US3867675A

US3867675A - Magnetic drive mechanisms for printing heads

Info

Publication number: US3867675A
Application number: US388664A
Authority: US
Inventors: Norbert Kitz; James John Drage
Original assignee: Bell Punch Co Ltd
Current assignee: Bell Punch Co Ltd
Priority date: 1972-08-18
Filing date: 1973-08-13
Publication date: 1975-02-18
Anticipated expiration: 1992-02-18
Also published as: DE2341754A1; IT998388B; GB1392010A; IE37985B1; LU68258A1; NL7311402A; FR2196606A5; IE37985L

Abstract

There is disclosed a drive mechanism for a printing head of a printing machine of the kind where the print head moves with respect to the material to be printed, the mechanism including an elongated electromagnet to provide a magnetic field, a coiled electrical conductor slidably mounted for movement under the influence of the magnetic field and adapted for connection to a print head, and a circuit connected to the conductor for sensing the position of the electrical conductor with respect to a datum and for controlling the supply of electric current to the conductor so as to control the movement of the conductor. The supply of current to the electromagnet and the coil are independently variable. The position sensing circuit can be used to control the supply of current to the electromagnet and the coil.

Description

madam United States Patent 1191 Kitz et al,

1111 3,867,675 Feb. 18, 1975 1541 MAGNETIC DRIVE MECHANISMS FOR PRINTING HEADS a [75] Inventors: Norbert Kitz; James John Drage,

both of Uxbridge,-England [73] Assignee: Bell Punch Company Limited,

Uxbridge, England [22] Filed: Aug. 13, 1973 [21] Appl. No.: 388,664

[30] Foreign Application Priority Data Aug. 18, 1972 Great Britain 38759/72 Nov. 17, 1972 Great Britain 53193/72 [52] US. Cl 318/119, 318/127, 318/135, 310/l4,3l0/15 [51] Int. Cl l H02k 33/00 .HO2k41/00 [58] Field of Search 318/119, 122, 126, 127,

[56] References Cited UNITED STATES PATENTS 3,688,035 8/1972 Cless .1 310/13 X Primary Examiner-Gerald Goldberg Attorney, Agent, or Firm-Laurence R. Brown [5 7] ABSTRACT There is disclosed a drive mechanism for a printing head of a printing machine of the kind where the print head moves with respect to the material to be printed, the mechanism including an elongated electromagnet to provide a magnetic field, a coiled electrical conductor slidably mounted for movement under the influence of the magnetic field and adapted for connection to a print head, and a circuit connected to the conductor for sensing the position of the electrical conductor with respect to a datum and for controlling the supply I of electric current to the conductor so as to control the movement of the conductor.. The supply of current to the electromagnet and the coil are independently variable. The position sensing circuit can be used to control the supply of current to the electromagnet and p the coil.

5 Claims, 12 Drawing Figures Pmmgmimsmrs 3,867,675

CONTROL INPUTS OUTPUTS FROM 5 22 5 T0 POSITION SENSORS 2' Hm I PRINT HEAD 46 44 42 W 47 /PRINT 1 MAGNETIC DRIVE MECHANISMS FOR PRINTING HEADS This invention relates to printing machinesof the kind where on operation a print head is reciprocated with respect to the material which is printed by the print head. The definition ofprint head includes heads which print by the impact of movable members, for example hammers and solenoid driven needles, heads which print by heat, light and other forms of electromagnetic waves, heads which print by the projection or attraction of charged particles to mark the material and any other form of head by which material is marked.

The definition of print head includes a head using a combination of any of the above material-marking methods. Examples of such printing machines are typewriters which use the gold ball print head and those digital printers, such as line printers in which the print head moves with respect to the material to be printed. it has been previously known to produce the relative motion by means of a drive mechanism having an electric drive motor geared to an endless belt to which is attached to the print head. The drive motor is controlled by mechanically actuated control circuits to control the extent of movement in one direction and reversal of movement after movement of a predetermined extent. The known drive mechanism has the disadvantages of complexity, high cost, and speed limitations because of the inertia forces in the mechanism. It has been proposed to produce the relative motion by means of a bar of high-permeability magnetic material which passes through a solenoid which is connected to a print head. Thisproposal suffers from the disadvantages that the print head is limited to a low weightbecause of level of flux density of the magnetic field; which flux density cannot be increased to accelerate a standard print head and solenoid so as to overcome the inertia and static friction forces to give a speed comparable with prior drive mechanisms. Furthermore, the bar of magnetic material is heavy and is expensive in the amount required to produce the standard carriage movement or stroke.

