Abstract:
A driver circuit is provided for a stepper motor which utilizes a processor that outputs a PWM signal. The driver circuit includes an H-bridge circuit having first and second inputs, and a switching circuit having an input and an output. The first H-bridge input and the switching input connect to the PWM signal output from the processor, and the switching output connects to the second H-bridge input. The switching circuit performs an inversion of a duty cycle of the PWM signal when the PWM signal is present, and a non-inversion of the duty cycle when the PWM signal is not present.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to stepper motors systems in general, and more specifically to an improved circuit for driving a stepper motor and a method of controlling the circuit and driver. 
   Stepper motor systems and drivers are known in the art. Conventional stepper motor systems include a stepper motor and a driver for the motor, the motor driver typically including an H-bridge circuit.  FIG. 1  illustrates a conventional circuit  10  for driving a stepper motor. 
   An analog voltage signal is fed into a voltage-to-duty-cycle converter  12 , which is clocked by an oscillator  14 . An output of the converter  12  is then fed into a first input  18  of a first AND gate  20 , and into a first input  22  of a second AND gate  24 . A phase signal is fed into second respective inputs  26 ,  28  of the first and second AND gates  20 ,  24 . The phase signal into the second input  26  is first passed through an inverter  30 . An output of the first AND gate  20  is then fed into an input  32  of an H-bridge first half  34 , and an output of the second AND gate  24  is fed into an input  36  of an H-bridge second half  38 . Respective outputs of the H-bridge halves  34 ,  38  drive motor windings  40 . 
   The conventional circuit  10  functions to change a current in the motor windings  40  by actively selecting successive coil currents, one after the other, in a sinusoidal manner, to drive the motor by a series of discrete microsteps. At any given moment, a selected coil current is actively controlled via a separate integrated circuit (not shown). The integrated circuit dynamically achieves a desired coil current by using a closed-loop control and a feedback of the motor coil current. Conventional stepper motor drive circuits thus are generally closed-loop systems with respect to where they connect to a stepper motor. Such conventional circuits experience several disadvantages in operation. 
   One disadvantage experienced by the conventional circuit  10  is noise. At certain micro-step values, a sampling nature of pulse width modulation (“PWM”) generation gives rise to audible sub-harmonics of the oscillator  14 . Imprecise frequency control of the oscillator causes an undesirable audible frequency jitter, or “fizzing.” Furthermore, long wires in the feedback network connected to the stepper motor windings cause an undesirable electrical ringing noise, which can also occur audibly. 
   Another disadvantage experienced by the circuit  10  occurs when the phase control is switched. Ideally, the phase should be switched when the current through the motor windings  40  is zero. However, at typical operating speeds of the motor, there is always at least a finite amount of current flowing through the windings  40  due to the inductive nature of the windings. Switching the current polarity when the current is not actually zero causes a shaft of the motor (not shown) to rotate in a non-uniform manner. The non-uniform rotation results in an increased vibration in the motor, which further increases the audible noise experienced. 
   Still another disadvantage experienced with the circuit  10  rises out of the fact that H-bridges experience a “dead zone” when the voltage driving the motor windings  40  crosses through zero. The dead zone causes a noticeable and undesirable pause of the motor motion at slow shaft speeds. The pause results in a jerky shaft rotation of the motor, which makes precise positioning of the motor shaft difficult to impossible at a dead zone location. The dead zone crossings also further increase the generated noise experienced by the motor. 
   An example of a method to reduce noise from a stepper motor is described in patent to Peeters, U.S. Pat. No. 5,440,214. Peeters describes a voltage PWM drive which generates a sinusoidal drive signal using PWM timing, approximating a sine wave using discrete levels. Straight line coding of each PWM timing step is implemented in a central processing unit (“CPU”) to generate an approximate PWM pulse corresponding to the desired sine waveform. Peeters uses a single drive speed for quiet operation, and a closed-loop feedback system to dynamically measure and control the stepper motor. 
   Although useful for balancing the duty cycle of the PWM signal, the discrete, single-speed operating method described by Peeters is unable to compensate for different, or variable, operational drive speeds. This method is also unable to compensate for dead zone anomalies within a drive step, and therefore cannot accurately position the motor at lower drive speeds, where the inertia of the system is weaker, and thus unable to mask the dead zone anomalies. 
