Tuesday, October 5, 2010

Drastic Improvement: new Mobile Platform

After a lot of stress tests with the original platform, it was time for something better. The original platform was no more than a toy RC car (entirely made of plastic) with some hobby RC parts on top of if, such as refurbished steering system (with a real servo) and a home made ESC (Electronic Speed Controller) for the original toy car motor. During a demonstration with my 4 year old nephew, showing him how fast the roving robot would go in an open area, suddenly something happened: it started running full speed, totally out of control, both forward and backward (in my mind I immediately tought one of the ESC FET's had fried). I rushed to grab it, and suddenly smoke started coming out of the motor. Worried about the LiPo battery, with the motor still running and smoking, I centered all effort in disconnecting the battery. My nephew started to cry with the stange situation. After making sure no fire would occur, I had a confused infant to comfort.

Obviously, the motor was completely fried. The home brew ESC had at least the two FET's burned (I hadn't spent more time doing a thorough diagnosis so far). Other than this there was fortunately no more damage. In spite of the full throttle collisions against the wall, everything else survived the event.

Given the accumulated ammount of effort dedicated to this robotic vehicle, I decided to instead of trying to repair the current version of the car, to move to a more decent roving platform.

After some searching in the robot and hobby RC markets, I found a reasonable solution, with all the conditions to ensure durability and stability: an electric hobby RC car - the HPI Maverick Strada XB. With 4 Wheel Drive, a 200 A Brushed Motor ESC, 540 size brushed motor, and 7.2 Volt 1800 mAh NiMh battery, this solution meets the conditions to perform better in demanding terrains:

Besides being cheaper than equivalent platforms for robots, it comes complete, whith everything necessary to run: steering servo, ESC, motor, and a radio transmitter and receiver (the later being redundant, given the fact that I already had another transmitter/receiver kit).

The integration with the existing robot core module (the board with all the electronics, including sensors, computer, camera, and digital/analog I/O controller) was relatively smooth, with the exception of the modifications needed for adapting to the lower main battery voltage: 7.2 Volts instead of 11.1 Volts.

Given the ammount of necessary changes, I took the opportunity to improve the computer (Fonera) power supply. Instead of powering it directly from a pack of 4 AA batteries, I added support for another 4 batteries to optionally connect in parallel, and booster circuitry to extract more energy from the batteries (the Fonera would not like when the voltage would drop below 5 volts), ensuring the correct voltage supply for as long as possible.

With all the mechanical changes required to adapt to the new frame, the final result is the following:

In spite of the aesthetics not being the high point of this project, I have to say that in my opinion it looks a lot better than before.

The AA battery packs are located in the replacement aileron. In spite of not being the optimal location considering the center of mass, it was the only adequate location, considering the fact that the sensors and camera could not have the corresponding field of view compromised.

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