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Loki   by David Buckley December 2002
Loki is the first of the Aesir series of Biped Robots each using four servos, two servos per leg. These explore the abilities of the different geometries obtainable using just two servos per leg and hence just two degrees of freedom per leg.

The robots are much squatter than those previously built such as BigFoot and Ambler but, because of the extra two servos, have far greater agility.

The goals of the Aesir project were for them to be able to play with a ball and collect bricks.

Loki was designed in December 2002 and was inspired by David Steadman's Sted-e-Man.

(The Aesir are a group of Norse gods.)

Loki is a four servo 7 inch (175mm) high biped walking robot, controlled by a PicAxe40x2.
Loki is driven by four servos and powered by four AA cells.
Loki can walk Forwads, Backwards, turn Right and Left, and Kick.
Loki can get up from any position if it falls over.
On the legs there are forward facing high power orange LEDs for illuminating obstacles which then can be detected by the black shrouded phototransitors next to them.
There are two ground contact switches on each foot for detecting table edges and whether there is secure footing.
There are two tip sensors so Loki can tell if it is tipped forwards or backwards.
Long range sensing can be done by the Ultrasonic ranger module.
Loki can play sounds and Ring-tunes from its onboard speaker.
Loki can store sixteen user inputted 16 character command strings in its memory.
There is a socket for extra memory so hundreds of commands could be stored.
Loki can be instructed by commands from a Sony IR remote-control.
The full set of instructions can be sent serially as text over cable or a plugin radio module.
Loki has an on-board command-help file.

Originally Loki was controlled by a Parallax BS2 on an Ambler board but in 2021 I changed it for a PicAxe40m2 on a new board which allowed IR control from a Sony handset, and extended commands from a Serial Terminal.
Now all the Aesir have the same control board and the same software.

Loki operation

One consequence of the design is that because Loki does not use weight shifting the ankle servos need to be more powerful than the servos used in BigFoot and Ambler.

Unexpected abilities - After Loki was built I was using a remote pot. connected to the Stamp to investigating the servo pulse values for various leg positions and discovered that Loki can walk sideways! I can't imagine ever discovering this in a CAD or modelling package.

Loki in various poses

Loki Electronics

Loki used a Parallax Stamp BS2 as its controller (the photos show an OEM version).
Servo power was supplied by four AA cells while a 9v PP3 powered the BS2.
Now the controller is the PicAxe40x2 and power is 4AA cells.
On the rear is a 1.5mm coax power socket for an external power supply which automatically cuts out the battery, and there is a voltmeter with a 'press to test' switch to check the battery voltage.

Loki Mechanics

Loki is 16 centimetres high by 17 centimetres wide by 13 centimetres front to back and weighs 620 grams including batteries
(6.25" x 6.75" x 5.25", 22.5oz)
Each leg at the hip is pivoted around a vertical axis while the foot is pivoted about an horizontal fore and aft axis.
The distance between the ankle pivots is 130mm and so to lift one foot off the ground the torque required is:
620/1000*13/2=4.03Kgcm
A standard size Supertec S03 servo has a rated torque of 3.4Kgcm at 4.8volts
A standard size Supertec S06/2BB servo has a rated torque of 7.2Kgcm at 4.8volts
Loki uses standard size Supertec S03 servos at the hips and the higher torque Supertec S06 servos at the ankles.
Material is 4mm birch ply.

The eagle eyed amongst you of course will have noticed that the hip servos are Hitec HS-615MG!
I intended to use the Supertec servos but just happened to have lying around two old HS-615MGs which are 7.7Kgcm @4.8v.
However on 20th September 06 one of the 615's electronics failed and I replaced them with S06s.
The measured rotation of S06s on 5v was 0.8ms/90deg anticlockwise as opposed to the 615's 0.9ms/90deg clockwise.
(The manufacture's figures on 4.8v are S06 speed 0.33sec/60deg and the 615 0.23/60deg.)
After installing and updating the program with the new direction and Max Min and Mid servo settings I was surprised that Loki walked much better than before.


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