Patent Publication Number: US-2007098373-A1

Title: Dc motor drive unit

Description:
TECHNICAL FIELD  
      This invention relates to a direct-current (DC) motor drive unit capable of securely starting up the motor with suppressed startup current and running the motor at the speed in accord with an external speed instruction.  
     BACKGROUND ART  
      Motors are used in game controllers and toys to drive or vibrate their moving parts. Mostly, DC motors are used for this purpose for the reason that they can be powered by batteries and their drive circuits are simple in structure.  
       FIG. 5  shows a circuit arrangement of a widely used conventional DC motor drive circuit having an open loop control system. As shown in  FIG. 5 , a DC motor  1  is connected between a power supply voltage Vcc and the ground via a switching transistor  2  for switching on/off the motor. Since the speed of the DC motor  1  is proportional to the current I flowing through it, the DC motor  1  can be driven at a predetermined speed by controlling on-off operation of the switching transistor  2  using a drive control IC  4  providing pulse width modulation (PWM) pulses. A resistor  3  is provided to adjust the base current of the transistor  2 .  
      In the DC motor drive circuit shown in  FIG. 5 , exceedingly large startup current Ip will flow through the motor  1  at its startup starting at time t 0 . In the example shown in  FIG. 6 , the level of the startup current is more than three times larger than the stationary current level Ic. Therefore, it is necessary to configure the transistor  2  and the power source to withstand such large startup current level Ip, which raises the cost of the drive circuit.  
      To rotate the motor  1  at a low speed, the duty ratio of the PWM pulses must be reduced. Since the startup current is reduced in accordance with the duty ratio, a startup failure can take place if the startup current is too small to generate a necessary startup torque. Therefore, it is not possible to arbitrarily set the minimum rotational frequency of the DC motor  1 , so that the range of controllable speed of the motor is limited.  
      As a solution for reducing the startup current of such a DC motor as stated above, Japanese Patent Application Laid Open No.H11-230045 (referred to as Patent Document 1) discloses a method of reducing the startup current of a DC motor in which a bias current, small enough not to rotate the motor, is passed through it even when the motor is not in operation.  
      In the method of Patent Document 1, the startup current can be sufficiently lowered. However, the motor consumes wasteful electric power since the motor is provided with current even when it is not in operation. Moreover, in the method of the Patent Document 1, the range of the rotational speed of the DC motor that can be regulated by adjusting the duty ratio of the PWM pulses is limited like the conventional drive circuit shown in  FIG. 5 , due to the fact that a bias current, though it is small enough not to rotate the motor, is flowing through the switching transistor.  
      It is, therefore, an object of the present invention to provide a DC motor drive unit having an open-loop control system, capable of ensuring startup of the motor with sufficiently reduced startup current, thereby allowing not only reduction of the withstand voltage of the switching transistor used, but also broadening of the range of controllable rotational speed of the motor.  
     DISCLOSURE OF THE INVENTION  
      A DC motor drive unit of the invention for driving a DC motor, adapted to control switching means connected in series to the DC motor, comprises:  
      acceleration setting means for setting a predetermined acceleration period and acceleration stage data in association with the acceleration period at the time of startup of the DC motor; and  
      means for generating PWM pulses (hereinafter referred to as PWM pulse generation means) having duty ratios in accord with the acceleration stage data or in accord with a prescribed rotational speed of the motor, wherein  
      the switching means is controlled by 
          the PWM pulses having duty ratios in accord with the acceleration stage data during the predetermined acceleration period; and     the PWM pulses having the duty ratio in accord with the prescribed rotational speed after the predetermined acceleration period.        

      The DC motor drive unit of the invention may further comprise a data judgment means for judging whether or not an externally supplied speed instruction data instructs driving of the DC motor. When a judgment is made that the speed instruction data instructs driving of the motor, the switching means is controlled by the PWM pulses having duty ratios in accord with the acceleration stage data during the predetermined acceleration period, but, after the acceleration period, controlled by the PWM pulses having a duty ratio in accord with the rotational speed instructed by the speed instruction data.  
      The acceleration period may include a sequence of N (N≧1) acceleration stages each set to have PWM pulses of a predetermined duty ratio over a predetermined acceleration time in such a way that the duty ratio increases in the successive acceleration stages.  
