact printers in general and to serial character or serial dot type of matrix printers in particular.
PRIOR ART Numerous serial printers exist in the prior art utilizing moving print heads, drums, disks, belts, etc., but virtually all of these are of the linear type in which either the print head or the printing type fonts are distributed or moved transversely along a linear path equal to the maximum length of a printed line for the device. It has, therefore, been necessary to either provide print heads for each character station along the line or to provide linear movement for the printing head, or for a hammer which strikes the print media against font characters, depending on the design chosen. This has been an inherent problem with many prior designs since the complexity of building accurate linear drive apparatus for a moving head entails a mechanically cumbersome and expensive lead screw or similar drive system using a tension tractor cord, linear rack escapement, etc., to drive a carriage carrying the print head or hammer back and forth. Similarly, and particularly in chain, belt and disk types of printers, high speed printing has necessarily involved very high speed movement of the character font carrying element or of the print head. This, in turn, has required very sophisticated and accurate timing and positioning mechanisms to provide for quality alignment of the final print and accurate registration between the moving print element and the medium or font set as it moves by. These features, too, have been expensive and difficult to build and maintain. Additionally, the linear type of printer has, as a consequence of its design, taken a relatively wide horizontal space always equal to, and usually greater than, the maximum width of the printed line, whether there be few or many characters in the printed line. This is not a desirable attribute in todays trend toward compactness, lightness, and generally streamlined design in business and commercial data handling equipment. Furthermore, the horizontal or linear layout of traversing apparatuses in general requires such things as line shafts or extended runs of a lead screw or tractor tension cables and numerous pivots, bearings, and miscellaneous smaller parts at either end of the machinery to support and sustain the traversing apparatus. In the alternative, where a plurality of hammers or type fonts are utilized with one for each, or for every few, character positions, numerous expensive hammers or character wheels must be provided (or their equivalent depending on the type of printer contemplated) which raises the cost and the complexity of the equipment and requires even more sophisticated timing and coordination between the moving elements of the various mechanisms. High speeds in the mechanical sense have required extremely fine workmanship and necessarily some great expense in designing and manufacturing high quality long-lived equipment to operate under such environmental design criteria as high character print throughput and reliable, low maintenance service. All of these problems, and more, are generally those with which printing machine designers have long been familiar as can be appreciated from the huge volume of designs and constructions appearing in the prior art. These problemsand others find alleviation in the present invention.
OBJECTS OF THE INVENTION In light of the above and other shortcomings and deficiencies with the printer designs of the prior art, it is an object of this invention to produce an improved, more compact serial printer of simpler and more reliable construction.
It is also an object of this invention to provide a moving head printer which is made relatively insensitive to shock and vibration external to the machine in an improved and simplified manner.
Yet another object of this invention is to improve printing machine design by eliminating expensive highspeed bearings, high speed motors, and high power requirements necessary to achieve high-speed printing in some prior designs.
A final object of this invention is to simplify the construction and maintenance by improving the design reliability and eliminating unnecessary and unwanted complexity in the structure and in the synchronization of various coordinated elements making up the printer.
SUMMARY OF THE INVENTION The foregoing objects and others are. met by providing a serial dot or serial character print head with a rotating or arcuately traveling carriage. The carriage pivots about a central point, and printing occurs at the outer periphery or arc described by the moving print head against a curved platen backing up the media to be printed upon. The print line length so provided, is equal to a much greater horizontal run and can be confined to lie within a horizontal dimension less than that of the total line length. The invention also makes possible the counter balancing of the print head to make it insensitive to shock and vibration introduced into the framework from outside the printer itself. Simplified position sensing and reduced power requirement are also realized due to the direct rotational motion between the main power drive input and the print head motion output. The reduced speed requirement which comes from having to drive the main mass of machinery at a relatively slow rotational speed while the printing head tip or print element traverses the print line at a relatively higher linear velocity also greatly aids the power reduction. These factors and others reduce the total complexity of the unit. Because of the relatively low speeds and stresses involved, the type of bearings, motors, clutches, and other mechanical elements can be greatly simplified and made of reduced cost materials without sacrificing performance or maintainability over a long life.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates a partially cut-away pictorial view, in simplified form, of the preferred embodiment of the invention and clearly shows the major working elements.
