EP0395189B1 - Temperature control device for a printhead or a hammerbank with electromagnets - Google Patents

Abstract

The needle matrix printer has a head (20) that has a large number of needles (1) and activated by a separate electromagnetic stage (3). Each needle actuator receives a drive signal from a drive circuit (21). Coupled to the drive circuit is a temperature monitoring circuit that has a constant current source (25) switched in during a pause period to cause a voltage drop that can be compared with a reference (27). A high or low signal can be latched into a 'D' type stage (30) and if the temperature is two high a cooling fan can be emergised.

Description

The invention relates to a printer with a print head or with a hammer bank of the electromagnetic coil type, with a device for temperature monitoring, in particular for serial matrix dot matrix printers, a temperature increase due to the power loss of the electromagnetic coil being able to be determined by comparison with a standard temperature and a switching signal derived therefrom either for Switching off the print head or for a reduction in the operating frequency or for the on-off control of a fan motor can be used.

A printhead or hammer bank of the electromagnetic coil type represents the interface between the mechanics of a matrix printer and the electronics (the printhead control). The demand for ever higher print speeds and service life places an extremely mechanical and electrical load on the printhead. These requirements make the printhead one of the most expensive assemblies in a printer. Particularly with impact printers, considerable control powers are required for the impression (usually by energizing a coil), which ultimately means heat losses. These losses increase proportionally with the number of printing elements and their activation frequency. It is therefore necessary to monitor the temperature of the printhead in order to avoid thermal destruction of printhead components. When the critical temperature for the print head is reached, the print speed is usually reduced (by lowering the operating frequency), the print head is temporarily switched off until the print head temperature has cooled to a defined value, or a fan motor is switched on for cooling.

Up to now, a temperature-sensitive sensor has been used for monitoring, which converts the print head temperature into a corresponding measured variable. For space and cost reasons, only one sensor is installed centrally, which determines the heat losses of all individual magnetic coils in a mixed temperature. The determined value depends on the temperature gradient between the electromagnetic coils and the sensor, the elapsing time for heat transfer, the number of controlled electromagnetic coils, so that this can only be a very inaccurate measurement.

It has already been proposed (DE-OS 26 18 224) to increase the number of print elements in the print head by at least one additional print element in an arrangement for extending the life of a print head for printing and writing devices, in particular for mosaic printers, so that the available write performance is rarely fully used. However, problems arise here in that the frequency distribution of the energization of the electromagnetic coil associated with a pressure element cannot be determined in advance.

Another known solution (US Pat. No. 4,496,824) controls the fan motor via a digital bus system, the temperature measured via a central thermistor first being determined via a voltage difference and an analog-digital converter, and the voltage difference being digitized. This measurement is also relatively inaccurate due to the individual measurement result because the thermistor of the control circuit is mounted on the head surface. It is a non-impact print head that generates heat even for printing. In this respect, the feature of the electromagnetic coil is missing here.

In the presence of a digitized temperature measurement, the input and Switching off the printhead is done by a printer according to another known proposal via special programs (European patent application 0 176 732).

A particular problem is a printer operating in graphics mode, in which the printhead can be subjected to a particularly one-sided load because only one printing element is constantly actuated. This causes local overheating of the electromagnetic coil, whereby any type of temperature monitoring fails.

From JP-A-62-62 770 it is also known, for use in a printer, to provide a device for monitoring the temperature of a print head on the basis of a constant current source. A constant current driver is used within a constant current circuit and a temperature monitoring time is selected. Within this, a test signal is generated and connected to the base of a transistor. An input voltage is compared with a normal time voltage by means of a resistor and a Zener diode, so that the respective temperature can be monitored. Such a system therefore does not provide an automatic means of intervening in the temperature profile of a printhead. In addition, such a temperature monitoring device is not sufficiently fast and safe and does not work precisely enough.

The present invention is therefore based on the object of providing a considerably faster, safer and more precisely operating device for temperature monitoring of a print head or a hammer bank of the electromagnetic coil type.

