Stepping motor control system and recording apparatus using the same

Abstract: In this invention, a stepping motor control system includes a PWM unit (102) for generating a PWM signal corresponding to a changing duty ratio, a low-pass filter (104) for receiving the PWM signal from the PWM unit, and converting staircase voltage levels represented by the changing duty ratio of the PWM signal into a smooth, substantially continuous voltage signal, and an arbitrary current value setting type constant current driving circuit (105), which receives the voltage signal from the low-pass filter, excites the excitation coil (107) of the stepping motor by a current value set in correspondence with the voltage signal, and maintains the current value set in correspondence with the voltage of the voltage signal. Thus, a stepping motor control system which can eliminate wasteful power consumption and can realize high-precision, constant-speed rotation can be provided. When the stepping motor control system is used for driving a carriage of a recording apparatus, a recording apparatus which is free from printing nonuniformity and can achieve power saving can be provided.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a stepping motor control system and a recording apparatus using the same. Related Background Art Conventionally, stepping motors are widely used as driving sources for industrial equipment in recent years due to their high rotational position alignment precision. In particular, stepping motors are popularly used as driving motors for so-called OA (office automation) equipment such as recording apparatuses. As a typical control system of a stepping motor, a constant voltage driving system is known. This system requires a constant voltage circuit for preventing a variation in torque caused by a change in motor current corresponding to a change in voltage. As a result, (1) the size of the entire circuit becomes large and cost increases. Furthermore, since constant voltage driving is performed, (2) a vibration is generated in the motor, resulting in large rotation noise, and (3) electric power which is not used for driving is wastefully consumed, thus generating heat. On the other hand, a constant current driving system is known as a driving system which is not influenced by a variation in voltage. In this constant current driving system, the value of a current flowing through a motor winding or coil is detected, and a switch element such as a transistor is pulse-width-modulation-driven so that the detected current value becomes a preset current value. The constant current driving system can solve the problem (1), but cannot solve the problems (2) and (3) due to constant current driving. In order to solve the problems of the above-mentioned constant current driving system, the present applicant has proposed an open PWM control system in Japanese Patent Application No. 4-203863 which corresponds to EP 0 581 300 A2. In the open PWM control system, a driving current value set signal for setting a variable duty ratio by pulse-width modulation (PWM) is generated, and is supplied to each winding of a motor, so that a current value which corresponds to a vector component matching an arbitrary rotational position of the motor to some extent is supplied to each winding. More specifically, since it is theoretically ideal to supply a current value, which changes in a sine waveform pattern, to each winding, a current value with a staircase waveform which is approximate to the sine waveform as much as possible is supplied to each winding of the motor. Two-phase excitation driving in the open PWM control system will be explained below. Fig. 10 shows a driving circuit of a stepping motor. The driving circuit shown in Fig. 10 comprises a micro-controller 201 for performing motor control, a pulse-width modulation unit (to be referred to as a PWM unit hereinafter) 202 which is incorporated in the micro-controller 201 and outputs pulse signals E and F whose frequencies and duty ratios can be set, an output port 203 which is incorporated in the micro-controller 201 and generates coded stepping motor control signals A, B, C, and D, a unipolar-coupled two-phase stepping motor 204, transistors 205 for exciting the stepping motor 204 in accordance with the control signals A, B, C, and D, current control transistors 206 for controlling currents flowing through the stepping motor 204 in accordance with the pulse signals E and F, fly-wheel diodes 207 for forming current paths when the current control transistors 206 are turned off, diodes 208 for preventing reverse currents due to induced voltages at the windings of the stepping motor 204, a programmable timer unit 209 incorporated in the micro-controller 201, and a ROM 210 which stores data such as the driving speed, PWM duty ratio, and the like of the motor 204. The micro-controller 201 reads out such data from the ROM 210. Fig. 11 shows the waveforms of the control signals for controlling the stepping motor 204. The micro-controller 201 generates the control signals A, B, C, and D for performing two-phase excitation driving of the stepping motor 204 via the output port 203. When these control signals are at H level, the transistors 205 connected to these control signals are turned on, and the