FIG. 2 illustrates, in cross section, a preferred motion detector and quantizer as used in the preferred embodiment.
Turning now to FIG. 1, the basic elements of a preferred embodiment of the invention, their arrangement and mode of operation will be discussed. In FIG. 1, the
main pivot shaft 1 is shown in a partially cut-away view in its vertically mounted position. Shaft 1 is driven by a gear 2 which is, in turn, driven from the gear 3 mounted on the end of a shaft connecting it to a speed reduction gear box connected to a drive motor 4. Motor 4 turns continuously and provides output power through gear 3 to drive gear 2 at an angular velocity equal to that desired for the printing sweep. Gear 2 turns freely on shaft 1 when it is not engaged by a clutch 5 to apply power to shaft 1. Clutch 5 is not shown in detail in the figures but a preferred form of clutch 5 may be seen in the IBM Technical Disclosure Bulletin, Vol. 14, No. 2, July, 1971, pages 455-456 which, for purposes of description of a suitable clutch system for the preferred embodiment, is made a portion hereof. The relatively slow rotating input from gear 2 is coupled by clutch 5 to the shaft 1 by pulling on the end of a spiral gripping spring in the clutch 5 which is of a type well-known and utilized in many spring clutches. A solenoid, not shown, is energized when print head motion is desired. The solenoid pulls on the end of the spring and causes engagement by wrapping of the spring onto the shaft and holds the clutch engaged until the solenoid is released. By the means of clutch 5, the rotary input motion of gear 2 is connected to shaft 1 and to the elements mounted on shaft 1 to cause them to rotate at the angular velocity of gear 2.
Main support carriage 6 is mounted on the top of shaft 1 in a fixed orientation so that it will move whenever shaft 1 moves. Carriage 6 supports print head 7 and counterweight 8. The print head 7, which in the preferred embodiment is a wire matrix print head similar to the type shown in U.S. Pat. Nos. 3,108,534, 3,592,3ll and 3,672,482, (which for the purposes of describing an operative print head, are made a part hereof,) is disposed in a fixed position on carriage 6 and is held in place for example, by screws 9 on either side of the print head housing. Nose portion 10 of print head 7 is placed in close proximity to the platen or platens ll, several of which are illustrated disposed around a circumferential path and in horizontal coplanar alignment with the nose 10 of print head 7. Arcuate platens 11 are supported in the main frame 12 in a fixed position during printing as illustrated. Platens 11 may be longer or shorter, more numerous or fewer than those platens 11 illustrated.
It is not essential that a wire matrix print head of any specific type be utilized as it is well-known that serial dot wire matrix print heads and serial whole character wire matrix print heads can be built and are available which could be equally utilized to advantage in the present embodiment of the invention. Similarly, electro discharge heads for printing on electrically sensitive paper, such as is well-known in the art, or similarly wellknown heat sensitive paper and hot wire matrix devices could easily be utilized in place of the preferred wire matrix print head.
In FIG. 1, three printing stations A, B, and C have been illustrated as utilized in the preferred embodiment to provide multiple copies of the same data. This feature is desirable for use, for example, in a retail store environment where customer purchase receipts, a store record or journal list maintained in the machine, and a special document printing or franking station, (for example, for imprinting credit slips or checks) are all required to have information printed on them. -A paper tape or document sheet 13 is shown at station B and would be wound onto a take-up roll 14 by suitable mechanisms not illustrated, with paper 13 being supplied from a supply roll not shown in FIG. 1. The document 13 is driven by suitable means such as driving the take-up roll by a shaft 15 or by friction wheel drive 16 illustrated in printing stations A and C. Friction wheels 16 bear against one surface of paper or document in the station and force it upward or downward when the wheels are turned by suitable means such as a stepping drive or continuously turning motor driving a clutch system.
