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Monthly Archives: April 2011

New Robot – Strategus_Aloeus

45 degree view of Strategus_Aloeus robot

Strategus_Aloeus

This robot is called Strategus_Aloeus, largely because I’m not great at naming things (also, it looks kinda beetle-esque and the Ox beetle looks kind of squareish).  It is a four motor walker which means that it uses just four motors as part of it’s gait. It is currently powered and controlled by (don’t wince – it is a prototype) an Arduino. Each motor is an unmodified servo. After some research I ‘d like to move from C to something more exciting involving neural nets.

The Strategus_Aloeus platform was made for a number of reasons, non the least of which  was a desire to investigate this fairly odd way (four motors, one for each leg) of creating a walking robot. I’m not too keen on designs that need dozens of high powered precision servos to get moving like hexapod and octopod  designs (most with upwards of 18 locomotory degrees of freedom) – they need intense processing and power just to move and while they are impressive engineering feats they are also delicate and demanding. In short I’m not convinced that designs with fewer moving parts aren’t worthy of study.

A key advantage of this design over more complex designs is it’s low cost and parts count – it can be made from four servos, and Arduino and needs little else. I wouldn’t be too worried about it’s safety if I left it to explore a desk or floor.

One of the main advantages of this design over two and three motor walking robots is that there is considerable scope in the design for changes in body position or “angle of attack”. This could be exploited by the robot in competitive fighting – in much the same way that some horned beetles fight to knock or push each other off something or turn each other over. The angle of attack of the body can be changed while maintaining a forward moving gait, although currently the robot does not do this (it’s something that I will develop later).

Construction

Currently the robot is operated by a tether to an Arduino to reduce weight and complexity. Apart from the servos the robot consists of simple leg pieces (plastic) and two flat chip boards bolted together. The servo mountings are friction fit with strategically placed rubber pads on the chipboard. The completed robot will contain some very  simple sensory equipment to investigate how little sensory data is needed to (autonomously) competently navigate an environment (or not as the case may be). My suspicion is that measuring changes in body position (e.g. with an accelerometer) while walking is all that is needed to detect things like varying terrain depth, voids, slippery surfaces and so on.

Leg Alignment

Strategus_Aloeus in an Aggressive Stance (side view)

Aggressive Stance

When designing and building Strategus_Aloeus I was aiming at creating a platform with which I could look at some unusual fixed leg/hip quadrupedal gaits. To this end you will notice that Strategus_Aloeus diverges from classic four motor topologies in one important aspect – the leg alignment (axis of rotation) is not parallel across the robot and yet the robot is still strongly directional. As far as I know previous designs focus on either forward leg alignment and strong directionality or symmetric leg alignment and omni-directionality.

Strategus_Aloeus in an Neutral Stance (side view)

Neutral Stance

This topology maximizes the “yaw” (side to side) component of the gait. This is advantageous because this  allows the robot to project it’s front legs forward further when walking while reducing the torsional stresses experienced at each joint. Needless to say this topology also makes the robot much more stable on its side axis (tipping it over sideways is harder). This also allows us to tinker with the amount of torsion that the robot exerts throughout it’s gait.

Gaits

I have used “gait” to refer to a cyclic series of static positions applied one after the other that, when applied one or more times result in forward movement (verbosely, lets call it a “transient static gait”). A more apt description might be a “static gait” however this definition diverges from the accepted meaning of a static gait insofar as each static position is not guaranteed to be gravitationally stable. At this early stage I have been able to identify 3 separate gaits for this topology, each with benefits and costs associated with it. Each gait can be operated on a range of step widths and body attitudes relative to the ground – which may allow the gait to perform differently. Other ‘active’ gaits such as running, jumping, skipping or hopping cannot be demonstrated in the current model, largely due to the limitations on the speed of the servos used and the in-elasticity of the leg materials.

Strategus_Aloeus Top View

Stepping forwards

The most high speed walking modes (irrespective of actual gait, in other words electing for short leg travel range) require the flattest terrain and any small features in the terrain strongly effect direction of travel. Larger (lower speed) modes are afflicted with a high forward motion coupled with a high reverse motion to each step cycle, but typically have much greater odds of surmounting an obstacle.

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