Four-Motor Crawler

[30YrsL8r]

I'll admit that I don't know anything about crawlers or crawling, but I was thinking about their construction this morning and I wondered:

If 1:1 cars benefit from traction control, would a crawler?

Does anyone know if a four-motor crawler has been tried? Four motors (one per wheel) each with it's own ESC.

I've played around with arduino-type boards in the past and I don't think it would be hard to control the ESCs from such a board, with a sensor on each wheel to tell the board how the wheels are turning in relation to each other.

Is this a crazy idea, or old news?

[waterbok]

I think that concept gets used on work trucks, have seen a build somewhere of a dumptruck and a crane which had gears and motors on all 4 or 6 wheels.

[bromvw]

this is a common design on large dump trucks . The engine drives a generator that then supplies power to each wheels drive motor . In principal this would work well on a rc but designing and building the axels would take some doing IMO. its not a crazy idea at all so long as you have the skill to make it work .

[Psylence]

Its a crazy idea alright... CRAZY AWESOME.

Like what they use on gigantic mining dump trucks and whatnot. Also a few car manufacturers have played with this for hybrid/electric vehicles. Volvo comes to mind, they prolly get a lot of crossover from their heavy equipment division on that front!

[speedy_w_beans]

Mike_o had a thread on a two-motor mini he did awhile back; I really liked what he was doing with it:

http://www.tamiyaclub.com/forum/index.php?showtopic=64820&hl=mike_o#entry439125

I'd like to try a four-motor, four-servo touring chassis someday because it would provide a lot of opportunity to experiment with Ackerman, torque vectoring, active rear steering, etc. It wouldn't be that hard to do with an 8-channel radio setup. I have a Turnigy 9X radio I reflashed with Open9X firmware that allows for lots of custom mixing; it's entirely possible to use 4 motors and 4 servos with it and program the relationships with custom mixes.

http://www.tamiyaclub.com/forum/index.php?showtopic=68469&hl=%2Bturnigy+%2B9x

Just some ideas to get the juices flowing...

[Tamiya1/10]

on paper and on actual working model would work well. put the power where there is traction and not wasting it.

on the other hand given today's high torque motor in relations to the weight of the rc, a locked front and rear differential will do just as good of a job. locked diff is crude and old school vs electrically controlled limited slip for each wheel. cost wise would be expensive 4 esc + 1 cpu to control the esc and 4 motors vs 1 esc and 1 motor + some puddy or 2 parts to lock the diff.s. Good engineering exercise but poor marketing potential. i guess if only the cpu is available..some of us will put it together since already have several motors and esc just sitting around.

[Daves956]

How would you handle the dig function or being able to spin the axles in opposite directions?

[DuaneB]

Hi,

It would be simple to handle dig or any other function provided your transmitter has enough channels to select the modes your require.

I have built a lot of similar projects including-

Active yaw control which senses unintended rotation and adjusts the throttle and steering in real time to stabalise an M04

http://rcarduino.blogspot.ae/2012/07/rcarduino-yaw-control-part-2.html

Mapping steering and throttle (car type) controls into completley different signals for tracked vehicles

http://rcarduino.blogspot.ae/2012/05/rc-arduino-robot.html

Measuring the relative speeds of four wheels to detect a loss of traction

http://rcarduino.blogspot.ae/2013/06/rc-arduino-traction-control-traction.html

In the short term I will use this in my M03 to test different traction control strategies but its long term future will be in my twin motor M07 (m03 + m04)

http://rcarduino.blogspot.ae/p/cars.html

Finally to see if a traction control system really works, you need a personal lap timer

http://rcarduino.blogspot.ae/2012/10/lap-timer-build-along-part-4-adding-ir.html

These rcarduino projects are widley known and used within the Arduino community.

Duane B (rcarduino)

[mr crispy]

Best 1:1 vehicle I can think of that used 4 electric motors, one per wheel is the NASA Luna Rovers or moon buggies. 4wd, 4ws you can run them just front or rear wheel drive. If you don't mind collecting them I know where there's 3 just sitting around doing nothing, never seen the rain and barely 100 miles on the clocks

[30YrsL8r]

I have built a lot of similar projects

Thanks, Duane - that's some excellent work and brilliant blogs; very interesting.

[30YrsL8r]

Just some ideas to get the juices flowing...

And they are, thanks.

[30YrsL8r]

Will probably take a while, but I think I'll combine these two projects and see if I can get something to work:

http://www.tamiyaclub.com/forum/index.php?showtopic=64820

http://mud4fun.wordpress.com/rc-rock-crawler/

[30YrsL8r]

Best 1:1 vehicle I can think of that used 4 electric motors, one per wheel is the NASA Luna Rovers or moon buggies. 4wd, 4ws you can run them just front or rear wheel drive. If you don't mind collecting them I know where there's 3 just sitting around doing nothing, never seen the rain and barely 100 miles on the clocks

Bit out of my way, sorry.

