THERMAL PRINT HEAD HEATING CIRCUIT FAULT DETECTION DEVICE

Abstract: A thermal print head heating circuit fault detection circuit comprises providing a switching circuit (47) between the print head heating circuits (S1,...Sn) and the power supply circuit (43) of the print head, connecting a heating circuit fault detection circuit (42) in parallel with the said switching circuit (47), and using a control circuit to switch the switching circuit between printing operation and print head fault detection operation, and which in the fault detection mode also carries out fault detection by checking the value of the current flowing through the heating circuit fault detection circuit (42).

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a thermal print head heating circuit fault detection device, and particularly to a device circuit configuration which is simple and which can perform detection stably at high speed without requiring adjustment. Description of the Prior Art The print heads used in thermal printers are provided with a plurality of heating circuits, each of which is provided with heating elements, gate circuits, transistors, and so forth. The heating circuits will not function if a heating element, gate circuit or transistor fails, which will therefore cause part of a dot raster to be output with the printing in a non-functional state. The most frequent cause of such failure is circuit line breakage in the heating elements. As techniques for solving such problems there have been proposed, as in Japanese Laid-open Pat. Appln. No. 58 (1983)-28391, providing a current sequentially to the heating circuits which is of such a level that printing does not take place, and whether there is a faulty heating element is determined by detecting whether the current has passed through each of the heating circuits. With this technique, resistors having a low resistance value are connected in series on the common terminal side of the heating circuits, and when current is sequentially supplied to the heating circuits, it is detected whether the current has passed through the said resistors. However, in order to detect the very small currents that are used, it is necessary to provide an amplification circuit with a high amplification factor, and adjustment of the amplification factor is also necessary, and as a result the circuitry becomes complicated in addition to which the working speed of the amplification circuit is slow, so that there has been a drawback that time is required for the checking process. Further, because there is a considerable manufacturing variation in the resistance values of the heating elements, it has been necessary to carry out fine adjustment of the amplification factor for each thermal print head, and it has not been possible to deal with variations in the resistance values of the heating elements of an entire thermal print head board. SUMMARY OF THE INVENTION The object of the present invention is to provide a thermal print head heating circuit fault detection device which has a simple circuit configuration, does not need adjustment even when there exists manufacturing error in the resistance values of the heating elements, and which can perform detection of circuit faults with speed and reliability. To achieve this object the present invention comprises providing a switching circuit between the print head possessing the heating circuits and the power supply circuit of the print head, connecting a heating circuit fault detection circuit in parallel with the said switching circuit, and controlling by means of a control circuit the switching of the switching circuit between printing operation and print head fault detection operation, and which, when in the fault detection operation, also performs fault detection by checking the value of the current flowing through the heating circuit fault detection circuit. The heating circuit fault detection circuit according to the present invention comprises providing a constant current circuit connected in parallel with a switching circuit, and a current detection circuit which detects current passing through the constant current circuit, and in the fault detection operation, fault detection being performed by the control circuit checking the output of the current detection circuit. In addition, in the current detection circuit of this invention a photocoupler is employed, the emitter portion of which is driven by current flowing in the constant current circuit, and the output from the photocollector portion of which is checked by means of the control circuit to thereby perform detection of heating circuit faults. The invention will now be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of the circuit of a printer employing the thermal print head heating circuit fault detection device according to the present invention; Fig. 2 is a circuit diagram showing principal parts of the present invention; and Fig. 3 is a flowchart showing the operation of the thermal print head heating circuit fault detection device according to this invention. DESCRIPTION OF THE PREFERRED EMBODIMENT In Fig. 1, connected to a CPU 30 are a program ROM 31, a data RAM 32 and an I/O port 33. The CPU 30 performs overall control of printer functions, such as printing, and heating circuit fault detection in accordance with a program stored in the program ROM 31. Stored in the data RAM 32 are printing