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Gimlee - U8
8-Legged Walking Robot with Upright Posture
Index:
|> Introduction
|> Movies
|> Background
|> Basic Design
|> Implementation
|> Design Modifications

<| INTRODUCTION

Gimlee-U8 is a walking robot with 8 upright legs, and is an extension of the design of our quadruped Nico. Most real-life quadrupeds have "upright" leg designs (oriented downwards under the body), and this contrasts with all invertebrates which typically have 6 or 8 legs, and which are of the "cantilevered" design (extending outward from the body).

The original concept for Gimlee was to build an 8-legged walking platform with more stability and versatility than a quadruped with only 4 legs, but having the mechanical advantages of the vertebrate "upright" leg design, even though this meant twice the cost and mechanical complexity. The results with Gimlee have been mixed so far. Getting it to walk proved somewhat more difficult than with Nico, as a result of several factors .... much greater weight, more problems with leg mechanics and ground clearance, and greater challenges in getting leg / joint timing to work properly. It also involved a complete re-design of the controller, to make it more powerful and versatile.

Gimlee can be programmed with up to 126 different behavioral repertoires and / or gaits, and is used as a testing platform for the OricomTech WMC20 Walking Machine Controller.


<| MOVIES

After some time, we have gotten Gimlee-U8 taking its first few steps.

  • (06 Oct 2004) - download gimwalk1.avi - "First Steps" (760 Kbytes). In these shots, Gimlee has a noticeable "limp", which was traced to the lower servo of right-front leg R1. This resulted from operating the servo, when the leg was fully-extended, too close to its limit - in this case with a pulsewidth of 628 usec. We have since re-positioned the servo horn. The loop response of these servos (Cirrus CS-71BB) becomes somewhat wimpy when operating close to their mechanical limits, and operating further from the limit greatly tightens up the response.

    The gait shown in the First Steps movie is an Alternating Tetrapod gait, where 2 sets of 4 legs each work together as (L1, L3, R2, R4) alternating with (L2, L4, R1, R3). See the description of octopod gaits on the gait timing analysis page. The alternating tetrapod gait of an octopod is a simple extension of the tripod gait of a hexapod.


    <| BACKGROUND

    Gimlee-U8 is a robot with 8 upright legs. As such, it differs markedly from the usual concept of an octopod or hexapod robot. The usual creations emulate nature's design, in that they use sprawled postures, and cantilevered legs. These designs, however, have the same built-in limitations as do nature's. The legs must typically expend energy simply in holding the body off the ground. The range of motion and power strokes are limited. The total size and weight of the creatures is also limited. No arthropods, eg beetles, spiders, crabs, etc, in nature ever grew to 20 or 80 feet in length as did the dinosaurs, or to weigh 200 or 2000 pounds like dogs or elephants, or could run at speeds to 60 MPH like antelopes and cheetahs. The basic body plans, and especially the leg designs, imposed hard limitations on how far the arthropods could evolve, in terms of size, weight, speed, and power. In the perspective of these limitations and comparisons, Gimlee-U8 is an attempt to capitalize on the inventions of nature by "combining" some of the best traits of both vertebrate and invertebrate animal design.

    See the comparative anatomy page for background material.

    Gimlee-U8 was introduced at the joint get together between the Denver and Front Range Robotics Clubs, 15 Feb 2003 at Acroname in Boulder CO, but it wasn't quite walking coherently at the time.


    <| BASIC DESIGN

    The design of Gimlee-U8 is actually an extension of the design of our quadruped Nico, the background of which can be found on our page about 4-legged locomotion. In the summer of 2002, while designing Nico and earlier researching animal locomotion, we got to wondering why no invertebrates had as few as 4 natural legs, and also why no robots with greater than 4 legs had upright postures, with rotated leg arrangements. Nature doesn't seemed to have provided such creations, but luckily, when designing Mecha, we are able to devise new designs never seen in the natural world.

    The following figures show the concept drawing for an octopod robot with upright posture that we conceived in July 2002. Top, side, and bottom views. The design of Gimlee-U8, as recently implemented, follows this closely. [pictures soon]

    [basic leg arrangements] [basic leg arrangements]


    Self-Stabilization. Robert Full and his associates of U.C. Berkeley have done extensive studies of locomotion in arthropods, such as the cockroach, as well as in many other animal species. He has found a similarity in the way all animals walk, in terms of leg timing and synchronization during gaits. He also discovered that invertebrates have self-stabilization features built-in, by virtue of their sprawled postures with legs projecting outwards from the body. This arrangement, along with mechanical spring-compliance in the legs, produces "pre-flexes" in the anatomy, such that natural forces acting on the legs tend to stabilize the body during locomotion.

