Templates For Simple Legged Locomotion
Would you like to scratch-build a walking robot or design one using a LEGO© Mindstorms™ or Radio Shack VEX™ robot kit? This article will help you understand the simplest methods for constructing bipedal, quadrapedal and six-legged walking robots. For added fun, we also describe how to hack a Tamiya Insect so that it can be controlled using an RC transmitter. ROBOT magazine will explore the locomotion geometry of more advanced walking designs in future issues, so stay tuned! Videos of Tamiya walking robots in action can be seen at www.botmag.com. —the Editors
The Tamiya Robocraft Series of robotic animals are an excellent starting point should you be looking for locomotion templates for your own walking, crawling, creeping or running robot. Because these bots have semi-transparent colored plastic body parts and housings, the mechanics of these critters are easily observed, inside and out.
These kits are not only fun to build, they are, in a word, easy. They include the motor, gear boxes (often with choices of gear ratios) and all necessary parts, and even a screwdriver and grease. You will only need a hobby knife and a pair of wire cutters. The provided directions are clear and easy to follow. Numbered, full-size pictures show mounting points for each screw and part. Parts are numbered and bagged separately and neatly, and wires come pre-stripped. Kits will take about an hour to complete.
With any of these kits, you first assemble the battery compartment, on/off switch box and the gearbox. Then attach the motor and do the wiring, and, finally, assemble the body. All parts are either screwed or snap-locked and no messy gluing is necessary. No soldering is required. Since parts are prepackaged for multiple kits, there are occasionally extra gears and some of the plastic trees may have unused parts.
Fine tuning of locomotion is possible in several by changing the gear ratios and/or crank plates or by selecting different mounting sockets for the linkage rods on the crank plates. These options allow you to alter your robot’s speed and stride. These kits are very affordable, most falling in the $15 to $20 range.
FROM ROTATION TO STRIDING
Motor revolutions are reduced in a gearbox and transferred to a rotating or reciprocating movement via crank plates and linkage rods. For many of the models, movement is made even more fluid and realistic by crossing link rods that transform purely rotational motion into an elliptical striding gait.
Sockets on crank plates are like adjustable cams; changing gear ratios can also change the walking speed or “personality” of the moving robot. You can choose from up to eight different speeds and stride styles, depending on the bot. Most even have a reversing switch that is included to watch it all happen backwards! Look closely at the photos and captions, and you will be a leg up on the geometry of locomotion for your next simple walking robot design.
Unlike the dog’s gait, the Racehorse’s forelegs and hind legs move in unison. Here, the crank plate at the back leg is about to roll forward. This will force the foreleg shoulders forward, causing the front feet to push down and back. At the same time, the top of the hind leg is pushed back by the fixed, second linkage rod, which will force the hind feet forward.
The Racehorse’s front legs have rolling feet and the rear legs have gripping feet. This produces a realistic galloping style as the hind legs push back and the front legs sweep forward. A high gear ratio of 16.6:1 is used, and you can choose between long and short strides.
The Pig’s gripping front legs pull the rear legs on rollers. A simple connecting rod between front and rear legs creates a purely circular rotational stride. The front feet rapidly thump down using a high gear ratio of 16.6:1. A center post between the front legs stabilizes the gait.
The Dog was the first in the Tamiya animal robot series. The left front foreleg is pushed back to its aft limit. The center of the foreleg is mounted to a socket on a crank plate. A linkage rod connected to the foreleg shoulder is also connected to the hind leg’s fixed pivot point. This rod forces the shoulder forward and thereby extends the sweep of the left foot rearward. The counterclockwise rotating plate is about to bring the left foreleg up and around to take another step.
The left foreleg has moved forward; the linkage rod connected to its shoulder pulls back on the top of the leg, causing the foot to extend forward. When the foreleg pushes back and physically moves down owing to the cam action of the crank plate (A), the body of the Dog will roll slightly to its right so that the left hind leg can sweep forward (B) without hitting the ground. The crossing linkage rods extend the stride and have the effect of changing the rotational motion to an elliptical gait for smoother locomotion. Eight types of strides and running speeds are selectable by changing the configuration of the gear box and crank plate holes. A low gear ratio of 58.2:1 or high gear ratio of 16.6:1 can be selected. The attached front roller stays enable these dogs to race on a course with tracks that have guide walls. Variations of the crossing-linkage-rod geometry are used in the Dog, Giraffe, Tiger, Rabbit, and Horse.
The Rabbit’s rear legs produce a forward hopping motion with gripping feet; the front legs have rollers as feet. The motor and battery case have been shifted to the front and weight has been reduced by using a AAA battery. The result is a smooth and agile hop. The Rabbit uses a high gear ratio of 16.6:1.
The Turtle’s locomotion geometry differs significantly from that of the other Tamiya robot animals. The front and rear pair of legs are each molded as one piece. From the top, you can see that the front and rear feet sweep forward and back in a seesaw motion.