According to the present invention there is provided a drive mechanism for a print head of a printing machine, the mechanism including a magnetic circuit having an elongated member and having at least one electromagnet circuit to provide a magnetic field normal to the longitudinal axis of the elongated member, an electrical conductor slidably mounted relative to the elongated member for movement under the influence of the magnetic field and adapted for connection to a print head, an electric circuit for supplying electric current to the conductor, and switch means connected in at least one of the circuits so as to control the extent of movement of the conductor.

Preferably the electric circuit for supplying electric current to the conductor includes means for controlling the variation of the electric current supplied.

According to a further aspect of the present invention there is provided a drive mechanism for a print head of a printing machine, the mechanism including an electromagnet having a first elongated member and at least a second elongated member connected parallel with the first member to provide a magnetic field between the members, an electrical conductor slidably mounted on a member for movement under the influence of the magnetic field and adapted for connection to a print head, and a circuit connected to the conductor for controlling the supply of electric current thereto so as'to control the extent of movement of the conductor. V

Preferablythe electrical conductor is a solenoid "of at least one helix wrapped aroundan elongated member.

Preferably the electromagnet; includes a circuit .for controlling the variation of the electric current supplied to the electro-magnet.

Preferably at least one of the circuits supplying electric current to the electric conductor and to the electromagnet includes means for changing the polarity of the electric current supplied.

Preferably the control circuit includes a means for sensing the position of the electrical conductor with respect to a datum.

According to a still further aspect of the present invention there is provided a drive mechanism for a print head of a printing machine, the mechanism including means to provide a magnetic field, an electrical conductor slidably mounted for movement under the influence of the magnetic field and adapted for connection to a print head and a circuit connected to the conductor for sensing the position of the electrical conductor with respect to a datum and for controlling the supply of electric current to the conductor so as to control the movement of the conductor.

Preferably the electrical conductor is a solenoid of at least one helix wrapped around an elongated member.

Preferably the magnetic-field-providing means is an electromagnet; and preferably the control circuit includes a circuit for controlling the variation of the electric current supplied to the electro-magnet.

Preferably the electric current supplied to the electro-magnet circuit can be varied independently of the current supplied to the electrical conductor.

Preferably at least one of the circuits supplying elec' tric current to the electrical conductor and the electromagnet includes means for changing the polarity of the electric current supplied.

Preferably the circuit for sensing the position of the electrical conductor causes the control circuit to control the supply of electrical current to the electrical conductor and to the electromagnet so as to control the movement of the conductor.

Preferably the electrical conductor can be'automatically returned to an initial position from any point on the extent of movement of the conductor.

Preferably the electrical conductor can move by at least one character position after an extent of movement equal to at least one character to facilitate reading what has been printed so far.

Preferably the electrical conductor can move an extent equal to at least one character position to print at least one character, can reverse the movement by at least the same number of character positions to facilitate reading what has been printed so far and can then reverse to move in the original direction to print at least one further character.

Constructional embodiments will now be described, by way of examples only, with reference to the accompanying drawings wherein:

FIG. 1 shows an isometric view of a first drive mechanism made in accordance with the invention;

FIG. 2 shows a plan cross-section taken on the line A-A on the drive mechanism shown in FIG. 1 together with a diagram of the circuits which control the operation of the drive mechanism;

FIG. 3 shows a vertical cross-section taken on the line 8-8 or the drive mechanism of FIG. 1;

FIG. 4 shows an isometric view of a second drive mechanism made in accordance with the invention;

FIG. 5 shows a vertical cross-section taken on line A-A on the drive mechanism shown in FIG. 4 together with a diagram of the circuits which control the operation of the drive and the print head mechanism;

FIG. 6 shows a vertical cross-section taken on the line B-B on the drive mechanism of FIG. 4 with the side cover removed from the position sensor;

FIG. 7 shows a view of a third drive mechanism made in accordance with the invention;

FIG. 8 shows a plan cross-section taken on the line A-A on the drive mechanism shown in FIG. 8 together with a diagram of the circuits which control the operation of the drive mechanism;

FIG. 9 shows a vertical cross-section taken on the line AA.on the drive mechanism of FIG. 8;

FIG. 10 shows an isometric view of a fourth drive mechanism made in accordance with the invention;

FIG. 11 shows a vertical cross section taken on line AA on the drive mechanism shown in FIG. 10 together with a diagram of the circuits which control the operation of the drive and print head mechanism; and

FIG. 12 shows a vertical cross-section taken on line BB on the drive mechanism of FIG. 10 with the side cover removed from the position sensor.

