Let's Make Robots!


Okay, I broke the rules. Yes, I'm ashamed. But I don't give up. Hopefully this is the right place for this article...

TSWLhexaPOC? Well, Two Servo Watt's Linkage hexapod Proof Of Concept would have been a bit too long for a name, wouldn't it? I stumbled over a few hexapods controlled by three servos. Well, it should be possible to walk over a plane with only two of them. If this is possible using some simple gear it would save money when building small robots. How did I find a solution?

I had a closer look at the usual mechanism of those three servo hexapods then you will notice that one servo tilts them while the other servos move two legs in a bow while the thirs one is not the center of these bows. This makes their feet slip and slide a little when moving. Can we improve this? First idea was to use Watt's linkage to move them linearly. Or some other straight line linkage - there are lot's of them. While digging the net I found out that Watt's linkage only moves approximately linearly in the middle and also produces bow movement in the ends. This gave me the idea how to get rid of one serve.

 TSWLhexaPOC - first sketch

In the image you see the first concept. One servo exchanges the three standing feet by tilting the bot. the other servo moves three feet through Watt's linkage with the third foot coupled to the other two. Yes, the third one will also slip in this construction. One reason for the POC in its name. If the servo only turns a small angle around the center this bot will move a straight line. I the servo turns a small angle near to one end it will turn to one side, if the servo turns near the other end the bot will wurn to the other side. cute, isn't it? As a side remark, if the servo turns the whole way the bot will make large straight steps. However, take care that the linkage doesn't turn the wrong way.

So far for the idea. A number of improvements are still necessary. The tlting meachnism in th eimage is not really feasible. Has to be done better. Hey, I'm a software guy! Extremely creative with electronic and mechanic knowledge. My current problems are that I don't have enough spare time to work on it - and robot construction is not my primary job - and that I am still thinking about the materials for a first prototype. Well, I have a few servos lying around, a propeller demo board, a few PICs, some transistors and other discrete electronic parts but I am not yet sure what to use for the body, the gear and the legs. Cardboard? Wood? Plastic? Metal?

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All I can say is that I am looking forward to see it in real life :)

Uuuuhhh... I apologize! This was an image export my first construction in Google Sketchup. You are right, the essential parts are invisible. Well, I was (and still am) a little handicapped because I'm currently wearing my left hand in a cast because I recently broke its scaphoid bone. I try again from the beginning...

I've attached another perspective. Here you can also see that there are two servos and get an impression what they do. The servo in the the rear (let's call it the "hip-servo") simply exchanges the three of six standing feet of the robot. You can see that three of them are connected rigidly with the large plate representing the body. In the meantime I've got a better idea how to mount them: mount them to a second plate placed which is mounted to the robot's body via a Sarrus linkage. This would allow for easy up-down-movement controlled by a servo and concentric mount that the two leg triples maximally overlap. Yeah, I know, an image would help also here. But I am still lacking time to draw one with one hand.

The other three feet can move horizontally But they cannot move freely, they only have one DOF. The leverage controlling the legs is a combination of two Watt's linkages. This makes them move in a partial lemniscate.

If the servo moves only a little the feet move almost on a straight line. This can be used for straightforward stepping of the robot.

If the deflection of the servo is larger the foot on the side where it turns to is moved towards the servo while the other foot is moved to the other side. This can be used to turn while walking - or even to turn on the spot making small steps back and forward. In theory, the robot could also run straightly with giant steps deflecting the servo by 180° since the feet move to the middle again, but you have to ensure that the levers turn to the correct direction when they turn back.

I hope this helps. Do you get the idea? Goal is to minimize the number of actuators by using some gears not too complicated.

The same idea seen from a different angle

I'm sorry, but could you explain this mechanism again, I can't understand where the rotation of the servo takes place, and not which part move...