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4-Legged Creatures page.
Slower Gaits
|> The Walk
|> The Trot and the Pace
|> The Turn
|> Note Added in 2004

The Creep.

The Creep gait is slower and more stable than the basic walk, and involves lifting just one foot at a time. We have some background info on Creep Gait on our Nico the Quadruped page.

<| The Walk
Once you have a bot with 4-legs, of course, the next step is to get it to walk, and Jake the Terrier illustrates here how to walk with a fine grace.

Jake walks with a 4-time gait, LF (left-front), RR (right-rear), RF (right-front), LR (left-rear), then repeat. Presumably, most dogs prefer to start the walk with a front leg.

Notice that balance and support are maintained by the LR+RF "diagonal" while the LF and RR legs are suspended [positions 1, 2], and by the opposite diagonal for the other 2 legs [positions 5, 6]. At the start of each step [positions 1, 5], the legs of the support diagonal are vertical, and the COG (center of gravity) of the dog is in the middle of the diagonal. Then the COG shifts forward as the stepping leg is extended [positions 2, 3, 6, 7], giving forward momentum to the body.

Regarding the suspended legs, the front leg precedes the rear leg [evident in positions 1-3 and 5-7] slightly - thus, the 4-part cadence. Furthermore, during initiation of the succeeding steps [positions 4, 8], the front leg of the new step lifts slightly before the rear leg of the previous step touches down. This prevents the feet on the same side from banging into each other during the transition between diagonals, since for a normal stride, the rear pad comes down near the front pad mark.
Click here for a short animation
sequence of a dog doing a
rapid walk (160 Kbytes).

Robots. With a robot, first you learn to stand stably, then you learn to walk tentatively. Nature has endowed real animals with what probably amounts to a near-optimal step, given their anatomy. The challenge is how to successfully adapt this to a mechanical being. See also, below: Note Added in 2004.

<| The Trot and the Pace
The Trot. Faster gaits than the walk are the Trot [positions 1-4] and the Pace [positions 5, 6], which our German Shepherd friend Jessie illustrates here. The Trot is basically a direct extension of the Walk shown above, used for greater speed, while the Pace is what a tired dog might use heading home after a long day on the range.

The Trot is a 2-time gait, LR+RF alternating with RR+LF. Like the Walk above, the Trot gets its stability by using alternating "diagonal bracing" under the torso [positions 1, 3], but here the legs work more closely in unison, the strides are longer and the forward lean is greater.

In practice, the phase of the front feet is slightly ahead of the rear - which keeps the feet on the same side from banging each other [positions 2, 4]. Quite obviously, the length of the stride with respect to the length of the torso and legs, as well as the precise phase relationship between front and rear legs on the same side, distinquish a successful trot from a bang, stagger and stumble. Dogs with longer legs, compared to torso length, do not trot well.

The Trot is an example of dynamic stability, where the animal could not hold its balance when stopped. However, because the Trot always keeps the COG of the torso straddled by diagonal bracing, and the order of stepping is similar to that of the Walk, one should think this gait would be easily implementable in a quadruped robot, once a successful Walk has been achieved.

The Pace. In the Pace, the dog uses "lateral" support, where both legs on each side work together, and the sides alternate. In the Trot, the COG of the torso is not directly over the legs, but is centered within the diagonals. With the Pacing gait, the COG will be offset from the supporting side unless the animal significantly leans its body sideways and angles its legs inward - an obvious stability problem.

Interestingly, humans also have something akin to a Pacing gait. We have discovered empirically that, at the end of a long tiring hike, when struggling up the final hills, it turns out to be very comforting to twist and lean the body over onto the supporting leg with each step. This produces a kind of ducky waddling movement which is not a good means for covering level ground at speed, but clearly takes some of the stress off tired muscles by shifting the COG directly over the grounded leg, so that the bones rather than the muscles take the effort. In a normal walk, the COG is normally kept inbetween the feet, so the leg muscles must take more of the effort. [dog trot]

Because of the instability inherent with the Pacing gait, a pacing robot would probably fall over and go to sleep at the first opportunity. The Trot, however, looks eminently doable in the robot.

More detailed trotting sequence pictures. Nice pictures of trotting dogs.

<| The Turn
Every now and then, a dog must turn. Turning involves several phases, and complicated angular movements of bone and muscle groups throughout its entire body:

  • initiation phase - turning is normally initiated into the leading leg.
  • conduct phase - as the body moves forward, and the leading leg straightens and moves backwards through its arc, the shoulders twist, the body pivots and shifts its weight, the non-leading front leg cross reaches, and the head and neck angle into the turn.
  • completion phase - the back leg opposite the leading leg pushes forward.
  • follow-through - the next step is taken onto the opposite diagonal.