I've been trying to think of a way of doing a proof of concept and I found a cheapish Heng Long tank gearbox on ebay (item number 290923182142).

I saw a post of yours on the Heng Long tanks and I wondered if you thought I might be able to run the two 380 motors off a pair of the cheap Hobbyking brushed ESCs?

Also, do you have an idea of what the Final Drive Ratio is of these gearboxes?

[mr crispy]

I think the HL tanks use 380 motors already, if not the motors they do use should be strong enough for your needs, especially if you're using 4 of then. I ran a HL Tiger through two TEU101s which gave it a nice smooth low speed that was easy to control. If those hobbyking ESC are similar they should work fine.

I've no idea on the final ratio but at full speed on the tanks your get a brisk walking speed. Of course the final gearing element there would be the drive sprocket which is maybe 1.5 inches max. So using larger diameter wheels should increase your top speed.

I've been thinking about using these gearboxes to make a 6 wheeled argocat type vehicle with skid steering like a tank.

The gearboxes are a bit bulky though, you could try fitting a GRU onto a 540 motor and driving direct from that? You'll have a lot more choice with motors and pinions.

[30YrsL8r]

Two GRUs would be way out of my budget for a test of an idea, but the info on the speed of the tank gearboxes is very useful - thanks. About 4mph at top speed with that size sprocket isn't wildly out for a FDR to play with.

Any idea on the diameter of the final drive shaft?

[30YrsL8r]

I'd be happy for crawler gurus to set me straight (the following is based on a little reading and a lot of thinking), but:

In order to navigate an obstacle, a crawler needs to overcome the force caused by gravity acting on its mass.

To do this, it imparts its own force on the obstacle using traction. If this force is greater than the gravitational force, it climbs.

So, I assume the idea is to have as much surface area and all four wheels in contact with the ground at all times. This spreads the force around and, hopefully, ensures that each tyre's share of the force is not greater than its traction.

If you lift a wheel with a locked diff, the force can now only be shared between the remaining three wheels. This is made worse if the diff isn't locked, as the other wheel on that axle won't turn either. So, there's more chance that the traction won't be enough and the wheels will skid.

Hence the need for lots of articulation, which makes sure that a wheel going over an obstacle doesn't lift its lower partner, too. In case it does, people use a locked diff to lessen the impact.

If I manage to get traction control to work, the crawler will intelligently share the force between the wheels based on how much they can handle. I also overcome the spinning wheel problem that drives the need for a locked diff.

Here's my question, though: do I need to still have a solid axle, or would independent suspension be better because it doesn't have a camber problem?

[TWINSET]

Make this;

It gets interesting at 2:39!

Generally, for crawling, you want solid axles as they force the chassis up and out of the way when the suspension cycles - IS means one wheel goes high but everything else stays low, causing fouling on chassis, diff etc.

It all depends on how much articulation you're after though, what terrain you'll be crawling, where you want the centre of gravity etc.

Don't get too engrossed in having loads of suspension articulation though - when crawlers were first 'marketed' having the front axle twist perpendicular to the rear axle looked great, but it caused a lot of problems with wheels dropping into holes etc.

The action of the suspension is just as important - if you're driving over uneven ground, you don't have to have all the wheels totally follow the terrain as, if the 'diffs' are locked, having a tyre in mid-air isn't a complete disadvantage if the other three tyres have traction - you could stop the wheel that's in mid air, or run it at reduced speed once it detects it's not under any load, so it will at least realise when it's back on the ground.

[bromvw]

Having driven many 1:1 off roaders ie land rovers , jeeps , hilux , land cruisers ect ect I would say that solid axels are the way to go . independent suspension is all well and good but the amount of articulation is limited by the length of the upper and lower arms . A solid axel using 4 links and coil springs will give far better articulation every time IMO .

[30YrsL8r]

It gets interesting at 2:39!

Great, I was already considering what would happen if I tried to turn a crawler like a tank - now I'm thinking about the fact that I can put each wheel into reverse individually and maybe I need a steering servo per wheel, too. Thanks!

I'd thought about the fact that I wouldn't want to completely stop a wheel that was in the air, as I might not know when it was down again - especially if I was using a drag brake on the ESC.

On the suspension front, maybe I could have a combination of independent and solid axle - IS up to a point then the whole axle articulates beyond that. All this before I even try out my traction control!

[MadInventor]

As we are talking technicalities of 4wd, etc here, 30Yrsl8r, have you ever seen or been aware of the 4wd transmission used in County tractors ? These use a single rear diff, with 4wd taken to the front wheels via dual propshafts, i.e. one each side of the vehicle. This makes it a lot more difficult to break traction without resorting to a diff lock. It's the very good reason these machines are still used widely in forestry still today, despite the company going bust in the early 80s.

