Posted 28 May 2022

After getting the IR homing capability working with Wall-E3, I started working on the ability to recognize the charging beacon during normal wall-following operations, and then transitioning (or not, depending on battery level) to the charge station docking procedure.

The ‘transition-to-charge’ feature assumes that the robot is tracking the right-hand wall within a few meters of the charging station, and detects the beacon. To connect to the charger, the robot must first navigate away from the wall to a point on the IR beacon centerline, turn to line up with the beam, and then follow it to the charger.

As I was testing this feature, I noticed at one point the robot detected the IR beacon while tracking the wall opposite the charger, not the wall where the charger was installed. This is a real problem, because my current ‘transition-to-charger’ algorithm assumes the geometry associated with the common wall case. In the common wall case, the robot makes a 90 deg turn away from the wall and moves to a spot calculated to put it on the beam centerline, but this won’t work at all if the robot is starting from a point on the opposite wall. In that case, the robot needs to travel forward or backward parallel to the opposite wall by a distance calculated to put it on centerline. So, the first thing the robot needs to be able to do is to detect which case (common wall or opposite wall) it is dealing with.

The primary difference between the two cases is how much the robot has to turn away/toward the wall it is currently tracking in order to point directly at the beacon generator; if it is on the common wall, then it may not need to turn at all, or may even have to turn a bit toward the wall to point directly at the generator. If it is currently tracking the opposite wall however, it will have to turn 30-60 deg away from the wall. So, the determination as to which case is in play, the robot needs to measure the number of degrees of heading change required to face the charger; if the heading change is 30-60 degrees, then it is the opposite wall case. If it is just a few degrees, then it is the common wall case.

The following short video shows the two cases and the heading change required to point to the charger in each case.

21 June 2022 Update:

Progress has been a bit slow over the last month, as I’ve been doing other things, including recovering from an eye/scalp injury sustained at the Senior Games in Florida (those old guys can still pack a wallop!). However, I now think I have the charging station homing operation working, at least for the right-wall-tracking (the most common and probably only) case.

As it turns out, I didn’t really need to worry about the problem of the robot picking up the IR homing beacon while tracking the far wall (i.e. the wall perpendicular to the one associated with the charging station). As I noted above, the robot can sometimes ‘see’ a beacon signal above the threshold, but when it does, the steering signal (goes from -1 to +1) is always off scale on one side or another. So, by adding the requirement that the steering signal be within the range of -0.8 to +0.8, this case is entirely eliminated, leaving only the ‘same-wall’ homing case – yay!

I made a number of improvements to the IR homing code, in particular the ‘initial approach’ code that places the robot optimally with respect to the IR beacon beam, so that the subsequent ‘home-to-charging-station’ procedure is almost always successful.

In the following short video, the robot tracks the opposite wall, makes the turn to track the adjacent wall, and then detects the IR homing beacon at the conclusion of the offset capture phase for the adjacent wall. Then the robot transitions to ‘I’m hungry – feed me’ mode.

In ‘feed me’ mode, the robot first aligns itself with the beacon in a two-stage (coarse and fine) procedure. Once this is accomplished, the robot determines its distance from the beacon, and then uses this information to determine how far off the adjacent wall it should be to perfectly line up with the center of the IR beam, makes a turn to be perpendicular to the adjacent wall, and then moves forward or backward to achieve the desired offset measurement. Once this is accomplished the robot re-aligns itself with the beacon again, using the same two-stage procedure. Once all this is done, the robot then homes on the beacon to connect to the charger.

The above algorithm is shown in action in the following short video. When viewing the video, keep the following in mind:

• The homing beacon is only recognized at the conclusion of the wall offset capture maneuver for the adjacent wall, even though the IR beacon signal is above the threshold as soon as the robot make the turn to the adjacent wall. This is done to place the robot at a known location to start the process.
• While aligning itself to the beacon, the robot turns ON its red laser pointer to allow us humans to follow the action. Although the ‘red’ laser looks mostly white in the video, it is indeed red.
• The beacon alignment takes place in two stages – coarse, then fine. This can be see by the speed at which the red laser dot moves back and forth.
• In this particular video, the robot is already at the proper wall offset, so the ‘move to desired rear distance’ operation is truncated – the robot makes the turn to be perpendicular to the wall, determines it is at the proper offset, and so immediately turns back to align to the beacon signal.