Another Newbie needing advise


My team is currently building a tank drive, horizontal flywheel robot and our current [393] motor allocations are:

• 4 Motors for drive
• 4 Motors (possibly) for flywheels
• 1 motor for the intake

That leaves 1 motor to spare, which made us think: “Should we invest the last motor for a ramp/lift system, or spend it on the intake to lose the risk of stressing the motor?” Which led me to do research on forces and ramps. In the short time I researched, I sifted through diagrams and equations as saw

Force In Newtons = (mass in kg) x (gravity in m/s^2) x (sin slope of ramp) + (friction) x (mass in kg) x (gravity in m/s^2) x (cos slope of ramp)

Now, I have a few questions:
• How much do your robots weigh right now and later approximately based on previous games?
• Has anyone ever calculated the newtons exerted by the standard VEX motor?
• What is the friction of a VEX steel bar? or How do I find it?
• How are your motors allocated?
• Is the task of lifting a robot feasible with 1 motor and a ton of gearing and hope?
• Is having 4 motors on flywheel system impractical, or good for the long run or should we shoot for 2?
• Are gears more preferable than chain and sprockets for the flywheels? If so, why?

I am new to posting on this website since I usually just read up on what people are saying. This is a very amazing community and I hope that I am welcomed here. I have much to learn from you guys with a LOT more experience in robotics than I, and would be very much be open to suggestion.

I’m going to guess that a lot of robots this year will weigh around 15 pounds. But that’s just a guess. Clawbots will weigh less, of course.

Check out this:

Which is discussed here:

Could you clarify what you mean by that?

I’ve heard a number of teams are having some success with using two motors on their flywheel shooters. I think to get two motors to work, however, you need to be obsessive about getting your friction down to as low as possible.

Personally, I would suggest trying to use two motors for a lift. But, on the other hand, if you’re deploying a ramp that somebody else must drive up, perhaps you could get away with one motor, especially if you have pneumatics. Creativity counts more than hope here. :slight_smile:

I’m guessing that gears are better than chains because of the friction that chains create. Chains will exert a force on your axles that is one-sided (The tension of the chain pulling on a sprocket “wants” to bend your axle toward the direction the chain is coming from). If you must use chains, I would suggest using them closer to the motors rather than near the flywheels.

You’ll probably tear up a few motors trying to lift robots with only one motor. I’d suggest just keeping the last motor for intaking, or for changing your angle of firing.

Also, I wouldn’t suggest using 2 motors for a flywheel if you plan on shooting across field. Your motors will quickly burn up as our team has discovered with many ratios thus far. Your setup of 4 motors is more practical.

I can give you some help with a few of these things but will need some more info for others.

in order to ■■■■■ the feasibility of your ramp we need to know how you plan on using in. if you are using your one motor to fold down the ramp, you’re good. If you’re trying to use 1 motor to hall a robot up a ramp, that’s much more tricky.

about finding the friction, f you want to find the friction between a vex robot and vex steel bar here is what you do,take a vex robot (really any basic drive train), and try to weight it appropriately, I would say robots will weigh close to about 10 pounds this year. so you set this robot on two pieces of the steel channel, so that the wheels are all one the steel channel. then you raise one end of the steel channel, creating an angle with the ground, and you continue doing this until the robot starts to move downhill. then you can calculate how much force gravity applied to the robot to get it to move and thus, you get the force of friction. the equation for this is: Friction = Sin(angle)gravityrobotmass

Thank you so much for your help and for the link reference, it really helped a lot.

As for the weight, I decided to calculate from 10-15 lbs since most lifted robots would be designed to be lighter.

I apologize for the ambiguity. I was asking for the friction of any standard VEX steel piece like a baseplate or the many C-channels. I was under the misconception that since they had the same materials, that they would all have the same friction.

How are your teams gearing your flywheels? Are your teams using Power Expanders? Our current friction seems low (I think), when we spin my wheels quickly, the remain spinning on their own well after 20-30 seconds.

