I’ve heard that mecanums tend to drift without intention to do so. Is it true that mecanums are so inaccurate?
I don’t know that I’ve seen mecanums behave like you are describing. People do complain about mecanum wheels for a few reasons though:
- they take up a lot of space
- they are slower. Honestly, I’m still a bit shaky on the details of this one, though it seems to have been largely remedied by the superior power of V5 smart motors.
- They don’t have quite as much traction as omni wheels, though they largely make up for this by being able to actively resist pushing from the side.
My verdict is that if you are using v5 they are a viable choice. If you use them, make sure you take advantage of the holonomic movement though. Some drivers instinctively integrate strafing into their movement while others naturally drive as if it were a normal chassis. If you are going to use the space they take up, just make sure it gets used and doesn’t go to waste.
Ya they do, after a while the error will build up and be really noticeable. You can use potentiometers to help the chassis move straight but that’s pretty much it.
Ok hold up. Do NOT use a potentiometer for this. Potentiometers are only meant to measure a range of angles and should not be forced to turn any farther. You want optical shaft encoders, which measure the change in angle from the starting position.
We use separate tracking wheels to measure the position of the robot rather than the rotation of the robot’s wheels. No matter what wheels you use, if you are measuring the drive shaft then error will accumulate fairly quickly.
If you do use Mecanum, add a 5/32" diameter hole thru the hub 1" away from and paralle to the axel hole. If the wheel is already mounted to a vex angle or channel, use the hole in the angle to line up the drill bit. The decorative dimple in the wheel is 1" from the center.
With a long Allen wrench, this makes mounting or changing the motor a lot easier. There is enough plastic in the wheel hub, that adding this should not weaken the wheel.
Sorry, thanks for the verification.
Yes, the mecanums do tend to drift not necessarily in regards to traction but because the rollers are at an angle, compared to normal omni-wheels with rollers that are straight. Mecanums should never be used for tracking; have a separate and unpowered wheel just for tracking (must be omni-directional wheel) if you are trying to measure distance. Because of how mecanums are built, they will move and turn slower but it can be solved by increasing the drivetrain gear ratio to 200 RPM or 600 RPM. Movement speed should rarely be a considered option if we are talking about wheels in VEX, just change the gear ratios for them and you’ll be fine. If you have a problem with speed then you need to see how other teams make their drivetrains
Right. To clarify, my original response assumed you were talking about drifting while trying to drive straight during driver control. It seems you were referring to position tracking, in which case the answer is yes, they are. However, it is not because you are using them on your drive, it is because you are using them for tracking. Again, I would recommend dedicated unpowered tracking wheels no matter which drive type you are using. The limitation of using the powered wheels for tracking is that if your wheels slip you have no way of measuring it.
thank you for your help. also does mecanums have undesirable sideways motion when just driving forward?
I have never noticed mecanum a drifting sideways while driving. In fact, if you use breaktype: hold with v5 then the mecanums actively resist being pushed to the side.
thank you again and what exactly does braketype:hold do? is it a pd lock?
It makes the motor actively resist position changes. It locks down the bot, but drains battery really fast
Well, yes. The battery does drain faster, but you should be starting each match with a full battery anyway and 2 minutes is nowhere near long enough to noticeably drain the battery.
Well yes, but it’s annoying during testing and practice as you constantly have to keep swapping
Yeah. Somehow I forgot to put that in my reply. I do remember that my robot wouldn’t last long at all during practice in turning point. I meant to put that in my post but I guess it slipped my mind after I started typing.
I’ll take extra function but changing the battery more frequently over ignoring a powerful tool any day though. We have like 6 batteries, so it’s not the end of the world. It’s not something I’d use on a chassis as much this season because it can be jerky while stopping, but last year it was nice because there was so much defense.
will a PD control loop to lock the chassis in place function the same? does it drain less electricity?
The v5 motor already have their own pid so they already do that.
To elaborate, breaktype: hold uses the motor’s internal PID to hold its rotation. It runs at a higher clock speed than a custom control loop would, but I highly doubt that makes any noticeable difference in power draw.
do i have to have the unpowered wheels on a hinge and band it so it sticks to the floor?
Although you are not required, it is extremely recommended to do that.