WO2008100224A1

WO2008100224A1 - Improved linear-rotary servo actuator

Info

Publication number: WO2008100224A1
Application number: PCT/SG2007/000045
Authority: WO
Inventors: Yong Peng Leow; Yong Min Kong
Original assignee: Yong Peng Leow; Yong Min Kong
Priority date: 2007-02-12
Filing date: 2007-02-12
Publication date: 2008-08-21

Abstract

An in-line hollow shaft direct drive independently driven linear-rotary servo actuator, having a linear voice coil and a rotary slot-less motor sharing a common hollow shaft. Both the linear voice coil and slot-less rotary motor are independently energized to linearly move and rotate the shaft. The longitudinal axis of the voice coil motor is in-line and co-axial with the longitudinal axis of the rotary slot-less motor. There are no stacking of the linear voice coil motor portion onto the rotary slot-less motor portion, or vice verse. The common shaft is supported on one end by a bottom bearing allows for linear and rotary motion, and the other by the top bearing assembly built into the coil of the voice coil motor. The two axes are coupled together by the common shaft but the two motions (linear and rotary) are independent.

Description

IMPROVED LINEAR-ROTARY SERVO ACTUATOR

FIELD OF INVENTION

The invention relates to an improved actuator which provides both linear and rotary motion independently. These actuators are useful for operations such as pick and place; and orientation for assembly lines in automated manufacturing environments.

DESCRIPTION OF THE RELATED ART

There are many actuators that are available to produce either linear only; rotary only; or combined linear and rotary motion. The minaturisation of components used in the consumer products such as our mobile phones, computers, etc had made automated assembly of parts during the manufacturing process more demanding. The need to keep cost in check and improve productivity demands that automated assembly machines are now running faster and more precisely. For this reason, many conventional drive method like pneumatics and hydraulics are being replaced by electric driven devices such as linear motor, rotary motor, stepper motor and voice coils.

The need for handling smaller parts at higher speed give rise to the challenge for making smaller and more compact actuator - so that they can be gang together together to form multiple actuator to increase through put. The second aspect of high speed imply that the inertia from moving parts must be kept to the minimum. Conventional methods of stacking different actuators (eg. One rotary axis onto a linear axis to form a two degrees of freedom actuator) to get the required degrees of freedom is therefore, not good enough to fulfill manufacturing needs.

Portegies et al described in US Pat No. 5,789,830 an in-line linear-rotary drive mechanism for a voice coil actuator which includes an electro-magnetic drive motor having a rotatable drive shaft. Both the rotary motor and its rotational drive shaft are mounted on the linear voice coil for translational movement therewith. Additionally, an actuator probe is connected to the rotational drive shaft via a coupling, thus the actuator probe moves in rotation in response to rotations of the drive shaft. It is noted that the voice coil is positioned to the side of the rotary motor. Essentially, only the actuator probe and the rotary motor are in-line, not the voice coil and the rotary motor.

Similarly, Neff et al described in US Pat No. 5,685,214 a mechanism for precise translational positioning of a rotatable rod which includes a reciprocating piston. A rotary motor is mounted on the piston for establishing rotation about a first axis, and a flexible coupling connects the rotary motor with a rod for precise rotation of the rob about an axis substantially parallel to the first axis. These two inventions are essentially a stacked configuration. One of the main disadvantage of this configuration is that the linear axis need to carry both the load and the rotary motor. This heavy load increases the inertia which limits its ability to accelerate/decelerate thus the impact is the response time, which leads to a slower actuator or lower throughput.

Bryson, III et al described in US Pat No. 5,105,932 a precision magnetic manipulator for use in positioning targets, and the like, within a vacuum chamber, employs separate mechanisms for linear and rotational motion and makes them substantially independent. This invention achieves its substantially independent motions by a clever combination of magnets, bearings and co-axial shafts. It is truly an in-line design, however, it is not a mechanized device. Furthermore, the many co-axial shaft needs to be carefully aligned to minimize rotary run out.

Hammer described in US Pat No. 5,093,596 a combined linear-rotary direct drive step motor, having a rotary section and a linear section in a single housing, comprising a cylindrically shaped variable reluctance linear step motor; a modified hybrid permanent magnet rotary step motor; and a common shaft shared by sail linear step motor and sail rotary step motor. This is indeed another inline design, however, iron laminations are present in the rotor thus increasing the inertia which limits the acceleration/deceleration of the actuator. Furthermore, being an open loop design, the positioning repeatability is not deterministic.

