EP0730972A2 - Printhead thermal control - Google Patents

Abstract

The power electronic system for the pressure control unit (14) regulates the amplitude of the pressure head voltage corresponding to the ambient temperature. The power electronic system is combined with a control unit (6) which operates with pre-heating and pressure impulses. A voltage distributor (R8,R9) incorporates the temperature sensor (R9) in the form of a thermistor for measuring the ambient temperature.

Description

The invention relates to a print head thermal control for a thermal printing device according to the type specified in the preamble of claim 1. For example, the invention can be applied to a direct thermal, thermal transfer or ETR print head or ink jet print heads with associated print control. Such printing devices can be used advantageously, for example, in franking machines or other mail processing machines.

From DE 38 33 746 A1, the preheating of the ink ribbon before printing is known. By preheating the printing element to close to its printing temperature, the energy required to initiate a printing process is to be minimized. As a result, the heating and cooling times of the printing element remain short. By comparing the target and actual temperature on the thermal head, the variables (pulse height and pulse width) of the clock frequency of the preheating pulses can be adapted to the required heating energy. However, the thermal state of an individual pressure element cannot be taken into account. If local temperature differences are not to produce pressure together with the preheating, a larger safety distance from the printing temperature must be maintained. The greater the fluctuations in the ambient temperature, the more so.

It is already known from DE 40 26 896 A1 to supply excitation energy depending on the thermal state of each printing element in order to carry out a printing operation with the printing element of the thermal print head. The printhead is thermally coupled to a temperature sensor. The consideration of time constants of the heat conduction in the print head from the heating elements to the temperature sensor device of the print head leads to the correction of the print pulse duration. A capacitor circuit is also required to simulate the effect of the time constant. However, the influence of the ambient temperature cannot be simulated.

For a thermal print head, DE 39 21 217 A1 has already presented a calculation based on a recording energy, a compensation energy and a heat storage energy which already acts on the printing element before printing.

Based on this calculation, the excitation time or voltage level of the pressure pulse is then reset. Such a calculation, in which the energy state must be taken into account before each printing process and new heat storage data for the subsequent energy state is generated and stored, can either only be carried out with additional complex hardware or the printhead cannot be used for higher printing speeds.

From EP 329 369 A2 a method for controlling the supply of a thermal print head is known, in which the print data of the previous and current control processes are evaluated in order to determine the thermal state of a print element of the thermal print head. After the analysis, the corresponding pressure elements are specifically preheated or corrective heating is carried out. Such a subsequent analysis (so-called historical control) ties up computing time. This is always disadvantageous if a particularly high printing speed is to be achieved. In none of the aforementioned thermal print head controls is the influence of the ambient temperature or the change in print head parameters, for example due to aging, directly taken into account.

From EP 421 353 A1, a thermal transfer print head can be operated in different printing modes. A thermal measuring element is attached to the head in order to regulate the pressure energy supplied to the head as a function of the head temperature and as a function of the head stress (pressure mode). The pulse width is reduced with increasing head temperature, without the need to use data processing (historical control) for preheating pulses or for heating pulses. The method therefore works less precisely than a method with historical control, so that only a combination of both has the required accuracy can be achieved. The disadvantage of time-consuming data processing arises again.

A disadvantage of these thermometer elements would be, on the one hand, that they respond too slowly to temperature changes (due to the thermal storage of the head) and, on the other hand, that the application of the thermopills on the head means additional effort. Influences of the ambient temperature are only recorded via the head temperature, although they can have a quality-reducing effect even without triggering the printing process on the ink ribbon.

This energy supply according to the method mentioned in EP 421 353 A1, i.e. the energy supply, which decreases with increasing temperature, now considerably impairs the print quality, since the ink ribbon tends to smear when the printhead is not preheated and the ambient temperature is high, especially when - as was already proposed in DE 42 25 798 A1 - in "saving mode" is worked. On the other hand, the maximum achievable printing speed is already limited by the variation in the heating pulse duration.

