Abstract:
A meter such as a speedometer for use in an automobile instrument panel includes a pointer driven by a stepping motor. A pulse voltage having a wider pulse width is supplied to the stepping motor to obtain a higher induced voltage to be compared with a threshold voltage for detecting the pointer-zero-position. After the pointer-zero-position is detected, the stepping motor is driven by a pulse voltage having a narrower pulse width to obtain a quicker response of the pointer. In this manner, the pointer-zero-position is accurately adjusted without sacrificing the quick response of the meter pointer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims benefit of priority of Japanese Patent Application No. 2000-224089 filed on Jul. 25, 2000, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a meter for use in an automotive instrument panel, a pointer of the meter being driven by a stepping motor. 
     2. Description of Related Art 
     A stepping motor for driving a meter pointer includes a magnet rotor and field coils. The magnet rotor is usually magnetized to have plural poles, and therefore, the magnet rotor has plural stable positions in its rotation. If an overload is imposed on a rotational axis of the magnet rotor, the magnet rotor tends to take a wrong rotational position which is shifted from a right position to a neighboring stable position. In a speedometer for use in an automotive vehicle, this phenomenon causes an erroneous indication of a vehicle speed. 
     To alleviate this problem, in conventional meters, the pointer is over-driven toward a stopper positioned at a zero-position of a scale plate upon starting or terminating meter operation, and the pointer is forced to stop at the zero-position. Power supply to the stepping motor has to be stopped when the pointer hits the stopper. If the power supply to the stepping motor is not stopped when the pointer abuts the stopper, the pointer vibrates at the stopper position and generates noise in hitting the stopper repeatedly. Therefore, the halt of the stepping motor has to be detected. In conventional meters, the halt of the stepping motor is electrically detected based on a voltage induced in the field coils. That is, a voltage is induced in the field coils when the magnet rotor is rotating, while no voltage is induced when the magnet rotor is stopped. 
     However, it is not easy to detect the induced voltage in the field coils because the level of the induced voltage is too low. Usually, alternating pulse voltage is supplied to the field coils to drive the stepping motor, and its pulse width is made narrow to obtain a faster response of the stepping motor. The pulse width corresponds to a renewal time of the meter indication. Usually, the pulse width is set in a range of 0.5 to 1.5 milliseconds. The level of the induced voltage is not high enough to detect the motor halt based on the induced voltage. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved meter in which the pointer zero-position is properly adjusted. To surely detect the abutment of the pointer with the stopper, the level of the voltage induced in the filed coils is enhanced by increasing the pulse width of the driving voltage in the pointer zero-position adjustment mode. 
     A pointer of a meter such as a speedometer for use in an automotive instrument panel is disposed in front of a scale plate having an analog scale thereon. A stopper for stopping the pointer at its zero-position is provided on the scale plate. The pointer is driven by a stepping motor disposed behind the scale plate. The stepping motor is composed of a stator having field coils and a magnet rotor. 
     For adjusting the zero-position of the pointer, a driving pulse voltage having a wider pulse width (W 2 ) is supplied to the field coils thereby to rotate the magnet rotor. When that driving voltage is at zero level, the voltage supply to one of the field coils is shut off, and a voltage (Vi) induced in that field coil is detected. The induced voltage (Vi) is compared with a predetermined threshold voltage (Vth). 
     If the induced voltage (Vi) is lower than the threshold voltage (Vth), it is determined that the pointer has stopped at the zero-position by abutting the stopper. At this point, the voltage supply to the field coils is discontinued. Thus, the zero-position of the pointer is accurately set. After the pointer zero-position adjustment is completed, a driving pulse voltage having a narrower pulse width (W 1 ) is supplied to the field coils to operate the meter in an operating mode. Preferably, the pulse width (W 2 ) is set to a level which is about two times the pulse width (W 1 ). The pointer zero-position adjustment may be performed upon commencement of power supply to the meter or when the power supply is shut off. 
