Tracked Carrier for the Homestead

[mdeaves]

I'm planning on building a remote control tracked carrier robot to help on our homestead. We will use it to carry materials, plow snow, zamboni the pond in the winter, power tools all over the site with 120 V auxilliary power, power a winch for pulling stumps, etc.

I'm thinking something like this,

Fero ETP-10 Electric Tracked Carrier | 1,000 kg carrying capacity

Fero ETP-10 is a remote-controlled tracked machine that makes transporting heavy materials more efficient and safer than ever.

www.ferocarrier.com

Here's my plan so far.

- 48V, 200 Ah, LiFePo4 batteries
- Dual hub motors, one on each track for tank turns
- 250 kg max payload capacity
- 200 kg unloaded mass
- Able to climb a max grade of 20%
- 6 km/hr top speed

I was very surprised by how small the hub motors actually need to be. The simulator at ebikes.ca says that two RH212 hub motors will be able to climb a 20% grade, fully loaded (450 kg total weight, 225 kg per track), at 5.4 km/hr without breaking much of a sweat. Here i've set the track drive wheel diamter to 4", which seems reasonable.



One thing I really need to consider is braking the tracks when the power is shut off for what ever reason, or just when the carrier is resting halfway up a hill. I really don't want this thing rolling down our hill, with 250 kg of bagged cement, completely out of control!

One of the main benefits of the hub motors is that it is easy to mount a brake disk to them, so the natural braking option is to use hydraulic brake calipers built for ebikes. They're already rated for use outdoors, are cheap and easy to find, and have great holding torque. The trick is making them normally on unless power is applied (fail safe). For this I plan on using a spring to hold the caliper on, and a hobby servo motor to relieve the spring force when the robot is commanded to move. I think that by adjusting the position of the servo so that it is near top dead center at the end of it's travel there won't be much servo torque needed to hold the brakes open. Hopefully this means I don't burn the servo motor with constant current when operating the robot.

My main question is about how I can control all of this? I know very little about how these motors are controlled, and I know even less about integrating servo control with hub motor control. I'd like to avoid any sort of custom coding if possible, but I can do a bit if necessary.

I'd also love for you all to give me a sanity check on my plan to make sure there are no large oversights.

My other main question is about sourcing or building the tracks, but we'll cross that bridge when we come to it.

Thanks so much for your help.

 

[Epithemeus]

I don't quite understand, are you using the rh212 as the hub of the track wheels directly? Or using some sort of gearing system/belt? Cause the RH212 is like 9 inches in diameter by itself so can't be a hub for a 4 inch diameter wheel.

 

[mdeaves]

That's a good point. I honestly hadn't thought about packing at all, I just chose a motor from ebikes.ca that I thought would work.

The plan would be direct drive. So that means either I increase the drive wheel size or choose a smaller diamter hub motor. Here is the Bafang G62 with an 8" drive wheel, and it also seems to work, with a higher top speed too!



I'm finding it a bit over whelming to select a motor. There's so many options and the seller never seem to list the specs that I want to see. That's why I'm using the ebikes.ca tool, but the motor selection is limited to the ones they carry. Even still they don't list the continuous current rating of the Bafang G62. Will 40 Amps burn this thing out? I don't know. If anyone knows of a decent resource for detailed motor specs please drop it below.

One other option I have is to make the tracks triangular like below



This would allow more clearance of the drive wheel, and allow for a larger motor. But it ruins the nice flat top of the carrier that I was hoping to use to moving large sheet goods.

 

[Apartunis]

Here is the Bafang G62 with an 8" drive wheel, and it also seems to work, with a higher top speed too!

That is a gear reduced hub motor that can not drive in reverse.

Direct drive hub motors that can drive both ways and give you enough torque to do the job are going to be large in diameter like 10 inches or more.

