Author Topic: Beginning Robot  (Read 785 times)

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Offline KlaytonTopic starter

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Beginning Robot
« on: October 07, 2011, 07:18:31 PM »
First things first. I've never built a robot before and doing a google search yielded this forum. I have a B.S. in Physics but that doesn't necessarily translate to being helpful with robotics, except for being able to think outside the box and do math. As the title says I want to build a beginning robot. I want to make a 6 wheeled robot because it will be the trial for a larger robot I want to build. This one doesn't need to be as large or as powerful as the end robot but I want it to be a debugging model that I can build on with sensors and such. I'm looking at making a 6x12" robot so it has room to expand with sensors and such as I get more familiar with robotics. A few things I'm Looking for in my beginning robot:

6 Wheels
Differential Drive
Rc Controlled
6" diameter wheels
Fast Acceleration
Cruising speed of ≈10 mph
NiMH Battery
Brushless motors (unless recommended otherwise)
12V but I'd like 24-36V

Questions I have. I realize with bigger tires you can't carry as big a load which is fine this is going to be a driving robot to begin with

1. Where do I start to estimate: Weight, Voltage, Torque, Acceleration, Gear Ratios, etc etc? Most say to guess but this isn't good enough for me. I need formulas or some way of estimating. I want to avoid over weighting the first robot so cruising velocity can be drastically reduced, since this will mainly be tied to dc motor weight x6, for fundamentals and functionality.

2. What is the recommended dc motor for robots? I'm thinking brushless design, as stated above, but in reading up on them they need a special controller. I'm thinking High torque as I want the robot to be fast.

3. Recommended chassis material? I was thinking plate aluminum as it's light but a plastic would work as well. It needs to be able to hold up to a beating as it will be tested outside and plastics can be rigid and crack under the strain of a vehicle not having a suspension.

4. Recommended voltage? I'm Thinking 24V as I'm looking at a 6-12V Motorx6

5. If a brushless motor is used what micro controller is recommended?

Offline hamzaman

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Re: Beginning Robot
« Reply #1 on: October 08, 2011, 01:54:29 AM »
If you want to use brushless motors, you should buy those that come with a controller.

If you do this, then answers to some of your questions are:

3. Aluminum is the best. It is easy to work with and if you work it well, you wont have the problem of having some of your wheels not touching the ground.

4. Choose the battery voltage after you have chosen the motors. If you choose 12v motors, get 12v batteries. Keeps things simple.

5. If you buy motors with controllers and all your machine does is drive, then you don't need a microcontroller. Just get a programmable RC module. If you use motors without controllers and want to implement them yourself, you'll need multiple microcontrollers to handle 6 motors.

Also I recommend building a small 2-3 wheeled robot first. Goodluck :)

Offline Soeren

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Re: Beginning Robot
« Reply #2 on: October 08, 2011, 08:42:42 AM »
Hi,

First things first. I've never built a robot before and doing a google search yielded this forum. I have a B.S. in Physics but that doesn't necessarily translate to being helpful with robotics, except for being able to think outside the box and do math.
Ahemm... In my experience, the more schooling => the more inside the box - it's the untrained, who doesn't know there is a box in the first place, that transgresses the walls (unfortunately much too often in fruitless directions)  ;D


There are quite a number of tutorials on this site that should help you get started.


As the title says I want to build a beginning robot. I want to make a 6 wheeled robot
Doesn't go well with differential drive and if you're planning doing odometry, add a seventh trailing wheel just for that purpose.


I'm looking at making a 6x12" robot [...]
6 Wheels
6" diameter wheels
So, the middle wheel will be touching the surrounding two wheels?


I realize with bigger tires you can't carry as big a load
Wrong. Tire size (diameter) has nothing to do with that.
You need to view the transmission as a whole, from the rotor of the motor to the tire on the ground.
Wheel diameter only changes the needed gear ratio.


1. Where do I start to estimate: Weight, Voltage, Torque, Acceleration, Gear Ratios, etc etc? Most say to guess but this isn't good enough for me. I need formulas or some way of estimating. I want to avoid over weighting the first robot so cruising velocity can be drastically reduced, since this will mainly be tied to dc motor weight x6, for fundamentals and functionality.
As mentioned, plenty of tut's here, but B.S. or not, this is the real world, so prepare to hone your guesstimating skills. (Or does the B.S. just stand for what it usually does  :P :D)


2. What is the recommended dc motor for robots? I'm thinking brushless design, as stated above, but in reading up on them they need a special controller. I'm thinking High torque as I want the robot to be fast.
Brushless equates to the most power in a given packet size AND to the most involved driver.
Since you haven't mentioned your skill level in mechanics, electronics and programming, I'll advice you to go with regular brushed DC motors, either with integrated gearheads, or with an external gear reduction.


3. Recommended chassis material? I was thinking plate aluminum as it's light but a plastic would work as well. It needs to be able to hold up to a beating as it will be tested outside and plastics can be rigid and crack under the strain of a vehicle not having a suspension.
Plastics is a very wide area and you can get just about any specs you want. Some plastics are very heavy though.
For the Non Plus Ultra design, choose carbon fiber and aramides.
For the cheap experimental platform, plywood is my preferred choice, as it allows faster changes than aluminum, takes less tools (easy to carry what's needed for changes on location) and is much cheaper.


4. Recommended voltage? I'm Thinking 24V as I'm looking at a 6-12V Motorx6
I'm confused. You have mentioned up to 36V (which IMO is way beyond sensible with NiMH).
Just hits me... You need to study basic electronics - 3 separate 6V motors takes... 6V to run...
I have a feeling you added their voltages to 18V?

Go with max. 12V for starters.


5. If a brushless motor is used what micro controller is recommended?
The choice of motor topology doesn't dictate a specific microcontroller, but you'll need a (quite pricey) brushless controller for each motor.

If you, in reality, just want to make a ROV (a glorified R/C car) rather than a robot, you don't need a microcontroller at all - just build it like any other R/C vehicle.


Your post is a bit self contradictory in places... You want it to be very fast, but speed doesn't really matter and so on. Better sit down and sketch some rough (but consistent) plans, refine and distill a few questions out of that for posting here.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline KlaytonTopic starter

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Re: Beginning Robot
« Reply #3 on: October 08, 2011, 07:01:54 PM »
First off, I do apologize for the inconsistency of the post as i was interrupted while writing it to pick up a part for my car and when I came back decided on changing somethings w/o going back and editing the earlier part for clarification.

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Ahemm... In my experience, the more schooling => the more inside the box - it's the untrained, who doesn't know there is a box in the first place, that transgresses the walls

Abstract thinking does not come intuitively to the vast majority of people and thus must be taught. Any decent physics program will teach you this and NOT the ability to be a number muncher with a calculator.

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I'm looking at making a 6x12" robot [...]
6 Wheels
6" diameter wheels

One of the things I changed half way through writing but didn't change the body size to go with it as you pointed out. The size was originally 2-3".

Quote
Wrong. Tire size (diameter) has nothing to do with that.
You need to view the transmission as a whole, from the rotor of the motor to the tire on the ground.
Wheel diameter only changes the needed gear ratio.

While I was talking about bigger wheels, not just diameter but width and weight, you are wrong either way. While a larger diameter wheel would need a changed gear ratio to optimize the torque produced by the motor, a bigger wheel in any aspect would reduce the torque available to to carry a load. While the amount of torque reduced might not be a very significant amount with a thin wheel of a simply larger diameter it would reduce the amount of torque free to carry a load. Seems that B.S wasn't such bs after all  :o.

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As mentioned, plenty of tut's here, but B.S. or not, this is the real world, so prepare to hone your guesstimating skills. (Or does the B.S. just stand for what it usually does  Tongue Cheesy)

When is the world ever not "Real". I didn't say I was working in a frictionless vacuum with perfectly smooth surfaces at absolute zero. I merely asked for a way to calculate the the values I asked for, not a smart aleck comment. While guesstimating has it's place if it is beyond the scope of beginning robotics that answer would have sufficed or as you mentioned there is a tut for it a link would have been better.

Quote
I'm confused. You have mentioned up to 36V (which IMO is way beyond sensible with NiMH).
Just hits me... You need to study basic electronics - 3 separate 6V motors takes... 6V to run...
I have a feeling you added their voltages to 18V?

Go with max. 12V for starters.

The confusion here is understood. I should have clarified I was going for, if possible, two separate circuits of 12V for each of the three wheels to slightly less than double battery life for a total of 24 V on the vehicle but not in one continuous circuit. Or, a 12V total with 2 6V circuits, But since you said a 6 wheeled design isn't conducive of differential drive this drastically reduces what I'll be building in size and cost.

Quote
If you, in reality, just want to make a ROV (a glorified R/C car) rather than a robot, you don't need a micro controller at all - just build it like any other R/C vehicle.

Stated at the beginning: This one doesn't need to be as large or as powerful as the end robot but I want it to be a debugging model that I can build on with sensors and such. Thus, it's not a Rov though for drive train fundamentals this will start as that before sensors are added, thus the need for the micro controller at some point.

Quote
You want it to be very fast, but speed doesn't really matter

I also said: I want to avoid over weighting the first robot so cruising velocity can be drastically reduced..... for fundamentals and functionality.
Want and need are two different things. In the list I expressed things I'd LIKE to have not HAVE to have on the robot and since I've never built one and I was looking for recommendations, that I would hope would accommodate what I'd like, the design aspects are able to be wildly changed for fundamentals and functionality for a first robot.

The final thing is your several mentions about my B.S. in physics. While I wasn't try to brag about it, but give a general ability to learn difficult material and think abstractly, I addressed at the beginning of my post that, "I have a B.S. in Physics but that doesn't necessarily translate to being helpful with robotics." Thus why all the crap with the bs?

The helpful things from your post though are that my idea for a 6 wheeled design will need a steering mechanism, a brushed motor to begin with on a plywood platform. So for this first robot 6 wheels and brush less motors will be thrown out in favor of a three wheeled, 2 driven 1 castor wheel, design to implement a differential drive.








 


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