Recording apparatus

Abstract: There is provided a recording apparatus in which a stepping motor (6) is used as a driving source to reciprocate a carriage (2) on which a recording head is mounted and the recording is executed by the recording head in accordance with the timing when the stepping motor (6) rotates, comprising: a rotational position detector (16) to detect a rotational position of the stepping motor (6); a current switching circuit (25) to switch energization currents to the stepping motor (6) on the basis of a detection signal from the rotational position detector (16); a motor speed control circuit (24) to closed loop control a rotational speed of the stepping motor (6) through the current switching circuit (25); and a controller (20) for detecting a load corresponding to the stepping motor (6) by a speed control output from the motor speed control circuit (24) and for controlling so as to change output torque characteristics of the stepping motor (6) in accordance with the load.

Description: Field of the Invention The present invention relates to a recording apparatus and, more particularly, to a serial type recording apparatus in which a stepping motor is used as a driving source for effecting at least the movement for the recording and scanning of a recording head. Description of the Prior Art Generally, in a serial type recording apparatus, a stepping motor or a brushless motor of the hybrid type or the PM (permanent magnet) type is frequently used as a motor to drive a carriage for conveying a recording head in order to record and scan. For instance, in the brushless motor, for example, a Hall element is ordinarily used to detect the position of the magnetic pole of a rotor to control a current supply. An optical or magnetical type encoder is used to detect the speed of the rotor. However, such a brushless motor has the following drawbacks. (1) It is necessary to match the position of the stator magnetic pole and the Hall element. (2) If the current supply is switched by the Hall element, since the position of the Hall element and the stator are unconditionally determined, the current supplying method of the motor is fixed. For instance, in the cases where what is called a 180° current supply control is executed and where what is called a 90° current supply control is performed, the position of the Hall element and the position of the magnetic pole of the stator electrically differ by 45°. Therefore, in order to execute two kinds, of current supply controls by using a single motor, the number of Hall elements must be doubled and the Hall elements must be arranged at positions suitable for the current supply control, respectively. For example, stepping motors in which the current supply control is executed by using an output of an encoder have been proposed in JP-A-62-193548 and JP-A-62-193549. However, only a motor structure in which an encoder is arranged at a predetermined position is merely disclosed in the above Official Gazettes and none of the drive control circuit and method for driving the motor and the like is disclosed. Therefore, in EP-A-0 313 046 on which the preamble of the claim 1 is based, there has been proposed a recording apparatus for executing a movement for recording and scanning of a recording head, comprising a carriage on which the recording head is mounted; a stepping motor for moving the carriage; detecting means for detecting a rotational angle position of a rotor of the stepping motor and for generating a pulse signal every rotation of a predetermined angle of the rotor; speed control means for controlling a speed of the stepping motor by a control output according to a time interval between the pulse signals from the detecting means; and current switching means for counting the pulse signals from the detecting means, and for switching and controlling energization currents which are supplied to coils of the stepping motor in accordance with the count value. However, the driving of the stepping motor is controlled by a closed loop. Hitherto, the driving control of the stepping motor has been executed by easily open loop controlling the frequency of the pulses of the same number as the number of driving pulses of the stepping motor. In the case where such a conventional stepping motor which is driven by the open loop control is used as a carriage driving motor, when the carriage is driven and run, particularly, in the case of the hybrid type motor, an annoying noise like "kee---n" which is caused by the vibration of the rotor of the stepping motor is generated. On the other hand, when the carriage is started, stopped, and reversed, that is, when the stepping motor is started, stopped, and reversed, the stepping motor is started or stopped while vibrating, so that a large noise like "Gatan" is generated. The above noises cause a problem in a printer which hardly generates noises such as an ink jet printer, particularly, like a bubble jet printer or the like. On the other hand, although the use of the above brushless motor as a carriage driving motor is also considered, in the case of the brushless motor, the rising time upon actuation is long and it is not suitable as a carriage driving motor in which the start, stop, reversal, and start of the carriage driving motor are repeated almost every line. In the case of using the brushless motor, the high-speed recording cannot be performed. Therefore, in US-4 928 050, there has been proposed a recording apparatus in which a stepping motor is used as a driving source and the recording head is moved to record and scan, wherein the recording apparatus comprises: detecting means for detecting a rotational angle position of a rotor of the stepping motor; and control means for closed loop controlling the driving of the stepping motor in accordance with the result of the detection of the detecting means. In the closed