Yeah, I was thinking if deploying a ramp and use the single motor for the deployment or a pulling system while having the robot on a mechanically deployed ramp.

Pneumatics would be nice;however, my school currently owns just one since our robotics program barely got the ball rolling three years ago. I would love to see a fully pneumatic lift system this year. :slight_smile:

My teams design is depending on a gear/sprocket train that doesn’t require a lot of horizontal space (perpendicular to the flywheels). Using gears wouldn’t work out well for that, but I fear that if I go the wrong route now, it’ll be harder to fix later on.

I don’t think there’ll be a need to change the angle very much. We decided to keep the flywheel system at a static, 45-degree angle. For different distances, I will have my programmer add a speed adjustment command for the remote

We thought shooting across the field doesn’t matter very much since the robot will be able to move around a lot. But we think that we might just stay with the 4-motor launcher system. :slight_smile:

From reading, we figured that the best way is to use the former option (motor to deploy the ramp with the help of rack gears and linear slides. However, we can also find a way to mechanically deploy the ramp and have a pulling system on it using the remaining motor. But since we are still relatively early in the season, we are trying to be flexible.

Thank you for this equation. Now, from very little of physics I know from middle school (Our highschool decided to put physics in our senior year), rolling friction is different from static friction. Since the robots have wheels (duh), that would mean that there would be different friction values if the robot is moving and when it isn’t. Is there a way to calculate the friction while taking into account the RPM of the wheels?

Last time I checked, they were using gears on their flywheels and no chains. They were using two 393 motors that were internally geared for torque mode and had an external gear ratio of something like 1:27, I think. They have a power expander so I’m guessing they will use it eventually. If your wheels can spin freely for 20-30 seconds, that’s a very good sign. I think they do overheat their motors eventually, but they seemed to think that they wouldn’t overheat if they were in an actual match and not just testing things over and over. I don’t know about that. I guess time will tell if only two motors can really keep up for an entire match. They do all their testing as though they are shooting the entire hypotenuse of the field, which is not likely to be the case in a real match.

Well, when I mentioned pneumatics, I was thinking about using it to deploy a ramp, not actually lift a robot. The concept would be that a single motor or piston could be used to trigger some sort of ramp that is spring-loaded using elastics, etc. I don’t know if that’s possible and I haven’t seen anything like it yet, but it’s just a thought.

I think it would be very hard to drag the average robot up a ramp. If your alliance is DOF (dead on the field), then you are probably out of luck unless you have something like a forklift or crane AND you can somehow get hold of them without damaging them or yourself. The time factor would be a killer for a DOF alliance: time spent struggling with a DOF is time that could be spent shooting balls at the goal. If you go with a ramp, I think you’ll just have to hope and pray that the robot can drive up it.

As for the physics of it all: I think you can break down the general ramp problem into several distinct problems. First, assuming a specific robot weight and ramp angle and number of chassis motors, you can think about how much torque the robot needs to supply to the wheels to drive up it. The torque-speed curve of jpearman’s graph will help with that. You would need to allow so much time for the ramp deployment and so much time for the robot to drive up it, not to mention the time for everyone to drive over to their zone and get in position for this task. Get a stopwatch and have your team visualize out loud all of this taking place (without looking at the stopwatch) and then see how much time it takes.

Another part of the ramp problem: if the robot is still on a slope when the power stops at the end of the match, will the robot stay in place or will its weight vector pointing down the ramp cause it to roll back down. This would require you to measure what the friction of the motors is when they are not powered, or you can overcome this problem by designing some kind of mechanism that allows robots to drive up but not roll down.

Another part of the problem: if the robot is still on a slope at the end, and you don’t have a mechanism to prevent it from rolling down the slope, and the motor friction is enough to keep it in place, you still should check that the rubber tires are not going to skid down the slope. This is where your actual friction of rubber tire vs. metal would be a consideration. Since the Vex metal has little holes in it, it’s not going to be something you can easily look up. You’ll probably just need to test that.

