Abstract:
In a return-to-zero stepping sequence for a stepper motor that drives the pointer of a gauge, steps that could potentially cause the pointer to flutter are modified. At least the initial potential flutter step of the stepping sequence is divided into sub-steps to progressively attenuate the motor torque, the motor is deactivated during intermediate potential flutter steps, and at least the final potential flutter step of the stepping sequence is divided into sub-steps to progressively re-apply the motor torque.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an instrument cluster gauge having a pointer positioned by a stepper motor, and more particularly to a stepper motor control method for initializing the pointer to a designated zero position. 
       BACKGROUND OF THE INVENTION 
       [0002]    Stepper motors have been used to drive analog gauge pointers, particularly in motor vehicle instrument clusters. Normal movement of the pointer is typically accomplished by micro-stepping the stepper motor, and the controller determines the relative pointer position by maintaining a step count. This eliminates the need for a position sensor, but requires a known initial position of the pointer. Since the pointer can be off-zero at power up, a return-to-zero half-step sequence is utilized at power-up to establish an initial zero position of the pointer. A typical return-to-zero step sequence involves driving the stepper motor through a specified angle of rotation in order to move the pointer against a fixed stop. Unfortunately, this can produce perceptible flutter of the pointer, and even audible noise, because certain steps of the return-to-zero sequence produce off-zero movement of a pointer that has already returned to the zero position. This phenomenon is described in some detail in the U.S. Pat. No. 5,665,897 to Lippmann et al., assigned to the assignee of the present invention, and incorporated herein by reference. 
         [0003]    One way of addressing the pointer flutter issue is to simply deactivate the motor windings during the steps that might produce off-zero pointer movement. While such an approach can be simple to implement, the torque generated by the motor may be insufficient to reliably return the pointer to the rest position under certain conditions, and substantial errors can occur in gauges where the motor lacks a geartrain between its rotor and output shaft. The aforementioned Lippmann et al. patent discloses a reliable but more sophisticated approach involving a factory calibration learning procedure and a wake-up routine executed periodically during ignition off periods. What is needed is an improved return-to-zero control method that is both simple and reliable. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention is directed to an improved return-to-zero control method for a stepper motor driven pointer of a gauge having a fixed stop corresponding to a known energization state of the motor. The control involves activating the stepper motor in accordance with a half-step return-to-zero stepping sequence in which steps that can potentially produce pointer flutter are modified in a way that progressively attenuates and then re-applies the torque produced by the motor. At least the initial potential flutter step of the stepping sequence is divided into sub-steps to progressively attenuate the motor torque, the motor is deactivated during intermediate potential flutter steps, and at least the final potential flutter step of the stepping sequence is divided into sub-steps to progressively re-apply the motor torque. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a diagram of a gauge including a pointer driven by a two-winding stepper motor and a microprocessor-based controller for carrying out a return-to-zero control method according to this invention. 
           [0006]      FIG. 2  is a table depicting a conventional eight half-step return-to-zero stepping sequence for the stepper motor of  FIG. 1 . 
           [0007]      FIG. 3  is a table depicting a modified half-step return-to-zero stepping sequence for the stepper motor of  FIG. 1  according to this invention. 
           [0008]      FIG. 4  is a flow diagram of a routine executed by the microprocessor-based controller of  FIG. 1  for carrying out the return-to-zero method of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0009]    Referring to  FIG. 1 , the reference numeral  10  generally designates an analog gauge assembly such as the speedometer or fuel gauge of a vehicle instrument cluster. The gauge assembly  10  includes a pointer  12  having a hub  12   a  affixed to the armature of a two-winding stepper motor  14 , and a faceplate  16  featuring graphical indicia  18 . Additionally, the gauge assembly  10  includes an internal or external stop  20  that defines a known position of pointer  12 . A microprocessor-based controller  22  coupled to the stepper motor terminals  24  activates the windings of stepper motor  14  for positioning the pointer  12  to indicate a measured quantity such as speed or fuel level. 
