JVN Build Tip: Mechanism Loading

[JVN]

So if you're on one of those teams actually doing engineering and not just throwing together a robot you may have done some math. It is likely you're creating some sort of system to lift up the tubes.

I'm want to talk about the secret of elevation mechanisms... passive assistance!

When one designs an elevation mechanism one typically chooses a motor, determines how much load you're lifting (the object + the weight of the mechanism itself) then calculates the gearing such that the chosen motor can lift the load without drawing too much current.

What does this mean? What is the definition of too much current?
In my experience this is dependent on two things, firstly the circuit breakers. The 40 amp breakers we use can only run so much current continuously before tripping; surprisingly enough this amount isn't actually 40 amps. The time it takes for the breaker to trip varies depending on the amount of current applied. I tend to design for a "worst case" loading of 45 or 50 amps knowing that I'll beat my driver if he or she puts the robot through worst case loading for more than 5 or 10 seconds at a time.

The other thing which determines "too much" current draw is the motor itself. Some motors are air cooled by the motion of the motor itself -- they have built in fans which draw air across the motor windings when the motor is spinning -- no spinning, no airflow. These motors are designed to run fast and don't handle stall very well, even at low voltages -- you should design these motors such that they see very little load and require very little voltage to hold that load.

Some rough rules of thumb:
Don't stall any of the banebot motors except the 775, and be worried about the 775 because we're still getting to know each other. Be very careful with the FP motor. The Window motors have a built in thermal cutoff so they probably won't blow up, but they also probably won't do what you want in that situation. CIMs are champs -- do whatever you want, the breaker will trip before the CIM even flinches.

Ok, so let's assume you've calculated your gearing based on the applied load and the motors you've chosen...
What happens if your gearing calculation is too slow? What happens if you do your math such that your motor is handling the load (this is why we do the math) and it ends up taking 15 seconds to raise your elevator?

You have four options:
1. Deal with it.
2. Gear the arm faster and run more current through the motor and hope something bad doesn't happen.
3. Add more power to the system (bigger motor, more motors)
4. Reduce the load on the motor.

Option 1 sucks, option 2 is dangerous, option 3 might not be available, so I really like option 4.
So... how do we reduce the load?

Now picking up a lighter object isn't always possible (certainly not this year). Reducing the weight of the mechanism sometimes helps, but this isn't always an option either. So what is another way to reduce the load on the motor? Passive Assistance! There are two main ways people use passive assistance; add a counterbalance to their mechanism (i.e. hang a weight on the backside of an arm) or add some sort of spring loading to assist the mechanism's motion.

People often commented they were surprised how fast 148's arm moved in 2007 powered only by a single globe motor (a relatively low powered motor) while some other teams were using CIMs and other much more potent actuators. The secret was the surgical tubing on the arm. As far as the motor was concerned, the arm weighed NOTHING. To hold itself steady required minimal voltage on the motor which meant minimal current was drawn (even at stall in the max load configuration).

We use surgical tubing for our load balancing, mostly because of how easy it is. You just stretch it between a stationary point on the robot, and the moving mechanism (preferably with some mechanical advantage via lever arm to help move the mechanism).

Now surgical tubing doesn't provide a constant force -- the force applied varies linearly based on stretch. The load on an arm type mechanism isn't constant either (when the arm is straight out it has more load than when it is at the top or bottom of it's rotation). Does this matter for our purposes? Nah, not really -- you can get it "close enough". On 148, we just keep adding surgical tubing bands until the arm is "weighless" -- sometimes we'll add a few more for good measure beyond that.

We buy McMaster P/N 5234K74 as our surgical tubing, mainly because we like the size, and we like the color black. We just bend the tubing over at the ends and zip-tie it into a loop (make sure you stretch the tubing before you zip-tie it, otherwise they'll fall off later). We then run another zip-tie through this loop to attach it to the robot.

Other teams use gas shocks for their passive assistance -- but we find tubing simpler to use and simpler to adjust (you don't need to calculate all the loads exactly right, you can "play with it").

Experiment with using Victors in "brake" mode with this happily weightless arm, and you should be able to do some quick code which makes it VERY easy to control -- no need for anything fancy, unless you're the type of team who just really likes getting fancy...

If you're still designing your mechanism for this year, you can plan ahead knowing that the motor won't see a lot of load (just the weight of the game object, and the friction in the system). This means you can gear the mechanism very quickly, or you can use a much smaller motor for your mechanism.

Believe it or not, you don't need 2 CIM motors to lift a 1 lb tube 15 feet in the air, you can do it with MUCH less power...

Let me know if this tip helps you out, and as always, good luck in 2011!

Originally posted here:
http://blog.iamjvn.com/2011/02/jvn-b...m-loading.html

[David Guzman]

Thank you John.

I wish someone had shared stuff like this with me a few years ago.

[thefro526]

John, these tips are great for any team, rookie or veteran.

