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Hi, I am a newbie and would appreciate some help. My 11 year old has just joined his school robotics club. He will be using set 9794 (Mindstorms for School with ROBOLAB 2.5.4) and has to prepare a robot for a Tug-of-War competition (based on FLL (...) (18 years ago, 5-Jun-06, to lugnet.robotics, lugnet.robotics.edu, lugnet.robotics.rcx.robolab)
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| | Re: Newbie needs Help
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(...) Raj One major key to making a good Tug-of-War robot will be making it go the "right" speed. If it goes too fast, some of the "power" will be wasted on speed. If it goes too slow, it won't use all the power it has. To change the speed of the (...) (18 years ago, 5-Jun-06, to lugnet.robotics)
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(...) Steve, Thanks for the tips and good wishes. Currently this is what we have: - 3 wheels driven by 3 motors. Each geared down at 15:1 - the wheels do keep spinning when the robot is held in place - very generous size limit, a 250mm cube - so no (...) (18 years ago, 5-Jun-06, to lugnet.robotics)
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(...) I suspect you're far from breaking gears or axles. However, keep in mind it is always possible (so don't blame me if it happens) :) Keep the distance between gears & wheels small, so the axles don't twist. Sounds like you still have more (...) (18 years ago, 5-Jun-06, to lugnet.robotics)
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(...) Well, I'm not exactly an expert - but it seems to me (if the rules allow it) that attaching the tow rope above the center of gravity of the robot and putting the drive wheels at the end nearest your opponent would result in the force of your (...) (18 years ago, 5-Jun-06, to lugnet.robotics)
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Steve Hassenplug wrote: > You can (A) add more weight (B) make it go faster, or (C) add more traction. > Considering you're already at the weight limit (out of parts), option (A) is out. > > Making it go faster is a reasonable option. Geared down (...) (18 years ago, 5-Jun-06, to lugnet.robotics)
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(...) Steve, Philo's page on 'Wheels, Tyres & Traction' seems to imply that increased speed will not make much of a difference or am I reading that wrong? By 'mechanically connect' do you mean by using two differentials? The maximum 'safe' gearing (...) (18 years ago, 6-Jun-06, to lugnet.robotics)
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(...) Steve, Provided that my robot doesn't tip-over and that only the front wheels are driven. I have seen a video of a robot in last year's competition that opened up into a V-shape. The angle between the arms of the V being greater than 90 (...) (18 years ago, 6-Jun-06, to lugnet.robotics)
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(...) Steve, One rotation sensor is allowed. There are also two light sensors, so your idea is definitely doable. The situations that are confusing are when the pull of the other robot prevents your robot from making any headway, as well as when (...) (18 years ago, 6-Jun-06, to lugnet.robotics)
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(...) One rotation sensor and a differential will allow you to compare the rotation of two wheels. If the driven wheel and the undriven wheel come in the two sides of the differential, and the rotation sensor is connected to the drive ring, then the (...) (18 years ago, 6-Jun-06, to lugnet.robotics)
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You only need one rotation sensor and some fancy gearing. Using a differential in a way similar to the "South Facing Cart" or the stearing drive in a dual differential setup, you could directly measure the amount of spin between a driven wheel and (...) (18 years ago, 6-Jun-06, to lugnet.robotics)
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(...) Ummm.... that's not the way I remember the mechanics. There is a torque created that is force on the drawbar times the drawbar height from ground. That torque will lift your front end. Doubling your own drawbar height halves your pulling (...) (18 years ago, 6-Jun-06, to lugnet.robotics)
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(...) Dean, Thank you for your response. I am going to get on with the tow rope suggestion right away. You might like to know that your 2002 'Building Lego Robots for FLL' guide was the first thing that I read. Raj (18 years ago, 6-Jun-06, to lugnet.robotics)
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(...) Raj, There's a bit of confusion here. I believe the above quote actually came from Steve Baker (posting only as Steve). As Steve Baker pointed out, there is a difference between kinetic (sliding) friction and static friction. ((URL) In the (...) (18 years ago, 6-Jun-06, to lugnet.robotics)
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| | Re: Newbie needs Help (diff sensor)
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Some varied thoughts here: (...) I might be wrong, but my understanding of the Lego differential gearing is that if both shafts are turning at the same rate and direction, the outer gear/shell will turn at the same rate as the axles. Only when the (...) (18 years ago, 6-Jun-06, to lugnet.robotics)
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(...) What I'm saying is that if the robot is moving along at (say) 3 inches per second - then the drive wheels will get most traction if they are turning at a speed equivelent to 3 inches per second. If they are rotating faster than that (because (...) (18 years ago, 6-Jun-06, to lugnet.robotics)
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| | Re: Newbie needs Help (diff sensor)
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(...) I think you need another pair of gears on one of the wheels to reverse its direction. If you are going to use a rotation sensor to measure the slippage then you need to realise that the sensor is notoriously poor at very low speeds. You'll at (...) (18 years ago, 6-Jun-06, to lugnet.robotics)
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| | Re: Newbie needs Help (diff sensor)
