Basic Analog HeXapod
Click on the image below for the prototype video:
Completely Autonomous – no tether connecting it to an outside power source or computer
Object Avoidance – can sense obstacles in its environment and maneuver around them turning left, right, or even an optional reverse ability. This is a real robot – not a wind-up toy.
Optional IR Sensing – this ability is available if you want to replace the tactile sensors in the tutorial. I’ll show you how do it simply.
Limited Terrain Ability – or whatever you call it when a bot is able to step over stuff and operate in something other than a smooth as glass environment. It can’t climb a stair case (although if you make a larger model I don’t see why it can’t) but it can step over and through some bumps in its path.
Durability – no duct tape or glue here. Project is built using standard #4, and #6 hardware.
Walking – that’s right ladies, no wheels. This is the way the science fiction writers of old imagined the future of robots would be. No sliding of legs back and forth here. We’re talking picking those feets up off the ground and stepping out kind of walking.
Cheap – (as in inexpensive) I hope to keep the cost of assembly down to under $60. Other 3 motor hexapods will cost you $150 and even more. (seriously, – take a look around the web) And not only that, they don’t walk as nice as the BAHX bot you’re gonna build.
PCB – (but not necessary) I got away from the free-formed soldering of components and was able to organize everything including the motor drivers in a nice compact PCB.
Alternate power source option – in other words, you can add a separate power source for your goliath- sized motors in case you wanna build a 6 foot model (while powering your circuit with no more than 6V as required to prevent smoke)
The neat thing about using inverters in this way is that it takes your DC signal and switches the out put from high to low, (positive to negative) at an adjustable rate. For a regular DC motor, this translates to an alternating left to right motion. The resistors and capacitors determine the rate of the switching and duration of time the motor will be turning a desired direction.
I will include this section to give credit to the original designers of the circuit, and a bit about how it evolved. - More to come later
The circuit schematic above is shown in this way for "Freeform" purposes. The pins are down with the view being above the chips, an amateur therefore can see how the components solder together. The first time I tried this, I used a blank PCB project board from Radio Shack like THIS one:
...it was kind of messy and ended up looking like this:
For the B.A.H.X. project, I decided to go ahead and have some PCBs made professionally, since I wanted to explore this concept a bit with some bigger and smaller projects. I am really satisfied with the results:
THE PCB's VARIATIONS on SCHEMATIC:
I included some variations on the original schematic for my PCB:
1. It has a seperate power source input for the motor section. This is there in case I want to use motors requiring more than 6V. More than 6V will burn out the ICs. However, If I am using 6V or under, I have a jumper pin that connects the motors to the IC power source.
2. Led indicator Outputs - I've included a couple of ports to plug in LEDs in order to indicate "power on", the second LED is to indicate the power for the secondary power if there is one. (Resistors for the LEDs calculated and included in the PCB.)
3. Motor Drivers (6 transistor H-Bridge)- Unless you are using very, very efficient motors, the circuit will not put out enough power to run your gear motor. I've included three 6-transistor H-bridges to boost the power. The 6 transistor H-bridge is an old tried and true method of building a home made motor driver, invented by Mark Tilden himself. However, I changed it up a bit by including diodes to prevent a burnout.
BAHX is a six legged walker that is quite unique. All of its motor functions, including walking, turning, object detection, and even an optional reversing feature are HARDWIRED. In other words, this robot’s brain contains no microprocessor. There are no pre-programmed commands telling it what to do or when to do it.
The circuit is designed in such a way that it uses simple components to produce an electronic signal in order to give the motor(s) instructions that end up displaying the result desired. The B.E.A.M. philosophy of robotics follows this approach to its many designs with some very satisfying and brilliant models out there. I’ll not explain everything about BEAM robotics here, as that is done elsewhere on the web and is done much better than I could do.
Generally speaking though, the thinking goes that most organisms don’t put a lot of brain power or cognitive thought into its most basic functions. For example, most don’t think to consciously tell their lungs to take a breath each time, nor do they mentally tell the leg to stretch out in front when walking, Most really don’t bring these things into the rational thought much, and truthfully, a lot of yours and my most basic of functions are hardwired into our brains in such a way that we really don’t think about these kinds of commands (stepping forward, telling our heart to beat) at all.