corresponding windings of the stepping motor 204 are excited. The change timing of each control signal, i.e., step time, is determined by the micro-controller 201 using the timer unit 209. By adjusting the step time, respective modes such as acceleration, deceleration, constant-speed operation, and the like are controlled. The micro-controller 201 controls the PWM unit 202 to output the pulse waveforms E and F. These pulse waveforms are set to be pulse-output at a predetermined frequency (e.g., a frequency of 20 kHz or higher, which is higher than the audible range of a man) to have a predetermined duty at predetermined timings. The change timing of the duty ratio is also determined by the micro-controller 201 using the timer unit 209. When the pulse outputs are at H level, the current control transistors 206 are turned on, and supply electric power to the motor 204. When the current control transistors 206 are turned off, electric power accumulated on the motor 204 is discharged via the corresponding fly-wheel diodes 208. Upon repetition of these operations, the current to be supplied to the winding of the motor 204 can be controlled in accordance with the duty ratio of the pulse waveforms E and F. Fig. 12 shows an example of PWM duty data stored in the ROM 210. Numerals 1 to 8 in the upper row are ROM addresses which are assigned for convenience, and numerical values in the lower row represent PWM duty ratios stored at the respective addresses. Fig. 13 shows the motor driving waveforms based on the PWM duty ratio data shown in Fig. 12. Note that signal waveforms E and F are not actual pulse waveforms, but express the duty ratios of pulses by their signal levels. Referring to Fig. 12, the micro-controller 201 changes the driving signals A and C, and sets the duty ratio of the PWM pulse signal E to be 40% in accordance with the numerical value, "40", stored at address 1 in the ROM 210. Thereafter, when the time 1/4 the step period has elapsed, the micro-controller 201 sets the duty ratio of the PWM pulse signal E to be 60% in accordance with the numerical value, "60", stored at address 2 in the ROM 210. Similarly, the micro-controller 201 sequentially reads out PWM data from the ROM 210, and sets the duty ratios. As for the PWM pulse signal F, values which are phase-shifted by 90° from the signal E are set. Therefore, for example, when the signal E is set to have a value corresponding to data read out from address 1, the signal F is set to have a value corresponding to data read out from address 5. With the above-mentioned system, the current flowing through the motor can be controlled at a period 1/4 the step interval, and the same effect as that of a conventional driving system known as double 1-2 phase driving can be obtained. More specifically, when the current waveform of the motor is approximate to a sine waveform, the motor can be operated with high efficiency and low vibration. Since the current flowing through the motor is influenced by the inductance of the motor winding and the counter-electromotive force, the PWM duty ratio is normally not proportional to the motor current. However, when duty ratio data in which these influences are corrected in advance are stored in the ROM 210, the current waveform can be controlled while being approximate to a sine waveform. As described above, since the open PWM control system drives the motor based on a current value approximate to a sine wave, the problems (2) and (3) of the constant voltage driving system and the constant current driving system can be solved to some extent. However, these problems are not sufficiently solved, and the open PWM control system suffers the following problems. (1) Since the open PWM control is an open system having no feedback loop, the pulse width of each PWM signal must be set by experimental trial and error so that the current flowing through the motor coil has a desired waveform. (2) For example, a variation in value of a current flowing through each winding is large due to variations in characteristics of transistors and diodes constituting the motor driving circuit. When the variation in current value is large, the variation in torque becomes large, and rotation nonuniformity occurs. As a result, constant-speed rotation cannot be attained. The current value varies due to a variation in source voltage, and the torque varies. (3) In order to approximate the current waveform to an identical sine waveform as much as possible, a staircase waveform must be further smoothed. However, the number of times of switching of the level of the current value is eight per control signal, and a sufficiently smooth sine waveform cannot be obtained yet. (4) A PWM memory table (a memory table of ON/OFF data) for the motor rotational speed is required, and a large volume of data must be stored in the memory (ROM). On the other hand, a recording apparatus for performing recording on a recording sheet by scanning a carriage which mounts a recording head uses a stepping motor or a DC motor to scan the carriage. In a recording apparatus, when a stepping motor is used in the above-mentioned open PWM control system, since high-precision, constant-speed rotation cannot be sufficiently obtained, printed (recorded) image nonuniformity may occur. In addition, since