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Rotary table construction 

Torque motors are also
called "frameless" motors. They don't have housings,
bearings, or feedback devices.
In this sense the motor is a kit
and meant to be part of the
machine structure.
Torque motors are designed as
direct drives. They eliminate
the need for gearboxes,
wormgear drives, and other
mechanical transmission elements
and directly couple the payload
to the drive. This makes
rotary drives with high
dynamic responses and no hysteresis
possible. The large inner
diameter of a torque motor is a
big advantage in machine
tool design. Large
hollow shaft allows designers
to design rotary table to support bearings,
feedback devices and payloads.
Torque motors are available in a
wide range of sizes, with
diameters from smaller than 100
mm to larger than 1 m. Motor
diameter is similar to frame
size for conventional brushless servomotors. And for a given
diameter, several axial lengths
are available.
Torque motors have a
large number of magnetic
polepairs with many permanent magnets on
the rotor. Torque
motors can be built as thin
rings. They
have smooth velocity modulation
with low ripple. Eddycurrent losses in
torque motors constrains
the maximum practical number of
pole pairs and speed. As a result, torque
motors are primarily designed
for low/midspeed range applications,
generally below 1,000 rpm.
Torque motors produce high
torque at stall and can have
high dynamic stiffness. However,
the motor alone does not
determine dynamic stiffness or
precision. To exploit the full
benefits of directdrive,
an integrated
rotary table must also contain precision
feedback device and high stiffness
bearing to provide a
highperformance rotary stage.
Highprecision, highresolution
feedback is essential for
optimal performance of rotary
tables. Because loads are
directly coupled to the drives,
higher accuracy is possible.
Rotary table
positioning resolution is also
in direct relation to the
feedback's resolution, so it
takes an encoder with a
high line count and a
high resolution interpolation
factor. System resolution below 1 arcsec
is achievable in direct drive
rotary stages..
An
important difference between
directdrive rotary tables and those
driven by conventional dc
servomotors and gearboxes is
that torque motors are found
inside the rotary table axis and are part of
the machine. Most of torque motors include
provision for liquid cooling which effectively
increases the continuous torque
rating of the motor and higher
power rating of the rotary
table. Air
cooling, while an option, is
much less effective than
liquidbased cooling.
A
torque motor is only one
element in a complete rotary
stage. It
still needs a mechanical
structure with high rigidity and
precision
bearings. It is
an overall integration of
these elements that determines
complete rotary stage performance. 


Rotary Table
Accuracy  Glossary of terms 
Arc Second 
The prefix, arc,
is used solely to differentiate this angular measurement from a
second of time. By convention, it is 1 part in 1,296,000 of a
circle; or 1 part in 3,600 of a degree; or 0.0002778 degree.

Backlash 
The measure by
which the tooth space of a gear exceeds the tooth thickness of
the mating gear along the pitch circle. It is affected by both
the center distance at which the gears operate, any eccentricity
in the gears, and the variation in thickness of the teeth.

Error Bandwidth 
The total
measure of deviation between all readings of an instrument
throughout its measuring range.

Error 
The difference
between a measured value and the true value.

Flatness &
Wobble 
A composite
tolerance applied to a flat surface rotating about an axis, e.
g., the faceplate of a rotary table. It is defined such that the
entire rotating surface lies between two parallel planes
separated by a distance equal to the tolerance specified.

Hysteresis 
A bias resulting
from the reversal of direction.

Index Accuracy 
The agreement of
the result of an angle measurement between the actual reading of
the instrument and the true value, that is free of other error,
at that point.

Latent Motion 
The measure of
change, as measured at the platen in the axial and radial
directions, between the unclamped and clamped condition on a
table equipped with axis clamps.

Parallelism 
The state of two
planes parallel to each other; where two planes are
equidistantly spaced in three dimensional space.

Perpendicularity 
Condition of a
line in which all angles to a reference plane are at right
angles.

