Electricity: motive power systems – Constant motor current – load and/or torque control
Reexamination Certificate
2001-07-16
2003-06-17
Leykin, Rita (Department: 2837)
Electricity: motive power systems
Constant motor current, load and/or torque control
C318S254100, C318S132000, C318S434000
Reexamination Certificate
active
06580238
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a motor driver furnished with a function of controlling the driving of a motor, such as a stepping motor or spindle motor, through negative feedback control of the current supplied to the motor.
2. Description of the Prior Art
FIG. 13
shows the circuit configuration of a conventional, typical stepping motor driver. Reference numerals
401
A and
401
B represent output circuits. Reference numeral
402
A represents an operational amplifier. Reference numeral
402
B represents a direct-current voltage source. Reference numeral
402
C represents a controller. Reference numeral
402
D represents an externally fitted resistor. Reference numerals
403
A and
403
B represent coils of a stepping motor. Reference numerals
404
A,
404
B,
404
C,
404
D,
404
E,
404
F,
404
G, and
404
H represent NPN-type transistors. Here, it is assumed that the stepping motor driver shown in
FIG. 13
is for driving a stepping motor having two coils.
The output circuit
401
A is composed of four transistors
404
A,
404
B,
404
C, and
404
D. The output circuit
401
B is composed of four transistors
404
E,
404
F,
404
G, and
404
H. The collectors of the transistors
404
A,
404
C,
404
E, and
404
G are connected together to a terminal
405
A. The terminal
405
A is connected to a supply voltage V
CC
. The emitters of the transistors
404
B,
404
D,
404
F, and
404
H are connected together to a terminal
405
B. The terminal
405
B is connected through the resistor
402
D to ground GND.
In the output circuit
401
A, the emitter of the transistor
404
A and the collector of the transistor
404
B are connected together to a terminal
405
C, and the emitter of the transistor
404
C and the collector of the transistor
404
D are connected together to a terminal
405
D. Between the terminals
405
C and
405
D, one coil
403
A of the stepping motor is connected.
In the output circuit
401
B, the emitter of the transistor
404
E and the collector of the transistor
404
F are connected together to a terminal
405
E, and the emitter of the transistor
404
G and the collector of the transistor
404
H are connected together to a terminal
405
F. Between the terminals
405
E and
405
F, the other coil
403
B of the stepping motor is connected.
A direct-current voltage generated by the voltage source
402
B is fed to the non-inverting input terminal (+) of the operational amplifier
402
A, of which the inverting input terminal (−) is connected to the terminal
405
B so that all the current that flows through the stepping motor is converted into a voltage by the resistor
402
D and is then fed to the inverting input terminal (−).
The controller
402
C controls the turning on/off of the individual transistors constituting the output circuits
401
A and
401
B in such a way that the direction of the current that flows through the coils
403
A and
403
B is switched with appropriate timing. Moreover, the controller
402
C controls the current fed to the bases of the individual transistors constituting the output circuits
401
A and
401
B when those transistors are turned on in such a way that the output of the operational amplifier
402
A does not fall below a reference level. In this way, negative feedback control is achieved in such a way that the current flowing through the stepping motor as a whole does not exceed a predetermined value.
However, in cases like this, where the current flowing through a stepping motor is controlled through negative feedback, depending on the specifications of the stepping motor actually driven, the following problems may arise. As shown in
FIG. 14
, when the transistors constituting the output circuits are turned on/off, the inductance present in the coils of the stepping motor may cause overshooting or oscillation in the current I flowing through the coils of the stepping motor, or oscillation lasting for several microseconds to several milliseconds in the voltages V
A
and V
B
across the coils, both leading to increased noise. This often causes problems in particular in electronic devices such as floppy disk drives that comply with the USB standard, because such devices receive electric power from their host and thus need to meet strict requirements with respect to the current that they consume.
Moreover, there is a type of problem in which, although an abnormal current is flowing through the coil of one phase of the stepping motor, the current flowing through the motor as a whole appears quite normal. In such a case, with the conventional stepping motor driver described above, which controls the current flowing through the motor as a whole with a single negative feedback circuit, it is not possible to overcome the problem, and thus it is not possible to drive the stepping motor with satisfactory accuracy.
Moreover, since a current so high as all the current supplied to the stepping motor is controlled through negative feedback, the resistances of the conductors that connect the output circuits to the supply power terminals affect the feedback factor greatly. Thus, problems tend to result from the fact that the resistances of conductors vary from one IC chip to another.
FIG. 15
shows a plan view and a connection diagram of the conventional stepping motor driver described above. As this figure shows, the output circuits
401
A and
401
B are connected by way of a typical conductor
407
to the terminal
405
B, which in turn is connected, outside the stepping motor driver, through the resistor
402
D to ground V
GND
. However, the two output circuits
401
A and
401
B use different lengths of this conductor
407
, and thus the resistance of the conductor
407
from the output circuits
401
A and
401
B to the terminal
405
B differs between them, with the result that the currents flowing through the coils
403
A and
403
B differ significantly from each other. Furthermore, the switching noise that is generated in the output circuits
401
A and
401
B tends to affect a current limiter circuit
406
A and other internal circuits adversely. This often causes problems in particular in electronic devices that comply with the USB standard, because such devices need to meet strict requirements with respect to the current that they consume.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a motor driver that is furnished with a function of controlling the current supplied to the coils of a motor through negative feedback and that is designed to minimize the noise accompanying the switching operations performed in output circuits.
Another object of the present invention is to provide a motor driver that can drive a motor such as a stepping motor with satisfactory accuracy.
Another object of the present invention is to provide a motor driver that is less prone to problems resulting from the fact that the resistances of conductors vary from one IC chip to another.
Another object of the present invention is to provide a motor driver that can supply individual coils of a motor with as equal currents as possible.
Another object of the present invention is to provide a motor driver that can minimize the adverse effect of the switching noise generated in output circuits on other internal circuits.
To achieve the above objects, according to one aspect of the present invention, in a motor driver furnished with a function of controlling the driving of a motor through negative feedback control of the current supplied to the coil of the motor, the motor driver permits the setting of a target value of the current that is negatively fed back, and sets the target value in such a way that the target value at a time point when the current starts being supplied to the coil of the motor is smaller than the target value after the current starts being supplied to the coil of the motor.
In this circuit configuration, the current that flows through the coil increases more gradually than in conventional circuit configurations in which the target value is fixed at the maximum value
Arent Fox Kintner Plotkin & Kahn
Rohm & Co., Ltd.
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