Method of driving stepping motor

Abstract: A method of driving a stepping motor comprises providing a chopping signal to a drive current of the stepping motor when a chopping condition is on, and synchronizing output of the chopping signal with output of a drive signal for driving stepwise the stepping motor.

Description: The present invention relates to a method of driving a stepping motor according to the preamble of claim 1. Conventionally, the stepping motor is used in an automotive suspension system disclosed, for example, in JP-A- 5-193327. A variable-damping-force-type shock absorber used in the automotive suspension system includes a rotary value having changeable damping force characteristic, and a stepping motor mounted at an upper end of the shock absorber so as to rotate the rotary valve. However, the conventional variable-damping-force type shock absorber has the following inconvenience: When driving stepwise the stepping motor drive current is always turned on, i.e. in the continuous turn-on state, actual current input to the stepping motor is heavy, and thus drive torque is great. As a result, great vibration noise is produced upon drive of the stepping motor, which may cause a seat occupant to have a strange feel upon low-speed cruising or standstill of a motor vehicle in particular. DE 40 02 158 A1 discloses a method of driving a stepping motor directed to reduce its torque variations. In order to achieve this, a synchronized clock current is provided for the stepping motor. More specifically, an oscillator clock impulse signal and its corresponding current signal are periodically synchronized with the rising edges of step impulses. DE 42 26 051 A1 discloses a method of controlling the position of a stepping motor within a vehicle for adjusting the damping coefficients of a shock absorber. Vibration of the stepping motor is reduced by enabling a specifically controlled transient process when the stepping motor is moved from one position to another. It is the object of the present invention to provide a method of driving a stepping motor which contributes to a restraint of generation of vibration during drive thereof. This object is solved by the subject matter of claim 1. In the following, the invention is described in more detail by referring to the accompanied figures. Figure 1 is a time chart showing a method of driving a stepping motor according to the present invention; Figure 2 is a flowchart showing an operation of the method of driving a stepping motor; Figure 3 is a diagrammatic sectional view showing a variable-damping-force type shock absorber to which the present invention is applied; and Figure 4 is a waveform chart showing vibration waveforms of a stepping motor when driven stepwise in accordance with the method of the present invention. Referring to Fig. 3, there is shown a variable-damping-force-type shock absorber for a motor vehicle, generally designated by SA, to which a method of driving a stepping motor according to the present invention is applied. The shock absorber SA includes a cylinder 1, a piston 2 serving to define an upper chamber A and a lower chamber B of the cylinder 1, an outer tube 4 serving to form a reservoir chamber C at the outer periphery of the cylinder 1, a base 5 serving to define the lower chamber B and the reservoir chamber C, a guide member 7 serving to slidably guide a piston rod 6 connected to the piston 2, a suspension spring 8 interposed between the outer tube 4 and a vehicular body (no shown), and a bumper rubber 9. Moreover, the shock absorber SA includes a rotary valve 10 having changeable damping force characteristic arranged within the piston rod 6, and a stepping motor 3 mounted at an upper end of the shock absorber SA so as to rotate the rotary valve 10 through a control rod 11. Referring next to Figs. 1 and 2, a method of driving stepwise a stepping motor according to the present invention will be described. Referring to Fig. 2, at a step 101, it is determined whether or not a chopping condition is on. It is noted that the chopping condition is on upon low-speed cruising or standstill of the motor vehicle. If the answer at the step 101 is YES, control proceeds to a step 102 where switching is carried out to the current turn-on state with a chopping signal, i.e. the discontinuous current turn-on state, then, control proceeds to a step 103. On the other hand, if the answer at the step 101 is NO, control proceeds to a step 107 where switching is carried out to the continuous current turn-on state, then, control comes to an end. At the step 103, it is determined whether or not a drive signal of the stepping motor 3 was switched. If the answer at the step 103 is YES, control proceeds to a step 104 where current turn-on with a chopping signal is reset to have the off-start state. After a lapse of a predetermined period of time T, at a subsequent step 105, the stepping motor 3 is driven stepwise by current turn-on with a chopping signal, i.e. discontinuous current turn-on. On the other hand, if the answer at the step 103 is NO, control comes to an end. At a step 106 subsequent to the step 105, it is determined whether or not the stepping motor 3 was driven stepwise up to a target position. If the answer at the step 106 is YES, control comes to an end, whereas if the answer at the step 106 is NO, control proceeds to a step 108 where switching of a drive signal is carried out to drive the stepping motor 3 by one more step, then, control returns to the step 102. Referring to Fig. 1, drive current for driving the stepping motor 3 is chopped upon low-speed cruising or standstill of the motor vehicle so as to decrease an actual current value. This enables a reduction in drive torque of the stepping motor 3, restraining vibration of the stepping motor 3 during drive thereof, resulting in a reduced noise level transmitted in a vehicular room. Further, output of a chopping signal is synchronized with that of a drive signal for driving stepwise the stepping motor 3. Thus, a period of time from a point that a drive signal is output to a point that a chopping signal is reset in the turn-off state can be constant, having no potential difference between drive steps in a ratio of a current turn-on time, i.e. an on-duty ratio. This can prevent generation of noise due to a dispersion of a vibration level of the stepping motor 3 during drive thereof, resulting in a further reduction in a noise level. Furthermore, upon output of a drive signal for driving stepwise the stepping motor 3, chopping current is first reset in the turn-off state or put in the off-start state, then synchronized with a drive signal to be turned on after a predetermined period of time T. This contributes to a reduction in the number of chopping signals, enabling a further reduction in drive torque of the stepping motor 3, resulting in a further reduced vibration level thereof. Referring to Fig. 4, there is shown vibration waveforms of the stepping motor 3 when driven stepwise in accordance with a flow as shown in Fig. 2. These waveforms correspond to waveforms of acceleration G in the direction of rotation of the stepping motor 3. In Fig. 4, a broken line indicates a vibration waveform in the continuous current turn-on state, and a fully-drawn line indicates a vibration waveform in the discontinuous current turn-on state with a chopping signal. It will be understood from Fig. 4 that the vibration waveform in the discontinuous current turn-on state with a chopping signal is stabilized at a lower level as compared with that in the continuous current turn-on state.

Claims: A method of driving a stepping motor, comprising the steps of : inputting a drive current to the stepping motor; providing a chopping signal to the drive current when a chopping condition is on for producing a discontinuous turn-on of the drive current; and synchronizing the chopping signal with a drive signal to step-wise drive the stepping motor; characterized in that the chopping signal is turned off upon output of the drive signal, and is turned on after a predetermined period of time (T). A method as claimed in claim 1, characterized in that the chopping signal is in the form of a pulse having a constant width and repeated with a predetermined period.

Inventors: HIRAMOTO MICHIYA, SIMIZU HIROYUKI, HIRAMOTO, MICHIYA, SIMIZU, HIROYUKI
Applicants: ATSUGI UNISIA CORP, UNISIA JECS CORPORATION
Application year: 1995
Main classification: H02P8
IPC codes: H02P8, B60G17, F16F9