From DE 41 33 207 A1 a method for controlling the supply of a thermal printing heating element is known, in which the printing data for several future printing columns are taken into account in order to selectively determine a number of preheating pulses before printing for each printing element. The thermal print head control has no temperature measuring means.

In EP 279 637 A2, on the other hand, the use of a second thermistor in the vicinity of the air inlet opening for determining the temperature of the ambient air of the device has been proposed. This information is also like the information about the Head temperature supplied to a microprocessor, which reduces the energy supply to the print head according to an increasing ambient temperature. The inequality of the characteristic curves of both thermistors is problematic for a more precise calculation.

DE 32 36 150 A1 has already proposed a control element for controlling the feed currents from the thermal head driver stage in accordance with the ambient temperature for a heat transfer printer. A thermistor is preferably connected into the supply voltage feed line to the printhead.

The thermistor has a temperature characteristic which corresponds to that of the ink ribbon. The feed currents thus change in a simple manner in a certain ratio to the ambient temperature. The voltage drop across the thermistor in the supply voltage supply leads to energy being lost for the pressure. Efficiency is particularly important for higher resolution printheads, i.e. deteriorated with many pressure points (dots). In addition, a fluctuation in the print image that is dependent on the print image content cannot be corrected.

The task is to find a technical solution for an electronic thermal control of printheads that avoids the disadvantages of the prior art and can be implemented inexpensively.

The object is achieved with the features of claims 1 and 8.

The different causes for a necessary thermal control lie on the one hand in the print image data and on the other hand in the conditions that influence the print. It is assumed that for the electronic control of a print head Control means, usually a microprocessor, is present in the system, which is loaded with other data in addition to current machine parameters and print image data.

The ink ribbon or the recording medium has a much lower heat capacity than the print head and consequently reacts faster to temperature fluctuations. It has been found that a temperature measurement on the printhead can advantageously be dispensed with and that in addition bypass temperature measurement, i.e. if the printhead is not thermally coupled to the temperature sensor, a much faster response tent is guaranteed, in contrast to thermal sensors conventionally used on the printhead.

The solution to the problem according to the invention is that the print head thermal control is a combination of power electronics assigned to a print control unit, which regulates the amplitude of the print head voltage according to the ambient temperature, and a control unit which, according to a predictive control method, for supplying individual print elements with print pulses and with preheating pulses with a variable pulse duration works, has.

It is provided that a thermal sensor is used for the slow temperature changes of the ambient temperature in a manner known per se, which is inexpensively arranged in the vicinity of the power electronics and existing ventilation slots in order to regulate the pulse amplitude. It is furthermore provided that, decoupled from this, the total content of the energy sent to the printhead (which is uniquely determined by the image data) is evaluated in advance by the microprocessor and depending on the pressure energy supplied to it expected temperature profile on the head is adjusted.

With a predictive control method, future control parameters are advantageously calculated so that the influences to be expected due to temperature changes also remain relatively small. This means that the control deviation remains low and strong fluctuations in the control are avoided, i.e. overshoot does not occur.

Through a storable presetting of parameters, a potentiometer can be replaced by a circuit called software potentiometer. Such a software potentiometer can be set by a service technician via the actuating means of the keyboard 2 in the service mode without the franking machine having to be opened.

A corresponding print head thermal control variant is provided for a print head which has a multiplicity of print elements and which is connected to power electronics and to a print control unit and to a control unit, the control unit being connected to the input means and to the output means via an I / O control and Storage means is coupled. The power electronics regulate the amplitude of the printhead voltage in proportion to the measured temperature and is assigned to the pressure control unit. A large number of printing elements are controlled by the microprocessor of the control unit according to a predictive control method. Means are provided for electronically presetting the amplitude of the printhead voltage and for supplying control parameters. The means for the electronic presetting include storage means, the microprocessor of the control unit, an I / O control and actuating means of the connected input means, and power electronics connected to the microprocessor of the control unit a, the latter having a digital / analog converter and a control circuit. The presetting voltage can be set by means of actuating means of the input means in the service mode in accordance with the printhead parameters required by the customer, and the setting can be stored in a non-volatile manner in one of the storage means.

Figur 1,

Blockschaltbild einer Frankiermaschine mit der erfindungsgemäßen Druckkopfthermosteuerung,

Figur 2,

Schaltbild der Leistungselektronik der erfindungsgemäßen Druckkopfthermosteuerung,

Figur 3,

Prinzip einer vom Mikroprozessor umschaltbaren Widerstandsanordnung,

Figur 4,

Schaltbild einer weiteren Variante der erfindungsgemäßen Druckkopfthermosteuerung mit Realisierung eines Software-Potis,

Advantageous developments of the invention are characterized in the subclaims or are shown in more detail below together with the description of the preferred embodiment of the invention with reference to the figures. Show it:

Figure 1,

Block diagram of a franking machine with the print head thermal control according to the invention,

Figure 2,

Circuit diagram of the power electronics of the print head thermal control according to the invention,

Figure 3,

Principle of a resistance arrangement switchable by the microprocessor,

Figure 4,

Circuit diagram of a further variant of the print head thermal control according to the invention with implementation of a software potentiometer,

FIG. 1 shows a block diagram of a franking machine with the print head thermal control according to the invention. A thermal direct, thermal transfer ETR or ink jet print head 1 with associated print control unit (DS) 14 can be used as the printing device of the franking machine. This pressure control unit 14 is used to control printing elements for digital printing.

The print control unit 14 stands with the print head 1 for data transfer by means of lines DÜ and also beyond a power electronics LE for power consumption by means of lines LÜ in connection. For the sake of simplicity, only one line has been shown.

The arrangement also consists of a control unit 6 which is connected to input / output means 2, 3, 4 and volatile storage means 7 and non-volatile storage means 5, 8, 9, 10, 11 and which operates according to a predictive control method which works without direct temperature measurement on the Printhead 1 manages. The control unit 6 is also connected to an encoder 13 as a displacement sensor and to a transport device 12 for the mail item or to a strip dispenser including strip release 12. The individual print columns are printed according to the speed of transport of the mail item until the franking stamp image is complete. In addition to the operating program, the pixel image data for the unchangeable image parts are stored in the program memory 11. The variable pixel image data, which are entered into a non-volatile working memory in accordance with the input via a keyboard 2, are stored in the character memory 9. The clock / date module 8 supplies further input data for the postmark, for which pixel image data are generated in the same way. The finished pixel image is stored in the volatile memory means 7 and evaluated in advance by the microprocessor of the control unit 6. The volatile storage means 7 is, for example, a RAM module or an internal RAM of the micro-processor in the control unit 6. The control unit 6 reads the pixel image data from the connected pixel memory 7 and processes this data in order to supply the print control unit 14 with print image data in accordance with the predictive control method. The corresponding operating program is stored in the program memory 11.

The predictive control method according to DE 41 33 207 A1 is advantageously used. If a printing process is to be triggered with the printing element in the near future, then in preparation for the printing process, the printing element is already subjected to preheating current pulses at times in which it does not contribute to printing. The energy content of the preheating pulses is continuously increased, thus achieving a high printing speed. The aforementioned preview of the future print data means that less computing time is used during printing if the calculation begins before the entire stamp image is printed.

Normally, a thermal transfer printhead does not require a thermal sensor on the printhead to detect and regulate strong temperature fluctuations because the control works with sufficient accuracy.

However, this does not exclude that an additional switchover (not shown) of the heating pulse height is provided for protecting the print head when the print head heats up considerably. Then a second thermistor (relatively expensive thermistor pill) arranged on the printhead in a manner known per se would be required. Practice has shown, however, that the print heads are manufactured so reliably that such protection can generally be dispensed with.