The first drive mechanism 10 shown in FIG. 1 has a base 12 which consists of a first elongated member in the form of a bar 14 of rectangular cross-section and of magnetic material such as mild steel, second elongated members in the form in bars 16 of rectangular crosssection and of magnetic material such as mild steel and a pair of end plates 18 which connect the bar 14 to the bars 16. At each end of the base 12 a coil former 20 of non-magnetic material is secured on the bar 14 and a coil of wire 22 is wound on the coil former 20 to make the carriage bar 14 act as an electromagnet.

As shown in FIG. 2 the coils 22 are connected to a smoothing circuit 24 and to a full-wave rectifier bridge 26 which is connected to the mains voltage of 230 volts a.c. through a switch 28.

A coil former 30 having shock-absorbing material 34 on the cheeks of the former is slidably mounted on the bar 14 and an electrical conductor in the form ofa solenoid coil 32 is wound on the former 30. A standard sev en-byfive matrix print head 36 is secured to the coil former 30 to clear the upper edges of the bars 16. The

solenoid coil 32 is connected to the output of a control to position control circuitry (not shown) and the other inputs connected to the output of a full-wave rectifier bridge 42. The input of the rectifier bridge 42 is connected to the output of a step-down transformer 44 the output of which is connected through a switch 46 to the mains voltage of 230 volts a.c.

In operation, the energised coils 22 generate a magnetic field between the bar 14 and the bars 16 and this magnetic field urges the solenoid coil 32, the former 30 and the print head 36 parallel with the longitudinal axis of the

bars

14 and 16 at a speed and a direction which depends on the magnitude and direction of current flow in the solenoid coil 32 determined by the control circuit 40. The control circuit 40 controls the speed of reciprocation of the print head 36 between a position where the coil former 30 abuts one of the coil formers 20 and a position where the coil former 30 abuts the other coil former 20 by generating a repeated symmetrical waveform in which the shape of the waveform determines the speed and direction of the print head 36.

The control input 41 of the control circuit 40 is controlled by other circuits (not shown) which drive the material to be printed (not shown) and the print head 36 so that seven successive reciprocations of the print head 36 print a line of characters on the material.

The circuit connected to the coils 22 can be controlled instead of, or as well as, the circuit connected to the solenoid coil 32. The second drive mechanism 10, shown in FIG. 4, has the base 12 having the

bars

14 and 16, the end plates 18, the coil formers 20, the coils 22 as previously described and previously shown in FIGS. 1, 2 and 3.

As shown in FIG. 5 the coils 22 of FIG. 4 are connected to the smoothing circuit 24 and to a full-wave rectifier bridge 26 which is connected to the mains voltage of 230 volts a.c. through a switch 28.

A coil former 51 is slidably mounted on the uppermost bar of the bars 16 and an electrical conductor in the form of a solenoid coil 48 is wound on the former 51. The former 51 is connected to an n-shaped saddle 50 of nonmagnetic material which saddle is slidably mounted on the bar 14 by the four pairs of rollers 52. The print head 36 is secured to the saddle 50.

A tape 53 of rigid non-magnetic material is attached at each end to a check of the coil former 20 respectively. The tape 53 passes through a tape senser 54 which comprises a line of seven light-emitting diodes 56 on one side of the tape 53 and a line of seven photoelectric diodes 58 on the other side of the tape and parallel with the diodes 56. The tape 53 is perforated longitudinally in seven lines to form a seven bit tape in which each signal of seven bits is a code representing the position of this signal from a zero signal sensed when the saddle 50 abuts each end of the coil formers 20 respectively. The connections to the light-emitting diodes 56 and the photo-electric diodes 58 are connected by a cable 60 to a control circuit 62. The control circuit 62 also has connections to the solenoid coil 48, and the print data input connections which transmit the codes representing the characters to be printed and has connections to the outputs of the print head 36. The control circuit 62 also has a pair of inputs connected to the output of the full-wave rectifier bridge 42 which is connected to the output of a transformer 44 the input of which is connected through the switch 46 to the main voltage of 230 volts a.c.

