Let's Make Robots!

Part 2: Speed Writer

Write on the pavement at high speed!

Speed writter races around with a spring loaded piece of jumbo chalk writting words. Using a hacked "Rebound" RC car for the body and an Arduino Nano for the brains.

Part 1 of this robot page is my tip/walkthrough on hacking the Rebound for robot use. This lets me use the original PCB for motor control and left me with a clean chassis to build upon.

Since this photo was taken I have made several modifications to reduce noise and am now deciding what sensors to uses. For now I am trying 2x IR sensors.

Hopefully these will stop speed writter crashing into anything while writing. They are just simple digital sensors and are being used like invisible antenna. They have a pot on the back where you can set their sensitivity/range. As the robot is very quick I've set them to their maximum range of 80cm in hopes the robot will detect an object in time to turn before crashing. Fortunately the rebound dual "H" bridge has electronic braking :D


Ok, I had some trouble with power dropping when both motors started suddenly or changed directions. Using a low dropout regulator instead of the 7805 made a big improvement. The IR sensors still give a brief false reading due to noise when the power spikes so I will use a simple RC filter to eliminate spurious signals.

I wanted to use "Jumbo Chalk" on the back of this but I couldn't find any locally so I decided to tape 6 smaller sticks together. Remembering too late that chalk is weak and brittle.


Guess I'll have to chalk that up to experience :P

Using some mecano from the original "Mr. Tidy" arm and a caterpillar antenna spring I made a new spring loaded arm to hold the chalk. The spring is mounted in such a way that it holds the arm up when not in use and applies gentle pressure to the chalk when the arm is down.

Now I just need to program it to write "LMR".

 

 

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Put a spring in a tube with one of it's ends sealed, insert chalk, and the spring should make it come out as it wears... though this way it must be on the ground all the time... :S

Originally I was going to use the big spring that normally ejects the battery from the rebound for that purpose. The spring loaded arm is better because it still puts pressure on the chalk but also holds the chalk up when it isn't needed.

I would put servo to hold and control the angle of arm. At the end of arm i would add chalk holder which would have ove contact sensor on upper side of chalk which would give feedback if chalk have freaction with ground which would be base for correcting the arm angle and compensate the wearness of chalk :)

rocking the 'duino!
Cool!
Any video yet? :D Can't wait to see his writings :)))
Because the motors draw so much current on start up and these sensors draw about 130mA continuous I have trouble with the Arduino reseting. Once I get the power issues sorted I just have to buy some chalk and make a polymorph arm for it.
Ithink adding a big capacitor between gnd and vcc of 'duino will help. Also a capacitor at rst would be nice.

The Arduino's on board regulator would fail when I added the caps to Vin and Gnd (don't know why yet) which is why I made an external regulation circuit. My problem now is the battery voltage drops too much when both motors kick in despite the 2x 4700uF caps.

Because I only had 7805 regulators which really need more than 7.2V to begin with they are not performing well. I will change to low drop out regulators today.

I know noise is not the problem because I can run it off a power supply @ 7.5V and everything is fine.