For the thermistor, which only detects the ambient temperature, a cheap design can be used instead of the expensive thermistor pill because the arrangement is no longer carried out on the print head. The advantage of the solution according to the invention that the thermistor no longer has to be placed directly on the printhead results on the one hand from the use of the aforementioned predictive control method.

On the other hand, the invention has succeeded in reducing the thermal storage elements of the measuring arrangement with the thermal sensor in such a way that a sufficiently rapid readjustment takes place when the bypass temperature changes without overshoot. In addition, both the type of tape used or, in the case of direct thermal printing, the type of paper used can be considered more easily as a further influencing variable.

A subordinate slow control of the amplitude of the pressure pulses for slow changes in the ambient temperature and a superimposed extremely fast and precise temperature control, which works in a predictive manner depending on the current print image contents and the associated overall energies of the system, are advantageously combined, and thereby the circuit - And component costs further reduced.

FIG. 2 shows the power electronics LE with the temperature sensor R9 for the temperature-dependent adjustment of the pressure pulse voltage level. Due to the dimensioning, different control behavior can be realized. The measuring arrangement can be arranged in the vicinity of the electronics and thus inexpensively in the system (omission of supply lines) and plug connections.

It is provided that the power electronics have a voltage divider R8, R9 containing the temperature sensor R9 for measuring the ambient temperature, the tap of which is connected to the non-inverting input of a proportional controller N9 and that the inverting input of the proportional controller N9 is connected to a reference voltage source. and that the proportional controller N9 is connected to the control input of at least one constant voltage module N1, N2, N3.

It is further provided that the temperature sensor R9 is a thermistor and the voltage divider R8, R9 is fed from a second constant voltage module N4 with a second constant voltage.

The control input of the at least one constant voltage module N1, N2, N3 is connected to an adjusting means R. The pressure pulse voltage level is set at a defined temperature to a voltage, for example +16 V in the thermal transfer printing process, by means of an adjusting means R. For example, three constant voltage modules N1, N2, N3 are connected on the output side via current distribution resistors R1, R2, R3. The output of the proportional controller N 9 is connected to the adjusting means R, preferably an adjusting resistor, via an RC element R7, C3, which is connected with its inverting input to the tap of the voltage divider R8, R9 (containing the thermistor R9). The second constant voltage module N4 supplies the required +5 V supply voltage for the electronics. The other non-inverting input of the regulator N9 is at the tap of a further voltage divider R5, R6 to reference potential.

The superimposed rapid thermal control by the microprocessor depends on various system parameters (such as printing speed, printing mode). The microprocessor, fed with this information, can implement any control curves and any control behavior.

An advantageous variant consists in that the setting means R is a D / A converter controlled by the microprocessor of the control unit 6 or that the setting means contains a resistance arrangement which can be switched over by the microprocessor.

• im Fall a): $\text{Rges = Rs + Ra}$
  
• im Fall b): $\text{Rges = Rs + Ra∥Rb}$
  
• im Fall c): $\text{Rges = Rs + Ra∥Rb∥Rc}$
  
• im Fall d): $\text{Rges = Rs + Ra∥Rc}$
  

FIG. 3 shows such a resistance arrangement which can be switched over by the microprocessor. The resistors Rb and Rc can thus be switched on via the data D by means of the switches Sb and Sc in accordance with the desired preset of the first constant voltage. The total resistance Rges is:

• in case a): $\text{Rges = Rs + Ra}$
  
• in case b): $\text{Rges = Rs + Ra∥Rb}$
  
• in case c): $\text{Rges = Rs + Ra∥Rb∥Rc}$
  
• in case d): $\text{Rges = Rs + Ra∥Rc}$
  

The belt speed or the printing speed can thus be taken into account or a basic contrast can be set for the printed image. The data D required for setting the basic contrast are stored in the non-volatile memory area H of the main memory 5 and can be entered using the keyboard 2.

