Field of the Invention
The invention relates to a linear actuator of a motion
system for driving an actuation arm, and in particular to a linear actuator that
is fitted with a velocity sensor for monitoring the velocity of the actuation arm
that is being driven.
Background and Prior Art
Ejector systems are used in the semiconductor industry
for a variety of purposes. One use is in the area of automatically transferring
a semiconductor substrate onto a work-holder for processing. For example, such an
ejector system is used to automatically transfer substrates from their containers
to wire bonding machines that make wire connections between electronic components
attached on the substrate and electrical connection pads on the substrate.
More specifically, in a typical wire bonding machine, an
ejector system is used to eject substrates (commonly in the form of leadframes)
from a magazine onto a work-holder channel. The substrates are pushed one at a time
onto the work-holder and held in position by clamping devices. Thereafter, a bonding
tool performs wire bonding on the substrates on the work-holder while the substrates
are being held by the clamping devices. An indexing system associated with the clamping
device is responsible for precisely indexing the substrate and presenting it to
the bonding tool. The bonding tool makes interconnections between the connection
pads (such as leads) of the substrate and connection pads on the semiconductor device
mounted on the substrate.
Conventionally, the ejector system comprises an ejector
arm that is driven by a traction drive using a DC motor. A prior art ejector system
100 is illustrated in FIG. 1. The ejector system 100 has an ejector arm 102 to push
a leadframe 104 into a work-holder channel (not shown). The ejector arm 102 is driven
towards the leadframe 104 by frictional force from rubber rollers 106 pressing on
the ejector arm 102. The rubber rollers 106 are mounted directly on a motor shaft
108 of a geared DC motor (not shown). Deep-grooved ball bearings 110 are positioned
along the route of motion of the ejector arm 102 to provide smooth and precise linear
motion to it. At least one of the ball bearings 110 is mounted on a preload arm
114. A spring 112 is coupled to the preload arm 114 to apply a certain preload on
the preload arm 114 and the at least one ball bearing 110 to maintain steady and
stable movement of the ejector arm 102. System initialization and motion control
are achieved through a home sensor 116 and a limit sensor 118 positioned adjacent
to the ejector arm 110.
Due to variation of friction between the leadframe 104
and the magazine or misalignment between the leadframe 104 and the work-holder channel,
it is possible for jamming of the leadframe 104 in the magazine or work-holder channel
to occur. Thus, a jam detecting sensor 120 is placed just above the leadframe 104
to sense any lateral deformation of the leadframe 104 that would occur during jamming
of the leadframe 104. Jam detection is important because semiconductor substrates
are generally fragile and vulnerable, and excessive external forces acting on their
components during a jam can result in damage.
There are certain drawbacks in this conventional design.
Since it adopts a traction drive mechanism, its reliability and lifespan are limited
by premature wear. Also, the thrust force is generally non-programmable and uncontrollable
for different types of leadframes. A similar thrust force would therefore lead to
different impacts on leadframes that comprise different materials, thicknesses and
widths. Even with stringent control over manufacturing tolerance and assembly, force
consistency is difficult to achieve for the above reason. Further, jam detection
and protection capability depends considerably on the sensitivity of the jam detecting
sensor 120. Especially for thick leadframes, the deformation in the leadframes may
be small, making it very difficult to sense deformations in the leadframes if they
are jammed. Finally, this jam sensing module entails significant additional cost,
which is likely to be even higher than the cost of the ejector system itself.
Summary of the Invention
It is therefore an object of the invention to provide an
improved linear actuator that avoids some of the disadvantages of the prior art
linear actuator systems, and offers a more effective and cost-efficient mechanism
that allows jam detection.
Accordingly, the invention provides a linear actuator for
driving an actuation arm comprising: a magnetic field; a first coil that is controllably
movable within the magnetic field by adjusting a current flowing through the first
coil whereby to drive the actuation arm; a second coil that is movable with the
first coil and configured such that a voltage is inducible in the second coil that
is proportional to its velocity; and a controller device connected to the second
coil for receiving feedback indicating the voltage induced in the second coil whereby
a velocity of the actuation arm is determinable, wherein the controller device is
further connected to the first coil and is operative to adjust the current flowing
in the first coil based upon said feedback for controllably driving the actuation
It would be convenient hereinafter to describe the invention
in greater detail by reference to the accompanying drawings which illustrate one
embodiment of the invention. The particularity of the drawings and the related description
is not to be understood as superseding the generality of the broad identification
of the invention as defined by the claims.
