GB2292545A - A method of adjusting the head gap of a wire dot impact printer - Google Patents

Abstract

The method comprises determining the thickness of a printing medium (p) on the basis of the time taken for a printing wire (30, Fig. 4) to return from a printing position to its rest position during a test printing. A head gap (g) is optimised by monitoring the speed of a print wire armature (32, Fig. 4) during printing onto a printing medium (p) and determining both the time taken for the print wire (30, Fig. 4) to reach a printing position from a rest position and the time taken for a print wire (30, Fig. 4) to return from the printing position to a rest position. <IMAGE>

Description

Method of Adjusting the Head Gap of a Wire Dot Impact Printer Description The present invention relates to a method of adjusting the head gap of a wire dot impact printer and a printer configured to operate according to the method.

In a conventional wire dot impact printer, a print head is disposed opposite a platen. An ink ribbon and a printing medium are located between the head and the platen. Printing on the medium is performed by impacting with printing wires, mounted in the head, against the platen thereby forcing ink from the ribbon onto the medium. A wire dot impact printer of this kind can used for printing on media of various types and the distance between the tip of the print head and the printing medium, that is the head gap, can be adjusted to an optimum value when thickness of the printing medium or a number of sheets thereof is changed.

Fig. 18 is a flow chart showing a conventional method of adjusting the head gap of a wire dot impact printer; Fig. 19 is a diagram showing sample prints produced during the conventional method. In Fig. 19, (a) is a diagram showing the printing data for a first line of printing; (b) is a diagram showing a test printing pattern; (c) is a diagram showing a reprinted pattern.

Referring to Fig. 18, the power of the wire dot impact printer is turned on at step S1. At step S2, a judgment is made as to whether or not a printing medium is present, and if so, the program step goes to step S3, otherwise, the program step waits for a medium to be inserted. Then, printing data from a host computer (not shown) is received at step S3. The position of the wire head is adjusted to set the head gap g at a reference head gap gA (for instance 0.5 mm) for test printing at step S4. The reference head gap gA is defined as the head gap g under the conditions that an ink ribbon (not shown) and a printing medium P, whose thickness is previously known, are installed. The standard printing time Ts under these conditions is pre-stored in a table in a ROM.At step S5, a test printing of several dots to several tens of dots of a first printing line is performed as shown in Fig. 19(b), and during the test printing, the printing time T is determined. At step S6, the difference between the determined printing time T and the standard printing time Ts, stored in the ROM, is calculated. The difference Ag between the standard head gap gA and an actual head gap g is then calculated using the relationship that a difference of 3 ctsec in the printing time T corresponds to a change in head gap g of 0.01 mrn. The thickness of the current printing medium or media is calculated, on the basis of the thickness of the printing medium P, used to establish the standard printing time Ts, and the difference Ag.At step S7, the change in head position, required to establish the optimum head gap gR, is calculated. Then, printing is performed for the line, on which the test printing is done, as shown in Fig. 19(c) at step S8. Thereafter, printing continues in the normal manner (step S9).

In the conventional method of adjusting the head gap of a wire dot impact printer as described above, it is difficult to adjust the head gap g accurately since there are differences between heads in the standard printing time Ts used for calculating the head gap g and in the determined printing time T.

It is an object of the invention to provide a method of adjusting the head gap of a wire dot impact printer in which the problem, occurring in the conventional method of adjusting the head gap, is solved, in which the printing time of a current print head can be readily determined and memorized by the wire dot impaa printer, itself, and in which a high accuracy in the determination of head gap and improved printing quality are obtainable.

According to the present invention, there is provided a method of adjusting the head gap of a wire dot impact printer, having a print head including a plurality of print wires, the method comprising determining the thickness of a printing medium on the basis of the time taken for a printing wire to return from the limit of its excursion to its rest position during a test printing, and optimizing the head gap on the basis of the determined medium thickness.

In an embodiment, a method according to the present invention comprising the steps of: (a) monitoring the speed of a print wire armature during printing onto the printing medium; (b) determing the time taken for a print wire to return from the limit of its excursion to its rest position from the monitored armature speed; (c) calculating the thickness of the printing medium on the basis of said determined time; (d) calculating the required change in the head gap for optimization thereof on the basis of the calculated printing medium thickness; and (o) resetting the position of the wire dot head on the basis of the calculated required change.

