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This article explains the usage and advantages of using the Mechanical Pivot Point operand during a Tolerance Analysis workflow. For that two show case applications, a single gauss and the Hubble Telescope, are used as walk through examples. Through out these walk through examples, the article also touches on the File Preview and the Draw Origin tools.
Note: This feature is only available in Ansys Zemax OpticStudio Premium/Enterprise.
Authored by Flurin Herren
- See sample file references
Introduction
Tolerancing is a vital part of the optical system development. It is a method to validate the probability that the optics will perform at the required level, considering manufacturing and assembly errors. A specific example: A 1000-piece series of a particular lens mount is manufactured out of an aluminum alloy by a CNC milling machine. The general accuracy of a CNC milling machine is between ± 0.025 mm and ± 0.005 mm. Therefore, each lens mount in the production run will be slightly different dimensions, which will affect the positing of any lens held in the mount. Even if we assume the lens itself is perfect, it will have 100 slightly different positions. This is called position tolerance. Ansys Zemax OpticStudio (OpticStudio) has a sophisticated tolerancing approach in which each optical component is affected by several tolerance operands (such as position tolerance operands for X, Y and Z). Those operands feed into an overall Monte Carlo analysis. In the end a statistical output is generated of how likely it is that a systems performance metric is violated.
- An in-depth training course on Tolerancing can be found here: Ansys Zemax OpticStudio: Tolerancing - Ansys Learning Hub
An evolution of the first pass single lens position tolerance, is to consider that several lenses can be mechanically connected, which subsequently has an impact on the tolerancing analysis. The most common mechanical component for this is a metal lens barrel.
In addition to retaining the lenses at the desired position, lens barrels can have other key benefits such as protection from environmental factors such as temperature or stray light reduction.
The two most common barrel designs are straight-barrel and a stepped-barrel. The straight-barrel utilizes spacers to maintain the lens positions while the stepped-barrel places the lenses on mechanical seats in the steps.
Figure 1. Straight-barrel (L) and a stepped-barrel (R)
Both designs usually retain the individual lenses in so called subcells. These subcells ensure individual lenses are centered in their subcell, which is then fitted into the parent barrel. Mounting the lenses into subcells not only improves the centering accuracy, which allows to achieve high-performance requirements, but also allows for easier assembly.
Figure 2. Individual lens cells
In OpticStudio, users can use an array of tolerancing operands to account for position and tilt variation. If a group of lenses is connected mechanically by a component such as a barrel the Mechanical Pivot Point Operand (MPVT) can be used in addition to the tilt operand (TETX/Y) of the individual lenses. MPVT automatically defines the desired center of the barrel and accounts for potential tilt of the lens barrel. The following section is a walk through on how the MPVT operand can be used in a tolerance analysis with OpticStudio.
Mechanical Pivot Point Tolerance
As a specific example, let’s look at a single Gauss lens.
- …\Documents\Zemax\Samples\Optomechanical\Optical Data Exchange
The mechanical housing barrel (dark component) on the right in the image below shows that all lenses in this system are connected via the barrel.
Figure 3. Ansys Zemax OpticStudio sequential mode (L) and mechanical housing in Creo Parametric (R)
To ensure the optical performance criteria is met, it is important to factor in the tilt tolerance of the barrel itself. To tolerance the tilt of the barrel without the use of the MPVT operand, a lot of manual steps are required. Which would include inserting a dedicated Coordinate Break for tilting that is moved to the center of the barrel and back with dummy surfaces, without violating the sequence aspect of the sequential mode of OpticStudio. With the Mechanical Pivot Point Operand (MPVT) all this is automatically taken into consideration. The MPVT servers the user as a control operand, which defines the rotation origin of a TETX/Y operand (Pivot Point).
Back to the Single Gauss Lens barrel example – if an MPVT operand is inserted all subsequent TETX/Y operands will use the MPVT defined center point as the center of rotation. MPVT has several options such as front and back vertex, mechanical edges or the geometric center. In this example the MPVT operand is used to achieve the following tolerancing setup:
- Section 1: The MPVT operand is set to 4 and the subsequent TETX operand spans from Surface 1 to Surface 9. The TETX operand simulates a tilt tolerance about the x axis over all surfaces in this range. This TETX serves to simulate the tilt tolerance of the barrel.
- Section 2: The MPVT operand is set to 0, and the subsequent three TETX operands include the surface that define lenses L1 to L3. All three lenses have an individual tilt tolerance about the front vertex (which is also the default TETX origin of rotation).
- Section 3: Let’s imagine the last lens (L4; surfaces 8-9) in the system is retained by a mechanical part of the main body of the imaging system. Hence it has its origin of rotation about the rear mechanical edge. To achieve that MPVT operand is inserted before the TETX operand and the “Pivot At” parameter is set to 3. All subsequent TETX operands now use the rear mechanical edge as the origin of rotation.