Repeatability 
The measure of
accuracy by which an instrument permits the return to a specific
point. In a normal or Gaussian distribution, the results are
spread roughly symmetrically about the central value, and small
deviations from this central value are more frequently found
than the large deviations. The normal curve can be represented
by
The standard
deviation, denoted by
is found by taking the difference between each observed
particular value and the mean, then squaring the difference,
adding all the squares, dividing by the number of readings, and
then taking the square root.

Resolution 
The smallest
increment of measure to which an instrument can respond.

Rotary Axis
Definition 
A measurement
which compares two full consecutive axis rotations to a known
standard of roundness.

Roundness 
A characteristic
that all parts of a circle are identical. The measurement of
roundness is essentially a measurement of the change in radii,
and the roundness error is the measurement between the minimum
and maximum radii in one lateral plane.

Runout T.I.R. 
Total indicator
deflection as measured over on revolution of the spindle.

Squareness (Orthogonality)

A condition of
being at a right angle to a plane or to a line. 
Straightness 
A deviation from
a line of sight which is generated by a reflected light–as for
example, an autocollimator and mirror.

Traceability 
Documentation to
establish that standards are known in relationship to
successively higher standards, culminating with the National
Institute of Standards & Technology, or equal.

Wobble (Axis) 
The measure of
deviation of a rotating plane to a reference plane. The
measurement is usually performed with a mirror and an
autocollimator. 

Rotary table  Accuracy vs Resolution 
A common misconception
is that the resolution of the device must also be
its accuracy. For example, if a digital readout
displays to four decimal places (0.0001), then it
must also be accurate to that same value. That is
usually not the case. Although high resolution is a
prerequisite for high accuracy, it does not
guarantee it. Consider the two graduated scales:
Both scales have 15
graduations over equal arcs; therefore, both have
identical resolutions of 1/15th arc. For arc A
the resolution increments are equal; however, for
arc B the resolution increments are obviously
not the same. That difference, scale accuracy, is a
component of position accuracy, and while both
examples have the same resolution, each will provide
very different results.
Accuracy
is the difference between the actual position and
the position measured by a reference measurement
device. Stage accuracy is influenced by the feedback
mechanism (linear encoder, rotary encoder, drive
mechanism (ball screw, lead screw, linear/torque
motor), and straightness/parallelism/runout of the
bearing guideway. IntelLiDrives uses laser
interferometers (for linear axes) and
autocollimators (for rotary stages) as a reference
measurement tools.
Repeatability
is defined as the range
of positions attained when the system is repeatedly
commanded to one location under identical
conditions. Unidirectional repeatability is
measured by approaching the point from one
direction, and ignores the effects of backlash or
hysteresis within the system. Bidirectional
repeatability measures the ability to return to the
point from both directions.
Resolution 
The smallest possible movement of a system. Also
known as step size, resolution is determined by the
feedback device and capabilities of the motion
system.
Low Accuracy
High
Repeatability

Low Accuracy
Low
Repeatability

High Accuracy
High
Repeatability







Rotary table
 Angular
vs. Linear Accuracy

The relationship between
angles and linear dimensions is defined by
trigonometry. Sine, cosine and tangent functions
allow to calculate these relationships. Because an
angle diverges as distance from the origin
increases, so too, increases the tangential
component. Therefore, it is not appropriate to
specify a rotary tables accuracy in linear values,
unless a maximum envelope (diameter) is also
specified.
It is not difficult to
determine the linear relationships to their
respective angles, within a specific envelope. Let's
equate 1 arc second (see definition in Glossary) to
some common linear measurements. For our discussion,
we'll use 0.000004848 as the tangent of 1 arc
second. Since the trig functions provide
dimensionless ratios, 0.000004848 (usually rounded
to (0.000005) applies to any unit of linear measure,
just be consistent.
To use the charts below,
locate your required linear tolerance, and the
maximum radius to be worked, the resulting
intersection shows the corresponding nominal index
accuracy, in arc seconds (unless specified, i.e.,
[°] = deg.; ['] = min.), required for the
application. If your tolerance zone or working
radius falls between two values, use the tighter
requirement.
Tolerance 
Working Radius (Inches) 

