Visual Line Tracking is Hard

Filed under: FANUC TP Programming Line Tracking Vision

I spent last week helping an end-user with their two-robot visual line tracking system. As far as systems like these go, this one’s pretty simple: just two robots, one backlit conveyor, one camera and basically one product. The entire codebase comes in well under a thousand lines. We had issues that were hard to solve despite the simplicity. What follows is my process while debugging several common visual tracking errors.

TLDR version: visual line tracking is hard.

Issue #0: What’s the Issue?

Customer complaint: the robots are missing roughly 1 out of 8 or 9 products.

What you mean by “miss?” Is the robot innaccurate or is vision failing to identify parts? Could it be a load-balancing or boundary issue?

Duplicating the issue in the customer’s presence is the best way to save a lot of wheel-spinning.

Issue #1: Trigger Distance

We watched the system for a few minutes, and sure enough, the robots let roughly 1 out of 8 or 9 parts through the system undisturbed. They looked pretty accurate otherwise, so it’s probably vision-related.

The vision runtime showed a problem immediately: we weren’t taking pictures often enough. Depending on the product spacing, the system might see half a part on the upstream side of the camera’s field of view (FOV) and then the trailing half on the downstream side.

     +-----------------+    +-----------------+
     |                 |    |                 |
     |                 |    |                 |
  +--|---+             |    |             +---|--+
  |//|///|   FOV       |    |       FOV   |///|//|
  +--|---+             |    |             +---|--+
     |                 |    |                 |
     |                 |    |                 |
     +-----------------+    +-----------------+
            Snap 1                 Snap 2

Solution: take more pictures by decreasing the trigger distance.

Issue #2: Vision Overruns

Unfortunately decreasing the trigger distance caused us to start getting a bunch of vision overruns. These errors occur when the system cannot process the current image before it needs to start processing the next one.

By taking pictures twice as often, we consequently cut the amount of time vision has to process images by 50%.

A quick look at the Track Sensor status page Status > Type > Track Sensor confirms that the vision processing time was almost always longer than the trigger interval.

Since we can’t slow down our conveyor or increase the trigger interval (that would put us right where we started), we have to try to improve our vision performance: simplify the vision process. Unfortunately, this vision process was about as simple as it gets: one “Blob Tool” with a pretty small range of acceptable area values.

I would expect a vision process like this one to take ~50ms, but this one varied greatly: sometimes <100ms, but often over 250ms. This indicates an overloaded CPU. Next stop: $idle_cpu_min.

Issue #3: CPU Load

The main robot has a lot on its plate: vision processing, queue management and ethernet encoder master. Even with all that work to do, I’d still expect to see some spare cycles available. Instead the available CPU was consistently less than 1% while the system was running. There must be a programming issue somewhere.

After a few minutes of sifting through the code, I found a spot where the robot is thrown into a tight loop while waiting for parts. By simply changing this action to be a one-time thing, we started seeing ~30% available CPU. With a few extra cycles to spare for vision processing, the vision process started running on the order of 50ms like I’d expect, and we eliminated the overruns.

Don’t celebrate just yet. The vision system is seeing the parts so well now that the robots have decided to handle the same parts twice: the classic “ghost-pick.”

Issue #4: Ghost-picking

Ghost-picking is pretty common and occurs when the same part is snapped multiple times by the camera. Luckily there’s a simple fix for this: increase the overlap tolerance. Overlap tolerance defines the minimum acceptable gap between parts when they are put into the tracking queue. If vision tries to throw a part into the queue that will end up right on top of an existing part, it’s thrown out as an overlap.

I encountered a red flag as soon as I went to increase this setting. It was already set to 500mm (way too high). With this setting, the system should discard any parts that appear within 500mm of each other. Then how the heck are the robots picking parts that are essentially right on top of each other? Is overlap tolerance broken?