    In designing Gimlee, we attempted to capitalize on Full's findings by using a low-slung, wide-body posture with compliance in the legs. Although we chose upright legs [for reasons described in the next section], rather than spawled and cantilevered legs, the number of legs plus body design help provide Gimlee with inherent self-stability. The biggest problem with our earlier quadruped robot Nico was the lack of stability at low speeds. Living quadrupeds, like cats and mountain goats, have 1000s of sensors, feedback circuits, and muscle fibers to aid in "active stabilization", but it is difficult to add all of this to a simple quadruped robot. Therefore, short of adding more complexity to Nico's control system, we chose to try a design with more inherent [and passive] self-stability in Gimlee. The use of 8 legs rather than 4, in itself, provides much better stability at slow speeds over broken terrain.

    Upright Posture. Of note, the legs of Gimlee-U8 are of much different design than previous hexapod and octopod robots. They are arranged vertically to give Gimlee an upright posture, rather than the "sprawled" posture seen in living arthropods and other multi-legged robots. When upright, the legs project directly down beneath the body, and the weight is borne mainly by the "skeleton" and "joints" [servo axles and gears], rather than by the "muscles" [servo motors and batteries]. This reduces energy requirements during standing, and precludes the servo motors from having to actively hold the body up against gravity. The legs move forward and backwards under the body, rather than projecting outwards and rotating in an arc about the perimeter. Although Gimlee looks like an invertebrate, regards leg count, it has the leg design and energy efficiency of a vertebrate.

    In summary, Gimlee-U8 uses the wide-body low-slung design with built-in self-stabilization of arthropods, but with the upright leg design found in the vertebrate kingdom.


    <| GIMLEE-U8 IMPLEMENTATION

    Body Details

  • basic octagon shape, 12" (30 cm) in diameter.
  • leg design - 8 legs with 2 DOF, shoulder/hip and knee/elbow joints.
  • leg movement - mirror-image bending based upon natural animal movements [vertebrate, that is].
  • motive power - 16 R/C servos for locomotion, eventually 2 more for body angulation.
  • power from 10 on-board AA NiMH batteries (6 vdc, 3600 mAH).
  • high-level control bootstrappable using subsumption techniques.

    Fabrication
    Gimlee originally had upper and lower decks of 1/16" aluminum plate, with the leg servos mounted under the lower using foam padding for shock absorption - ie, mecha-cartilage. However, the upper deck was left off, after it was discovered how heavy was the total weight of the device. A 12" x 12" x 1/16" aluminum deck plate weighs 14 oz (400 gm), and the total weight without the upper deck is about 70 oz (2000 gm), most of which is in the 8 legs.

    Gait Control
    The basic gait-control concept is a CPG [central pattern generator], which creates trapezoidal-shaped servo motion trajectories. Each trajectory is defined by 9 or more parameters on position and velocity, so an individual gait can be created by setting 144 parameters over 16 servos. More details of this are described in reference to walking in the quadruped Nico. (see also, Design Modifications section below). The gaits of Gimlee-U8 are adapted from those of natural arthropods, spiders and insects, as described on the multi-legged gait page.


    <| DESIGN MODIFICATIONS

    [added 07 Oct 2004]. In the first implementation of Gimlee-U8, we used legs similar to those of Nico, namely legs with short femurs (1" long, upper leg segments) and longer tibias (4" long, lower leg segments). As before, the tibial-servos were mounted directly on the femur-servos, and rigid plastic tube extensions were used for the tibias themselves. However, it was discovered empirically that, with this design, the rear legs of Gimlee had a ground clearance problem during the recovery phase of each step. Nico probably had the same problem, but it was not as noticeable with only 4 legs.

    The solution to this problem involved both re-design of the legs, plus use of more complicated leg movements. We spent some time studying how large dogs walk, and use their "elbow" joints to increase foot-to-ground clearance. This is discussed on the Dog's Elbow page. As can be seen in the movies and pictures above, the legs were re-designed by mounting the tibia servos on servo mounts (SKU Number: SRVMNT-201) obtained from Gordon McComb at Budget Robotics, and by using shorter-length tibias made of #6 metal screws with rubber bumper feet on the ends. Use of screws makes it easy to adjust the legs, to make them all equal length. In the new design, both femur and tibia lengths are about 2.5" (6 cm), and the ground clearance during leg bending is much better than in the original leg design.

    In addition to leg re-design, we also needed to re-design the servo controller. Nico's gaits used only 2 movements per servo, but to implement the type of leg movements shown on the Dog's Elbow page, we needed a more versatile controller, which would allow more movements during the recovery phase of each step. Details on the new controller are given here: Walking Machine Controller. With the new controller, we can use 3 movements for each femur servo, with the third movement added to draw the upper leg segments up close to the body, in order to raise the knee and elbow up as high as possible above the ground - similar to what the dogs do.

    <| TOP


    © Oricom Technologies, Feb 2003, updated Nov 2006