Abstract:
A method for controlling the speed of a motor driven by an unregulated voltage supply is disclosed. The disclosed method comprises obtaining a ripple signal (V R ) representative of the ripple component of the supply voltage, and modulating the motor current (I M ) in response to at least the ripple signal to maintain the speed of the motor at a prescribed target speed.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to methods and apparatus for controlling the speed of a motor. More particularly, the invention relates to an inexpensive system that drives a DC capstan motor with an unregulated voltage supply and controls the motor speed by employing a simple yet elegant supply ripple feedback into the servo control loop without additional microcontroller intervention or sophistication. 
     BACKGROUND OF THE INVENTION 
     Prior art systems for driving a DC motor of the type employed in a computer tape or disk drive typically employ a relatively expensive regulated DC voltage supply. For example, in the system depicted in FIG. 1, a voltage regulator 10 regulates the unregulated DC supply voltage V IN  and provides to a motor 12 a regulated supply of DC voltage V REG  and a motor driving current I M , wherein the latter is a function of the voltage across the motor input and output supply terminals and the motor impedance. The motor impedance is a function of the resistance of the motor windings and the speed of the motor. 
     In the system depicted in FIG. 1, a single Hall effect sensor 14 provides position feedback signals to a microcontroller circuit 16 for use in determining the motor speed. The microcontroller 16 provides to a pulse width modulator (PWM) circuit 18 a signal indicative of a target voltage to be applied to the motor. The microcontroller 16 controls the voltage across the motor, and thus the motor current I M , by generating a signal (called TARGET in the drawings) that is pulse width modulated by PWM circuit 28 and employed to control the actuation of a switch unit 20. A current sensing device 22 provides a signal V(I M ) indicative of the motor drive current I M , and this signal is fed back to the PWM circuit 16 and employed by the microcontroller 16 and PWM circuit 18 to limit the motor current to a predetermined maximum level. 
     The PWM signal controlling the switching device and thus modulating the motor current I M  is generated by the PWM circuit 18 under the assumption that the motor supply voltage is well regulated and that a given duty cycle of the PWM signal will result in a certain motor current (note that motor speed increases as motor current increases, and the number of Hall transition signals per unit time increases as the motor speed increases). The microcontroller 16 and PWM circuit 18 deal with any low frequency variation of the load, such as, e.g., a variation due to the winding of a cartridge tape (e.g., a DC2000 tape cartridge), which might otherwise cause the motor speed to vary unacceptably, by periodically monitoring the motor speed and adjusting the PWM signal to increase or decrease the current I M  as needed. In the exemplary system of FIG. 1, the microcontroller 16 checks the motor speed approximately every 10 ms, or at a rate of about 100 Hz. 
     In the highly competitive computer peripherals industry, it is extremely important to produce products at a low cost. Therefore, it would be advantageous to reduce the cost of a motor control system for use with a tape or disk drive by eliminating the need for a regulated DC voltage supply. However, it is important that the recording frequency (and thus the motor speed) of tape/disk drives be well controlled. If one were to simply eliminate the voltage regulator from the system of FIG. 1, a ripple in the supply voltage, typically having a frequency of about 100-120 Hz and an amplitude of several volts (peak-peak), would cause an unacceptable motor speed variation. This variation could be avoided by increasing the speed and complexity of the controller, particularly by increasing the speed of the main servo loop to about twice the expected bandwidth of the ripple component of the supply voltage, but this would increase the cost of the controller and thus nothing would be gained from eliminating the voltage regulator. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a low cost motor control circuit particularly suited for use in connection with a computer tape or disk drive. A more specific object of the invention is to provide a mechanism for obviating the need for a high cost/high performance controller in a motor control circuit that employs an unregulated power supply. The present invention achieves these goals by employing the ripple component of the unregulated supply voltage in a nested loop within the main servo control loop to make high speed adjustments to the signal modulating the motor current I M . 
     In a presently preferred embodiment of the invention, the inventive method comprises the steps of obtaining a ripple signal (V R ) representative of the ripple component of the supply voltage, and modulating the motor current (I M ) in response to at least the ripple signal to maintain the speed of the motor at a prescribed target speed. The presently preferred embodiment described herein further comprises the step of providing a first modulating signal (PWM 1 ) for modulating the motor current to maintain the target speed. The step of modulating the motor current comprises employing PWM 1  to obtain a second modulating signal (PWM 2 ) for use in modulating the motor current. In this embodiment, the ripple voltage has a predetermined, or predeterminable, bandwidth (i.e., the bandwidth can be measured or calculated), and the second modulating signal PWM 2  is adjusted at a frequency greater than the expected maximum frequency of the ripple voltage. 