So, in summary for a ramp: you have motor torque considerations (can you drive up a ramp at that angle in the time allowed?), you have wheel/gearing friction (will the robot roll down a slope after the power is shut off?), and you have actual sliding friction to worry about (will the robot wheels, even if locked, skid down the ramp after the power is shut off?).

If you hope to hit the high goal, keep in mind the geometry of the field, especially when you’re very close to the goal. :wink:

That is the one thing I do not understand from what people are doing. They are repeatedly testing how far their robot can shoot and the speed, but not the adjustments of the speed as the distance changes from the robot to the goal. Even with the driver loads available, it is still strategically better to have at least 1 robot going after the bonus balls.

I was just point out if they managed to lift a robot with pneumatics using the same concept as hydraulics, it would be an awesome feat. Also, spring loaded ramps I believe, should be the go-to thing since the huge demand for motors.

It is always hard to have a fallen alliance partner, that was one reason I pushed against having a stationary robot while the rest of my team was for it.

Yes I think I will probably hold off on the ramp until later in the season, since it would take a significant amount of coordination with other alliance members.

Yet another feat to overcome. Use of ratchets in a pulley system maybe? I have yet to see someone effectively using ratchets.

We are thinking of a robot of similar design. Our “teacher/supervisor” thinks we should do a stationary robot. This is because he thinks we are waiting motors for a drive system. We think that we need to have the ability to move in case of a DOF or no show. To compromise, we have a 2 motor chain drive system that worked wonders for us in Toss UP. We still have to mount our flywheel to the chassis so I am unsure how well it will work.

I agree that you should save the ramp for later. I think it would be a good idea to focus and fine tune the flywheel first.

I also don’t understand why people are doing this. Why attempt to score 160 in driver loads (that’s with the 8 autonomous loads) when there are 10 stacks of 25 points (250 in all; high goal only) laying on the field that wouldn’t require as much work on the motors to score?

We are currently designing our robots to work together in a way that both can shoot full field, and both could be adjusted to shoot from different points on the field. Our third robot for the new kids will probably be a bonus ball hoarding bot, it’ll be useful for pushing bonus balls into the loading zone in a legal way, while holding at least 4 other balls. Keeping all 10 bonus balls from the other alliance could have a big impact on them especially during the early season. Later on in the season though you would have to deny more balls than just the 10 bonus balls.

Of course this would be only for early competition to teach them the fundamentals of the robots.

Directly replying to the question:

Put that one motor on either intake or flywheel. You can do a single big flywheel with five motors driving it. As a beginner, like what VEX’s starting guide said, try to focus on one thing. My opinion is for you to focus on shooting. If you can build a robot with great control to guarantee accuracy, then you’re doing a fantastic job without any elevation concern.

For the speed control, I know at some point in the future teams are gonna start asking: How do I make my flywheel’s speed consistent? I probably will play with more PID velocity stuff and make more PID velocity programming videos aiming at regulating flywheel speed, as when the game came out and people were guessing about it, I was everywhere on the forum yelling :PID velocity for flywheels! haha.

Flywheels that can launch balls across field are dangerous stuff that can easily tear down motors without some proper control. At least try some slew rate control on the flywheel motors to reduce the stress. I am sure Mr. Pearman will reiterate this in the future.

Also, an issue for the stationary robot is accuracy of the ball launching. One may assume that you only need to keep track of one arch and keep it at that point; however, in a thread posted recently, you can see how the ball density really affects the trajectory of the ball.

For the Slew rate issue, would you recommend using quad encoders to regulate the RPM or gearing the opposite sides of the flywheels together in a way that they turn only when the other is turning.

Also for the speed loss on the flywheels fix, should I have more wheels on my flywheel since it would have more mass making it harder for the ball to slow it down? I plan to have the motors running the entire match (maybe slowing it down a bit for battery consumption when I’m not launching).

In my opinion Don it would be best to have the two sides geared together. Tech can glitch but mechanics are reliable. for your second question i think it is important to find a balance between weight and a mass that your motors can reliably spin,

Ahh, Good point. How many wheels do you have on your launcher? Dude you take off the tires or…?