In operation the second drive mechanism operates similarly to the first drive mechanism. The print data transmitted to the control circuit 62 includes control signals such as print on, print off, space, Tab, and carriage return and these control signals are compared with the seven control inputs from the tape sensor 54 to control the amplitude and polarity of the dc. voltage transmitted to the solenoid coil 48 and the operation of the print head 36.

The method of mounting the print head 36 of the second drive mechanism 10 has the advantage that there is greater mechanical stability.

The print data transmitted to the control circuit 62 can be in any of the standard codes, such as the ASCll code.

The voltages transmitted from the control circuit 62 to the solenoid coil 48 can vary in polarity and amplitude to control the direction and speed respectively of the print head 36 and can have a pulsed duty cycle and pulsed amplitude in response to tabulate or carriage return signals.

The circuits connected to the solenoid coils 22 can be controlled instead of, or as well as, the circuit connected to the solenoid coil 48.

The third drive mechanism shown in FIGS. 7 to 9 has a rod 11 of magnetic material such as mild steel, a tube 14' of magnetic material such as mild steel with an axial slot 16 (FIG. 1) in the tube 14' and a pair of circular end plates 18 which are of magnetic material such as mild steel and which connect the tube 14' coaxially to the permanent magnet rod 11. On the rod 11 and adjacent the end plates 18 are circular coil formers 20 each with a coil of wire 22 which is connected to the smoothing circuit 24, the rectifier bridge 26 and the switch 28 previously described with reference to FIGS. 2 and 5 so that the rod 11 forms an electromagnet.

A coil former 30 is slidably mounted on the rod 11 and an electrical conductor in the form of a solenoid coil 32 is wound on the former 30. A standard sevenvertical matrix print head 36 is secured by a pair of L- shaped brackets 33 made of non-magnetic material to the coil former 30 to clear the upper edges of the axial slot 16' in the tube 14'. A pair of non-magnetic roller bearings 34 (FIG. 3) are mounted between the print head 36 and the tube 14'. The solenoid coil 32 is connected to the output of a control circuit 37 having inputs 38 which are connected to solenoid-positionsensing circuitry (not shown) which senses the position of the coil former 30 with respect to a datum position so as to control the supply ofelectric current to the solenoid coil 32. The control circuit 37 has outputs 39 which are connected to the print head 36 and inputs 40 which are connected to a source of print data. The other inputs 41 of the control circuit 37 are connected to the output of a full-wave rectifier bridge 42. The input of the rectifier bridge 42 is connected to the output of a stepdown transformer 44 the input of which is connected through a switch 45 to the mains voltage 230 volts a.c.

In operation, the magnetised rod 11 generates a magnetic field between the rod 12 and the tube 14' and this magnetic field urges the solenoid coil 32, the former 30 and the print head 36 parallel with the coaxial longitudinal axes of the rod 12 and the tube 14 at a speed and a direction which depends on the magnitude and direction of current flow in the solenoid coil 32 determined by the solenoid-position sensing circuitry (not shown) and the control circuit 37. The control circuit 37 controls the speed of reciprocation of the print head 36 between a position where the coil former 3,0 abuts one of the coil formers 20 and a position where the coil former 30 abuts the other coil former 20 by generating a repeated symmetrical waveform in which the shape of the wave-form determines the speed and direction of the print head. The control circuit 37 automatically controls the supply of current to the solenoid coil 32 so that the print head 36 is automatically returned to an initial position which is at either of the farthest extents of movement of the print head along the axis of the rod 11. The initial positions to which the print head 36 is automatically returned may be at preselected positions along the axis of the rod 11. The control circuit 37 is controlled by other circuits (not shown) with control the drive of the material to be printed (not shown) and the print head 36 so that at seven successive character positions the print head 36 prints a five-column character and leaves a space of two character positions on the material. The control circuit 37 can control the supply of current to the solenoid coil 32 so that the print head 36 is moved automatically by one character position after a movement equal to at least one character position so as to facilitate reading what has been printed so far. The control circuit 37 can also control print head 36 to move automatically by one character position. to reverse automatically to move back by at least two character positions so as facilitate reading what has been printed so far, and then to reverse automatically to move in the original direction by at least three character positions.

