Android 9

Although this isn’t the 9th actual android I wanted to keep the numbering as it was, so this will be known as Android 9. I have decided to call him ‘Featherweight’ as the design will be based around light weight materials and low power actuators.

I’ll be using mainly balsa wood for this biped but I’m still aiming to make the legs about 50cm long so the whole thing comes to about 90/100cm (approx 3ft) tall. It will use some of the high torque linear actuators although I haven’t seen them in real life yet so I’ve no idea if they’re up to it. The plan is to move the leg joints as little as possible for walking in order to ‘gear down’ the actuators and increase power through levering.

I may still use aluminium tube for some of it but the main concept will be a balsa box construction with heavy card glued on with PVA to make the structure rigid. Anything that needs extra strength will be reinforced with 0.5mm aluminium sheet. All the joints will be made from 3mm steel rod rotating inside polythene bushes so I’m hoping that thehole construction will be light but relatively strong.


Today I have recieved the majority of the parts that will be required for the android. So far it has all come from which is an educational website that supplies schools, but also the general public. They are located in the UK and have some very reasonable prices.

So far I have the following items:

-2x A4 sized sheets of 0.5mm thick aluminium.
-50x 30cm long, 3mm steel rods.
-8x packs of assorted balsa wood:

-120x 3mm polythene bushes (which you snap off the spurs they come on).
-40x ‘hub caps’ that push fit to the end of the 3mm rods:

-7x ‘high torque’ linear actuators:

The actuators look pretty small and low power, but having tested them on 4.5v they are actually pretty strong, at least they will be more than sufficient for the balsa wood and card design. They move pretty slowly but as they have enough power there is no longer a requirement to lever them down further within the leg mechanisms. In fact, they will have to levered up slighty to move the joints fast enough, this means that for a small amount of actuator movement the joint will move a long(er) way.

Although sell various sheet material, I wanted to feel what it was like before I bought it, so I will get something from a craft/stationary shop – it will probably be like thick mounting card/board. Then I can start construction…


I sat down and made some parts tonight, the construction is easy going and very quick as everything has been cut with a sharp knife and a steel ruler – much less messy than cutting through wood or steel too. I’ve made all the front and back sections for both legs already:

The front of the leg will be the curved section, the back is the one with the hole in which is where the motor will be mounted:

The blue stuff is thick mounting card/board from my local craft supplier, it’s probably 1mm thick:

As each piece is braced with balsa wood across the section and kind of coated with a card skin, the pieces are really very rigid, they don’t have much twist in them at all. I’ve glued each piece together with wood glue which will never come undone. Each section is 23cm / 9″ long, here’s a size comparision with a full size Roboraptor:

I’ve added the polythene bushes to the corners of the leg pieces where they will hinge, these will be covered on the outside with the hub caps pictured above:

Next will be the knees and the tops/botoms of the legs, this android will work in exactly the same way as the others since android 3 – with double hinged joints and parallel vertical sections.


I’ve made the knee pieces which allow the top and bottom of the leg to hinge as per the previous androids:

I need to cut the steel axels shorter but the parts basically look ok and leave space for the motor in between the vertical leg pieces:

So far the leg is 49cm / 19″ long and it still has the feet to go on. I’ll make the ankles and hips next… I intend to make the whole android in the same way so the body sections will be just as rigid and hollow to allow for the control electronics to fit inside.


I’ve now made the ankles and some feet, which I may modify later to make them a nicer shape, but they will do for now:

Here’s some of the leg parts laid out together, using the other knee as the hip:

It’s about 48cm/2ft long as it is (bent) in the picture, you can see the red marks on the tape measue are in feet. It’s a bit longer than anticipated (as usual) but the motorss should still be more than adequate.

On with the propper hips next… and then the lower body.


Small update today but I’ve decided the motors I had were too slow (although powerful enough) so I’ve ordered some larger ones which are plastic so they are still quite light weight. I’ll post the pics when they arrive.

For now I’ve mainly finished both legs:

They Bend enough for most things although the range is fairly limited, but this is fine because the motors will be levered to provide maximum torque for the movement that’s required. I estimate that the leg joints will bend about 45 degrees for a full 180 degrees rotation of the motors. Even though each half of the leg can only bend 45 degrees, if they both bend in the same direction it sets the knee at approx 90 degrees as pictured:

I’ve also made a start on the hips although I don’t want to do too much until I know for definate how large the motors are as these need to be mounted somewhere on/in them:

Basically each one fits to the top of each leg to allow sideways leaning at the hips, then these parts will in turn attach to rods that rotate vertically to allow the legs to rotate.


I recieved the motors on Friday which I bought from British Robotics. They come as a kit which is quite easy to assemble, you can make 3 different gear ratios depending on which of the parts you select, I made the highest ratio using all the gears. These are theTamiya 70093 3-Speed Crank-Axle Gearboxes which are the smallest ones available. If you are outside the UK then Here are the kits, one set of parts and one assembled motor/gearbox:

Here’s the motor and gearbox on it’s own:

The motors in them are basically the smallest ‘budget’ type hobby motors which are quite small:

The gearbox is about 7cm / 2+3/4″ long. The output shaft is about 10cm / 4″ long and is a 3mm hex shape, but it comes with the ‘servo arm’ style levers which have metal screws in to lock them to the shaft:

All together, they are not undersized for the robot and don’t look too out of place next to one of the legs as each leg section is only 20cm / 8″ long:

However, the lack of worm gear in the gearboxes means that they are quite easily back-driveable. They have enough power to move the balsa wood legs but I think they will suffer once the body and potential electronics and batteries etc are on top.