It's usefulness to you would be that you could run each side of the vehicle on a motor, one ESC a piece, and have steering through both conventional and skid steering if you use a v tail mixer on the escs and steering. I've used the V tail mixers for driving my vintage tanks on 2 esc, but can still drive it like a buggy (I.e. 1 stick for throttle and 1 for steering)

Skid steering, combined with normal steering.

I've found running walking beams on the back of my 6x6 truck gives great grip without resorting to diff locks. It's not that I can't get stuck, but it's considerably better than plain 4wd, and the walking beam articulation does wonders for keeping the wheels on the ground:

Here's a vid of it, you have to persevere a couple of minutes in to get to the off road stuff. Also bear in mind, no locked diffs and towing a trailer with 2Kgs+ of weight.

I think a 2 motor crawler is worth a try, with one motor driving each side of the vehicle.

[30YrsL8r]

Thanks for the information, MadInventor. Yet more terminology and technology to get my head around!

My Heng Long gearboxes arrived just now. Over the weekend I raided my bits box and found/made enough to get the front end of a DT-01 together.

Not exactly a classic crawler base, but enough for me to mess around with traction control (once I steal my son's kit DT-02 front tyres when the new ones arrive).

I researched (watched youtube videos about) how servos work and they look VERY low tech, so not much to fear there.

The picture below should give you an idea of how I want it to hang together.

I was hoping that the 380 motors would pass each other when each wheel's suspension acted, but they are too close and their magnets attract each other.

As a result, I'll have to swap the output shafts so that the motors and wheels are on the same side (and relocate one of the motors to clear the wheel).

Should have gone for the other type of Heng Long gearbox. Oh well....

[MadInventor]

The other thing that I forgot to mention is articulated chassis. If the chassis pivots in the middle this can greatly increase manoeuvrability. Looking at the pic above, if you can get skid steering to work on the rear wheels this will greatly decrease the turning circle, although it will be more effective with a shorter wheelbase.

These are worth looking at as they only cost a few quid:

http://www.ebay.co.uk/itm/RC-Model-Aircraft-Plane-TURNIGY-V-Tail-Mixer-Ultra-small-/150964672805?pt=UK_ToysGames_RadioControlled_JN&hash=item2326321925

If you connect this to CH1 & 2 of the RX to this gizmo, and the outputs to 2 identical ESC to drive the tank motors, the rear end will effectively work like a tank. Full throttle and no steering gives full throttle on both motors, adding steering adds less throttle and then braking to the motor you are steering into. You can also get one motor to operate just on the steering, and if you want, pivot steering where both wheels operate in opposite directions.

Connect in the steering servo with a Y lead as well and you effectively have a 2wd tractor with skid steering, with some added flexibility of being able to rotate the rear wheels in opposite directions. (Not much use with 2wd, but go 4wd and it's very useful.

They are what I used to be able to drive my old vintage tanks 'buggy style' without using tank MFUs.

[MadInventor]

Have just thought of another way you could get your chassis up and running for this. 2 clod busters. 4 clod buster gearboxes. Remove the axle off 1 side of each gearbox (Cut off the drive axle short so it just pokes out of the gearbox by about 5mm, then use a custom spacer with a bearing in the middle to bolt 2 gearboxes side by side to each other. Lock all the diff assemblies with hot glue, and you have 1 motor per wheel, 4 steerable hubs, plenty of room for 4 servos (kits are already available for mounting servos on the axle supports). 4 link kits already available for custom mounting, just need a double width chassis. Only other mod required is to fit the motor on the wrong side of 2 of the gearboxes , so they don't impede the other gearbox bolted to it.

Ta-da !

[30YrsL8r]

You're really giving this a lot of thought, MadInventor! I should be able to incorporate the v-tail mixer function in software once I get my prototyping board connected up to the buggy.

I intend to get a bit more adventurous with the hardware once I've proven that my micro-controller programming skills are up to the traction control challenge but, for now, I'm sticking with my low-budget DT-01/tank hybrid. A custom crawler build will (might) follow.

I've swapped the output shafts and raided a Meccano box to mount the gearboxes to the chassis - will do a better job now that I know they fit.

I didn't bother swapping the offending motor as I need to extend the output shafts to give a DT-01 sized track, which means the motor won't foul the wheel (see comparison with a DT-01 gearbox in the photo.

[MadInventor]

Yeah, I like these sort of projects that try to do things differently. I put a lot of thought into the possibility of building a scalish county tractor model a couple of years ago, and was trying to toss up which way to go between twin motor driving each side of the transmission (not very scale but easy to do skid steering), and a more scale 3 speed box with a diff attached, and a diff lock working on the rear axle off a dog clutch. Never made any real progress on it other than building the trailer, and then ended up building my 6x6 instead to pull the trailer. I'm just starting on my next project, which is a home made half track based on a king tiger chassis, I am doing the opposite to you and trying to mount a solid rear king hauler axle with tank sprockets .

I'm going to follow this thread with interest.