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  2. 4 years ago

    FirePhoenix

    21 Mar 2012 Pleasanton, California 1000B

    Why don't you just put the wheels the way they are supposed to be. Mecanum wheels were created so that they could be put in that orientation. The 2 left wheels should be on the left side and the 2 right wheels should be on the right side. Then gear it 5:3 instead of 5:1 which is crazy fast. A 3 gear is the one smaller than the 5 one.

  3. Brian He

    21 Mar 2012 Vancouver,BC,Canada

    @FirePhoenix Why don't you just put the wheels the way they are supposed to be. Mecanum wheels were created so that they could be put in that orientation. The 2 left wheels should be on the left side and the 2 right wheels should be on the right side. Then gear it 5:3 instead of 5:1 which is crazy fast. A 3 gear is the one smaller than the 5 one.

    the robot cannot "slide" left and right fast enough on the mats if putting those wheels in the normal drive.
    And ya, it's mostly a torque problem that the robot cannot move forward from resting on the mats. But weirdly it did successfully do so for the first time when we test it.

    For additional details: The robot later on can travel left and right although slowly but hardly forward and backward. It's like at first it's fine for travelling forward/backward but not left/right then afterwards is the other way around without replacing the Mecanum wheels.

  4. FirePhoenix

    22 Mar 2012 Pleasanton, California 1000B

    @Brian He the robot cannot "slide" left and right fast enough on the mats if putting those wheels in the normal drive.
    And ya, it's mostly a torque problem that the robot cannot move forward from resting on the mats. But weirdly it did successfully do so for the first time when we test it.

    For additional details: The robot later on can travel left and right although slowly but hardly forward and backward. It's like at first it's fine for travelling forward/backward but not left/right then afterwards is the other way around without replacing the Mecanum wheels.

    That's because you also need to power the back mecanum wheels when moving forward and if you want to move left to right, you will need to power the front 2 wheels as well as the back.

    However, this design will not work because the speed of the back wheels moving from front and back will be much slower than the front wheels moving forward and vice versa when you are moving sideways. The best you can do is just use the mecanums how they were made and deal with the slowness of moving sideways. Otherwise make a different omnidirectional drive because this will not work.

  5. Oliver W

    22 Mar 2012 Auckland, New Zealand AURA

    There is nothing wrong with how you have arranged your wheels.
    Mecanums have an effective 1:1 ratio when strafing. For every one time the wheel turns, it will strafe by one wheel circumference. So you won't have problems with your motors running at different speeds.

    The robot won't strafe straight, but then people opt for non-straight strafe quite often anyway.

    You aren't doing it the normal way, but all the people telling you "OMG YOU'RE DOING IT WRONG!!!" aren't really justified in saying that.

    Your current gear ratio is way too high.
    Demonstrating this mathematically is tricky. To work out an optimum gear ratio you need quite a lot of information. For example if you want to know which gear ratio will be fastest, you need to know:

    • the robot's weight
    • the wheel diameter
    • the characteristics of the motor or combination of motors (for motors with a linear speed-torque curve, this means free speed and stalling torque)
    • the distance the robot will need to drive

    and that's only if you ignore friction. The last point is relevant - the optimal gear ratio will increase if your robot needs to travel further in one go. Because this distance changes depending on the game state and the driver's style and your and the opponent's strategy and any number of other factors, it isn't something that can be easily determined.

    Because of this, because no one to my knowledge has accurately determined the actual speed-torque curve of a vex motor, and because friction has a large effect and is difficult to model, gear ratio calculations aren't usually used by vex teams. It's easier and works better to base your gear ratio decisions on the intuition that comes with several years of experience. Maybe someday AURA will get around to making a tutorial on how to do it as well as is reasonably possible in VEX, but since we don't actually use motor torque calculations ourselves we're not in the best position to do this. We would just be making our best guess as engineering students at what would work.

    Use a gear ratio somewhere between 1:1 for speed and 2:1 for speed depending on robot weight and strategy. In your case go for somewhere towards the lower end of the range, 1.2:1 or 1.4:1 because mecanum wheels require extra torque to overcome roller friction when strafing.

    As for the more idiosyncratic problems you've mentioned, those aren't as easy to comment on. Change your gear ratio and see if they keep happening.

