walker robot

[smart.cat]

The Coat Hanger Walker
By Gareth Branwyn
Photos by Jay Townsend
Illustrations by Mark Frauenfelder

In this project, we'll build a little critter out of surprisingly simple, minimal parts. We'll even make use of one of those coat hangers that seem to breed in the bedroom closet. This project is an ingenious little hardware hack dreamt up by a Canadian BEAM enthusiast named Jérôme Demers. It wonderfully illustrates a number of principles of bottom-up, BEAM-friendly robotics. Here are a few:



* It revels in the K.I.S.S. principle ("Keep It Simple, Stupid"). It uses a simple control circuit, a single servomotor, and a single gear to create a four-legged walker.

* It demonstrates use of the popular BEAM Bicore circuit, a two-node timer circuit that tosses a control signal back and forth between the nodes, creating a reciprocating back and forth motion in the motor and gears, and therefore, in the legs attached to the gears.

* It makes use of techno-junk. Okay, so a coat hanger isn't very “techno,” but if you have a decent techno-junk stash (and who among us doesn't have access to old computers, motorized toys, analog modems, old answering machines, etc. at this point?), you likely have some of the other components detailed in the parts list below.

* This project is a good example of the relatively level playing field found on the Internet for the sharing of robot designs and innovative ideas, regardless of who or where you are. I was truly impressed with this project, and many others on Jérôme’s Web site Insectroïdes, even before I discovered that he was only 15-years-old when he ceated the walker! Dang precocious kids today! When I was 15, I was in the backyard playing chicken with a Bowie knife and few of my mullet-headed friends.


FIGURE 1: What our finished coat hanger walker will look like. Cute, ain’t it?

The persnickety among you will be quick to point out that by many definitions of a robot, this ain’t no stinkin’ robot. It doesn’t gather, process, and respond to feedback of any kind. It’s really a walking machine rather than a proper robot. We still think it's a cool walker, because of the reasons bulleted above, and because it teaches many of the fundamentals in construction and electronics that can be used in other bot building projects. You also end up with a nifty little “monitor pet” that'll merrily motor across your desk and impress all your geek friends (especially when you tell them: “I made it out of a coat hanger and some junked electronics!”

We’re not going to lie to you. Although this critter has relatively few parts, the Bicore control circuit can be a little harrowing to solder (you’re up for a challenge, right?). The dual inline package (DIP) IC socket that the control circuit plugs into is very small and the pins are closely populated. Soldering these pins will take some patient effort. That’s the bad news. The good news is that DIP IC sockets cost about 10 cents a pop, so you can mess up quite a few in practice and it’s no big deal. Hopefully, you’ll have logged plenty of soldering practice time by now and be ready for some big boy/big girl soldering. If not, go ahead and read through this entire project, order any parts you need, and then do some soldering practice sessions while you wait for the components to arrive.


Gathering the Parts
This project requires some materials and parts you might already have lying around the house and some components you’ll likely need to purchase at your local electronics store, an online retailer, or some other source. Our pals over at Solarbotics have kindly put together a Parts Bundle of most everything you need for this project. It does not include the 40-tooth gear. We got ours from Jameco.

The Parts List
The Coat Hanger Walker requires the following parts:

* (1) hobby servo motor with “Servo Horn”
(Solarbotics Part #GM4)

* (1) 1.5 inch (4cm) plastic gear (around 40 teeth are good)

* (1) 2 feet of 8 to 10–gauge wire; can use coat hanger wire or 10-guage copper wire (Solarbotics Part #Wleg)

* (1) short piece of 1cm diameter plastic tubing (can use “jacket” from preceding leg wire)

* (1) terminal block (Radio Shack Part #274-678)

* (2) AAA battery holders (each holds two AAA batteries)

* (1) length of 1cm diameter heat-shrink tubing (can use “jacket” from preceding leg wire)

* (2) .22 µF monolithic capacitors

* (1) 100K to 10MΩ resistor (we recommend 3.2MΩ)

* (1) 74HCT240 integrated circuit (IC)

* (1) 20-pin DIP IC socket

* (1) on/off toggle switch (smallest you can find)

* (2) leg mounting pads (Solarbotics Part #LMP2); optional, but recommended

* pieces of scrap plastic (from product packaging) or .030" plastic sheeting

* several feet of stripped hook-up wire (or other 22-guage wire)




FIGURE 2: All the parts you’ll need to build your first robo-critter.

The Tools and Supplies List
You’ll also need the tools and supplies listed here:

* Needlenose pliers

* Hobby knife

* Wire cutters

* Screwdriver set

* Soldering iron and related soldering tools
and supplies

* Micro-torch or heat gun

* Breadboard and hook-up wire (a.k.a. Breadboard Jumper Kit)

* 2-part epoxy

* Superglue

* Some recycled component leads

* Rotary (Dremel) tool (optional, but highly
recommended)

* Metal ruler

* (4) AAA batteries


Freeforming the Bicore Control Circuit
The first thing we’ll want to do in building our robot is to assemble its brain. The Coat Hanger Walker makes use of the ingenious BEAM Bicore circuit. It’s prefixed bi because it has two states, or nodes, and core because, well, it’s the central part of the robot’s nervous net. Our Bicore uses the 74HCT240 integrated circuit. This chip is an inverting octal buffer. That’s just a fancy way of saying that it is a chip with eight logic gates that invert the signals going into them. Whatever goes in each gate gets inverted, so a low signal becomes a high signal and a high signal becomes a low one. By combining the three gates on one side and three gates on the other (by soldering their pins together), we end up with two “teams” inverting gates that “buffer” the signal and make it more powerful. Bicore! The signal passing back and forth between the two nodes sends high and low (or “on” and “off”) pulses to our servo motor. The result is back and forth movement of the motor shaft, which is transferred to our gears to create a reciprocating walking motion. By the way, if you’re wondering, the remaining two gates are used as sort of the controller for the two three-gate teams.

Breadboarding the Bicore

Before we actually heat up our soldering iron and start dripping molten metal all over components, we want to breadboard our circuit to make sure that all the components are working properly and that we have a sound design for our Bicore circuit (If you don’t know how to breadboard, read the “Thumbnail Guide to Breadboards” later in this project doc before continuing).

You’ll want to hook up the wires (from your breadboard jumper kit), the resistor (whichever value you decide to start with), and the two .22µF capacitors to the following tie points on the breadboard. For these numbers, assume that the pin in the upper-left corner of the IC is pin 1 (the pin to the left of the little dimple). Then it’s pins 2–10 on the left side, straight across (from 10) to 11, and then up to pin 20.

Connect jumper wires to the follow tie points:

* Left side of IC: 4-6, 3-4, 5-7, 6-8, 7-9

* Right side of IC: 14-16, 13-15, 12-14, 11-13, 15-18

* From left side to right side: 1-19

* Capacitors: 2-3 and 18-17

* Resistor (across IC): 2-17

.)
and thx