     According to the present invention, a higher induced voltage (Vi) is obtained because the driving pulse having a wider pulse width (W 2 ) is supplied to the stepping motor in the pointer zero-position adjustment mode. Therefore, the zero-position is easily and accurately detected by comparing the induced voltage (Vi) with the threshold voltage (Vth), while avoiding pointer vibration otherwise caused at the zero-position. Further, the pointer response is quicker because the driving pulse having narrower pulse width (W 1 ) is supplied to the stepping motor in the normal operation mode. Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view showing a meter for use in an automotive instrument panel as a speedometer; 
     FIG. 2 is a partial cross-sectional view showing the speedometer shown in FIG. 1; 
     FIG. 3 is a cross-sectional view showing a stepping motor for driving the speedometer; 
     FIG. 4 is a diagram showing a circuit for driving the stepping motor; 
     FIG. 5 is a flowchart showing a process for controlling the stepping motor; 
     FIG. 6 is a timing chart showing waveforms of a driving voltage supplied to the stepping motor in its normal operation mode; 
     FIG. 7 is a timing chart showing waveforms of a driving voltage supplied to the stepping motor in its pointer zero-position adjustment mode; 
     FIG. 8 is a graph showing an induced voltage in the pointer zero-position adjustment mode; and 
     FIG. 9 is a graph showing an induced voltage in the normal operation mode. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present invention will be described with reference to the drawings. FIGS. 1 and 2 show a structure of a speedometer for use in an automotive instrument panel. A pointer  20  is disposed in front of a meter panel  10  having a scale plate  10   a  thereon. A scale  11  showing a vehicle speed from 0 km/h to 180 km/h is formed on the scale plate  10   a . The pointer  20  is connected to a pointer shaft  31  via a pointer base  21 . The pointer shaft  31  is connected to a driving unit  30  that drives the pointer shaft  31 . A stopper  13  is provided in front of the scale plate  10   a , so that the pointer  20  stops at the zero-position by engaging with the stopper  13 . 
     The driving unit  30  containing a driving mechanism  32  therein is fixed to a rear surface of a wiring board  40  which is positioned behind the meter panel  10 . The driving mechanism  32  includes a stepping motor M shown in FIG. 3 and a reduction gear train (not shown) that transfers rotational force of the stepping motor M to the pointer shaft  31  with a reduced speed. The pointer shaft  31  extends from the driving unit  30  to the front surface of the meter panel  10  through a hole  12  formed in the meter panel  10 . 
     As shown in FIG. 3, the stepping motor M is composed of a stator  30   a  and a magnet rotor  30   b . The stator  30   a  is composed of a yoke  33  and a pair of filed coils  34   a ,  34   b  wound around poles  33   a ,  33   b  of the yoke  33 , respectively. The magnet rotor  30   b  is rotatably supported within the yoke  33  and is magnetized to form plural N and S poles alternately on its outer periphery. The outer periphery of the magnet rotor  30   b  faces the tips of poles  33   a ,  33   b  with a certain air gap. 
     FIG. 6 shows driving pulse voltages supplied to the field coils  34   a ,  34   b  in the normal operating mode. A-phase driving voltage is supplied to the field coil  34   a , and B-phase driving voltage is supplied to the field coil  34   b . Magnetic flux of a cosine waveform, the phase of which is different from each other by 90 degrees, is generated in each field coil  34   a ,  34   b , and flows through the yoke  33  and poles of the magnet rotor  30   b . Thus, the magnet rotor  30   b  is rotated. The pulse width t 1  shown in FIG. 6 corresponds to a renewal time of the speed indication, which is set in a range from 0.5 to 1.5 milliseconds. The field coils  34   a ,  34   b  are referred to as A-phase field coil and B-phase field coil, respectively. 
     Referring to FIG. 4, a circuit for driving the stepping motor M will be described. A microcomputer  50  is connected to a battery Ba through an ignition switch IG. A vehicle speed sensor S supplies signals indicating vehicle speeds to the microcomputer  50 . The microcomputer  50  controls the stepping motor M according to a computer program stored therein. The control process is shown in FIG. 5 in detail. A driving circuit  60   a  connected to the field coil  34   a  supplies the A-phase driving voltage thereto under the microcomputer control. A driving circuit  60   b  connected to the field coil  34   b  supplies the B-phase driving voltage thereto under the microcomputer control. 
     The driving voltage is supplied from the driving circuit  60   a  to the filed coil  34   a  through a pair of switches  70 ,  80 . Both switches  70 ,  80  are analog switches which are simultaneously controlled by the microcomputer  50 . The switch  70  includes a fixed contact  71  connected to an output terminal  61  of the driving circuit  60   a , an open contact  72  and a movable contact  73  connected to one end of the field coil  34   a . The switch  80  includes a fixed contact  81  connected to an output terminal  81  of the driving circuit  60   a , a fixed contact  82  connected to an input port  51  of the microcomputer  50  and a movable contact  83  connected to the other end of the field coil  34   a . Both switches  70 ,  80  are simultaneously brought into a first position (contacts  71  and  73  connected; contacts  81  and  83  connected) and simultaneously brought into a second position (contacts  72  and  73  connected; contacts  82  and  83  connected). The A-phase driving voltage is supplied to the field coil  34   a  at the first position, while the voltage supply is discontinued at the second position. At the second position, a terminal voltage of the field coil  34   a  is supplied to the microcomputer  50  through the input port  51 . 