      The DC motor drive unit may be adapted to: measure the time that has elapsed from the beginning of the sequence of acceleration period to determine the current stage in the acceleration period; and determine the duty ratio associated with the stage and/or the duty ratio associated with the speed instruction data in accordance with a lookup table.  
      Further, the DC motor drive unit may be adapted to execute acceleration of the motor in the acceleration period only if a determination is made that the speed instruction data instructs driving of the DC motor and the DC motor is not in operation.  
      The DC motor drive unit may be adapted to stop the DC motor if a judgment is made that the speed instruction data does not instruct driving of the DC motor.  
      As described above, a DC motor drive unit of the invention sets up a predetermined acceleration period in which switching means (e.g. a switching transistor), connected to a DC motor having an open loop control system, is controlled by PWM pulses of predetermined duty ratios at the time of startup of the DC motor, which permits suppression of the startup current of the motor, and hence reduction of the withstand current of the switching means and the cost of the DC motor drive unit while ensuring secure startup of the motor.  
      The invention sets up N (N≧1) acceleration stages in the acceleration period, with each stage having a predetermined acceleration time (duration) and a prescribed duty ratio of PWM pulses in such a way that the duty ratio increases in the successive stages, which enables quick startup while suppressing the startup current of the motor.  
      Moreover, since the DC motor is driven by PWM pulses of predetermined duty ratios during the acceleration period and by PWM pulses of a duty ratio based on speed instruction data after the acceleration period, the startup capability of the DC motor is improved and the minimum permissible rotational speed of the motor can be reduced. That is, the motor can be securely started up at all times, and the range of controllable speed of the motor after the startup can be broadened.  
      In addition, since driving instruction, rotational speed instruction, and stopping instruction for the motor can be discerned based on the magnitude of the speed instruction data supplied to the DC motor drive unit, an upstream or superior control unit can give the motor drive unit instructions on different drive conditions by simply sending the speed instruction data to the motor drive unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows an arrangement of a DC motor drive unit in accordance with one embodiment of the invention.  
       FIG. 2  is a flowchart describing operation of the DC motor drive unit of  FIG. 1 .  
       FIG. 3  is a graph showing an exemplary operational scheme of the DC motor drive unit shown in  FIG. 1 .  
       FIG. 4  is a graph showing another exemplary operational scheme of the DC motor drive unit shown in  FIG. 1 .  
       FIG. 5  shows an arrangement of a conventional DC motor drive unit.  
       FIG. 6  is a graph showing an operational scheme of the conventional DC motor drive unit. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      An inventive DC motor drive unit will now be described in detail by way of example with reference to the accompanying drawings.  FIG. 1  is a block diagram showing a circuit arrangement of a DC motor drive unit  10  in accordance with one embodiment of the invention.  FIG. 2  is a flowchart describing the operation of the circuit shown in  FIG. 1 .  FIG. 3  shows an exemplary operative condition of the DC motor drive circuit shown in  FIGS. 1 and 2 .  
      The DC motor drive unit of  FIG. 1  is controlled by an open loop control system. A DC motor  21  and a switching transistor  22  are connected between a power supply voltage Vcc and the ground. The switching transistor  22  has a base, which is supplied with PWM pulses Pwm from the motor drive control circuit  10  to turn on and off the switching transistor  22 . An adjustment resistor  23  is a variable resistor for adjusting the base current of the transistor  22 . The resistor  23  is provided as needed. A free wheel diode  24  is provided to restore electric power or to reduce noises, so that the diode can be omitted if it is necessary to reduce the cost of the circuit.  
      Through the DC motor  21  flows current I, the magnitude of which depends on the on-off duty ratio of the switching transistor  22 . In a stationary state operation where the DC motor  21  is driven to rotate at a fixed speed, the on-off duty ratio of the switching transistor  22  is controlled by the duty ratio of the PWM pulses Pwm. At the time of startup, however, an exceedingly larger current than stationary current will flow through the motor, as is the case with conventional drive units, unless the startup current is limited. Thus, the invention limits the startup current to ensure reduction of the withstand current of the switching transistor  22  while ensuring secure startup of the motor, whereby broadening the range of controllable speed of the motor.  