The backup platens 11 are preferably of a hard steel, such as Cu-Ni sintered steel of 7.1 specific gravity, but if another type of printing head such as an electro discharge head were utilized, it should be understood that cooperating platen of suitable design would be placed in position instead of the backup platen l l illustrated. For purposes of description, a dotted arrow 17 has not been illustrated on the top of counterweight 8 to show the direction of motion to be expected when clutch 5 is engaged. In this example, the counterweight and print head attached to carriage 6 would move in the counter clockwise direction upon energization of the clutch 5. Counterweight 8 offsets the mass of the overhung carriage 6 and of the print head apparatus born on carriage 6 so that the totality of the elements attached to the end of drive shaft 1 are dynamically balanced against unwanted vibration or motion due to external shocks and movements. Print head 7 will, therefore, maintain a steady and constant speed whenever clutch 5 is engaged. Printing, as is well-known to those familiar with any of the currently known wire matrix print heads, is accomplished by driving one or more of a plurality of fine wires outward against either a pressure sensitive document or against an ink carrying (or similar) marking material ribbon and against the document. An alternative arrangement is to provide an inked platen 11 or other marking means disposed behind the document relative to the print head so that back printing is accomplished. In the preferred embodiment, an ink impregnated ribbon is stored in a cassette (not shown) which is mounted on top of counterweight 8. The cassette is utilized to feed a ribbon out of the cassette around the nose 10 of print head 7 and back into the cassette for storage with suitable drive means being provided to withdraw the ribbon from the cassette at one side and stuff it into the cassette at the opposite side. A typical ribbon cassette of this type which may be utilized in the preferred embodiment is shown in U.S. Pat. application, Ser. No. 209,684, filed l2,-20,-7I.
As print head 7 moves counter clockwise, it scans an arcuate path on which are disposed the various print stations A, B, and C as are illustrated. A continuous print path and continuous platen could be used, if desired. Some means must be provided for notifying the electronic system providing the wire matrix wire driving pulses that a specific location for printing has been reached.
Such a means is shown in FIGS. 1 and 2 and consists of an angular shaft position sensor or motion quantizer 18 which contains a light source 24 and a photo sensor 25 disposed on opposite sides of an interposer l9 having numerous apertures 20 and 21 disposed about its periphery. The use of such optical position sensing systems is well-known in the art and need not be discussed in great detail. The theory of operation is that light impinging on one side of interposer 19 shines through apertures as they move between the source of light and a photo sensor disposed on the opposite side of the interposer. Each time such an aperture passes between the source of light and the photo sensor, an electrical pulse is produced. These pulses, as indicia of quantized motion, may be counted by suitable counting means to determine, in relative numbers of steps corresponding to the placement of the apertures on the interposer 19, the relative position of the printing head along its printing path to any desired degree of accuracy. Equivalent systems may utilize mark sensing optical means instead of light transmitting optical means, magnetic mark sensors, mechanical brushes or the like. It is, of course, well-known that either the source and sensor may move with the interposer fixed, or vice versa, since it is relative motion between the elements that is sensed. A preferred embodiment, however, utilizes a perforated interposer 19 carrying a plurality of finally spaced apertures 20 for producing pulses corresponding to character width distances traversed by the head. Apertures 21, of somewhat larger size and of widely disposed location, are utilized to provide information for block positioning such as, in the preferred embodiment illustrated, notification to the system that that print head is located opposite the beginning of. a new scan at station A, station B or station C. The block position signals correspond to the notches 21 and the large blank area 22 taken from the periphery of interposer 19.
Some means must also be provided for returning the print head and carriage back to the home or start position by reversing the rotation of drive shaft 1 when printing has been completed at the last station at which printing was desired. In the preferred embodiment, this function is satisfied by the provision of a helical return spring 23 which stores energy as drive shaft 1 is coupled by clutch 5 to the gear 2 to drive the print head around its printing path in a counter clockwise direction. Then, when clutch 5 is finally released at the end of printing, spring 23 utilizes the energy stored in it to return and print head and counterweight by driving shaft 1 in the clockwise direction. Excessive rebound forces are taken up by providing a suitable dash pot or other damper means 24 in a position to contact carriage 6 as it returns to the home position opposite the end of station A.
