Updated 1/9/05

P.A.A.M.I's Docking Station and Charging Contacts

 Last time we discussed the electrical schematics and circuitry for building a battery charger for your robot. In this part two of a series of three articles on self charging robots, we illustrate our method of making contact with the charger, and a docking station for your robot to dock with and charge its batteries when needed.

Contact types

There are three basic ways to connect your robot to a charger, first the robot basically plugs itself in to a standard connector arrangement, and second a frictional contact array or spring loaded separate contacts. A third method, which is seldom used is to use charging coils in a base and robot to inductively connect the robot to the charger. While the first method seems to be the obvious choice, the accuracy required to plug two connectors together makes it a difficult task. The third method involves large and heavy inductive coils and is generally less suitable for home robots. We therefore found that the second method is the best for around the home and is easy to make.

Contact plates

This method works with a wide range of misalignment , and is simple to construct. We tried spring loaded copper contacts, assorted guitar springs, pressure plates, contact fingers, carbon brushes, and even rolling metal wheels with metal axles. Each has its merit, but none was more successful and reliable for us than the copper plate and spring ball method. The idea is to have either the robot or base have a two plate flat copper or brass board, in which the spring loaded balls slide onto. The problem with putting a copper plate on the front of a robot such as is used in the Octobot, is that that valuable real estate on the front is taken up, where we would normally put IR sensors and bumpers. The same would be true with spring loaded contacts on the front of the bot. The answer then is to put the copper plate on the BOTTOM of the robot, and have it slide over the spring loaded balls. That way your not dragging the contacts around the house on the robot, picking up socks, and cat toys on the connections.

But does it work?

Using this arrangement, we have had 95 percent success on the first attempt when docking, and 100 percent on the second pass. When the robot fails to connect for some reason, it simple backs up and goes forward again and gets it on the second try. Our next article will detail the programming for docking, charging and escaping the charger in more detail. Now on to the hardware photos and descriptions!

 The docking / feeding station. The T shaped base contains the contacts and is stable with a three point support. Above it is the metal electronics box, containing the beacon array, 18vdc 1.6A power supply used by the robots charger, and the bumper to prevent impact with the clear beacon dome. On the top is a 20 pound steel weight to keep the charger in place. (Almost looks like a counterweight for a telescope, doesn't it?) The beacon sends a beam with 5 high power IR LEDs that is modulated at 38.5 kHz. The beam width is narrower than the docking acceptance angle for the robot - this is critical to insure a successful docking.
 Contact arrangement. Two brass balls from the hardware store used in lamp shades were drilled out (they are tapped for #8) to press fit onto the expansion springs. These are the type that are all squeezed shut and you pull on them to expand them like a rubber band. They sit about 1/8 inch above the bottom of the robot and will drag across the copper plates underneath the bot.
 Side view. To connect the wires to the springs, we soldered the wires first to brass shim stock, and wrapped it around the base of the spring. Then a tie wrap secures it to the spring. The springs can be bent over 90 degrees and be even run over by the robot with out any problem of springing back. And believe me, your robot WILL run over them from time to time!
 The underside of PAAMI shows the copper plate on the front for contact electrically. The left side is ground, and the right side is for the +18vdc. Note the barrier between them that has no connection for safety. The balls obviously must not contact both and short them so that why the barrier was put in. The width is about 3 inches and depth about 2 inches - plenty of "runway" for the balls and to stop the rolling robot in time.
 Close up of the plates. I used 4-40 pan heads to secure it to the lexan, and the balls just bounce off them when these hit.
 Looking at the front of the robot, just underneath to show the copper plates are flush with the bottom lexan. Of course the drive motors and stasis sensor wheel hang below too.
 Not an easy shot to take, You can see the balls on the springs leaning forward as they contact the underside of the robots copper plates. The black boxes contain the IR proximity sensors (IS417 type)
 PAAMI cruising along in an area of the kitchen where the robot seems to visit most frequently. The charger is just in the sun. IF she is hungry (low battery) AND the beacon sensor sees the beacon THEN the charge level in the 8 layer priority arbitration architecture kicks in and causes the docking behavior.
 First, she rotates toward the beacon using the beacon sensor. By rotating rather than banking the turn, we don't have to worry about hitting anything and therefore the charge processor subsumption level does not have to look at the IR prox or bumpers.
 Heading directly toward the charger. It may make a correction or two in direction along the way, but it generally heads right for the beacon within about an inch accuracy.
 Looking over the top of the bot as it heads directly for the beacon in the dome on the feeding station. The beacon sensor is also in a dome on the top of the robot to protect it from the evil coffee table shelf. It has a guitar string bumper in front of it as well. T'Pring watches as the robot nears the charger.
 DOCKED! The robot sees the voltage on its copper plates, then stops, shuts down everything except the charge processor and soaks up the energy. Takes bout an hour, then it turns systems back on, backs up and does a charger escape maneuver to clear the charger. Although the beacon sensor will see the charger many times in the hour or so It runs, it will simply ignore it until it has a low battery. Ahh, the wonders of Priority Arbitration Architecture !
MPG move Clips with sound of the robot in action docking.

Still from actual Movie
 385 Kb - for standard modem connections. In these movies, taken with my Sony P10 digital camera, the sound reveals the robots response to the charger. First, you hear the "Pip.....Pip" of the sonar of the level 0 subsumption level scanning the area ahead for obstacles. The when the robot sees the charger, it makes the "Happy Sound" and homes in on it.

Still from actual Movie -  half sized
 5.3 Mb - for high bandwidth connections. You'll like this one.
Still from actual Movie - half sized 6.9 Mb - for high bandwidth connections. Shows the bot hunting for the beacon on the way in.

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