      The motor drive control circuit  10  is supplied from a superior control unit with speed instruction data Dsp instructing the rotational speed of the DC motor  21 . The superior control unit includes a CPU serving as, for example, a game controller and a main control unit for a toy. The superior control unit provides speed instruction data Dsp for controlling startup, rotational speed, and stopping of the DC motor  21 .  
      The motor drive control circuit  10  has: a controller  11  that includes data register means  11   a , data judging means  11   b , and rotation detection means  11   c ; acceleration setting means (hereinafter also referred to as acceleration time counting means)  12  adapted to count, upon receipt of an acceleration instruction signal Sacc, the time that has elapsed since the beginning of the acceleration period to determine the current acceleration stage and output relevant acceleration stage data Das for the acceleration stage; PWM duty generation means  13  for generating a pulse generation signal Ipwm for establishing PWM pulses, the PWM duty generation means  13  being supplied with speed instruction data Dsp, an acceleration stage data Das, and a stop instruction signal Soff; and PWM pulse generation means  14  for generating PWM pulses Pwm upon receipt of a pulse generation signal Ipwm and for supplying the PWM pulses Pwm to the switching transistor  22 .  
      The data register means  11   a  can store speed instruction data Dsp in a readable form, and updates the stored speed instruction data Dsp every time new speed instruction data Dsp are supplied from the superior controller.  
      The data judging means  11   b  reads out speed instruction data Dsp from the data register means  11   a  and, based on the speed instruction data Dsp, judges whether the speed instruction data Dsp instructs driving of the DC motor  21 . For example, if the speed instruction data Dsp exceeds a predetermined value, the data judging means  11   b  makes a judgment that the speed instruction data Dsp instructs driving of the motor, but otherwise makes a judgment that the data does not instruct driving.  
      When the speed instruction data Dsp is judged as instructing drive, the data Dsp is supplied to the PWM duty generation means  13 , or, at the time of startup, an acceleration instruction signal Sacc is supplied to the acceleration time counting means  12 . When the speed instruction data Dsp is judged not instructing driving, a stop instruction signal Soff is supplied to the PWM duty generation means  13  to stop the DC motor  21 . It is noted that the function of the stop instruction signal Soff can be substituted for by the speed instruction data Dsp supplied to the PWM duty generation means  13  and/or the acceleration instruction signal Sacc supplied to the acceleration time counting means  12 .  
      When a PWM pulse Pwm is received from the PWM pulse generation means  14  as a rotation detection signal Rdet, the rotation detection means  11   c  judges whether the DC motor  21  is rotating or not.  
      If a judgment is made (at the time of startup) that the DC motor  21  is not rotating, an acceleration instruction signal Sacc is supplied from the controller  11  to the acceleration time counting means  12  on condition that the speed instruction data Dsp instructs driving of the motor. When a judgment is made that the DC motor  21  is rotating (namely, it is in normal operation), speed instruction data Dsp is supplied from the control means  11  to the PWM duty generation means  13  on condition that the speed instruction data Dsp instructs driving of the motor.  
      It should be understood that the rotation detection signal Rdet could be any signal that indicates rotation of the DC motor  21 , so that a pulse generation signal Ipwm can be used for this purpose.  
      The acceleration time counting means  12  sets up N (N≧1) sequential acceleration stages, for example three acceleration stages S 1 -S 3 , in the acceleration period and outputs an acceleration stage data Das associated with the acceleration stages S 1 -S 3 . Upon receipt of an acceleration instruction signal Sacc, the acceleration time counting means  12  starts counting the time that has elapsed since the beginning of the acceleration period to output the acceleration stage data Das (integers  1 - 3  for example) over the respective prescribed times T 1 -T 3  for the respective acceleration stages S 1 -S 3 . The numerical acceleration stage data Das (e.g. integers  1 - 3 ) representing the respective acceleration stages S 1 -S 3  can be replaced by data similar to the speed instruction data Dsp representing the speed of the DC motor  21 . Upon completion of the Nth acceleration stage (e.g. acceleration stage S 3 ), the acceleration time counting means  12  ends outputting the acceleration stage data Das.  