1 
2 
3 
4 
5 
6 
10 
12 
18 
20 
24 
30 
36 
40 
48 
0.010" 
½° 
¼° 
10' 
8' 
6' 
5' 
3' 
2' 
2' 
1' 
1' 
1' 
30 
30 
30 
0.005" 
¼° 
10' 
5' 
4' 
3' 
2' 
2' 
1' 
1' 
30 
30 
30 
20 
20 
15 
0.003" 
10' 
5' 
4' 
3' 
2' 
2' 
1' 
1' 
30 
30 
30 
20 
20 
15 
10 
0.002" 
5' 
3' 
2' 
2' 
1' 
1' 
30 
30 
30 
20 
15 
10 
10 
10 
10 
0.001" 
3' 
2' 
2' 
1' 
30 
30 
20 
20 
15 
10 
10 
10 
5 
5 
5 
0.0005" 
1' 
1' 
30 
30 
20 
15 
10 
10 
5 
5 
5 
5 
2 
2 
2 
0.0003" 
1' 
30 
20 
15 
10 
10 
5 
5 
5 
3 
3 
3 
2 
1 
1 
0.0001" 
20 
10 
5 
5 
3 
3 
2 
2 
1 
1 
1 
0.5 
0.5 
0.2 
0.2 
0.00005" 
10 
5 
2 
2 
2 
1 
1 
1 
0.5 
0.5 
0.5 
0.2 
0.2 
0.2 
0.1 
0.00001" 
2 
1 
1 
0.5 
0.2 
0.2 
0.2 
0.1 
0.1 
0.1 
* 
* 
* 
* 
* 
Table 1 [inch values]
Tolerance 
Working Radius (Millimeters) 

25 
50 
75 
100 
125 
150 
200 
250 
375 
500 
750 
1000 
1500 
0.25 mm 
½° 
¼° 
10' 
8' 
6' 
5' 
4' 
3' 
2' 
1' 
1' 
30 
30 
0.1 mm 
10' 
5' 
4' 
3' 
2' 
2' 
1' 
1' 
30 
30 
15 
15 
10 
0.05 mm 
5' 
3' 
2' 
1' 
1' 
1' 
30 
30 
20 
15 
10 
10 
5 
0.025 mm 
2' 
1' 
1' 
30 
30 
30 
15 
15 
10 
5 
5 
5 
3 
0.01 mm 
1' 
30 
20 
15 
10 
10 
10 
5 
5 
3 
3 
1 
1 
0.005mm 
30 
15 
10 
10 
5 
5 
5 
3 
3 
2 
1 
1 
0.5 
0.0025 mm 
15 
10 
5 
5 
3 
3 
2 
2 
1 
1 
0.5 
0.5 
0.2 
1 µm 
5 
3 
2 
1 
1 
1 
0.5 
0.5 
0.5 
0.2 
0.1 
0.1 
0.1 
0.5 µm 
2 
1 
1 
0.5 
0.5 
0.5 
0.2 
0.2 
0.2 
0.1 
* 
* 
* 
Table 2 [metric values] 


Accuracy
vs Radial Runout


When a rotor turns in
its bearings, it typically results in a radial
runout; essentially the roundness of the rotor
rotation. For air bearings this can be as
little as 1 micron T.I.R.; for some rolling contact
bearings, it can be as high as 25 microns and more.
This runout is a linear translation along a plane
which is perpendicular to the axis' centerline.
Consider this geometry:
Where A is the
theoretical center of the rotary table, and B is the
runout error. This runout results in an offset
error of ±S. If B is small relative to the part's
tolerance band, it can be ignored. However, if B is
large this error must be considered because the
rotary index position is generated about A; whereas,
the part's index angle is generated from its own
center, which is located on B. A part can not be
centered better than the radial runout value. If a
table has a 10 microns runout, the part can not be
centered better than 10 microns. The smaller the
radius L, the greater will be the resulting
difference between Θ and Θ', and the greater the
index error.
The following formula
allows to calculate the effective angular error,
based on the runout error of the rotary table
bering, and the diameter of the part or feature:
where Θ_{e}
is in arc seconds, B is the runout
and D is the diameter, with both being
expressed in either inches or millimeters




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