Re: Articulated arm vs Forklift style arm
 

For those with electrical arm and elevator experience, how do you keep your arm or forklift in the position you want it without power. How do you make the up and down appear so easy and controlled?

Our students built a four-link arm in the off-season for a game they made up. (We simulated a build season with them to get them ready for this season) They used one of the locking Dewalt transmissions and a CIM to drive the arm and while going up was good, controlling the down was difficult. We did try to do some balancing, but overall, once the brakes released on the Dewalt, the arm was too quick going down to control nicely.

My question to the veterans with really successful mechanisms, is how do they control their arms and forklifts so nicely? Do you build custom worm gear drive trains, or use really inefficient gearboxes to establish the same level of control going up and down? For balancers do you use torsion springs or gas shocks?

Re: Articulated arm vs Forklift style arm
 

Fork-Lift
Pros:
- Compact, even in the scoring zone
- Consistent positioning in the y & z axes in relation to the scoring
- 1 Sensor can determine multiple positions
- Extremely simple to adjust height for minor corrections

Cons:
- Must be very rigid to prevent torsion & binding when the lift is at maximum height
- Generally heavier due to (typically) multiple stages
- Requires upkeep every match -- PROPER tension on cabling, cabling remains on pullies, ensuring the winch hasn't jumbled the cabling, etc
- If linear slides are used, they limit the ability to powder-coat (not a con for most in FRC, however in the real world this is a consideration since EVERYTHING must deal with corrosion if it's a long-lasting product -- so we teach it to our students as well)

Rotating Arm
Pros
Simple to build if it's 2 joints
Very easy to program so long as the joints are geared slowly enough (standard PIDs)
No need for a braking mechanism if it's geared enough (due to lightweight game piece this year)

Cons
- The drivers have to be different distances from teh scoring pegs depending on how high the arm is
- It must be 2 joints to reach a game piece on the floor (most 2007 designs make the bumper configurations illegal under 2011 rules)
- Different orientations of the shoulder joints may cause undesiered re-orientations of the game piece -- in 2007 many teams took advantage of it (see 330's diagram in the Behind the Design book) yet this year that's a bit more difficult (IMO).

Re: Articulated arm vs Forklift style arm
 

In a nutshell: Balancing. Make sure your arm is counterbalanced either through stored energy or weight and it will be a lot easier to hold up.

I've heard some people say they use a PID loop for this purpose and some people say not to, depends on who you ask...

Re: Articulated arm vs Forklift style arm
 

Suction devices will not work, even if you generate enough power to keep hold of the tube, it will make the rest of your robot lose speed and power. Besides, the slightest collision knocks the tube off of the vacuum. So either way, it is not efficient

Re: Articulated arm vs Forklift style arm
 

Our team has an idea for a dual conveyor belt system that pulls in the tubes, then it flips the tubes so they are vertical. At which point a telescoping arm with a hook grabs the tube and lifts the tube up so it can place it on a scoring grid.

Re: Articulated arm vs Forklift style arm
 

We have built a couple big arm bots and 1 forklift. There is a really good article with pictures of our 2007 bot in the Behind the Design book. It also discusses the controls. On both the 2005, and 2007 machines, we had a high axis arm with a large drive sprocket. We also used a 3-4" lever off the back of the arm and tied multiple loops of bungee. The bungee was very very tight. For 2005 there was approximately 200 lbs of downforce on those joints. make sure the bungee is fresh for that year (it degrades quickly especially if stressed). Also, when operating in cold environments, the bungee will stiffen. Warm it up to room temp before operating your arm otherwise funny stuff might happen.
The resolution with the shoulder joint was not the best due to the locking pins in the dewalt, and the use of FP motors which burn up in stall very easily. Because of this we had a forearm on both of those bots that would adjust relative to the shoulder position. Ideally you want angular feedback right at the actual joint for rotation. This will give you true position vs. using a sensor in teh chain loop that will have a little error from chain lash.

I don't remember what we used as the drum brake for our elevator in 2008. In fact, I am not sure that we had one.

Re: Articulated arm vs Forklift style arm
 

I suggest you look to team 1771 in 2008 picking up a 48 inch trackball that weighed orders of magnitude more than a tube:
http://www.thebluealliance.net/tbatv.../2008arc_qf1m1

I'll go a step further. I will bet you a steak dinner (this is the accepted bet on our team, to be collected at an away-regional or competition) that not just 1, but several teams do a vacuum system, do it well, and consistently win matches with their robots.

Nearly every idea we brainstorm in our team meetings ends up being implemented by someone successfully on the field. That includes the ones we've decided not to use because of some perceived technical problems - like vacuum in '08, a dumper bot in '09 (but you'll need to be able to shoot at different angles so a turret is perfect!), and linear-motion kicker in '10 (binding killed ours).

Re: Articulated arm vs Forklift style arm
 

I'll take that bet. See you in St. Louis. :P

Re: Articulated arm vs Forklift style arm
 

We actually made a 3 pronged suction claw and it worked pretty darn good but we still shot the idea down. But the tubes did stay connected to it while jostling the holder.

Re: Articulated arm vs Forklift style arm
 

An arm would pick it up faster but u would have to position more to pick the tube up than with a forklift. so i would go with a forklift. hope this helps.:cool:

Team 639

Re: Articulated arm vs Forklift style arm
 

Sometimes when asking for advice it is hard to make a good decision based others people’s opinions, as there are so many of them. A great tool for sorting out the opinions and making a sound decision is to create a decision matrix.

On the left hand side are all of the your possible decision alternatives or options. Next enter the determining factors or requirements that you are interested in along the top. You then need to decide the weight (scale 1-10) for each factor.

The final entries are for your opinions (scale of 1-10) of what your think a given factor ranks or meets your factor or requirement.

By multiplying the rank by the weight and totaling up all the factors for each option, it is easy to see what option will meet you requirements.

Example: