I’m brand new to the forum and to robots. Despite that I’m hoping to get started on a project idea right away which means I’ve got a lot to learn and a lot of questions. In general I’d like to develop a rope climbing robot which seems like it shouldn’t be too difficult. It seems like the most challenging aspect (and feel free to set me straight about this if I’m wrong) is the power requirement when you’re always working against gravity. The first iteration will not involve any moving parts beyond the drivetrain, so no grabbers or anything like that.
Not really knowing the best first step to take, I’ve started with the Drive Motor Sizing Tool to get an idea of what I’ll need for a drive train based on my initial estimate of the performance specs. Is that a reasonable place to start? Is there a recommended process for designing a robot? e.g. determining drive requirements, sourcing motor and battery, fitting them into the frame etc?
The specs I have so far used are:
]weight: 16 kg/:m] ]1 drive motor/:m] ]wheel diameter: .0508 m/:m] ]robot velocity: 1.34 m/s/:m] ]incline: 90 degrees/:m] ]supply voltage: 48 v/:m] ]acceleration (besides gravity): .2 m/s/:m] ]operating time: 600 minutes/:m] ]efficiency: 65%/:m]
Those specs gave the following motor/battery specs:
]motor rpm: 250/:m] ]torque: 12.42 N-m/:m] ]total power: 328 W/:m] ]max current: 6.8 A/:m] ]battery: 68 Ah/:m]
I’ve also come up with a required drive wheel rpm of 840. So I’ll need to reduce speed from motor rpm and pick up some torque with gearing.
So my first question is regarding the efficiency input. The default value on the tool is 65%. Is that a typical number to use? What factors would determine whether the estimated efficiency could be more of less than 65%?
Do the numbers that the drive tool produced seem realistic?
I’ll explain the steps that seem to make sense to me as far as designing this project and then maybe someone with more experience can comment:
Determine what the operating requirements are.
In my case I need a robot that can climb a rope/cable straight up at a max speed of around 5 mph, although typical operating velocity will be less. I am estimating that the weight will be around 16 kg and that I will have one motor driving one drive wheel through a gearbox with a spring loaded idler wheel applying pressure to the other side of the rope. With a 2" drive wheel I know the drive wheel rpm will be around 840 rpm.
Determine required drive wheel torque, motor torque and rpm and battery requirement
For this I have plugged in the estimated requirements from Step 1 and come up with the numbers I posted in my first post.
Source motor and battery
I’m not 100% sure how this works. I know the torque and rpm I need at the drive wheel but there are a lot of different ways to get there. I’m GUESSING I could look for a 2500 rpm motor with 1/3 the torque I need and that way, with some 3:1 gearing, I’d end up with roughly 840 rpm and the required torque at the drive wheel. So I start looking through supplier catalogs for a motor that fits. Then ditto for finding the right battery…just use the number that came out of the Drive Motor Sizing Tool to find it in a catalog?
Yes? Can anyone tell me if my methodology is reasonable so far?
I was wondering if you have any drawings or diagrams that can better explain the mechanism you are using to attach and climb the rope. Also, the weight of the robot is pretty high. Is this suppose to be a rover robot that can find a rope and then use some sort of mechanism to switch to a rope climbing mode?
The robot would spend all of its time climbing up and down a rope so no need to consider any other mode of locomotion. It will simply be lifted onto the rope when it is needed and will ascend or descend as instructed. It’s is a working robot so it has to be rugged. I’m estimating the weight right now based on the ruggedness requirement. It will also require battery life of 10 hours or more so I’m counting on fairly hefty battery weight. And I’m guessing too that a rugged motor will be heavier as well.
I will try to post a diagram although my drafting skills are pretty terrible and the general idea is quite simple. I’ll try to explain better and you can let me know if you still need a diagram.
So just imagine a half inch rope dropped vertically in front of you. The robot is hanging on the far side of the rope form you so that from your perspective, there is one wheel on the left side of the rope and one on the right. They are on a spring tensioning system that pulls them towards each other so that the rope is squeezed between them. One of those wheel is driven so that when it turns it will drive the entire robot either up or down the rope. It might be easier to imagine this as a skinny pole that the robot is climbing so that you don’t have to think about keeping the robot or the rope from twisting etc.
OK I’ve attached a very simple diagram showing the basic layout. Hopefully that will help with visualization. Just remember that the rope is essentially pinched between the ropes to maintain traction.
A picture, even a bad one, really is better than a thousand words.
OK so LiPo batteries are much lighter, last longer, provide more consistent power but are more expensive? Is that pretty much the trade-off?
Besides being able to move itself around, I’d also like some excess lift capacity built into this thing in case it comes in handy at some point to be able to use the robot to move cargo so 16 kg is good for now.
In any case, do the numbers I’ve posted from the Drive Calculator look reasonable?
I’m assuming a gear motor is the best option to get the torque and rpm I need? If so, where is a good place to start looking for something that will work for my application? Ditto for batteries.
Anything I haven’t considered that would cause problems with this project, e.g. problems inherent in always fighting gravity?
Most of the losses in efficiency come from the battery pack, motor controller, gearing and motor(s).
For example if your battery can provide up to 90% efficiency (meaning roughly 10% of the energy is lost between the battery and the motor controller), your motor controller is 85% efficient, your motor is 70% efficient and your gearing (connected to the motor) is around 80% efficient, the total efficiency of your system would be:
0.9 * 0.85 * 0.7 * 0.8 = 0.4284 = 42.84%
Ideally your battery will be more efficient, but motors and gearing are far from perfect. If you use “cheap” products, you can see that a lot of the battery’s energy will be lost to heat.
OK so LiPo batteries are much lighter, last longer, provide more consistent power but are more expensive? Is that pretty much the trade-off?
Comparing the same values with something slightly off 90 degrees gives around 2000oz-in.
For something weighing 16Kg traveling straight up using just one motor, it’s a good start.
The 16Kg needs to include the weight of the battery and motor and payload etc.
robotshop.com/en/12v-5310-rp … motor.html (343oz-in stall, so more like 70oz-in continuous.
You’ll need at least a 30:1 gear ratio: robotshop.com/en/banebots-p8 … ox-36.html
This means the max (unloaded) rpm will be around 147rpm, which is well below what you’re looking for.
Note too that this is a 12V motor - not 48V. We don’t sell any 48V motors which would be suitable for your needs.
Most smaller mobile robots operate at 12V (a few at 24V), so we don’t carry 48V batteries.
According to the calculations, you’ll also need an 80Ah, 48V battery pack, and even in LiPo, this is incredibly heavy and expensive.
Gravity is your enemy, so make it as lightweight as possible.
Friction between the drive wheels and rope will mean success or failure.
Thanks for the input. I have been through the tutorial as you suggested and found some very useful information there. What I haven’t been able to find anywhere is a good description of a general protocol for designing a robot. The way I’ve been going about it, for lack of knowledge of any better method, is:
determine operating requirements such as speed, effect of gravity for vertical operation, etc
guess at total weight based on estimated weight of robot + payload
use online tools to estimate motor torque and battery requirements
find motor and battery to suit calculated requirements
build prototype and test
refine
EFFICIENCY
So what is a typical efficiency? I understand of course that it will depend on application but by your example it seems like in most situations 65% is a really optimistic efficiency estimate. Do you usually work with sub 65% estimates?
MOTORS
From the little research I’ve done so far while I try to confirm that the numbers I’m working with are realistic, it looks like there aren’t many 48V motors period. I’ve found some 24V and 36V motors for electric wheelchairs and scooters that look like they might work. Hope you don’t mind links to other suppliers:
So on that page there is one motor that is 36V, 450 W that, with gearing, gives output of 450 rpm. If I increase my wheel diameter from 2" to 4" that gets me the speed I need but then changes my torque requirement correct?
Is it common for people to build their own reduction gears? Or do you generally just keep looking until you find the right motor/gearing combo for your application?
I’m thinking I might need to operate this thing on a steel cable rather than rope. By using either a hard urethane wheel or a steel sheave it should reduce friction while still giving decent traction.
Yep gravity is not my friend in this case but that’s why I want to design this robot well so that it thrives as it fights gravity.
We put 65% as a typical overall efficiency for smaller sized robots.
Torque equals force multiplied by the radius (put very simply). Change the radius and you affect the torque as well.
Really rare for people to create their own gearing, except when it’s low so they can use spur gears, timing belt/pulley etc.
As an after-thought, you might consider a worm gear which will (ideally) lock in place in the event that the robot loses power. robotshop.com/en/vertical-sh … arbox.html (motor sold separately)
Thanks Coleman. So it look like that motor doesn’t have the oomph I’d need.
A question about the Motor and Battery estimator, where it asks for number of minutes of operation, does that assume operation at full throttle? Because my robot will be running at close to full throttle for only 8% of operating time, I’m wondering if the numbers I’m getting, especially for the battery requirement, are too high?
Yes, the tool is more of a “worst case scenario”, so the operating time is also worst case scenario - essentially climbing for 10 hours straight.
If you really only need 8-15% of this value, then change the numbers accordingly.