data, the location of faulty heating circuits determined on the basis of checking for heating circuit faults, and the like. Also connected to the I/O port 33 are a print data input circuit 34 for the input of the data to be printed, a drive circuit 35 for driving a pulse motor 36 used to transport the printing paper, a drive circuit 37 for driving a display 38, such as a CRT, a drive circuit 39 for driving a buzzer 40, and a thermal print head 41. A print head power supply circuit 43 is arranged so that the power it supplies to the print head 41 for printing operations goes through a heating circuit fault detection control circuit 42. This heating circuit fault detection control circuit 42 and power supply circuit 43 are each connected to the I/O port 33 so as to be suitably controlled by the CPU 30 in accordance with the program. Fig. 2 shows details of the print head 41, the heating circuit fault detection control circuit 42 and the power supply circuit 43. The print head 41 is provided with a data register 44 comprised of shift registers, a latch circuit 45 and n heating circuits S₁, S₂, S₃, ..., S n . Each of the heating circuits S₁, S₂, S₃, ..., S n is comprised of AND gates G₁, G₂, G₃, ..., G n , transistors HTr₁, HTr₂, HTr₃, ..., HTr n , and heating elements HR₁, HR₂, HR₃, ..., HR n . The data register 44 is for storing one dot-line of printing data, the data input DI being input one bit at a time via I/O port 33 by means of a clock signal CLK and output to a latch circuit 45. The latch circuit 45 is arranged so that when a latch signal LAT is input via I/O port 33 the data stored in the data register 44 is read out. The output terminals of the latch circuit 45 are connected to one of the input terminals of the AND gates G₁, G₂, G₃, ..., G n . The other input terminal of each of the AND gates G₁, G₂, G₃, ..., G n is connected to the input terminal of the I/O port 33 for the input of a strobe signal STR. The output terminal of each of the AND gates G₁, G₂, G₃, ..., G n is connected to the base of the corresponding transistor HTr₁, HTr₂, HTr₃, ..., HTr n . The emitter of each of the transistors HTr₁, HTr₂, HTr₃, ..., HTr n is grounded, and the collector is connected to one side of the corresponding heating element HR₁, HR₂, HR₃, ..., HR n . The other side of each of the heating elements HR₁, HR₂, HR₃, ..., HR n is connected to a common terminal 46 of the print head 41. The power supply circuit 43 supplies the required electrical power to the print head 41 and is arranged so that the output voltage is varied by the HV input via the I/O port 33. In addition, when the constant current circuit 48 described below is employed, there is no special need to vary the output voltage. A switching circuit 47 is connected between the print head 41 and the power supply circuit 43, and the said constant current circuit 48 is also connected in parallel with the switching circuit 47. Switching circuit 47 consists of a D type flip-flop circuit FF, transistors Tr₁, Tr₂, resistors R₁, R₂, and a diode D1. A switch control signal SWC output from the CPU 30 is input ot the input terminal of the flip-flop circuit FF via the I/O port 33. When a clock signal is input when the switch control signal SWC is High, the output is held high, and when a clock signal is input when the switch control signal SWC is low, the output is held low. The output terminal of the flip-flop circuit FF is connected to the base of the transistor Tr₁ via the resistor R₁ and the diode D1. The emitter of the transistor Tr₁ is grounded and the collector is connected to the base of the transistor Tr₂. The emitter of the transistor Tr₂ is connected to the output terminal of the power supply circuit 43 and the collector is connected to the common terminal 46 of the print head 41. Resistor R₂ is connected between the base and the emitter of transistor Tr₂. With the switching circuit 47 thus constituted, when the flip-flop circuit FF output is high, transistor Tr₁ and Tr₂ both come on, and electrical power is supplied directly from the power supply circuit 43 to the common terminal 46 of the print head 41. When the flip-flop circuit FF output is Low, transistor Tr₁ and Tr₂ go off and power is supplied from the power supply circuit 43 to the common terminal 46 of the print head 41 via the constant current circuit 48. The constant current circuit 48 is comprised of transistor Tr₃, resistors R₃, R₄, and a Zener diode ZD. The photodiode PD used as the photoemitter portion of a photocoupler 49 is connected to the constant current circuit 48. Specifically, the collector of the transistor Tr₃ is connected to the output terminal of the power supply circuit 43 and the emitter is connected to the common terminal 46 of the print head 41 via the resistor R₄ and the photodiode PD. In addition, the resistor R₃ is connected between the base of the transistor Tr₃ and the output terminal of the power supply circuit 43, the anode side of the Zener diode ZD is connected to the common terminal 46 of the print head 41 and the cathode side is connected to the base of the transistor Tr₃. The electrical potentials VC and VD of the constant current circuit 48 thus formed are approximately the same, and the difference in potential between the anode and cathode of the Zener diode ZD, which is to say the difference between VB and VC, becomes constant. Therefore, as the difference in potential between VB and VD