Load-control-type actuator

Details Load-control-type actuator Load-control-type actuator

2001-06-18

2003-06-24

Le, N. (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

C324S760020

Reexamination Certificate

active

06583637

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a load-control-type actuator for use in handling equipment for mounting and assembling components and measuring and sorting equipment for measuring electrical characteristics of electronic components.
2. Description of the Related Art
Hitherto, when mounting small-size electronic components on a circuit board, component assembling apparatuses using voice-coil motors have been widely used. The voice-coil motor is excellent in positioning accuracies more than those of mounters using pneumatic power and solenoids and is also excellent in reducing a force applied to components.
FIG. 1
shows an example of a conventional handling apparatus using such a voice-coil motor. A movable part
120
of a Z-axis-drive mechanism is driven by a voice-coil motor in the vertical direction. A nozzle
121
is arranged at lower end of the movable part
120
and a built-in spring
122
is arranged interposing between the movable part
120
and the nozzle
121
. In the nozzle
121
, a sucking port
121
a
is formed, which is communicated with a vacuum sucking device (not shown) via a sucking port
120
a
of the movable part
120
a
. A workpiece W is thereby held at tip end of the nozzle
121
. On the external periphery of the nozzle
121
, a flange
121
b
is unitarily formed, which is downwardly urged by a spring
122
in touch onto a receiving portion
120
b
formed at the lower end of the movable part
120
and the flange
121
b
is held thereto.
An initial load is applied to the spring
122
in order to hold the nozzle
121
when the movable part
120
moves vertically as described above. Accordingly, an impact load produced when the nozzle
121
touches the workpiece W or when the nozzle
121
having the workpiece W held thereon touches an object (circuit board, etc.) is certainly lager than the initial load, product defects such as cracks and chips are caused during handling or mounting small-size and thin elements.
Such problems are not limited to handling equipment; in measuring and sorting equipment, for example, the problem also rises when a measuring probe is urged onto an electronic component so as to measure electrical characteristics of the electronic component.
In component-assembling equipment having the sucking nozzle
121
, when air leakage is generated due to non-existence of a sucked component or disagreement in sucking, a sucking force by air and the initial load of the spring
122
are out of balance so that the initial load becomes larger causing damages of workpieces during the handling. Furthermore, since the sucking port
121
a
is released when a workpiece W is to be picked up by sucking, the initial load does not become large so much; however, when placing the workpiece W on a circuit board, etc., after picking it up, because the sucking port
121
a
of the nozzle
121
is closed, the nozzle
121
is pulled toward the movable part
120
so that the initial load becomes larger. Therefore, there is a problem that the load applied to the workpiece W during the placing is larger than during the picking.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a load-control-type actuator in which an impact load applied to a workpiece can be reduced so as to prevent damages in the workpiece.
In order to achieve the above-mentioned object, a load-control-type actuator is provided, which comprises a contact part which comes into contact with a workpiece, a movable part for supporting the contact part via an elastic member, and operating means which can control a thrust driving force to the movable part, wherein provided that F
1max
denotes the maximum impact load at a time of contact between the workpiece and the contact part and F
2
denotes a spring reaction force of the elastic member within a response time t
k
of the actuator, a spring constant k
b
of the elastic member is set to satisfy the condition: F
1max
≦F
2
.
A component assembling apparatus having a sucking nozzle for picking up and placing components will be described as an example. In the sucking nozzle having a spring built therein, the impact force to a workpiece is broken down into three forces shown in FIG.
2
.
A first force is an initial load (A); a second force is an impact load (B) due to rigidity parameter of a colliding object; and a third force is a load (C) due to a reaction spring force of a built-in spring.
From
FIG. 2
, it is understood that when the initial load (A) included in a conventional apparatus is eliminated, the entire impact load can be reduced.
However, since the initial load (A) is for holding the nozzle to be stable, a spring having high (hard) rigidity has to be built-in in order to have the zero initial load. When the rigidity is too high, however, since the load (C) due to a reaction spring force of a built-in spring shown in
FIG. 2
influences thereon significantly, the spring constant thereof needs to be suitably designed.
Therefore, according to the present invention, the spring constant k
b
of the elastic member is set so that the spring reaction force F
2
of the elastic member within the response time t
k
of the actuator is reduced smaller than the maximum impact load F
1max
between the workpiece and the contact part.
Thereby, the impact load applied to the workpiece is reduced enabling damages in the workpiece to be reduced small. Moreover, since the contact part can be held to be stable while reducing the initial load to be small, the movable part can be operated in a high speed.
The spring constant k
b
of the elastic member can be obtained from the equation:
F
2
=k
b
e
−&egr;t
V
0
/q
sin(
qt
),
provided that 0≦t≦t
k
, and the equations:
e=c
b
/2
m
v
, q=[&ohgr;
2
−&egr;
2
]
½
, and &ohgr;
2
=k
b
/m
v
,
wherein m
v
is the mass of the movable part and c
b
is the spring viscosity.
The maximum value of the spring constant k
b
of the elastic member can be thereby calculated.
When the operating means is formed by a voice-coil motor, it is preferable that the response time t
k
range from 0.001 to 0.1 second.
A voice-coil motor is a type of a linear actuator and produces a thrust driving force in proportion to a current. When a current I is passed through a voice coil from a controller via a current amplifier, a force of F=B I L is produced in the coil by a magnetic flux density B of a magnetic circuit and a length L of a coil conductor. By the force F, the nozzle (movable part) connected to the coil via a connecting mechanism is moved. When a nozzle mechanism including the coil is lowered for handling the workpiece, for example, the nozzle can be precisely positioned (below 0.1 mm) based on a signal of a positional sensor. As described above, the impact load to the workpiece is suppressed by controlling a collision velocity and by controlling a current after collision, a precise static load is applied when approaching the workpiece.
In addition, when using the voice-coil motor, the practical range of the response time t
k
is approximately from 0.001 to 0.1 second.
Preferably, the operating means sequentially performs positional control so that the movable part is advanced to a position immediately before the contact part touches a workpiece, velocity control so that the movable part is advanced at a constant velocity from the position immediately before the contact to the contact position at which the contact part touches the workpiece, load control so that the movable part is further advanced while controlling the contact pressure between the contact part and the workpiece after the contact part touches the workpiece, and positional control so that the movable part is retracted so as to separate the contact part from the workpiece.
By controlling the operating means sequentially in such a manner, a load applied to the workpiece is reduced, enabling the high-speed operation to be performed with a high degree of accuracy.
The spring constant k
b
of the elastic member may prefe

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