ME250 EXPERIMENT OBJECTIVES

Experiment # 4 Straightness Measurements

using Autocollimator

Goal: An autocollimator is an optical instrument that is used to measure small angles with very high sensitivity. We will setup an autocollimator and study the straightness of a linear guide bearing system.

Learning Objectives

Upon completion of this lab, the student shall be able to:1. Explain how an Autocollimator works2. How to setup the autocollimator to measure the small

errors in alignment of precision components3. How to measure linear straightness using an

autocollimator4. Determine the straightness of a linear guide bearing

system

Instruments and Apparatus 1. Davidson Optronics D656 two axis autocollimator2. Linear bearing guide adapter plate (A) SJSU ME25053. /10 2 in. Aluminum mirror -Thorlabs PF20-03-F014. Kinematic mirror mount for 2 in. optics -Thorlabs

KM#2005. THK linear motion guide SHS15R 22 in. long6. Linear guide rail adapter plate for optical table SJSU

ME25037. McMaster Polyester 1m x 0.5" (L x W) measuring tape-

item#8965A61

Introduction to autocollimator

An autocollimator is an optical instrument that is used to measure small angles with very high sensitivity. As such, the autocollimator has a wide variety of applications including precision alignment, detection of angular movement, verification of angle standards, and angular monitoring over long periods. The autocollimator projects a beam of collimated light. An

external reflector reflects all or part of the beam back into the instrument where the beam is focused and detected by a photodector or is focused on a reticle target and detected by the eye. The autocollimator measures the deviation between the emitted beam and the reflected beam. Because the autocollimator uses light to measure angles, it never comes into contact with the test surface.

Reticle

Condensing lens

Beam splitter

Objective lens

Eyepiece lens

Reflector

Introduction of Apparatuses

eyepiece

Rotate the angular dial to change the initial readout

focusing optics

Figure 3 - Theory of operation: autocollimator uses collimated light beam to project the reticle’s image on a reflector (mirror) and measures the angle deviation between the incident and return image with an angular dial.

Figure 1 - Davidson D656 2-axis autocollimator provides 30 feet measuring range and 0.1 second precision of angle measurement.

Figure 2 - CCD camera and lens transform image to the monitor for better resolution.

camera and transformer assembly

light source

Experiment Procedure

A. Autocollimator alignment demonstration (pre-lab)

The functionality of the autocollimator will be demonstrated with the use of a linear slide, a flat mirror and the crosshair of the autocollimator. The demonstration will show how the autocollimator can be used to detect small angular errors in precision components.

B. Autocollimator setup and coarse alignment (Refer to Figure 4 & 5)

1) Clean the sliding surface of the THK linear motion guide with a micro fiber cloth.

2) Screw the kinematics mirror mount on the linear bearing guide adapter plate, which was attached to the slider of THK linear motion guide SHS15R.

3) Place the autocollimator about one foot away from the front of the THK linear motion guide. Turn on the light source.

4) Position your eye at the opposite end of the THK linear guide looking toward the autocollimator at the position where the mirror will be located (as shown in Figure 5). Look for the projected image of the reticle (as shown in Figure 8).

5) Use the horizontal and vertical adjustment knobs to adjust the autocollimator scope up/down and left/right so that the projected reticle image is centered at the position where the mirror will be located. This will require two people.

6) Slide the mirror mount assembly created in step (2) on to the THK linear guide with the mirror facing towards the autocollimator. Verify the pointer is on the side of where the measuring tape was located. Position the mirror at the 10 cm position.

7) Set the autocollimator angular measurement dial (see Figure 9) to mid range with the seconds dial at zero. Use the parallax control feature for accurate setting.

8) Tip and tilt the 2” Aluminum mirror using the adjustment knobs (See Fig. 6) on the mirror mount so that the rings of the Autocollimator image is centered

about the crosshair as shown in Figure 8. When the crosshair is centered on the ring, the mirror has been oriented to be perpendicular to the optical axis of the autocollimator. The next section will show you how to measure angular deviation.

Figure 4 - Autocollimator straightness measurement setup

Figure 5 – The vertical and horizontal adjustment knobs change the angles of incident collimated beam onto the reflector.

y

z

xThe centers of the front autocollimator’s lens and the reflector should be about the same height.