loop control of the stepping motor, an encoder is attached to the rotary shaft of the stepping motor, an output signal of the encoder is counted, the rotational position is detected, and a stepping motor energization signal is switched when the count value coincides with a predetermined count value, thereby controlling the rotation of the stepping motor. As mentioned above, in the case of driving the stepping motor by the closed loop control, it is necessary to execute the speed control and the position control in order to improve the recording accuracy. When the printer is designed, it is necessary to determine a control gain and a phase as parameters for the speed and position controls in consideration of the stability of the carriage speed, the response speed of the stepping operation, and the like. In the recording apparatus in which the carriage is driven by the stepping motor which is controlled in a closed loop, if a load torque which is applied to the carriage motor increases due to an environmental change or an aging change, the objective values of the rising time to actuate the carriage, driving speed, speed change amount, and the like cannot be satisfied, so that there is a fear such that the carriage stops in the worst case. The load torque increases in the following cases. For instance, under a low temperature environment, a viscosity of lubricating oil for reducing the friction which was coated onto the carriage sliding shaft deteriorates, or coefficients of thermal contraction are not matched due to a difference of materials of the parts, or paper particles, dusts, and the like enter between the sliding shaft and the carriage, so that a friction load increases and the load torque of the stepping motor is increased. Actually, a load torque margin is provided so that the normal operation can be also executed even if the driving system changed. However, to provide a torque margin, it is necessary to use a stepping motor which can output a larger generation torque or to reduce the load by using a reduction gear or the like. Thus, the number of parts increases, the costs rise, and the like. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a recording apparatus which is capable of effectively controlling a stepping motor within a wide range of load torque. According to the present invention this object is achieved by the measures as indicated in the claim 1. Thus, according to the present invention the current switching means corrects the count value by the load change signal, thereby changing the output torque of the stepping motor, if the control output exceeds a predetermined maximum value. The advantages of the invention will become apparent and obvious to those skilled in the pertinent art upon referring to the following description provided in connection with the accompanying drawings, of which: BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a carriage driving section of a recording apparatus according to the present invention; Fig. 2A is an internal constructional view of a carriage driving motor shown in Fig. 1; Fig. 2B is a cross sectional view of Fig. 2A; Fig. 3 is a circuit constructional diagram of a drive control system of the motor shown in Fig. 1; Fig. 4 is a flowchart for the circuit shown in Fig. 3; Fig. 5A is a waveform diagram of an ordinary control state of an energization switching signal in Fig. 4; Fig. 5B is a waveform diagram in the case where the phase of the energization switching signal in Fig. 4 was shifted; Fig. 6 is a torque characteristic graph of the carriage driving motor in Fig. 1 in the case where the phase was shifted; and Fig. 7 is a characteristic graph showing a change in control output according to a preferred embodiment of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail hereinbelow with reference to the drawings. Fig. 1 shows a carriage driving mechanism. Reference numeral 1 denotes a recording head of, for example, the ink jet type; 2 a carriage on which the recording head 1 is mounted and which moves along guide shafts 3A and 3B; 4 a timing belt whose both ends are coupled to the carriage 2 and which is reeved between pulleys 5A and 5B; 6 a carriage driving motor to drive the carriage 2 through the timing belt 4; and 7 a recording sheet which is held at the opposite position of the recording head 1 by a platen or the like (not shown). A shielding plate 8 is attached to the carriage 2. When the carriage 2 is moved in the R direction in Fig. 1, that is, to the left and arrives at the initial position, the shielding plate 8 is inserted into a slit 9A of a photo sensor 9. Thus, the initial position is detected and an encoder (not shown) attached coaxially with the carriage driving motor 6 is initialized as "0". As the carriage 2 is moved from the initial position in the F direction, namely, to the right, the current position is successively detected by counting the signal from the encoder and, at the same time, the recording is executed onto the recording sheet 7. On the other hand, after the carriage 2 was run by a distance corresponding to the recording of one line, the recording sheet 7 is fed by an amount corresponding to only one line by sheet feeding means (not shown). An example of the driving conditions which are required for the carriage driving motor 6 in such a recording operation will now be explained. In the case of a recording density of 360 dots/inch, a rotational speed of the carriage driving motor 6 corresponding to such a recording desity is set to about 800 r.p.m. in the high speed mode and is set to about 400 r.p.m. in the low speed mode. Further, a time which is required from the start of the carriage 2 until the arrival at a constant speed running (rotational speed: 800 r.p.m.) in the high speed mode is set to about 60 msec, a constant speed running time is set to about one second, and a time which is required from