In another embodiment, the determination of the thickness of a printing medium is made additionally on the basis of the time taken for a printing wire to reach the limit of its excursion from its rest position during the test printing. Preferably, this embodiment comprises the steps of: (a) monitoring the speed of a print wire armature during printing onto the printing medium; (b) determing the time taken for a print wire to reach the limit of its excursion from its rest position and the time taken for it to return from the limit of its excursion to its rest position from the monitored armature speed; (c) calculating the thickness of the printing medium on the basis of said determined times; (d) calculating the required change in the head gap for optimization thereof on the basis of the calculated printing medium thickness; and (o) resetting the position of the wire dot head on the basis of the calculated required change.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a block diagram illustrating a wire dot impact printer to which a method of adjusting a head gap according to the invention is applied; Fig. 2 is a plan view showing gap shifting means of the wire dot impact printer; Fig. 3 is a side view showing the gap shifting means; Fig. 4 is a vertical cross section showing a print head of the printer; Fig. 5 is a plan view showing a printed circuit board of the printer; Fig. 6 is a perspective view showing an essential portion of the printed circuit board; Fig. 7 is a diagram illustrating a sensor circuit of the printer; Fig. 8 is a block figure of the sensor circuit; Fig. 9 is a diagram of waveforms of signals for the sensor circuit;; Fig. 10 is a time chart showing the speed waveform of an armature the magnetic flux in the magnetic circuit is changed; Fig. 11 is a time chart showing a speed waveform an armature when the timing of the drive voltage is changed; Fig. 12 is a time chart showing a speed waveform of an armature when the hardness of printing media is changed; Fig. 13 is a flow chart showing a method of adjusting a head gap for a wire dot impact printer according to the invention; Fig. 14 is a diagram showing a comparison of return speeds; Fig. 15 is a diagram showing the relationship between thickness of printing medium, printing time, and return time; Fig. 16 is a diagram showing the relationship between thickness of printing medium, printing time, return time, and printing and return time; ; Fig. 17 is a time chart showing the speed waveform of an armature when the hardness of the printing medium is changed; Fig. 18 is a flow chart showing a conventional method of adjusting the head gap of a wire dot impact printer; and Fig. 19 is a diagram showing a printing sample according to the conventional method of adjusting a head gap for a wire dot impact printer.

Referring to Fig. 1, a wire dot impact printer includes a head driver 3a for driving a print head 4 having a head coil 3b, a motor driver 5 for driving a spacing motor 6 for shifting the print head 4 in the widthwise direction of printing medium, a motor driver 7 for driving a line feed motor 8 for feeding the printing media in a direction perpendicular to the widthwise direaion of the medium, and a motor driver 13 for driving gap shifting means, or gap shifting mechanism, 15 having a pulse motor 14 for changing the head gap of the print head. These drivers 3a, 5, 7, 13 are respectively connected to a control circuit 2 for controlling operation of the entire printer to be controlled thereby.The control circuit 2 includes an interface LSI 2a for inputting printing data through an interface 1, an interface LSI 2b for outputting the printing data, a CPU (Central Processing Unit) 2c for performing processing operations, such as for calculation of the head gap g from detected printing time T, a RAM (Random Access Memory) 2d used as a back-up memory for storing the printing data and an average value Tpa of standard printing times Tp of respective pins, and a ROM 2e for storing control programs and print fonts. The control circuit 2 is also connected to a selector switch 11 for selecting a printing time detection mode, a control switch 9, and printing time detecting means 10 having sensor elearodes 10a provided at the print head 4 and a sensor circuit 10b.

Referring to Figs. 2 and 3, the print head 4 is positioned opposite a platen 25 and is disposed on a carriage 22, which is supported on guide shafts 23, 24 arranged perpendicularly to side frames 26, 27, so as to be movable in direction A. The carriage 22 moves in direction A by receiving power from the spacing motor 6 (shown in Fig. 1) and shifts the print head 4 in the widthwise direction of the printing media P. The platen 25 rotated by power from the line feed motor 8 and conveys the printing medium P in a lengthwise direction, perpendicular to the widthwise direction of the medium.

When printing, the print head 4 moves in the widthwise direction of the printing medium P with a predetermined speed and impacts on an ink ribbon (not shown) at a printing position of the printing medium P with printing wires (not shown). When reaching the end position of the printing medium P and the printing of one line is finished, the print head 4 consequently moves in the opposite direction to return to its initial position. At that time, the platen 25 rotates to feed the printing medium P by one line, and then, printing starts for the next line.

