I built a frame that paint bot can be mounted on for testing. I built it out of scrap lumber from the CMU Robotics Club. The frame has the motors mounted near the top with the cables running through guides at the top of the frame.
I currently do not have any sort of painting mechanism mounted to paint bot. I’m thinking about building a marker holder so I can test it a little easier than using spray paint. Currently I just have a weight attached to the two cables to simulate the painting mechanism.
I’ve been working on trying to learn more about stepper motors. In the past month or so I’ve learned that there are two varieties of stepper motor: Unipolar and bipolar (and sometimes both!). In all of the research I’ve been doing I came across a lot of great resources. They are listed at the end of this post.
All of my interest in stepper motors comes from the fact that the Robotics Club recently received a large supply of nice stepper motors and because the original spray painting graffiti bot, Hektor, as well as some other recent Hektor-esque bots, used stepper motors for its control. The particular motors that the Robotics Club obtained are 6-wire stepper motors. This means they can be used as either unipolar or bipolar steppers. This is very useful because it gives me a larger range of control options. However, the motors run at an odd voltage (3.68 volts) which limits the range of available drivers ( I’ve only found 2 capable of driving these motors). Due to the lack of available motor drivers for all these stepper motors we now have, I decided to build my own.
After doing some looking, I decided to control the motors in unipolar mode because the control circuit is simpler. The circuit I build, which is based on this one, is basically 4 Darlington transistors which are used to toggle the coils on and off and some diodes for protection. The stepper motor driver turns each one of the four half coils in the stepper motor on and off sequentially to drive it. The original circuit uses a L297 stepper motor driver to handle sending signals to the Darlingtons, but, I didn’t have any on hand, so I just sent the signals sequentially from and Arduino.
To test the circuit, I didn’t want to potentially harm one of the nice stepper motors we have, so I was using an old stepper motor out of an IBM floppy drive. I couldn’t find documentation so I couldn’t figure out what order to toggle the coils to properly step the motor. I was able to get it to step some, but never rotate continuously. It would step a bit one way, and then step back. I believe my control circuit was ok, because the motor was able to step some. The issue at this point may be the motor itself (just having it hooked up incorrectly) or my code.
I’m going to keep working with this circuit, maybe get an L297 to try out, or I’ve know some people who have used shift registers to accomplish the stepping. I’m also going to try this out on some other motors as well. Hopefully I will soon have a nice inexpensive stepper motor driver that other people in the club will be able to use for these nice motors we have.
List of stepper motor resources:
- HobbyCNC
- http://www.hobbycnc.com/
- Has some nice information, and points at more
- Wikipedia Labs/Books
- http://en.wikibooks.org/wiki/Practical_Electronics/Stepper_Motors
- Some good general information on steppers, and some control circuits
- Northwestern University LIMS wiki
- http://hades.mech.northwestern.edu/index.php/Unipolar_Stepper_Motor_Driver_Circuit
- Great control circuit for unipolar steppers. Mine is based on this one
- Iowa University Stepper Motor Tutorial
- http://www.cs.uiowa.edu/~jones/step/
- Lots of info about steppers
Over the past month or so I have been working on building a painting robot. This robot will be able to paint a wall using spray paint in a graffiti-esque manner. My initial inspiration, and thus similarity in design, was a robot called Hektor. Hektor was a graffiti robot built as a project by Jürg Lehni with help from Uli Franke at écal, the University of Art and Design Lausanne. More about Hektor here and écal here.
I just finished some testing on my spray paint can holding/spraying mechanism. The video below shows the second test of the spraying mechanism. In the initial setup the servo used to depress the spray nozzle couldn’t provide enough downward force all the time. The second video shows an attempt at getting around this issue by the servo pressing on the nozzle from a different direction.
Second Spray Test:
Initial Spray Test:
I will probably end up modifying the spraying mechanism further so it is less of a hack to get it to depress the spray can nozzle.
The following pictures show the can holder and spraying mechanism during the build:
I’ve also finished building motors to move the spray can around. When I get time, I’ll post something about the motor build.
During the previous semester, as part of a class project, we built urban search and rescue robots. We built the robots using the LEGO NXT set. For our robot I built a custom pan-tilt mechanism to mount the camera we were provided with.
The pan-tilt used two servos and a custom microntroller board to interface the servos with the NXT. The controller board also allowed for LEDs (used for illumination) to be turned on and off.
We controlled the robot over a bluetooth link via joystick. It turns out that the NXT brick times out after 15 min no activity and that an open bluetooth does not count as activity.
In short, this project ruined my childhood and made me hate LEGOs. The constraints of the project made it very difficult to build an effective robot. However, a simple combination of wheels and treads let a build a robot to fit the constraints.
As part of a class project, for Micro/Nano Robotics, we’re building an ornithoper. The above video shows an example of the flapping mechanism.
For this project we will be building an ornithopter that uses passive wing pitching to increase the amount of total lift. As the video shows, the flapping mechanism is complete. The wings should be built within the next few days.
We’ve mounted Stairbot in a hanging position so we could perform some testing on its legs. We’ve discovered some issues with our servos, and we keep blowing some of them up. We’re not sure if it’s an issue with the design, or if the servos are bad.
A video of the legs cycling through some motions:
We also have some pictures of the hanging setup:
Stairbot is getting closer and closer to completion. The above render is what the final version should look like. We’ve decided to laser cut a lot of acrylic to build most of the leg and body structure.
We’ve ordered all of our electronics, and we should be receiving them soon. Then we can start actually programming and getting gaits set up. Our goal is to have the construction mostly completed by the end of the month, and start working on some basic motion control.
Below are some images from the build process.
I managed to get a gallery of a step-by-step build of the MicroColony/Build18 sensor board uploaded. This post details the sensor board itself a bit better.
We plan on building a better one that uses a multiplexor to allow us to have more than 5 sensors on the board.
During Build 18 I managed to slap together a simply chassis for our micro colony robots. It was super simple, made of some plastic and hot glue, but, it was able to hold everything on it.
Here are some pictures:
As you can see it isn’t very big, not much larger than my cell phone. I used 3 4-40 machine screws which let us slide the sensor board into place and tuck all of the electronics underneath. I really like the 4-40 screws for stacking feature and it is something I would like to retain for the next revision of our chassis.
The next revision will also have a mount for the servos, instead of them just being tacked in place with hot glue. It was a quick and dirty build, but, it got the job done.
Lately I’ve been really really busy, so I haven’t had a chance to keep updated on everything. I’ve managed to do quite a bit lately, and in the next few days I’ll be posting updates on everything I’ve done.
First off, Build 18 has come and gone. We were able to get everything mostly built, but part delays kept us from finishing our micro-colony. I’ll be posting about the finished product, with pictures, soon. We’ve already started work on redesigning it to be better. Hopefully by the end of the semester we’ll have a nicely built finished product.
A lot has also happened with Stair bot. We’ve been doing a lot of work machining parts to build the legs. We will be ordering the remainder of our raw materials as well as some electronics. We have also received the remainder of our funding in the form of a SURG grant. The donation for our grant came from Boeing. The pace of work on Stairbot just keeps picking up. Hopefully we’ll have it built and working soon.
I’ve also become head Mechanical Engineer for the RoboClub’s air hockey robot. I’m working on a redesign for it so we can get it up and working. In addition to this I’ve been building some things for some of my classes, for Into Robotics my team built a Rube goldberg machine, I’ll post about that later. And for Micro/Nano Robots my team will be building ornithopters, so I’ll keep updated on those.
Additionally, I’m working on changing some back end stuff of my blog, so things will be changing the way they work, which will make everything nicer I hope.
