1. Field of the Invention
The present invention relates to a motor control circuit and a motor drive system using the same and, more particularly, relates to a motor control circuit which smoothly controls rotation of a motor incorporated in electronic devices such as a floppy disk drive (FDD) and a hard disk drive (HDD), imaging devices such as a video camera and a video recording and reproducing device, and many kinds of office automation devices, and further, is suitable for making a portion a motor drive unit thin and small sized.
2. Description of Related Art
When roughly classified, methods of driving a motor and controlling the rotation thereof include a so-called open control method which controls motor rotation independently of actual motor rotating conditions, and a so-called feedback control method which detects motor rotating condition and controls the motor rotation based upon the detected signal.
Corresponding to methods of detecting the rotating condition, the feedback control methods include method of performing a control by detecting rotating condition of a rotor by means of a detection sensor such as a Hall element, and a method of performing a control by detecting a state of drive current and drive voltage, for example, constituting an input to the motor without using a sensor such as a Hall element.
The latter feedback motor control method is called a Hall-less motor method, because the method cases such sensors as a Hall element which are adapted to directly detect motor rotating condition. This Hall-less motor method is adapted to detect a state of, for example, drive current and drive voltage, which reflect a rotating condition of a motor, for example, an influence due to a counter-electromotive force from a coil of the motor, which is indirectly detected from the drive voltage. A detection signal representing the motor rotating condition is obtained therefrom.
FIG. 6 (a) shows a block diagram of a motor control circuit according to a conventional Hall-less motor method.
Herein, numerals 11 , 12 and 13 are detection circuits which generate pulses having a pulse: width dependent upon a rotating condition, numeral 2 drive signal producing circuit which produces signal, numeral s 31 , 32 and 33 are drivers and numeral 4 is a three phase motor.
The drive signal producing circuit 20, which is usually composed of a logic circuit and, depending upon necessity, a phase shift circuit, produces pulse-like (including pulse-like waveforms in both positive and negative directions) driving signals Ud, Vd and Wd response to the detection signals Up, Vp, Wp, having different respective phases, which serve as models of output waveform to the motor 40.
The drive signals Ud, Vd and Wd are respectively applied to the drivers 31, 32 and 33 and are amplified an amplifier in the respective drivers to produce output currents Iu, Iv and Iw having waveforms according those o f the drive signals Ud, Vd and Wd . These currents are fed to respective coils in a star connection to drive and rotate the motor 40. A waveform of the output current Iu is illustrated in FIG. 6 (b) as a representative of output currents Iu, Iv and Iw. Phases of the output currents Iu, Iv and Iw deviate successively by 120.degree. from each other. However, the actual current flowing through the motor coil varies in relation to a load applied thereto.
At the output sides of the drivers 31, 32 and 33 output voltages Vu, Vv and Vw are generated depending upon the output currents Iu, Iv and Iw. These output voltages vary dependent upon the variation of a counter-electromotive force generated in the coils in response to a motor rotating condition. Accordingly, these voltages reflect a rotating condition of the motor 40.
Therefore, using comparators for the detection circuits 11 , 12 and 13, the output voltages from the respective drivers 31 , 32 and 33 are compared with a neutral point voltage at the star connected coils . Thereby, a rotating condition of the motor 40 can be detected in a form of a pulse width. The detection results are respectively outputted from the detection circuits 11 , 12 and 13 as the detection pulses Up, Vp and Wp of which leading and trailing edges are determined in response to the rotating condition. In case of three phase motor control, the phases of the detection pulses usually deviate by 120.degree. , from each other and the duty ratio of the respective pulses is about 50% .
Based upon these detection pulses Up, Vp and Wp the above drive signals Ud, Vd and Wd, which reflect a rotating condition of the motor 40, are produced through logical processing, for example, by the drive signal producing circuit 20 .
Thereby, control loops starting from the outputs of the drivers and returning to the inputs of the drivers via the detection circuits are formed. With the function of these feedback loops a control in response to a rotating condition of the motor 40 is carried out and the rotating speed thereof is maintained at a predetermined speed.
In this kind of conventional motor control circuit the detection signal is obtained in a form of a pulse and logical processing is applied to the detection pulse to produce a pulse-like drive signal .
However, since the waveform of the drive signal which serves as a model waveform of a drive current or drive voltage for the motor is one as illustrated in FIG. 6 (b) , the waveform of a drive current or drive voltage for the motor is also a pulse shape. Therefore, the motor drive torque is also generated in a pulsing manner, and a sufficiently smooth rotating condition of the motor cannot be obtained.
On the other hand, triggered by an improvement such as in recording density on a recording medium to and from which information is written and read in the field of HDD and FDD, the motor which controls the rotation of the medium is required to be controlled to rotate smoothly in accordance with the improvement in recording density The same tendency exists in such fields as video devices, wherein a recording density on a magnetic tape has been also improved such that it is desirable to rotate a motor as smoothly as possible.