Although the carriage 22 moves along a pair of the guide shafts 23, 24, a rear portion of the carriage 22 is supported by the guide shaft 24 through a level adjustment mechanism 29. That is, the rear portion of the carriage 22 is fixed to the pulse motor 14, whose spindle 14a directly couples with a screw gear 14b. A guide pin 22a is formed at a bottom face of the rear portion of the carriage 22 so as to protrude therefrom and is inserted, so as to be movable up and down, in a guide hole 28a of a slider 28, mounted on and being slidable along the guide shaft 24. The slider 28 is formed with a gear or gears (not shown), which mesh the screw gear 14b. Accordingly, the carriage 22 is supported on the guide shaft 24 by the slider 28, the screw gear 14b, the spindle 14a, and the pulse motor 14.When the pulse motor 14 rotates, the rear portion of the carriage 22 moves up and down in direction C, namely, along the guide pin 22a guided by the guide hole 28a, thereby rotating the carriage around the guide shaft 23. As a result of this operation, the tip 4a of the print head 4 shifts in direction B to change the head gap g formed between the tip 4a and the printing medium P. It is to be noted that other means, for example, shifting the platen 25, in addition to what has been described above, can be used as means for changing the head gap g.

The printing time detecting means 10 will now be described in detail.

Referring to Fig. 4, the print head 4 is comprises a plurality of printing wires 30 (only two are shown in Fig. 4) provided within the head, a front casing 31 having guide holes 31a for guiding the printing wires 30, a plurality of armatures 32 formed of a magnetic material, a plate spring 33 supporting the armature 32, a base plate 34, a plurality of electromagnets 35, each composed of a core 35a and a head coil 35b winding around the core 35a, a printed circuit board 36 for feeding current to the electromagnets 35 and a connector terminal, a permanent magnet 37, a base 38, a spacer 39, a yoke 40, a printed circuit board 41, and a clamp 42.The clamp 42 clamps the base plate 34, the permanent magnet 37, the base 38, the spacer 39, the plate spring 33, the yoke 40, the printed circuit board 41, and the front casing 31 together, one above another, so as to form a unitary body. The armature 32 is supported on a side of an unfixed end 33a of the plate spring 33, and a proximal portion 30a of each printing wire 39 is jointed to a tip 32a of the armature 32. The tip 30b of the printing wire 30 is arranged so as to be guided by the guide hole 31a of the front casing 31 to impact on the printing medium P. As shown in Figs. 5 and 6, a plurality of sensor electrodes 10a, formed by a copper foil pattern, are disposed at positions, corresponding to the armatures 32, on the printed circuit board 41.These sensor electrodes 10a are connected to a connector terminal 41a, provided at the edge of the printed circuit board 41, by the printed wiring. The printed circuit board 41 is coated with an insulating film for isolating the yoke 40. Therefore, static capacitance occurs between the sensor elearodes 10a and the armatures 32. The static capacitance becomes small as the spacing between them becomes large, and large as the spacing between them becomes small.

In a print head 4 thus construaed, when the head coil 35b is not energized, the magnetic field of the permanent magnet 37 attracts the armature 32 toward the base plate 34, or downward in Fig. 4, in opposition to the elastic force of the plate spring 33. If the head coil 35b is energized under these conditions, the magnetic flux of the elearomagnet 35 cancels the magnetic flux of the permanent magnet 37, thereby releasing the armature 32 from attraction of the permanent magnet 37, so that the armature 32 moves toward the front casing 31, or upward in Fig. 4, by the energy stored in the plate spring 33. The printing wire 30 is then projected through the guide hole 31a according to the motion of the armature 32, thereby impacting on the printing medium P.The yoke 40 comprises a part of a magnetic circuit formed by the electromagnet 35, and serves for preventing mutual interference between the sensor electrodes 10a.

In Figs. 7, 8, the sensor circuit 10b is constituted by a digital IC 50 having MOSFETs (Field Effect Transistors) 50a, 50b, an oscillator 51, a resistor 52, an integrator 53, an amplifier 54, a differential circuit 55, and a comparator 56, and is connected to the sensor electrode 10a built into the print head 4. In the sensor circuit 10b thus constructed, the output end of the digital IC 50 is connected to the sensor electrode 10a, and the input end of the digital IC 50 is connected to the oscillator 51. When the digital IC 50 receives a square wave signal Sosc (shown in Fig. 10) from the oscillator 51, a current Ic flows through the output of the digital IC 50. The MOSFETs 50a, 50b turn on and off alternately in dependence on the square wave signal Sosc, so that the current Ic charges and discharges of the sensor electrode 10a. Discharging current Is flows to the ground through the MOSFETS 50b and the resistor 52.