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(...) Quite true. My bad. -dave (18 years ago, 6-Jun-06, to lugnet.robotics)
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(...) I don't think Dean was talking about torque from the rope tipping the robot; I think he was talking about the fact that a raised attachment point for the rope can result in a downward component of the tension, increasing the force down on the (...) (18 years ago, 6-Jun-06, to lugnet.robotics)
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(...) Sure, I understand the argument. But this old farm boy has both a few hours in the driver's seat of a farm tractor, as well as a couple of engineering degrees, both of which prompted my cautionary statements. Let me start by saying that I (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) (Or vice-versa!) (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) Yeah - bad for a tractor with front-wheel steering - and a need to steer - not so bad for a robot with either no steering or rear-wheel skid steering. This robot really doesn't care if it's front end gets light - but for a tractor it's a major (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) Thought provoking question. It led me to the conclusion that when the bot is generating it's maximum possible drawbar pull (as limited by the motor and drive train), the weight on the undriven front wheels (I'm assuming a rear wheel drive (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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| | Re: Newbie needs Help (diff sensor)
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(...) Actually it does - barely. With the increased current, RCX output voltage drops, so does motor stall current. See these compared charts: (2 URLs) Philo (18 years ago, 7-Jun-06, to lugnet.robotics, FTX)
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Philo, That's interesting, and looks to explain why it is that I've never been able to observe the RCX output going to thermal or current protection cutoff? I've one largish sumo type bulldozer style robot with 4 treads that can easily push 4-5 (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) It does happen - if you use more that 2 motors or if you try to use a RC motor. The driver circuit limits current around 500mA, and at that rate it is not long before going in thermal shutdown mode. (...) No, you won't see smoke... RCX motor (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) A rather inventive young man that I know did try exactly that - using the ribbed tubing from the Mindstorms set inside the Mindstorms motorcycle tire. It's a good fit, and a good idea, but I'm not sure how effective it was. (...) One simple (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) I would have expected that compression would be a good thing because it would increase the area of the 'contact patch' - which increases traction - which is a good thing - right? (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) The problem is that this is measuring the static friction ("stiction"). That's the wrong thing to measure if your strategy is to jam all the motors full on and progress forwards (we hope!) with all wheels spinning. To measure the dynamic (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) Steve Hassenplug, Sorry, my mistake. Thank you for the Wiki link and all your help. Raj. (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) Dave, Actually I am very happy and grateful. I am getting a crash course. Couldn't have asked for more. Raj. (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) That makes sense. (...) Yeah - I agree. So we have to sum some force vectors here. If our robot is pulling to the left, we have a horizontal 'drive' force to the left and a force in the tow rope going off to the right - plus a force due to (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) As you note in your mini-cooper example, *if* you can keep the wheels from "spinning out", but instead are always in rolling contact with the ground, static friction is what's important... and generally, the static coefficient of friction is (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) hmm. This sounds like a good project... (...) All things being equal, I'd put my money on the robot with ten spinning wheels, over a couple (or even ten) stationary ones. I'd also mechanically connect all the motors together, so they drive a (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) Hmmm - that's unfortunate...but if the 'right' strategy is to go with a non-slipping robot, the measurement of sliption is really a lot less important. (...) Yeah - so many of those basic mechanics equations are only approximations - yet they (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) I'll get to that, eventually... there's just too much fun stuff going on, and... (...) A historical point on this. The first time I met Steve was at a sumo event, where I had carefully calculated the correct gear ratio, given the torque of a (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) Not so obvious. If you increase the surface the pressure per surface unit decreases proportionnaly (at least on hard surface)... Actually I would expect little or no variation in traction power. Philo (18 years ago, 7-Jun-06, to lugnet.robotics)
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(...) I'm with you until we get here. Attaching at the front of a long nose feels like a bad idea. Any side force on the tow rope will tend to pull the robot off course, and once slightly off course the force only gets worse. In any case, a (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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| | Rotation Sensor Accuracy [was Re: Newbie needs Help (diff sensor)]
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Steve wrote on Tuesday, June 06, 2006 5:59 PM <<snip>> (...) <<snip>> I think what is really meant is that most RCX firmware implementations are poor at filtering hardware transient errors. And the Rotation sensor is very "good" at generating (...) (18 years ago, 8-Jun-06, to lugnet.robotics)
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