End game latching and leaving motors enabled

[Coach_Dave]

Hello fellow Coaches/Mentors!

My team wants to start hanging on the lander and finish latched on. However, due to the gear reduction not being high enough, the weight of the bot is able to back drive the lifting mechanism if the motor is not holding position.

The team has been able to program the motor in position control so as long as the motors are enabled, it works fine. However, when we reviewed the match sequence flowchart (1.5.7 in Game Manual 2), it appears that we have an issue at the end of the runs because the team has to press the Stop button before the referees do their inspection.

I think at the start, they should be fine because during the autonomous program, the Initialization portion of the code can lock the motor. So the team can get the robot in position and press the Init on the Driver Station and it should stay hanging before the start. (Can someone confirm that this is a correct assumption on my part)

The concern is at the end. The bot can hang as long as the program stays running. However, if the team has to press the stop button before the judges inspect end game latching, the bot will drop down.

I suspect that we will need to find a mechanical solution to the issue but I wanted to confirm this first. Thanks in advance for any advice.

[jkenney]

Regarding end game: our assumption, and our experience at two tournaments, has been that the 50 points are awarded for latching as long as the robot is off of the floor at the moment the match ends. Our robot (and many others we've seen) drifts back down as soon as we press "Stop". The only problem I can imagine is if you are only lifting the robot a few millimeters off the floor at the end, and the scorekeepers can't clearly see that it's off the floor.

[NEOFTC]

The game manual part 2 and the ref manual (available for all to see) make it clear that the refs will make a visual determination immediately. They will not do any evaluation after stop is pressed. So you had better make it obvious that your bot is off the floor.

[Coach_Dave]

Thanks jkenney and NEOFTC. I am nervous because I recall seeing a ref hunched down and sliding a piece of paper under the bot checking for contact with the ground at the end of the match. I don't know if Stop was pressed before or after this inspection. I will talk to the team about making sure to let the scoring ref know that the robot will drift down at the end.

NEOFTC - Can I ask for a favor? Can you point me to the section in the game manual and the ref manual where "a visual determination immediately" is made before Stop is pressed? I've looked through both manuals and have not been able to find this. The ref manual I looked through is this one:

https://www.firstinspires.org/sites/...ad-referee.pdf

Thanks again!

[jkenney]

Regarding initialization for autonomous, see the following (which discusses risks associated with what you are proposing, as well as an alternative strategy):

https://ftcforum.usfirst.org/forum/f...ng-before-play

Also see answers #4 and #5 in this game rules thread:

https://ftcforum.usfirst.org/forum/t...-match-answers

There's not a way to "lock" the motor, but you can tell it to maintain a specific position using run-to-position mode. It will be running and draining the battery during the entire initialization period, which could be prolonged. That may or may not be sufficiently motionless to satisfy answer #4. If you decide to go that way, you might want to pose a very specific question about it on the rules forum.

[NEOFTC]

From the ref manual:
Scoring Latching End Game Achievement The definition of latched includes language that advises teams to make sure that their latching action (and elevation of their robot) are obvious and unambiguous. We will NOT be jumping into the field to perform

[Coach_Dave]

Thanks again jkenney and NEOFTC.

In case someone is following this, the answer to my initial question is:

• Locking motor during init of Autonomous is OK as long as the robot is stationary. Caveat is that it may be a long wait before the match starts and worry about battery drain. In our case, the gear reduction is enough that a small torque is needed to hold the bot up so this shouldn't be an issue.
• Locking motor at the end of the match after Stop is pressed is not required. Caveat here is to warn the scoring judges that the bot will drift down when Stop is pressed so they can take note of the latched state when the timer hit 0:00. It also needs to be clearly seen that the robot is above the ground while latched.

Also, as a Public Service Announcement, I didn't know about this particular area in the Forum that is exceedingly helpful.
https://ftcforum.usfirst.org/forum/t...d-game-answers

[RatLabGuy]

Keep in mind that not only is battery drain an issue, but heat may be as well. Depending how you're running it this is a classic "stall" situation and 100% of the energy going to the motor becomes heat since it has nowhere to go. This can burn out a motor.

A suggestion:
If your lift mechanism is run by a gear that is connected to the motor via chain, rather than directly driven (which I strongly recommend, to avoid too much direct stress on it) then you can use a servo to swing a little arm down that adds pressure to the chain, like an old school brake. Our rookie team had the exact same problem as you and did this on their bot this year and it seems to work flawlessly.

Another option is to convert to a worm gear to drive the lift. It cannot be backdriven.

[Coach_Dave]

Thanks for the tip RatLabGuy. Overheating the motor brushes with the motor in the stalled condition was a concern so the team tested what current level was required to hold the bot up. It turned out to be 10% so it was good to see it being that low. The team will be doing a longer term test of motor in this stalled condition to make sure we don't run into surprises.

The mechanical solution to the problem I mentioned is similar to what you describe with the servo. Unfortunately, worm gear is not an option at this time (we don't have one).

Thanks again!