The stated object is achieved according to the invention in accordance with the invention at the outset by the fact that the solenoid coil assigned to each pressure element can be connected to a constant current source during a pause in the current supply, that a voltage drop at the solenoid coil generates a signal which is sent to a comparator with a reference signal which defines the temperature limit is compared and that the output signal of the comparator is adopted as a binary logic signal in a data buffer and that the logic signal serves as a control signal for a temperature reduction in the print head. This solution ensures that each electromagnetic coil is monitored much more effectively. By measuring the coil resistance as a function of temperature, a conclusion can be drawn as to the respective coil temperature. In the event that the measuring intervals are very short, short-term local overheating can also be measured. A particular advantage of the marked device is therefore the inertial detection of the actual coil temperature, regardless of the inductance or

Self-inductance and the monitoring of all coils at the same time. In a development of the invention it is provided that a delay element is switched on in the transmission of the measurement pulses of the binary logic signal into the data buffer, the response time of the system corresponding approximately to a measurement interval. On the basis of this proposal, only error-free measurement data can get into the data buffer, so that initial erroneous measurements are eliminated.

According to the further invention it is proposed that the comparator, the constant current source, the data buffer memory and the delay element are enclosed in an integrated circuit containing a needle driver circuit. An increase in circuit complexity with the individual measurement of each electromagnetic coil is in no way disadvantageous because the components mentioned can also be integrated in the redesign of an integrated circuit for a needle driver.

For the description of the invention, it is expedient to consider the temperature monitoring of an electromagnetic coil. This consideration is repeated for every further electromagnetic coil. The principle of the invention is based on the utilization of the temperature-dependent resistance of a technical coil. With Ohm's law it can be shown that at constant current the voltage drop across the coil resistance is directly proportional to the temperature. During regular operation of the printhead, however, there is a constant change in the field, which induces a voltage in the electromagnetic coil. This induction voltage is added vectorially to the voltage drop across the coil resistance and would falsify the measured value. Only when the magnetic field has stabilized is the induced voltage zero and the terminal voltage proportional to the coil temperature.

Fig. 1

einen Matrixnadeldruckkopf der Elektromagnetspulenbauart mit der schaltungsmäßigen Einrichtung für die Temperaturüberwachung und

Fig. 2

die gemäß Fig. 1 jeder Elektromagnetspule zugeordnete Einrichtung für die Temperaturüberwachung in vergrößertem Maßstab.

An embodiment of the invention is shown in the drawing and will be described in more detail below. Show it:

Fig. 1

a matrix needle printhead of the electromagnetic coil type with the circuitry for temperature monitoring and

Fig. 2

1 assigned to each electromagnetic coil device for temperature monitoring on an enlarged scale.

The matrix needle printhead shown corresponds in the exemplary embodiment to the so-called folding anchor type (FIG. 1) and has e.g. 9, 12, 18 or 24 printing elements 1. Each pressure element 1 is assigned a pair of magnetic yoke legs 2, an electromagnetic coil 3 and a hinged armature 4, the electromagnetic coil 3 being plugged onto a radially outer magnetic yoke leg. There are 9, 12, 18 or 24 such systems.

The pressure elements 1 are guided in a pressure needle guide housing 5 by means of individual guide walls 6, the bores 7 of which define the arcuate course from a needle engagement point 8 of a pressure needle head 8a and the hinged anchor 4 to a mouthpiece 9, the mouthpiece 9 being at a distance from the pressure abutment, not shown Druckelementhubes lies on the pressure abutment recording medium.

The pressure needle guide housing 5 forms a shell 5a corresponding to the outer diameter of the matrix needle print head 20 in the area of the hinged armature 4. In the center of the shell 5a is a guide bush 10, which forms an abutment for a compression spring 11, each Pressure element 1 is assigned.

A rear housing 12, which accommodates the systems and is made of plastic or aluminum (magnetically non-conductive materials), centers a one-piece electromagnetic coil carrier 13 made of sintered or investment casting. The one-piece, very precise and high-quality electromagnetic coil carrier 13 is magnetically insulated by electrically non-conductive fillers 14 within the rear housing 12. The housing 12 made of aluminum is also provided with a central, heat-resistant, damped washer 15 on which the radially inner armature arms 4a lie in a basic or rest position.