remains constant even if there is a change in VB produced by a change in the potential VA or in the resistance of the heating elements HTr₁, HTr₂, HTr₃, ..., HTr n , I ss current flowing in the photodiode PD stays constant and the photocoupler 49 operates stably at high speed. The emitter of the phototransistor PTr₃ which forms the photocollector of the photocoupler 49 is grounded and the collector is connected to the power source via resistor R₅. The collector of the phototransistors PTr is connected to the I/O port 33, and the CPU 30 performs the detection of faults in the heating circuits S₁, S₂, S₃, ..., S n by detecting the potential of the said collector. That is, when transistor TR₂ is in the Off state, current coming from the power supply circuit 43 goes to the print head 41 via the constant current circuit 48, but if there is any fault in the heating circuits S₁, S₂, S₃, ..., S n that are the object of the detection process, there is no I ss flow in the constant current circuit 48, and accordingly the collector side of the phototransistor goes High, while if there is no fault there is in I ss flow and the collector side of the phototransistor PTr goes low. If the CPU 30 which is monitoring the phototransistor PTr collector potential detects the potential has gone High, the CPU 30 determines that a circuit fault has occurred, while if the potential is low the circuit is determined to be normal. The heating circuit fault detection circuit is thus formed by the addition to the constant current circuit 48 of the photocoupler 49 and the resistor R₅. The diode D connected between the common terminal 46 of the print head 41 and the output terminal of the power supply circuit 43 is provided for the protection of the transistor Tr₂ of the switching circuit 47 and the constant current circuit 48. The working of the heating circuit fault detection in the thermal printer shown in Fig. 1 and Fig. 2 will now be described with reference to Fig. 3. In step 1, the number of heating circuits S₁, S₂, S₃, ..., S n , that is, the total number of dots N of the print head 41, is placed into a specific address of the data RAM 32. Following this, in step 2, the switch control signal SWC is made to go Low and a clock signal is input to set the flip-flop circuit FF output to Low and the transistors Tr₁, Tr₂ to Off. In step 3, by inputting a single clock signal CLK with the input data DI in the High state, a binary "1" signal is set into the first stage of the data register 44. Next, in step 4, a latch signal LAT is input to latch the contents of the data register 44 with the latch circuit 45, and a strobe signal STR is input to obtain a current flow only in heating circuit S₁. In this state, the collector-side potential of the phototransistor PTr is checked, and if it is Low, it is determined that the heating circuit S₁ is normal, while if it is High it is determined that it is faulty. That is, when heating circuit S₁ is in a faulty condition owing to a circuit line break or the like in the heating element HR₁, current does not flow in the heating circuit S₁ or the constant current circuit 48, so there is no emission by the photodiode PD and the potential on the collector side of the phototransistor PTr₃ goes High. When the heating circuit S₁ is working normally, current flows through the constant current circuit 48 and the transistor HTr₁, so there is emission by the photodiode PD and the collector-side potential of the phototransistor goes Low. If it is determined that there is no fault in the heating circuit S₁, the process moves on to step 8. If it is determined that there is a fault, in step 7 a value N corresponding to the number of the faulty heating circuit is stored in a specific address of the data RAM 32, after which the N value is decremented by just one in step 8. Next, in step 9, by inputting a single clock signal CLK with the input data DI of the data register 44 in the Low state, the binary "1" signal is shifted from the first to the second position of the data register 44. In step 10 it is determined whether N equals zero or not. If N does not equal zero it is determined that checking of all of the heating circuits S₁, S₂, S₃, ..., S n has not yet been completed, and the procedures of step 4 through step 10 are repeated. When the checking for circuit faults has thus been completed for all of the heating circuits S₁, S₂, S₃, ..., S n , N becomes zero and the process advances to step 11. In step 11 it is determined whether there is a faulty heating circuit. If it is determined that there is no faulty heating circuit, the print head 41 heating circuit fault detection operation is terminated and printing or other such operations are proceeded with. When it is determined that there is a faulty heating circuit, reference is made to the faulty heating circuit number stored in the data RAM 32 to determine the extent of the fault in terms of printing capability, i.e. whether printing is possible. Then, in step 13, the display 38 is used to indicate whether printing is possible and also to show the number of the faulty heating circuit or circuits, and a buzzer sounds to signal the completion of the heating circuit fault detection operation. As the printing operation does not form part of the gist of the present invention, details thereof will be omitted, except to say that after the switch control signal SWC is made to go High and a clock