     Referring to the flowchart shown in FIG. 5, a process for controlling the stepping motor will be explained. Upon closing the ignition switch IG, the microcomputer starts its operation and the vehicle starts running. At step S 100 , both switches  70 ,  80  are brought into the first position thereby to connect the driving circuit  60   a  to the field coil  34   a . At step S 110 , the driving voltage is supplied to both field coils  34   a ,  35   b . The A-phase driving voltage having a pulse width W 2  (corresponding to the renewal time t 2 ), shown in the upper portion in FIG. 7, is supplied to the A-phase field coil  34   a  from the driving circuit  60   a . The B-phase driving voltage having a waveform shown in the bottom portion in FIG. 7 is supplied to the B-phase field coil  34   b  from the driving circuit  60   b . The renewal time t 2  corresponding to the pulse width W 2  is set to two times of the renewal time t 1  (the renewal time under the normal operation mode). 
     The magnet rotor  30   b  is rotated by the driving voltage supplied to both field coils  34   a ,  34   b . According to rotation of the magnet rotor  30   b , voltages are induced in both field coils  34   a ,  34   b . The induced voltages in both filed coils  34   a ,  34   b  have phases different from each other. The meter pointer  20  is rotated according to rotation of the magnet rotor  30   b , and the rotational angle of the pointer  20  is renewed with the renewal time t 2 . 
     At step S 120 , whether the voltage level of the A-phase driving voltage is zero or not is determined. If the level of the A-phase driving voltage is not zero, the process proceeds to step S 121  where both driving voltages continue to be supplied. The magnet rotor  30   b  is rotated according to the phase difference between the A-phase driving voltage and the B-phase driving voltage. Accordingly, the pointer  20  continues to be rotated. On the other hand, if it is determined that the level of the A-phase driving voltage is zero at step S 120 , the process proceeds to step S 122 . At step S 122 , both switches  70 ,  80  are brought into the second position to discontinue voltage supply to the field coil  34   a . At the same time, one end of the field coil  34   a  is connected to the input port  51  of the microcomputer  50  through the movable contact  83  and the fixed contact  82 . At step S 123 , the voltage Vi induced in the A-phase field coil  34   a  is fed to the microcomputer  50 . 
     At step S 130 , it is determined whether the induced voltage Vi is lower than a predetermined threshold voltage Vth. In this embodiment, the threshold voltage Vth is set to 0.5 volts, considering the fact that the renewal time t 2  under the zero-point adjustment mode is set to two times of the renewal time t 1  under the normal operation mode. If it is determined that the induced voltage Vi is not lower than the threshold voltage Vth at step S 130 , the process at steps S 123  and S 130  is repeated. If it is determined that the induced voltage Vi is lower than the threshold voltage Vth at step S 130 , the process proceeds to step S 131 , where it is determined that the meter pointer  20  has engaged with the stopper  13 . Then, at step S 132 , the driving voltage (having the pluse width W 2 ) supply to both field coils  34   a ,  34   b  is discontinued. 
     Since the renewal time t 2  under the pointer zero-position adjustment mode is set longer than the normal renewal time t 1 , the induced voltage Vi is sufficiently high compared with an induced voltage V 1  under the normal operation, as shown in FIGS. 8 and 9. Therefore, the pointer engagement with the stopper is surely and accurately found out, and the pointer zero-position adjustment can be made without fail. Further, since the driving voltage supply to the stepping motor is discontinued at the substantially same time as the pointer  20  abuts the stopper  13 , the pointer vibration otherwise occurring at that time is avoided. 
     Then, the process proceeds to step S 150 , where whether the ignition switch IG is ON or OFF is checked. If the ignition switch IG is ON, the process proceeds to step S 140  where both switches  70 ,  80  are brought to the first position. At step S 142 , the driving voltages having the pulse width W 1  (corresponding to the normal renewal time t 1 ) shown in FIG. 6 are supplied to both field coils  34   a ,  34   b . The A-phase driving voltage is supplied to the field coil  34   a  and the B-phase driving voltage is supplied to the field coil  34   b . The stepping motor M is rotated by the driving voltages, and the pointer rotational angle is renewed with the renewal time t 1 . Since the renewal time t 1  is set to a sufficiently low level, the pointer quickly responds to vehicle speed changes. If it is determined that the ignition switch IG is OFF at step S 150 , the process comes to the end. 
     The present invention is not limited to the embodiment described above, but it may be variously modified. For example, the stopper  13  may not stick out from the front surface of the scale plate  10 a. The pointer  20  may carry a member that abuts a stationary member when it comes to the zero-position. The present invention may be applied to other meters than the speedometer, such as engine rotation meters or fuel gauges. Those meters may be used for other purposes than automotive use. 
     While the present invention has been shown and described with reference to the foregoing preferred embodiment, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.