Patent Application
20050248307
1. Field of the Invention
The present invention relates to a positioning control for an actuator using an induction motor.
2. Description of Related Art
Japanese Unexamined Patent Publication (Kokai) No. 2001-175334 discloses a conventional position control system in which an induction motor and an inverter are combined as described in detail in Paragraphs [0002] to [0017] and shown in FIGS. 7 to 10.
A load position control speed disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2001-175334 is as follows. An absolute position detector 26 outputs an absolute position signal in accordance with a position of a movable table driven by an induction motor 21. In a position controller 27, there are provided an absolute signal converter 35 which supplies absolute position data produced based on the absolute position signal, to control a position of a movable table; and a pulse train output converter 36 which supplies, as a speed feedback pulse for the induction motor 21, an incremental pulse produced based on the absolute position signal, to a vector control inverter 29.
As described above, a conventional system shown in
The present invention provides a method for controlling an induction motor, in which the speed of the induction motor is controlled, by an inverter, to move and stop an actuator at a predetermined position, wherein a speed command to be input to the inverter is switched to an intermittent pulse signal when the actuator reaches a position, a predetermined distance before the predetermined position. Thus, the system can be constructed at low cost, and sufficient practical positioning and stopping accuracy can be obtained.
An embodiment of the present invention will be described in detail with reference to the drawings.
A clamping device 1 is used for an injection molding machine or the like, and opens/closes and clamps a die 5 to which a molten material is injected from an injection device (not shown). The clamping device 1 is composed of a stationary plate 2 and a movable plate 3 to which the die 5 is attached; a plurality of tie rods 4 that are inserted in the periphery of the movable plate 3 and receive a clamping force; a die thickness adjusting device 22 that is provided on the other ends of the tie rods 4 and includes an attachment plate 8 into which a threaded portion 11 is inserted; and a publicly known toggle mechanism 6 that is provided on the opposed surfaces of the attachment plate 8 and the movable plate 3, and generates a clamping force. The toggle mechanism 6 is driven, by a servomotor 19 secured to the back side faces of the attachment plate 8, using a ball screw 7 forwardly or backwardly driven via a gear 18, a belt 17 and a ball nut 16. The toggle mechanism 6 may be replaced with the other mechanism such as a hydraulic cylinder.
The die thickness adjusting device 22 is composed of the attachment plate 8; the threaded portions 11 that penetrate through and project from the four corners of the surface of the attachment plate 8, which is opposed to the surface to which the toggle mechanism 6 is attached; nuts 9 that are engaged with the threaded portions 11 and rotatably provided on the attachment plate 8; gears 10 provided on the outer peripheries of the nuts 9; an annular gear 12 engaged with four gears 10; a drive gear 13 that drives the annular gear 12; an induction motor 14 that is secured on the front side face of the attachment plate 8 and drives the drive gear 13; and an encoder 15 that is connected to the drive gear 13 and detects a position of the die thickness adjusting device 22 functioning as an actuator.
The induction motor 14 is provided with a speed reducer having a ratio of 90:1, but has no brake. The rotation speed of the induction motor 14 is controlled by a general purpose inverter 21 which arbitrarily converts the frequency of three-phase alternating current. A speed command 23, a normal rotation command 24 and a reverse rotation command 25 are transmitted from a controller 20 to the inverter 21. The speed command 23 is an analog voltage signal of 0 to 10 V, and is preset and stored in the controller 20, along with one of the normal rotation command 24 and the reverse rotation command 25 that are alternatively selected in accordance with the speed command 23. The inverter 21 outputs the three phase alternating current, which has a frequency proportional to that of the speed command 23, and a phase sequence in accordance with the normal rotation command 24 or the reverse rotation command 25, to the induction motor 14.
The rotation of the induction motor 14, which has reduced to 1/90 by the speed reducer, is transferred to the drive gear 13. The drive gear 13 rotates the four gears 10 and the four nuts 9 via the annular gear 12. Thus, the die thickness adjusting device 22 is relatively moved in forward-and-backward directions, with respect to the tie rods 4, and along the axes of the rods. Accordingly, the toggle mechanism 6 coupled to the die thickness adjusting device 22, and the movable plate 3 coupled to the toggle mechanism 6 are moved. When the die thickness adjusting device 22 is moved so that a stationary die and a movable die of the die 5 are abutted with each other with the toggle mechanism 6 being extended most by the servomotor 19, the clamping force applied to the die 5 is zero. In this state, after the toggle mechanism 6 is once flexed to slightly open the die 5 as shown in
As described above, the clamping force varies based on an extension amount of the tie rod 4, i.e., a displacement of the die thickness adjusting device 22 with respect to an abutment position of the die 5. The displacement of the die thickness adjusting device 22 depends on the diameter and, length of the tie rod 4. In this embodiment, for example, the displacement is about 2 mm to obtain a clamping force of 110 ton which is a specified value of the clamping device 1. The lead length of the threaded portion 11 is 4 mm, the diameter of the drive gear 13 is substantially identical to that of the gear 10, and the encoder 15 outputs 4096 pulses per one rotation. Accordingly, the displacement of the die thickness adjusting device 22 with respect to the encoder 15 per 1 pulse is about 11 m.
An embodiment of the control method according to the present invention will be described below with reference to
When the die thickness adjusting device 22 reaches the position A, the controller 20 switches the speed command 23 to a predetermined speed reduction signal 26, to reduce the speed of the die thickness adjusting device 22. The distance between the position A and the predetermined position C is about 1 mm in this embodiment.
When the die thickness adjusting device 22 is sufficiently decelerated and reaches a position B provided between the positions A and C, the controller 20 switches the speed command 23 to an intermittent pulse signal 27 in which pulse-shapes intermittently appear. The distance between the positions B and C is about 0.1 mm in this embodiment. As shown in
The controller 20 constantly outputs one of the normal rotation command 24 and the reverse rotation command 25 so that the die thickness adjusting device 22 is moved in only a forward direction (rightward direction in
Thus, the die thickness adjusting device 22 which moves in the forward direction, at extremely low speed, is stopped and positioned when the encoder 15 detects the predetermined position C. In this respect, an overrun of the die thickness % adjusting device 22 from the predetermined position C corresponds to 2 to 3 pulses of the pulses in the encoder 15, i.e., 2 to 31 m in this embodiment. This value is remarkably improved in comparison with a value of 101 m in a conventional method.
The above movement of the die thickness adjusting device 22 at an extremely low speed cannot be obtained by the speed command 23 to be output at an extremely small value, due to problems in the properties of the inverter 21, the induction motor 14 or the actuator. Contrary to this, the intermittent pulse signal 27 allows the speed command 23 to be output at a high value to which the inverter 21, the induction motor 14 or the actuator can appropriately respond, even if the high value corresponds to an extremely low speed on average. Thus, the movement of the actuator at an extremely low speed can be easily realized at low cost. As the speed command 23 is switched to the intermittent pulse signal 27 to accomplish the above object, the shape of the intermittent pulse signal 27 is not limited to this embodiment, and the pulse may have several shapes including a general rectangular shape.
The present invention is not limited to the above-described embodiment, and several modifications may be made therein without departing from the gist of the invention. In the above embodiment, for example, the actuator is represented by the die thickness adjusting device 22. However, the present invention can be applied to the actuator other than the die thickness adjusting device 22.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-137969 | May 2004 | JP | national |