    There are also a couple of small nitpicks I have with some of the other stuff in this thread:

    @devinc Yes. Actually there is a tutorial on how to do it on my teams Youtube channel.
    www.youtube.com/user/nhrc569c[/url]

    You can't use static friction to determine an optimum gear ratio. The best you can do is use it to determine a ratio below which there's no point gearing down because the motors already have the power to spin your wheels. This might be helpful on a wallbot, but the majority of the time it won't be a helpful thing to know for Vex applications.

    @Brian He [url]http://www.vexmen.com/2011/11/whats-your-robot-drive-style/)

    Where this link says "other holonomic drives": only the first is actually holonomic. The others only have two degres of freedom out of the three required to be holonomic.

  6. Brian He

    22 Mar 2012 Vancouver,BC,Canada

    The gear ratio I shell admit, accordingly to most of the responses people provided, is extremely high...
    VEX Mundi, your analyzations above really make sense to me, except I need more help about Mecanum wheel; Could you provide more details about the Mecanum strafing? Because I think the Mecanum wheels that runs or spins must have the same motor speed in order for the robot to travel straight, at least in the H/U drive arrangement that we got. Isn't the faster it spins the more it strafe even it got 1:1 strafing ratio, so if the motor for each wheel aren't running in the same speed the robot may not travel straight.

    @Vex Mundi For every one time the wheel turns, it will strafe by one wheel circumference. So you won't have problems with your motors running at different speeds.

    Which this also lead the another question of mine where if each Mecanum wheel travels exactly 45 degree, and when they are placed parallel or perpendicular with each other, they are able to travel/strafe straight as long as the weight distribution of the robot is quite balanced plus same motor speed?

    @Vex Mundi The robot won't strafe straight, but then people opt for non-straight strafe quite often anyway.

    Our team has finished our last competition at BCIT in March 9th. And hopefully our robot hasn’t being disassembled yet so we can still play it after the Spring Break…

    I started this thread after all because I felt like I couldn’t live with all the unknowns and confusions in my head lol, and you’ve cleared up a lot of things from your respond above.

    In my opinion, I guess the intuition thing you’ve mentioned above is perhaps what I need to avoid when doing some relatively higher level of engineering. We started our actual robot construction quite late, around 2 weeks before the competition date and it was during that period of time I found that there are significant number of obstacles when transforming theories/conceptual ideas into actually working mechanicals.

    *And thanks everyone for all your responses!*

  7. devinc

    23 Mar 2012 Murrieta,CA 569C

    @Brian He Thank you for the tutorial.
    The equation you've provided: F=UMG has the "U" represents the static friction, but why when the value that plug into "U" is "Tan 30 degree" ?

    What's happening when in theory it's 12:1 gear ratio but has to reduce 40% and then becomes 7:1 in the end?

    You are correct. "U" does represent the static friction but the way you find the value is to take the tangent of the 30 degrees, but you need to do preliminary testing on your robot and not take my values. The ratio in my video is actuallly backwards. It is suppost to be 1:12 and 1:7. It goes from 12 to 7 because it was reduced by 40% to compensate for stalling, wear, friction, etc. I have a PDF of the tutorial that may make more sense of you want me to e-mail it over to you. Just let me know.

  8. Brian He

    23 Mar 2012 Vancouver,BC,Canada

    @devinc You are correct. "U" does represent the static friction but the way you find the value is to take the tangent of the 30 degrees, but you need to do preliminary testing on your robot and not take my values. The ratio in my video is actuallly backwards. It is suppost to be 1:12 and 1:7. It goes from 12 to 7 because it was reduced by 40% to compensate for stalling, wear, friction, etc. I have a PDF of the tutorial that may make more sense of you want me to e-mail it over to you. Just let me know.

    Sure!
    So I guess "U" is found by using another equation.
    Is the purpose of using the ramp is just to find the weight of the robot or there's more in it?

  9. devinc

    23 Mar 2012 Murrieta,CA 569C

    The ramp is to find out at what angle your robot will slide. that is where the degree comes into play. the robot can be a rough estimate. It will effect the equation but what I do when I dont have a scale is do 2 estimate (light and heavy weight) and then just take the average of the two. What is your email? I will send it right over.