      When an acceleration stage data Das is supplied, the PWM duty generation means  13  generates a pulse generation signal Ipwm, which is set to increase the duty ratio (D 1 -D 3 ) of the PWM pulses Pwm in the successive acceleration stages S 1 -S 3 . When speed instruction data Dsp is supplied, the PWM duty generation means  13  generates a pulse generation signal Ipwm in accord with the speed instruction data Dsp. The pulse generation signal Ipwm can be any signal that can determine, for example, the timing of rise and fall of a PWM pulse Pwm.  
      The speed instruction data Dsp may be solely supplied to the PWM duty generation means  13  even when the acceleration stage data Das is not supplied to the PWM duty generation means  13 , but the speed instruction data Dsp may be supplied to the PWM duty generation means  13  simultaneously with the acceleration stage data Das. When the speed instruction data Dsp and the acceleration stage data Das are supplied simultaneously, the PWM duty generation means  13  is controlled to prioritize the acceleration stage data Das. When a stop instruction signal Soff is supplied from the controller  11  to the PWM duty generation means  13 , the PWM duty generation means  13  stops outputting a pulse generation signal Ipwm, irrespective of whether the acceleration stage data Das and the speed instruction data Dsp are supplied or not.  
      Since the PWM duty generation means  13  generates a pulse generation signal Ipwm in accordance with the speed instruction data Dsp and the acceleration stage data Das, it is preferable to provide the PWM duty generation means  13  with a lookup table. As an example, given a speed instruction data Dsp in an 8-bit digital form, the lookup table determines the duty ratio of the PWM pulses Pwm such that the duty ratio of the PWM pulses Pwm is zero when the speed instruction data Dsp is less than a predetermined lower limit, but not zero when the speed instruction data Dsp exceeds the lower limit.  
      In this manner, driving, stopping, and rotational speed of the motor can be controlled by the speed instruction data Dsp supplied from the superior control unit. If there is a nonlinear relationship between the rotational speed of the DC motor  21  and the duty ratio of the PWM pulses Pwm, an apparently different relationship can be established between the speed instruction data and the duty ratio on the lookup table by taking account of the nonlinear characteristic in the lookup table. For example, an apparently linear relationship can be desirably established between the speed instruction data Dsp and the rotational speed of the DC motor  21 .  
      The PWM pulse generation means  14  generates PWM pulses Pwm having a duty ratio in accord with the pulse generation signal Ipwm supplied from the PWM duty generation means  13 , and outputs it as a drive signal to the switching transistor  22 . In the example shown herein, the PWM pulses Pwm is supplied to the controller  11  as a rotation detection signal Rdet.  
      Functions of the motor drive control circuit  10  described above can be implemented in hardware as well as in software.  
      Referring to the flowchart of  FIG. 2 , along with  FIGS. 1 and 3  respectively showing the arrangement and operative conditions of the DC motor drive circuit, operation of an inventive DC motor drive unit will now be described.  
      The operation starts in step S 101 , in which speed instruction data Dsp specifying the rotational speed of the DC motor  21  is set in the data register means  11   a  by the superior control unit.  
      In each of steps S 102  and S 103 , the data judging means  11   b  reads out the speed instruction data Dsp from the data register means  11   a  and compares the speed instruction data Dsp with a predetermined value N 1 . In step S 102 , if the speed instruction data Dsp is found to be smaller than the predetermined value N 1 , the speed instruction data Dsp is not considered to be drive instruction data, thereby executing no startup operation for the DC motor  21 . If in this case the DC motor  21  is already in stationary rotation, an action is taken to immediately stop the DC motor  21 . If in step S 103  the speed instruction data Dsp is again found to be smaller than the predetermined value N 1 , the procedure returns to step S 101  to repeat this operation.  
      When the speed instruction data Dsp is larger than the predetermined value N 1 , the procedure proceeds to step S 104  through steps S 102  and S 103 , since the speed instruction data Dsp then instructs driving of the motor.  
      In step S 104 , it is judged by the rotation detection means  11   c  whether the DC motor  21  is rotating or not. The rotation of the DC motor  21  is judged, or estimated, based on a determination as to whether PWM pulses Pwm are supplied to the DC motor  21  or not, or whether a pulse generation signal Ipwm has been outputted or not to generate the PWM pulses Pwm. Since the rotation of the DC motor  21  is detected based on, for example, the PWM pulses Pwm, a rotation sensing device such as a tachometer is not required.  
