Moving a human using VEX EDR

VEX V5 Technical Support
#1

Morning all, so a few weeks back I started building a VEX EDR tank to move me around (95kg). I had quite a lot of experience doing this with LEGO and VEX IQ. However I am finding it a real challenge which has really surprised me since VEX EDR is Metal, and more powerful than LEGO and IQ.

First off I tried to run 8 motors off 1 Cortex + Power expander (direct motor to wheel connection). This ended up shutting down pretty fast. https://www.youtube.com/watch?v=ojPM1XSDD9M

I then doubled everything (2 cortex, 16 motors, 2 power expanders). This worked better but still no where near workable

I spoke to a few people and they said the motors where shutting down and so I should gear it down. I really did not want to lose speed but went for a 2.3 to 1 ration (84, 36 if I remember). I then went back to a single cortex, 2 power expanders and 8 motors, the motors drive 2 omni wheels each side and the rest are free wheeling. This so far has been the best but still not great. I have now added 4 extra omni wheels to help with the weight.

Tonight I plan to add more motors and power expanders (using the Y 3 wire cable) and put power through more wheels.

Anyone got any ideas?

#2

MOAR MOTORS

#3

i think without sacrificing much speed, you can change the motors to torque, and have more wheels to spread out the pressure evenly so as to reduce the normal reaction forces

#4

In brief, you need to spread the load over the available supply circuits such that the 4 amp limit of each supply circuit is never exceeded. Just looking at the calculation of power required, you need about 11 or more supply circuits to run 1/1 motors at full speed (which is what you’re trying to do in direct drive) or 5 or more supply circuits if you gear it down to 50% speed at the drive axles. (Note I didn’t yet do the torque requirement to determine number of motors needed; just number of 4 amp circuits.)
Given my choices, I’d do it with one cortex, driving some motors from each half of the cortex. I’d use power expanders for the remaining circuits.

Details:

Take speed s as, say, or 0.25 m/s, half the unimpeded (not to say unladen) speed of a VEX 393 motor. (Assuming 50% gear reduction drive. 1/1 is given lower in the calculations.)

Need mass converted to a force, so:
weight in Newtons Earth = 95KG * 9.81 N/KG = 932

Coefficient of friction for 4 inch omni wheels=0.714,

from @LegoMindstormsmaniac post in this thread: https://vexforum.com/t/anyone-have-friction-coefficient-of-vex-wheels/19261/1

So, taking some simplifications (like ignoring air resistance and starting friction, and assuming level ground)

Total Power required (watts) = coefficient of rolling friction * weight in newtons earth * speed

TPR = 0.714 * 932 * 0.25
= 166 watts
Taking high charged batteries, and noting I=P/V, then:

amps required = 166/8, or 20.75 amps

Note to drive the motors at full speed, you’d need:

TPR = 0.714 * 932 * 0.53
= 353 watts

amps required = 353/8, or 44 amps.

Each cortex half (ports 2-5, ports 4-9) can source 4 amps before the PTC device will shut you down.
Similarly, each power expander can source 4 amps.

So, you need one cortex and three power expanders to move with 1/2 gearing, once the system is moving.
Or one cortex and nine power expanders to move with no external gearing.

You might need just a bit more to get it moving.

Also, you need to spread the load over an appropriate number of motors. That, too, is calculable.

[Edited to clarify axle speed assumptions. @KyPyro]

#5

Note I assumed 4 inch omni wheels, which is what you show in the video, I think.

#6

I’m curious, how did it work out with the LEGO and Vex IQ sets? Were you able to move yourself?

#7

Interesting @kypyro , how did you get the coefficient of friction for 4 inch wheels? Is that given somewhere?

#8

@Yerayrobotics, Here’s the excerpt where I reference an older thread.

You can click on that link and read the thread for details, but near the bottom of the thread @LegoMindstormsmaniac has a nice list of some readings he took. He averages the calculations for 4 inch omni wheels and comes up with 0.714.