  10. Brian He

    23 Mar 2012 Vancouver,BC,Canada

    @devinc The ramp is to find out at what angle your robot will slide. that is where the degree comes into play. the robot can be a rough estimate. It will effect the equation but what I do when I dont have a scale is do 2 estimate (light and heavy weight) and then just take the average of the two. What is your email? I will send it right over.

    Oh, I though you could sent it through the private message system.
    wait isn't all you need is to find the friction force (on the ramp) so the torque value must be greater than that? Not sure on how to get the stalling's affect...

    @Vex Mundi [B]Tyou need to know:

    • the robot's weight
    • the wheel diameter
    • the characteristics of the motor or combination of motors (for motors with a linear speed-torque curve, this means free speed and stalling torque)
    • the distance the robot will need to drive

    and that's only if you ignore friction.

    Would this provide a way for calculating the optimal gear ratio? :
    Weight the robot then put it on a ramp and adjust to the maximum ramp angle -> calculate the friction force -> divide by the diameter of the wheel -> then compare the result force value with the motor's stall torque value and see the ratio.
    but the ramp surface must be the same material with the competition field
    stall torque value is available on the product webpage

  11. Oliver W

    23 Mar 2012 Auckland, New Zealand AURA

    @Brian He Could you provide more details about the Mecanum strafing? Because I think the Mecanum wheels that runs or spins must have the same motor speed in order for the robot to travel straight, at least in the H/U drive arrangement that we got. Isn't the faster it spins the more it strafe even it got 1:1 strafing ratio, so if the motor for each wheel aren't running in the same speed the robot may not travel straight.

    This is correct. If your motors are not the same strength your robot will not go straight - we had this exact problem on our mecanum drive recently. This is the case for any drive, holonomic or not, although it may be more noticeable with mecanums because strafing them requires a reasonable amount of torque and weak motors will struggle with that. You just have to hope that all the motors you use are up to standard. If they aren't then, if you can, swap them out for ones that are.

    @Brian He Which this also lead the another question of mine where if each Mecanum wheel travels exactly 45 degree, and when they are placed parallel or perpendicular with each other, they are able to travel/strafe straight as long as the weight distribution of the robot is quite balanced plus same motor speed?

    Each wheel will exert the same force. If the wheels are in the standard rectangular configuration and the centre of gravity is centred between them, each one wheel will exert the same torque on the robot because the moment arms will be the same length. If the wheels are at different distances from the centre of gravity, as it looks like yours might be, then the moment arms will be of different lengths so there will be a torque imbalance and the robot may rotate.

    Another consideration is that while mecanums are designed to exert force at 45 degrees to the orientation of the wheel, roller friction means this isn't always the case. As roller friction increases the direction of the force exerted by the wheel will become closer to the wheel's orientation (i.e. the wheel behaves more like a normal wheel). This doesn't affect the torque balance of a normal mecanum drive, but it will affect yours because your wheels are mounted in different orientations. This is the reason why I said your drive would produce non-straight strafe. It's possible that several conflicting factors will cancel each other out and make the robot strafe straight. If not, it's not a big problem because your driver can adjust for the wonkiness.

    @Brian He In my opinion, I guess the intuition thing you’ve mentioned above is perhaps what I need to avoid when doing some relatively higher level of engineering. We started our actual robot construction quite late, around 2 weeks before the competition date and it was during that period of time I found that there are significant number of obstacles when transforming theories/conceptual ideas into actually working mechanicals.

    This is true. Engineering involves some mathematical analysis and some intuition, with more of the former as complexity increases. There is a tendency when building Vex robots to forgo any kind of mathematical analysis and use intuition based on experience because Vex systems are simple enough that it's usually a more effective approach.

    This is a bit of a pity from a engineering education point of view. I think the best approach if you're interested in learning more about the engineering (or perhaps if you're a mentor interested in teaching students more about the engineering, but that's not where my experience is) is to see mathematical analysis as complementary to an experience-based approach and to check whether the results of your analysis match up with what works. If not then your analysis is mising something and maybe you can learn from your mistake, if so then maybe you found a good solution quicker than you would have through trial and error.