A fourth drive mechanism 10' shown in FIG. 10 has a base 12 which consists of a first elongated member in the form of a bar 14 of rectangular cross-section and of magnetic material such as mild steel, second elongated members in the form in bars 16 of rectangular crosssection and of magnetic material such as mild steel, and a pair of end plates 18 which connect the bar 14 to the bars 16. At each end of the base 12 a coil former 20 of non-magnetic material is secured on the bar 14 and a coil of wire 22 is wound of the coil former 20 to make the base 12 act as an electromagnet.

As shown in FIG. 11 the coils 22 are connected to the smoothing circuit 24 and to a full-wave rectifier bridge 26 which is connected to the mains voltage of 230 volts a.c. through a switch 28.

A coil former 51 is slidably mounted on the bar 14 and an electrical conductor in the form of a solenoid coil 48 is wound on the former 51. The former 46 is connected to a U-shaped saddle 50 of non-magnetic material which saddle is slidably mounted on the bars 16 by the eight pairs of rollers 52. The print head 36 is secured to the saddle 50.

The fourth drive mechanism 10' includes solenoidpositioning circuitry in the form of a rigid strip 53 of plastics material which is attached at each end to a respective cheek of the coil former 20 and which carries transverse lines (not shown) each line several thousandths of an inch wide and spaced such that the lined and non-lined portions of the strip 53 are of equal width, and a casing 54 which is attached to the saddle 50 and which has the strip 53 passing through. The casing 54 contains a short strip 55 of plastics material which is attached to the casing 54 with its longitudinal axis parallel with and spaced a few thousandths of an inch from the longitudinal axis of the strip 53. The strip 55 carries parallel lines (not shown) each equal in width and spacing to the lines (not shown) on the strip 53, but inclined by the width of half a line to the normal of the longitudinal axis of the strip 55. The casing 54 also contains a lamp 56 and two

phototransistors

57 and 58 arranged so that the strips 53 and 55 separate the lamp 56 from the

phototransistors

57 and 58. The

phototransistors

57 and 58 are on opposite sides and are equispaced from the transverse axis of the strips 53 and 55. The lamp 56 and the

phototransistors

57 and 58 are connected by a cable 60 to a control circuit 62 which is similar to the control circuit 37. The control 7 circuit 62 also has connections to the solenoid coil 4-8 and the print data input connections which transmit the codes representing the characters to be printed and has connections to the outputs of the print head 36. The control circuit 62 also has a pair of inputs 41 connected to the output of the full-wave rectifier bridge 42 which is connected to the output of the transformer 44 the input of which is connected through the switch 45 to the mains voltage of 230 volts a.c. In operation the fourth drive mechanism operates similarly to the third drive mechanism. ln operation, the relative movement of the strip 55 with respect to the strip 53 causes the generation of a succession of moire fringes which are parallel with longitudinal axes of the strips 53 and 55 and which travel towards the longitudinal edges of the strips 53 and 55 at a speed dependent upon the relative speeds of the strips 53 and 55. The

phototransistors

57 and 58 count the fringes to generate pulse trains in which the distance between each pulse is equal to one column distance and in which the phase of the train of pulses from the phototransistor 57 with respect to the train of pulses from the phototransistor 58 (which train of pulses acts as a datum) is related to the speed and direction of motion of the solenoid 48. The datum pulses of the train of pulses from the phototransistor 58 are counted by character counter circuit (not shown) in the control circuit 62, which character counter circuit transmits a start of track signal at a zero pulse count and in effect counts the number of column positions between the position of the solenoid 48 and the start of track signal. The pulses of the train of pulses from the phototransistor 58 are also divided into successive groups of seven pulses which pulses control circuits (not shown) within the control circuit 62 so that the first five pulses allow the printing of five columns by the print head 36 is one complete character, and the other two pulses do not allow printing so that there is a space of two columns between successive characters.

The speed of the solenoid 48 is maintained approximately constant by the action of speed control pulses formed by differentiating the trains of pulses from the

phototransistor

57 and 58 upon the circuit (not shown) in the control circuit 62 which supplies current 62 which supplies current pulses to the solenoid 48. The differentiated datum pulses from the phototransistor 38 control the end of the solenoid current pulses and the differentiated pulses from the phototransistor 57 override the normal beginning of the solenoid current pulses at approximately 700 usec from the end of the current pulses so as to control the width of the solenoid current pulses. The control action is a form of negative feedback in that as the solenoid speed increases the width of the solenoid current pulses decreases to supply less current to the solenoid 48 and when the solenoid 48 slows down, the width of current pulses increases to increase the speed of the solenoid 48. The speed of the solenoid 48 is thus maintained within two speed limits.