Swift described in US Pat No. 6,798,087 a rotary-linear actuator system having a spindle assembly and a housing assembly. The spindle axis can be movable and/or rotatable along a longitudinal axis extending through the spindle assembly. It is an in-line design. However, the rotor shaft is made to carry two sets of magnets. Each sets of magnets are different in shapes and sizes to produce the two different motion when interacting with two sets of different coils. Furthermore, magnets from the rotor tends to stick out of the housing when it is extended. The feedback devices are also not commonly available.

Chitayat described in a series of similar patents US Pat No. 5,952,744; US Pat No. 5,982,053; US Pat No. 6,137,195; US Pat No. 6,215,206; and US Pat No.6,719,174 an actuator with two independent degrees of freedom, rotates a stage about an axis and moves the stage along the axis. A set of Z-axis coils interacting with the magnets on the plunger moves the plunger in the z direction (linear). A second set of phi-coils interacting with the magnets on the plunger moves the plunger in the phi direction (rotary). In this invention, the number of magnets used is very large. For example, for an embodiment cited in the patent, a ring would comprise 12 magnets. With 4 rings shown in this embodiment, there would be 48 magnets. As high energy magnets are probably the most expensive component in a permanent magnet actuator, this would constitute a relatively high cost from the magnet volume involved. Moreover, the assembly costs is also increased with so many magnets involved. Furthermore, the weight of the magnets are high, making the inertia high which directly impact the respond of the actuator. Moreover, the packing density of the magnets is only 50%, this also means that the effective working portion of the coil is also 50%. In other words, 50% of the current in the coils are not producing force or torque. This results in higher ohmic loss (in the form of heat) compared to effective work done. Additionally, due to the nature of this design, each of the coils (coil that gives vertical force and coil that provides torque) cannot occupy more than half of the circular space of the actuator. One of the coil may occupy half of the circular space and the other coil occupies the other half. Hence, the effective working "area" of the flux cutting the current in the coils is very much compromised (reduced). Lastly, the feedback devices are also not commonly available.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved in-line hollow shaft direct drive independent driven linear-rotary servo actuator in which the coils in the linear and rotary sections are capable of being independently energized to realize two independent motions, one linear and one rotary.

It is still a further object of the invention to provide an in-line hollow shaft direct drive independent driven linear-rotary servo actuator having a linear voice coil motor portion and a rotary slot-less motor portion sharing a common shaft and a common axis within the same housing. The longitudinal axis of the voice coil motor is in-line and co-axial with the longitudinal axis of the rotary slot-less motor. The benefits of the in-line configuration with a common shaft allows for better accuracy in positioning as there are no unwanted moment load (which causes compliant in the structure and bearing) due to off-axis driving.

It is still a further object of the invention to provide an in-line hollow shaft direct drive independent driven linear-rotary servo actuator wherein the common shaft is however a hollow shaft capable to be prepared to accept fittings for vacuum connection to the object to be manipulated.

More specifically, it is an object of the invention to have an improved in-line hollow shaft direct drive independent driven linear-rotary servo actuator with a set of coils in the linear motor and rotary motor sections sharing a common shaft within the same housing, the set of coils in the linear motor section and rotary motor section capable of being independently energized to realize two independent motions, one linear and one rotary, the rotary motion produced by a rotary slot-less motor section and the linear motion produced by a linear motor portion which is a linear voice coil motor.

Preferably, the improved linear-rotary servo actuator does not have any stacking of the linear voice coil motor portion onto the rotary slot-less motor portion , or vice verse.

Preferably, the improved linear-rotary servo actuator has both linear voice coil portion and rotary slot-less motor portion sharing a common shaft which is supported on one end by a bottom bearing, and the other end by the top bearing assembly integrated into the coil of the voice coil motor. Preferably, the improved linear-rotary servo actuator has two motions (linear and rotary) which are independent with the common shaft rotating about the inner race of a top bearing assembly integrated into the coil of the linear voice coil motor, but the outer race of the top bearing assembly which is integrated into the coil of the linear voice coil motor is prevented from rotating.

Preferably, the improved linear-rotary servo actuator has additionally a runner block and a guide rail which further prevents the coil of the linear voice coil motor from rotating.

A subsidiary object of the invention is to have an improved linear voice coil drives wherein the position of the linear motor portion is measured directly by a high resolution optical encoder and linear scale assembly.

Another subsidiary object of the invention is to have an improved linear-rotary servo actuator wherein the angular position of the slot-less motor is measured directly by a high resolution optical encoder and rotary scale assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. IA depicts, in accordance with a preferred embodiment of the invention, a plan view. Fig. 1 C is a longitudinal section view with the cover removed taken along A-B of Fig. 1 B.

