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