    @Brian He Would this provide a way for calculating the optimal gear ratio? :
    Weight the robot then put it on a ramp and adjust to the maximum ramp angle -> calculate the friction force -> divide by the diameter of the wheel -> then compare the result force value with the motor's stalling torque value and see the ratio. The ratio represents the gear ratio...
    but the ramp surface must be the same material with the competition field

    This would calculate the minimum gear ratio (i.e. the one providing the most force) that you could use without the robot losing traction. This is because static friction measures the amount of force you can put on stationary object A which is in contact with stationary object B (usually the ground) before A will move. In the case of wheels it is the amount of force you can put on the wheel before the wheel will slip.

    The optimal gear ratio will not involve static friction since your wheels should not be slipping - the force required to slip a Vex wheel on Vex foam is outside the bounds of what a normal robot will exert under normal (non-pushing) conditions. Calculating the optimal gear ratio for driving would involve making sure that your motors are giving as much power as possible. Power is torque multiplied by rotational velocity, so the motor gives zero power output while stalled (zero rotational velocity) or while freely spinning (zero torque). The optimum rotational speed for a motor is about halfway between these values, depending on the individual motor's characteristics. For a Vex motor halfway will be a good approximation because Vex motors are permanent magnet DC motors.

    The optimal gear ratio will be the one that gives the greatest energy output when driving for a given amount of time, or the one that maximises the integral of power with respect to time over the time travelled. As I mentioned in my previous post the parameters of this calculation would be difficult to get accurate measurements for.

  12. Brian He

    23 Mar 2012 Vancouver,BC,Canada

    @Vex Mundi Each wheel will exert the same force. If the wheels are in the standard rectangular configuration and the centre of gravity is centred between them, each one wheel will exert the same torque on the robot because the moment arms will be the same length. If the wheels are at different distances from the centre of gravity, as it looks like yours might be, then the moment arms will be of different lengths so there will be a torque imbalance and the robot may rotate.

    Seems like our robot does curved a bit when we test the strafing, haven’t seen it from strafing across the field yet. We have our robot’s base tilted forward, so the supporting beams for the robot’s arm and the arm are leaning forward as well, I guess that reduced the amount of curving by making the distances between each wheel and the center of gravity closer to even.

    @Vex Mundi Another consideration is that while mecanums are designed to exert force at 45 degrees to the orientation of the wheel, roller friction means this isn't always the case. As roller friction increases the direction of the force exerted by the wheel will become closer to the wheel's orientation (i.e. the wheel behaves more like a normal wheel). This doesn't affect the torque balance of a normal mecanum drive, but it will affect yours because your wheels are mounted in different orientations. This is the reason why I said your drive would produce non-straight strafe. It's possible that several conflicting factors will cancel each other out and make the robot strafe straight. If not, it's not a big problem because your driver can adjust for the wonkiness.

    This sounds true… So if the robot wants to travel diagonally, the diagonal angle may not even be 45 degree anymore…
    Maybe I will upload a driving video soon after we changed the gear ratio.

    @Vex Mundi There is a tendency when building Vex robots to forgo any kind of mathematical analysis and use intuition based on experience because Vex systems are simple enough that it's usually a more effective approach.
    This is a bit of a pity from a engineering education point of view. I think the best approach if you're interested in learning more about the engineering (or perhaps if you're a mentor interested in teaching students more about the engineering, but that's not where my experience is) is to see mathematical analysis as complementary to an experience-based approach and to check whether the results of your analysis match up with what works. If not then your analysis is mising something and maybe you can learn from your mistake, if so then maybe you found a good solution quicker than you would have through trial and error.

    Okay, I just realized VEX is not rocket science yet, things doesn’t have to be exactly accurate and it is probably more fun to try it with intuition first. I was thinking about some engineering procedures that minimize the mistakes people can make and the robot can come out in its best possible condition, like using the most suitable gear ratio accordingly to the design. =/

    @Vex Mundi Calculating the optimal gear ratio for driving would involve making sure that your motors are giving as much power as possible. Power is torque multiplied by rotational velocity, so the motor gives zero power output while stalled (zero rotational velocity) or while freely spinning (zero torque). The optimum rotational speed for a motor is about halfway between these values, depending on the individual motor's characteristics. For a Vex motor halfway will be a good approximation because Vex motors are permanent magnet DC motors.
    The optimal gear ratio will be the one that gives the greatest energy output when driving for a given amount of time, or the one that maximises the integral of power with respect to time over the time travelled. As I mentioned in my previous post the parameters of this calculation would be difficult to get accurate measurements for.

    thanks for the help!

