Thermal print head

Abstract

 Thermal print head with a thermal print substrate on which are arranged a plurality of print resistors (R1-R416) which are adapted for selective electric heating, for generating of a dot pattern on a paper web which is in contact with the print resistors. Each resistor is provided with a control unit (B1-B32) included in the print head. The control units comprise members for storage of information (DATA) which is supplied to the print head and which defines the selected resistors. A control pulse (STR) is supplied to the print head for activation of the selected resistors. The control units comprise delay members (Hi) for successive delay of the control pulses which are supplied to the control units of the different resistors, thus obtaining stepwise switching on and off of the print resistors.

Description

[0001] The invention relates to a thermal print head according to the precharacterising part of claim 1.

[0002] Thermal print heads of this kind are previously known. In such a print head the substrate is usually formed as a rectangular plate. On the plate a number of small print re­sistors - up to several hundred - are arranged adjacent to each other along a straight line. A paper web of heat-sensi­tive paper is fed in a stepwise manner past the print head in such a way that the paper web makes contact with the print resistors. For each "row" on the paper web, a desired set of print resistors are activated by sending a short cur­rent pulse through the selected resistors, upon which these are heated and generate a row of dots on the paper. The pa­per web is then moved one step, a new set of print resistors are activated, and so on. In this way, characters (letters, figures, graphic information, etc.) may be printed on the paper web.

[0003] In those cases where all the print resistors, or a large number of them, are activated at the same time, the total supply current pulse to the thermal print head will have a high amplitude which may amount to several tens of amperes. The flank of the current pulse therefore becomes very steep, both upon switching on and switching off. These steep flanks tend to cause considerable electrical interference and to give rise to electro-magnetic radiation. It is known that, when the print resistors are switched on and off, the tran­sients may be reduced by a suitable arrangement of capaci­tors or inductors. However, these must be dimensioned for both the full supply voltage and for maximum amplitude of the load current. Components of these kinds therefore entail a considerable complication and an increase in price, and it is difficult to obtain a good transient attenuation in this way.

[0004] In thermal print heads of the kind referred to here it is known to use groupwise, non-overlapping current supply or groupwise, non-overlapping activation ot the print resistors with a view to reducing the amplitude of the total supply current. However, this requires a plurality of additional contact surfaces of the thermal print head, which entails a complication and a cost increase as well as a reduction of the reliability of the system of which the thermal print head is a part. Furthermore, this principle of current sup­ply reduces both the printing speed and the quality of the printout.

[0005] The invention aims at providing a thermal print head of the above-mentioned kind which is operable with a considerable reduction of the steepness of the flanks of the supply cur­rent pulse and hence with a considerable reduction in elec­trical interference radiation, and this without any limita­tion of the amplitude of the supply current and with only a minimum number of contact surfaces for the current supply to the resistors and of control signals.

[0006] To achieve this aim the invention suggests a thermal print head according to the introductory part of claim 1, which is characterized by the features of the characterizing part of claim 1.

[0007] Further developments of the invention are characterized by the features of the additional claims.

[0008] By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in

Figure 1a a thermal print substrate of a thermal print head according to the invention,

Figure 1b in detail how the print resistors are arranged one after the other at the thermal print substrate in Figure 1a,

Figure 1c a thermal print head according to the invention with a thermal print substrate according to Figure 1a and Figure 1b,

Figure 1d another embodiment of a thermal print head accor­ding to the invention with two separate sub­strates,

Figure 2 in the form of a block diagram a circuit diagram for the thermal print head of Figure 1a,

Figure 3 one of the 13 control circuits of the thermal print head,

Figure 4 in more detail the control circuit for one of the print resistors,

Figure 5 as a function of the time certain signals occurring in the thermal print head as well as the supply current to the print head,

Figure 6 an example of how the delay circuit shown in Figure 4 may be designed.

[0009] Figure 1 shows an example of a thermal print substrate 1 used in a thermal print head according to the invention. On the substrate, which is a rectangular plate of insulating material, there are arranged a row -f print resistors R, a set of control circuits A1-A13 and contact members K with a plurality of contact surfaces for the supply of control si­gnals and supply current to the substrate. On the substrate are further arranged the necessary electrical connections between the units arranged on the substrate. For the sake of simplicity, however, these connections are not shown in the figure.

[0010] The print resistors R comprise 416 small resistor spots R1, R2,... R416. These are arranged adjacent to each other along a straight line in the manner shown in Figure 1b. The print resistors may suitably be designed in thick-film technique or thin-film technique and have the square shape shown in Figure 1b with a side length of, for example, 0.25 mm.