Fig. 2 depicts, in accordance with a preferred embodiment of the invention, an isometric view with cover attached.

Fig. 3 A depicts, in accordance with a preferred embodiment of the invention, one front view and one side view, in which the moving shaft is in the retracted position.

Fig. 3B depicts, in accordance with a preferred embodiment of the invention, one front view and one side view, in which the moving shaft is in the extended position. Fig. 4 in accordance with a preferred embodiment of the invention, an isometric view of a section of the actuator.

Fig. 5 in accordance with a preferred embodiment of the invention, an isometric view of the rotor assembly of the actuator.

Fig. 6 in accordance with a preferred embodiment of the invention, an isometric view of the stator assembly of the actuator.

DETAILED DESCRIPTION OF INVENTION

Figure 1 shows an in-line hollow shaft direct drive independently driven linear-rotary servo actuator, in accordance with the present invention which is designated 100. hi order not to obscure the details, a cover 30 is removed. Figure 2 shows in accordance with the present invention, the actuator 100 with its cover 30 in place. Figure 3 a shows the actuator 100 in the retracted position, while Fig. 3B shows the actuator 100 in the extended position. However, the actuator 100 is not limited to position in only this two extreme positions. Referring back to the actuator 100 in Fig. 1, it includes a base assembly designed 200 and shown in detail in Fig. 4 ; a rotor assembly designed 300 and shown in detail in Fig. 5; and stator assembly designed 400 and shown in detail in Fig. 6.

The main object of this invention is to have control movement of the common hollow shaft 8 independently in a linear and rotary manner. Referring to Fig 5, a cylindrical iron core 9 is epoxy onto the common hollow shaft 8. Onto this iron core 9, four magnets of which 10a and 10b have their north pole facing outward, while 10b and 1Od have their south pole facing outward are epoxy in place in an alternate manner, that is, N-S-N-S. Mounted opposite of the magnets 10, are two sets of rotary coil 26. The wires in the rotary coils 26 are oriented such that it runs longitudinally and parallel to the axis of the common hollow shaft 8. The back side of the rotary coils 26, are back up by a cylindrical coil iron core tube 25. The magnetic circuit starts from 10, via a small an air gap through the rotary coil 26 into the coil iron core tube 25 and makes a U-turn back into the rotary coil 26 into the iron core 9 before ending back at the magnet 10. When a properly commutated current (normally done by a digital amplifier) is passed through the coils 26, a tangential force is created and it turns the common hollow shaft 8. The circular motion of the common hollow shaft 8 is guided on one end by a ball cage bearing. The ball cage bearing comes in two parts. An outer shell 23 which is retained securely by a clip 22 in the spindle housing 21, and a moving ball cage 24, which is capable of rotating about the common hollow shaft 8 and at the same time allows linear movement within the length of the outer shell 23. The circular motion of the common hollow shaft 8 is further guided by another bearing 27. This bearing 27 can be a deep groove ball bearing if the load to carry and spin in low. In cases where heavier load is expected, two bearings can be used. The position of this bearing is constrained by a collar 31 secure by set screw and the hub 13. In our preferred embodiment, the single bearing configuration is used. Mounted further upstream to the common hollow shaft 8 is a circular scale 12 which is turn mounted to a hub 13 and secured onto the common hollow shaft by set screw. The circular scale 12 consists of lines etch onto a substrate (commonly glass for high precision application). The rotation of the common hollow shaft 8 can now be measured directly by a rotary encoder 11, which transform the reflected signal returned from the circular scale 12 into pulses and feedback these signal to the driver or controller. These feedback pulses are decoded into angular positions, thus making precise angular positioning possible. Thus a complete description of the hollow shaft slot-less rotary motor is given.

