Balancing Explorer

Inspiration

Balancing Explorer is an attempt to dive into the world of self balancing robots and Mapping/Environmental Awareness robots. Eventually the robot will be equipped with ultrasonic sensors and other guidance systems yet to be defined. This is all dependent upon weather or not I can actually get the thing to balance properly. The project has been stalled due to lack of understanding on how to properly filter the data coming from the sensors. If you think you can help me on that one, please let me know.

Chassis

The chassis is made from four lengths of 6-32 threaded rod and three round pieces of Plexiglas. The bottom plexi layer is 1/4" for strength and the top two layers are 1/8" to reduce weight. The layers are held in place the two 6-32 nuts on the top and bottom of each layer. I enjoy working with clear plastics as it really gives the robot a personality of its own.

Propulsion

The motors are mounted to the bottom 1/4" plate of the robot chassis. There are two Hsiang Neng GH12 50:1 Gear Motors with Lynxmotion Green Dot Sumo Tires. The hub adapters and motors mounts were also obtained from Lynxmotion.

Motor Drivers

The GH12 gear motors have a pretty heft stall current and from balancing I expected the motors to consume a lot of power. To prevent overheating I decided to use two LMD18200 from National to drive the motors. The LMD18200 can handle far more current then the L293D and offers a large tab to better dissipate the heat generated from rapidly switching the motors direction.

Balancing Sensors

In order to keep upright the robot must know which ways is up and how fast the ground is approaching. This is usually accomplished using a gyro and an accelerometer. The gyro does a fantastic job of determining angular speed and can be integrated over time to give and but he gyro has a drift. The accelerometer in a static state can give a good angle but in a dynamic system other accelerations cause the data to be error prone. When the two sensors are properly combined they can produce a very reliable depiction of the robots attitude and angular speed. This is the part that I have yet to get right. The accelerometer used is a ADXL311 from Analog Devices and the gyro is a ADXRS150 also from Analog. Both parts are on SparkFun breakout boards.

Robot Brain

The robot is driven by a PIC18F452 running at 48MHz. The micro does.. tries to keep the robot in the upright position. The code from the robot is written in C to avoid having to do math in assembly.

Circuit Boards

As I expected that this project would be a long term project I decided to create circuit cards for the systems rather then just hand-wiring on proto-boards. The boards turned out pretty well.

Controller Board

The control board houses the micro, the motors drivers, the voltage regulator and some misc. house keeping circuits.

Sensor Board

When I created the original sensor board I did not put any filtering on the circuit card. This proved to be a problem. So I had to create a second sensor board that has filtering on the sensor data. The accelerometer and gyro mount in sockets on the sides.

Balancing Explorer Data Sheet
Robot class	Balancing/Inverted Pendulum
Size	8.5 in(W) x 5.0 in(L) x 9.0 in(H)
Weight	800 grams
Frame	1/8" and 1/4" Plexiglas combined with 6-32 threaded rod.
CPU	Microchip PIC18F452
Programming Language	C written in the SouceBoost IDE
Power	14.8V from a four cell 2200mAH lithium-ion battery pack. A linear regulator steps the voltage down to 5V for the PIC and sensors.
Motors	Two 12V Hsiang Neng GH12 50:1 Gear Motors.
Motor driver	Two National LMD18200 driven by microprocessor
Speed Control	Hardware generated Pulse Width Modulation.
Drive System	2WD using two motors
Tires	Two Green Dot Sumo Tires from Lynxmotion
Sensors	One ADXL311 and One ADXRS150