Thursday, January 15, 2015

GERALD microscope control system

While my optical microscopes are capable of sufficient resolution for imaging larger-process ICs, taking massive die images (this one, for a comparatively small 3.2mm^2 die, is about 0.6 gigapixels) has been beyond my capability because I have better things to do with my time than sit in the lab for a week turning the stage knob a little, clicking a button to take a picture, turning the knob a little...

John has a computer-controlled microscope that he uses for large imaging jobs and it seemed like it'd be a good idea to make one of my own. The first step was to write some control software. John's user interface is very much a "programmer's GUI" and I figured something with a bit more eye candy wouldn't be too hard to do.

The result was a system called GERALD. (I couldn't think of a name for it, asked my girlfriend for suggestions, and this was the first she came up with...)

The prototype is using my old AmScope microscope because I didn't want to take my main Olympus out of service for an extended period of time during development. Yes, I'm aware that the "support" structure under the stage isn't very rigid... this is a software development testbed and I won't be using this microscope in the final deployment so there's no sense wasting time machining nice aluminum brackets. I just have to be careful not to move around too much when using it ;)

Prototype GERALD system on my desk. The breadboarded MCU is generating step/direction pulses from USB-serial commands and sending them to the stepper controller under the textbook.

The camera in use is an AmScope MD1900. It uses a proprietary USB protocol and only has Windows driver support, and I try to run a 100% Linux shop. This was a problem... In keeping with John's unofficial motto "Open source by any means necessary", I corrected the problem ;)

A bit of Wireshark and libusb coding later, I had a basic working driver. The protocol is closely related (but not identical) to the MU800 that John reversed a few months ago, which helped me get my feet in the door. There's a bit of trivial obfuscation (XORing control transactions with 0x55aa or 0x5aa5) which I fail to understand... The average user isn't going to notice anything, and anyone with the skills to reverse engineer USB transactions or a kernel driver will see right through it, so why bother?

I plan to try making a kernel V4L2 driver in the future but for now it works so it's not a huge priority.

The current GERALD system is very much a WIP, most basic features are there but automated image capture and some other things aren't implemented yet.

GERALD UI overview
The basic UI layout is modeled in large part on the FEI Versa FIB's control system, which is currently my favorite control system for either optical or electron microscopes.

The upper left panel is an overview of the current camera feed, scaled down to fit. The upper right view, meanwhile, shows the center of the feed at native (1:1 pixel) resolution. Both views have a footer that includes a scale bar, date/time, the objective in use, and magnification. (The magnification is the actual value based on calibration of the camera and my 24" 1080p display.)

The lower right view is a webcam pointed at the sample stage. I haven't had the time to make a bracket to hold it in the right spot so it's just sitting on my desk for now. This is the equivalent of a SEM chamber cam and I hope to eventually use it to avoid crashing the sample into the objective in full-remote-control operation.

The lower left view is a navigation display showing the current sample. Unlike the Versa's nav-cam (a single static image taken of the stage when the sample is loaded) my navigation view is a composite of actual microscope images. Every time a new video frame comes in when the stage isn't moving (to avoid motion blur) it is plotted in the navigation view at the current physical position of the stage.

As of now the navigation view is non-interactive; all it does is show the current field of view on the sample. My plan is to support clicking to move the stage to a specific point, as well as drawing to define a rectangular area for step-and-repeat imaging.

In typical use I envision the user moving to the upper left and bottom right corner of the sample manually with the joystick, then selecting an objective and drawing a box around the sample to initiate a high-resolution imaging run.

In order for all of this to work properly, the system must be calibrated. The first step is to calibrate the camera so that it knows how large each pixel is. I've done this manually in the past by taking pictures of a calibration slide and counting pixels, but it's high time I automated the process.

GERALD pixel size calibration
The algorithm is quite simple and is designed to work with the pattern on my calibration slide (10um pitch short lines and 50um pitch long lines) only. As of now the are limited sanity checks so if there's no calibration slide in view the results can be somewhat strange :)
  • Convert the image to grayscale using NTSC color weights
  • Compute a gray-level histogram
  • Median-filter the histogram to smooth out spikes and find peaks
  • The distribution should be approximately bimodal (white lines on dark background). Take the mean of the peaks and threshold the image to binary.
  • Do a median filter on the binary image to smooth out noise
  • Scan across the image horizontally, one scan line at a time. Note the start and end locations of each white area. If the width is too small (less than two pixels) or too large (more than 10% of the screen) discard it. Otherwise save the width and centroid as a slice down a potential line.
  • For each slice, check if the next row down has a line slice within a couple of pixels. If so, add it to the growing polyline and remove it from the list of candidates.
  • Fit a line segment to the points in each polyline using least-squares, set its width to the mean of all slices used
  • Extend each line segment until it hits the edge of the image. If the projected line gets within a very short distance of BOTH endpoints of a second segment, the two are collinear so merge them. (This will smooth out gaps in the detected lines from dust specks etc.) The resulting lines are plotted in green in the calibration view.
  • Project the lines onto the X axis and sort them from left to right.
  • For each line from left to right, measure the perpendicular distance to the next one over (displayed in red in the calibration view).
  • Compute the median of these line lengths. This is considered to be the pitch of the lines, in pixels.
  • Find the median width of the lines.
  • If the pitch is less than three times the line width, we're looking at fine-pitch lines (10 μm pitch). Otherwise the fine pitch lines were too small to resolve and we're looking at coarse pitch (50 μm).
  • Compute the physical size of one camera pixel from the known physical pitch and pixel pitch.
Now that that the camera has been calibrated, we know how big a camera pixel is - but we don't have the scaling and rotation factors needed to transform between camera and stage coordinates. This is the second half of the calibration.

Rather than trying to compute a full transformation matrix, the current code simply represents a motor step for each motor as a 2-vector describing the distance (in nanometers, referenced to the camera axes) the stage moves during one motor step. We can then easily compute the distance moved by any number of motor steps as a linear combination of these two 2-vectors.

This algorithm is built on top of the same CV code used for the camera calibration.
  • Find the rightmost line on the calibration slide.
  • Locate the midpoint of it. (This is marked with a blue dot in the debug overlay.)
  • Check if this keypoint is in the left 1/4 of the camera's FOV.
  • If not, move left 50 steps, take a picture, and repeat until it is.
  • Record the position of the keypoint.
  • Move right, 50 steps at a time, until the keypoint is in the right 1/4 of the FOV.
  • Record the 2D distance the keypoint moved and divide by the number of steps taken to get the X axis step vector.
  • Repeat for the Y axis, except using 1/3 as the threshold instead of 2/3 since the Y axis FOV is smaller than X.

The system isn't quite finished but it's coming along nicely. Hopefully I'll have time in the next couple of months to finish the software, make a PCB for the control circuit, and machine brackets to hold all of the parts onto my Olympus scope.

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