signal is input to set the flip-flop circuit FF output to High and the transistors Tr₁, Tr₂ to Off, printing proceeds as the print data is being input into the data register 44. While in the foregoing embodiment the heating circuit fault detection circuit comprises a constant current circuit and a photocoupler the emitter of which is driven by current flowing in the said constant current circuit, it may also be formed using a fixed resistance and checking the value of a current flowing in the said fixed resistance. With such an arrangement, current would not flow in the fixed resistance during printing, so a high resistance could be employed, and there would therefore be no need to provide an amplification circuit having a high amplification factor, and fine adjustment would also be unnecessary. Also, when a constant current circuit is used in the heating circuit fault detection circuit, because the supply voltage does not require to be set at a certain value, there is no need for power source voltage control by the print head power supply circuit. It is also possible to connect a comparator of the type having an output which goes Low when the potential at the common terminal of the print head is within a specific range, and High when it is outside the said range, and connecting this output to an I/O port to enable detection of heating circuit line breaks, short circuits, and circuit resistance anomalies. Although in the embodiment shown herein transistors are used for the switching circuit, it is also possible to use thyristors or the like. Thus, as described above, the present invention comprises providing a switching circuit between the print head possessing the heating circuits and the power supply circuit of the print head, connecting a heating circuit fault detection circuit in parallel with the said switching circuit, and controlling by means of a control circuit the switching of the switching circuit between printing operation and print head fault detection operation, and which, when in the fault detection operation, also performs fault detection by checking the value of the current flowing through the heating circuit fault detection circuit. Therefore, with the present invention there is no need to provide an amplification circuit having a high amplification factor, and fine adjustment also becomes unnecessary, so the circuitry and operation are extremely simple and it is therefore possible to manufacture it at very low cost, and in addition it is also possible for the detection operation to be performed at high speed, and as such it has high commercial utility. Moreover, the heating circuit fault detection circuit according to the present invention is comprised of a constant current circuit connected in parallel with the said switching circuit, and the photocoupler connected so that the photoemitter thereof is driven by current passing through the constant current circuit, with fault detection being performed by the control circuit checking the output of the photocollector of the photocoupler during fault detection operation. Therefore, even if there is variation in the resistances of the heating elements, the current flowing in the constant current circuit remains constant at all times and the emitter of the photocoupler can function reliably, so circuit fault checking can be done reliably at high speed. With the addition of a circuit which detects the potential of the common terminal of the print head, it also becomes possible to detect heating circuit line breaks, short-circuits, and circuit resistance anomalies.

Claims: 1. A thermal print head heating circuit fault detection device comprising a power supply for supplying electrical power to a thermal print head, a switching circuit provided between the print head and the power supply circuit, a heating circuit fault detection circuit connected in parallel with the said switching circuit, and a control circuit which controls the switching circuit to switch between printing operation and print head fault detection operation, and which in the fault detection operation also performs fault detection by checking for the presence of a flow of current in the heating circuit fault detection circuit. 2. A thermal print head heating circuit fault detection device comprising a power supply for supplying electrical power to a thermal print head, a switching circuit provided between the print head and the power supply circuit, a constant current circuit connected in parallel with said switching circuit, a current detection circuit which detects current flowing in the constant current circuit, and a control circuit which controls the switching circuit to switch between printing operation and print head fault detection operation, and which in the fault detection operation also performs fault detection by checking the output of the current detection circuit. 3. A thermal print head heating circuit fault detection device as defined in Claim 2 in which the current detection circuit is provided with a photocoupler the emitter portion of which is driven by current flowing in the constant current circuit, and the output from the photocollector portion of which is checked by the said control circuit to perform the detection of heating circuit faults.

Inventors: TAKAHASHI, KOUETU
Applicants: KABUSHIKI KAISHA SATO
Application year: 1986
Main classification: B41J2
IPC codes: B41J29, B41J2