Driving the Turtle’s movement is a vertical center post that revolves in a circular motion around a turning axle. The post rides in an angled slot in a belly plate that, in turn, causes the fore and aft leg sets to rotate forward and backward via offset pivot points on the legs.
The forward leg set is mounted on a ball joint. The angled slot in the belly plate and the belly plate’s offset pivot point at the front legs causes the front legs to move slightly in the vertical axis and grip the ground as it walks—pulling the Turtle forward. The result is stepping front legs and sliding rear legs—for a very “turtle-like” gait. A gear ratio of 58.2:1 is used.
A link rod attached to the Beetle’s front right leg causes the wagging horn to move from side to side. When this horn bumps into an obstacle, it physically turns the Beetle, redirecting its path away from the obstacle. A low gear ratio of 203.7:1 or high gear ratio of 58.2:1 can be selected. Unlike the Pig, there is no center post to stabilize the gait, so the Beetle rocks as it walks in an insect-like fashion.
The Wall Hugging Mouse has a sensor rod on its left side. The mouse pivots to its left until the sensor contacts a wall or other vertical surface, then it drives straight. This has the effect of causing the Mouse to “hug” the wall or outside of a box as it moves. We have included it here because it could also be used to create a “wall hugging” walking bot if the robot has a separate motor drive on each side, like those in the Tamiya Insect.
The Kangaroo has a unique hopping gait that can be adjusted.
The six-legged Insect uses a middle-leg crank plate and cross-braced linkage rods. During locomotion, two legs support one side while a single leg is supporting the other. This stable and efficient walking system is a lot of fun to watch in action. Brian Lynch pursues the included soccer ball with the stock Tamiya Insect Robot.
The Ostrich is a bipedal robot with a foot-over-foot stepping action that transfers weight from one leg to the other. Multiple socket choices on the crank plate allow you to select stride length. A low gear ratio of 203.7:1 results in a relaxed, longlegged walking style.
Convert the Mechanical Insect to 2-Channel R/C
by Deborah Wiegle, Brian Lynch and Russ Pribanic
The Mechanical Insect takes the cross-braced walking system of the Tamiya Dog and extends it to an additional pair of legs to create a very “insect-like” gait. For fun, we hacked the Insect by converting it to RC for unfettered walking at low and high speeds. In slow motion, it demonstrates the careful stepping motion of a praying mantis. At high speed, the hacked Insect runs faster than the stock setup and appears eerily bug-like.
INSECT DUAL DRIVE
The stock Mechanical Insect is equipped with two independent motors and gearboxes that drive separate drive systems on each side of the bug. The rotating movement from the powered center pair of legs is transferred in a reciprocating movement to the front and rear legs.
The drive systems are controlled with a pre-wired, tethered 2- channel remote control with two control sticks, one for each side of the bug. Push the sticks in unison in the same direction and the Insect will move forward or backward in a straight line. Push them in contrary directions, and the six-legged Insect will rotate clockwise or counterclockwise, spinning about its own axis. If both “leg-drives” are moving forward but one is operating at a faster pace, the Insect will walk along an arc with the faster drive on the outside.
The very small Novak XXtra synthesizing 75MHz receiver is a perfect fit for the Insect. Two LRP Quantum Reverse speed controls power each drive system. The speed controls need to be set up–simply push the appropriate buttons and follow the flashing lights. Program in high, neutral and brake points (Place the brake at the neutral stick point so that it “disappears”).
A Futaba 9C transmitter on 75MHz (ground frequency in the U.S.) controls the RC Insect without any special programming. By selecting the right receiver ports, the two-stick control system of the stock Insect controller is perfectly emulated.
The stock system comes with two AAA batteries for a nominal 3-volt power system. The hack uses a Thunder Power 2S (twocell) 7.4 volt 730mAh lithium polymer battery. We used Radio Shack 10-Ohm, 1-watt resistors (No. 271-151) to drop the voltage from 7.4 volts to about 4V. This is above the stock 3 volts but very workable as it provides extra power to support the weight of the RC gear and it enables the Insect to run faster. You will want to moderate the throttle so as not to overburden the motors. Finally, we used some Du-Bro plastic pushrod linkage tubing to hold the receiver antenna.
This is a great hack that anyone can duplicate. Add another set of legs, some “fur” and 14 eyes or so, and you’ll have a terribly creepy spider at your beck and call. If you build a project along these lines, please send your pictures and comments to ROBOT at email@example.com. To see a video clip of the hacked Insect in action, visit www.botmag.com/issue1.
www.dubro.com, (800) 848-9411
Futaba, distributed exclusively by Great Planes Model Distributors
www.futaba-rc.com, (800) 682-8948
www.legomindstorms.com, (800) 835-4386
LRP, distributed by Team Associated,
www.teamassociated.com, (714) 850-9342
Novak Electronics Inc.
www.teamnovak.com, (949) 833-8873
Radio Shack VEX
Tamiya America Inc.
www.tamiyausa.com, (800) 826-4922
Tamiya Robot Animals
Thunder Power Batteries
www.thunderpower-batteries.com, (702) 228-8883