Motors/Gearboxes required to move a 20 kg/44 lb robot

[ArchVince]

After the last drive system was too weak to move the robot, it seems prudent to check the current plan before proceeding.

Robot Specs:
This isn't for use in a FIRST event, so the drive system doesn't need to comply to FIRST regulations. The robot is ~20 kg (44 lbs), with a 38 cm x 38 cm (15" x 15") wheelbase. We're looking for top speed of 1.5-2 m/s (5-6.5 ft/s).

Drive System Specs:
Four 5" wheels, of which the two rear wheels are powered by one BaneBot RS775 motor each. The wheels will be attached directly to the output shaft of the gearboxes on the motors, which will be VersaPlanetary 50:1 gearboxes. The current plan is to use the 18V RS775, but to run them at 12V.

Parts:
RS775 Motor
VersaPlanetary Gearbox

If someone could confirm that this will be sufficient power, I'd appreciate it. If any other information is needed, let me know.

Suggestions of other drive systems are also welcomed, as no one working on this specializes in this area. If you do want to suggest something different, here is some potentially helpful information:

Wheel size is limited to between 4"-6" due to the frame construction.
The motors will be on 40 amp fuses (10 amp fuses if we can get away with it, but that seems unlikely).

Side question, as long as I have a thread: If I have the wheels as close to the gears on the output shaft as possible, would I still need to worry about supporting the shaft to protect the planetary gearbox?

Thanks.

[magnets]

Oops, I was wrong.

[protoserge]

A couple things:

There's more to speed on the motor. You need to be more concerned with torque. The BaneBot RS775 18V operating at 12V is a bit different.

Can you post a sketch of your drivetrain geometry? Is it a square? Wide base? Please provide dimensions, wheel material, and tire width

You will need to answer these questions:

1. What is the scrub force of the robot?
2. What is the torque/force of each wheel needed to turn the robot to overcome the scrub force?
3. What is the torque needed to accelerate the robot to maximum speed in an acceptable time?
4. What motor and gearbox combination will achieve this?
5. Anything I missed

You've already got a good starting point with the requirement of 1.5-2 m/s. From here, you need to figure out how much force provided by each wheel is required to rotate the robot in place (this will be the most demanding operation of your setup). Also, keep in mind that the motor torque ratings are given at a certain RPM. This RPM may be at full power. If you use a full-power rating, you may not have control when throttling down (low speed or fine motion).

Originally Posted by ArchVince

Side question, as long as I have a thread: If I have the wheels as close to the gears on the output shaft as possible, would I still need to worry about supporting the shaft to protect the planetary gearbox?

Post a photo or CAD file of your design. My initial suggestion would be to support that end since the shaft is only supported by one 3/8" thick bearing on the output of the gearbox if I recall correctly. This may be easy or difficult depending on your design. It may be possible to stack a bearing block with the appropriate bearing press fit on the output end of the gearbox to help alleviate the moment arm.

[artdutra04]

Originally Posted by magnets

I would try to support the shaft on the other side. The versa planetary manual says you don't need to, but they are talking about sprockets, not wheels.

I would also try to have more reduction if possible.

The banebots rs775 free speed is 19500 rpm.
Lets say that you have the motor spinning at 15000 rpm. With a 50:1 reduction, that's 300 rpm for the wheels. The wheel circumference is about 7.9", so that gives you a max of 2370 feet per minute of travel, or 40 feet per second, which is too much.

If I were you, I would go for a larger reduction in the versa planetary (maybe 100:1) and I wouldn't direct drive. Then, you could do a 2:1 reduction with chain, and you could drive both the front and back wheels.

The RS775 spins at 19500 rpm free speed at 18v. At 12v, the free speed is approximately 13000 rpm.

[KrazyCarl92]

Originally Posted by magnets

Lets say that you have the motor spinning at 15000 rpm. With a 50:1 reduction, that's 300 rpm for the wheels. The wheel circumference is about 7.9", so that gives you a max of 2370 feet per minute of travel, or 40 feet per second, which is too much.

Aside from the motor spec being incorrect, this math is also flawed. Divide by 12 to convert from inches to feet. Also the incorrect formula for circumference was used.

C = pi*d = 3.14*5 in.
C = 15.7"
300 rpm = 5 rps
speed (inches per second) = 5 rps * 15.7"/revolution = 78.5 in./s
speed (feet per second) = 78.5/12 = 6.5 fps

This is on the slow side of somewhat reasonable speeds for a robot using FRC motors. That being said, I have no practical experience with RS775-18 in a drive system, so use at your own risk.