wasteful consumption power cannot be sufficiently eliminated, the service life of a battery cannot be prolonged in, e.g., a portable recording apparatus which receives electric power from a battery. The DC motor is free from rotation nonuniformity at high speeds, and is suitable for constant-speed rotation. However, when a DC motor is used in a recording apparatus, a linear encoder for position detection, and a control circuit for processing a signal from the linear encoder and performing position control are required, resulting in higher cost than an apparatus using a stepping motor. EP 0 311 095 A2 discloses a pluse motor control apparatus for driving a pulse motor or stepping motor, respectively. The control apparatus comprises a digital-to-analog converter being connected to an input terminal for converting a digital input signal to an analogous voltage signal. The analogous voltage signal is applied to a comparator for controlling the stepping motor. The comparator controls in a closed loop a current which is applied to said stepping motor, wherein the current value is set on the basis of the analogous voltage signal. This known control apparatus does not generate a PWM signal. SUMMARY OF THE INVENTION The object of the present invention is to provide a stepping motor control system, which can solve the above-mentioned problems, can eliminate wasteful consumption power, and can attain high-precision, constant-speed rotation. The object is solved by a stepping motor control system comprising the features of claim 1. Advantageous embodiments are defined in the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram for explaining the principle of a stepping motor driving device according to the present invention; Figs. 2A and 2B are waveform charts for explaining the function of an arbitrary current value setting type constant current driving circuit (to be simply referred to as a constant current driving circuit hereinafter) used in the stepping motor driving device; Figs. 3A to 3C' are waveform charts for explaining the function of a low-pass filter; Fig. 4 is a circuit diagram of a stepping motor (unipolar motor) driving device; Figs. 5A to 5H are waveform charts for explaining the function of the overall stepping motor (unipolar motor) driving device in the case of two-phase excitation; Fig. 6 is a circuit diagram showing a stepping motor (bipolar motor) driving device as another embodiment; Figs. 7A to 7D are waveform charts of control signals used in the stepping motor (bipolar motor) driving device in the case of two-phase excitation; Fig. 8 is a circuit diagram of a low-pass filter using an operational amplifier; Fig. 9 is a perspective view showing the outer appearance of the recording apparatus of the present invention; Fig. 10 is a circuit diagram showing a conventional stepping motor driving circuit; Fig. 11 is a waveform chart showing control signals for controlling a conventional stepping motor; Fig. 12 is a table showing an example of PWM duty data stored in a ROM as a memory of a micro-controller for the conventional stepping motor; and Fig. 13 is a waveform chart showing the motor driving waveforms based on the PWM duty ratio data. BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. Fig. 1 is a block diagram for explaining the principle of a stepping motor driving device according to the present invention, Figs. 2A and 2B are waveform charts for explaining the function of an arbitrary current value setting type constant current driving circuit (to be simply referred to as a constant current driving circuit hereinafter) used in the stepping motor driving device, and Figs. 3A to 3C' are waveform charts for explaining the function of a low-pass filter. The following description will be given with reference to Fig. 1. Fig. 1 is a diagram for explaining the principle of a stepping motor according to the present invention, and illustrates only one current control transistor and associated motor winding portion as a motor winding excitation circuit for the sake of descriptive convenience. Referring to Fig. 1, a micro-controller 101 for controlling a stepping motor incorporates a PWM unit 102. The PWM unit 102 outputs pulse signals E and F whose frequencies and duty ratios can be set. An output port 103 is incorporated in the micro-controller 101, and outputs coded stepping motor control signals (to be simply referred to as control signals hereinafter) A, B, C, and D. The micro-controller 101 also incorporates a programmable timer unit 109. The timer unit 109 is used for setting the change timing of each control signal, i.e., the step time, and the like. A ROM 110 stores data such as the driving speed, PWM duty ratio, and the like of the motor. The pulse signal E output from the PWM unit is input to a low-pass filter 104 constituted by a resistor R1 and a capacitor C1. Note that the function of the low-pass filter 104 will be described later. The output from the low-pass filter 104 is input to a constant current driving circuit 105, and the output from the circuit 105 is input to a current control driving transistor 106. A motor winding 107 is connected between the current control transistor 106 and a power supply, and a sensor resistor 108 is connected between the current control transistor 106 and