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(Click on the following titles to see detail.)
- 1. Linear motor vs. Rotary motor
- Schematic diagram:
The operation principle of the linear motor and the rotary motor are
almost the same. A linear motor can be realized by cutting a rotary motor
to the center of the shaft and flat it off as shown in the following figure.
So, there is always a counterpart for linear motor as
compared to the rotary motor. For example, we have an induction rotary motor, then
a linear induction motor will exist. Similarly, the brushless rotary motor has a
counterpart of the brushless linear motor. Here, we do not want to explain the
detailed operation principle of various kinds of motors, which can be
found in many text books, instead, we point out the main difference
between linear and rotary motors
--- the end effect , a considerable side effect.
- End effect:
The most different feature between a rotary and a linear
motor is the end effect. In the following figure, you can see that there is no end along the rotated direction in a rotary motor. However, obviously, a linear motor mover has two
ends along the moving direction.
Such a end will produce complicated phenomena including
the considerable cogging force due to reluctance variation, the deformation of
magnetic field at high speed...etc. As a consequence, the design of the linear
motor and the control algorithm of the motor drive should take this into account to suppress
these side effects, especially when accurate and fast positioning is
concerned.
- 2. Linear motor stage vs. Ball screw stage
- The following table list the comparison of the
linear motor stage and the ball screw stage.
| |
Linear motor stage |
Ball screw stage |
| Advantage |
Easy to operate at
high speed and high acceleration
Less
maintenance even at high speed and
high acceleration operations
Easy to achieve
fast and accurate
positioning because of direct
drive
Higher
dynamic stiffness and servo bandwidth
Stroke
is no limit |
Thrust
is larger under a given limitation of stage size
Price
is lower because of mass production
|
| Weakness |
Thrust
is lower under a given limitation of stage size
Price
is higher
Application
is difficult (?) |
Speed and
acceleration is limited
Frequent
maintenance is necessary if operated at high speed and
acceleration because of wearing
Backlash lowers
the dynamic stiffness and complicates the positioning control
Stroke
is limited |
What we'd like to say here is the linear motor stage,
although, has higher price currently because it has not been mass production yet. However,
considering the excellent performance that upgrades your machine and the low
maintenance that cuts your long-term cost, it is worthy to employ linear motor
stages in your machine as soon as possible. As to the application, we think the
most suitable machine to work with linear motor is the one with the
demands of high speed , high accuracy, and low external payload, such as pick and place applications.
- 4. Important issues on designing a good linear motor system
- Cogging force
Due to the end effect and slot-teeth alignment, the
brushless iron-core linear motor has considerable cogging force. Such a
force must be minimized to under 2% of rated thrust so that the short
settling time of positioning and smoothness of velocity profile can
be achieved. The ways to reduce cogging force include skewing the magnet
with respect to winding core, using large least common divider of
pole and slot number, employing field compensation technique in drive
control algorithm. As a consequence, a good linear motor for fast and
precision positioning must have low cogging force. You can use hand to
move the slide table to feel the extent of cogging force. A good design
should lead to a smooth feeling.
Force ripple
This term is different from the above cogging force, but it has the
similar outlook as compared to the cogging force. This force is mainly
due to the commutation of 3 phase motor current and the non-perfect sine
distribution of the magnetic field. It will affect the fast positioning
capability and the low-speed performance. Note that the epoxy-core
linear motor also has this drawback even without cogging force. This
term only can be measured by the special test facility. The maker should
perform this test to verify the quality of the motion stage.
- Ratio of thrust to moving mass
The ratio of thrust to moving mass determines the
load capacity of a linear motor. Less moving mass means more capacity
for external load. In addition, the reaction force due to moving mass
under high acceleration and deceleration will results in considerable
vibration to your machine and may lead to unpredictable resonance. So, a
good linear motor stage must keep its moving mass as small as possible.
- Decoupled mechanism
In some circumstance, we need to move 2 axes simultaneously.
Most of product just mount one axis on the other axis, which leads to
unequal bandwidth of motion. For example, if X axis is mounted on the Y axis, the motor of
X axis only need to move the moving part of itself, but the motor of Y
axis not only move the moving part of itself but the whole X axis stage. Such a scheme is called "stacked type" XY stage
(Our module: XYS). In
contrast, the equal bandwidth of motion can
be achieved by employing decoupled mechanism. The motor of each axis
only need to move the moving part of itself plus a light weight of
common slide (See our products of [XYD],
[YZD]).
- Duty cycle
The value of duty cycle is very important in
determining an adequate rated power for a linear motor. In most cases, you
will not run the linear motor all the time, instead, the linear motor
will stop moving for a period of time and wait for other
activities such as vision calibration or the movement of other axes.
Note that the size of the linear motor is related directly to its rated
power not maximal power. So, you need to determine the duty cycle or
specify the motion profile carefully for your application, otherwise, the linear motor will be over
sized or over
heated.
- Moving cables
The moving cable is the critical issue on the life
cycle of a linear motor stage in addition to the linear guide for suspension.
A good linear motor stage must make its number of moving cables as few
as possible. In the case that stroke is smaller than the length of moving
parts, we can eliminate all the moving cables by employing moving magnet
scheme and stationary encoder head. This is important when the stage is
operated frequently under high acceleration and deceleration.
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