   A different stepper motor driver circuit is described in a patent to Labriola, U.S. Pat. No. 5,977,737. Labriola employs an H-bridge driving circuit, and controls the circuit using a closed-loop feedback system which utilizes a predicted motor current value. The predicted motor current is calculated on the basis of empirically-derived characteristics of the motor itself, and measured values for the motor angular velocity and physical angle. Similar to Peeters though, Labriola also requires dynamic measurements of some variables with a closed-loop feedback system that can produce undesirable noise. 
   Accordingly, it is desirable to construct a driver circuit for a stepper motor system which reduces the operational noise of the motor, while also avoiding the problems associated with closed-loop feedback, dead zone anomalies, and switching the current-limited waveform. The desired circuit should be operational for a continuous range of rotational speeds of the motor shaft. 
   SUMMARY OF THE INVENTION 
   The above-listed objects are met or exceeded by the present driver circuit for a stepper motor system. Major advantages of this circuit include an open-loop system, and a switching circuit which can provide both PWM phase inversion and PWM loss protection through the hardware of the circuit itself. The driving method for the motor additionally corrects for significant anomalies experienced by conventional motor systems. 
   More specifically, the present invention provides a driver circuit for a stepper motor which utilizes a processor that outputs a PWM signal. The driver circuit includes an H-bridge circuit having first and second inputs, and a switching circuit having an input and an output. The first H-bridge input and the switching input connect to the PWM signal output from the processor, and the switching output connects to the second H-bridge input. The switching circuit performs an inversion of a duty cycle of the PWM signal when the PWM signal is present, and a non-inversion of the duty cycle when the PWM signal is not present. 
   In another embodiment, a stepper motor system is provided and includes a stepper motor, a motor driver circuit to drive the stepper motor, a switching circuit, and a processor. The motor driver circuit has first and second driver inputs, and the switching circuit has a switching input and a switching output. The switching input connects to the first driver input, and the switching output connects to the second driver input. The processor outputs a PWM signal to the first driver input and the switching input, and the switching circuit functions as an inverting circuit when the PWM signal is present, and as a buffer circuit for the motor driver circuit when the PWM signal is not present. 
   In a further embodiment, a method for driving a stepper motor having a motor driving circuit is provided. The method predicts a rotational acceleration of a rotatable shaft of the motor, and computes a shaft rotational position based on the predicted shaft acceleration. A present deviation value of the computed shaft position is then predicted based on the computed shaft position. A rotational speed of the motor shaft is also computed based on the predicted shaft acceleration, and the predicted present deviation value is then adjusted based on the computed shaft speed. The adjusted deviation value is corrected for a zero-crossing anomaly, and then output to the motor driving circuit. 
   Under normal motor loads and standard driving speeds, the present invention is particularly effective at eliminating operational noise from the motor, while also allowing the motor to operate smoothly over a continuous range of operational speeds. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a conventional driver circuit; 
       FIG. 2  is a block diagram of a stepper motor system according to an embodiment of the present invention; 
       FIG. 3  is a schematic diagram of the switching circuit illustrated in  FIG. 2 ; and 
       FIG. 4  is a flowchart diagram of a method of calculating a PWM value according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIGS. 2 and 3 , a stepper motor system of the type suitable for use in the present invention is generally designated  50 , and includes a stepper motor  52  and a motor driver  54  for driving the stepper motor. The stepper motor  52  is preferably a two-phase motor, and the motor driver  54  is preferably an H-bridge circuit, as are known in the art. The motor driver  54  includes separate first and second drivers  56  and  58 , respectively, for driving both respective ends of a motor winding  60  of the two-phase motor  52 . A PWM voltage control signal is applied directly from a drive control CPU  62  to an input  64  to the first driver  56  of the motor driver  54 , as best seen in FIG.  3 . 