The hinged anchors 4 are fixed to the radially inner arm 4a by this damping disc 15 and pivotally mounted approximately in the middle by an elastic damping ring 16 with a square cross section and by the radially inner magnet yoke leg of the pair of magnet yoke legs.

The folding anchors 4 perform pivoting movements in order to give the pressure elements 1 a lifting movement. Such stroke movements (forward and return stroke are approx. 0.3 mm) are carried out with a frequency of 2,000 to 3,000 Hz.

From the electromagnetic coils 3 (FIG. 1), connecting lines 17 are routed through the filling compound 14 to a printed circuit board, not shown. The shell 5a and the rear housing 12 are connected to one another by means of screws 18 and threaded nuts 19.

Each of the electromagnetic coils 3 is now assigned a special device for temperature monitoring of the print head 20.

On account of a switching signal generated by it, this special device can either cause the print head 20 to be switched off or the operating frequency to be reduced or the control of a fan motor to be switched on or off.

The device includes the following circuit parts that perform the specified functions:
Each solenoid coil 3 is supplied with power current pulses 22 via a known needle driver circuit 21 for actuating a pressure element 1 (such a needle driver circuit 21 is described, for example, in German Patent 31 51 242). This application of current to the electromagnetic coil 3 causes heat loss, which heats the print head 20 and, above all, can heat the entire heating of the electromagnetic coil 3 or individual turns of the copper wire until it is destroyed.

As soon as a pressure element 1 is not actuated, there is a pause in the current supply. A switch 24 is closed by a measuring pulse 23 and the electromagnetic coil 3 is connected to a constant current source 25. This current causes a voltage drop Us at the electromagnetic coil 3, from which a reference signal 26 is generated at a comparator 27. The comparator 27 compares the voltage difference Up - Us with the reference voltage 28 at the comparator 27. At the output 29 of the comparator 27, depending on whether the measuring voltage is above or below the reference voltage, there is a "logic high potential" or a "logic low" Potential ". The reference voltage 28 corresponds to the temperature value at which the device is required to act. The binary signal of the logic high potential / logic low potential is stored in a data buffer memory 30. Because of the energy potential of the induction or in the electromagnetic coil 3 during a last energization pulse 22

In order to avoid falsification of the measurement result, self-induction is later given the measurement pulse 23 via a delay element 31 in the data buffer memory 30. The binary data stabilize before it is saved, so that incorrect values are avoided. The measurement process is ended when the data information is transferred to the data buffer 30. Depending on the measurement result, a decision is made as to whether the print head 20 as a whole or individual electromagnetic coils 3 are to be switched off or whether the operating frequency is to be reduced or whether a fan motor is to be switched on.

The comparator 27, the constant current source 25 (which can also be designed as a discrete component), the data buffer memory 30 and the delay element 31 and the needle driver circuit 21 can be contained in an integrated circuit 32.

Claims (3)

1. A printer with a print head or a hammer bank of the electromagnetic-coil type having a means for monitoring temperature, in particular for serial wire matrix printers, in which a temperature increase due to the power loss of the electromagnetic coil (3) can be detected by comparison with a standard temperature and a switching signal derived therefrom can be used either for switching off the print head or for reducing the operating frequency or for the switch-on/-off control of a fan motor,
   characterised in that
   the electromagnetic coil (3) associated with each print element (1) can be connected each time to a constant-current source (Up) during a pause in current supply, that a voltage drop (Us) at the electromagnetic coil (3) produces a signal which is compared on a comparator (27) with a reference signal (26) which establishes the temperature limit and that the output signal (29) of the comparator (27) is taken over as a binary logic signal (log high or log low) into an intermediate data store (30) and that the logic signal (log high or log low) serves as a control signal for lowering the temperature in the print head (20).
2. A printer according to Claim 1, characterised in that a delay element (31) is inserted into the relaying of the test pulses (23) of the binary logic signal (log high or log low) into the intermediate data store (30), with the reaction time of the system corresponding approximately to one measuring interval.
3. A printer according to Claims 1 and 2, characterised in that the comparator (27), the constant-current source (25), the intermediate data store (30) and the delay element (31) are enclosed in an integrated circuit (32) containing a wire-driver circuit (21).

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