We've experimented with Passive assist for the First time that I know of, and it made a WORLD of difference. We're still searching for that point of near weightlessness, but even now, a few bands of tubing is helping us a lot.

From what we've found, and it sounds like you're coming to the same conclusions, the RS775-18 is a GREAT motor. We were running ours for around 20 minutes, with as a lift or two each minute and a solid ten seconds or more of stall with each lift and they didn't heat up very much at all.

[spacepenguine]

Wow. Thanks John! That's just the solution I needed to counterbalance the arm without blowing the 120lb limit or introducing the complexities of a ratcheting system. That's just too simple.

[Gary Dillard]

We planned to use the FP motor with it's gearbox and a rack-and-pinion for our vertical lift, and knowing how the FP's do not like to stall (I think they were the original source of "magic smoke" back in the days before CIM's when they were one of the choices for drive train motors) we decided to use constant tension springs available in a wide range from McMaster. We assembled everything that was going to be lifted, weighed it, divided by 2 and ordered 2 springs in that range. No close fits required - the ID of the spring is over an inch so we just stuck a 3/8" bolt through it, attached it to structure and screwed the other end to the arm. Works great - without the motor even installed you can move the arm to any position and it stays there.

[Aren_Hill]

We cheated to learn this by just copying 148's arm in 2007, works quite well

[Chris Fultz]

Originally Posted by Aren_Hill

We cheated to learn this by just copying 148's arm in 2007, works quite well

So did we. Can we claim collaboration?

[Aren_Hill]

Originally Posted by Chris Fultz

So did we. Can we claim collaboration?

Well we did it in 07 during the fix it windows, this years pretty different on our end.

So if collaboration can be applied over long time periods maybe

[dtengineering]

Originally Posted by JVN

CIMs are champs -- do whatever you want, the breaker will trip before the CIM even flinches.

This is (as you'd expect from John) a great write up. There is one conditition, however, where our CIM's have become hot enough (outer case over 60 degrees C) that I started to worry about them. That was using them as drive motors in repeat practice matches. One year we were through tech well before most other teams and the field crew wanted robots on the field to test their systems and make sure everything was "broken in". Our drive team was MORE than happy to oblige. At some point (on the third battery change, if I recall... we'd been going at full "match" power for over half an hour) I mentioned to one of the students that we should check the motor temperature.

The student replied "Ouch!" We grabbed our IR thermometer and I can't remember exactly what the temperature of the case was, but it was hot enough that even if the CIM wasn't flinching, I was. It took about half an hour to cool down to the point where I felt good about firing the motors back up again.

Now, the good news was that the CIMs were still fine, so I'm not disagreeing with the quote. But there may be occasional circumstances where even the mighty CIMs shall succumb to heat.

Of course, if you use them in an arm, and counterbalance the arm like John is suggesting (that's kind of the point of the whole post) then you shouldn't have a problem.

Jason

P.S. If you want to see some cleverly counterbalanced arms, just check out a VEX tournament.

P.P.S. Oh, the many benefits of being through tech at 9:00am!

[JVN]

Originally Posted by dtengineering

Now, the good news was that the CIMs were still fine, so I'm not disagreeing with the quote. But there may be occasional circumstances where even the mighty CIMs shall succumb to heat.

You should see how hot a CIM gets after 6 hours of Robowrangler practice.... but they keep running.

[dtengineering]

Originally Posted by JVN

You should see how hot a CIM gets after 6 hours of Robowrangler practice.... but they keep running.

I want a video of the hot-dog roast taking place over the drive train at the end of the day!

I wonder if anyone has actually smoked a CIM... maybe they ARE indestructible in an FRC context.

Jason

[Aren Siekmeier]

Our kicker mechanism from 2010 was the linear type using a lead screw and surgical tubing, and while the idea had merit, it was poorly executed. There was no shortage of binding on the system, and the CIM driving the lead screw would get very hot, besides the Jag faulting every 5 seconds, until we got everything tuned just right.. (Only to be knocked out of tune 30 minutes later! )

Of course, the CIM has shown no sign of permanent damage, and just keeps chugging. Yay CIMs!

[MrForbes]

We still have the original little banebots motor driving the arm of our 2007 robot. It's a long arm on that robot, too. We used a gas spring.

This year we're using a single window motor to raise the arm...it's also a long arm.

Here's the lower attaching point for our passive assistance--easy to adjust, just push it up or down the pole, it stays where you put it, because of the magic of friction and leverage.



Great post JVN! as usual

[s_forbes]

Hooray for passive assistance! Our 2007 bot was able to lift a tube quickly with the assistance of a gas spring, powered by a single 540 Banebots motor. This year's arm is more interesting, but fortunately we have math to compensate (and bigger motors!).

[ThirteenOfTwo]

We learned passive assistance in VEX, specifically last year when we wanted to design a 254-esque arm to fling the game objects without any pneumatics. We actually gave our basket "negative weight". Footballs flew.

Since then, passive assistance on every arm we've done, including on the VEX robot that made it onto JVN's blog. It's that useful.