Abstract:
The frequency of the pulse width modulator within a step motor control circuit is increased above a base frequency under defined conditions to enable more accurate construction of the phase current waveform for preventing mid-range resonance. The PWM frequency is stepped between frequencies by a fixed amount above the base frequency, to prevent the excitation of system harmonics and prevent step motor operational instability.

Description:
Continuation in Part of U.S. patent application Ser. No. 10/689,828 filed Oct. 22, 2003. 

   BACKGROUND OF THE INVENTION 
   Step motor systems sometimes experience an operational instability known as “mid-frequency” or “mid-range” resonance. 
   This instability which often causes loss of motor torque and leads to motor stall, is caused by an interaction between the step motor drive, power supply and step motor load. When observing the step motor phase current, the shape and magnitude thereof are unstable. 
   Previous methods to prevent mid-range resonance include modifications to the power supply, connection of choke coils to the step motor and circuits designed to produce signals indicative of error. The drawbacks to these methods lie in the extra complexity involved. These methods also may need to be tuned to the specific system of step motor, step motor drive, power supply and load 
   U.S. Pat. No. 5,264,770 entitled “Stepper Motor Driver Circuit”; “U.S. Pat. No. 4,675,590 entitled “Stepping Motor Driver with Mid-frequency Stability Control” and U.S. Pat. No. 4,319,175 entitled “Stabilized Stepping-motor System” each describe early circuits relating to step motor controllers. 
   One purpose of the present invention is to reduce midrange resonance in a multiphase step motor for improved step motor performance. 
   SUMMARY OF THE INVENTION 
   The frequency of the pulse width modulator, “PWM”, within a step motor control circuit is increased above a base frequency under defined conditions to enable more accurate construction of the phase current waveform for preventing mid-range resonance. The PWM frequency is stepped between frequencies by a fixed amount above the base frequency, to prevent the excitation of system harmonics. 
   The PWM is synchronized to the incoming step input once per cycle to prevent the motor step clock from ‘beating’ against the PWM frequency. Improved current control leads to less phase current lag resulting in greater stability. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of the step motor control circuit in accordance with the invention; 
       FIG. 2  is a diagrammatic representation of the signal waveforms within the circuit of  FIG. 1  showing the PWM_OSC frequency change; 
       FIG. 3  is a diagrammatic representation of the signal waveforms showing the synchronization of the PWM oscillator within the control circuit of  FIG. 1 ; and 
       FIG. 4  is a flow chart diagram depicting the logic for changing the PWM-OSC frequency in accordance with the teachings of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The SET POINT GENERATOR  11 , within the step motor control circuit  10  of  FIG. 1 , creates a FRONT_SLOPE current signal  24  ( FIG. 2 ) on conductor  12  and a SIGN current signal  29  ( FIG. 2 ) on conductor  13  in response to the STEP signal input on conductor  14 . The FRONT_SLOPE signal on conductor  12  occurs when the SIGN signal is present on conductor  13  and the STEP signal on conductor  14  causes the step motor phase current to increase for one quarter of a cycle. 
   These signals connect to and influence the operation of a PWM OSCILLATOR, “PWM_OSC”  15  that creates the PWM_OSC signal  30  ( FIG. 2 ) on conductor  16 . These signals, along with others (not shown), are inputted to the BRIDGE CONTROL LOGIC, “LOGIC”  17 . The PWM_OSC signal on conductor  16  along with others (not shown) direct the operation of the LOGIC,  17 . The LOGIC  17 , through conductors  18 – 21  operate the H BRIDGE  22  that controls the flow of step motor phase current through MOTOR PHASE COIL, “COIL”  23 . Although one COIL  23  is shown, the other two COILS (not shown) are connected in a similar manner. 
   Referring now to  FIGS. 1–3 , the PWM_OSC  15  creates the PWM_OSC signal  27  at frequencies that range between a low or base frequency up to a specified maximum frequency. The PWM_OSC  15  starts at the base frequency and increases the frequency, if required, to maintain a specified minimum number of edges  27  of the PWM_OSC signal  26  during the FRONT_SLOPE period until the specified maximum frequency is reached. Having a minimum number of edges enables more accurate construction of the phase current for preventing mid-range resonance. 