      When a judgment is made in step S 104  that the DC motor  21  is not rotating, the procedure proceeds to an acceleration phase (steps S 111 -S 114 ), but otherwise the procedure proceeds to a stationary rotation phase (steps S 121 -S 122 ).  
      In the example shown herein, the acceleration phase (steps S 111 -S 114 ) incorporates an acceleration period that includes a first through a third acceleration stages S 1 -S 3  (N=3 in this example), so that the drive unit outputs acceleration stage data Das in the respective acceleration stages S 1 -S 3 .  
      In step S 111 , acceleration is executed while the acceleration stage number is 0, 1, and 2 in accordance with the respective acceleration stages S 1  through S 3 , and then the procedure proceeds to the stationary rotation stage (steps S 121 -S 122 ) when the acceleration stage number becomes 3.  
      The acceleration stage number is 0 at the beginning of a startup. Conditions for the first acceleration stage S 1  are set in step S 112  (for example, “acceleration time=T 1  ms and the duty ratio of the PWM pulses=D 1 %”). The DC motor  21  is turned on and off (that is, the switching transistor  22  is turned on and off) in step S 103  under this acceleration condition.  
      Development of the acceleration of the DC motor  21  is shown in  FIG. 3 ( a )-( b ). The first acceleration stage S 1  starts at time t 0  with the duty ratio of D 1 % and lasts a period of T 1 . The level of the current I provided to the DC motor  21  in the first acceleration stage S 1  remains a little higher than the stationary current level Ic of the motor  21  (under duty ratio of 100%). This current I decreases in the course of time from time t 0  to t 1 . At time t 1 , the first acceleration stage S 1  ends. At this point of time t 1 , the acceleration stage number is incremented by 1 in step S 114 , that is, the count is incremented from 0 to 1.  
      When the acceleration stage number is 1, conditions for the second acceleration stage S 2  are set (for example, “Acceleration time=T 2  ms and duty ratio of PWM pulses=D 2 %”). The DC motor  21  is driven in step S 103  under the acceleration conditions. As seen in  FIG. 3 ( a )-( b ), the second acceleration stage S 2  starts at time t 2  with the duty ratio of D 2 % and lasts a period of T 2 . The level of the current I provided to the DC motor  21  in the second acceleration stage S 2  also remains a little higher than the stationary current level Ic of the DC motor  21 , and decreases in the course of time from t 1  to t 2 . At time t 2 , the second acceleration stage S 2  ends. At this point of time t 2 , the acceleration stage number is incremented by 1 in step S 114 , that is, the count is increased from 1 to 2.  
      When the acceleration stage number is 2, conditions for the third acceleration stage are set (for example, “acceleration period of time=T 3  ms and duty ratio of PWM pulses=D 3 %”). The DC motor  21  is driven in step S 103  under the acceleration conditions. It is seen in  FIG. 3 ( a )-( b ) that in the third acceleration stage S 3  the acceleration starts at time t 2  with the duty ratio being D 3 % and lasts for a period of T 3 . The level of the current I in the second acceleration stage S 2  also remains a little higher than that of the stationary current level Ic of the DC motor  21 , and decreases over a period from time t 2  to t 3 . At time t 3 , the third acceleration stage S 3  ends. At time t 3 , the acceleration stage number is incremented by 1 (step S 114 ), which increases the count from 2 to 3.  
      When the acceleration stage number is 3, a judgment is made in step S 111  whether the third acceleration period has expired or not, and, if it has, the procedure proceeds to the stationary rotation stage. Shortly after time t 3  when the motor entered the stationary rotation phase, the level of the current I rises to a level (peak level Ip in the example shown) which is a slightly higher than the stationary current level Ic of the DC motor  21 . The current then decreases in time towards the stationary current level Ic.  