the constant speed run until the stop of the carriage 2 is set to about 60 msec. Figs. 2A and 2B show an example of a construction of the carriage driving motor 6 mentioned above. Reference numeral 10 denotes a rotor; 11 a rotor shaft; 12A and 12B stators arranged around the rotor 10; and 13A and 13B coils. A detecting disk 14 of the encoder is attached coaxially to the rotor shaft 11. A photo interrupter 15 is attached on the stator side. Therefore, the rotational position of the carriage driving motor 6 can be detected by counting output pulses from a rotary encoder 16 comprising the detecting disk 14 and the photo interrupter 15. A motor drive control system to execute a closed loop control of the carriage driving motor 6 will now be described with reference to Figs. 3 and 4. In Fig. 3, reference numeral 20 denotes an MPU (microprocessor unit) to control the whole recording apparatus. In accordance with control programs stored in an ROM (read only memory) 21, the MPU 20 drives and controls driving sources of the other mechanisms (not shown) by using an RAM (random access memory) 22 for processing recording data and also controls the carriage driving motor 6 to drive the carriage 2. For this purpose, the MPU 20 has a counter constructed by a hardware or software (not shown) and detects the position of the carriage 2 by counting output pulses 23 from the rotary encoder 16. The MPU 20 controls the rotational speed of the carriage driving motor 6 to foregoing speed in the high or low speed mode through a motor speed control circuit 24. The MPU 20 controls the start, stop, and rotating direction of the carriage driving motor 6 through a current switching circuit 25 for switching energization currents to the coils 13A and 13B of the carriage driving motor 6, thereby starting, stopping, and moving the carriage 2. On the other hand, the motor speed control circuit 24 closed-loop controls the rotational speed of the carriage driving motor 6 in accordance with a detection output of the encoder 16. Practically speaking, a time interval between the output pulses 23 from the encoder 16 is compared with a preset reference time. In accordance with the result of the comparison, a control output 26 to the carriage driving motor 6 is adjusted so as to eliminate the time difference. When the MPU 20 instructs the rotational speed of the carriage driving motor 6 to the motor speed control circuit 24, the motor speed control circuit 24 selects the comparing reference time corresponding to the instructed speed in response to such a speed instruction and compares the reference time with the pulse interval, thereby controlling the rotational speed of the carriage driving motor 6 to the speed in the high or low speed modes, On the other hand, the current switching circuit 25 starts the switching operation of the energization currents by a start signal 27A which is input from the MPU 20, thereby starting the carriage driving motor 6. On the other hand, the carriage driving motor 6 is stopped by a stop signal 27B which is input from the MPU 20. Further, as a point regarding the invention, the current switching circuit 25 controls the switching timing of the coil energization currents of the carriage driving motor 6 by a closed loop in response to the detection output of the encoder 16 in accordance with a procedure, which will be explained hereinlater, by the MPU 20. For this purpose, the current switching circuit 25 has a counter 28. The output pulses from the encoder 16 are counted by the counter 28 and the energization currents are switched at a time point when the count value of the counter 28 coincides with a predetermined value. In the embodiment, since a stepping motor of double phases is used as a carriage driving motor 6 as shown in Figs. 2A and 2B, the energization currents are switched 48 times per rotation of the rotor 10 by a pattern of a single phase. On the other hand, the number of output pulses from the encoder 16 is set to 288 per rotation. Therefore, since the rotor 10 rotates by only an equal angle every progressing of one energization pattern, assuming that the rotational angle is set to one step, the number of pulses which are output from the encoder 16 every step is set to 288/48 = 6. Therefore, the rotor 10 can be rotated at regular intervals if the energization currents are switched each time six output pulses from the encoder 16 are counted. However, in this case, since the carriage driving motor 6 is not rotated unless a predetermined relative positional relation is held between the magnetic pole of the rotor 10 and the magnetic poles of the stators 12A and 12B, it is necessary to match the relative positions between the magnetic pole of the rotor 10 and the magnetic poles of the stators 12A and 12B as an initial operation. Therefore, actually, in a state in which a predetermined phase was energized, the counter 28 in the current switching circuit 25 is reset to a predetermined numerical value. After that, the pulses from the encoder 16 are counted by the counter 28 and the energization currents are switched every other predetermined value (six pulses in the case of the embodiment). Consequently, when a relative positional relation was obtained between the magnetic pole of the rotor 10 and the magnetic poles of the stators 12A and 12B, the energization currents can be switched. For instance, in the case of the single phase energization, a ring counter which can count 24 pulses of one torque cycle, that is, four steps is used as a counter 28. Assuming that the count values for switching of the energization in this case are set to 6, 12, 18, and 0, energization waveforms as shown in Fig. 5A are obtained. Subsequently, a control procedure for allowing the carriage driving motor 6 to execute the optimum driving according to the load, which is a feature of the invention, will now be described in accordance