Before crying wolf to FANUC, I took a few logs from the tracking queue. VQLOGs show when parts are put into the queue, allocated, acknowledged, overlapped, skipped, etc. These are extremely useful for debugging load-balancing and inconsistent picking issues.

A simple way to test if all parts are getting into the queue is to feed a known number of parts and compare it to the VQLOG. I fed 10 parts into the system and watched the robots handle them all, sometimes more than once. There must be some interesting stuff in the VQLOG.

The VQLOG indicated that many more parts were actually getting into the queue. A little baffled, I remembered a similar issue from a few years back where a customer had accidently put the robot’s tracking boundaries within the camera’s FOV. Let’s see if it happened again.

Sure enough, the upstream robot’s allocation boundary extended into the FOV. The VQLOG suddenly makes sense. Here’s what was happening:

  1. Vision snaps a part on the upstream side of the FOV.
  2. Part passes into the robot’s allocation window and it’s allocated to the robot.
  3. Vision snaps the part again on the downstream side of the FOV. The part is put into the queue because overlap tolerance doesn’t care about allocated parts.

By simply pushing the allocation window out of the FOV, the robots stopped picking the same parts twice. Unfortunately we’re now back to square one: lots of parts getting through untouched.

Issue #5: Overlap Tolerance too High

Remember the 500mm overlap tolerance? Now that we’re not stealing parts out of the camera’s FOV, overlap tolerance is working as intended, and it’s way too high. I set it to something more reasonable (like 50mm) and the picking finally started to look good.

Issue #6: Kiss of Death

Of course as soon as you fix one issue, the less crucial/annoying things need to get fixed. Evidently these robots have run into each other in the past. Sounds like we need to look at the boundaries again.

One of the many problems with tracking very high-speed conveyors is that the robot requires a pretty long track window to actually perform its tracking operation. Depending on the conveyor speed vs. the performance of the robot, it may track for quite a while. This particular system was using LR Mate 200iCs: great robots, but ~120ft/min is pretty quick no matter what robot you use.

At this fast conveyor speed, the robot will probably never have to reach the upstream boundary. By the time it’s tracked a part, it’s probably 200 or 300mm downstream, but what if the conveyor stops at just the right time?

Likewise on the downstream side: if you have the discard boundary set correctly, the robot shouldn’t hit it and get a “TRAK-005 Track destination gone” error. But what if someone lowers the robot speed or increases the conveyor speed? What if the robot goes for the next part instead of skipping one?

This system had the upstream robot’s downstream boundary almost right on top of the other robot’s upstream boundary. With this condition, if the upstream robot tracks out and the downstream robot attempts to pick at the very upstream side of its window, they’re going to crash. Moving R1’s downstream boundary a bit upstream and R2’s upstream boundary a bit downstream left ~300mm between them in the worst-case scenario which should be plenty. I also set both robots’ discard boundaries to almost the length between upstream and downstream boundaries to ensure that they only go for parts at the upstream-side of their work areas.

The Rabbit Hole

This is how I think these issues started:

  1. The integrator tried to increase picking performance by adjusting boundaries and accidentally pushed one into the FOV.
  2. Robots start ghost-picking, so they increased the overlap tolerance.
  3. A reasonable overlap tolerance did not work, so they kept increasing it without any luck.
  4. As a last resort, they set the trigger distance to roughly the length of the camera’s FOV so parts can’t be snapped more than once. As a result, some parts were never fully in the FOV: our original problem.

Visual tracking applications (and debugging them) is hard. It’s just one of those things with so many moving parts (see what I did there?) that a small issue in one place can cause other things to go haywire somewhere completely different. A pretty straight-forward customer complaint led me down the rabbit hole of trigger distance, vision overruns, CPU overloading, ghost-picking, overlap tolerance and eventually the line tracking boundaries before finally resolving it.

I’ve been unfortunate enough to frequent this trail a few times in the past. They’re always painful at the time, but once you struggle through issues like these, you won’t forget. Keep breaking (and fixing) things so you can fix them when they break unintentionally.

There's more where that came from.

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