Android 3

After much thought about making my last android stronger and more stable (it did just about hobble along but was a bit bendy), I decided to rethink the leg design and start yet again.

The concept of this android will be rather like those tool boxes with fold out trays that always stay level (a ‘cantilever tool box’). The knee, hip and foot will all be held parallel by two shin and thigh parts hinged twice at each point. This means that only two motors are required per leg to move all three joints, one above and one below the knee. The android’s upper half will also be held bolt upright, although the body may hinge in turn above the waist to allow it to balance backwards and forwards.

Here’s the model to explain it:

I decided to paint the parts as I went along as the legs will be wooden. This will give a low centre of gravity, whilst the body will be made of lighter aluminium parts or a minimal wooden frame.

I have made these parts so far, can you guess where they go?, the blue parts are 30cm long each:

28/11/04

I have now made the ankles and the first part of the hip joints, these will allow the android to lean from side to side which is crucial for walking:

The existing hip parts will connect to the intermediate hip parts which will allow the legs to turn so the android can walk round a corner or turn on the spot:

01/12/04

I have now completed the intermediate hip sections and the rest of the feet. The android is lying down in this shot as is has nothing to hold the joints in any position. The android currently stands 88cm / 2’11” tall, when it is completed it should be just under 5′.

The hips hinge sideways against the body so the legs can turn outwards:

The black stump on top will have a cage attached to hold the electronics and have the rest of the body mounted on it::

04/12/04

In the following pictures the legs have been blocked out with wood so they hold straight and the android stands up. Eventually the motors will hold the legs in place, these are due to arrive next week.

I have added some bicycle brake cords to each leg to synch the sideways tilt of the hips with each foot:

These are attached to the outside off each foot:

and to the inside of the hip:

A further two brake cords will be attached to each hip so that a motor can pull the android from side to side. The android can already balance on one leg using this method to keep the foot at an angle and stop it from leaning back under it’s own weight. In this picture the right leg is off the side of the bench, the android is fully supported on it’s left leg:

07/12/04

I have now received and fitted the motors to the legs, there is one more motor to go on it’s back that will make the android lean from side to side. In this shot the android is posed in ‘freeze frame’ with it’s left leg raised off the ground:

Close up, just to prove it:

Here is a close up on the motors fitted above and below the knee. They have cams on them that fit into a slot attached to the outside of each leg:

23/12/04

I have now added the electronics to the Android that will enable it to be controlled from a computer or any microprocessor with an RS232 output:

This is the same as the last android, the servos at the top move a potentiometer to produce a reference voltage, this is compared with the pots on the joints and the motor is turned in the relevant direction until both positions match:

The servos are standard r/c hobby servos controlled by a 12 port serial servo controller board, a close up of this can been seen below. This will leave me 6/7 spare servo ports that can control other features of the robot directly with servos or in the same manner as the legs:

Next I have to fit the last motor to the left leg which is missing (gears stripped), some current limiting is required – the whole thing is powered from a 13.8V/7A bench supply adjusted down to 9V, but sparks can be seen in the motors when they turn so I’ll just get some big wire-wound resistors to drop about 4 volts and prevent it getting carried away. This sounds crude but will essentially be the same as a R/C car ‘old style’ speed control – speed and timing are not to critical on this android as it can ‘freeze frame’ at any time in it’s walking motion, unlike the previous models which used the inertia of the sideways movement to momentarily lift one leg off the ground.