Suitable anti-friction bearings 25 are also included at either end of the drive shaft 1 so that free rotation of the balanced system described can be attained.
Turning now to FIG. 2, a more detailed description of the position sensor 18, the interposer 19 and the apertures 20 and 21 will be provided. As stated previously, the interposer 19 is attached at one end of rotating main shaft 1 while the matrix print head 7, attached to its carriage 6, is rigidly affixed to the opposite end of shaft 1. Shaft 1 was chosen for the attachment point of the interposer 19 because its angular rotation is relatively slow and because the print head 7 is firmly attached to the other end of the shaft, thus eliminating print position variations due to tolerances in the drive train and variations in the motor speed which could cause misreading of the exact head position relative to the position of the interposer 19. interposer 19 has apertures that can be termed a grid and, in the preferred embodiment, the grid is made of etched metal, although it could be made of plastic, photographic film,
or any other material opaque to light. Similarly, if a light transmissive grid system is not desired, reflective material such as used by mark sensing means of an optical type or magnetic material such as used by mark sensing means of the magnetic type could be easily substituted in place of the transmissive photosensor system. As illustrated in FIG. 1, the grid on interposer 19 rotates in an arc of slightly over as the head accesses the three separate print stations shown. The actual linear velocity at the tip 10 of the matrix print head is approximately 8.8 inches a second. The surface velocity at the grid on interposer 19 is somewhat less because it is at a shorter radius from the center of shaft 1 than the tip 10 of print head 7.
The grid portion of interposer 19 has three groups of 30 slits each with a nominal width per slit of 0.02 inches. The slits just named are denominated 20 in the figures and each slit corresponds to a timing mark for the beginning of a character printing cycle. These slits are approximately 0.042 inches apart and are placed at a three inch radius from the center of shaft 1; the actual character spacing produced at the print head is 0.083 inches because of the greater radius at the tip of the print head which has approximately a 3 inch greater radius (a total of 6 inch radius approximately), and the grid was chosen at a 3 inch radius smaller than the radius of the print head tip to minimize weight and torque problems when the rotating shaft was returned to the rest position.
A second series of notches or slits 21 and an open space 22 on the interposer l9 determine the home position and the end of line position for each of the three print stations A, B, and C. This section of the grid is placed at the outer periphery of the interposer 19 and is radially outside the character position grid of apertures 20. The using or host electrical system requires that a home position be distinguished from the three end of line positions in this embodiment. The notches 21 and the open space 22 are made so that two photo detectors 25, one for the grid of apertures 20 and one for notch or space 21, or 22, are activated only at the home position so that the home position can be distinguished from the three end of line positions indicated by notches 21. Thus, pulses are produced due to the movement of the interposer which corresponds to each character spacing position and a separate pulse is produced by notches 21 and finally, a double pulse is produced by a proper placement of a final notch 21 and one of the apertures 20 so that two detectors will be on simultaneously when the head is in the home position illustrated in FIG. 1.
Photo detector 18 is a commercially available device (such as part No. 08-592 5-060, made by HE! Corp. of Chaska, Minn.) which has two light emitting diodes 24, specified as spaced .02 apart, which could as well be lamps, positioned on one side of a U-shaped block of material 18 and facing towards the opposite leg of the U. On the opposite side of the U is positioned a lense 26 to focus light from the light emitting diode onto a photo diode or photo cell which produces electrical pulses whenever light from the light emitting diode source passes through the interposer either through a slit or through one of the notches, and is focused by the lense onto the sensor. The sensor gives a character timing pulse to the using host system for every 47 minutes and 45 seconds of arc traveled at the aforementioned radius and slit size. This are corresponds to approximately one pulse every 9.4 milliseconds at the speed of travel utilized in the preferred embodiment. These pulses are fed over an interface line to the controlling system which will then generate specific wire firing signals at every 0.9 milliseconds to generate proper characters by the repeated firing of the wires in the wire matrix print head 7 between character impulse signals from the detector 18. This, of course, assumes that a character is to be printed; the system connected to the printer will determine whether or not at a given character impulse signal from the detector 18, any character is to be printed at all.