   The PWM-OSC  15  counts the number of PWM_OSC edges  27  during the occasion of the FRONT_SLOPE signal  24 . If there are fewer than the minimum number of edges specified and the maximum frequency has not been reached, the PWM_OSC frequency is increased by a fixed amount when the FRONT_SLOPE signal ends as indicated at  25 . 
   If the number of edges  27  of the PWM_OSC signal  26  is greater than or equal to the number specified during the occasion of the FRONT_SLOPE signal  24 , the PWM_OSC frequency is decreased by a fixed amount when the FRONT_SLOPE signal ends as indicated at  25 . 
   When the FRONT_SLOPE signal  24  is above the base frequency and is stable or changing slowly, the PWM frequency will repetitively step between two frequencies. This step between frequencies occurs when the frequency, in one cycle, is increased creating more PWM_OSC edges  27 . In the next cycle, (not shown) the number of edges  27  of the PWM_OSC signal  26  will be greater than or equal to the minimum number of edges specified, therefore causing the frequency to decrease. The frequency is increased within the each of the following cycles. The stepping between frequencies prevents the excitation of system harmonics, thereby preventing mid-range resonance. 
   In the case where the signal period of the FRONT_SLOPE signal  24  is decreasing rapidly, the PWM-OSC  15  will increase the frequency by a fixed amount each cycle until the maximum frequency is reached or the period of the FRONT_SLOPE signal  24  becomes stable, whichever occurs first. 
   In the case where the period of the FRONT_SLOPE signal  24  is increasing rapidly and the frequency of the PWM_OSC signal  26  is above the base, the PWM-OSC  15  will decrease the frequency by a fixed amount each cycle until the base frequency is reached or the period of the FRONT_SLOPE signal becomes stable, whichever occurs first. 
   If at the end of a cycle, the frequency of the PWM_OSC signal  26  is increased to the maximum, the frequency of the PWM_OSC signal will be decreased at the end of the next cycle, even though there may be fewer than the number of specified edges  27  of the PWM_OSC signal  26 . On the following cycles, the PWM_OSC frequency is increased back to the maximum frequency. This continues the beneficial stepping between PWM_OSC frequencies even at the maximum frequency limit. 
   It is to be noted that the counting of PWM_OSC edges  27  could be performed during any portion of the motor operating cycle. 
   As shown in  FIG. 3 , the phase current  28  transitions through zero, as indicated in phantom, twice per cycle, although only one cycle is shown in  FIG. 3 . At one of the transitions, indicated by the SIGN signal  29  on conductor  13 , the PWM oscillator  15  resets its frequency generator as indicated  30  thereby synchronizing the PWM oscillator to the phase current. The phase current changes in response to a step input, such that the PWM oscillator is synchronized once per sine cycle to the incoming step input. This synchronization prevents the frequency of the step input from “beating” against the PWM oscillator frequency, preventing the potential of midrange resonance. It is to be noted the synchronization could occur at any point within the sine cycle. 
   A flow chart diagram  31  is depicted in  FIG. 4  for controlling the LOGIC  17  of  FIG. 1 . A count is made of the number of PWM-OSC edges during FRONT-SLOPE ( 32 ) and a determination is made as to whether the number of PWM-OSC edges is less than a predetermined minimum ( 33 ). If the number of PWM-OSC edges is less than a predetermined minimum, a determination is made as to whether the PWM-OSC frequency is at a predetermined maximum ( 34 ). If the PWM-OSC is not at a predetermined maximum, the PWM-OSC frequency is increased ( 36 ) and the number of PWM-OSC edges during FRONT-SLOPE is re-counted ( 32 ). If the PWM-OSC frequency is at a predetermined maximum, the PWM-OSC frequency is decreased ( 37 ) and the number of PWM-OSC edges during FRONT-SLOPE is re-counted ( 32 ). 
   If the number of PWM-OSC edges is not less than a predetermined minimum, a determination is made as to whether the PWM-OSC frequency is above a base value ( 35 ) and if not, the number of PWM-OSC edges during FRONT-SLOPE is re-counted ( 32 ). If the PWM-OSC frequency is above a base value, the PWM-OSC frequency is decreased ( 37 ) and the number of PWM-OSC edges during FRONT-SLOPE is re-counted ( 37 ). 
   It has herein been shown that careful control of the PWM-OSC frequency to construct the phase current waveform in a step motor prevents mid-range resonance and eliminates motor stall.