Scoutbot V0.1

Well everything is finally wired up and ready to rock and roll. 3 Ultrasonic sensors facing forward, left (45deg) and right (45deg) as well as a forward facing Sharp IR sensor and a rear IR sensor.

Not enough space

Sensor wires are small. So small in fact that screw terminals sometimes can't grip them properly and the available pin crimps that I had just wouldn't crimp correctly. I needed some kind of breakout board for them to attach to the Mega screwshield.

In addition to this there wasn't any space left on the Wild Thumper to mount anything without building a 2nd level. A re-design was needed.....

Not enough pins! (MEGA Screwshield)

After connecting up the ultrasonics and IR sensors to the board I started looking into how many I/O pins I had left on the Arduino Duemilanove board. The VNH 5019 Motor shield requires 8 digitals and 2 analogue pins. This left 6 digital pins and 4 analogues, of which 3 ultrasonics needed 3 digital pins and the IR sensor an analogue, not much room for expansion!

Something bigger and better was needed....the MEGA :) 54 digital pins and 16 analogues should allow plenty of expansion!

Sensors!

While thinking about how to mount my ultrasonic transducer onto the chassis I started thinking about other sensors that I would like to use and an actual purpose for the robot. To start with I wanted the robot just to be able to do basic collision avoidance. As such would a single ultrasonic sensor be enough? Should I mount it to a servo motor so it could pan, or should I buy a number of the sensors? Would I just want ultrasonics or should I investigate IR (Infra red) as well.

I decided as the Seeedstudio sensors were relatively cheap I'd buy a few more along with a Sharp IR sensor. I also needed some kind of mounting bracket. Active Robots had a range of brackets, but as with most things on the internet I had no idea what they would actually be like once I received them. I decided on sampling a few types.

• Lynxmotion MPSH-01 Multi-purpose Bracket
• Active Robots UB1S3 Bracket (All Sharp IR and Devantech Ultrasonics)
• Active Robots SPB1S3 Panning Bracket

The Sharp GP2Y0A21YK IR sensor mounted perfectly on the UB1S3 bracket as advertised, however the Seeedstuido Ultrasonic module was a close fit to the panning bracket but not a perfect fit (Nothing a needle file couldn't handle). 

Motor Tests

Finally I have power and a robot chassis so time to start work on putting everything together and testing it!

The robot is powered by a 7.2V NiMh 4300mAH battery, this is connected to a Dagu High Power switch. This switch allows a manual on/off or one to be fed from some other electrical source, either a switch mounted elsewhere or from a microprocessor controller. The switch is then wired into the VNH5019 motor shield. This in turn powers the Arduino and gives power to the motors.

Batteries

Power requirements for this robot are going to be slightly more than a few AA's in a battery holder. The chassis can carry unto 2 sub C packs with are often found in RC cars. The Arduino requires an input between 7v and 12v as it has its own voltage regulation to 5v and 3.3v and the motors in the chassis required around 6v - 7v, so I decided on a 7.2v pack. The VNH5019 can power the Arduino however it features no regulation so the voltage input to it must be suitable for both the motors and Arduino, which 7.2v is.

I naively plunged straight in and ordered some Lipo (Lithium Polymer) batteries as most robot websites seemed to stock them in their various forms. I picked out a 7.2v Sub C sized pack, and Lipo charger. However after placing the order I decided to look into the types of batteries a little more and found to my horror Lipo maybe wasn't the best choice. It seems they require a lot of care and attention with regards charging and discharging. Batteries had to be kept balanced and couldn't be fully discharged.

After looking at various forums it became clear this was something that really should be used outside the home in a garage or workshop rather than a spare bedroom due to "abused batteries" potentially catching fire. Lots of sites recommended charging in flameproof bags or buckets of sand. Now a lot of this info was written when Lipo's first came out and I know things have probably moved on since then, but I really didn't want to risk it.

Robots! Part 2

Well it finally arrived, thanks to Robosavvy and the Royal Mail :). The kit comes with the main chassis already built along with the motors and suspension system. All that I had to do was fit the motor axial mounts and the wheels. I left the 2DOF arm off for now, mainly as the chassis is a little bigger than expected and it may have trouble moving around with the arm on. This kit has the 75:1 motor gear boxes and the specification sheets says it will do upto 3km/h. This chassis also has the option of the 34:1 gearbox that will do 7km/h which is a little fast for indoors! Perfect for an outside robot though.

The motors with the Wild Thumper are also quite beefy. Rated at 6VDC (usable between 2VDC and 7.5VDC) with a stall current around 6amps, the Arduino is not going to be able to drive these directly.

Once I started looking around at the various motor shields for the Arduino, one thing quickly became apparent. Motors upto 2A were fine but anything above this was going to require a special motor driver board. The problem was also compounded by the fact the Wild Thumper's motors where connected together in pairs (Both right side act as one, as do the left side). This meant a potential 12amps should the motors stall, on the plus side I could get by with a single or dual motor board.

Pololu seemed to offer a wide variety of motor control boards and after looking around I found the VNH5019 which would control both motors and was an Arduino shield, BONUS! As it was a dual motor driver it also worked out cheaper than buying 2 separate boards. In addition to this Pololu have also written an Arduino library to use with the shield.