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In case you have missed my earlier posts: the article below is part of the large, fully automated train layout called "Sioux.NET on Track". You can watch a video of our presentation of our layout at Lego World Utrecht 2016 at our Youtube channel: Replacement of a slow candy crane As you can see in the video, the loading of the four wagons is done by the so-called candy crane. A nice and eye-catching structure, but is is slow. Really slow. When the visiting parents asked me at Lego World what the layout was about, my answer was usually “for children it is a candy delivery machine but for the parents it is a Zen machine” ;-). The delivery of four candies took in total about 13 minutes; most of the time the candy crane was fetching the containers with candies and loading them into the train. When we walked around Lego World, we noticed the following robot arms at the Mindstorms stand. We all had the same idea: that robot arm would be our next building and the replacement of our crane. The robot arm would be responsible for moving the containers from the pickup position to the wagons. The robot arm on the photo is originally designed by Mike Dobson and a LDD file is available. But I don’t find it a challenge to build these things from a building instruction (although, rebuilding these large objects from an LDD file is quite a challenge in itself ;-). So I started to build a robot arm from scratch. Of course, you will see some parts that look similar and yes: I have stolen these ideas from the master ;-) Robot arm, six axis DOF (some background info) The robot arm that we are designing, is a so called six-axis DOF robot (DOF is an abbreviation for Degrees of Freedom). The six axis that it can move are shown on the following picture: This axis, located at the robot base, allows the robot to rotate from left to right. This axis allows the lower arm of the robot to extend forward and backward. The axis extends the robot's vertical reach. It allows the upper arm to raise and lower. Working in conjunction with the axis 5, this axis aids in the positioning of the end effector and manipulation of the part. Known as the wrist roll, it rotates the upper arm in a circular motion moving parts between horizontal to vertical orientations. This axis allows the wrist of the robot arm to tilt up and down. This axis is responsible for the pitch and yaw motion. The pitch, or bend, motion is up and down, much like opening and closing a box lid. Yaw moves left and right, like a door on hinges. This is the wrist of the robot arm. It is responsible for a twisting motion, allowing it to rotate freely in a circular motion, both to position end effectors and to manipulate parts. It is usually capable of more than a 360 degree rotation in either a clockwise or counterclockwise direction. The gripper to pick-up the parts, candy containers in our case. Since this is not a movement, it is not seen as a separate axis. So, let's start building! Building of the robot arm, work in progress (first prototype) We started with the upper arm of the robot arm. For the first prototype, we used a a copy of the crane grabber for the gripper part. Three motors were placed in the upper arm (for movement 5, 6 and 7). The result can be seen below: And all worked fine, apart from one major disadvantage: to make the movement "axis 4" possible, the upper arm needed to be connected using a turntable (art. 4624645). But the weight of the upper arm was too high for the turntable. It started to bent a bit, making the rotation (axis 4) almost impossible. So, we needed a new upper arm but much more light-weight. Building of the robot arm, work in progress (second prototype) We discussed in the team what could be improved. And if the weight of the arm is too much, it needed to go on a diet. So, we looked for a way to put the motors in the upper part of the arm and to get three axles through the turntable. That would save lots of weight: 1) because three motors are not needed in this part of the arm, and 2) because the arm could be much shorter. But is it possible to get three axles through one turntable? Yes, you can. I found a video that does the trick, you can find it here: Another solution uses non-Lego parts: Quite a nice solution but we have a restriction that we don't use non-Lego elements. If three axles is not really possible, let's step back to a two-axle solution. And so we did: we created an upper part of the robot arm with only one motor and a simple fix to get two axles through the turntable: The result of the second prototype can be seen here. We said goodbye to the grabber and made a two-finger gripper. And indeed, the second prototype was better than the first time. Take a look at a (kind of) complete upper arm: But another major disadvantage: if the arm made a movement around axis 5 or axis 6, the gripper opens or closes as well. The reason is that the gears that control the movement of the opening/closing of the gripper, are also rotating when the gripper is rotated (axis 6) or when the wrist rotates (axis 5). You can power the motor that controls the gripper to compensate, but it is not accurate enough. So... goodbye to prototype 2. Building of the robot arm, work in progress (third prototype) How can you prevent gears to turn when you don't want them to....? Remove the gears! So the next (and hopefully final) prototype uses pneumatics to control the gripper. We added two touch sensors to the upper arm to detect the position of movement 5. In the photo above, the read L shaped peaces are pressed against the touch sensor when it reaches the end position. At the other side, the same principle is used to detect the other end position. Next to build: a sensor to detect the rotation position of the gripper. And the motor(s) to control the movements 5 (wrist) and 6 (gripper rotation). If that is finished, the upper arm is ready and we can continue with the part that holds the upper arm. That's all for now, I'll keep you posted. Please let me know what you think of it. Enjoy, Hans -
Sioux.NET on Track @ Lego World 2016
Hanso posted a topic in LEGO Technic, Mindstorms, Model Team and Scale Modeling
At our Flickr page you can view the photos of our visit at Lego World 2016 in Utrecht: https://flic.kr/s/aHskMd9EAj. A video is also available: Enjoy, Hans