ROTARY TABLES

 

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Rotary table construction
 
rotary tables

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, worm-gear 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 brush-less servomotors. And for a given diameter, several axial lengths are available.

Torque motors have a large number of magnetic pole-pairs with many permanent magnets on the rotor. Torque motors can be built as thin rings. They have smooth velocity modulation with low ripple. Eddy-current losses in torque motors constrains the maximum practical number of pole pairs and speed. As a result, torque motors are primarily designed for low/mid-speed 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 direct-drive, an integrated rotary table must also contain precision feedback device and high stiffness bearing to provide a high-performance rotary stage.

rotary stage = rotary tables + rotary stage + rotary tables

High-precision, high-resolution 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 arc-sec is achievable in direct drive rotary stages..

An important difference between direct-drive 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 liquid-based 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 lightas 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/run-out of the bearing guide-way. 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. Uni-directional repeatability is measured by approaching the point from one direction, and ignores the effects of backlash or hysteresis within the system. Bi-directional 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"

10'

8'

6'

5'

3'

2'

2'

1'

1'

1'

30

30

30

0.005"

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

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 m

5

3

2

1

1

1

0.5

0.5

0.5

0.2

0.1

0.1

0.1

0.5 m

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 run-out; 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 run-out 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 run-out error.  This run-out 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 run-out value.  If a table has a 10 microns run-out, 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 run-out error of the rotary table bering, and the diameter of the part or feature:

Angular error

where Θe is in arc seconds, B is the run-out and D is the diameter, with both being expressed in either inches or millimeters

 
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