Position Sensor II

The finished design of the position sensor uses an Atmel ATMega 16 microcontroller, that reads in voltages from the pressure transducer, a yaw rate gyro, and a 3 axis accelerometer mounted directly below the gyro. The Z data is inverted. A temperature sensor monitors the PCB temp to aid in correcting the values read by the ADC. We have added an X and Y gyro to complete our 6 axis IMU.. The sensor runs on a 1/16 tick simple OS, that allows the global registers used by the host cpu for communications to be continuosly updated with the latest data 16 times per second. The Host CPU queiries individually or core dumps the 8 values.

Depth 0.1 cm steps max depth 65 meters auto surface compensation

Temperature .01 degress F

Rotation X, Y, Z with respect to earth surface, 0.1 degrees

Aceleration, X, Y, Z 0.01 G .

We leave the higher order displacement and velocity for the main DSP which is the Central controller. This circuit draws 30mA while operating, so it doesnt need to sleep during the proposed 1hr operating window the team has chosen.

The Guts, Putting it all together...

The above photo is the thrusters, controllers, position sensor and barely visible servos mounted in the main body, thois assembly is made of delrin and makes up the inner sealed cavity. We would tell how we intend to insure water tight ness, but decided the competition would benefit too much from our example. So this will remain a secret. We are now ready for main sensors, accoustic, and optical, the main CPU cammera and the battery compartment, The above design can manuver, control prop speed 2 stern palnes, and rudders each and accurately calculate 6 axis position sub temperature and depth to .1cm... all queried from the main cpu.

Next stop the brains...

Thruster Design

We did not have sufficient donations to purchase proffesional thrusters, what we did have was free machine time at a very prestigious CNC manufacturing facility. AXIS FAb inc. With laser cutters and 5 axis precision machine at our disposal, we took a crack at a custom design. On our team was Mindy Ren, our Mech Eng student starting at UT Knoxville this fall. We opted to try a design we saw on thrusters used on tug boats. This was essentially a differential drive, propellers turning opposite directions at either end of the tube. This way the water entering the tube would begin to rotate in the same helical pattern as it continued through the thruster cowl, at the exit port it encountered a second propeller that slapped againts the water rotation adding to the thrust instead of just pushing it along. Using a thin shaft cowl also allowed the drive shaft cross section to be rather small. certainly smaller than if the motor and cover was on axis. The propellers chosen came from a dutch RC subamarine outfit are molded brass 2.5 " diameter and we have both right and left hand versions for the counter rotation. The shaft cowls are light weight delrin, and the gear housing is a 1/2" x 3/4" x 3/4" copper pipe"T" from a local hardware store (they are not a sponser so we wont metion their name) . The motor is 90 degrees so it mounts internal to the sub body.

The motor gear is the pinion in our simple differential, we used belvel washers for tension and back filled the cavitys with silicone grease, The shafts have bearings in either end and turn free. The gear ratio is 3 : 1 so the props turn 3000 RPM max when the motor is most efficient at 9000 rpm. No cavitation occured on the propellers at these speeds due mostly to the design of the props, they are 7 blade swept propellers modeled after a russian acula class submarine.

The cowls and assemblys shown have o-ring seals, so any leakage will be along the shaft through 3" of grease past another 2" of grease and finally into the motor cavity which is sealed and pre presurized. Tets so far show no leakage, as the contest is only to 16-30 ft this hopefully wont be a problem,

Preliminary tests show a current draw of 1.5A at 3000 rpm prop speed, 0.25 A at slow 400rpm prop speed, for manuvering. The whole assembly weighs under 2 lbs. its a little noisy but smooth action the real test is in the pool !!!...

Thruster Controller

The team chose to use brushless DC motors due to their small size and high efficientcy, (relatively low loss) Brushless DC, or BLDC have no electrical losses due to the rotor as this is made up of high power permanent magnets. The motor is rotated by applying current to three coils set around the rotor at 120 degree offsets. This type of setup insures that the magnets will be centered on one coil while between the other two. this way there will always be at least one coil that can pull the motor so short bursts of speed are nearly always at maximum and there are virtually no dead spots.

We start by experimenting with BLDC motors and controllers using an evaluation board from Atmel corp, it uses a microprocessor simular to the one we used on the position sensor. After experimenting we will decide what we like , dislike about this approach and suggest improvements with our own design.

Eventually we were sucessful writing the code to commutate the motor, it was driven via interrupts on the comparator inputs from the hall sensors. However being busy moving the motor at speeds of 8000 rpm and higher left communications rather slow, so we considered a second microprocessor, to aid communications, cost increase only $5 or using an available BLDC commutation controller, to offload the actual motion of the motor to logic, and the micro to providing PWM, and comm support. We choses the latter as we found a controller from Allegro micro systems that also had 3A mosfet bridges built in. THe unused PWM channels were diverted to controlling 2 servos, one for planes and onefor rudders.

The thruster design is to have a port and starboard thruster, each with its own plane and rudder. The new controller shown on top, was a low power 3A BLDC controller with RS485 port, motor temperature sensor, and servo controller. in a 1" x 2" package.

Position Sensor 1 (Pressure)

The first experiment was with pressure sensors, The above photo consists of a 0 - 5 psi transducer with a voltage output. the circuit uses a precision 5Vdc regulator to ensure stability during measurement. The team used a large syringe connected with a 7" length of 1/8" tygon tubing to simulate increased presure and noted the change in the output voltage with increase in pressure applied. A simular sensor will be placed on our "position microprocessor board" and we will use the pressure to calculate the depth of the submarine.

The data from the manufacturer's data sheet plus the experimental results were used to formulate the sensors reponse and calibrate our device to calculate depth