Electricity: motive power systems – Open-loop stepping motor control systems
Reexamination Certificate
2000-11-07
2002-10-01
Donels, Jeffrey (Department: 2837)
Electricity: motive power systems
Open-loop stepping motor control systems
C400S283000
Reexamination Certificate
active
06459229
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to motor control apparatuses for driving motors, such as stepper motors, by switching the excitation phase thereof, and more particularly, to a motor control apparatus having a plurality of operation modes, such as a ramping up/down mode (acceleration driving/deceleration driving), a constant-speed driving mode, and a holding operation mode.
2. Description of the Related Art
A constant-voltage driving method and a constant-current driving method are generally employed for driving stepper motors. The constant-voltage driving method is widely used because the circuit implementing this method has a simple structure and is inexpensive. When the rotation frequency of a motor becomes high, however, the current of a motor winding has a long rising time due to the inductance of the motor winding. Therefore, motor torque is reduced at high-speed rotation to make high-speed rotation impossible.
In the constant-current driving method, the voltage to be applied and the winding inductance are set so as to make the time constant of the motor winding small and the current flowing through the winding is kept constant by switching on and off the current by the use of a switching device, such as a transistor, while the current is being detected, such that the current matches the detected current. When the constant-current driving method is employed, high-speed motor rotation can be performed although the circuit implementing this method has a complicated structure and is expensive. A constant-current driving circuit has been integrated into an IC these days, and therefore, constant-current driving is allowed at a low cost.
When high-speed driving is applied to a stepper motor with the above-described condition, rotation control is generally achieved in control zones, such as an acceleration zone where the motor is accelerated to a target rotation speed, a constant-speed zone where the target rotation speed is maintained, and a deceleration zone where the motor is decelerated and stopped. In the acceleration zone and the deceleration zone, it is necessary to set the rotation torque of the motor higher than in the constant-speed zone in order to change the rotation speed within a relatively short period of time. A method for changing the driving current in the acceleration zone, the constant-speed zone, and the deceleration zone is used.
FIG.
9
and
FIG. 10
are views showing the relationship between the target current and the motor winding current in a conventional constant-current driving method.
FIG. 9
shows the relationship obtained near the start of the acceleration zone, and
FIG. 10
shows the relationship obtained near the end of the deceleration zone. In each figure, the upper part indicates the relationship in phase A of the motor and the lower part indicates the relationship in phase B. A solid line illustrates the target current in each part, and a dotted line illustrates the motor winding current in each part. The maximum winding-current values required to obtain necessary rotation torque in the acceleration zone in the phases are indicated by +IA
1
, −IA
1
, +IB
1
, and −IB
1
, and the maximum winding-current values required to obtain necessary rotation torque in the deceleration zone in the phases are indicated by +IA
2
, −IA
2
, +IB
2
, and −IB
2
.
It is understood from
FIG. 9
that the rotation frequency increases in the direction from the left to the right in the figure as an acceleration operation is achieved, the winding current flows more than necessary against the target current near the start of acceleration, and the winding current approaches the maximum target winding current as the rotation frequency increases. It is also understood from
FIG. 10
that the rotation frequency is reduced in the direction from the left to the right as a deceleration operation is achieved, the winding current is close to the maximum target winding current near the start of deceleration, and the winding current flows more than necessary as the rotation frequency is reduced.
Ideal control can be achieved so that the winding current always matches the target current, by improving the switching characteristic of a driving circuit and by reducing the winding resistance and inductance to improve the response of the current flowing through the winding. To rotate the motor at a high speed, expensive components are required, and the cost of the motor control apparatus increases.
Therefore, as shown in FIG.
9
and
FIG. 10
, the maximum target current is specified for the motor winding current such that an appropriate rotation torque is generated at a high-rotation-frequency area if high rotation torque is required at acceleration and deceleration. Since the winding current flows more than necessary near the start of acceleration and near the end of deceleration in this method, however, a large-capacity power supply having a large maximum rating is used to drive the motor with the maximum supply current being taken into account. This type of power supply is expensive.
In addition, since the winding current flows more than necessary near the start of acceleration and near the end of deceleration, wasteful motor heat is generated and motor vibration occurs due to wasteful applied rotation torque, and noise also occurs.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a motor control apparatus for preventing a motor-winding current from flowing more than necessary to allow stable motor rotation.
Another object of the present invention is to suppress the driving current of a stepper motor near the start of an acceleration mode and near the end of a deceleration mode to allow the stepper motor to be rotated at high speed with low cost.
According to one aspect, the present invention that achieves at least one of the foregoing objects relates to a motor control apparatus comprising a stepper motor, driving means, excitation-signal generation means, a timer, pulse generation means, storage means, and control means. The driving means is for driving the stepper motor. The excitation-signal generation means is for applying an excitation signal to the driving means to drive the stepper motor when the excitation signal is switched. The timer is for measuring a predetermined period of time at the end of which the excitation signal is switched. The pulse generation means is for applying a pulse signal having a specified duty cycle to the driving means to control the driving current of the stepper motor. The storage means is for storing motor-control-table information for various driving modes of the stepper motor including at least an acceleration mode and a deceleration mode. The control means is for specifying for the pulse generation means information required for generating a pulse signal having a duty cycle specified according to the motor control-table information of a driving mode. The information includes information used by the driving means to suppress the driving current of the stepper motor near the start of the acceleration mode and near the end of the deceleration mode. The control means is also for controlling the timer to start measuring the predetermined period of time and for controlling the excitation signal generation means to switch the excitation signal at the end of the measured predetermined period of time.
Among the driving modes for which the motor control-table information is stored in the storage means, the acceleration mode has a current-suppression table for acceleration and a normal table for acceleration, and the deceleration mode has a current-suppression table for deceleration and a normal table for deceleration. The control means makes the timer start measuring the predetermined period of time and controls the driving means to drive the stepping motor according to the various driving modes for which the motor control-table information is stored in the storage means, according to the time measured by the timer
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