Non-contact measurement machine for freeform optics

Why the non-contact measurement machine?

To eliminate aberratinos, fewer components, smaller systems, lower system mass, more design freedom.

field of view and resolution

field of view: how much it sees

resolution: how sharp it sees

What are all the manufacturing processes for mirrors? What do they have all in common?

Glass: spherical (conventional polishing), asoherical/freeform (deterministic polishing)

Metal: (a)spherical (diamond turning), freeform (slow/fast-tool servo)

Plastic (injection moulding)

Extreeme accuracy (IBF)

Each proces has characteristic removal function and dwell time can be calculated when error map is known. For aspherical and freeform surfaces, a high density error map is needed.

What is the current state of the art for the measurement machines and what is overall goal?

Profilometer: fast, universal, small, contact, not accurate enough

Interferometer: fast, non-universal, small, non-contact, accurate (in/out of phase)

CMM: slow, universal, large, non-contact, not accurate enough

Goal: fast, universal, large, non-contact, accurate

precision and accuracy

precision: measure for random (unpredictable) errors

accuracy: measure for systematic (constant) errors

uncertainty* : includes both

calibration

comparison of system performance with known reference

1. adds uncertainty statement to a measurement result

2. result can be used of on/off line correction

traceability

an unbroken series of comparison, each with specified uncertainty

repeatability and reproducilbilty

repeatability - same conditions

reproducibility - varying conditions (lower limit of what a machine is capable of)

uncertainty

= reproducilbity + calibration (+model based corrections)

What are the three types of loops?

1. structural loop (=stiffness or position loop) for dynamics: the assembly chain from one machine element to another which provides the pyhiscial support and constraint for each element

2. metrology loop (= measurement loop) for stability: the shortest part that carries info about the relative position of two or more measurement locations and consistent of a series of solid objects measurably connected by a position measurement sensor or a calibrated sliding mechanism

3. force path for strength: the assembly chian from one machine element to another which conducts forces

Describe the error sensitivity analysis and how the problem was reduced to a 2D problem.

In total, there are 13 DOFs: 6 DOF of product, 6 DOF of probe and distance measured by the probe.

Errors in normal direction are critical, error in tangential directionhave second order effect. → 2D probelm of 6 DOFs

What rails are used in the design? Describe them.

Crossed roller bearing: high stiffness, high repeatability, limited travel, friction, 2 rails needed, preloading needed

Recirculatin ball/roller bearing: lower stiffness, lower repeatability, unlimited travel, preloade and seals give high friction

Rails always do 1 DOF motion.

Describe the air bearing used in the design.

• compressed air in a small gap

• high preload give a small gap with high stiffness

• frictionless

• need self-levelling

• flat air bearing can do 3 DOF motion

Describe the motors for linear actuation.

Rotary motor with recirculating ball scre: lowe fricition, high stiffness

Direct drive linear motor: no gear, no backlass, no friction

• iron core: high attraction force, exposed magnetic field, good efficiency, high actuation force, cogging

• ironless: no attraction force, no cogging, only small magnetic stray field, lower actuation force

Describe the encoders in the system.

• readhead with linear or circular scal that produce sinusoidla signal

• lightsoure, reticle and detector

• singles sensor (no direction detection, sensitive to light source power variations) vs multiple detectors (direction sensitivity, less disturbance sensitivity)

• incremental counting (one reference) vs abolsute (pseudo-random lines)

lasr interferometers

=displacement measure interferometer

Left: feedback into source due to 50% beamsplitter and very sensitive to tilt

Right: use polarization for better efficiency, use retroreflector to reduce tilt sensitivity of flat mirror, double resolutions but double the air path

Overall, accuracy suffers from air properties and alows for sidewats motion when measuring to plane mirror.

What is the overall design made of?

• all air bearing motion system

• ironless linear motors, encoders

• vertical stage aligned with 3 air bearing

• separate positon and preload frames for minimal hysteresis

• separate metrology ststem in the measurement plane

Described the motion system.

• position product in Theta

• position probe in R, Z, Psi (minimal hysteresis and heat production)

• granite base - stable, high damping

• vibration isolators - large, need levelling

• base frame - for transport

• air bearing spindle

• brake - pneumatic holding brake

• product table with reference edge

Z-stage components

• aligned to vertical plane with 3 air bearings

• ceramic tube

• preload frame

• weight compensation (custom made air bearing with a safety latch and air reservoir)

• linear motor through COG

R-stage componenets

• far apart bearing for tilt stiffness

• stiff box structure

• linear motor through COG

• holding brake

• cable routing

Psi-axis componenets

• kinematic mount on Z-stage

• custom air bearing

• cylinder lenses for metrology system

• encoder

• direct drive motor

• counterweight for probe

Metrology system

measures probe and product relative to a metrology frame

Interferometry system

• R, Z, Psi

• probe tip is point of interest, but difficult to measure (Abbe error)

Upper metrology frame

• holds reference mirrors

• straight beams, stiff frame

• constrained in 6 DOF

specific stiffness

determines eigenfrequency and vibration amplitude

thermal sensitivity

determines deformation as function of non-symmetric heat load for a given temperature difference

volumetric thremal diffusivity

determines temperature distribution due to asymmetric heat load

volumetric thermal stability

determines deformation for a given asymmetric heat load

Lower metrology fram

• measure error motion of spindle

• 3 DOF (R, Z, tilt)

Total number od DOFs?

• length of probe

• R, Z, Psi axis

• R, Z, Psi of table

probe

dual stage, differentiial confocal principle, interferometer for objective displacement, position dependent for correction of tilt dependency