FIG. 4 shows a further circuit variant with which an electronic presetting of the voltage value can be implemented using a digital / analog converter (DAU). Such a circuit variant is also referred to as a software potentiometer.

The franking machine can be switched to a service mode. Depending on the actuation of a corresponding actuating means, preferably a button for an up function and a button for a down function, the preset voltage value can be increased or decreased. The digital / analog converter (DAU) consists, for example, of an HC-Latch 20 which can be controlled by the microprocessor and an R2R resistor network R51 to R58 and R61 to R 67. The DAU output currents are generated by the aforementioned R2R resistor network, which has a ground potential switched resistor R6 forms a voltage divider, into a preset voltage implemented, which is present at the non-inverting input of a controller N9.

This voltage can be converted, for example, with a step size of 0.01 V by an 8-bit DAC. The aforementioned controller N9 is connected as a subtracting amplifier. The output voltage of a temperature sensor amplifier N8 is present at its inverting input. The non-inverting input (+) of a non-inverting adjusting amplifier N10, V3, R13 and R14 is present at the output of the controller N9 via an RC element R7C3. The output of the aforementioned setting amplifier is present via a resistor R15 at the control input of the at least one constant voltage module N1, N2, N3. According to the selected circuit of the temperature sensor measuring amplifier N8, it is provided that an NTC termistor with negative temperature coefficients is used as the temperature sensor, which forms a voltage divider with a series resistor R8 and a base resistor R18 providing a ground connection, the termistor R9 of the voltage divider comprising a second constant voltage module N4 a second constant voltage U4 is fed. The voltage divider tap is present at the non-inverting input of the temperature sensor amplifier N8 and is stabilized with a capacitor C9 connected in parallel with the base resistor R18. With increasing temperature, the voltage divider at its tap supplies an increasing voltage to the non-inverting input of the temperature sensor amplifier N8, the output voltage of which increases. As a result, the output voltage of the temperature sensor amplifier N8 to be subtracted from the preset voltage also increases and the control voltage decreases. A first voltage U1 is thus generated at the output of the at least one constant voltage module N1, N2, N3, which allows the required compressive stress level to be generated.

The second constant voltage module N4 supplies a second voltage U2, for example the required +5 V supply voltage for the printhead electronics.

The power electronics are connected to the printhead electronics of the printhead 1, which are not shown in more detail in FIG. 1 and which generate pressure voltage pulses in accordance with the control by the pressure control unit DS. Such printhead electronics of the printhead 1 contain at least the driver gates for loading the individual print elements of the printhead 1 with data on data transfer lines DÜ from the print control unit DS on the one hand and energy, for example first and second voltages U1 and U2, via power takeover lines LÜ from the power electronics LE.

For another temperature sensor, a different circuit for the temperature sensor amplifier can be selected to boost the voltage before it is sent to the controller. The controller adds a negative voltage to the preset voltage or subtracts a positive voltage from the preset voltage depending on the selected circuit type. The control voltage at at least one constant voltage module N1, N2, N3 is set by means of the preset voltage as a function of a parameter, for example the resistance value of a termotransfer print head.

The power electronics LE shown in FIG. 4 can also be expanded with a switch means and a comparator that can be queried by the microprocessor of the control unit 6. The switch means, not shown, is provided to switch off or bridge the temperature sensor. This is advantageous for comparing the DAU. The compressive voltage U1 is then compared by a comparator, not shown, with an accurate reference voltage and thus a Offset determined for the DAU. This adjustment is provided during production and can be repeated at the customer to compensate for a drift.

The invention is not limited to the present embodiment. Rather, a number of variants are conceivable which make use of the solution shown, even in the case of fundamentally different types.