ground. The constant current driving circuit 105 serves to supply a current determined by a voltage input to its input terminal to the motor winding. As will be described in detail later, the circuit 105 compares a voltage (to be referred to as a reference voltage hereinafter) determined by the current value flowing through the sensor resistor 108 and a voltage input to the input terminal using a comparator (to be described in detail later with reference to Fig. 4). When the current value flowing through the motor winding becomes larger, the circuit 105 decreases the current value; when the current value flowing through the motor winding becomes smaller, the circuit 105 increases the current value, so as to supply a constant current determined by the input voltage value to the motor winding independently of a variation in source voltage of the current control transistor. Since the current value is determined by the input voltage value, as described above, the current value changes upon changing the input voltage value. Therefore, the circuit 105 is an arbitrary current value setting type constant current driving circuit. A supplementary explanation as to the above-mentioned contents will be given with reference to Figs. 2A and 2B. Figs. 2A and 2B show the input waveform (voltage waveform) of the constant current driving circuit (Fig. 2A), and the waveform of the winding current value obtained based on the input waveform (Fig. 2B). For example, when a staircase voltage value is input as an input voltage, a current corresponding to the input voltage value, i.e., having a current value determined by the input voltage value, is supplied to the motor winding. The supplied current includes small ripple components (shown in an enlarged scale in Fig. 2B), since the constant current driving circuit 105 compares the input voltage and the reference voltage and ON/OFF-controls the input voltage, as will be described later. The function of the low-pass filter 104 will be described below with reference to Figs. 3A to 3C'. For example, if a PWM signal (Fig. 3A) is designed to generate a staircase voltage signal (a voltage signal whose duty ratio changes stepwise), the low-pass filter 104 which received the PWM signal smoothes the steps of the staircase voltage (indicated by a dotted curve) upon operation of its capacitor, as shown in Fig. 3B, and outputs a voltage defined by a smooth curve. For example, when such a voltage is input to the above-mentioned constant current driving circuit to control the current of the motor winding, a winding current value determined in correspondence with the voltage signal is obtained, as shown in Fig. 3C. In the present invention, in order to smooth a staircase signal as much as possible, the number of times of switching of steps is preferably increased as much as possible, as shown in Fig. 3B' (in Fig. 3B', the number of times of switching of steps is small for the sake of convenience, but in practice, switching of steps is performed a large number of times), i.e., a micro-step voltage signal is preferably generated, so as to further smooth the winding current value, as shown in Fig. 3C'. The entire stepping motor driving device of the present invention will be described below with reference to Figs. 4 to 7D. Fig. 4 is a circuit diagram of a stepping motor (unipolar motor) driving device (note that reverse flow prevention diodes and fly-wheel diodes are not shown in Fig. 4 since they are the same as those in the prior art), and Figs. 5A to 5H are waveform charts for explaining the function of the entire stepping motor (unipolar motor) driving device in the case of two-phase excitation. Fig. 6 is a circuit diagram of a stepping motor (bipolar motor) driving device as another embodiment (note that reverse flow prevention diodes and fly-wheel diodes are not shown in Fig. 6 since they are the same as those in the prior art), and Figs. 7A to 7D are waveform charts showing control signals used in the stepping motor (bipolar motor) driving device in the case of two-phase excitation. Referring to Fig. 4, the pulse signals E and F output from the PWM unit 102 are respectively input to low-pass filters 104a and 104b, and the outputs from the low-pass filters 104a and 104b are respectively input to one input terminals (the non-inverting terminals in Fig. 4) of comparators 111a and 111b of the constant current driving circuit 105. The other input terminals (the inverting terminals in Fig. 4) of the comparators 111a and 111b receive voltage signals (reference voltages) which are detected by sensor resistors 108a and 108b and are determined by currents flowing through the sensor resistors. Each of these comparators 111a and 111b compares the voltages input to the non-inverting and inverting terminals. When the input voltage to the non-inverting terminal is higher than that to the inverting terminal, each comparator generates an H (high) output; when the input voltage to the non-inverting terminal is lower than that to the inverting terminal, each comparator generates an L (low) output. The output from the comparator 111a is input to one input terminals of A- and A -phase AND gates 112a and 112b, and the other input terminals of the AND gates 112a and 112b receive control signals A and C. Therefore, when the control