Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Patent
1988-07-11
1989-06-06
Dobeck, Benjamin
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
318571, 318636, 318578, 164155, 364476, G05B 1942
Patent
active
048374903
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control method for an injection motor of an injection molding machine, and more particularly, to a control method for an injection motor, which is capable of preventing the motor from overheating.
2. Description of the Related Art
In an injection molding machine, as is generally known, an injection motor is used as a drive source for an injection unit, and processes of injection, hold, and application of back pressure for metering are executed by the injection motor. In the injection molding machine of this type, the output torque of the injection motor is small, so that a very small driving current flows through the injection motor when the back pressure is applied. In the injection and hold process, a large output torque must be produced, so that a very large driving current flows through the injection motor. Meanwhile, the injection motor is selected in consideration of various design conditions, such as economic efficiency. It is not common, therefore, to use, as the injection motor, a large-capacity motor such that its continuous rated current is never exceeded by the driving current. As a result, the driving current during the injection and hold processes may possibly exceed the continuous rated current by a large margin, so that the average value of the driving current flowing through the injection motor during one injection molding cycle may sometimes exceed the continuous rated current. In such a case, since the amount of heat produced is in proportion to the square of the driving current, the injection motor may overheat or be subject to other troubles.
Control methods of an injection motor of a type capable of determining whether or not an injection motor can overheat have been proposed by the present applicant (Japanese patent application Nos. 61-72416 and 61-155174). These methods were created on the basis of recognition that a large driving current flows during the hold process and a value of the driving current during the same process is in proportion to a preset hold pressure. According to these methods, on one hand, the square of a preset hold pressure is multiplied by a hold time to obtain a first value which is in proportion to an amount of heat produced in one injection molding cycle, and on the other hand, the square of a pressure produced when an injection motor delivers its maximum torque or when a continuous rated current flows in the motor is multiplied by one cycle time to obtain a second value determination is made as to whether a ratio of the first value to the second value is less than a predetermined value to determine whether or not the injection motor can overheat.
However, according to these proposed methods, only approximation of heat produced by the injection motor is obtainable.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a control method for an injection motor of an injection molding machine which is capable of controlling the injection motor on the basis of an amount of heat produced by the injection motor, which is obtained from the driving current flowing through the same motor, so as to prevent the motor from overheating.
In order to achieve the above object, a control method for an injection motor, according to the present invention, comprises steps of: (a) sampling and detecting, at intervals having a predetermined period, a value of current flowing through the injection motor; (b) adding respective squared products multiplying an associated one of the current values and multiplying the sum by the predetermined period, to obtain a first value; (c) determining whether or not the injection motor is in an overheating state, on the basis of a ratio of said first value to first predetermined determination value; and (d) setting a quiescent time to lengthen an injection molding cycle time, when said step (c) that the injection motor is in the overheating state.
According to the present invention, as described above, a determination is made as to wh
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patent: 2832108 (1958-04-01), Vossberg
patent: 4279151 (1981-07-01), Anderson
patent: 4437046 (1984-03-01), Faillace
patent: 4493362 (1985-01-01), Moore et al.
patent: 4558421 (1985-12-01), Shriver
patent: 4597048 (1986-06-01), Mazur et al.
Dobeck Benjamin
Fanuc Ltd.
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