The toilet paper wars have been raging for decades, with two sides deeply entrenched. I’m curious if there is a similar preference for rope climbing robots.
Do you go overhand or underhand?
For TP, overhand means that the accessible portion is on top of/in front of the roll; underhand means the accessible portion is behind/on the bottom of the roll.
Similarly, an overhand robot, as it drives into the rope, pulls the rope over the top of the spinning apparatus, while an underhand robot, as it drives into the rope, pulls the rope under the spinner.
I think all robots end up underhand by this definition. I think both spinning directions will end up pulling the rope under the spinner, so I’m not sure you can differentiate. with TP, there is a wall to use as a reference plane.
I think if you aren’t doing underhand, you are just making it more likely that your rope won’t catch. Eventually overhand turns into underhand, but it has to loop to the backside of your drum before you get a full wrap, during this part it is possible for your rope to disengage from your drum.
The moving robot is your reference plane. As I said, as the robot drives into the rope, it either takes the rope over or under the drum. The direction of travel creates the reference.
With TP, you typically have a fixed point of reference - a wall behind it, with the user in front. So from that fixed point of reference, you can say it either goes overhand or underhand (goes over the top towards the front, versus under the bottom when pulled towards the front).
I think we need to first define our point of reference here. Are we talking about the bulk of the robot (for team’s where the climbing mechanism isn’t in the center of the robot) as being the “back”? Or maybe referencing the airship, assuming you always approach it from the same direction? And what about teams that drop the rope inside their robot, then back away to snag it?
Maybe a better way is to reference what happens with the rope itself. In one direction, you grab the rope and start pulling up, with the tail end of the rope just hanging out until it eventually gets caught underneath the rest of the rope. In the other direction, you immediately fold the rope over so you’re winding it onto the tail end. I’ve attached a crappy image of these two methods.
Underhand. We have our “spool” on an arm, and thanks to the ratchet, when we turn the empty spool overhand, it raises the arm into the frame perimeter. Because the arm is stopped only go a few degrees inboard of upright, the underhand spin pushes the arm out as well as powers the climb.
I guess I’ll offer the dissenting opinion for our overhand rope grab.
Our robot’s lifter is build into the chassis, down low. The highest point of the lifter is at about 4" and the robot’s center of gravity is about 4-4.5" off the ground. If we ran the lifter underhand, the rope would come out from the bottom edge of the robot, and the robot would tilt backwards when climbing. This would mean resting the robot weight on the fuel intake during the beginning of the climb and landing on the fuel intake and hopper if we fall.
The first moment that a rope touches the drum, it has almost no hold. Running underhand means that we’re fighting gravity to get the initial wrap, going overhand means that we’re taking advantage of gravity for it.
How many wraps do you get before the robot starts going up?
To be safe, we want 1-2 wraps around our climber before reaching full tension on the rope. With a 2" climber diameter, we need about a foot (Happy Pi Day everyone!) of rope to get two wraps. We achieve these wraps by having a slipknot come loose as the robot pulls on the bottom of the rope.