I’ve edited my previous post to clarify a couple of assumptions, and straighten out some language.

#9

Whoops. Definitely did not see that link. Thanks @kypyro ! I’ll give it a read.

#10

I think @kypyro math is too pessimistic: 353 watts would require about 90 of 393 motors, which is clearly way too many!

The way I understand @LegoMindstormsmaniac post from a linked thread is that 0.714 is a static friction coefficient of omni-wheels vs soft foam tiles, where they would make a depression in the foam. Then rolling friction coefficient would be somewhat dependent on the robot weight and speed because robots would have to constantly climb out of that depression. Here is a rolling friction illustration from an older thread:
https://vexforum.com/index.php/attachment/56464dba339d2_rolling_resistance_friction.jpg

However, I don’t think @burf2000 is planning to drive on the foam tiles and, therefore, we don’t need to worry about rolling friction as much. There is some deformation of the omni-wheel’s rollers but, I think, the major friction source is from the wheel axles vs bearing blocks and from the bending of the axles under the heavy load.

I couldn’t tell from the OP what is the exact motor layout, but another issue is, probably, uneven load distribution, where motors on the back of the vehicle are carrying most of the load, burn out first, and then it cascades to the front motors. To counter that you would need to either concentrate the direct drive motor-wheel pairs where the load is the highest or, even better, link all motors on each side together to form a power sharing pool (chains are an easy way to do load balancing).

So considering both the high axle load and load balancing, if I had to build a human transponder, I would put most of the motors to power tank threads on the back and a few powering a couple of omni-wheels at the front. Essentially, it would be a Half-track vehicle similar to this:

http://vignette3.wikia.nocookie.net/tractors/images/6/6f/Half-track_light_truck_at_Lincoln_08_-_DSC00036.jpg/revision/latest?cb=20100212214955

VEX tank thread kit includes double bogie wheels, that have round axles and should be able to carry a lot of weight with low friction when properly lubricated.

Once the chassis is balanced, I would try to power track driving sprockets either directly with multiple force geared motors or with speed or turbo geared motors externally geared down 3:1. Since you don’t need to obey VRC game limitation of 12 motors and one power expander, you can simply keep adding power expanders and motors until you are happy with the resulting torque and speed.

If you want to get fancy you could do a transmission to start in a low gear, accelerate and then switch to a high gear. This video demonstrates, probably, the simplest option you could do, unless you want to go with more complex pneumatics or differentials type:

However, the transmission would require to add quad encoders and control program in order to synchronize output and motor velocities during the shifting between the high-torque and high-speed modes. Otherwise it would be very easy to ruin transmission gears under such a large load.

#11

It’s really not that hard to do. Two years ago we built a pushbot that I could stand on top of and drive around. It had a mecanum drive powered by 8 393 torque motors geared externally 1:9 (also for torque), and a two-layer thick 35x35 plate of steel c-channel on top (it was originally to add weight for traction). Never skipped gears and took a while to eventually twist the shafts going into the wheels (we didn’t have HS shafts), but it did carry a person on top without even slowing down (or browning out the motors), albeit veeeerrryyy slowly.

#12

As what I see, the majority of the weight is supported by only four wheels instead of the 10 because of where you would be sitting. How about go with more and smaller wheels in that area to reduce the axles from bending easier? How about more power expanders to handle the immense amount of motors as well as adding more motors? By the way, an extra CORTEX isn’t really needed if you just add more power expanders (Since all the CORTEX will do is send the info to the expander, then the expander supplies the power needed). In the area in the center, what about putting a very small 2" wheel smack dab in the center of the area you are sitting at to reduce boeing from the immense amount of weight?

#13

Just an extra note, Those wheels don’t seem like 4" wheels. How you can tell; the 4 inch wheels has the left side rollers parallel to the right side rollers on the wheel, as the 3.75" wheels has the left side rollers shifted.
Here’s the pictures:
276-2185-hs-shaft.jpg
276-3526-single_1_1.jpg

#14