Metrology Artifact Design

1

• Metrology Artifact Design
• Presented by
• Karalyn Folkert
• Advisor
• Dr. Thomas A. Dow

North Carolina State University
2
Project Objective

• GOALS
• Fabricate one or more artifacts
• Predict measurement uncertainty
• Determine a transfer function
• Validate equipment performance

3
Background
Ring Gauge

• Calibrated diameter and circularity
• Probed data compared with calibration data
• Lobing error based on geometry data comparison
• Machine errors determined from error pattern of
  circular error

4
Ball Bars

• Rigid bar with a sphere at each end
• Free-standing ball bar is held by a gauge stand
• Magnetic ball bar 1 magnetic socket fixed to
  CMM table, 1 magnetic socket takes
  place of probe
• Probe errors cannot be determined

5
GeostepTM

• Calibrated hole distance
• Measure different orientations in the working
  volume

• Spheres mounted in center of each hole
• Center to center distances varied to separate
  systematic errors from step standard

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Hole Plate
Ball Plate

• Ceramic or stainless steel spheres arranged in
  space
• 4 points of each sphere probed

• Precision-machined holes at qualified distances
• 3 points of each hole probed

7
Space Frame

• Triangular plate with 3 magnetic ball links
  joined by a sphere at the apex
• Only magnetic ball links and plate calibrated
• Indicates error tendencies of machine

8

• Use calibrated regular geometric shapes to
  simulate freeform object
• Use CMM to find relative position of shapes
• Create a calibrated CAD model of geometric shapes

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Considerations

• Coordinate Measuring Machine
• 3 mutually orthogonal carriages
• Various configurations from machine to machine
• Probes

Touch trigger
Scanning
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Initial Artifact Features

• 3- 4 radius with 1 wall thickness
• Representative size of parts
• Aluminum
• Ease of machining
• Multi-wave feature
• 0.0125- 0.25 wavelengths
• ? 2.5?m amplitude
• Simulate affect of small features on a part
• ? reference
• For measurement process

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Features
Swept sine wave is on the ID of ring gauge
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Mounting

• Disk mounted to vacuum chuck on diamond turning
  machine
• Spacers allow for clearance of FTS
• Each surface turned

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FTS Open Loop Control

• Look-up table included inputs of amplitude,
  starting and ending frequency, length and number
  of data points
• Function box linearly corrects displacement of
  FTS as voltage is sent

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Reference Signal Generation

• Sine wave with varying frequency from 25 to 500
  Hz
• Based on 20 RPM spindle speed

• Amplitude depends on counts of encoder
• Phase delay will occur

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Fabrication

• The four surfaces of the ring were diamond
  turned
• The swept sine wave machined onto the ID of ring
  with fast tool servo (FTS)
• A reference groove machined on the ID/OD of the
  ring

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LVDT Measurement

• Probe tip radius was 1 mm
• Dips may be attributed to high frequency of
  the waves that the LVDT could not electronically
  measure

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Ring Distortion

• PV surface values of ring with bolts tightened
• 380 nm, 712 nm, 858 nm

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Ring Distortion

• PV surface values of ring with bolts untightened
• 3.3 µm, 4.4 µm, 4.7 µm

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Oak Ridge CMM

• Scanning of swept sine wave on ID

• Measurement setup

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CMM Scan

• Dips may be reflective of actual shape
• Attenuation in the servo

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Roundness

• Measurement taken on the flat portion of the ID

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Sources of Error

• This section describes the possible sources of
  error in the trial fabrication of the ring gauge
  artifact.
• They are
• Tool clearance
• Size of LVDT probe
• FTS dynamics

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Tool Clearance
Where A 2.5 µm L 0.317 mm

• Clearance angle of the tool is 6
• Theoretical maximum slope is 2.84
• With actual values
• 0.186 mm (cylinder) gt 4.84
• 0.287 mm (ring gauge) gt 3.14

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Probe Compensation
Blue Perfect wave Magenta Comp. data Yellow
LVDT data

• Begins to decrease in amplitude at a spatial
  wavelength of 0.204 mm
• Minimum wavelength is 0.186 mm

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Fast Tool Servo

• 18 mm long, hollow cylindrical piezoelectric
  actuator (25 mm OD and 12 mm ID) drives the
  device to a maximum displacement of 18 µm
• Cap gage provides feedback on the position of
  the tool holder by looking at the back of it
  through the hollow actuator
• A commanded position is turned into a positive
  input voltage sent to a HV amp where it is
  amplified by 100 before being sent to the FTS

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System Dynamics
Operating range for fabrication

• Natural frequency occurs around 5100 Hz
• Higher order system

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PZT Hysteresis

• Cap gage can measure displacement of 10 µm
• Piezos will displace a maximum of 18 µm

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Closed Loop Concept

• Spindle position captured
• Look-up table assigns disp value to position
  value
• Displacement is converted into voltage
• Voltage from high voltage amplifier sent to FTS
• FTS synchronized with encoder position

Open loop Closed loop
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Deconvolution

• Deconvolution uses the closed loop dynamics of
  the FTS to determine a modified input signal.
  When the modified signal is applied to the FTS,
  the motion of the tool will be the desired shape.
  
• The plot demonstrates the correction for phase
  and magnitude errors.

30
Method Comparison

• Demonstrates the error using open loop, closed
  loop, and closed loop w/ deconvolution as
  measured by the cap gage

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Future Plans

• Fabricate final artifact
• 17-4 PH stainless steel
• Heat treated
• Electroless nickel plated
• Closed loop with deconvolution input
• Finalize measurement mounting
• Specify use of artifact