- Mechanical construction of the robot

The design is best described by the drawing and the pictures.

Main frame is composed of two wooden poles (1) connected by plywood panels (2). The side-legs servos are connected to two aluminum sheets (4), which are connected to the wooden poles by four M3 screws. The center servo is attached to aluminum block (3) which is held together with the sheet (4) by the same screws which attach the sheet to the poles (1).

It would be better to construct the body from aluminum profile rather then wood, but wood was just at hand when I needed it.

The legs are made of aluminum (dural) sheet 11 x 2 mm which I bought in Hobby market OBI. One meter of this sheet costs about 2 Euro.

drawing1

Fig. 2.1 - Drawing of the robot frame

drawing 2

Fig. 2.2 - Drawing of the robot legs 

 

You can download the drawings in full size here.

 

Instructions

First cut the wooden poles (1) to desired length. Then cut the plywood panels (2), shape them and finish with sand paper. Drill the holes for the wooden sticks.

Apply glue on the sticks and/or holes in the panels and put it all together. Make sure the poles are parallel and the panels are perpendicular to them.

 

Measure and drill the holes in the wooden poles for the traverse sheets (4). Then measure (or mark through the holes in the poles) the holes onto the sheets (4) and drill them. Make sure the servos will fit on the sheets without much free play! The dimension 42 mm is just approximate.

Fix sheets (4) to the poles with M3 screws.

Put the two side servos on the sheets and mark the holes for screws which will attach the servos to those sheets.

Detach sheets (4) and drill the holes. If you have the equipment, you may drill holes 2.5 mm and then cut M3 threads instead of using nuts. If not, just drill holes 3.2 mm and use nuts.

 

The center servo is attached to aluminum part (3), about 6x10x45 mm which I found in my storage. Alternatively, you may use piece of aluminum U-profile which may be easier to get. The part (3) is attached to the front sheet (4). The dimensions are not critical, it is only important that it can hold the center servo in proper position in the center of the frame. See the drawing and photos. Drill the holes for the servo now.

 

Attach the sheets (4) (also with the part (3)) to the frame with M3 screws.

 

Cut the aluminum sheets (5) and (6) for the side legs. You need four pieces of legs. The numbers 5 and 6 are used just to distinguish the “top” and “bottom” leg. On each side of the robot (that is, attached to each side servo) is one “top” and one “bottom” leg. See the drawing - round servo “lever” is number (7) on this drawing. Before bending the legs I suggest you make a model legs from carton and try them out on the frame to be sure that you know which way to bend the real legs. When ready, bend them in vice according to the drawing.

 

Legs 

Prepare round “levers” for the side servos. These you should get with the servos - there are usually several levers included in the box with a servo. If you do not have such a round “lever” for your servos, you will need to improvise with attaching the legs. If you have a lever with four arms, this will work well too.

 

Drill holes at the ends of the legs (5) and (6) – but JUST THE HOLE AT THE END of each leg - and put the legs onto the servo lever (7) as shown on the drawing. Drill ONE hole through (7) for the screw at the ends of legs and attach the legs temporarily with M3 screw. Mark the positions for the other 2 holes on (7) and drill them. These holes should be located so that there is enough material between the holes and edge of (7) and also there is enough material on both sides of the legs. Mark the position of these holes on the legs (5) and (6) through the holes in (7) which you just drilled.

 

Detach the legs and drill holes in them. Put the legs with part (7) together. Note which leg is up and which down as can be seen on the drawing! Do not tighten the screws too much.

There could be pieces of small pipe or similar under the higher leg to keep the distance from part (7) but it will also work without it – as on my robot. Just don’t over-tighten the screws.

 

Prepare the middle leg (9). This is quite a complicated construction. Feel free to use your own imagination to simplify it. Here is how I build mine.

Bend the leg sheet (9) to shape using vice and some piece of wood or metal to help you maintain the center space. Cut small hole on side of the top of the leg for the servo lever (8) – see drawing.

Prepare part (10) from aluminum, drill hole into the leg (9) and then put the part (10) in place and mark the position of the hole onto it. Drill it taking care to keep it straight.

Attach the servo lever (8) and mark position of hole which will be used for screw attaching the lever to the leg. Be sure not to put it through the traverse screw (for which you just drilled a hole through (9) and (10)) but little above it!

Drill the hole and put it all together using M3 screws.

 

Connect the servos to the robot frame and then connect the leg assembly to each servo. If you have a servo tester or similar tool, you can set the servos to neutral position before doing so, but it is not necessary at this stage.

 

Put the robot on flat surface and bend the legs as needed so that they all touch the ground and the robot is straight. Be careful not to put force on the servos. For small adjustments you can use two pairs of pliers – one to hold the leg and second to bend it. If bigger adjustment is needed, don't be lazy to remove the lever from the servo and bend the leg in vice.

Check if the legs can move freely. They should be arranged so that the top leg (5) on one servo (e.g. right-side) is above the lower leg (6) of the second-side servo, see the drawing and photos. Don’t worry if the top and bottom legs are slightly touching each other at some place. As long as they are easy to move (which should be, as the aluminum sheet is smooth), it's fine.

Be careful when moving the servos by hand. For standard size servos this is usually not a problem but some miniature servos have the gears constructed so that it is not possible to move them from outside by pulling the lever. They can be moved only by the motor from inside and if you pull too much on the lever you can break the gears easily. So in case you feel strong resistance, do not push the servo lever. You can check the free movement later when we have the controller board ready.

The mechanical construction is finished for now.

 

Bottom view of Krabos

Fig. 2.3 – bottom view of the robot

 Krabos - center leg

Fig. 2.4 – Detail of the center leg and servo attachment

 Krabos - leg attachement

Fig. 2.5 – Detail of the side legs attached to the servo

 Krabos - front view of legs

Fig. 2.6 – Legs of the robot. Front view.

 

Part list for mechanical construction (see the drawings for Frame and Legs)

 

  1. Pole (wooden stick, diameter 10mm) - 2 pcs

  2. Panel (plywood 5 mm thick) - 2 pcs

  3. Center servo holder (aluminum block) – 1 pc

  4. Side servo holders (aluminum sheets 11x2 mm) – 2 pcs

  5. Side legs, bottom (aluminum sheet 11x2 mm) – 2 pcs

  6. Side legs, top (aluminum sheet 11x2 mm) – 2 pcs

  7. Servo lever, round, standard accessory for servo – 2 pcs

  8. Servo lever, straight, standard accessory for servo – 1 pc

  9. Center leg, (aluminum sheet 11x2 mm) – 1 pc

  10. Center leg block, (aluminum) - 1 pc

  11. M3 screws and nuts