 

[Epithemeus]

That is a gear reduced hub motor that can not drive in reverse

A welded clutch could solve that. I'm not sure a hub motor is going to be the right motor for the job though. I feel like you'd be better off doing something more like a go cart with a central motor driving an axle with a gear reduction that can be carefully chosen. But I understand the appeal of the simplicity of hubs. Maybe a scooter or skateboard hub could work size wise but those probably wouldn't give you the torque you want. Grin also sells the wheelbarrow hub designed for lower speeds, maybe look into that.

 

[mdeaves]

I see your point about being able to change up the gear ratio. My problem with a chain drive, other than the added cost and space, is that the ratio needs to be so large that I'm back to having a giant sprocket dragging on the ground. Unless I do a two stage chain drive, but thats added complexity I'm not really interested in. The drive wheels need to spin in the 200-300 rpm range at max speed, so that's usually a 10:1 ratio from any mid-drive motor without a gear box.

Maybe a motor/planetary gear box combo would work. Something like this,

1500W BLDC Planetary Gear Motor, 36V/48V

Compact 1500W DC planetary gear motor works at 36V or 48V power supply. It comes with 3000 rpm rated speed, 4.78 rated torque and 3:1 to 250:1 gear ratio. Small brushless motor with a planetary gearbox is designed for your electric cars. The power, voltage, speed, shaft, motor length and lead of...

www.brushless.com

This site also allows me to order it with a brake, which I'm guessing negates the need for my brake caliper/servo mechanism. I'm not sure if it's good to use outdoors though. Again, the site is very sparse on information.

Does anyone know of any commonly used motor/planetary gear box combos? If they have a built in brake and are rated for outdoor use I'm set!

 

[Apartunis]

Electric wheelchair motors are reversible and often use worm gear reduction. That gives the advantages of large reduction ratio and locking the wheels in place when the motor is not running. They are also small in diameter compared to hub motors.

 

[mdeaves]

All the wheel chair motors I could find are 500 W of less, likely too small for what I want.

I did find these motors though, 800 W, 48 V, with a brake disk mount, and they look like they are less than 8" in diameter. I could use two motors on each side to beef it all up.

disc braking 48v 800w reversible geared 14 inch electric wheelbarrow motor – UU Motor Retail

retail.uumotor.com

 

[offGridDownUnder]

Not the entire solution, but something to throw into the mix, I connect my DD Grin All-Axle to my rear drive wheel with a Gates Carbon belt.

Gates provides sprockets in the range of 22T-26T (maybe other sizes by now) and I use Gates chainrings which are available from 39T-74T to provide reduction. The small sprockets mount to 44m BCD ISO brake rotor mounts on the motor - which will be available from many bicycle motors. For the larger, I cut spiders from 4mm steel or Al to mate the 44m BCD ISO brake rotor mounts (on the hub) to the 104mm BCD or 130mm BCD chainring standard.

This would permit mounting the motor above the tracks and offer some reduction. The Gates belts are also resistant to dirt, water and have a longer lifespan than chains with much less fuss. Tensioning is not a problem as Gates belts are designed to operate under tension.

 

[offGridDownUnder]

Also, I have seen small tracked machines with a place for an operator to stand while controlling the machine. I'm wondering if starting with one of those would save you fabrication time.

 

[floydr]

All the wheel chair motors I could find are 500 W of less, likely too small for what I want.

The Wheel chair ( I picked a chair up for free a couple years ago) Motors I have are 1082W 24V gear reduction brushed Motors . Ebay has them or a Thrift store might have some. An actual wheelchair with 14" or larger wheels should have 1000W motors.
later floyd

 

[mdeaves]

Thanks for your input everyone. After thinking about it for a while I think that perhaps it's best to not use tracks because they are difficult for me to build and I'm not sure how they would do over the very rough and rocky terrain that we have on our property (see below).



Since the top speed requirements are so low, I'm leaning towards a more Mars Rover type Bogie suspension system like this.

6 independently driven wheels allows me to use smaller motors and still meet the power requirement. Worm driven wheel chair motors could work, and they automatically solve the braking problem since they can't be back turned as Apartunis pointed out. The Rover from OSURC still uses tank steer and seems to manouver just fine, so I won't need to steer the wheels like the perserverance rover does.