Electricity: motive power systems – Positional servo systems
Reexamination Certificate
2002-03-04
2003-10-28
Masih, Karen (Department: 2837)
Electricity: motive power systems
Positional servo systems
C318S603000, C318S599000, C318S632000, C318S446000, C318S663000, C388S811000
Reexamination Certificate
active
06639374
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This present invention relates to a method and a device for controlling a DC servomotor for driving a rotating load such as a valve.
BACKGROUND OF THE INVENTION
When a rotating load such as a valve is driven by a motor, a rotating load driving device using a DC servomotor as a driving source is often used.
A valve being controlled to open and close according to various control conditions is often used in order to adjust characteristics of various machines and control operations of the machines. For example, in a two-cycle combustion engine, a valve for adjusting timing of exhaust in order to improve output characteristics of the engine and to purify exhaust gas of the engine, a valve for opening and closing a cavity connected to an exhaust pipe to adjust a resonance frequency of the exhaust pipe in order to improve exhaust efficiency and a valve for adjusting air-intake timing are provided as needed. These valves are controlled according to various control conditions such as a rotational speed and an opening degree of a throttle valve.
As a device for driving a valve provided in a combustion engine, a device is used which is provided with a DC servomotor, a driven member to be driven by the DC servomotor, an operation member attached to an operation shaft of the valve in order to rotationally operate the valve and a wire for coupling the driven member and the operation member.
In this kind of driving device, with the rotation of the driven member to be driven by the motor, the operation member is rotated while being pulled by the wire, and the operation shaft of the valve is rotationally operated by the rotation of the operation member.
If the valve is driven by such a driving device, a driving current is supplied to the motor at a duty ratio of 100% at the start of control to cause the valve to carry out an operation of moving a valve body of the valve from one limit position (e.g., fully closed position) to the other limit position (e.g., fully opened position) and an operation of moving the valve body from the other limit position (e.g., fully opened position) to one limit position (e.g., fully closed position). In this way, self-cleaning of the valve is performed. Foreign matters such as carbon and dusts adhered to a movable portion of the valve are removed by this self-cleaning operation to allow the valve to move smoothly in subsequent control operations.
In addition, when this self-cleaning operation is performed, a stop position of the driven member at the point when the valve body of the valve stops by reaching the fully closed position and a stop position of the driven member at the point when the valve body of the valve stops by reaching the fully opened position are detected and stored as a fully-opened-side final stop position and a fully-closed-side final stop position of the driven member, respectively.
During the operation of the combustion engine, a position of the driven member attached to the output shaft of the motor is detected by a position detector such as a potentiometer. Then, a duty ratio of a driving current of the motor is controlled such that a deviation between the detected current position of the driven member and a target position of the driven member corresponding to an optimal position of the valve body of the valve, which is determined by various control conditions such as a rotational speed of the engine and an opening degree of the throttle valve, becomes zero. In this way, the position of the valve body of the valve (hereinafter referred to simply as the position of the valve) is coincided with the optimal position.
As described above, the valve driving device is comprised such that the driven member that is driven by the motor and the operation member attached to the operation shaft of the valve are coupled by a wire and the valve is driven while the operation member is pulled via the wire with the rotation of the driven member. In the valve driving device, when a driving current with a duty ratio of 100% is supplied to the motor and the operation shaft of the valve is driven toward the limit position (the fully opened position or the fully closed position), even after the operation shaft of the valve reaches the limit position, the driven member is displaced to a certain degree by the extension of the wire due to a large output torque of the motor caused by the driving current with a duty ratio of 100%. Thus, the final stop position of the driven member and the limit position of the operation shaft of the valve cannot accurately correspond to each other. Therefore, in the conventional device, a position that is beyond the position of the driven member corresponding to the limit position of the operation shaft of the valve may be detected as the final stop position of the driven member.
However, during the operation of the engine, when the driven member is controlled to reach the final stop position with the target position of the driven member as the final stop position in order to displace the operation shaft of the valve to the limit position, it is not possible to always flow the driving current with a duty ratio of 100% in order to have good controllability. As the position of the driven member is approaching the final stop position, it is necessary to reduce the duty ratio of the driving current. Thus, when the operation shaft of the valve reaches the limit position and stops, the motor also stops. Therefore, if the final stop position of the driven member detected when the control is started is a position that is beyond a position corresponding to the limit position of the operation shaft of the valve, the motor stops and is locked in the state in which the driven member cannot be displaced to the final stop position. If the motor continues to be locked, since the driving current continues to flow into an armature coil while the motor is stopped, it is likely that a coil of the motor is burned and a switch element of a circuit for controlling the driving current of the motor is damaged. Thus, it is necessary to provide means for detecting that the motor is locked and immediately block the driving current of the motor when it is detected by this means that the motor is locked. However, since it requires a certain period of time to detect that the motor is locked, protection of the motor and protection of the circuit for controlling the motor cannot be accurately realized.
In addition, in the case in which the final stop position of the driven member is shifted from the position corresponding to the limit position of the operation shaft of the valve at the start of the control, if the driving current of the motor is blocked when the operation shaft of the valve reaches the limit position and the motor stops, the driven member may be pulled back by tension of the wire. If the driven member is pulled back in this way, since the operation shaft of the valve may also be pulled back from the limit position, the control for causing the operation shaft of the valve to reach the limit position cannot be accurately performed.
As described above, in the driving device in which the driven member attached to the motor side and the operation member attached to the valve side are coupled by the wire, it may be difficult to accurately correspond the final stop position of the driven member to the limit position of the operation member due to extension of the wire. Thus, the control for displacing the operation shaft of the valve to the limit position cannot be accurately performed.
Further, although a load of the driving device is assumed to be a valve in the above-mentioned example, the same problem occurs in the case in which a driven member to be driven by a DC servomotor and an operation member for operating a rotating load other than a valve are coupled by a wire to control the DC servomotor, thereby controlling a stop position of the load.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a control method and a control device for a DC servomotor for driving a load, whic
Kokusan Denki Co. Ltd.
Masih Karen
Pearne & Gordon LLP
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