Brief Description of the Drawings
An example of a preferred embodiment of a linear actuator
in accordance with the invention incorporated in an ejector system will now be described
with reference to the accompanying drawings, in which:
Detailed Description of the Preferred Embodiment of the Invention
- FIG. 1 is a schematic side view of a prior art ejector system;
- FIG. 2 is an isometric view of an ejector system comprising a linear actuator
in accordance with the preferred embodiment of the invention;
- FIG. 3 is a schematic cross-sectional side view of the ejector system looking
along sectional line A-A of FIG. 2;
- FIG. 4 is a schematic cross-sectional plan view of the ejector system looking
along sectional line B-B of FIG. 2;
- FIG. 5 is a schematic control diagram showing a control system for the linear
- FIG. 6 is a graphical illustration of a jam detection and leadframe protection
process of the ejector system of FIG. 2.
The preferred embodiment of the present invention will
be described with reference to FIGs. 2 and 3 in relation to an ejector system, although
it should be appreciated that the invention is also applicable to other actuation
systems. FIG. 2 is an isometric view of an ejector system 10 comprising a linear
actuator in accordance with the preferred embodiment of the invention. FIG. 3 is
a schematic cross-sectional side view of the ejector system looking along sectional
line A-A of FIG. 2.
The ejector system 10 includes an actuation arm in the
form of an ejector arm 12 for pushing an object, such as a leadframe 14. The ejector
arm 12 is mounted onto a housing 16 and is configured to be slidable with respect
thereto. The housing 16 houses components for driving the ejector arm 12 and for
sensing a velocity at which the ejector arm 12 is driven.
The main component for driving the ejector arm 12 consists
of a voice coil motor (VCM), which comprises a first coil, such as a movable motor
coil 30, and magnets 36 that are arranged about a central yoke 18 that is oriented
parallel to a direction of motion of the motor coil 30. The magnets 36 and central
yoke 18 produce a magnetic field such that the motor coil 30 is controllably movable
within the magnetic field by adjusting a current flowing through the motor coil
30. The ejector arm 12 is attached rigidly to the movable motor coil 30 via a bracket
26 such that movement of the motor coil 28 will drive the ejector arm 12 to move
with respect to the housing 16.
A velocity sensor is also coupled to the motor coil 30
and bracket 26. The velocity sensor preferably comprises at least a second coil,
such as a sensor coil 28, and a third coil, such as a compensation coil 32, which
are arranged to interact electromagnetically with the magnets 36. The sensor coil
28 is movable with the motor coil 30 and is configured such that a voltage is inducible
in the sensor coil 28 that is nominally proportional to its velocity. The sensor
coil 28 may be disposed in the same magnetic field as the motor coil 30 for electromagnetic
interaction, but it would also be disposed in a separate magnetic field by locating
it away from the motor coil 30. In the former set-up, the sensor coil 28 is preferably
wound on top of the motor coil 30, as in the present embodiment.
The motor coil 30, sensor coil 28 and bracket 26 are movable
as one unit relative to the central yoke 18. To facilitate movement by these components
relative to the central yoke 18, the bracket 26 includes linear bearings that are
mounted onto two shafts 20, 22 which are arranged to extend in directions parallel
to the central yoke 18, and therefore to a direction of motion of the motor coil
30. The linear bearings 24 allow the bracket 26 to be slidable with respect to the
shafts 20, 22 and help to provide smooth and precise linear motion to the bracket
The ejector arm 12 initially rests at a home or retracted
position. When activated, it is extended from the housing 16 in the direction of
the leadframe 14 to push the leadframe 14 until it reaches a predetermined extension
limit. A home sensor 34 and a limit sensor 35 positioned adjacent to the ejector
arm 12 detect reference indicia formed on the ejector arm 12 so as to determine
a position of the ejector arm 12, especially to determine when a fully retracted
position and a fully extended position respectively are reached. The home and limit
sensors 34, 35 are useful for system initialization and motion identification.