   The same PWM signal is also applied directly from the CPU  62  to an input  66  of a switching circuit  68 . The switching circuit  68  preferably includes an exclusive OR gate  70  and a charge pump  72  as its main components. A first input  74  of the exclusive OR gate  70  connects directly to the switching circuit input  66 , and thus the applied PWM signal as well. The first input  74  of the exclusive OR gate  70  connects to a second input  76  of the exclusive OR gate through a series connection of an input capacitor  78  and a first steering diode  80 , which are both portions of the charge pump  72 . The remainder of the charge pump  72  preferably includes a second steering diode  82  connected between ground and the connection of the first input capacitor  78  to the first steering diode  80 , and an output capacitor  84  and a bleeder resistor  86  both connected between the second exclusive OR gate input  76  and ground in parallel. An output  88  of the exclusive OR gate then preferably connects directly to an input  90  of the second driver  58 . 
   Drive control software to operate the motor driver  54  and the switching circuit  68  is executed by the CPU  62  from a program memory  92 . A sine waveform for the motor driver  54  is stored in a sine table  94 , which may be a part of the program memory  92 , or a separate memory component accessible by the CPU  62  (FIG.  2 ). The drive control software and the operation of the CPU  62  and memory components are further discussed below with respect to FIG.  4 . 
   The switching circuit  68  advantageously can perform several functions for the motor driver  54 . According to this preferred circuit configuration, the switching circuit  68  functions as a PWM phase inversion circuit for each of the first and second drivers  56 ,  58 , driving either end of the motor winding  60  out of phase with its other end when the PWM signal is present. A PWM signal is considered to be present when it provides a switching at a periodic rate, along with an associated duty cycle. The switching circuit  68  also functions as a PWM loss protection circuit, which will disable the motor driver  54  when no PWM signal is present. By disabling the motor driver  54 , large, excessive currents can be prevented from flowing through the motor driver and the stepper motor  52 . 
   Instead of switching the direction of the H-bridge (that is, the direction a current would flow in a motor winding connected to the H-bridge) every sine wave half cycle, as conventionally done, the present invention operates to switch the direction of the H-bridge at a much higher frequency, preferably 19.5 kHz. A frequency of 19.5 kHz is low enough to allow for a maximum number of steps to approximate a pure sine waveform, but still high enough to be beyond the audible range of human hearing. The duty cycle of the 19.5 kHz switching waveform is thus used to control both the direction and the magnitude of the current in the motor windings. A 50 percent duty cycle of the 19.5 kHz direction control waveform then preferably corresponds to a coil current of zero. Similarly, a zero percent duty cycle would then correspond to a maximum winding current in one direction, and a 100 percent duty cycle would then correspond to a maximum winding current in the other direction. 
   The present invention produces a sinusoidal current in the stepper motor windings  60  by varying the duty cycle of the 19.5 kHz H-bridge switching waveform in a sinusoidal manner, where a 50 percent duty cycle also represents zero voltage across the windings. The amount of duty cycle sinusoidal deviation (“swing”) from 50 percent then corresponds to an amplitude of the sine waveform. The frequency of the sinusoidal deviation in duty cycle, on the other hand, then corresponds to the rotational speed of the stepper motor drive shaft, where one sine wave cycle corresponds to one step. The higher the frequency of the sinusoidal variation of the H-bridge control waveform, the higher then will be the speed of the motor shaft. This configuration allows a direct voltage drive of the motor driver  54  and the switching circuit  68 , which in turn allows the present invention to effectively function for any speed of the motor. 
   Direct, bipolar voltage drive is accomplished by driving each end of the stepper motor winding  60  out of phase with the other, and applying a 50 percent duty cycle of the PWM signal as a zero voltage drive condition. Greater than a 50 percent duty cycle will cause a positive current flow through the winding  60 , while less than a 50 percent duty cycle will cause a negative current flow. By this configuration, there will be no additional need to switch the phase of the winding current. One side of the winding  60  is driven directly by the PWM signal (through the first driver  56 ), while the other side is driven (through the second driver  58 ) by an inverted PWM signal from the switching circuit  68  when PWM is present. This configuration allows the present invention to accomplish inversion through a simplified hardware circuit, what is typically required to be encoded in CPU software in the conventional method. 
   When direct voltage driving according to the present invention, a voltage is produced across the motor winding  60  when PWM is present, and each end of the winding is at an opposite phase. When PWM is not present (fails), however, the advantageous configuration of the switching circuit  68  also performs a PWM loss protection circuit. 
