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SickSack
The SickSack Controller
The active parts of the SickSack robot are eight standard size RC servos:
Futaba RC sponsored our project with ten of these S3010 servos which feature ball bearings and high torque and speed. This project would not have been possible without this support.
The servos need a power supply of 5 to 6 volts with adequate current capabilities and a PWM control signal. A controller board and battery system had to be designed for supplying this.
Power Supply
The power supply problem was solved by taking advantage of a Texas Instruments Turbo Trans power module: PTH08T220W and having eight AA size NiMH cells in series. The power module is a very compact DC to DC buck converter able to step the down from 14 V to 5.5 V delivering an incredible 16 amperes.
The Texas Instruments PTH08T220W power module embedded on the controller board.
In the photo, to the right of the power module, an 7805 can be seen, which powers the microcontroller and other stuff. We felt it was a good idea to isolate the digital power supply from the servo power supply.
The battery system had to be capable of delivering a couple of amperes, and we discovered that battery containers often adds considerable ohmic resistance. That was unacceptable, so the solution was to get cells with solder tags, and simply solder all battery connections. A breaker switch and a connector for charging the batteries were mounted.
The Controller
A single Atmega32 microcontroller was utilized for all control tasks. We had already verified that it was possible to generate eight servo PWM signals without taxing the MCU very much, so we thought that a single controller should be able to do the job.
For sensing the black tape line on the floor, three photo transistors and illumination with red LEDs were employed. By measuring the light level with illumination on and off alternatingly, the reflectivity of the floor could be estimated. The photo transistors provided plenty of amplification in themselves, and no other gain stage was required before the AD converter. Actually saturation was a problem with the high level of infrared energy in the lightning on the RoboCup track.
The user interface was simply two buttons and two signal leds. Have a look at the complete schematic:
The controller board layout, as routed from the schematic:
The PCB was ordered at Olimex, and the result was quite satisfactory as always. The completed SickSack controller:
From below, the LED illumination and photo transistors can be seen. The copper whiskers were installed to eliminate ESD problems.
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