signals A and C input to the AND gates 112a and 112b are at H level, the AND gates 112a and 112b supply the output (H or L) from the comparator 111a to current control transistors 106a and 106b to supply a current to the motor winding (motor coil). Note that the output from the comparator 111b is input to one input terminals of B- and B -phase AND gates 112c and 112d, and is similarly gated in accordance with control signals B and D input to the other input terminals of the AND gates, as described above. The operation of the constant current driving circuit 105 will be described below. Assume that the control signal A is at H level, and the pulse signal E as a PWM signal has a 50% duty. The output voltage from the low-pass filter 104a is 2.5 V (source voltage of 5 V). A voltage of 2.5 V is input to the non-inverting input terminal of the comparator 111a of the constant current driving circuit 105. At this time, if no current is supplied to the motor winding due to an initial state, and a voltage detected by the sensor resistor 108a (e.g., 1 Ω) is equal to or lower than 2.5 V, since the voltage value applied to the non-inverting input terminal of the comparator 111a is higher than that applied to the inverting input terminal, the output from the comparator 111a goes to H level. For this reason, since the two inputs of the AND gate 112a are at H level, the output from the AND gate 112a changes to H level. As a result, an H voltage is input to the base of the current control transistor 106a, and the transistor 106a is turned on. A current is supplied to the motor winding until the current value reaches 2.5 A. When the current value exceeds 2.5 A, the voltage detected by the sensor resistor 108a becomes equal to or higher than 2.5 V. As a result, the voltage at the inverting input terminal of the comparator 111a becomes higher than that at the non-inverting input terminal, and the output from the comparator 111a changes to L level. Since this L output is input to one input terminal of the AND gate 112a, the output from the AND gate 112a changes to L level to turn off the current control transistor 106a. For this reason, the current decreases, and the voltage detected by the sensor resistor 108a decreases to a value equal to or lower than 2.5 V. When the voltage value becomes equal to or lower than 2.5 V, the output from comparator 111a changes to H level again, and the above-mentioned cycle is repeated. In this manner, the current value (e.g., 2.5 A) determined by the voltage (e.g., 2.5 V) input to the constant current driving circuit is maintained independently of the presence/absence of a variation in source voltage. For example, when the duty ratio is set to be 80%, a current value of 4 A corresponding to an input voltage of 4 V is maintained to be supplied to the motor winding. The above description has been given as to the A phase, and the same applies to the A -, B-, and B -phases. The function of the entire stepping motor (unipolar motor) driving device in the case of two-phase excitation will be described below with reference to Figs. 5A to 5H. Although two-phase excitation will be exemplified below, the present invention is not limited to two-phase excitation, but may be applied to other excitation systems such as one-phase excitation, 1-2 phase excitation, and the like. In the case of two-phase excitation, waveforms A (A phase), B (B phase), C ( A phase), and D ( B phase) of control signals are rectangular waves having a 90° phase difference, as shown in Figs. 5A to 5D. For this reason, in Fig. 4, in the ON state of the waveform A of the A phase, a predetermined current is supplied to an A-phase winding 107a via the constant current driving circuit 105 on the basis of the voltage from the low-pass filter 104a. After the 90° phase passes, in the ON state of the waveform B of the B phase, a predetermined current is supplied to a B-phase winding 107c via the constant current driving circuit 105 on the basis of the voltage from the low-pass filter 104b. Then, in the ON state of the waveform C of the A phase, a predetermined current is supplied to an A -phase winding 107b via the constant current driving circuit 105 on the basis of the voltage from the low-pass filter 104a. Thereafter, after the 90° phase passes, in the ON state of the waveform D of the B phase, a predetermined current is supplied to a B -phase winding 107d via the constant current driving circuit 105 on the basis of the voltage from the low-pass filter 104b. In this manner, a torque required for rotating the motor is supplied. Figs. 5E and 5F show the pulse signals E and F as PWM signals to be input to the low-pass filters 104a and 104b (for the sake of convenience, Figs. 5E and 5F illustrate one ON state of each of the control signals A and B, and other states are not shown since they are repetitive states of the illustrated states). Each of the signals E and F is set to have a sine waveform since theoretically the voltages obtained via the low-pass filters 104a and 104b optimally have sine waveforms. However, square curve waveforms may often be suitable depending on the characteristics of the motor and other factors. Either the sine or square waveforms may be adopted in correspondence with situations, or may be slightly modified (e.g., the ramp-up