[0011] For control of the print resistors, 13 control circuits A1, A2, ... A13 are arranged on the substrate. These consist of silicon chips, i.e. integrated circuits manufactured in a conventional manner, which are applied on the substrate 1. The control circuits are mutually identical and each control circuit is connected to and controls 32 print resistors.

[0012] Figure 1c shows a thermal print head according to the inven­tion. The head has a substrate 1, which is formed in the manner described in Figures 1a and 1b. Only two of the con­trol circuits of the substrate, Ai and Ai+1, are designated.

[0013] The substrate is arranged between two light-metal sections 11 and 13. The section 11 constitutes a cooling body for the substrate, and a layer 12 of, for example, thermal high-con­ductivity silicone is arranged between the section 11 and the substrate. The section 13 constitutes a protective hou­sing and has a projection 131 which fits into a groove 111 of the cooling body (section 11). The sections 11 and 13 are held together by screws, for example 17a, 17b, for which ho­les (e.g. 132a, 132b) are arranged in the sections 11 and 13, the substrate 1 and the layer 12. Hoses or strips 14, 15 of elastic material, for example silicone plastic, are ar­ranged between the section 11, 13 and the substrate 1 to take up any deviations in shape and dimensions between the sections and the substrate. A flat contact strip 16 is ar­ranged in communication with the contacts K of the substrate and is provided at its opposite end with a contact member 161.

[0014] The thermal print head shown in Figure 1d differs from that in Figure 1c by having two substrates, namely, a thermal print substrate 1a and a second substrate 1b. The substrate 1a supports the print resistors R and all or certain of the control circuits (Ai, Ai+1). In its simplest form the sub­strate 1b may only be provided with wires (for simplicity not shown) which connect the substrate 1a to a contact mem­ber K1 for connection of the print head. The contact member K1 fits into a recess 112 in the section 11.

[0015] Alternatively, certain or all of the control circuits of the print head may be arranged on the substrate 1b, as shown by the dashed squares B1, B2.

[0016] The necessary connection means for transmitting signals and supply currents and voltages between two substrates are not shown in Figure 1d.

[0017] The dimensions of the print head may be, for example, 45 mm in the direction of feed of the printing paper (horizontally in Figures 1c and 1d) and, for example, 100 mm in a direc­tion perpendicular thereto (parallel to the extension of the print resistors R). The section 13 may have a width of only 35 mm in the direction of feed mentioned, leaving the surfa­ces of the substrates 1 and 1a, respectively, uncovered up to a width of about 10 mm along the edge where the print re­sistors R are arranged. The printing paper is maintained in contact with this free surface of the substrate and hence with the print resistors R with the aid of a roll or the like.

[0018] Figure 2 shows 3 of the 13 control circuits, namely A1, A2 and A13. The circuit A1 is connected to and controls the re­sistors R1-R32, the circuit A2 is connected to the resistors R33-R64, and the circuit A13 is connected to the resistors R385-R416. One end of each print resistors is connected to a supply voltage U, which is supplied to the substrate via one of the contact surfaces K. The supply voltage flowing to the substrate is designated I. The other end of each resistor is connected to one of the control circuits. This end of the resistor is normally kept by the control circuit at a poten­tial which is equal to the supply voltage U, thus preventing any current flow through the resistor. When a resistor is to be activated, that end of the resistor which is connected to the control circuit is switched to a potential deviating from U, for example to the potential 0 V, for a certain pe­riod of time, and a current pulse for heating the resistor will then traverse the resistor.

[0019] To the substrate there is supplied, via one of the contact surfaces K, a signal DATA which defines which of the print resistors are to be activated at a certain moment. This in­formation consists of a digital word, preferably supplied in serial form, containing 416 bits, one for each one of the print resistors. In a manner which will be described in more detail below, the control circuits contain units which toge­ther form a shift register of 416 bits. When writing the mentioned word into the print head, the information is suc­cessively shifted further from the control circuit A1 to the control circuit A2 and so on, until all 416 bits have been shifted into the control circuits. Each control circuit then contains information as to which of its 32 associated resi­stors are to be activated. The information which is shifted through the control circuit A1 to the circuit A2 is designa­ted D33 in Figure 2. The information which is shifted through the control circuits A1 and A2 to the control cir­cuits A3 is designated D65 and so on, and the information shifted through all the preceding control circuits to the last control circuit A13 is designated D385. The reading of the information just mentioned is controlled by clock pulses CL, which are supplied to the print head. After reading in the information, this is stored in each control circuit in holding circuits, the storing being initiated by a pulse L0 supplied to the print head. Thereafter, the selected print resistors are activated with the aid of a control pulse STR which has a duration equal to the desired heating time of the resistors.