  13. Brian He

    27 Mar 2012 Vancouver,BC,Canada

    Ok, so after we changed to 1:1 gear ratio, the robot can now drive on the mats. Haven't have time today for programming the strafing...
    The turning looks like drifting, which is pretty cool, but the motion is slow...

    1:1 ratio also solved the noncontinuous spinning of the back wheels as well, so does this means the gear ratio also affects the amount of currents drawn from the motors which can trigger the circuit breakers?

  14. Oliver W

    27 Mar 2012 Auckland, New Zealand AURA

    The strafing motion will be slow, it always is for mecanums. They are more versatile than ordinary wheels but they can't do everything.

    It looks like while strafing you aren't turning your front wheels, so the drifting behaviour you have is exactly what you shoule expect. You should be spinning the front wheel on the side that you want to travel towards backwards, and the front wheel on the side you want to travel away from forwards, when you want to strafe.

    @Brian He 1:1 ratio also solved the noncontinuous spinning of the back wheels as well, so does this means the gear ratio also affects the amount of currents drawn from the motors which can trigger the circuit breakers?

    Yes. Motors that are rotating more slowly draw more current.

    [A physical explanation: the electicity in motors flows through very long coils of copper wire with very low resistance. This causes the coil to become magnetised, and this repels/attracts the permanent magnet that surrounds the coil so the motor spins.

    When a coil moves relative to a magnet a voltage is induced in the coil. In the case of a motor, this voltage opposes the voltage powering the motor (which makes sense, because if the opposite were the case then the motor would power itself indefinitely). This back voltage means that as the motor goes faster, less current is "pushed" through it.

    The free speed is where the motor is spinning fast enough that the back voltage equals the supply voltage, so no current flows (except in reality there is friction, so really the back voltage almost equals the supply volatge and very little current flows).

    When the motor is forcibly held still (stalled) there is no back voltage so the current is high. the power is the current times the voltage so the input power to the motor is also high. However, the motor is not generating any mechanical power because it isn't moving so all that energy becomes heat.

    This is why motors that are geared too high will trip both circuit breakers and temperature sensors. Motors that are geared too low won't; they will just give less than their optimum output power.]

  15. Brian He

    31 Mar 2012 Vancouver,BC,Canada

    Um... I guess it may also be the programming problem, we reprogrammed it and the base now drive more smoothly. (The video below is old)
    The new programming we have now seems to eliminates the noncontinuous spinning problem.

    noncontinuous spinning (1:1 gear ratio)

    Mecanum Wheels - ZigZag move
    [url]http://youtu.be/TZxVBtUYkm4

    We took the arm off so the robot is not heavy enough and those wheels slip on the mats. And we did the ZigZag move for fun :D

    I shell upload another video about the final programming of the base and how the robot drives around, forward/backward, left/right/diagonal strafing, stationary turning, "drifting", and maybe zigzag move as well all in one programming. Mecanum wheels are awesome :)

    Thanks.

  16. Brian He

    5 Apr 2012 Vancouver,BC,Canada

    Here's the result after we reprogrammed the robot's base. The movement it can produce is quite substantial depends on which wheel's motor is on/off when the robot is driving around, we now changed it to 4 wheels rotating rather than 2 wheels only. It can do things like rotating during its strafing, like the robot is floating on the ground, but has a very low speed when in 1:1 gear ratio, even on the hard ground instead of on the mats.

    Any improvement that you could suggest?

    link:

  17. bgambsky

    5 Apr 2012 Neenah, WI 1200

    At viewing the set up of your wheels, I like the thought, because this actually lets the speed remain the same no matter which way you look at it, but the only problem that I see with the set up is the diagonal driving. There is probably a motion of it when you push it on the ground, but the programming will not allow for you to move in "every" direction.

    As for the gear ratio. High speed gearing is the only option other than this. if not torque. I know your weight is light but you should use 1:1 ratio with the highs geared for speed at the very most.

    good luck :D

 

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