side is set to be less steeper than the ramp-down side). Figs. 5G and 5H show the voltage outputs from the low-pass filters 104a and 104b in the case of square curve waveforms. When such a voltage waveform is used for driving the motor via the constant current driving circuit 105, as described above with reference to Fig. 4, a current with a square curve waveform corresponding to this voltage waveform is supplied to the motor winding. As a result, constant-speed rotation with higher precision than that achieved by a staircase current in the conventional PWM control is realized, and wasteful consumption power can be eliminated. Fig. 6 is a circuit diagram of a stepping motor (bipolar motor) driving device. A micro-controller 101, a PWM unit 102, an output port 103, a timer 109, a ROM 110, low-pass filters 104a and 104b, and a constant current driving circuit 105 are the same as those in the embodiment shown in Fig. 4. Transistors 106 are used for selecting a phase to be excited and the current direction. As in the above embodiment, when two-phase excitation is performed, control signals A, B, C, and D shown in Figs. 7A to 7D are used. Currents supplied to the transistors 106 are switched by these control signals, and two-phase control similar to that in the above embodiment can be attained. In this embodiment as well, pulse signals E and F and the low-pass filters 104a and 104b are controlled by the same waveforms (Figs. 5F to 5H) as in the above embodiment. Another embodiment of a low-pass filter will be explained below with reference to Fig. 8. Fig. 8 is a circuit diagram of a low-pass filter using an operational amplifier. A resistor 305, a capacitor 307, a resistor 306, and a capacitor 308 are used as external elements of an operational amplifier 304 to constitute the low-pass filter. Since the arrangement of this embodiment can provide steeper low-pass filter characteristics than those in the above embodiment, a smoother output can be generated. When the input impedance of the constant current driving circuit 105 which receives the output from the low-pass filter is low, the output from the low-pass filter is influenced by the low impedance in the above embodiment. However, according to this embodiment, since the output signal from the low-pass filter has a high impedance, the output from the low-pass filter is not influenced by the impedance of the constant current driving circuit. In this manner, the low-pass filter of this embodiment is very effective for the constant current driving circuit with a low input impedance since it has an impedance conversion function in addition to the low-pass filter characteristics. A recording apparatus which is suitable for adopting the above-mentioned stepping motor driving device of the present invention will be described below. Since the stepping motor driving device of the present invention is particularly suitably applied to a chargeable (portable) ink-jet recording apparatus, the following description will be made in association with a chargeable ink-jet recording apparatus. However, the recording apparatus of the present invention is not limited to this. Fig. 9 is a perspective view showing the outer appearance of the ink-jet recording apparatus IJRA to which the stepping motor driving device of the present invention is applied. A carriage HC engages with a spiral groove 5004 of a lead screw 5005, which is rotated via driving force transmission gears 5011 and 5009 in synchronism with forward/reverse rotation of a driving motor 5013. The carriage HC has a pin (not shown), and is reciprocally moved in the directions of arrows a and b. A paper pressing plate 5002 presses a paper sheet against the platen roller 5000 across the moving direction of the carriage. Photocouplers 5007 and 5008 constitute home position detection means for confirming the presence of a lever 5006 of the carriage in a corresponding region, and performing, e.g., switching of the rotating direction of the motor 5013. A member 5016 supports a cap member 5022 for capping the front surface of the recording head, and a suction means 5015 for drawing the interior of the cap by suction performs suction recovery of the recording head via an intra-cap opening 5023. A cleaning blade 5017 is movable in the back-and-forth direction via a member 5019, and these members are supported on a main body support plate 5018. The blade is not limited to this, but a known cleaning blade can be applied to this embodiment. A lever 5021 is used for starting suction of the suction recovery. The lever 5021 moves upon movement of a cam 5020 which engages with the carriage, and the driving force from the driving motor is subjected to movement control by known transmission means such as clutch switching. These capping, cleaning, and suction recovery members are arranged to perform required processing operations at their corresponding positions upon operation of the lead screw 5005 when the carriage reaches the home position side region. As described above, the stepping motor driving device of the present invention using the low-pass filter and the constant current driving circuit can provide the following effects. (1) As compared to the constant voltage driving system or the constant current driving system, the entire circuit can be