RC08Sensors

The 2007 robots had no on-board sensing and while using the camera to do most of the sensing worked the process is improved by adding sensor to the robots themselves. There are three new sensors for the robots in 2008: an IR LED ball sensor array, an encoder, and an accelerometer.

Ball Sensor

Ball handling is just as important in robot soccer as it is in the real thing. The best soccer players are known for their fancy footwork and hopefully ours will be too! As such, a big goal for the 2008 team is to increase the ball handling skills of the robots. The electrical team's contribution to this goal is the IR (infra-red) ball detection system which detects if a ball is in-front of the robot and both activates the dribbler and notifies the host when there is.

Encoders & Accelerometers

Besides being excellent ball handlers robot soccer players need to be agile and highly manuverable while being precise in those movements. In order to increase the precision of the robots movement a closed-loop wheel velocity controller is used in the 2008 system to implement a velocity control mechanism to reduce transient error in wheel velocity and maintain steady-state wheel velocities. Most of this mechanism resides in software in the form of either a PID loop, a state feedback controller, or a state estimator (only if we are going to use a mouse see discussion tab). The physical components of this system are wheel encoders and the accelerometer.

On each wheel there are a pair of sensors that use some method (not fully spec'd) to discretize the rotation of each wheel. This discritization can be used to calculate the wheels velocity. With that information four software based feedback controllers can be used to decrease transient and steady state error due to the motors and the carpet. As another level of controller accelerometers are used to get acceleration which is integrated back to velocity. This information is fed into another controller strictly for velocity control of the robot and not the wheels. If the accelerometers are not used then the feedback controller for velocity is done using the vision system.

Tasks

IR LED Ball sensor

• [X] Find an IR LED and a IR transistor - See part Matrix
• [X] Design the bandwidth filter and the Schmitt trigger circuits - considering not using
• [ ] Build and test
  • [X] Create a test-stand to test and verify ball sensor array performance
  • [ ] Build a prototype ball sensor system
  • [ ] Write code to interface with AD, read values, and make decisions
• [ ] Design 1st revision board.
  • [ ] Debug
  • [ ] Software calibration routine

Encoders

• [X] Get final speed and wheel specs from ME team - 5:1 Gear ration
• [X] Deceide on an encoder scheme - Use the built-in encoders on the motors
• [ ] Write firmware to interface
• [ ] Integrate into velocity controller

Accelerometers

• [ ] Search for a cheap part
• [ ] Write firmware to interface
• [ ] Integrate into velocity controller

Specifications

IR Ball Sensor

In order to be effective ball handlers the robots must be able to postion themselves relative to the ball for kicking and dribbling. In 2007 this was accomplished entirely using the camera and the off-field controller, but problems with the accuracy of locating the ball proved that a more robust solution was needed. In 2008 it was proposed that the tasks of detecting the ball's location relative to the robot and aligning the robot to the ball would be moved from the off-field controller to the robots, In order to achieve this goal the electrical team will design a two part system for ball sensing: a breakbeam placed in front of the dribbler, and an LED array for detecting the ball's location relative to the robot's x axis. Both the breakbeam and LED array will operate in the IR band and will be offset in wavelength so as to avoid cross-talk. The break-beam will be built using discrete parts and the LED will be designed as an expansion board for the main robot controller. The LED array will take advantage of the extra IO pins available on the both the FPGA and the LPC. Below are the design requirements for both.

Break-beam

Design	IR LED and IR Transistor
Voltage	3.3V
Current	25-50mA
Wavelength	Different from LED array

LED-Array

Design	Array of 8 LEDs and transistors.
Voltage	3.3V
Current	250-400mA
Wavelength	Different from break-beam
Protocol	SPI
Size	?

• Matrix of Potential Ball Sensor Components

Accelerometers

• Still defining specification
• Matrix of Accelerometers

Schematics

Schematics

Articles

Filter Design

• Filter Design in Thirty Seconds
• Filter Design on a Budget

Links

• Electrical System