A feisty little 3D printed MeArm.
Tired of helpful robots that make a positive impact in society? Wondering where Skynet is after all this time? AngryBot might be right for you.
AngryBot is a MeArm: a small, dexterous, open source robot design under development for many years now. While it's usually meant to be laser cut on a sheet of acrylic, 3D printing is also a viable option. MeArm just has 4 motors which dictate its movement. With the design, some very smart people have solved the inverse kinematics, which is usually a pretty difficult problem for larger robots. The inverse kinematics allow you to provide the robot with an arbitrary point in 3D space to reach. The equations can be solved in a way to test if that point is reachable and further equations can be used to plan the path to bring the robot there.
AngryBot improves upon the MeArm design by adding a ping sensor, which works similar to bats and submarines (SONAR). As a happy accident, the sensor itself looks surprisingly like a set of eyes.
Most of the assembly follows available instructions from previous MeArm builders. Here are the different websites I used:
- Parts (3D Model)
- Assembly
- Wiring (without a shield)
- Control
- Configuring
- for configuring the servos and working with the IK
The construction of the physical arm was based entirely off the instructable. That being said, 3D printing the model added a series of difficulties that made construction time painfully long. First, the 3D printer I used couldn't fit the entire model. To make it work, I cut the model into two halves and printed each separately. To make the cut, I used MeshLab, finding no other helpful software for deleting parts of STL files. Then, to prevent pieces flying off the plate during printing, I built plate adhesion. For these sorts of parts, it meant that the whole file printed essentially as one big plate, each of the smaller components connected by thin layers of plastic. To separate the pieces, I used an exacto-knife. Then, since the parts were printed instead of cut, they were not the exactly correct size. The printing process left some of the holes just a bit too small, which was the hardest part. Some of the servo holders took up to a half hour to drill out each to fit in the part. The chassis of the robot has parts that fit closely together to make a stable cage. To make these right, I used a series of files to slowly shave down to the exact size I needed. Overall, if you can just laser cut it, you'll be cutting down your build time by a major factor. If you must print it, keep a dremel and files close at hand.
Wiring it was not too bad. Not having a motor shield isn't a big deal at all. Just separating the battery supply from the arduino's power is enough (but make sure to connect grounds, or else you'll get a floating voltage that could fry something). The above link was all I needed to get it all hooked up. The addition of a ping sensor is no big deal too. The only nuance is that you must connect it to the common ground between the motors and arduino.
Configuring the motors was the next part that got me stuck. Knowing that you must calibrate the motors to get good results from the IK can save you much time and frustration. The calibration itself is very easy. The last link from above describes it pretty well, except that they refer to some mysterious module called a "MeBrain." Skip that. You can just hook in a standard potentiometer. If you don't have one of those and you're feeling pretty clever, you could certainly just use the Serial monitor to give different PWM values to the motors with the arrow keys. So what is the calibration? It's just matching PWM values on the arduino pins to angles of the arm. Check out the config folder for more.
The wiring here is pretty close together. In fact, these pictures are pretty useless. Instead, here's the wiring diagram:
Overall, AngryBot is not very complex at all. It has two states: scanning and attacking. The scanning state consists of looking in four predefined locations. If at any point the sensor reads something that's close enough, it'll go into the attacking state. The attacking state first checks if the location is reachable. If it is, the arm sticks out and tries to reach that point. Once it does reach that point it'll snap the gripper (mouth) a few times for effect. You can see all of this code in the code folder.
Another small detail: I'm using ino to build and upload my code. Essentially
it's like an arduino build tool for the command line. That's how my code is
structured; the library is sym-linked into the lib/ directory so I can use
the IK. Building and running this code can go like this:
$ cd code/
$ ino build
$ sudo ino upload
$ sudo ino serial(You only need the serial part for configuration though.)
AngryBot was fun, but it's still not impressive enough to be really notable. In the future, I'd like to deck it out a little bit. I'd like to have an RGB LED on it that changes colors to red when it's gonna attack. I'd also like to have multiple personalities. Maybe on a clock it changes from really angry to really shy and runs away every time it sees something. That sort of Jekyll and Hyde relationship could give the character a whole lot more depth.
To make these changes, I think I'd have to laser cut the robot. For whatever reason, the IK are off by a noticeable amount. I think the blocky construction by 3D printer is too coarse to be controlled finely. Some errors in the lining of up the gripper hands, for example, make the robot stop working correctly after a few attempts at biting.