rendered compact, cost can be reduced, and a motor vibration is not generated to reduce rotation noise. (2) Furthermore, as compared to the open PWM control system, since each phase of the motor can be excited by a smooth current waveform (e.g., a sine or square curve waveform) approximate to an ideal waveform in principle, high-precision, high-speed constant rotation can be realized. Since current components which consume electric power are cut, wasteful power consumption can be avoided, and hence, the device of the present invention is suitable for, e.g., a battery-driven recording apparatus. In addition, a high torque can be obtained as compared to consumption power. (3) In addition, the excitation current is not influenced by a variation in power voltage. On the other hand, the stepping motor driving device of the present invention which uses the D/A converter and the constant current driving circuit has the above-mentioned effect (1) as compared to the constant voltage driving system or the constant current driving system, and can provide the following effects as compared to the open PWM control system. (1) The excitation current is not influenced by a variation in power voltage. (2) A voltage waveform for generating an excitation current can be easily set. The recording apparatus of the present invention which uses the above-mentioned stepping motor driving device (especially, using the low-pass filter and the constant current driving circuit) has the following effects. (1) Since high-speed, high-precision constant-speed rotation can be realized, the printing (recording) speed can be increased, and printed image nonuniformity can be eliminated. (2) Since a high torque can be obtained, the motor can be rendered compact, and hence, the entire apparatus can be rendered compact.

Claims: A stepping motor control system, which comprises signal generation means (101) for outputting a control signal for driving a stepping motor to driving means (105, 106) for driving said stepping motor, and outputting pulses, whose duties are set in correspondence with a plurality of periods obtained by dividing the one step driving time of said stepping motor, to said driving means, wherein said system comprises a PWM unit (2) for generating a PWM signal corresponding to a duty ratio and a low-pass filter (104a, 104b) for receiving a PWM signal from said PWM unit (102), and converting staircase voltage levels represented by a changing duty ratio of the PWM signal into a smooth, substantially continuous voltage signal, and said driving means (105, 106) comprises an arbitrary current value setting type constant current driving circuit (105), which receives the voltage signal from said low-pass filter (104a, 104b), excites an excitation coil (107a, 107b, 107c, 107d) of said stepping motor (5013) by a current value set in correspondence with the voltage signal, and maintains the current value set in correspondence with the voltage of the voltage signal. A system according to claim 1, wherein said low-pass filter (104a, 104b) comprises a resistor (R1, R2) inserted between said PWM unit (102) and said driving means (105, 106), and a capacitor (C1, C2) arranged between a terminal, on the driving means side, of said resistor and ground. A system according to claim 1, wherein said low-pass filter (104a, 104b) comprises an active filter using an operational amplifier (304). A system according to any one of claims 1 to 3, wherein said arbitrary current value setting type constant current driving circuit (105) comprises a comparator (111a, 111b) which receives the voltage signal at one input terminal thereof, and receives a reference voltage obtained in correspondence with a detected current value flowing through said stepping motor at the other input terminal thereof. A system according to claim 4, wherein said arbitrary current value setting type constant current driving circuit (105) comprises a gate (112a, 112b, 112c, 112d) which receives an output from said comparator (111a, 111b) at one input terminal thereof, and receives the control signal from said control signal generation means at the other input terminal thereof. A system according to any one of claims 1 to 5, wherein said PWM unit (102) generates said PWM signal to approximate a waveform of the voltage signal output from said low-pass filter (104a, 104b) to a sine waveform. A system according to any one of claims 1 to 5, wherein said PWM generates said PWM signal to approximate a waveform of the voltage signal output from said low-pass filter (104a, 104b) to a square curve waveform. A recording apparatus, which comprises a stepping motor (5013) for moving a carriage (HC) which mounts a recording head to attain a recording scan, and a PWM unit (102) for controlling a driving current of said stepping motor by applying pulses, whose duties are set in correspondence with a plurality of periods obtained by dividing one step driving time of said stepping motor, to driving means (105, 106) for driving said stepping motor, characterized in that said apparatus comprises a stepping motor control system as claimed in any precedent claim.

Inventors: IKEDA TETSUHITO, IKEDA, TETSUHITO, IKEDA, TETSUHITO, C/O CANON K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: H02P6
IPC codes: H02P8, G11B21