An integral value of the discharging current Is for one cycle is equivalent to the amount of charge Q stored in the sensor electrode 10a.

Where the static capacitance of the sensor electrode 10a is Cx; the frequency of the oscillator 51 is f; the resistance of the resistor 52 is Rs; the gain of the amplifier 54 is a; and the power supply voltage is VDD, the mean value of the discharging current Is is defined by f. Q - f. Cx .VDD and output voltage Vo of the amplifier 54 is defined by Vo - Cx. Rs. a. f. VDD so that an output voltage Vo proportional to the static capacitance Cx, to be determined, is obtained. The output voltage Vo is fed to the differential circuit 55, from which a voltage which is a proportional to the speed of the armature 32 (shown in Fig. 4) is outputted.The output is fed to the comparator 56, so that the sensor circuit 10b outputs printing time T, at the end of which the printing wire 30 impacts on the printing medium P (shown in Fig. 3). In practice, the amplifier 54 is used as an AC amplifier and responds only to the movement of the armature 32, while neglecting the voltage shift (DC components) due distributed capacitance existing in addition to that due to the sensor electrodes 10a.

If changes of magnetic characteristics of the material of the wire dot head 4 or changes of the structure due to wear occur, the magnetic resistance in the magnet circuit changes and the printing time T also changes.

Referring to Fig. 10, I1 represents the current flowing through the head coil 35b, shown in Fig. 4; V1 represents the speed waveform of the armature 32 before the magnetic flux changes; V2 represents the speed waveform of the armature 32 after the magnetic flux has changed; VREF represents the slice level; DTA represents the drive voltage application time; T, represents the printing time before the magnetic flux changes; T2 represents the printing time after the magnetic flux has changed. As shown in Fig. 10, if the magnetic resistance in the magnetic circuit becomes small, reducing the magnetic flux, the speed waveform of the armature 32 changes from V, to V2, and the printing time T changes from T1 to T2, becoming longer.

Referring to Fig. 11, I, represents the current flowing through the head coil 35b, shown in Fig. 4, before the drive voltage application time DTA (Fig. 10) changes; I2 represents a current waveform flowing though the head coil 35b after the drive voltage application time DTA has changed; V, represents the speed waveform of the armature 32 before the drive voltage application time DTA changes; V2 represents the speed waveform of the armature 32 after the drive voltage application time DTA has changed; VREF represents the slice level; T1 represents the printing time before the drive voltage application time DTA changes; T2 represents the printing time after the drive voltage application time DTA has changed.As shown in Fig. 11, if the drive voltage application time DTA becomes short for some reason, the printing time T changes from T1 to T2 and becomes longer.

Referring to Fig. 12, II represents the current flowing through the head coil 35b (shown in Fig. 4); V, represents the speed waveform of the armature 32 before the hardness of the printing medium is changed; V2 represents the speed waveform of the armature 32 after the hardness of the printing medium has been changed; VREF represents the slice level; T, represents the printing time before the hardness of the printing medium is changed; T2 represents the printing time after the hardness of the printing medium has been changed.

For example, in the case when the printing medium P is hard, for example drawing paper not winding around the platen 25 but floating and contacting to the front casing 31of the wire dot head 4, the operation of the printing wire 30 and the armature 32 is restricted from the start of printing, thereby changing the speed waveform of the armature 32 from V, to V2, and, accordingly, the printing time T changes from T, to T2, becoming longer.

The head gap g, the speed v of the armature 32, and the printing time T are related as follows:

Where the speed v is approximately constant, the formula can be approximated by g = T. v - a (2) a: : Constant However, since the printing time T changes, as described above, according to changes such as the magnetic flux in the magnetic circuit, the drive voltage application time DTA, and the hardness of the printing medium, the relation between the head gap g and the printing time T becomes nonlinear.

Consequently, the head gap g can not be calculated accurately.

An embodiment of the invention in which: time between attracting of the armature 32 to the core 35a and returning of the armature 32 (hereinafter, called "return time" is deteaed after printing is done; the thickness of the printing medium P is judged based on the returning time; and the head gap g is calculated, will be described Referring to Fig. 13, Il represents the current flowing through the head coil 35b shown in Fig. 4; Ví represents the speed waveform of the armature 32; VREFR represents a slice level; TR represents the return time. In this case, the speed waveform Vl has a constant absolute value during the return time TR, and, therefore, Formula (1) above can be approximated by Formula (2) above.