Mounted even further upstream is a set of linear coil 16. The coil 16 is done by winding copper wires layer by layer concentrically onto a circular aluminum drum 17. Mounted to the drum 17 is bearing 27. The bearing 27 is mounted in such a way that the inner race 27a rotates with the common hollow shaft 8. The outer race 27b does not rotate but is force fit into the drum and further retained in position by a clip 29. Mounted to the drum 17, is a L- bracket 14. On the L-bracket 14, the rotary encoder 11 is mounted via a mounting bracket 15. Mounted onto the L-bracket 14 is a runner block 19. The runner block 19 when sliding on the guide rail 2, allows only linear motion, and prevented the drum 17, and the L-bracket 14, the rotary encoder 11, mounting bracket 15 from rotating. This non-rotating properties of the drum 17 and rotary encoder 11 are very desirable. It enables non-stop all round rotation of the common hollow shaft 8 without having to worried about entanglement of electrical cables. Referring to Fig. 4, it shows the main base 1 of the actuator 100. On this base 1, the linear guide rail 2 is mounted. Also mounted on the base 1, are two limit sensors 3a and 3b. These two sensors when connected to the controller (not shown) or driver (not shown) has the ability to decide what to do when the limits are triggered by the magnets 20a and 20b. The magnet 20a is positioned to activate the limit sensor 3a when the common hollow shaft 8 reaches the upper limit position as shown in Fig 3 a.

Similarly the magnet 20b is positioned to activate the limit sensor 3b when the common hollow shaft 8 reaches the upper limit position as shown in Fig 3b. Mounted to the rear of the base 1, is the core mounting 4 for the linear voice coil. The core mounting 4 houses the core 5. A through hole circular permanent magnet 6 is attached concentrically inside the core. On top of the magnet 6, a hollow circular flux plate 7 is attached. The magnet 6, flux plate 7, coil 16 and the core 5 forms a close magnetic circuit. When a current is passed through the wire in the coil 16, the section which is at the portion where the magnetic flux cross from the flux plate 7 to the into the core 5, the interaction between the current and magnetic flus produces a force. This force is linearly guided by the linear guide rail 2 and runner block 19 to produce the second independent motion. This independent motion is possible because the inner race of the bearing 27a rotates with the common hollow shaft 8. The outer race of the bearing 27b which is fixed to the drum 17 is prevented from rotating but is allow to move linearly by the linear guide rail 2 and runner block 19. There is however, a clearance between the common hollow shaft 8 and the drum 17, the flux plate 7, the magnet 6, the core 5 and the core mounting 4. Mounted on the base 1 is a second encoder 28. It is mounted below a linear scale 18 which moves with the drum 17.

The linear scale 18 consists of lines etch onto a substrate (commonly glass for high precision application). The linear movement of the common hollow shaft 8 can now be measured directly by a linear encoder 28, which transform the reflected signal return from the linear scale 18 into pulses and feedback these signal to the driver or controller. These feedback pulses are decoded into linear positions, thus making precise linear positioning possible. Thus a complete description of the invention, being an in-line hollow shaft direct drive independently driven linear-rotary servo actuator, is given. ADVANTAGEOUS EFFECTS OF THE INVENTION

A linear voice coil motor uses is lighter in weight and therefore more efficient. Furthermore, the improved linear-rotary servo actuator costs less to manufacture because it uses fewer magnets.

Although the improved linear-rotary servo actuator produces two motions (linear and rotary) which are independent,

The linear voice coil drives the common shaft directly without any couplings or transmission like gears, belt, etc., and its linear position is measured directly by a high resolution optical encoder and linear scale assembly. The rotary slot-less motor also drives this same shaft directly and its angular position is also measured directly by a high resolution optical encoder and rotary scale assembly. These direct drive techniques coupled with direct measurement techniques allows the invention to manipulate objects with high speed, short response time, high bandwidth, and high precision.

Claims

1. An improved in-line hollow shaft direct drive independent driven linear-rotary servo actuator with a set of coils in the linear motor and rotary motor sections sharing a common shaft within the same housing, the set of coils in the linear motor section and rotary motor section capable of being independently energized to realize two independent motions, one linear and one rotary, the rotary motion produced by a rotary slot-less motor section and the linear motion produced by a linear motor portion which is a linear voice coil motor.

2. An improved linear-rotary servo actuator as claimed in Claim 1 wherein the linear voice coil motor portion is not stacked onto the rotary slot-less motor portion , and vice versa.

3. An improved linear-rotary servo actuator as claimed in Claim 1 wherein the common shaft shared by both linear voice coil portion and rotary slot-less motor portion has two ends, one end supported by a bottom bearing, and at the other end by a top bearing assembly integrated into the linear voice coil motor.

4. An improved linear-rotary servo actuator as claimed in Claim 1 wherein the two motions (linear and rotary) are independent with the common shaft rotating about the inner race of a top bearing assembly integrated into the coil of the linear voice coil motor, but the outer race of the top bearing assembly integrated into the coil of the voice coil motor is prevented from rotating.

5. An improved linear-rotary servo actuator as claimed in Claim 1 having a runner block and a guide rail which prevents the coil of the linear voice coil motor from rotating.