   PWM may fail when the CPU initializes and the instantaneous current is very high, or simply when the motor driver software stops for any reason. It is particularly important in such cases to protect the stepper motor  52  and the motor driver  54  from resultant high currents. The preferred configuration of the switching circuit  68  and charge pump  72  thus allows the charge pump to cease functioning when PWM is not present. In such a case, the second input  76  of the exclusive OR gate will go to “low,” and the circuit  68  will act as a straight buffer. Accordingly, the voltage (high or low) applied to each end of the winding  60  will be the same, and no current will therefore flow across the winding, thereby preventing excessive currents in particular from harming the motor  52 . 
   Direct voltage drive also allows a PWM master clock (not shown) to be derived from a stable crystal oscillator, which would not generally be usable in such a fashion according to the conventional method. As such, undesirable noise may be further reduced according to the present invention, and particularly as the drive voltage more closely approximates a pure sine wave, discussed below. The open loop configuration of the present invention also means that no feedback is necessary from the motor  52  to the CPU  62 , or from the motor to the motor driver  54 . 
   Direct voltage, open-loop driving also allows the present invention to avoid problems associated with use of closed-loop systems having current feedback. Excessive wiring required for feedback is avoided. Dead zone anomalies are corrected in the CPU  62  by pre-distorting the sinusoidal deviation in duty cycle such that the transition through the 50 percent duty cycle (zero) point occurs more rapidly at low shaft speeds. As the motor shaft speed increases, this effect is decreased and eventually removed as it becomes unnecessary. Such pre-distortion occurs real-time as the motor is running, and as a function of the motor shaft speed. It should be noted that this real-time function does not constitute a closed-loop, or feedback, control because the motor shaft speed is predicted by the stepper motor drive software, and not actually measured from the physical motor. 
   The predicted shaft speed is useful to compensate for other problems with stepper motor drivers identified by the present inventors. One such problem is that the effective sine wave drive amplitude must be increased with the drive frequency to overcome the increased winding impedance caused by inductance properties of the winding  60 . As frequency increases, so does the winding impedance. If the drive amplitude is not increased with frequency, an increase in the shaft speed will result in a decrease in the torque of the motor  52 , which is undesirable. The present invention automatically and advantageously compensates for this torque loss by using the predicted motor shaft speed to automatically adjust the duty cycle swing of the motor driver  54  (H-bridge). 
   Referring now to  FIG. 4 , calculation of the adjustment to the PWM signal applied to the motor driver  54  (and switching circuit  68 ) is shown, as preferably performed by the software coding of the CPU  62 , program memory  92 , and/or sine table  94 . The PWM voltage control signal preferably allows the stepper motor  52  to microstep with 1024 discrete steps per full motor step, to smoothly approximate a pure sine wave. 
   In step S 1 , the rotational acceleration (Alpha) of the motor shaft (not shown) of the motor  52  is predicted. The predicted value for Alpha is determined by an analysis of either the desired shaft angle versus the predictive deviated shaft angle for a position controller, or the desired shaft speed versus the predictive deviated shaft speed for a speed controller. These predictive deviational values are predetermined empirically, according to expected, known characteristics of the motor  52  itself, and are preferably accessed from the program memory  92  or sine table  94  by the CPU  62 . 
   These desired values (position or speed) represent the ideal values the stepper motor system  50  should achieve operationally. The predictive values, on the other hand, represent the expected deviations seen for motor systems of these types. The present inventors though, have discovered that these predictive values adequately and accurately substitute for the actual, dynamically measured values of the rotational position and/or speed under normal driving speeds and driving loads for such stepper motor systems. While the present invention eliminates the need any actual, dynamic measurements during operation of such a device, one actual calibrating measurement is preferably performed at the initialization, or startup, of the device, to physically sense and calibrate the device prior to normal operation. 
   In step S 2 , the shaft rotational angle, or position (Theta), is computed. The shaft positional computation is based on the present rotational speed (Omega), the previous rotational position (Theta  LAST ), and the desired acceleration Alpha. The following formula for the computation corresponds to the basic kinematics equation for computing a new position:
 
