Electricity: motive power systems – Motor-reversing – Armature or primary circuit control
Reexamination Certificate
1999-10-20
2001-09-11
Ip, Paul (Department: 2837)
Electricity: motive power systems
Motor-reversing
Armature or primary circuit control
C318S292000, C318S294000, C318S434000
Reexamination Certificate
active
06288507
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a current-controlled motor drive circuit for implementing, in a direct drive system, motor characteristics including high output, high efficiency, and minimized variations in revolution in an extremely low speed revolution range, e.g. 50 rpm to 100 rpm required of, for example, a drum drive motor of a copier or a main motor of a laser beam printer.
2. Description of the Related Art
Hitherto, drum driving motors of copiers or main motors of laser beam printers are generally controlled by pulse width modulation (PWM) using an H bridge. A motor drive circuit using the H bridge employs a system in which only a source side or a sink side is switched in an H-bridge switching mode for conducting the PWM control. In this system, an attempt to carry out sinusoidal current control (wherein sinusoidal information is added to a current command value to provide faithful sinusoidal waves while detecting motor coil current) as indicated by the solid line in
FIG. 1
develops a phenomenon in which a current fall delays as indicated by the dashed line in the graph. This causes a sudden drop in current when an energizing direction is switched. Such sudden change in motor coil current in the vicinity of zero cross at which the current direction is switched is not negligible because it worsens variations in revolution. Furthermore, the motor coil current deviates from a sinusoidal command value at the time of current switching or commutation, making it impossible to obtain a current waveform that is faithful to the command value. This happens because of the following reason. When only a source side shown in
FIG. 2A
or a sink side shown in
FIG. 2B
is switched, in either case, a regenerative current i produced by counter-electromotive force passes in an independent closed loop via a flywheel diode D
2
at the time of regeneration when either is switched OFF. At this time, a voltage V of a motor coil
7
is clamped by the forward voltage of a diode D
2
and a FET saturation voltage (and a voltage drop in a shunt resistor Rs is added in the case of switching of the source side), so that the reduction in the regenerative current i becomes slow or is delayed. In the circuits shown in
FIGS. 2A and 2B
, reference characters Q
1
, Q
2
, Q
3
, and Q
4
denote field effect transistors (FETs).
FIG. 3
shows a conventional H-bridge type current detecting circuit. In the drawing, when the source side and the sink side are switched simultaneously, there will be no problem in a powering mode because all motor coil current passes through the shunt resistor Rs. In a regeneration mode (at the moment of switching OFF), however, the regenerative current i from a counter-electromotive force flows over two paths (P
1
) and (P
2
) as shown in the drawing. In the case of path (P
1
), the current detecting circuit determines that no regenerative current is flowing whereas the regenerative current i is actually passing through the motor coil
7
because the current does not pass through the shunt resistor Rs in path (P
1
). In the case of path (P
2
), the current detecting circuit determines that the regenerative current is flowing in the reverse direction whereas the regenerative current i is actually flowing through the motor coil
7
in the same direction as that immediately before current supply was cut off because the direction of current passing through the shunt resistor Rs is reversed between the powering mode and the regenerative mode in the case of path (P
2
). Thus, the conventional current detection system has been posing a problem in that the current in the motor coil cannot be accurately detected.
There has been another problem. The regenerative mode means a switching OFF mode, providing a time zone or range wherein no control can be conducted. As a matter of fact, a PWM switching waveform directly appears or overlaps in the current detecting circuit of
FIG. 3
, inversely affecting the current control of the motor coil
7
.
As set forth above, the conventional current detecting system has been presenting serious problems in both a drive mode of a logic circuit for driving a switching device of an H-bridge and a current detection method employing a shunt resistor, which results in making impossible to detect motor coil current quickly and accurately. As a result, the conventional current detecting system has a shortcoming in which it cannot control motor coil current in strict accordance with a sinusoidal current command value.
SUMMARY OF THE INVENTION
The present invention has been made with a view toward solving the problems described above, and an object thereof is to provide a current detecting circuit capable of solving the problem of the delay of a motor coil current fall with respect to a current command value, which is a shortcoming of a conventional current detecting system, and also capable of accurately detecting motor coil current by removing a switching noise superimposed on a current detection waveform. It is another object of the present invention to provide a current-controlled motor drive circuit capable of controlling motor coil current faithfully to a current command value by employing the current detecting circuit.
To these ends, according to a first aspect of the present invention, there is provided a current-controlled motor drive circuit comprised of: an H-bridge circuit which is formed of a flywheel diode and a switching device, and connected to a motor coil; a current detecting circuit which is connected to the H-bridge circuit, has a plurality of shunt resistors, and computes a motor coil current value on the basis of voltage drops in the shunt resistors; a memory for storing an energizing direction of the motor coil and command value data regarding an energizing current waveform; a control circuit that issues an instruction regarding an energizing current amount of the motor coil so that an error between command value data stored in the memory and a detected current signal detected by the current detecting circuit always stays zero; and a switching device driving logic circuit that PWM-drives the switching device of the H-bridge circuit on the basis of an energizing current amount command signal from the control circuit and an energizing direction command signal from the memory; wherein the switching device driving logic circuit switches both source side and sink side of the H-bridge circuit when the motor coil is excited.
According to a second aspect of the present invention, there is provided a current-controlled motor drive circuit comprised of: an H-bridge circuit which is formed of a flywheel diode and a switching device and connected to a motor coil; and a current detecting circuit which is connected to the H-bridge circuit, has a plurality of shunt resistors, and computes a motor coil current value on the basis of voltage drops in the shunt resistors, the switching device of the H-bridge circuit being PWM-driven to control a current of the motor coil; wherein both source side and sink side of the H-bridge circuit are simultaneously switched when the motor coil is excited.
In a preferred form of the present invention, the current detecting circuit is inserted independently in a current path for a motor powering mode and in a current path for a regenerating mode, a shunt resistor for detecting powering current is connected in series to a diode for blocking regenerative current to prevent inflow of regenerative current, and a shunt resistor for detecting regenerative current is connected in series to a flywheel diode for blocking powering current to prevent inflow of powering current.
With this arrangement, regenerative current quickly attenuates, providing an advantage in which the uncontrollable time at switching OFF shortens owing to the quicker attenuation with a resultant quicker control response. Moreover, a detected current value can be accurately computed since independent shunt resistors are used to separately perform detection in the powering mode and the regenerating mode of t
Fujitani Sakae
Makino Kenichi
Suzuki Yuzuru
Howell & Haferkamp LC
Ip Paul
Minebea Co. Ltd.
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