The print data transmitted to the control circuit 62 includes control signals such as print on, print off, space, tab and carriage return and these control signals also control the amplitude and polarity of the do voltage transmitted to the solenoid coil 48 and the operation of the print head 36.

The method of mounting the print head 36 of the fourth drive mechanism has the advantage that there is greater mechanical stability than the mechanical stabilities of the first and second drive mechanisms.

The print data transmitted to the control circuit 62 can be in any of the standard codes such as the A511 code.

The voltages transmitted from the control circuit 62 5 to the solenoid coil 48 can vary in polarity and amplitude to control the direction and speed respectively of the print head 36 and can have a pulsed duty cycle and pulsed amplitude in response to tabulate or carriage return signals. The control circuit 62 can cause the automatic movement of the print head 36 previously described in respect of the control circuit 37.

The circuits connected to the solenoid coils 22 can be controlled instead of, or as well as. the circuit connected to the solenoid coil 32.

The solenoid-position-sensing circuitry can include photo-optical devices magnetic devices or sliding contacts to generate pulses representing characters as column position.

The circuit for controlling the supply of currents to the electrical conductor can decelerate or stop the movement of electrical conductor by open-circuiting or short-circuiting the electrical conductor or by suddenly reversing the polarity of the current flowing through the electrical conductor.

The circuit for controlling the supply of current to the electro-magnet can decelerate or stop the move ment of the electrical conductor by reversing the polar ity of the current flowing through the electromagnet either separately or in combination with a change in the current flowing through the electrical conductor.

The circuits for controlling the supply of current to the electrical conductor and the electromagnet can operate separately or together to bring the electrical conductor to a stop without mechanical impact at either end of its movement.

What we claim is:

l. A drive mechanism for moving a print head over a longitudinal path comprising in combination, a first elongated member in the form of a length of magnetic material such as mild steel disposed parallel to said path, a second elongated member in the form of a length of magnetic material such as mild steel disposed parallel to said path and spaced from said first member, means disposed for producing a magnetic field coaxially through the length of one said member and with the two said members located in a closed magnetic path for the magnetic field, and a movable print head carriage mounted on a selected one of said members for movement over said path comprising an electromagnetic coil disposed coaxially around the selected mem' her for control of movement of said carriage along said path by means of a magnetic field generated thereby reacting with said magnetic field through the length of said member.

2. A drive mechanism as defined in claim 1 wherein said means producing the magnetic field comprises at least one electromagnetic solenoid disposed coaxially about one of said members.

3. A mechanism as defined in claim 1 including a longitudinal member located parallel to and along said longitudinal path having defined therein codes for identifying different positions along said path, and means for selecting said codes to move the print head to selected positions along said path.

4. A mechanism as defined in claim 1 including a longitudinal strip member located parallel to and along said longitudinal path having defined thereon a set of being transparent, and wherein said means counting said lines comprises a set of phototransistors and a lamp arranged to sense a succession of Moire fringes parallel to the longitudinal axes of said two strip members.

Claims

1. A drive mechanism for moving a print head over a longitudinal path comprising in combination, a first elongated member in the form of a length of magnetic material such as mild steel disposed parallel to said path, a second elongated member in the form of a length of magnetic material such as mild steel disposed parallel to said path and spaced from said first member, means disposed for pRoducing a magnetic field coaxially through the length of one said member and with the two said members located in a closed magnetic path for the magnetic field, and a movable print head carriage mounted on a selected one of said members for movement over said path comprising an electromagnetic coil disposed coaxially around the selected member for control of movement of said carriage along said path by means of a magnetic field generated thereby reacting with said magnetic field through the length of said member.

4. A mechanism as defined in claim 1 including a longitudinal strip member located parallel to and along said longitudinal path having defined thereon a set of transverse lines and spaces of equal width, and means counting said lines as the print head moves along said longitudinal path.

5. A mechanism as defined in claim 4 including a second longitudinal strip member parallel to the first with a set of lines similar to that of the first, both said strips being transparent, and wherein said means counting said lines comprises a set of phototransistors and a lamp arranged to sense a succession of Moire fringes parallel to the longitudinal axes of said two strip members.