Electricity: motive power systems – Plural – diverse or diversely controlled electric motors – Plural linear-movement motors
Reexamination Certificate
2000-04-20
2001-08-28
Enad, Elvin (Department: 2834)
Electricity: motive power systems
Plural, diverse or diversely controlled electric motors
Plural linear-movement motors
C310S012060
Reexamination Certificate
active
06281644
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a combined linear motor in which a plurality of linear motors are operated in parallel, and in particular to a combined linear motor for driving a moving table of a machine tool.
2. Description of the Related Art
FIG. 4
shows a conventional combined linear motor for driving a moving table of a machine tool. A moving table here is defined as a table of a machine tool, to which a spindle or a tool rest of the machine tool is attached. In the following, a device having three linear motors operating in parallel will be taken as an example of a conventional linear motor.
The conventional device comprises a machine tool part and an NC control part. The machine tool part comprises a base
5
fixedly attached by a plurality of table stoppers
1
for limiting strokes of the moving table, linear guide rails
2
, a position detector
3
, and linear motor stators
4
a
,
4
b
,
4
c
, and a moving table
9
attached by a linear guide
6
, a position detector reader
7
, and linear motor movers
8
a
,
8
b
,
8
c
. The NC control part comprises an NC device
10
, a servo control section
11
, a current distribution operating circuit
12
, and current amplifiers
13
a
,
13
b
,
13
c
. In this drawing, only the outline of the moving table
9
is shown for explanation of installation parts beneath the moving table.
In each of the three linear motors employed in the above conventional example, a stator and a mover consist of a magnet and a wire, respectively. Alternatively, other combinations may be possible for a stator and a mover, such as wires and magnets, wires and wires, and so on.
The moving table
9
is moved under movement control, which is achieved in cooperation with the NC device
10
, the servo controller
11
, the current distribution operating circuit
12
, and the current amplifiers
13
a
,
13
b
,
13
c
. The NC device
10
outputs a position command. The servo controller
11
calculates and outputs a current command based on the position command from the NC device
10
and data on a present position supplied from the position detection reader
7
. The current distribution operating circuit
12
outputs current distribution commands for the respective motors operating in different phases, based on the data on the present position and the current command. The current amplifiers
13
a
,
13
b
,
13
c
output currents having respective phases based on the current distribution commands, to the movers
8
a
,
8
b
,
8
c
of the linear motors.
Instead of the three linear motors employed in the above conventional art for parallel driving, a fourth or more motor/motors may be provided in response to a demand for a larger thrust. Specifically, addition of a linear motor mover and stator and a current amplifier, which together constitute a fourth linear motor, could easily attain a table moving mechanism with a larger thrust.
In any case, in this conventional device, a single moving table
9
is driven by two or more linear motors, which are driven in parallel at the same time.
When a linear motor is used as a driving source of a moving device of a machine tool, cogging trust of the linear motor may affect machining accuracy. Therefore, as much cogging trust as possible must be suppressed especially in precise or highly accurate machining. Note that “cogging thrust” here corresponds to cogging torque with a rotary motor.
However, generally, suppression of cogging thrust is subject to limits as circular arrangement of movers is not allowed with a linear motor, of which strokes are inevitably limited due to the structure thereof, in other words, due to the existence of both ends of the motor all the time.
In particular, when two or more linear motors are driven in parallel to attain high thrust according to conventional arts described above, phases of cogging thrust of the respective linear motors are made coincident with one another. This may cause a problem of an increase in cogging trust of the linear motors in proportion to the number of linear motors in use.
SUMMARY OF THE INVENTION
In order to address the above problem, according to the present invention, there is provided a combined linear motor for simultaneously driving a plurality of linear motors in parallel to thereby drive a moving body, in which the plurality of linear motors are divided into groups of i in number, and a stator or a mover in each group is arranged such that its phase is stepwisely displaced from that in another group by 360°/(2*i)°.
Also, a combined linear motor of the present invention comprises a single position detector for detecting a position of the moving body, and a current distribution operating circuit provided for each group of linear motors. Each current distribution operating circuit calculates optimum supply current for a corresponding linear motor, based on the position detection information supplied from the position detector and displaced angels of the respective linear motors.
Further, in a combined linear motor (a linear motor driving device) comprising a guiding means and N (N: a natural number) units of linear motors, the N units of linear motors are divided into i (i: a natural number equal to or more than 2, or i≧2) groups, each group including j (j: a natural number) units of linear motors, holding the relationship N=I*j. A displacement angle &psgr;(k) of a motor electrical angle of a linear motor/motors in the k-th group (k: a natural number wherein 1≦k≦i) in the i groups, relative to motor electrical angle of a linear motor/motors in the first group, an be obtained from the following expression (1). Note that the motor electrical angle of the linear motor/motors in the first group an be obtained with k=1.
&psgr;(
k
)=360*(
k−
1)/(2
*i
)° (1)
By installing the respective linear motors such that their motor electrical angles are displaced by the calculated displaced angle &psgr;(k), cogging thrust associated with the relative linear motors can be offset as a whole.
REFERENCES:
patent: 4853602 (1989-08-01), Hommes et al.
Eba Koji
Kitagawa Katsuyoshi
Komatsu Toshiteru
Miyamoto Yasuhiro
Nagase Takashi
Enad Elvin
Jones Judson H.
Okuma Corporation
Oliff & Berridg,e PLC
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