Note: FRC Rebound Rumble Requirements

I have been thinking a little more about capabilities a robot needs to compete successfully in Rebound Rumble:

Shot accuracy tradeoff:

Comparing a robot that can make 100% of its 2 point shots with a lower accuracy 3 point shooter:

In hybrid a 3 point shooter can miss 1 out of 6 and still get the same points. (83 % accuracy)

In user mode a 3 point shooter can miss 2 out of six and still get the same points.   (67% accuracy)

To achieve the hybrid accuracy,  one can use manual sighting from the key line and design an accurate cannon.  Pending a lot of testing, lets assume that a ball trajectory variation of about 5 inches out of 100 inches or about 5 percent will give a 95% probability of success.   This translates to roughly 2.5% on a one sigma basis or about 1.5 degrees one sigma in angle accuracy.

If the robot moves forward to the fender, it can reduce the range to 4.5 feet from 9 feet and the % errors are inversely proportional to the range… so one could stand a one sigma error of around 5% or 3 degrees.     This makes for better scoring if you are aligned to the target… however, you lose you manual sighting and must rely on hybrid or autonomous targeting.  I am not sure that you can rely on encoders or gyros to keep you aligned to within 3 deg because you might bump a corner of the robot or slide a few inches sideways.   However,  in hybrid, a camera bore-sighted with the cannon can display the aiming error and the Kinect driver could make fine corrections where necessary or there could be auto aiming by closing a control loop adjust both range and sighting angle.   This is more complex and shot reliability may be degraded since you are doing more things.  Also when a robot moves toward the fender it takes longer to get to the bridge and retrieve possible extra balls to shoot especially if planning on capturing the ones on the cooperative bridge.

I believe there is great utility to scoring one or two extra balls in hybrid even if they are put in the 1 or 2 point baskets.  The 3 point bonus looms large.  So I recommend that shots be taken from a manual sighting from the key and then a quick maneuver to get the balls off the bridge.  I believe a robot can quickly align with the bridge using autonomous programming but if a ball doesn’t fall into the ball scoop then it is possible for a Kinect operator to retrieve a loose ball and if maneuvering is simple might still be able to score an additional ball.   Here  I think Mechanum wheels would have an advantage over a 6 wheel robot since the robot can keep its alignment a wall while collecting a ball and then make a quick run to the fender for a shot without making turning maneuvers.  This would simplify Kinect control since the robot is only moving forward or sideways.   … So need to have a requirement for arena based heading either from gyros or encoders.

That Darn Bridge:  The team chose to make getting the 40 point elimination balance bonus a priority.   However, I do not see a design that can achieve this reliably and therefore driving the design in this area at the detriment of scoring may not be the best trade-off.   Perhaps we should strive to be compatible with a super 3 robot balancer robot rather than be one and focus more on being a good shooter.

There are three basic approaches  to three robot balancing and each has its flaws:

1) Ride the rail with a skinny ,  half hanging robot.  This is great, but there must be a 10 in wheel base to support this.   This could be achieved by adding extra rider wheels that don’t normally contact the rug.  I suspect that these would have to be powered since using the power of just the two inside wheels would drive motor currents to roughly 50% of max for as long as you are on a tilted ramp.    Normally, we design a robot to have enough torque to accelerate at 1 g .. typically requiring 40 amps max.   To climb a 15 deg slope you need .25 g capability.  With all motors powering, this would require roughly 10 amps per motor.  If you have two wheels off the rail then we need 20 amps.    So it would be important to have the extra rider wheels powered.       I believe both 6 wheel and Mechanum drives could be adapted to use this concept.