When the VCM (comprising the motor coil 30 and magnets
36) actuates the ejector arm 12 to push the leadframe 14, the velocity sensor (comprising
the sensor coil 28, compensation coil 32 and magnets 36) senses the speed and direction
of the motion. Electrical signals relating to the speed and direction of motion
of the ejector arm 12 are continuously monitored to establish a velocity-based closed
loop control to ensure that the ejector system 10 tracks a prespecified velocity
profile. The velocity profile is field-programmable to suit different leadframes
14. Any undesired disturbance to the moving leadframe 14 results in a change of
motor current, as will be explained in further detail below. Once the current exceeds
a predefined threshold, the ejector system 10 may determine that movement of the
ejector arm 12 has been obstructed and a leadframe jam has been encountered. Any
current that exceeds the predefined threshold during acceleration and deceleration
is programmed to be as short as possible so that it may be simply excluded when
determining whether there has been a leadframe jam.
FIG. 4 is a schematic cross-sectional plan view of the
ejector system 10 looking along sectional line B-B of FIG. 2. For simplicity, the
housing 16 of the ejector system 10 is not shown. The drawing shows a cross-sectional
view of a VCM configuration that is adopted with due regard to the maximum travel
requirement in the present application. It should be appreciated that the VCM configuration
may vary with different travel requirements.
FIG. 4 shows a pair of rectangular box magnets 36 mounted
on back yokes 38 that are arranged with the indicated polarities. The back yokes
38 are affixed symmetrically on opposite sides of the central yoke 18 for mounting
the magnets 36. This arrangement of the back yokes 38 and the central yoke 18 in
between generates uniform magnetic fields 37 of nominally equal intensities between
each magnet 36 and the central yoke 18, so that the respective motor coil 30 and
sensor coil 28 experience a uniform magnetic field intensity while moving within
the magnetic fields.
When the motor coil 30 of the VCM is energized by an electric
current, a Lorentz force directly proportional to the size of the electric current
is generated in a direction nominally parallel to and co-linear with a longitudinal
axis 19 of the central yoke 18. Reversing the direction of the electric current
in the motor coil 30 reverses the direction of the Lorentz force.
The velocity sensor preferably uses the same magnetic as
well as mechanical configuration as the VCM. The sensor coil 28 is directly wound
on top of the motor coil 30 and moves together with the motor coil 30. Therefore,
when motion of the motor coil 30 is actuated by the VCM, a back-EMF is induced in
the sensor coil 28, which is directly proportional to the velocity of the motion.
This allows the velocity of motion of the ejector arm 12 to be determined.
At the same time, an undesirable transformer voltage is
also induced in the sensor coil 28 on account of its close magnetic coupling with
the motor coil 30, especially when the motor coil 30 is energized by a chopping
current. Thus, the compensation coil 32 has been incorporated in the preferred embodiment
of the invention to minimize the electromagnetic disturbances from the motor coil
30. As shown in FIG. 3 and FIG. 4, the compensation coil 32 is stationary and is
preferably wound on the central yoke 18. In this embodiment, the compensation coil
32 is disposed in the same magnetic field as the sensor coil 28 and motor coil 30.
It is electrically connected with the sensor coil 28 in anti-series so that transformer
voltages induced in both the sensor coil 28 and the compensation coil 32 cancel
out each other. Full cancellation of the induced voltages is hard to achieve due
to variations in the magnetic field induced by the motor coil 30 and the saturation
level in the central yoke 18 along the travel path of the motor coil 32. Incorporation
of a low pass filter is therefore desirable to suppress spikes and extract only
the usable velocity information that is detected.
FIG. 5 is a schematic control diagram showing a control
system for the linear actuator. The control system generally comprises a controller
device, which may include a microcontroller 40 and a proportional-integral (PI)
controller 42, a voice coil motor module 44 and a velocity sensor module 46. Inputs
that are received by the microcontroller 40 include a trigger input 48, home sensor
input 50, limit sensor input 52, velocity setting input 54 and jam threshold input
56. A communications port 58 may also be included to establish communications between
the microcontroller 40 and other devices.
The controller device is connected to the sensor coil 28,
as represented by the velocity sensor module 46, for receiving feedback Vfeedback
indicating a voltage induced in the sensor coil 28 whereby the velocity of the ejector
arm 12 is determinable. The controller device is further connected to the motor
coil 30, as represented by the voice coil motor module 44, and is operative to adjust
the current flowing in the motor coil 30 through current signals Icmd(t)
based upon said feedback for controllably driving the ejector arm 12 according to
a predetermined reference velocity profile that is preset.