That is, in the wire dot head 4, as described above using Fig. 4, the head coil 35b is energized when printing, thereby generating magnetic flux in a direaion such that the magnetic flux of the permanent magnet 37 is cancelled, and thereby releasing the armatures 32 and the printing wires 30 attached to the plate spring 33. In the case when the armature 32 moves by a certain distance, the relationship between attraaing force FM Of the magnetic flux generated by the permanent magnet 37 and the elastic force Fs generated by the plate spring 33 is controllable by adjusting the spring constant of the plate spring 33 and the magnetic field of the permanent magnet 37, and the speed v of the armature 32 can be relatively constant without being influenced by the attracting force FM and the spring force Fs. When the printing wires 30 impact on the platen 25 (shown in Fig. 3) through the printing medium P, since the platen 25 is formed of a material having a small repulsive coefficient, such as rubber or the like, the repulsive force FR received by the printing wires 30 while the force of the impact is substantially constant even if the thickness of the printing medium P changes.

Referring to Fig. 14, V, represents the speed waveform of the armature 32 when a rubber platen 25 (shown in Fig. 3) is impacted on by the printing wire 30 (shown in Fig. 4) through the printing medium P; VA represents the speed waveform of the armature 32 when a metal platen 25 is directly impaaed on by the printing wire 30; VB represents the speed waveform of the armature 32 when a metal platen 25 is impacted on by the printing wire 30 through the printing medium P. VB also represents the speed waveform of the armature 32 when a rubber platen 25 is direaly impacted on by the printing wire 30.

When the platen 25 is impacted on by the printing wire 30, the repulsive force FR forces the printing wire 30 to be returned. At this time, as shown in Fig. 14, if the metal platen 25 is directly impacted on by the printing wire 30, the return speed VA, when the armature 32 is attracted by the core 35a, is high, whereas the return speed VB is low when a thick printing medium P is placed between the platen 25 and the printing wire 30, so that the return speed VB is almost the same as the return speed V1 when a rubber platen 25 is used. The return speed varies between VA and VB in accordance with the thickness of the printing medium P, thereby affeaing the speed v of the armature 32 in the return period.On the other hand, the return speed VB in the case when a rubber platen is direaly impaaed on by the printing wire 30 is almost the same as the return speed Vl in the case of an intermediate printing medium P. That is, in the case of a rubber platen 25, the thickness of the printing medium P does not change the return speed. As a result, although the printing wire 30 is affected by the attraaing force FM, the spring force Fs, and the repulsive force FR through impacts during printing, the speed v of the armature 32 is approximately constant when the platen 25 is of a material with a small repulsive coefficient. On the other hand, the printing time T is affeaed by magnetic force Fc generated by the current fed to the head coil 35b, in addition to the attracting force FM and the spring force Fs.

Since it is produced by a transitional current flowing the head coil 35b, the magnetic channelling force Fe changes in a nonlinear manner corresponding to the increasing rate of transition. Furthermore, the magnetic channelling force Fc affects the speed v of the armature 32 more than the attracting force FM or the spring force Fs. Accordingly, the operations of the armature 32 and the printing wire 30 becomes nonlinear. Since the speed v of the armature 32 during the return time TR is lower than the speed v of the armature 32 during the printing time T, the time required for the printing wire 30 to move across the same head gap g becomes longer, so that as the detection region, or the dynamic range, of the return time TR becomes broader, the detection is done with high accuracy.That is, since a material having a small repulsive coefficient, such as rubber or the like, is used for the platen 25, the speed v of the armature 32 is reduced after the armature 32 impacts the platen 25.

Accordingly, the speed v of the armature 32 during the return time TR is less than that before the armature 32 impacts, so that the time required for the printing wire 30 to shift across the head gap g becomes longer than the printing time T.

Referring to Fig. 15, T represents the printing time and TB represents the return time. As shown in Fig. 15, if the thickness of the printing medium P changes between a and ss, the printing time T changes in a range TB whereas the return time TR changes in a range TA. In this case, since: TA > TB, the detection region of the return time TR becomes broad, so that the detection is done with high accuracy. Disturbances such as changes in the magnetic characteristics of the wire dot head 4 and changes in the drive voltage application time DTA have little influence on the detection of the return time TB, SO that effect of the thickness of the printing medium P can be calculated stably.That is, as described above, the printing time T is affected by the attracting force FM, the spring force Fs, the repulsive force FR, and the magnetic cancelling force Fc, whereas the return time TR is affected by the attracting force FM, the spring force Fs, and the repulsive force FR but not by the magnetic channelling force Fc. Accordingly, the influence of disturbances is suppressed. Although floating of the printing medium P does affea the printing time T before the printing medium P is pushed against the platen 25, floating of the printing medium P does not affect the return time TR after the printing medium P has been pushed against the platen 25 because the printing wire 30 is returned by an almost constant repulsive force FR.