Figure 4. Ansys Zemax OpticStudio sequential mode; Tolerance Data Editor
Section 3 shows that the MPVT cannot only be used to simulate the tilt tolerance of a barrel housing multiple lenses, but also if specific lenses are retained in a way other than about the front vertex. To set the origin of rotation back to “default” (front vertex) a last MPVT with the parameter 0 can be added.
With the Tolerance Data Editor filled out, the system is ready for a Tolerance Analysis Run with the Tolerancing tool. Before that the tolerance system perturbations can be previewed with the File Preview functionality. In this way the user can inspect the coordinate breaks and other changes which will be made by the Monte Carlo simulation before actually running a simulation.
File Preview
With the File Preview button, a new instance of OpticStudio with the name pattern of “CreatePreview_MC_T..” is opened. In this new file all the coordinate breaks and changes which are applied by the Monte Carlo simulation are carried out. It is a random Monte Carlo instance within the region defined by the operands in the Tolerance Data Editor.
Figure 5. File Preview
For clarity, the Preview instance file automatically hides specific surfaces in the Lens Data Editor. Looking at the displayed coordinate breaks, you can see they follow the sections 1-3 which were defined in the Tolerance Data Editor in the previous chapter.
- Coordinate break on Surface 2 colored yellow (+TETX 1 9) – Section 1
- Coordinate breaks on Surface 3, 8 and 13 colored red (+ TETX 1 2; 3 4; 5 6) – Section 2
- Coordinate break on Surface 20 colored turquoise (+TETX 8 9) – Section 3
The users can also unhide all rows to see how and where OpticStudio is applying the rotations. The surfaces which are initially hidden are all coordinate breaks which are used to travel to a defined pivot point and back.
Draw Origin
In addition to the color coding of the coordinate break surface rows, the 3D Layout plot in the Preview file also contains a visual indication of the pivot point. All the coordinate break surfaces have the “Draw Local Origin Point” Flag enabled, so in the 3D Layout a sphere is drawn at the location where the tilt is applied.
Figure 6. Draw Origin example
These local origin point spheres can also be individually colored and their size is adaptable in the “Line Thickness” drop down menu of the 3D Layout.
Mechanical Pivot Points with Mirrors
The Mechanical Pivot Point operand can also be used on reflective surfaces such as mirrors. Similar to the approach elaborated in the section before, to apply the MPVT operand on a mirror, the MPVT operand can be added in the Tolerance Data Editor and the pivot point can be chosen with the “Pivot At” parameter. The subsequent TETX/Y then needs to have set the Surf 1 parameter equal to the Surf 2 parameter, which has to be the surface which represents the mirror surface.
As another specific example let’s look at the Hubble Telescope sample file.
- …Documents\Zemax\Samples\Sequential\Telescopes
The system contains a primary and secondary mirror, lets assume the larger primary mirror (Surface 2) is retained at the back and hence has a mechanical pivot point at the rear mechanical edge. To achieve this, an MPVT operand is set to “Pivot at = 3” and the TETX is set to Surf 1 = Surf 2 = Target Surface, which is in this case Surface 2.
Figure 7. Hubble Telescope in Ansys Zemax OpticStudio sequential mode; Tolerance Data Editor
Same as in the previous section, the Preview File Tool opens a random iteration of the file in the Min-Max range of the defined tolerances. This is a good approach to check the tolerancing before running an actual full Tolerance Analysis.
In order to maintain a good overview in the Lens Data Editor, only the main coordinate break on Surface 3 is shown. Additionally, the local origin point of this coordinate break is drawn on the 3D Layout.
Figure 8. Hubble Telescope in Ansys Zemax OpticStudio sequential mode; Draw Origin Example
If the user wants to investigate the exact location of the Coordinate Break carrying out the mechanical pivot, all rows can be shown by right-clicking on the Lens Data Editor and selecting “Unhide all rows”.
Conclusion
The Mechanical Pivot Point Operand can be used to apply a tilt to a group of several optical components in the sequential mode of Ansys Zemax OpticStudio, hence can be used to simulate a mechanical part which holds several components such as lens barrel. Additionally, it can be used to apply tilts at different physical locations (pivot points) relative to the default front vertex of the TEDX, TEDY, TEDR, TETX, TETY, and TETZ operands. The later functionality can be used for individual components or for groups. The options to place the pivot point are Front Vertex (0), Rear Vertex (1), Front Mechanical Edge (2), Rear Mechanical Edge (3) and Geometrical Center (4).
References
- Katie Schwertz, James H. Burge; Field Guide to Optomechanical Design and Analysis; SPIE Field Guides Volume FG26