Theta=Theta LAST +Omega* t+ ½Alpha* t   2 
 
   In step S 3 , a stepper sine table lookup index is derived from the computed new shaft position Theta. In a preferred embodiment, the internal variable for motor shaft position, Theta, is actually scaled 2 12  (or 4096) times as large. The units for Theta are also preferably in microsteps, rather than degrees. According to this embodiment, Theta can be directly converted into an index from the sine table  94  simply by dividing Theta by 4096 (or, four times the number of microsteps per step). 
   In step S 4 , the present deviation of the shaft position is predicted. A desired motor coil voltage (for either phase of the motor winding  60 ) is accessed from the sine table  94 . This accessed voltage is represented by a number called the Amplitude. The Amplitude is an eight-bit signed (positive or negative) number, and corresponds to the swing of the duty cycle, discussed above. In other words, the Amplitude is determined within the following boundaries:
 
−255&lt;Amplitude&lt;255
 
   The variation of the Amplitude with shaft position Theta thus represents a pure (approximated) sine waveform. There are therefore 1024 entries in the sine table  94  for every full step of the stepper motor  52 . In other words, the stepper motor  52  is microstepped with 1024 microsteps per full step. 
   In, step S 5 , the present rotational shaft speed Omega is calculated according to the predicted shaft acceleration Alpha, and the predicted deviation of the shaft position (PWM value) is adjusted based on the calculated speed Omega. First, a PWM Frequency Adjustment is determined by multiplying the Amplitude, obtained from the sine table  94 , by a calculated Scale Factor. The Scale Factor is based upon the calculated shaft speed Omega, and dynamically changes as a function of the shaft speed according to the following formula:
 
Scale Factor=Base Level+(Slope*Omega)
 
   The Scale Factor compensates for the torque loss, described above, to keep the torque of the motor generally constant as speed varies. The Base Level and the Slope are both empirically determined characteristics of the stepper motor  52  itself. The Base Level determines the motor current at a shaft speed of zero. The Slope is a factor to compensate for the inductive components of the impedance through the winding  60 . This impedance increases as the drive frequency increases, being a linear function of the winding inductance. The effect of the Slope therefore is to keep the coil current relatively constant as the speed varies. The calculated Scale Factor is a thus a linear function of the motor rotational shaft speed, adjusted for impedance induction and scaled for a constant motor torque. 
   It is important to note that the exact value of the maximum coil current is dependant on both a power supply voltage, and the shaft speed Omega. The nature of the electronics for drivers of this type is such that, at a particular power supply voltage, the current in the motor coils typically increases as the shaft speed decreases, due to the inductive component of the motor windings. The software calculations shown here, on the other hand, automatically correct for this current effect “on the fly” by adjusting the Scale Factor as a function of the shaft speed Omega. The Scale Factor starts as the minimum value Base Level, and then increases as a function of the shaft speed. 
   The PWM Frequency Adjustment can then be calculated according to the following formula:
 