[0020] The information and the control signals to the print head may be generated, in a manner known per se, by suitable di­gital equipment, for example a microprocessor.

[0021] Figure 3 shows in more detail the embodiment of one of the 13 control circuits, namely the circuit A1. This consists of 32 identical units B1-B32, one for each one of the resistors R1-R32 connected to the control circuit. In a manner corre­sponding to that shown in Figure 2, Figure 3 shows how the word DATA is shifted on between the 32 units in the control circuit. From unit B1 the information D2 is shifted forward to the unit B2, from this unit the information D3 is shifted forward to the unit B3 an so on. The 32 units in the control circuit contain members which store the first 32 bits of the data word which defines those print resistors which are to be activated. When the whole data word has been read into the thermal print head, each one of the units B1-B32 con­tains a binary digit which indicates whether the associated print resistor is to be heated or not.

[0022] The pulses CL and LO are supplied to all the units shown in Figure 3. The control pulse STR is supplied only to the first unit B1. As will be described in more detail below, the control pulse STR is delayed in the unit B1 by a prede­termined time to form a delayed pulse STR2, which is sup­plied to the unit B2. In this unit the received pulse STR2 is delayed for a time equal to the delay time in unit B1 to form an additionally delayed pulse STR3, which is supplied to the unit B3, and so on. Thus, the unit B32 finally recei­ves the control pulse STR32 which has been delayed 31 times.

[0023] Figure 4 shows how each of the units B1-B32 in Figure 3 is built up. The unit is connected to and controls the print resistor Ri. When reading in the above-mentioned data word, the control information Di is supplied bit by bit to an edge-triggered bistable flip-flop Ei. Upon each received clock pulse CL, the output signal of the flip-flop is set equal to the input signal and is supplied in the form of a signal Di+1 to the corresponding bistable flip-flop E1+1 of the next unit. After 416 clock pulses, the entire data word has been shifted into the shift register which is formed by the 416 bistable flip-flop E1-E416 included in the print head. The output signal from the flip-flop Ei is also sup­plied to an additional edge-triggered bistable flip-flop Fi. When the whole data word has been read into the above-men­tioned shift register, the print head receives a pulse LO, which is supplied to the flip-flop Fi thus setting the flip-­flop Fi in accordance with its input signal. The output si­ gnal of the flip-flop Fi is supplied to an AND-circuit Gi with an inverted output together with that control pulse STRi which is supplied to the unit in question. Normally, the control pulse is a logic zero, which means that the out­put signal of the AND-circuit is normally a logic one, the output signal of the AND-circuit being "high" and no current flowing through the resistor Ri. During the desired heating time for the selected print resistors, the control pulse STRi is a logic one. For each selected resistor the word DATA contains a logic one, which after reading in is stored in the flip-flop Fi. When the control pulse STRi changes from a logic zero to a logic one, then output of the AND-cir­cuit G for the selected resistors changes from a logic one to a logic zero, and the selected resistors are traversed by current for the duration of the control pulse STRi. For the non-selected resistors, the word DATA contains logic zeros, the output signal from the flip-flop Fi for such a resistor is a logic zero, the output signal of the AND-circuit Gi is a logic one, and no current traverses the resistor.

[0024] The circuit shown in Figure 4 also comprises a delay circuit Hi which delays both the switching on and switching off of the control pulse STRi received by the delay circuit by the time tf. The output signal STRi+1 of the delay circuit thus consists of a control pulse of a duration equal to that of the input control pulse STRi but delayed by the time tf in relation thereto.

[0025] Figure 5 shows as a function of the time the pulses, CL, LO and STR arriving at the thermal print head, the control pulse STR2 which defines the heating time of the resistor R2, the control pulse STR32 for the resistor R32, and the total current I from the supply current source.