2) Fit three robots end to end on ramp.   Allowing 12 inches for balancing adjustments this leaves 72 inches to share among three robots.   Allowing another 12 inches for 4 bumper widths leaves 60 inches for three robots.   So one can fit three 20 inch robot frames on the ramp.     Typically, two robots must be sideways with narrow profiles and the other one could be length wise and hanging off half of the robot or one robot would be sideways and two robots must hang off. ( very unlikely to achieve).    If robots are sideways, it is hard to imagine that they must not have the capability to adjust sideways as the robots work their way up the ramp.  Hence, a 6 wheel robot would be at a disadvantage to a Mechanum robot here.   A 6 wheel robot could get towards the center of the ramp and turn sideways but it will likely have to reposition itself to help the balance.   In this scenario , an active auto balance system could work better with the Mechanum.    In a two robot balance, both would work ok with auto balance.

3.  Fit two robots end to end, one with a third robot stacked onto it.    This scenario probably will surface.  I can envision a specialized robot that is one foot high that will take a piggy back robot from the ramp onto its back and then join the remaining robot on the ramp for a two robot balancing act.   This to me is the most effective way to balance three robots but it requires careful coordination and several configurations of robots on the ramp… If it is a real threat I also believe that a good defense can be done by restricting access to the ramp and delaying the balance.      If an alliance tries to achieve this from their lane then several offensive robots must cross the bump or the ramp to get into the lane.  A good set of defensive robots can run interference to one or two of these robots to prevent a three robot balance.   If offensive robots try to get on the bridge opposite the lane then again they are vulnerable to blocking or temporary pinning.     As a defender, a six wheel robot probably has the edge over a Mechanum due to its traction and pushing strength but a Mechanum robot might better defend against a very agile robot.

Not probable but it will happen:  Based upon the difficulty either from execution complexity or good defense I believe that there will be a small number of successful 3 robot balances.   Maybe less than 1 in 10 tried.   So the average points gained would be more worth 4 rather than 40 points.  This is  based upon the fact that if you fail to balance three, then likely you will fail to get two or even one balanced.

Teams will only attempt this if absolutely necessary since it is very risky.  But if you are down by 6 to 10 points when the last 30 second period starts… then it is worth the risk.    If I was the alliance with that kind of lead, I would defend heavily against the three robot balance and probably make a tough job almost impossible.     If an alliance tries to set up before the thirty-second point, they give up opportunities to score.

Mechanum over 6 wheel drive for 599:

One can always make an argument for sticking with the tried and true 6/8 wheel drive.  Many robots will have it and one or two will likely be on the winning alliance.    Based upon the discussion above re the three bridge balance  I don’t see this requirement driving the 6 wheel drive.  If anything, I think the added maneuverability coupled with a skinny profile can make a Mechanum design more robust and also fit better with different types of robot designs to achieve a balance.   If it is a wash with the 3 robot balance, then the Mechanum would be favored for the targeting/ball retrieval tasks.

But… my main reason for favoring Mechanum’s is to gain experience with them.   We will never know unless we try them.   (Same with swerve drive which I think is highly favored by this game…but we can’t do this because we as a team have been reluctant to try these too. )

Skiff Design:  If the skiff is going to pull down the bridge it must turn around and go up the bridge to keep the skiff able to grab the next robot bumper to help keep it on the end of the bridge.  Perhaps the skiff should have the capability to swing completely to the other side for knocking down the bridge.  It  could have a foward slant and allow the bridge to slide down it even while you are driving with some speed.   This capability would require a clamp on the frame that would relieve the stress on the pivot joint.  The clamp could be pneumatically actuated….. at last an excuse for air on the robot.  Also, the arm could be use to lift the robot off the bridge and act as a brake if there was room to deploy it.

Having the ball scoop on the opposite end of the robot than the direction you are shooting has an advantage in that it extends the rear of the robot to allow you to shoot about a foot closer to the rim and still stay in touch with the colored key.   Also it allows robots to feed the scoop from the open field rather than having to go between you and the rim.

Braking requirement:  Wouldn’t it be nice to drop a foot or be able to lock the wheels once you get to the end of the bridge?  Using the motor current manually or with encoder position feedback seems iffy particularly if there is movement in the bridge that creates transverse inertial forces.


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