[Nate Laverdure]

See the BaneBots spec for the RS775-18 motor at 12V (pdf link).

[Nathan Streeter]

What will this encounter for high-torque situations? Pushing/Pulling? Driving up an incline? What other things does this drivetrain need to do well? Maneuver well? A lot of this will determine how much torque you’ll need and what wheels you want to use. Also, the surface it’ll be driving on is important for designing the frame and choosing wheels… will it be a hard, smooth surface? Tight carpet like an FRC field?

Since you have a pretty slow speed in mind, driven by a very fast motor I’d recommend going with a smaller wheel (4”, probably) so that you need less of a gear reduction. Also, it seems like a robot as small as 15”x15” would want to use smaller wheels.

If you have 4 relatively grippy wheels on the ground, in a semi-square layout, you’re probably going to have a very difficult time turning. I’d strongly recommend using either a low traction wheel at the non-driven end or an omni wheel.

So, if you’re using a 4” wheel to go 6 ft/s: (6ft/1s) * (12in/1ft) * (1rev/4*pi in) * (60s/1min) = 344 rpm
That means in order to go 6 ft/s using 4” wheels you need to spin them at 344rpm.

Your RS775-18 run at 12V should have a free speed of 13000rpm. When making a first pass for a top speed calculation of a drive train with low-to-moderate drag, using the max efficiency speed isn’t a bad idea, so I’ll use that. So, assume when you’re at top speed the motors are spinning at 11300rpm (per Ether’s helpful little .exe).

That means the ratio you need for the reduction is (11300rev/1min) * (1min/344rev) = 32.8:1

Go ahead and run these calculations by yourself using a different size wheel, if you want, but it certainly seems like a 50:1 would be ample reduction for 5” on a robot that weighs this little. Also, I doubt the RS775 with a 30+ reduction and 5” or 4” wheels on a <50 pound robot will have any trouble accelerating. I’d be happy to help you with those calculations if you’d like though.

How this drivetrain is being used is my big concern though… RS motors (and pretty much all brushed, fan-cooled motors) should hardly ever be stalled, as they’re very, very susceptible to burning out.

Also, if this is all pretty light-duty, I’d say you’re actually fine to leave the gearbox unsupported, as long as you put the wheel right against the motor.

So, please respond, answering some of the questions throughout my post and some of the above posts… about how the drivetrain is being used (and on what surfaces) and what wheels you’re thinking of using.

[protoserge]

Originally Posted by Nathan Streeter

How this drivetrain is being used is my big concern though… RS motors (and pretty much all brushed, fan-cooled motors) should hardly ever be stalled, as they’re very, very susceptible to burning out.

This is a good point. I would have to look at the VersaPlanetary box to ensure the cooling air intake for the motor is not blocked. Knowing IFI, this has probably been taken into account with the mounting kit. Does anyone have experience with this?

Lack of cooling flow guarantees you will case short a BaneBot in no time.

[ArchVince]

After looking at the relevant calculations and some discussion with the others working on the robot based on what you've posted, we've made several changes to the design. We decided to use omni wheels for the front (non-drive) wheels. We've also decided to switch to a 30:1 ratio on the VersaPlanetaries, and use 4" wheels. As such, this is the current layout for the drive base:

Labeled Diagram
Unlabeled Diagram

Current plan is to use the following parts:
Hub
Key
Omni Wheels
Drive Wheels

I can't seem to find a way to support the shaft of the VersaPlanetaries, though. The shaft is too short o get a wheel and a support on.

stinglikeabee:
Hopefully the diagram and links above provide enough information as to the geometry. It is slightly wide base. The drive wheels are made of thermoplastic polyurethane, 1" across, the omni wheels have Black SBR Rubber rollers with 0.75" OD rollers.

