Home BEAM background   Getting started Tutorials The "Miller" walker tutorial Nv nets The basic circuit Applications Leg mechanics Core / leg interface Convergence Mechanics tutorials Schmitt triggers, logic, and feedback "Hacking" a Mac floppy eject motor Reversing a motor without sensors Build an RJP   BEAM From the Ground Up is a BEAM Reference Library site.		Leg mechanics The tricky part I can't possibly teach all the skills one needs to make an effective and efficient pair of legs, but with perseverance the following section should give you the right start. We will concentrate on a 2-motor / 4-leg walker which may not be the most flexible design, but is the easiest to build and has proven its reliability and capability in a number of existing machines. 1: The motors This is probably the biggest consideration in a microcore Walker. The level of success you have with your walker is directly related to the type of motor used. The microcore itself gets an implicit feedback from the motors, this is what gives it the adaptivity. What to look for in a Motor.... EFFICIENCY: This is REALLY important both from a power consumption standpoint (better the motors the smaller the battery you need) and from a power to weight ratio. The better the motor you can get the better your likelihood of success. You should look for a motor with at least a 35% efficiency rating, good cassette motors and pager motors typically fall in this range, Mabuchi hobby motors are WAY off (typically 10%). Much higher efficiencies are possible (up to 88%)but this is usually found in expensive medical grade motors like "Escap" and "MicroMo". Keep your eyes open when perusing the surplus catalogues, these sometimes go on sale for as little as $5. SIZE: For the most part smaller is better, but it's not as important as efficiency. You also want to consider your own skill level, don't try to work with really small things for your first go at this. NUMBERS: Buy extras; even the best of us can really mess up a motor. 2: The gears You can't build a walker without them. Most DC motors usually run far too fast (1000's of RPM), and don't output enough torque. What to look for in a gearbox... Efficiency / Size / Numbers: For all the same reasons as above Compliance: This is really critical -- you should be able to grasp the output shaft with a pair of pliers and be able to turn it and have the gears spin back to the motor. If you can't make the motor spin then you have a gear train that is too inefficient (most likely) or too high a ratio. Worm drives are also OUT, they only go one way (motor to gear and not gear to motor) and they tend to choke under high loads. Output RPM: The ideal is about 30 RPM@ 5V. More than that means that you probably won't have enough torque (and if you do the damn thing will jump around so fast it's hard to figure out what it's doing). A lower RPM means that the machine may be moving too slow to be of use as well the ratio may be high enough that the legs can actually bend themselves under the torque load. 3: Interfacing motor and gears I STRONGLY suggest you find a factory motor/gearbox combination If you have to build your own then bear a couple of things in mind...... You want direct gear contact. Beltdrives, friction drives, flexible shafts etc. all have big problems with respect to efficiency and compliance. Keep everything clean. Glue, solder flux and metal fillings are deadly enemies to gearboxes 4: Materials Solder is our friend, and the better your materials solder the easier it will be to build a frame. Welding wire or filler rod is the best bet. Copper clad carbon steel rod 1/6" to 3/32" diameter is cheap and available at any welding supply place. An nice shiny option is High Nickel filler rod used for TIG welding cast iron but its MUCH more expensive brass tube and wire found at most hobby shops is a good bet as well. I suggest a solder with an Organic/water soluble flux, "Hydro X"by Multicore is my favorite. 5: Basic frame layout This is the basic layout, you want to keep the motors and output shafts lined up front to back and the front motor should be tilted at 30 degrees. This means the front motor will supply lift and push but we'll discuss that more later. You should mount the motors far enough apart to fit all your electronics including batteries in between (usually about 4"). 6: Adding the legs Leg shape and configuration will vary greatly between machines. A few things to bear in mind are: Contact point This is the most important aspect of leg design. The shape of the leg is less important as where it touches the ground. By placing you robot on a sheet of graph paper as shown here you can get symmetrical contact points. Width Try to make the legs at least 2/3 the length of your robot, this of course will depend on the available torque. It has also been shown that making the back legs slightly wider than the front helps in stability Connection Make sure that your legs are connected with something structurally sound, krazy glue doesn't cut it. If you can't solder the legs directly to the output shaft then try and find some sort of locking ring or set screw that will fit. Look for brass gears or pulleys that have their own set screw and then you can solder the legs directly to the brass. Angle By angling the legs slightly forward the legs will have the ability to "ratchet" over obstacles Your legs will change shape several times before you are done so its best to make a set of "test"legs that are easily recoupable before you use the good materials. 12 or 14 gauge household copper wire makes for effective reconfigurable "Gumby" legs. 7 Making it walk Time to make a minor detour here ( you may have noticed we don't have the microcore connected to anything yet ). Move on to the next section, Interfacing the MicroCore With the Leg Motors
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