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omnidrive vs. kiwi
I have been surfing around looking at lots of omnidrives (4 omni wheels) and kiwi drives (3 omni wheels), and I am wondering which of the two is better?
The omnidrives would probably have more available power, but perhaps more friction as well? Also, how do they drive? With the example of an omnidrive, do all 4 wheels spin during straight motion? Or do just the two oposite each other turn, and the other two coast? I would experiment, but at the moment i only own 2 omni wheels :rolleyes: . |
Re: omnidrive vs. kiwi
I'd rather use 4 omni-wheels. It would take more weight and all that but a rectangular chassis is easier to fabricate and deal with than a triangular chassis.
http://www.chiefdelphi.com/forums/sh...?postid=420541 To drive forward, all wheels drive forward. To go to the side, the wheels on the same side will drive in opposite direction. wheels in the opposite angles of eachother will drive in the same direction. To go left, the front right and back left would drive forwards and the front left and back right would drive backwards. This would be opposite to go right. I'd expect that if you were going in a direction parallel to a pair of wheels ie. Front left and back right, then the other two wheels would turn at all. I hope I didn't make that too confusing |
Re: omnidrive vs. kiwi
With the system you describe it seems that the robot is never driving in a direction which is perpendicular to one of the wheels, so why are the omni-directional wheels necessary? Unless you were traveling straight in a diagonal I guess... in that case would two of the wheels be turned off? For instance, if traveling diagonally forward and left, wouldn't front left and back right be off, and front right and back left be traveling forward?
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Re: omnidrive vs. kiwi
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I don't really think the omni-wheels are necessary, just strongly suggested. |
Re: omnidrive vs. kiwi
That sounds like it would be a nightmare to program. What is the typical way that the sticks control it anyway? For instance, does one control stick control straight, sideways, and diagonal movement, and the other stick (ch 4) control spinning? (that would be like a video game) Or would the right stick control forwards, backwards, and spinning like a normal bot, and the left stick (ch 4) control strafing (sideways motion)?
With the last mentioned approach one could possible leave the right stick in the normal 4 wheel drive/ 12 configuration, and just program ch 4 to mix in strafing (sideways motion). Does anyone have an existing program for this that they could post? My concern with the whole omnidrive idea is that a lot of the power is being wasted. Comparing forward motion, how much slower is an omnidrive bot compared to a conventional drive train design with otherwise the same weight and power? |
Re: omnidrive vs. kiwi
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Such a robot will travel forward and sideways at speeds comparable to other robots based on its gearing, though optimally, I think you'd like to gear it faster than other machines in those directions so it's not unusable when traveling at some other angle. When you're using a combination of wheels to move at some random angle, your speed will be something less than it's straight line speed. Where omnidirectional movement is concerned, "forward" becomes a relative term. You can make forward on the joystick any direction, really. Some are more efficient than others, however, when speed and power are considered. |
Re: omnidrive vs. kiwi
The drive train you discribe (in which 'forward' motion involves 2 omni wheels oposite each other spinning, and the other 2 ideling) could possibly be slower/weaker than the one in the link, because in forward motion only the power from 2 of the motors is being harnessed.
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Re: omnidrive vs. kiwi
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Re: omnidrive vs. kiwi
what do you mean by 'six of one and half a dozen of another'?
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Re: omnidrive vs. kiwi
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"It doesn't really matter", "It's your choice","Choose either way", "Neither choice is better", "Similar results with either". Get the picture? "Six of one"=6 "Half a dozen"=6 6=6 |
Re: omnidrive vs. kiwi
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Re: omnidrive vs. kiwi
does anyone have a program to control a 4 wheel omnidrive that they can share?
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Re: omnidrive vs. kiwi
Just a quick note first. Both 3 wheeled, and 4 wheeled systems are "omni-drives" (which also includes swerve ("crab") drives, mecanum drives, ball drives, and other omni-driections systems). The method of motion is referred to as a "holonomic" drive system, where the motion of the robot is based off of the vectors of the output of the motors. 3-wheeled holonomic systems are also referred to as kilo-drives, and kiwi-drives.
When discussing the 4-wheeled system, let's nickname each of the 4 motors/wheels North, South, East and West (assuming you are placing them in the cardinal directions, as opposed to the corners as shown in the picture, but the same principles still apply in either positioning). To go directly North, you power the East and West motors, but the N and S stay motionless. To go East, you power North and South, and E and W stay motionless. To go along a diagonal, you power all 4 motors equally. To travel anywhere in between, you power all 4 motors varying amounts. To get a curved path instead of a straight path of travel, you power one motor more than that on the other side. Say you want to travel northwards, with a slight curve to the west. You leave N and S idle, power East 100%, and West 90% (so it will curve like a skid steering, aka tank drive, system would). To spin in a 4 wheel system, you move two opposite facing motors in opposite directions, instead of the same (or all 4 if you wish to spin in place). Likewise in the 3-wheeled system. To travel in the exact direction of one of the wheels, you leave that wheel idle, and power the other two motors equally. To travel in other directions and curves, treat it basically the same way as a 4-wheeled system, just the amount of power to each wheel will be differnt based on direction. The difference in between systems is in the efficiency. In a 4-wheeled system, you get 50% efficiency from your motors. Think of it when moving along a cardinal axis. You have 2 motors powered, and the other 2 idle, therefore 50% efficiency (the same applies for any other direction, as some of the power is "wasted" as you are not travelling directly into it's output direction). In a 3-wheeled system, you draw 66.7% efficiency. You get 100% efficiency from both systems when spinning in place. .667x3motors=effective power of 2 motors .5x4 motors=effective power of 2 motors Therefore, unless your spinning, you get the same power with 3 motors as you do with 4, but you use 1 less motor. But with a 4 wheeled system, you have a greater "footprint" or support base, making you more stable. You also get a greater power output when spinning in place, although the value of that tends to be limited. The math to figure out the vectors (and therefore the programming) is also slightly easier though. As well as construction in a kit with all 90 degree angles, such as the vex kit (60 deg angles are obnoxious to bend). As for control, with the vex controller, i'd recommend the following. Use the left (or right, depends on your preferance) to control direction of travel. Up, the bot moves "north". Left, it travels "west". Right="East", down="south". The other stick would control spin. Left=counter-clockwise, right=clockwise. I'd leave out the curves for now, it's more difficult to program and to control, and they tend not to be very necessary. |
Re: omnidrive vs. kiwi
Oh, the 4-wheel omni drive in this video functions differantly.
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I suppose that there are just two ways to set up a 4 wheel omnidrive? |
Re: omnidrive vs. kiwi
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As for the robot in the video: the control is similar, but: to go north, all four go one way, south, all four go the other way. East or west, two go one way and two go the other way. To turn, all the motors spin like they were trying to spin the same turret (assuming it's a circle laid on top). |
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