So, the plan now is to keep these motors for the arms (2 DOF at the shoulder, 1 DOF at the elbow and use R/C type servos for the fingers/grippers). I have now ordered six of the the Tamiya 72004 Worm Gear Gearboxs for the legs. These gearboxes have a worm gear, a slightly higher possible gearing ratio and a larger motor. If the ones I have already are anything to go by then the next ones will be more than adequate. At least I have all my degrees of freedom covered for motors…


I have recieved the six larger motors. They are the Tamiya 72004 Worm Gear Gearboxes:

I’ll be using them at 336:1 gearing ratio which runs at an ideal speed at about 3/4 volts, they also cannot be back-driven due to the worm gear. They are slightly larger compared to the previous gearboxes and use the larger RE-260 Motor:

They come with various output arms and also another axel and bracket suitable for use as the front wheel of a car etc. I’ll be using these as the hinge point at the other end of the shaft from the motor arm, so this is ideal:

They look a bit more at home next to the leg assembly and certainly have enough power to drive it. I may even extend the output arm to get more travel in the leg once the motors and lever hinge points are positioned:


So I finally got around to building the motor mounts, here are some bits of aluminium:

They attach to the backs of the legs to stop the bolts pulling through:

The motors themselves are mounted on balsa blocks in the middle of the legs:

I’ve also added the push/pull-rods so that the legs will stand up on their own:

180 degree rotation of the motor makes them bend as far as they go, they don’t bend as much as I’d like but I’ll adjust it later, either by making the motor output lever longer or moving the other end of the push-rod to another postition:

The hips are attached to the tops also so the legs can lean sideways at the tops as well as at the ankles:

The hip section will also rotate against the body to give it all the degrees of fredom for turning / walking around corners. Next I will start on the lower body and plan the main torso which will hold the electronics…


I’ve made the lower torso / rest of the hips where each leg attaches. It’s basically an ‘H’ shape on it’s side with a bit sticking out of the front:

This allows the legs to rotate outwards:

and inwards:

The previous hip sections on each leg allow the android to lean sideways:

Each hip motor will attach on the sticking-out parts on the outside of the hips and have a push-rod across the gap between the hip and the leg:


The torso is well on its way. First I started with a base to build it on:

it will sit on top of the existing hip sections:

The whole thing is like a box of ‘quality steets’ or an upside down Dalek, angled outwards at the top:

There is a hole in the back so I can get inside and I’ve also left a hole in the front which I’ll fit a panel to later with some indicators/a display etc on:

Next I need to build the shoulders where the arms will mount, as well as the head. Not sure exactly what’s going to be in it’s head yet so I’ll probably leave some sort of platform… but I’d like it to look integrated into the body more than being just stuck on top so I need to give that some thought. Once that’s sorted and the arms are made I can pretty much start messing with the control system.


started the construction of the upper body now I’ve given the arms and head some thought:

here are some more bits being glued:

and this is the main construction finished:

The head is gong to sit just infront of the curved bits on top resting on the upper facing panel of the chest, it will be hinged at the curved parts so it can rotate upwards. The whole thing isn’t going to get much taller nowso the head will be pretty integrated with the upper body and will only hold a few things. It will have a mechanism inside the body for the movement which will be actuated by an R/C servo:

The arms will also have motors for the shoulders inside the body, there plenty of space for them here. Still a few extra bits to finish off there, the panels to the front of the very upper chest are missing, there will probably be speakers or lights etc in there:

I’ve fitted the extra motors to the hips to allow sideways leaning so it’s weight can be displaced side-to-side… to allow the android to stand on one leg etc:

They have enough motion to lean as far as is needed, again as with the other leg motors they operate a lever that is ‘across’ the joint so there is a reduction in speed and also power:

So here it is altogether so far:


I’ve made an approximate head so that the android gets its proportions, it doesn’t make it much taller and makes it look a bit ‘hunched’, but the arms will hang lower so it will just look like it has big shoulder pads which it needs anyway for the shoulder motors etc. Here’s the head:

It hinges up so it can look upwards to see humans:

The top of the head isn’t glued on yet so I can fit some sensors inside. I’m considering making a pan mechanism for a single camera so it can look side to side:

I’v also started the arms, they will use 3 motors each, two for shoulder rotation and outwards-lift, and one for the elbow joint:

The arms will be attached to shoulder parts that hinge against the existing body and the upper arm to give the 2 degrees of freedom:

Two motors will be mounted on the arm and one inside the ‘shoulder pad’ on the body, it looks a bit random right now but it should be clearer once it’s all together… not too many more bits to make:


I’ve pretty much finished the arms this week, here are the three sections hinged together to make the elbow and one DOF at the shoulder:

The arms have been attached to the android along with two motors each, one for the elbow joint and one for the shoulder joint:

So, the arm can bend at both joints:

they are also hinged for the other DOF at the shoulder, but I still need to fit the motors inside the ‘shoulder pads’ to actuate them:

That’s pretty much all the main parts made now so it looks like this:

Mechanical things left to build are:

-extra shoulder motor mounts.
-hands/grippers using R/C servos – maybe 2 servos per arm.
-head tilting servo.
-some motor/actuator arrangement to rotate the legs, or fix them in place for preliminary testing (currently held with buldog-clips).
-mounts for feedback pots on each motor, although this may use alternative methods for different joints.

Once anything out of that with any significant weight is done I can start testing some electronics which I’ll detail in another article along with some testing videos…


I took a few pics of the android around the house so you can get an idea for it’s physical size. It is exactly 1m/39in tall:

Dynamic stability…

The electronics and control system for this android can be found in the article ‘Dynamic stability testing’.