Servo - March 2008

electronics in the sport. The brain is a Nios soft processor, running inside a Cyclone FPGA, made by Altera. The processor runs a Web server that is hooked back through an 802.11 wireless link with user commands coming from a USB joystick hooked to a laptop. The laptop runs a custom application which allows a copilot to tweak various parameters during combat.

The toughest challenge the team hit with Cyclone Drive was maintaining directional information inside the machine. In order for the robot to process a command to go “North,” it needs to know where North is, in the driver’s reference frame. Quadrature encoders are used on the wheels, and this is pretty reliable for relative direction. However, the wheels slip, change diameter based on speed, and when the bots go flying on impact, all bets are off! Thus, some system is needed for absolute correction of heading, locking the robot’s reference frame to the driver’s reference frame.

Their first approach was to use a two axis magnetometer to measure the Earth’s magnetic field. Measuring that magnetic field in an environment

with 18 HP of brushed DC motors that see peak currents over 1,000A was extremely difficult and required extensive lab testing. They had to test a variety of different shielding, grounding, physical locations, wiring architectures, and algorithm parameters for the magnetometer to best insulate it from the motor, motor controller, and power line noise.

They found that on the second version of the bot (with its lower chassis), the magnetometer was only 2” from the arena floor, which swamped the ability of the magnetometer to read to Earth’s magnetic field. So, they changed over to a dual laser transmitter. One (a visible laser) is used for sighting, and the other (an infrared laser) for directional information.

After prototype testing using a simple eight bit microprocessor, they built their “beta” version using a DSP-based control system. The fairly advanced 16-bit DSP processor had all the usual motor control peripherals: quadrature encoders and pulse width modulation generators, in particular. This setup worked for them from the start. They

got some basic communications and controls running, but the system wasn’t flexible enough for all their needs.

So, they moved to the combination of the Nios processor and Altera Cyclone FPGA. The communications system uses a dual approach: 802.11b for the primary system and a robot-specific 900 MHz system for the backup system.

Operation at the laptop is intuitive. CycloneBot can be commanded with simple joystick input to move north, south, east, and west. Telemetry is passed back to the laptop through the data stream for real-time status checking and later diagnosis and evaluation.

The heavy computational lifting is handled by the onboard Cyclone/Nios combo, freeing the driver from the usual demanding relative-path-based control requirements. This intent-based control allows the pilot to focus his attention completely on the strategy of the match. SV

Material and photos were contributed by Ilya Polyakov, Team Carnivore; Rich Olson, Team Spambutcher; Michael Worry, CM Robotics; and Dale Heatherington, Dale’s Homemade Robots.

MAN UFACTUR IN G:

Win With Bulletproof

Planetary Gearb xes

● by Nick Martin

Most combat bot builders in the smaller weight classes start out using drill motors or small custom planetary gearbox boxes such as those marketed by BaneBots. Many of those builders have found, as I have, that the stress of combat will break the gearboxes at the most inconvenient times.

The failure is usually a broken pin in the second stage, which leads to

stripped gears, motor burnout, and crushing defeat. Fortunately, this cheap and easy upgrade will fix the problem, win battles, and save you money.

Parts and Resources List

You will need only a few consumable parts, some scrap

aluminium, and a few small tools that you may need to buy. You will also need some gearmotors; I chose the BaneBots 42 mm model for this project. For major tools, a drill press, grinder, and either a good bench vice or an arbor press is essential.