Measuring tape shows the position of the mirror

Incident beam

Returning beam

Eyepiece

pointer

adapter plate

x

Eye position for coarse alignment

Kinematic mirror mount

C. Measuring the straightness of motion of the THK guide

Note: Every team member in the group will need to collect an equal portion of the data.  Take 3 to 5 measurements at each mirror position as time permits.

1) With the carriage at the starting 10 cm position, adjust the autocollimator vertical and horizontal knobs to center the image very precisely on the crosshair. Look for symmetry of the image about either side of the crosshair lines. Make sure adjustment is secure.

2) Record the starting value of the angular dial in units of minutes and seconds of arc.

3) Record the initial position of mirror by using the measuring tape and pointer.

4) Move the mirror by 100mm (the base length, b, in the Figure 10).

5) Adjust the angular dial so that the elevation filar (horizontal line) bisects the center dot of the green reticle image. Record the dial reading for elevation. Adjust the angular dial so that the azimuth filar (vertical line) bisects the center dot of the green reticle image. Record the dial reading for azimuth.6) Record down the EL and AZ.

6) Repeat STEP 4 and 5 until the Aluminum mirror reaches the end of the THK linear motion guide at position 60cm.

Figure 6 - Adjustment knobs on the KM#200 mirror mount. Tip and tilt the mirror to align the image coincident with the reticle before measurement.

Tip/tilt adjustment knobs

Through holesfor mounting

Figure 7 – CCD Camera setup (left)

1. Remove the eyepiece.

2. Mount the bracket by screwing down camera.

3. Tighten down the screw to the camera to secure the assembly in

Figure 8 - Autocollimator image under the

scope (right)

The reticle under the autocollimator’s eyepiece

should match up with the reticle’s image

Figure 9 - Angular dial in minutes and seconds

D. Calculation and Results1. For the series of measurements at each mirror

position, convert angles from minutes and seconds-of-arc to radians and calculate the mean angular measurement. Normalize the mean angular measurements by subtracting the mean angle for all mirror positions. From the azimuth and elevation angle (slope) at each measurement position, calculate the offset in ∆X and ∆Y relative to the autocollimator optical axis using the formulas below.

2. Add each offset to the previous one to construct the horizontal and vertical deviation from straightness (straightness error) along the rail path. Remove the linear components from the data.

3. Plot:  (a) Mean straightness error X and Y versus Z (rail coordinate)

(b) Mean straightness error X and Y versus Z (rail coordinate) with linear components removed.

4. Calculate the 3-sigma error of all angular measurements. Calculate error as (measurement – mean) for each set of angular measurements at a given position (3-5 measurements). Calculate single 3-sigma value for errors of all measurements (50-100 measurements) combining azimuth and elevation measurement errors.

5.  Discuss the sources of error.

6.  How does this straightness measurement technique compare to the reversal and interferometer techniques for sensitivity, repeatability, accuracy, set up time, ease of use, and cost? (Answer this question only if you have completed the other labs first.)

Figure 10 – autocollimator’s straightness measurement setup

Formulas

The offset change relative to the optical axis for one measurement distance “b” centered on the measurement position:

∆Y = b*tan EL = b* EL if EL is in units of radians

∆X = b*tan AZ = b* AZ if AZ is in units of radians

The surface straightness error at each position:

n

Y(n) = ∑ ∆Y(i) i=1

n

X(n) = ∑ ∆X(i) i=1

Remove linear components:

Y(z) corrected = Y(z) –[m*(z) +c], where z denotes the rail position and

[m*(z) +c] is the equation of the trend line.

X(z) corrected = X(z) –[m*(z) +c], where z denotes the rail position and

[m*(z) +c] is the equation of the trend line.

Appendix 1: Autocollimator

http://www.davidsonoptronics.com/poa.htm

Measurement of non-parallelism in windows, laser rod ends, and optical wedges by the two-face reflection method.

Measurement of squareness of an outside corner by aperture sharing.

Angle comparisons by aperture sharing.

Checking right angle prisms for angular and pyramid errors

http://www.davidsonoptronics.com/poa.htm

http://www.micro-radian.com/Principles.html