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A clear understanding of specification definitions
is the most fundamental requirement to select a suitable linear motor
stage for your machine. Too severe specification requirement leads to
unnecessary high cost, while too loose one results in the failure of your
project. Hence, it is worthy to spend a little bit of time to read thru
whole the material before you go further ahead.
There are three parts of definitions of specifications
you need to figure out, namely, the definitions of the stage specifications,
the definitions of the linear-motor specifications, and the definition of
dynamic positioning.
(Click on the following titles to see detail.)
- Definitions of the
stage specifications
- Effective stroke
This term means the distance available for effective
positioning. It should exclude the length for over-travel limit switch
at the two ends of motion stage.
- Resolution
This term means the minimal step that the stage
can achieve positioning. For a well tuned and designed linear motor
stage, the motion resolution is supposed to be the same as the
resolution of the linear scale/feedback encoder. However, these two
terms are actually not equivalent. For example, there is one stage
equipped with a linear scale of 0.1 um resolution. If you send a step
command of 0.1 um, the stage may not move. It starts to move until the
command becomes 0.5 um. Then, the resolution of the stage is 0.5 um
rather than 0.1 um.
- External payload
This term means the mass of all the additional
components mounted on the moving slide of the motion stage.
- Moving mass
This term means the intrinsic moving mass of the
stage itself. It should exclude the external payload. This specification
should be provide by the stage maker.
- Accuracy, Repeatability, and lost motion
These three specifications are eventually the
"static" performance of the motion stage. They have close
relationship with the precision grade of the stage components. For
example, the accuracy is the synergy effect of the machining precision
of the stage mechanical components, the flatness and straightness of the
linear guide, and the intrinsic accuracy of the linear scale. The
repeatability is mainly related to the resolution of the linear scale,
the stiffness of the linear guide, and the rigidity of the stage
mechanical components. The lost motion for the linear motor stage should
be smaller than the resolution of linear scale, because of direct drive
without backlash. What we would like to mention here is that higher
requirements of these three specifications will lead to higher price of
the motion stage. Hence, it is important to carefully determine
these specifications of the motion stage for your application so that
the best performance can be achieved with lowest cost.
The test method of these three specifications mainly
uses the laser interferometer, as shown in the following figure. With
the motion stage fixed on a granite table, the moving slide is moved
back and forth for m times by the motor drive to position at n
predetermined points. Record the actual readings at the predetermined
position.
The detailed mathematical expressions of accuracy,
repeatability, and lost motion at the predetermined point are
illustrated as follows.
For the user of the motion stage, it should be noted
that the repeatability is the most critical specification for your
application. It has direct effect on the quality of your final product.
So, finer repeatability should be selected if the price is still
allowable. However, the accuracy is more or less tricky. The final
accuracy of the motion stage can be compensated according to the results
of the accuracy test (error mapping). In addition, if the modern vision calibration system
is employed in your machine, the requirement of accuracy can be looser
to lower the price.
- Straightness and Flatness
Straightness is defined as the maximal horizontal
deviation of slide motion within a stroke length. This term can be measured by using a
dial gauge with respect to the vertical reference plane parallel
to the moving direction. This specification also can be measured by
laser interferometer with special lens.
Flatness is defined as the maximal vertical deviation
of slide motion
within a stroke length. This term can be measured by using a dial gauge
with respect to the horizontal reference plane parallel to the
moving direction. This specification also can be measured by laser
interferometer with special lens.
 
- Definitions of the
dynamic positioning specifications
- Maximal velocity, maximal acceleration
This two terms are deduced by investigating the whole
motion profile. What we would like to mention here is that the
requirement of moving a given stroke within a predetermined period has
infinite combination of maximal velocity and maximal acceleration, as
shown in the following figure. In addition, the maximal velocity of the
stage is constrained by maximal output frequency of encoder, the linear
motor itself, and the speed limit of the linear guide. A higher maximal
acceleration always means a larger linear motor and drive power. Hence,
a reasonable tradeoff between the maximal velocity and maximal
acceleration should be made to result in a achievable maximal speed with
smallest motor thrust requirement.
- In-position specification
This specification is defined as achieving a given
stroke positioning within the specific tolerance in a predetermined
period. For example, the in-position specification of wire bonder XY
table is moving 2 mm stroke length and positioned within plus/minus 3 um
in 18 ms, as shown in the following figure. When the stage maker is also
in charge of the drive gain tuning with respect to a dummy external
payload, this specification should be clearly defined.
- Definitions of the linear motor specifications
- Rated thrust
This terms is also referred as
"Continuous force", which means the motor will produce this
force without over-heating the motor winding. A more clear definition of
this term should include the temperature rise when continuous power is
output. The value of this specification for an application should be
derived according to the motion profile. The following diagram
illustrates how to calculate this value.
- Maximal thrust
This terms is also referred as "Maximal
force", which is needed for the motor to achieve maximal
acceleration with both moving mass and external payload. A more clear
definition of this term should include the allowable period (e.g., 0.5
sec) for maximal force.
- Rated current
This term is defined as the line r.m.s.
current needed for producing rated force.
- Maximal current
This term is defined as the line r.m.s.
current needed for producing maximal force.
- Force constant
This
term is defined as the force produced per line r.m.s. current
- Voltage constant (Back emf)
This term is defined as the peak line-line
voltage that motor produced at the speed of 1 m/sec.
- Coil resistance
This term is defined as the resistance
measured between any two motor lines.
- Coil inductance
This term is defined as the inductance
measured between any two motor lines.
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