      Specifically, the acceleration times and duty ratios can be set as, for example, “T 1 =25 ms, D 1 =65%”; “T 2 =25 ms, D 2 =75%”; and “T 3 =25 ms, D 3 =85%”. The acceleration times T 1 -T 3  of the respective acceleration stages S 1 -S 3  can be identical or different from one another. However, in order to limit the current I below a certain level, it is necessary to increase the duty ratio (D 1 -D 3 ) in sequence in the acceleration stages S 1 -S 3  in the order mentioned.  
      Further, it is preferred that the duty ratio D 1  for the first acceleration stage S 1  is set independently of the speed instruction data Dsp that is given at the end of the acceleration period so that the motor  21  can overcome the static frictional torque acting on it. Thus, after the acceleration period, the DC motor  21  can be rotated at a low speed in accordance with speed instruction data Dsp no matter whether the speed instruction data Dsp gives 100% duty ratio as shown in  FIG. 3  or significantly small duty ratio as indicated by a broken line in  FIG. 3 ( a ). Thus, startup capability of the DC motor  21  is improved in the manner as described above, which in turn permits reduction of the minimum permissible rotational speed of the motor.  
      In the stationary rotation stage (steps S 121 -S 122 ), the PWM duty generation means  13  and the PWM pulse generation means  14  generate PWM pulses having a duty ratio in accord with the speed instruction data Dsp to control on-off operation of the switching transistor  22 . This makes the DC motor  21  to rotate at the speed in accord with the speed instruction data Dsp.  
      Subsequently, the steps S 101  to the stationary rotation step S 121  via steps S 102 -S 104  is repeated to keep the DC motor  21  in rotation.  
      When the speed instruction data Dsp is changed during a steady operation of the DC motor  21 , the operating condition of the motor  21  will be changed accordingly. If new speed instruction data Dsp has a value larger than a predetermined value N 1 , the duty ratio of the PWM pulses Pwm is changed in accordance with the new speed instruction data Dsp, thereby causing the DC motor  21  to continue its rotation at a speed set by the new speed instruction data Dsp.  
      However, when the new speed instruction data Dsp has a value smaller than the predetermined value N 1 , the new speed instruction data is not judged in step S 102  as giving drive instruction. The procedure further proceeds from step S 102  to a stop phase (steps S 131 -S 132 ), in which speed instruction data Dsp is stopped (step S 131 ), that is, not given to the DC motor  21 , and the acceleration stage number is reset to 0 (step S 132 ). Then, steps S 101  to the stop phase (steps S 131 -S 132 ) via step S 102  is repeated to sustain the motor in a standby mode.  
      Thus, drive instruction, speed instruction, and stop instruction are discerned from the magnitude of the speed instruction data Dsp. This implies that an upstream or superior control unit can give the motor drive control circuit  10  instructions on different operating conditions of the DC motor  21  using only the speed instruction data Dsp.  
       FIG. 4 ( a )-( b ) illustrates operation of a DC motor drive circuit for which N=2, that is, it has an acceleration period associated with two acceleration stages S 1  and S 2 . As seen in  FIG. 4 , the acceleration period differs from the foregoing example in that it involves only two acceleration stages S 1  and S 2 , but is similar in operation to that described above in connection with  FIGS. 1-3 . As an example, acceleration times and duty ratios can be set as, for example, “T 1 =50 ms, D 1 =60%”, and “T 2 =50 ms, D 2 =75%”. The acceleration times T 1  and T 2  for the respective acceleration stages S 1  and S 2  can be identical. However, in order to limit the current I below a certain level, it is necessary to increase the duty ratio (D 1 -D 2 ) in the successive acceleration stages S 1 -S 2 .  
      It will be apparent that the acceleration period can include more than three (N≧4) acceleration stages, or only one acceleration stage (N=1). What kind of acceleration stages be provided for the motor drive unit depends on, for example, the switching transistor  22 , DC motor  21 , and power source used.  
      It should be understood that the DC motor  21  could be a brush-type motor or a brushless motor. The switching transistor  22  is not limited to a bipolar transistor, and in fact it can be any switching element that can be switched on and off by a control signal.  
     INDUSTRIAL APPLICABILITY  
      The DC motor drive unit of the invention can control the rotation of a DC motor used in a game controller or a toy to drive and/or vibrate a movable element thereof in accordance with an external speed instruction. The drive unit can suppress the startup current of the DC motor.