with Fig. 4. The portion surrounded by a broken line simplifies the control operation by the motor speed control circuit 24 described before. This portion shows that the rotational speed of the carriage driving motor 6 is fed back and the closed loop control is executed so that the difference between the actual rotational speed and the instructed speed from the MPU 20 is set to 0. That is, in step S1, a check is made to see if the difference between the actual rotational speed and the instructed speed from the MPU 20 is 0 or not. If it is not 0, step S2 follows and a control output corresponding to the difference is calculated. In the next step S3, a check is made to see if the control output has exceeded a maximum value, e.g. 80% output or not. If NO, step S4 follows and the carriage driving motor 6 is driven by the control output. If it has exceeded the maximum value, e.g. the 80% output in step S3, the processing routine advances to step S5 and the energization switching value is changed and the processing routine is returned to step S1. That is, in the MPU 20, the energization switching value of the counter 28 in the current switching circuit 25 is corrected by only 1 in the direction reverse to the direction of the count by a phase advance signal 29. When the count value of the counter 28 is increased, the energization waveforms are as shown in Fig. 5B. The phase of the energization current switching signal is advanced, so that a current can easily flow in the winding. Fig. 6 shows a change in output torque characteristic of the carriage driving motor 6 in the case where the phase of the energization signal was advanced. As will be understood from Fig. 6, since the output torque of the carriage driving motor 6 increases as the switching phase is advanced, the carriage driving motor 6 can be also controlled to a predetermined speed even for a load in a wide range. Therefore, even in the case where the load torque increases and the output becomes maximum and exceeds the speed control limit, by advancing the phase, the output torque can be raised and the speed control can be properly executed. A situation such that when the control output arrived at 100%, and the carriage driving motor 6 becomes uncontrollable as in the conventional apparatus does not occur. Fig. 7 shows the relation between the load torque and the output of the control circuit. In the conventional control, the control output also increases in proportion to an increase in load torque and, finally, the control output is saturated as shown by a broken line. However, in the embodiment, when the control output has reached a predetermined maximum value (80% of the maximum output in the embodiment), the phase for energization switching is advanced, so that the output value is suppressed to a low value as shown by a solid line and, thereafter, the output value similarly increases in proportion to the load. However, by repeating the advance of the phase each time the output value has reached the maximum value, e.g. 80%, the output can be controlled to a predetermined value or less. On the other hand, by suppressing the output to a low value at the same load, there are obtained effects such that the electric power consumption of the carriage driving motor 6 can be suppressed and the heat generation of a motor driver IC can be suppressed. In the embodiment, the reason why the output characteristics of the carriage driving motor 6 have been switched in accordance with the load of the carriage driving motor 6 (also relating to a load degree of the motor speed control circuit 24) is because the optimum values of the various constants (for instance, loop gain and phase) for the speed control differ depending on the output characteristics of the motor and the load state. However, in the case of changing the output characteristics of the carriage driving motor 6 in the high load state as mentioned above, there is no fear of overshooting or hatching due to mismatching of the control constants.

Claims: A recording apparatus for executing a movement for recording and scanning of a recording head (1), comprising: a carriage (2) on which said recording head (1) is mounted; a stepping motor (6) for moving said carriage (2); detecting means (16) for detecting a rotational angle position of a rotor of said stepping motor (6) and for generating a pulse signal (23) every rotation of a predetermined angle of the rotor; speed control means (24) for controlling a speed of said stepping motor (6) by a control output (26) according to a time interval between said pulse signals (23) from said detecting means (16); and current switching means (25) for counting said pulse signals (23) from said detecting means (16), and for switching and controlling energization currents which are supplied to coils (13A, 13B) of said stepping motor (6) in accordance with said count value; said recording apparatus being characterized in that control means (20) are provided for generating a load change signal (29) when said control output (26) of said speed control means (24) exceeds a predetermined level; and said current switching means (25) corrects said count value by said load change signal (29) from said control means (20), thereby output torque of said stepping motor (6) being changed, if said control output (26) exceeds said predetermined level. An apparatus according to claim 1, characterized in that said correction of said count value is performed in a direction reverse to the direction of counting. An apparatus according to claim 1, characterized in that said load change signal (29) is a phase change signal advancing a phase of said energization currents.

Inventors: ITO NORIAKI, TORISAWA AKIRA, ITO, NORIAKI, TORISAWA, AKIRA, ITO, NORIAKI, C/O CANON KABUSHIKI KAISHA, TORISAWA, AKIRA, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: B41J19
IPC codes: B41J19, B41J29