Moreover, even though some nonlinearity may exist, the thickness of the printing medium P is judged on the basis of the printing and return time TT, comprising the printing time T and the return time TR, and the head gap g is calculated with high accuracy.

On the basis of the foregoing, the head gap g is set by monitoring the speed of a print wire armature during printing onto the printing medium and then determing the return time from the monitored armature speed. The thickness of the printing medium is calculated on the basis of the return time and then used to calculate the required change in the head gap for optimization thereof.

Finally, the position of the wire dot head is reset in the newly determined optimum position.

Referring to Fig. 16, T represents the printing time; TR is the return time and TT iS the printing and return time. As shown in Fig. 16, if the thickness of the printing medium P changes between a and ss, the printing time T changes in a range TB and the return time TR changes in a range TA, whereas the printing and return time TT changes in a range TA+B. Though the printing and return time TT assumes some nonlinear changes, since: TA+B > TA > TB, the detection region of the printing and return time TT becomes broad, so that the detection is done with high accuracy.

On the basis of the foregoing, the head gap g is optimized by monitoring the speed of a print wire armature during printing onto the printing medium and determining both the time taken for a print wire to reach the limit of its excursion from its rest position and the time taken for it to return from the limit of its excursion to its rest position therefrom. Then, the thickness of the printing medium is calculated on the basis of the determined times. The change required to optimize head gap g is calculated from these times.

Finally, the head gap g is set to its optimum position.

Furthermore, setting the slice level VREF to a higher level than an original point of the speed waveform allows detection of the float of the printing medium P. Referring to Fig. 17, 11 represents a current waveform flowing through the head coil 35b shown in Fig. 4; V, represents the speed waveform of the armature 32 before the hardness of the printing medium is changed; V4 represents the speed waveform of the armature 32 when the printing medium is hard and floating; VREF represents the slice level; Tsl represents the operation time from the start of the application of the drive voltage to the time when the speed v of the armature 32 reaches the value where the speed waveform Vl crosses the slice level VREF; TU represents the operation time from the start of the application of the drive voltage to the time when the speed v of the armature 32 reaches the value where the speed waveform V4 crosses the slice level VREF. Accordingly, the floating condition of the printing medium P can be detected by the difference between the operation times Tsl, T52.

This application is divided out of United Kingdom patent application no.

9314630.6

Claims (6)

1. Claims 1. A method of adjusting the head gap of a wire dot impact printer, having a print head including a plurality of print wires, the method comprising determining the thickness of a printing medium on the basis of the time taken for a printing wire to return from the limit of its excursion to its rest position during a test printing, and optimizing the head gap on the basis of the determined medium thickness.
2. 2. A method according to claim 1, comprising the steps of: (a) monitoring the speed of a print wire armature during printing onto the printing medium; (b) determing the time taken for a print wire to return from the limit of its excursion to its rest position from the monitored armature speed; (c) calculating the thickness of the printing medium on the basis of said determined time; (d) calculating the required change in the head gap for optimization thereof on the basis of the calculated printing medium thickness; and (o) resetting the position of the wire dot head on the basis of the calculated required change.
3. 3. A method according to claim 1, wherein the determination of the thickness of a printing medium is made additionally on the basis of the time taken for a printing wire to reach the limit of its excursion from its rest position during the test printing.
4. 4. A method according to claim 3, comprising the steps of: (a) monitoring the speed of a print wire armature during printing onto the printing medium; (b) determing the time taken for a print wire to reach the limit of its excursion from its rest position and the time taken for it to return from the limit of its excursion to its rest position from the monitored armature speed; (c) calculating the thickness of the printing medium on the basis of said determined times; (d) calculating the required change in the head gap for optimization thereof on the basis of the calculated printing medium thickness; and (o) resetting the position of the wire dot head on the basis of the calculated required change.
5. 5. A method of adjusting the head gap of a wire dot impact printer substantially as hereinbefore described with reference to Figures 10 to 16 of the accompanying drawings.
6. 6. A printer configured to perform a method according to any preceding claim.