     The first and second modulating signals are preferably pulse width modulated digital signals. The step of modulating the motor current preferably comprises converting the first modulating signal PWM 1  into a reference voltage and comparing the reference voltage to a sum of the ripple voltage and a voltage (V(I M )) representative of the motor current. In the preferred embodiment, PWM 2  is a binary signal which assumes a first level (e.g., high) when the reference voltage is greater than the sum and a second level (e.g., low) when the reference voltage is less than the sum. 
     According to another aspect of the present invention, a method is provided for controlling the speed of a motor of a tape or disk drive, where the motor is driven by an unregulated voltage supply. The method comprises the steps of determining a target motor speed and modulating the motor current to maintain the target speed, wherein, in a main servo loop performed at a first frequency, the motor speed is checked and the motor current is adjusted if necessary. In addition, the motor current and ripple voltage are measured and the modulation of the motor current is adjusted to maintain the target motor speed with greater accuracy. According to the invention, the latter steps are performed in a nested loop at a second frequency greater than the frequency of the main servo loop. The second frequency may be specified to be greater than approximately 200 Hz, at least 10 times larger than the first frequency, or at least twice the expected bandwidth of the ripple voltage. 
     Another important feature of the invention is that the cost of performing the nested loop is less than the cost of increasing the speed of the main servo loop to account for the ripple voltage and maintain the target speed. 
     Other features of the invention are disclosed below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically depicts a prior art motor control system. 
     FIG. 2 schematically depicts one presently preferred embodiment of a motor control system in accordance with the present invention. 
     FIG. 3 depicts details of the summing circuit 26 employed by the system of FIG. 2. 
     FIG. 4 is a flowchart of a method for controlling the speed of a motor in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 2 and 3 schematically depict one presently preferred embodiment of the invention. As shown, this embodiment omits the voltage regulator 10 of the system of FIG. 1, but it does include the motor 12, microcontroller 16, PWM circuit 18, switching device 20, and current sensing unit 22. In addition, the inventive system depicted in FIG. 2 includes a low pass filter (LPF) and a summing circuit 26 arranged as shown. The capacitive coupler 24 provides an AC ripple voltage V R  (i.e., the AC component of the unregulated input voltage V IN ), which is employed by the summing circuit 26, in combination with the voltage V(I M ), to generate a second modulating signal PWM 2 . The second modulating signal PWM 2  is employed to modulate the motor current I M  by actuating the switching unit 20. 
     Referring to FIG. 3, a presently preferred embodiment of the invention employs a summing circuit 26 comprising a low pass filter 30 and a comparator 32. The low pass filter 30 converts the first modulating signal PWM 1  to a reference voltage, and this reference voltage is input to a first input terminal of the comparator 32. A feedback signal is also provided, via a feedback path 33, to the first input terminal of the comparator 32. The sum of the ripple voltage V R  and the voltage signal V(I M ) is input to the other input terminal of the comparator 32. The output of the comparator is a binary, pulse width modulated signal PWM 2 , which is used as described above to modulate the motor current. The means for forming the sum of V R  and V(I M ) may include, for example, a pair of suitable resistors connected to form the necessary sum. 
     FIG. 4 is a flow chart illustrating another aspect of the invention. As shown, a first step S1 of the present invention is to provide an unregulated DC voltage to the motor. 
     The next step S2 is to provide a target motor speed. A main servo loop comprising steps S3 through S10 is then performed. 
     Step S3 determines a duty cycle for the modulating signal PWM 1 . A nested loop comprising steps S4 through S8 is thin performed. 
     Step S4 measures the motor current I M  and step S5 determines the ripple component of the supply voltage. 
     In step S6, the first modulating signal PWM 1  is employed along with the motor current I M  and ripple voltage V R  to generate the second modulating signal PWM 2 . 
     In step S7, the motor current is modulated with the second modulation signal PWM 2 . 
     In step S8, the system checks the Hall count (i.e., the accumulated Hall transitions) and at the same time loops back to step S4 as shown, to continue the nested loop, while in step S9 the motor speed is determined on the basis of the Hall count. 
     In step S10, the measured speed is compared with the target speed to determine whether the measured speed is within predefined limits. The method then loops back to step S3 to recompute the duty cycle and adjust the motor speed, if necessary. 
     Those skilled in the art will readily appreciate that the preferred embodiments disclosed herein may be modified in many ways, and that such modifications will perform the same functions and achieve the same results as described herein. For example, it is not absolutely necessary, although it is preferred, that the first and/or second modulating signals be PWM signals, or even that they be digital signals. It is also not necessary that the motor current be modulated by a switching device as disclosed in FIGS. 1 and 2. Other modifications will become apparent in view of this disclosure and the following claims.