Category Archives: Land Robomachines (A)

Bounce off walls behaviour

Circular omnidirectional hexapod exhibiting bounce-off behaviour

The hexapod has six non-contact infrared range finding sensors between each leg. As soon as an obstruction is sensed by any one of the sensors then evasive action is initiated to back off the robot from that obstruction.

Docking for battery charging

Circular hexapod omni-bot battery charging.1

Hexapod identifying its battery charging spike and then sitting on it, (oooh painful!). Just one robot shown at Singapore’s Nanyang Technological University inter-faculty research competition in 2003 where Frank’s students won 1st prize in the robot division

Charging system with robot shown reaching 6% charge

…after sitting on the battery charging spike, the charging system, which can also discharge the battery if necessary to put the battery in a known charge state, shown on the left, measures the battery status and starts to charge the hexapod on-board battery. The charging system integrates the electrical current with respect to time to indicate the charge status and this is shown as a percentage on a visual display. all done by Frank’s students as their Final Year Project

Hexapod as a 6dof platform

Hexapod as a mobile 6 dof Stewart platform

The hexapod consists of six legs. Each of the 6 legs has  3 degrees of freedom (3dof) which means that the leg tip can move in 3 orthogonal directions, e.g. (i) x, y, z Cartesian space,or (ii) R, theta, Z cylindrical coordinate space or (iii) phi, theta, R spherical coordinate space, or (iv) an arbitrary 3 dimensional warped coordinate space.
If 3 alternate legs are lifted off the ground, the remaining 3 legs on the ground can be programmed to move the robot body in 6 degree of freedom space, e.g. the body can be translated in the x, y and z directions and rotated about each of these axes, i.e. given motion in the A, B and C axes. In this case the robot body becomes a Stewart platform.
The robot can now be programmed in double tripod gate meaning that the robot walks on 3 alternate legs then walks on the remaining 3 legs. Thus the robot becomes a walking or mobile Stewart platform.

Compact in-line design

Omni directional compact beetle

Specialised design of an in-line hexapod with the batteries secreted inside a tubular aluminium central spine. It’s got forward facing non-contact infra red range finder whisker-like sensors and an algorithm designed to back off and keep its distance from objects

Tracking robots

Twin circular hexapod omnidirectional robots in master/slave radio control synchronism

Here the left hand master robot is sending instructions via radio control to the other slave robot so that the slave mimics the master

2dof leg hexapods, one stalking the other with a range and bearing sensing bar

Here a stalking robot is following a victim robot. A 6dof range and bearing sensing bar is used by the stalker to sense the range and bearing of the victim. The stalker robot maintains the length of the bar at a constant range and the bearing information dictates the turning action of the stalker.

Range and bearing 6dof device for robot tracking and stalking algorithm design

Here, Frank describes how the 6dof range and bearing sensing bar works

Behaviour patterns for omnidirectional hexapod

Some examples of complex physical computing applied to an omnidirectional hexapod.

The hexapod featured here has 3 degree-of-freedom legs thus giving the robot omnidirectional walking capabilities. Each leg has one microcomputer that controls 3 Futaba servomechanisms and carries out real-time inverse kinematics at an update rate of 50Hz, i.e. a new set of calculations is carried out every 20ms.  A set of five bytes is received by each leg microcomputer every 20ms. Each set is different for each leg. Each byte is, of course, an integer ranging from 0 to 255. The five bytes represent, for each leg, the following;

(i) the position, “n”, of the walking step in its digital locus cycle whose shape is a rectangle, (ii)the leg plan angle, “plangle”, i.e. the walking step direction, (iii) the amplitude of the walking step, “amp”, (iv) the height of the rectangle, “zag” and (v) the height bias of the rectangle, “zbias”.

1. Figure of eight sequence

Here the hexapod rotates in partial circles or arcs about two points that are separated. These points are called “Instantaneous centres of rotation”. “ICofR” and are specified as x-y coordinates with respect to the hexapod body. The +ve x-axis is to the right side of the body and the +ve y-axis is to the front of the body.

n varies from 0 to 255 where n=0 is the leg tip positioned at its origin, n=63 is the leg tip positioned at the end of its walking-on-the-ground stroke and just beginning its walking-in-the-air stroke, n=127 is the leg tip half way through its cycle and directly above the origin in the air, n=191 is the end of the walking-in-the-airstroke and just at the point of the leg tip placing its tip back on the ground, n=255 is just short of being back at the origin with the leg tip on the ground. Note that there are two values of n called bp1 and bp2, (bp=”breakpoint”), where bp1 is a computed value dependent on the height of the locus walking rectangle called “zag”, (Z air-to-ground value). For example, if the step amplitude, “amp” is amp=40 mm, (i.e. step length of 80 mm), and the height, zag=40mm then bp1=95 and bp2=223; and if zag=0 (not practical) then bp1 merges to 63 and bp2 merges to 191. The purpose of this number design is to ensure that the leg tip spends the same amount of time in the air as on the ground. Remember, that for a double tripod gait that this is the case, i.e. equal time in air as on the ground; but zag will always be greater than zero in order to walk so the path length through the air will be greater than on the ground, thus, the speed of the leg tip through the air has to be greater (and constant) than on the ground. Tricky eh! to compute in real-time. Anyway Frank has worked out a technique where you can still walk in all the other gaits other than double-tripod gait (=3-leg-support-3-swinging), i.e 4-leg support-2-swinging which has numerous variations and 5-leg-support-1-swinging which also has numerous variations.

2. Hexapod as an inside epicyclic gear

Here the hexapod has its rotation about an IC of R updated to a new location at every step

3. Hexapod as an outside epicyclic gear

Similarly the ICofR is being update at each step

4. Rotating about 4 points

..and once again the hexapod is rotating about 4 ICofR’s but the locaction of each ICofR is respecified after a few steps rather than after each step. Thus it can be seen that an infinite range of behaviour patterns can be programmed using the concept of rotating about an ICofR

Mechatronic omni-directional hexapod

..a demonstration programme carried out back in 2000 outside the Singapore NTU library

Fast retreat in corridor

..another example of playing around with the location of ICofR’s. Here the location of the ICofR is at infinity distance to the left of the universe which is equal to infinity distance to the right of the universe, i.e. the hexapod walks in a straight line; then the hexapod  rotates about an ICofR that is located at the point of touching of an imaginary horizontal line that is tangential to an imaginary circle passing through the outer perimeter of the leg tips.

3 degree of freedom leg in cylindrical coordinate motion

3dof robot leg demonstration

Three complex inverse kinematics equations are being solved in real-time (every 20msec) for the 3 angular displacement servomechanisms. Just one Basic Stamp2SX microcomputer solves these equations with only 2kbytes of memory and uses only 16-bit integers, no trigonmetrical functions; instead only truncated Taylor series are used.

Latest Hexapod, circular design

Latest hexapod design turning on the spot

Here the robot is turning fast on the spot…a future research project is to task a student with improving the stepping profiles to minimise the noise during walking…there should be smooth transition between one leg tip lifting off the floor and the other being placed on the floor.

Latest hexapod design walking in a corridor

Here the robot is walking through a set routine involving a sequence of walking in a straight line then turning on the spot through a prescribed angle, sometimes walking fast and sometimes walking slow and sometimes walking backwards