According to the preset reference velocity profile, a velocity
command Vcmd(t) is sent by the microcontroller 40 to the Pl controller
42, which in turn sends a current signal lcmd(t) to initiate the voice
coil motor module 44 to drive the VCM. As the ejector arm 12 is driven, the velocity
sensor module 46 senses the velocity at which the ejector arm 12 moves. A velocity
loop is facilitated by the velocity sensor module 46 sending velocity feedback Vfeedback
in the form of the voltage induced in the sensor coil 28 to the microcontroller
40 to track the reference velocity command Vcmd(t) and maintain the specified
velocity profile. Motor current that is sensed, Ifeedback, is also fed
back to the microcontroller 40 for jam detection. Velocity fluctuations should result
in quick changes to the motor current signal lcmd(t) to correct the velocity
error. Once the detected motor current lfeedback exceeds a certain value,
leadframe jam is presumed. However, since it is likely that the current feedback
signal will exceed the predefined threshold during acceleration and deceleration
of the ejector arm 12, the control system automatically pre-empts and excludes jam
triggers during acceleration and deceleration comprising current increases that
are of comparatively short durations. The predefined reference velocity and jam
thresholds are preferably programmable to cater for different situations.
FIG. 6 is a graphical illustration of a jam detection and
leadframe protection process of the ejector system 10 of FIG. 2. The graphs include
a reference velocity graph 60, a velocity sensor output graph 62 and detected motor
current graph 64. The values shown are negative due to a relative driving direction
initiated by the VCM, but the values may also be positive, the primary reference
being the absolute values. The relative driving direction does not affect implementation
of the preferred embodiment.
The ejector system 10 preferably operates in a controlled
velocity loop with a trapezoidal reference velocity profile through control by the
controller device of the current flowing through the motor coil 30, and it tries
to maintain this profile during its operation. When no jam occurs as in Fig 6(a),
the cruising motor current is small (about 240mA) since the moving components only
need to overcome friction during constant velocity motion. Troughs in the velocity
graphs 60, 62 show when the ejector arm 12 is traveling at constant velocity in
the direction of the leadframe 14. Peaks and troughs in the motor current graph
64 represent current increases to accelerate and decelerate the ejector arm 12.
FIG. 6(b) includes an illustration of a jam profile 66
between times t1 and t2. Once there is an obstruction to movement
of the ejector arm 12 and a jam is encountered, the absolute current in the motor
current graph 64 increases rapidly (to about 1.6A), which exceeds a predefined protection
threshold which is set according to the characteristics of the leadframe 14 in use.
The motor current exceeds the predefined protection threshold for the duration t2-t1.
This duration is longer than that which is expected during acceleration and deceleration
of the ejector arm 12. As a result, the system determines that there has been a
leadframe jam. Accordingly, the ejector system 10 can be stopped and remedial action
Alternatively, since the velocity sensed by the sensor
coil 28 is significantly reduced when movement of the ejector arm 12 is obstructed,
the ejector system 10 can be programmed such that the controller device is configured
to detect when a velocity of the ejector arm 12 is below a predetermined threshold
for a certain period of time as illustrated in the velocity graph 60 between t1
and t2. If so, this may also indicate an obstruction to movement of the
ejector arm 12 and appropriate remedial action can be taken.
The leadframe ejector system 10 using the linear actuator
comprising a built-in velocity sensor as described in the above embodiment of the
invention enables both the VCM and velocity sensor to share similar magnetic and
mechanical components. A compensation scheme has also been implemented to enhance
the sensing quality of the velocity sensor, which is very important to automatic
jam detection and leadframe protection. With this design, automatic leadframe jam
detection can be achieved without having to implement an external feedback device,
which would increase costs significantly. Since the current threshold is software-programmable,
the system can cater to different leadframes in use.
Tests using the described embodiment show excellent velocity
characteristics of the velocity sensor as well as adequately sensitive and reliable
jam detection and leadframe protection capabilities for different types of leadframes.
The invention described herein is susceptible to variations,
modifications and/or additions other than those specifically described and it is
to be understood that the invention includes all such variations, modifications
and/or additions which fall within the spirit and scope of the above description,