Claims (11)

Print head thermal control for a thermal print head which has a multiplicity of print elements and which is connected to power electronics and to a print control unit and to a control unit, the control unit being coupled to the input means and to the output means via an I / O control and to storage means, characterized in that the power electronics assigned to the print control unit (14) regulate the amplitude of the print head voltage according to the ambient temperature and is combined with a control unit (6) which operates according to a predictive control method for feeding individual thermal pressure heating elements with preheating and pressure pulses of variable pulse duration. Print head thermal control according to Claim 1, characterized in that the power electronics have a voltage divider R8, R9 containing the temperature sensor R9 for measuring the ambient temperature, the tap of which is connected to the non-inverting input of a proportional controller N9 and that the inverting input of the proportional controller N9 has a reference potential is connected, and that the proportional controller N9 is connected to the control input of at least one constant voltage module N1, N2, N3. Print head thermal control according to claims 1 and 2, characterized in that the temperature sensor R9 is a thermistor and the voltage divider R8, R9 is fed from a second constant voltage module N4 with a second constant voltage. Print head thermal control according to claims 1 to 3, characterized in that the control input of the at least one constant voltage component N1, N2, N3 is connected to an adjusting means R. Print head thermal control according to claims 1 to 4, characterized in that the proportional controller N9 is connected to the control input of the at least one constant voltage module N1, N2, N3 via an RC element R7, C3 and via the aforementioned setting means R. Print head thermal control according to Claims 1 to 4, characterized in that the setting means R is a D / A converter controlled by the microprocessor of the control unit 6 or that the setting means R is a resistance arrangement (Rs, Ra, Rb, Rc, Sb , Sc) contains. Print head thermal control according to claims 1 to 5, characterized in that the setting means R is a D / A converter controlled by the microprocessor of the control unit 6 or that the setting means R is a resistance arrangement (Rs, Ra, Rb, Rc, Sb , Sc) contains. Print head thermal control for a print head which has a multiplicity of print elements and which is connected to power electronics and to a print control unit and to a control unit, the control unit being coupled to the input means and to the output means via an I / O control and to storage means, characterized by - That the power electronics (LE) regulates the amplitude of the printhead voltage proportional to the measured temperature and is assigned to the pressure control unit (14), a large number of printing elements being controlled by the microprocessor of the control unit (6) according to a predictive control method and - That means are provided for electronically presetting the amplitude of the printhead voltage and for supplying control parameters, the means for electronically presetting storage means (5, 8, 10), the microprocessor of the control unit (6), an I / O control and actuating means of the connected input means (3) and power electronics (LE) connected via the pressure control unit (14) to the microprocessor of the control unit (6) and having a digital / analog converter (DAU) and a control circuit, the presetting voltage using actuating means of the input means ( 3) set in service mode in accordance with the customer-specific printhead parameters required and the setting can be stored in a non-volatile manner in one of the storage means (5, 8, 10). Print head thermal control, according to claim 8, characterized in that the power electronics have a voltage divider R8, R9, R18 containing the temperature sensor R9 for temperature measurement, the tap of which is connected to the non-inverting input of a temperature measuring amplifier N8, that the inverting input of a proportional controller N9 via a resistor R11 is connected to the output of the temperature measuring amplifier N8 and that the non-inverting input of a proportional regulator N9 is connected to a preset voltage which is provided by the digital / analog converter (DAU) and that the proportional regulator N9 is connected via a non-inverting adjusting amplifier N10, V3, R13 , R14 and via a resistor R15 to the control input of at least one constant voltage module N1, N2, N3. Print head thermal control according to claims 8 and 9, characterized in that the temperature sensor R9 is a thermistor and the voltage divider R8, R9 is fed from a second constant voltage module N4 with a second constant voltage U2. Print head thermal control according to claims 8 to 10, characterized in that the proportional controller N9 is connected via a RC element R7, C3 to the non-inverting adjustment amplifier N10, V3, R13, R14.

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