[0026] At the time t1 the reading of the word DATA into the print head starts, and after 416 clock pulses CL the reading is completed and the binary figures which define the selected print resistors are stored in the flip-flops Ei. The last one of the 416 clock pulses occurs at the time t2. Thereaf­ter, the print head receives a pulse LO at the time t3, the information stored in the flip-flops Ei thus being transfer­red to the flip-flops Fi. Thereafter, if desired, a new rea­ding of a word DATA to the flip-flops Ei may be started. When the flip-flops Fi are set, the thermal print head re­ceives the pulse STR which defines the heating time for the selected resistors. This control signal starts at the time t4 and terminates at the time t7. The heating of the first resistor (R1, R33, R65 and so on) in the group of 32 resi­stors associated with each control circuit (A1, A2, ... A13) thus starts at the time t4 and terminates at the time t7. At the time t5 the unit B1 (see Fig. 3) delivers the control signal STR2 to the unit B2. This causes resistor number 2 in each group (R2, R34, R66 and so on) to be switched on provi­ded that the respective resistor is selected. The process continues in a corresponding way with successive switching on of more and more print resistors. At the time t6 the con­trol signal STR32 is delivered to the last unit in each group. In this connection the following equations apply:
t5 = t4 + tf
and
t6 = t4 + 31 x tf
The supply current I to the print head will grow stepwise in 32 steps up to its maximum value, which is reached at t=t6. In a corresponding manner, the supply current will be redu­ced in 32 steps in the time interval between t = t7 and t = t9.

[0027] In the print head described above, the delay time tf may, for example, be 0.2/µs. The duration of the control pulses STRi should then be at least 6.4µs (32 x 0.2µAs). In the application shown in Figure 5, it is longer, thus obtaining interval t6-t7 with constant supply current. With this tech­nique the delay time tf may be adapted in such a way that a suitable flank steepness is obtained, which makes it possi­ble to minimize electrical interference and electromagnetic radiation.

[0028] The delay circuits Hi may suitably by designed in the manner shown in Figure 6. Each delay circuit is built up of two in­verters, IN1 and IN2. The control signal STRi which is to be delayed is supplied to the inverter IN1, and the delayed control signal STRi+1 is obtained as output signal from the inverter IN2. The delay time will be the combined delay ti­mes of the two inverters. This delay time may be adjusted depending on the relation between the current drive capacity of the two inverters.

[0029] With the aid of the print head according to the invention, controlled supply current pulses of low flank steepness are obtained, both upon switching on and switching off. In this way, the thermal print head provides, in a simple and advan­tageous manner, a very considerable reduction of the elec­trical interference caused by the supply current pulses. Further, as will be clear from the foregoing description, in a thermal print head according to the invention only a mini­mum number of contacts are required for the supply of con­trol signals and supply current, which results in lower costs and higher reliability of the contacting members.

[0030] The embodiment described above is only one of many possible ways of designing a thermal print head according to the in­vention. Thus, the control circuits included in the print head may be formed in a great many other ways while maintai­ning the function according to the invention. Also, of course, the number of control circuits may be different from that described above (13), and this also applies to the num­ber of print resistors (416). Further, a print head accor­ ding to the invention may contain additional control cir­cuits which influence the interface between the thermal print head and the controlling digital equipment. However, these additional control circuits do not change the stepwise control of the switching on and off of print resistors ac­cording to the invention. Further, in the example described above the print head has one single row of print resistors arranged one after the other; however, the invention is, of course, applicable also to print heads which have a plura­lity of parallel rows of print resistors for simultaneous generation of a dot matrix.

Claims

1. Thermal print head with a thermal print substrate (1) on which are arranged a plurality of print resistors (R1-R416) which are adapted for selective electric heating, with storage members (Ei, Fi) for receiving and storing informa­tion (DATA) which defines which of the print resistors are to be heated during a certain time interval, and with acti­vating members (Gi) for activation of the heating of the se­lected resistors in dependence on an activating signal (STR) supplied to the print head, characterized in that the thermal print head comprises members (Hi) adap­ted to achieve a stepwise switching on of groups of print resistors when initiating the heating during said time in­terval and a stepwise switching off of groups of print resi­stors when terminating the heating during said time inter­val.

2. Thermal print head according to claim 1, cha­racterized in that the activating signal (STR) is adapted to be supplied to the activating members (B1, B33, B65 and so on) for a first group of print resistors (R1, R33, R65 and so on) and that the print head comprises delay members (Hi) adapted thereafter, with successively incre­asing delay, to supply activating signals to the activating members for one additional group of print resistors at a time.

3. Thermal print head according to claim 2, charac­terized in that the delay members comprise delay circuits (Hi), which are each one adapted to be supplied with the activating signal (STRi) for one group of print re­sistors and to deliver their output signal (STRi+1) as acti­vating signal to another group if print resistors.

4. Thermal print head according to any of the preceding claims, characterized in that the members (Hi) for stepwise switching on and off of the print resistors are adapted, upon initiation of the heating of the selected print resistors, to switch on then resistors stepwise until all the selected resistors are simultaneously in the swit­ched-on-state.

Drawing