    The image at the right shows a dog at a point in the turn which appears to be right after it has stepped off the leading [left] leg, and the non-leading [right] leg has crossed over. Note the diagonal formed by the RF+LR legs, as well as the complex set of rotation angles between head, shoulders, spine, and haunches in the rear. The LR foot [seen on the ground, looking between the two front legs] has been placed at such an angle that the dog's rear end is leaning into the turn. The RR leg [unseen] is raised and trailing the others.

    The horse at the left illustrates essentially the same posture and point in the turn as the dog above - albeit here the turn is much more extreme, and the entire body is leaning heavily inside to counter balance centrifugal force. Although not obvious, the horse's weight is bearing heavily on the sharply-bent left rear leg. The front shoulders are twisted sharply into the turn.

    The dog at the right is also in a similar phase of the turn. Here the body lean, cross-over of the [right] front leg, and angulation of the shoulders is especially obvious. The next action will obviously be to extend the left front leg further into the new direction of travel.

    From a diagonal footfall viewpoint, these positions roughly correspond to positions 8-9 for the Terrier and position 4 for the Shepherd above. The positions here appear to be a relatively straight-forward extension of the basic gaits above, except that here the animals have incorporated significant body twisting and leaning into the movements. The twisting allows the front-end to move in a new direction, while the leaning helps counteract centrifugal forces acting on the body mass. In addition, the lean is a dynamic and off-balance movement, so the body is to some extent falling into the turn.

    Note that there is a way to turn a motorcycle known as "counter-steering", which is pseudo-inituitive, but also a dynamic almost-falling movement. Great fun too [if you're careful]. See: [1] [2] [3] [4] [5].

    Additional info from McDowell Lyon's dog book:

  • for high speed turns, the body catapults in an arc over the leading leg, whose pad remains fixed during the entire maneuver.
  • an animal turning into its non-leading front leg is likely to trip.
  • it is possible to initiate the turn from a rear leg, but this is less common.

    Dinos: Dinosaurs may have have some trouble turning, due to large rotational inertia the result of having long heavy necks and tails: [1] [2]

    Robots. Turning manuevers pose a challenge to roboticists. The dog and horse can bend the spine, twist the shoulders, cross reach the front legs, angle the head and neck, push at various angles with the rear legs, and lean the entire body into the turn. The question is, how many degrees of freedom must be built into the bot to attain success in turning? Hmmm .... [lost in thought].

  • <| References

  • "The Dog in Action", by McDowell Lyon, 1950, Howell Book House - excellent treatment of dog gaits.
  • "Horse Gaits, Balance, and Movement", by S.E. Harris, 1993, Howell Book House.

    <| Note Added in 2004
    Our first attempts to build a walking quadruped, and later octopod with upright legs, were modeled on Jake the Terrier's fine upright walk shown above. Furthermore, the original controller was designed to produce corresponding movements in the robots. However, the walkers' legs had significant ground clearance problems, as discussed elsewhere. We have since discovered that Jake's stance and walk are "more" upright, compared to most (especially larger) dogs, the reason for this being that terriers have a relatively short humerus (upper arm in front), and this gives the dog a very straight leg stance.

    In passing, we note that we would have done better to have more closely emulated Jessie the Shepherd's configuration, shown above. Jessie shows much more evident backwards and upwards movement to the humerus and front elbow at the end of each power stroke, and beginning of the forward movement of the legs - as well as much more bending in the elbows at the extreme front and back postions. And this greatly improves the ground clearance. Part of our new robot re-design has included devising a controller which can more easily provide "extra" movements to the leg segments. Specifically, it does a 2-stage recovery stroke which starts with a ground clearance movement, and followed by a forward movement. This is a subtle change which allows the leg to remain on the ground for longer periods at the end of the power stroke - which in turn gives the gait more stability - but which also allows better ground clearance at the start of the forward movement of the leg.

    In short, it's not clear from the pictures above, but during walking in most animals, there is a short period of time when all or most of the legs overlap on the ground simultaneously, and which increases the stability of the gait. But there is also, at least in larger dogs, a movement which specifically increases the ground clearance during the recovery stroke - although these movements all might blend into a smooth sequence during faster walking. However, to emulate this adequately in the robot takes a more sophisticated controller than we had originally devised.

    The overlap in footfall is illustrated on the Multi-Legger page. More discussion about ground clearance is given on the Dog's Elbow page.

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    © Oricom Technologies, Nov 2001, updated April 2004