We are using a winch in order to telescope this year, and the only realy problem with our setup is that when there is no powere given to the motor (once it is no longer stalling) the motor will backdrive. to eliminate that i thought i could use some kind of ratchet. So my real questions are:
Have other teams used ratcheting winches, how did they work ?
Did you only have it ratchet in one direction or did you do both ?
personally i would like to do both but i have no clue on how i can engage and disengage the pawl.
Is there anyother way to do it without ratchets?
worm gears are out of the question, because i cant really affor to reduce the motor anymore.
The only other setback is that we are not using pnumatics so they are basically out of the question.
Last year, our team used a telescoping arm/ winch system for our pull up arm.
We used a latch that would prevent it from back driving. The latch would engage/disengage from a notch in the winch drum whenever we wanted it not to back drive (the operator would press a button to latch the winch). It was spring loaded to pop into the notch, but we used a servo to hold it out of the way until we wanted it to go in.
It was very simple to make actually… we used two FP motors, with the FP gearboxes and “drums”. We bolted the two drums together to make one big one… so we had two FPs bolted together, mirror images of each other. We then notched out the the raised portion of the winch where the two drums met together… with a hacksaw. As for the latch, it was just a rotating metal bar, pushed down toward the notch by a spring, and help up and let down by a servo.
The whole concept and implementation of this mechanism was really quite simple. Let me know if you don’t quite understand how it worked so that I can find a picture or whip up a paint sketch.
My advice for a mechanical solution would be to include a worm gear to prevent being backdriven.
Other options would be to leave the calibration off-true just enough as to equal the force down so that at ‘rest’ the motors would turn to fight being backdriven. As a general warning, the first method is a tried and true method. The second is me thinking on my feet (ok more-so thinking on my tush).
On our arm we are using a worm gear to prevent it from being back driven. The smallest worm gear we could get was 20-tooth, and a 20:1 gear down on the van door motor is too slow, so we had to gear it up 6:1 (or should that be 1:6…) first. Now this is for our arm, which won’t move more than 300 degrees or so, you would probably need a lot more gearing up for a winch. Had to get some thrust bearings to support the worm.
It would probably be overkill for a winch, a servo controlled latch would probably be easier, but just sharing what we did.
I can’t seem to find any good pictures, but last year, I am proud to say, we had an extremely simple and effective ratchet design. We simply took a ratcheting socket wrench, cut fice slots in a large socket to mesh with the FP spool driving our winch, and bam, no more backdrive.
You could conceivably do the same thing, put a servo on the direction selecting switch on top of the wrench and control the direction it works in.
Simbotics used one of these last year on the telescoping arm; it worked by inserting a pawl into a groove, in order to keep the shoulder joint from unfolding when hanging.
With regard to the worm gear idea, you might want to think carefully about the disadvantages inherent to a reasonably unbackdrivable worm gear system. The amount of effort to backdrive a worm gear is a function of its efficiency; a 40%-efficient worm gear won’t backdrive easily, a 75%-efficient one will require much less effort. Since you don’t necessarily want to dump most of your power in an inefficient reduction stage, a worm might not be such a great idea. Some worm gear systems, like in power winches, are designed to be more efficient in one direction than the other–this would be ideal, but I would think that this would be difficult to engineer, without customizing the gear profiles.
I would, instead, try to use a ratchet & pawl system, maybe off of a hand winch, or something from SDP/SI (look under “Ratchets & Pawls”, appropriately enough). Attach it to a low-torque section of your reduction gearing, if possible, to avoid stripping teeth. Disengaging and engaging the pawl is probably best accomplished with a servo.
Alternatively, any of the methods used for dog-shifting transmissions (hex dog, square dog, dowel pins) could be employed, if you don’t need a wide range of locked positions.
We did this also last year on our rack and pinion. The real nice trick that we used was it was all done through software. When we moved the arm down, it ratcheted past it. When we moved the arm up, you would press the up button as normally, but he software would delay moving the arm, and instead first release the servo. With this, the drivers would barely even realize in a match there was a servo on there at all. Minimizing functions your driver performs I’ve always found to be crucial (Hey, we’re slow!)
Last year my team used a winch to lift our robot off the ground. We had a latching servo mechanism like a coupel teams have mentioned above. Just add a gear or sprocket to the pulley you want to lock. Then add a spring loaded servo with a piece of metal attached to the end of it that can jam up the sprocket or gear you placed on the shaft. Very simple. We jury rigged one up that was competition usable in an afternoon.
Now all that being said, i don’t think teams will need a ratchet like they did last year. Think about the requirements for last years extenders. They had to be able to hold 130 pounds of force and they had to hold that once power was cut. Specifically the second reason was why teams needed a ratchet. This year you’re probably lifting about 12 to 30 pounds depending on how heavy your manipulator is and how many tetras you’re carrying. At most of the stock geardowns the motors will able to generate that torque without drawing much power and thus generating little amounts of heat. Its defenitly within the, “It’ll surivive a 2 minute match and a victory lap” zone. That doesn’t mean you shouldn’t use a ratchet, just that its not necessary.
Finally if you don’t have a ratchet what you do need to do is put your victors in brake mode. They have two modes, brake and coast. If you have them in coast mode they will just let whatever they are connected to backdrive. If they are in brake mode they will counter and backwards motion automatically(motors backdriven act as generators, so the current they generate acts a signal for the victor and it just sends the power back.) This will give the arm or winch mechanism the ratchet feel without a ratchet. This is how you generate heat in motors though.
i will post a sketch becasue i helped design that piece on the arm last year. like jaine said the latch was held up by a servo, and pressed down by a spring, it was mounted using leftover air cylinder mounts. basically the metal bar would push aginst the flat part of the drum that we had notched out. the drawing is a cross cut of the drums that fit on the fp motors.
Last year, after finishing the gearboxes, I took charge of designing the lifting mechanism. I used the same principle that is found on boat trailers. It worked great. It was only one direction, spring loaded latch. I’ll include a picture, keep in mind the latch is back wards in the pic. Perhaps you could use a small pneumatic cylinder to control the latch instead of a spring, or some adaptation of that.
Do you understand the drawing that Dillon posted? The one he drew was a cross section of the winch drum.
The one I drew is a picture of the two FP drums bolted together. The black square is the notched out portion. The end of the rotating metal bar would be raised and lowered inside the notch, consequently allowing or disallowing the winch to rotate.