         [0010]    In the illustrated embodiment, the stepper motor  14  has a permanent magnet rotor and a stator wound with two coils, designated herein as Coil A and Coil B. An example of a suitable stepper motor is the PM20T stepper motor produced by NMB Technologies Corporation. A useful characteristic of that and other stepper motors is that the winding energization state for holding the pointer  12  at the rest or zero position can be known, whether by manufacturing design or post-manufacture testing. However, the initial position of the pointer  12  cannot be known for certain due to power interruptions and so forth, and the controller  22  will typically execute a return-to-zero stepping sequence at power-up for driving the pointer  12  against the stop  20 . 
         [0011]    The table of  FIG. 2  depicts a conventional half-step return-to-zero stepping sequence for producing counter-clockwise rotation of pointer  12 . Since there are two windings, a complete half-step sequence will entail eight different half steps (energization states), with the eighth step corresponding to the energization state that will hold or maintain the pointer  12  at the rest or zero position in abutment with stop  20 . In the table, the duration column represents the step duration in milliseconds, and the Coil A and Coil B columns show the respective coil states (on/off) and current direction (+/−). The drawback of the depicted step sequence is that certain energization states of the sequence will produce clockwise rotation of the pointer  12  if the pointer  12  is already at the zero or rest position, possibly resulting in perceptible flutter and noise. Specifically, the undesired clockwise rotation can occur at steps  4 ,  5 ,  6  and  7  of the illustrated stepping sequence. For example, if pointer  12  is at the rest position, the fifth half-step of the sequence will produce clockwise torque to move the pointer  12  away from the rest position. For convenience, these potentially flutter causing steps are referred to herein as potential flutter steps. 
         [0012]    The table of  FIG. 3  depicts a modified half-step sequence according to this invention. It differs from the conventional half-step sequence in two ways. First, stepper motor windings are both deactivated during the middle two of the four potential flutter steps (i.e., during steps  5  and  6 ). And second, the first and last of the four potential flutter steps (i.e., steps  4  and  7 ) are divided into sub-steps for reduced torque generation. Referring to  FIG. 3 , step  4  of the conventional step sequence is divided into sub-steps  4   a  and  4   b , and step  7  of the conventional step sequence is divided into sub-steps  7   a  and  7   b . Taken together, sub-steps  4   a  and  4   b  have a duration of 4 ms (i.e., the same as steps  1 ,  2  and  3 ), but both motor windings are deactivated during sub-step  4   b . Similarly, sub-steps  7   a  and  7   b  taken together have a duration of 4 ms, but both motor windings are deactivated during sub-step  7   a . Generally speaking, breaking the first and last of the potential flutter steps into sub-steps of reduced torque generation progressively attenuates and then re-applies the torque produced by motor  14  to minimize the likelihood of perceivable flutter while ensuring that the motor  14  will produce sufficient torque to reliably move the pointer  12  to the rest position. Of course, more than just the first and last potential flutter steps can be sub-divided if desired. Also, the step durations shown in  FIGS. 2 and 3  are only representative, and may vary depending on the pointer size, the motor torque characteristics, and other parameters. 
         [0013]    Referring to  FIG. 4 , the initialization routine  50  is executed by controller  22  when power is initially applied to the gauge assembly  10 , and the pointer position is completely unknown. The blocks  52 ,  54  and  56  are executed in order as shown to output the return-to-zero (RTZ) step sequence of  FIG. 3 , to pause for a prescribed interval to ensure pointer stabilization, and then to again output the return-to-zero step sequence of  FIG. 3 . This sequence can be repeated as necessary depending on the allowable range of pointer movement and the pointer movement that occurs for each activation of the step sequence. This ensures that the pointer  12  will be reliably returned to the rest or zero position even in cases where one complete step sequence is insufficient to ensure its full return. 
         [0014]    In summary, the present invention provides a simple and cost effective control method for initializing a stepper motor driven pointer of a gauge assembly to a zero position. While the method has been described with respect to the illustrated embodiment, it is recognized that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. For example, the return-to-zero step sequence can be configured to produce clockwise pointer rotation instead of counter-clockwise rotation, the method can be applied to micro-stepping as well as half-stepping, and so forth. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims.