Hexapod project

[DrEvil]

Hello,
I'm new to robotics, last quarter in college I took a class called 'C++ programming and robotics'. We worked mainly with the Parallax Scribbler & bluetooth, making it follow walls, chase people, etc. My background is in computer programming, and I am fairly good at programming ai. Anywho, this fall quarter I begin an independent learning contract of my own design. I plan to build a Hexapod robot. Not of the traditional insect-like design I have seen around, but more akin to a six-legged mammal. Actually, it will be more like a four-legged mammal, with arms, that will act as a third pair of legs when needed, for things like climbing stairs and such. My plan thus far is to construct the chassis and legs using the Vex robotics kits. I like the modularity of their products, and don't own a cnc machine, so I have no method of fabricating my own bits. For the microcontroller, my professor wants me to use Arduino, so I will be using the Arduino bluetooth model, which unfortunately is rather expensive. Eventually I want to use a microsoft kinnect for a sensor, but for this quarter I will simply use a proximity sensor. for movement, I plan to use servos and linear actuators. I'm not sure how to figure out how much torque is necessary, or what 25N means for an actuator. If any of you can help me out with this, I would appreciate it. Also, if anyone knows where I can find some strong, cheap actuators, that would be fantastic. I also want to rig the robot to be radio controlled, so that I can control the robot when I want, and then flip a switch and have the ai take over. If any of you experts and hobbyists out there have some advice, or tips, I would love to hear them. I plan on posting updates on here as my project progresses, and I will check back often.
 Thanks

[Soeren]

Hi,

If any of you experts and hobbyists out there have some advice, or tips, I would love to hear them.

First bit of advice that springs to mind is, to ask you to take a deep breath every now and then when you write (a.k.a. add whitespace).

Your post is close to unreadable as is and you just pour on, without considering the reader.

Try once more and then ask a few well defined questions, then it will be more "edible" and as such, will have greater potential for getting you the help you need.

Further, you cannot hold such diverse info unless you manage it a little at a time.
So please, give it another try

[DrEvil]

Hahahahahaha, you are so right.

Hello,
I'm new to robotics, last quarter in college I took a class called 'C++ programming and robotics'. We worked mainly with the Parallax Scribbler & bluetooth, making it follow walls, chase people, etc. My background is in computer programming, and I am fairly good at programming ai.

This fall quarter I begin an independent learning contract of my own design. I plan to build a Hexapod robot. Not of the traditional insect-like design I have seen around, but more akin to a six-legged mammal. Actually, it will be more like a four-legged mammal, with arms, that will act as a third pair of legs when needed, for things like climbing stairs and such.

My plan thus far is to construct the chassis and legs using the Vex robotics kits. I like the modularity of their products, and don't own a cnc machine, so I have no method of fabricating my own bits.

For the microcontroller, my professor wants me to use Arduino, so I will be using the Arduino bluetooth model, which unfortunately is rather expensive. Eventually I want to use a microsoft kinnect for a sensor, but for this quarter I will simply use a proximity sensor.

for movement, I plan to use servos and linear actuators. I also want to rig the robot to be radio controlled, so that I can control the robot when I want, and then flip a switch and have the ai take over.

I plan on posting updates on here as my project progresses, and I will check back often.

I have three questions:
1: How do I determine how powerful the servos and actuators need to be?

2: Are there any cheaper and better robot building components besides Vex metal?

3: Do I wire the servos and actuators directly to the Arduino Bluetooth board, or do I need a separate motor controller board?


 Thanks

[Conscripted]

Hello. That was much easier to read.

1. Your servos and actuators will need to be selected based on how strong they need to be. How much torque will they require. This will be much easier to determine once you figure out how heavy everything is going to be.

2. Cheaper and better are usually mutually exclusive unless price is the only qualifying factor. Regardless both "cheap" and "better" are subjective terms and will vary depending on a person's ability and comfort level.

3. Servos are a fancy gear motor with controll electronics already built in. You should be able to connect them directly to the Arduino.

I would love to hear about your progress.

Conscripted

[Soeren]

Hi,,

Wow, that was a huge improvement!
From unreadable to very readable in only one post - I think you hold some kind of a record here

I have three questions:
1: How do I determine how powerful the servos and actuators need to be?

As I am not sure how you're going to power the legs, I cannot give you any exact measures, but here's something general about the factors involved...

As Conscripted mentioned, weight is a major factor, so at least a reasonable estimate is needed (and remember that a battery can be quite a bit of the total weight).
How the weight is distributed over the 4 or 6 legs matters as well.

The step length is another parameter that you need in the formula and the needed speed of movement of each limb is just as important.

It's quite complex making calculations for a setup like yours, but if you start making some sketches (to scale) and make notes of how fast you want each limb to move (maximum) plus the total weight, assuming an evenly distributed weight, some rough approximations can be made from this as a starting point.

2: Are there any cheaper and better robot building components besides Vex metal?

I think Vex is perhaps the best modular prototyping tool for 'bots and I regret not buying a load of it before the recession hit, back when I could spend more than $1k month on my "toys", but I kept putting it off 
It sure is expensive, but for quick and strong prototyping I haven't seen its equal - Lego cannot even touch it.

I had tonnes of Lego since I was old enough to put two bricks together, as it's a national product that probably every Dane has had at least some encounter with during their childhood. Vex on the other hand... I have only seen it on photos, but still, I believe it to be the superior of all the construction "toys" I know.

So yes, Vex is the next cheapest, and what's cheaper... Everything else
Better, I don't think so.

You can use the Vex stuff with regular servos and R/C transmitter and receiver to save some (and get more channels and range).

3: Do I wire the servos and actuators directly to the Arduino Bluetooth board, or do I need a separate motor controller board?

R/C servos (there are many other kinds of servos) are controlled by a weak low voltage signal like that of a microcontroller I/O pin, but it needs the power and ground connected to the power source.

Since I don't know what linear actuators you plan on using, I canot say anything for sure, but most will need an H-bridge in between the microcontroller and the actuator. The voltage and (max.) current  is needed to device an H-bridge that can handle it.

[georgeecollins]

When you say you want to make a hexapod not like an insect, but like a mammal, do you mean that you want the leg actuators (or servos) to mostly be on the same plane as opposed to one servo on the horizontal plane and one on the vertical.  A very simple diagram would be very helpful since not many six legged.

Here's what I mean by a robot with actuators not on the same plane (which gives you more insect like legs).
Asynchronous with PropChip

A really common arrangement for hexapod legs is one servo in the horizontal plane, two in the vertical.   

Here's a robot with legs with servos in the plane of motion which is more mammal like:
Ripple Gait

Linear actuators are usually slow and expensive relative to servos, but you can find very powerful ones.  I would recommend using servos because it is so common. 

My rule of thumb for how strong the motors have to be is to take the torque (if motors) or force (if actuators), divide it by length of the leg that will be lifting then multiply that by the number of legs you expect to be supporting the robot at any time.  That number should be twice the weight of your robot. 

So if your servo has 36 ounce/ inch of torque, your leg is three inches long, and you plan on resting on three legs at all times.. 36/3*3= 36.  Your robot shouldn't weigh more than 18 ounces. 

People often disagree with me about doubling the weight because in theory you don't need to.  But trust me, I have done this a few times and your robot will work much better if each leg has double the force it needs.  Your robot will never really have all it's weight perfectly spread across each leg (although hexapods are much better for this) and you won't always be on a perfectly level surface.  Torque figures are sometimes not holding torque, which means that a leg could stall out if the robot is leaning on it and it can't move. 

Good luck!