BEAM robotics however, at least as far as most of the examples on the web seem to be, seems sometimes to limit itself to very simple and light designs, too many of them resembling more serious robotics projects only very, very slightly even by the most accommodating of standards. A walker that simply vibrates until it goes one direction or another, or one that slides its feet back and forth on a smooth surface to give the illusion of walking just doesn’t satisfy. In other words, I wanted to make something using this analog technology, but something that would be indistinguishable from a programmable robot in its walking, environment sensing, and navigating.
I found myself telling my sons to be careful when they touched one of my projects. I would sigh with relief when they finally got done inspecting it. Most of the things I was able to put together with my limited ability and materials were so fragile a puff a wind could make them fall apart. I wanted to build a bot with everything in the above paragraph in mind, but one that could withstand the attention of my 10 year old. I figured, if it could withstand that, it could be considered a truly durable design.
In 1996, I was watching the news and heard about two soldiers injured in a
Almost as soon as I heard this I couldn’t help but think that there had to be a better way of doing this kind of thing. I began to think that someone should design something that could scope out a dangerous mine field before any human set foot on it. Maybe design something that could travel all over a field, even setting off unexploded charges, yet cheap enough to be no big loss when blown up. Maybe even be cheap enough to release a whole swarm of them over a few acres of land.
"the cost of even limited-application robots is beyond what demining projects can afford, compared to the typical salaries of human deminers. Progress in robotics can change this situation, but the gap remains wide."
Even remote control vehicles would require too much man power in order to swarm a whole area in a short time. In order to do clear a mine field you would have to come up with something that could randomly cover an area, avoid obstacles, and be very, very cheap- requiring limited human interaction.
That's when I started thinking about a computer-less robot.
If you look at the picture of the ant below, you can see one lone ant simulating random movement in a designated area. Now, with even limited intelligence such as obstacle avoidance, you can see that one ant, even though he is moving around quite a bit, won’t cover the area very effectively:
What you need is a group of ants wandering around the designated area randomly. Much more ground can be covered, in less time. Of course, in order to do this you need a mobile, inexpensive, cpu-less bot so that a swarm technique can be cost effective:
These random wanderings, using a swarm approach to any task that would require ground cover such as search and rescue, or triggering surface mines, can actually spread over a given area quite effectively. Remember the swarm of ants in the above picture? Take a look at how much of the surface is covered by their wanderings
This is of course, is only one of the reasons such a design would be useful. I have always had a desire that amateur robot hobbyists move beyond glorified remote control cars and start putting some legs on their creations. This really was made evident for me after reading , the book by the designers of the first Mars Rover. I came to understand that a wheeled robot while quite functional, had some limited applications. A walking robot, while initially more difficult, could ultimately overcome these limitations. Maybe someday, with the advancement of walking robotic techniques and design, a road-less location would not have to go unexplored.
BAHX uses what is known as a “standard tripod gait” in order to move around. This means that the middle legs rotate on the verticle plane, and the front and back legs rotate on the horizontal.
The middle legs provide “lift” and the other legs provide the forward or backward motion.
This is a tried and true method of propulsion in hexapod robotics, and it enables the bot to turn either way when needed very easily when using three motors.
This gait is defined as “simple” because it uses only three motors to accomplish its task of walking. More complex hexapods are out there that mimic lifelike walking in a much better way, but require 12 or more motors in order to do so. If you look at the image below closely, you can see that the middle legs actually move forward and backwards, AS WELL AS up and down.
In other words, the legs are moving in two degrees of freedom. In order to accomplish this you would need two motors per leg, (or a complicated gear system)
The standard tripod gait can be accomplished with two motors, in which one motor controls the outside legs, and one for the middle leg assembly, but the turning ability is limited on such bots. There are even some examples of one motor hexapods out there and these are really quite a neat engineering feat:
I went with the three motor mechanical design because of its simplicity, but also because it seemed to have the ability to evolve in to a design that could navigate around obstacles effectively, and traverse over rough terrain. This design doesn’t stumble forward… it actually walks.