MODE OF OPERATION Although it has not been illustrated in the figures in any detail, it will be understood by those familiar with wire matrix printers, and printers in general, that the signals for causing the generation of dot matrix charac ters must be provided by some kind of logic system at the proper time and in the proper sequence to print out characters as desired. These systems are well-known in the art and do not form a part of the present invention which is directed only towards the printing or marking mechanism itself which produces the proper motions and signal pulses from which the using system can derive timings for the application of electrical currents to the wire firing circuits which drive the wire drivers in the print heads. Two modes of operation are generally possible in wire matrix printers: characters may be printed on demand and one at a time or, as is more usually the case and generally more economical, a plurality of characters, usually enough to make up a line of printing, are stored in a buffer or electronic storage device until a full load of data for printing has been collected. When a load of data for printing has been collected and the print head is sensed to be in the home position, control electronics logic systems will begin emptying the contents of the buffer and simultaneously therewith will engage the print head drive system by energizing the clutch so that as each character position pulse is produced by the position sensor 18, the specific wire firing pattern to generate the desired character then exiting from the buffer can be produced and applied to the wire matrix print head driving circuits to generate the wire impacts for forming the character desired.
In the present embodiment, the return of the print head to the home position energizes two photo sensors as previously discussed whose combined and simultaneous signal is utilized by logic systems having a controlling system to which the printer is attached to signal that the print head is returned to the home position and is ready to begin printing a new line of data on command. Data characters to be printed are then transmitted in serial dot form for each character as the print head moves and the sensor 18 detects the beginning of character printing positions as previously described. In the specific embodiment illustrated, the, same data is printed at one, two, or three stations and, in the normal mode of operation will print at stations A and B each time and station C only when a document to be printed upon has been inserted by hand into the printing area. Inserting such a document by hand in the station C closes a microswitch (not shown) to indicate to the system that a document is in place and that the head should be traversed the full width of its printing path to cause printing in station C as well. When no document is in station C, the clutch is disengaged upon sensing of the second end of line signal by sensor 18 and the return spring 23 will return the printing head to the home position.
ADVANTAGES Many advantages accrue from the specific design of the preferred embodiment of the invention. First, the bearings utilized in supporting the main shaft 1 and, although not shown, the shaft on which drive gear 3 is mounted, and further, in the reduction unit supplied with motor 4, all may be of an inexpensive, nonprecision type due primarily to the low stresses imposed on them due to the nature of the design. Specifically, since the motor is continually operating, loads of great magnitude are not presented and stresses are basically low at the motor shaft and throughout the gear train because the matrix print head 7 is relatively light in weight and is counter balanced by counterweight 8 so that only the rotational inertia of the system must be overcome in addition to the slight rewind torque necessary to store up sufficient energy in spring 23. The output speed delivered by gears 2 and 3 to shaft 1 through clutch 5 would, if the print head mechanism were allowed to rotate through a full 360, be at a speed of only about 13 RPM and thus the main bearings 25 which support shaft 1 need not be of a special high speed or heavy duty design.
Additionally, motor 4 need not be a large size motor since, at the reduced RPM output finally transmitted to shaft 1, very little force is required in the form of torque to accelerate the print mechanism and to store energy in spring 23.
Similarly, too, clutch 5, although it is of a specialized design as pointed out in the referenced IBM Technical Disclosure Bulletin, has low wear characteristics due to the fact that little slippage is experienced and stresses imposed on the clutch parts are quite low due to the low speed and the low force requirements produced in accelerating the print head and mechanism as described.