With omni wheels, the turning requires vastly less torque. I'll run through my calculations here, since I'm not entirely sure they're done properly (values are overestimated, to be on the safe side):
Us = Static Coefficient of Friction for Omni Wheels (sideways) = 0.27
Uf = Static Coefficient of Friction for Omni Wheels (forwards) = 1
L = Load on wheel = 5 kg
Fs = Frictional force (sideways) = L * 9.8 m/s/s * Us = 13.23 N
Ff = Frictional force (forwards) = L * 9.8 m/s/s * Uf = 49 N

Assuming a pivot point of the left drive wheel, with the right drive wheel pushing, and the two omni wheels resisting:
Dd = Distance from Right Drive to Pivot = 0.31 m
Dl = Distance from Left Omni to Pivot = 0.23 m
Dr = Distance from Right Omni to Pivot = 0.38 m
Drx = Dr Horizontal Component = 0.31 m
Dry = Dr Vertical Component = 0.23 m
Tl = Torque from Left Omni Wheel = Fs * Dl = 3.0 Nm
Tr = Torque from Left Omni Wheel = Dry * Fs + Drx * Ff = 18 Nm
Tt = Total Torque Required = Tl + Tr = 21 Nm
Fd = Drive Force Required = Tt / Dd = 59 N

R = Wheel Radius = 0.0508 m
Ta = Torque at Drive Wheel Axis = R * Fd = 3.0 Nm

At max efficiency, the motor outputs 0.1 Nm of torque, so, with the 30:1 gearbox, we should be able to turn fine.

I'm very hesitant to trust these calculations, as I wasn't entirely sure what to do for them. If someone wants to correct bad assumptions I'd appreciate it.

The acceleration time to maximum speed is a fairly moot point due to the use case. It's a display robot, not a competition one, so even if it takes a bit to get up to top speed there really isn't an issue.

Nathan Streeter:
It will likely need to go over small bumps, although that is most likely the extent of it. It's a display robot, so think of the typical school. It may need to get in and out of elevators, or over the bottom part of a door frame, but it shouldn't need to push or pull anything.

The surface it will run on will vary. The most common will be tile, concrete, and tight carpet.

My understanding is that 40A auto reset fuses will prevent the motors from being stalled, and thus prevent that burnout.

One thing I did forget to mention previously: I have no idea why this would be relevant to the drive system, but just in case there is an effect, the robot is actually fairly large vertically. It's about .9 m (3') tall. However, the top is very light compared to the bottom; the center of mass is about 0.33 m (13") off the ground. If we run into problems, we can rework the battery cage to get it down to 0.29 m (11.5"). I don't see this being an issue at any point but acceleration, so worst case, I can just limit acceleration to an acceptable level in code.

My apologies if I missed any of your questions, as there was quite a bit there. Thank you for the help, it's already proving beneficial.

[Nate Laverdure]

Originally Posted by ArchVince

I can't seem to find a way to support the shaft of the VersaPlanetaries, though. The shaft is too short o get a wheel and a support on.

If you really feel like you need to support the cantilevered shaft, you could add an additional hub to the outside face of each wheel (on the opposite side from the existing hub). You could increase the hub bore to fit a bushing which could ride on an additional cantilevered shaft fixed to the robot frame.

HOWEVER-- This design is poor because it requires all of the elements (hub bolt pattern, hub bore, bushing OD & ID, shaft) to be accurately placed on the axis of the versaplanetary. Stresses from misalignment of any of these things would be taken up (at least partly) in the gearbox.

EDIT-- or you could press a bearing into the opposite side of the wheel and have that ride on a fixed shaft. That would eliminate a number of uncertainties from the tolerance stackup. My point still stands about accurately locating the fixed shaft relative to the versaplanetary axis, though.

[cmrnpizzo14]

This is usually pretty helpful for questions of this nature.

http://www.chiefdelphi.com/media/papers/2059?langid=2

[Ether]

Originally Posted by ArchVince

Ff = Frictional force (forwards) = L * 9.8 m/s/s * Uf = 49 N

If someone wants to correct bad assumptions I'd appreciate it.

If the omni wheels are unpowered and free to rotate, Ff should be the forward rolling friction, not the forward traction.

Also, CoF varies greatly according to the surface the wheel is on.

[Paul Copioli]

The VersaPlanetary shaft does not need to be supported if you stay within the load limits described in this document: http://content.vexrobotics.com/vexpr...s_20130107.pdf

Additionally, there are vent holes in the motor mounting plate specifically designed for the BB RS-775 motor.

[ArchVince]

Ether: Ah, thank you, excellent points.

Paul Copioli: Looks like we should be well within the load limits, thanks for the link.

Nate Laverdure: Based on what Paul linked, we should be fine without it. Thanks for the ideas, though.

cmrnpizzo14: Good resource, thanks.

Thank you all for your help and information. We've settled on a design now that I'm confident will work.