 PWM  Frequency Adjustment=Amplitude*Scale Factor
 
   The Adjusted PWM Value is then easily determined from the PWM Frequency Adjustment as follows:
 
Adjusted  PWM  Value= PWM  Midpoint+ PWM  Frequency Adjustment
 
   The PWM Midpoint is the PWM value at the 50 percent duty cycle (zero swing), described above. The PWM Frequency Adjustment is therefore a positive or negative number, representing the amount of swing from the 50 percent duty cycle midpoint. 
   In step S 6 , the Adjusted PWM Value is corrected for zero-crossing anomalies. This correction accounts for the dead zone anomaly, described above, that occurs when the voltage waveform of the stepper motor crosses the zero point, or 50 percent duty cycle. The zero cross correction (Z-Fix) is an intentional distortion factor that is dynamically applied “on the fly” to the Adjusted PWM value before it is output to the motor driver  54  (and switching circuit  68 ). The Z-Fix correction factor compensates for the effects of the noticeable dwell experienced by the motor shaft at slower rotation speeds when crossing the zero point (reversing duty cycle). 
   First, a Zero Scale Factor is determined according to the following formula:
 
Zero Scale Factor=Zero Base Level−(Zero Slope*Omega)
 
   The Zero Base Level and the Zero Slope characteristics are empirically determined for the motor  52  itself, similarly to the Base Level and the Slope in step S 5 . The Zero Scale Factor though, can never be a negative number. The Zero Scale Factor will always equal zero whenever the product of Omega and the Zero Slope equals or exceeds the Zero Base Level Factor. In other words, the Zero Scale Factor disappears at high rotational shaft speeds (large Omega). 
   The actual zero cross correction factor Z-Fix can then be calculated as a function of the Amplitude and the Zero Scale Factor:
 
 Z -Fix=Zero Scale Factor−((Zero Scale Factor/255)*Amplitude)
 
   As shown in the formula, the Zero Scale Factor is first divided by 255 (which is the maximum value of the sine table  94 ) before being multiplied with the Amplitude. The correction factor Z-Fix is therefore a function of both the lookup value from the sine table  94  and the shaft speed Omega but, as noted above, Z-Fix will decrease as the speed increases, and be zero at very high shaft rotational speeds. 
   Z-Fix is then added to the Adjusted PWM Value to get a Corrected PWM Value:
 
Corrected  PWM  Value=Adjusted  PWM  Value+ Z -Fix
 
   At high rotational speeds, therefore, the Corrected PWM Value will simply be equal to the Adjusted PWM Value. The output software here corrects for the slow-speed, zero-cross effect by pre-distorting the waveform near the zero crossing point. Since this correction is only needed at low speeds, the pre-distortion is eventually removed as the motor shaft speed increases. 
   In step S 7 , the Corrected PWM Value is output to the motor drive  54  (and the switching circuit  68 ) as a present PWM signal. 
   As illustrated above, this embodiment of the present invention advantageously corrects for several anomalies typically encountered with motors and drivers of these types. Through software control in the CPU  62 , the present invention is able to effectively compensate for increased winding current by keeping the winding current relatively constant versus the shaft speed. The present invention also effectively compensates for zero crossing dead zone anomalies, which are particularly noticeable at low operating speeds of the motor. The correction features described herein also correct for the noticeable effects that appear when such devices operate at speeds which produce a natural resonance. 
   According to these features of the present invention, a stepper motor can be quietly and smoothly driven over a continuous range of operating speeds. An operator is not limited to single, or discrete, operating speeds or frequencies predetermined by other conventional devices. 
   While particular embodiments of the driving circuits and methods of the present invention have been shown and described herein, it will be appreciated by those skilled in the art that changes and modifications may be made thereto without departing from the invention in its broader aspects and as set forth in the following claims.