The power requirement for producing a traverse, or one full oscillation, of the print head from home position to end of travel and home again is probably similar to that required in a linear traversal printing head system, but the duty cycle is less stringent in time requirements because of the low speed motions at the output of the mechanicam system and because of the simplified return spring type of return drive. The system is, of course, more compact than the equivalent linear system would be to print in three adjacent stations because the traversing apparatus required in a linear system and the necessary side supports and bearings have all been eliminated. In addition, because the paper or documents fed through the printing stations take on the arcuate form of the platen, the paper develops a column strength due to its arcuate curvature transverse to a direction of feeding the paper. This allows a much simpler feeding mechanism to be utilized since the inherent column strength and stiffness of the paper are enhanced by the arcuate curvature imparted to it in the area of printing.
In addition, the print head and photo sensor relationship is a direct one-to-one motion control which, due to the fact that the position sensor is fixed and the interposer and the print head are both affixed at relative spacing to the ends of a rigid shaft,'creates an exceedingly accurate position sensing device which is not affected by backlash and manufacturing tolerances. Adjustment of the relative position of the interposer and the print head may be accomplished simply by means of a set screw affixing either element to the drive shaft 1.
Because of the relatively smaller size of the printer, the low power requirement and the simplicity of structure which is made possible by the arcuate platen and reciprocating, arcuately moving print head, the overall weight and expense of the printer and the ease of its manufacture are greatly improved over previous designs. Additionally, no expensive and hard to maintain stepping or incrementing motions are required which require complicated mechanisms and more complicated and expensive electronic systems. Infinite character printing ability within the limits of the dot matrix resolution is inherently provided by the matrix type printers, but the ability to produce a device in which the matrix type of print head is virtually insensitive to outside induced shocks and vibrations because of the balanced nature of the reciprocating rotary drive mechanism make for an improved performance at a relatively lower cost.
While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
What is claimed is:
1. Media marking apparatus, comprising:
at least one mark producing means for making visible marks on a medium;
support means for supporting said mark producing means in proximity to a medium to be marked and in a position to make a mark when said mark producing means is operated;
pivot means mounted at a fixed radial distance from said medium and supported in bearings for rotation about the central axis of said pivot, said axis being perpendicular to the radius establishing said radial distance;
said support means supporting said mark producing means being rigidly affixed to said pivot for rotation therewith to traverse an arc of a circle having its center on the axis of said pivot;
at least one arcuate platen means for supporting said medium at said fixed radial distance from said pivot in a position to be marked upon by said mark producing means while it traverses said arc, said arc of said platen means being cylindrical and having its axis coincident with said axis of said pivot means;
drive means connectable to said pivot means for turning said pivot through an arc;
return drive means coupled to said pivot means for turning said pivot means in an arc in an opposite direction to that produced by the action of said drive means when it is connected to said pivot means; and
motion sensing and quantizing means for indicating the rotation of said pivot and for signalling the relative amount of travel along said arc taken by said support means and said mark producing means in response to said drive means.
2. Marking apparatus as described in claim 1,
said mark producing means comprises a wire matrix printing mechanism; and
said return drive means comprises a resilient, stressed helical spring connected at one end to a fixed point relative to said pivot and at the other end connected to said pivot and encircling it, thereby applying a return torque to said pivot.
3. Apparatus as described in claim 2, further comprising:
a counterweight affixed to said support means in a position to counter balance said printing mechanism and said support so that the center of gravity of the assemblage so produced lies approximately on the central axis of said pivot.
4. Apparatus as described in claim 3, wherein:
a plurality of said arcuate platens for supporting a like plurality of markable media are placed at adjacent quantized distance locations along said arc; and
said motion sensing and quantizing means comprises a source of light, a sensor positioned relative to said source of light, a sensor positioned relative to said source of light to receive light therefrom, and an interrupter means having alternate light transmissive and opaque areas.
5. Apparatus as described in claim 4, wherein:
said source and sensor are affixed in a stationary position; and
said interrupter is adapted to be moved by the motion of said pivot to interrupt the flow of light from same.