It will need to interact with and sensors for Proximity Detection, Depth Regulation, Light Levels, Power Monitoring, Speed and are described within the design of the robot subsystems.

It will need control actuators including: Proximity Detection, Propulsion, Depth Regulation.

It must also handle Communications.

Input/Output requirements are:

• 1 x analogue input - depth measurement using pressure sensor
• 1 x analogue input - speed measurement using ???? sensor
• 1 x analogue input - proximity detection - fin voltage measurement
• 2 x digital inputs - depth regulation motor limit detection
• 1 x digital input - under voltage warning sensor
• 2 x digital inputs - light level and following (left/right, equal/unequal)


• 1 x digital input - serial port
• 1 x digital output - serial port


• 1 x digital output - proximity detection - activate high voltage field
• 3 x digital outputs - proximity detection - select fin for voltage measurement
• 2 x digital outputs - depth regulation motor forward/reverse/stop control
• 1 x digital PWM output - propulsion and steering for tail servo

The Jaycar PIC16F627-04/P microcontroller was chosen for its local availability, low cost chip and programmer, and its capabilities.

• PIC16F627-04/P - is a cheap controller which is available from Jaycar outlets in Australia. It only has 35 instructions, 0-20MHz clock, 18 pins (small), 15 I/O pins, 1k program space, 224 bytes of data and 128 bytes EEPROM (settings between power off), 3 timers, 2 analogue comparators, a serial port, sleep mode (1 uA @3V), in-circuit re-program via 2 pins, 3 to 5.5V, 2mA operating. The circuit diagram for the programmer is here

• AVR ATmega128 - See the application board for details
• OpAmp Circuits - See hyperphysics for theory, circuits and calculations.

Communications between the robot and external PC must support:

• Transmission of commands and requests for sensor readings from the external PC.
• Transmission of status changes from the robot.
• Bidirectional communications.
• Not require fixed wiring.
• Function through water from all locations in the tank and for all orientations of the robot.

• RS232 3 wire via Infrared - Serial communications using the RS232 protocol are simple and well supported in PIC devices. A single infrared detector/LED at each end of the link are all that is required to establish a connection. Direction and range could be a problem under water, so the software will need to retry communications. This is the simplest and cheapest approach that provides automonous operation.
  We need to consider power consumption.
  In order to send and receive, the detector and LED need to be mounted on the outer surface of the fish. You should check to see if you have an old mobile phone laying around with an integrated serial transceiver. TMR6101/TR2 pinout. TFDx4xxx pinout. TFDx6xxx pinout. HSDL-3201 pinout (most like the one in my old Nokia). The battery could also be of use in this project.

A useful resistor chart..

• Bluetooth -
  Cost and integration with controller?
  Since Acacia already has bluetooth connectivity to the LAN and internet, this would appear to provide a simple way to link the fish to a Linux host for monitoring and control.
• Cabling - Cheapest but least practical mechanism.

• Servo Operation of Mechanical Tail - The tail requires a cyclic push/pull force. It must be held in full left or right positions whilst the fish is moving to steer. This force will be supplied using an RC Servo mechanism. The speed will be set as a voltage from 0V to 5V which will cause the tail oscillation. The speed sensor will be used in a feedback loop in order to dampen acceleration and adjust propulsion to match the set speed.
• Braking - If speed is set to zero, propulsion will cease and water friction/turbulence will slow and stop the fish.
  This is unlikely to be effective enough if the fish is streamlined, so a better mechanism must to be developed.
• Reversing -
  ??? TBD ???.

• Air Muscle Operation of Mechanical Tail - Air Muscles appear to be ideal until you consider the air supply, valves and sensing mechanisms. They are easy and cheap to build at any size.
• Servo Controlled Side Fins - To achieve up/down and roll control, 2 servos would be required. The control of side fins is not required if the gas bladder is used for depth control and roll control is not needed.
• Water Jet Pump - The robotic fish would actually be a robotic single legged octopus if we used this type of propulsion.
• Magneto-Hydro-Dynamic Propulsion - This is a simple but impractical mechanism using the interaction of electric and magnetic fields to generate thrust directly on the water with no moving parts. It would only work in salty water (conductive) and would require superconductors for the extreme magnetic field and very high voltages.

• Depth - Measured voltage from a MPX2010DB pressure transducer. This transducter outputs a 0-25mV variation over 0-10kPa pressure differential and so will require a differential amplifier to boost the output voltage to 0-5V. We can use part of a LM348 quad op-amp for this purpose.
• Speed - Differential pressure between a front and side facing tubes will provide a measurement of speed. Another transducer as used for depth measurement will be used with an amplifier as above.

• Accelerometer - Feedback on movement
• System Voltage and Current - Energy conservation, refuel
• Speed - can be determine by:
  
  • The voltage from a generator attached to a free rotating propeller in the water stream (anemometer).
  • Some relationship to the energy expended less water resistance (not practical)
  Doppler shift of light or sonar reflected from surrounds. This is the ground speed. Differential pressure between a front and side facing tubes (see manometer).
  • Also see Speed for other options

Underwater positioning is a difficult problem to solve at any reasonable cost. Assuming that we can accurately map the sides of the tank, we still need to determine which end of the tank the fish is actually facing.

• Proximity Left/Right/Top/Bottom/Front/Rear - Provides information about distance to surrounding objects such as tank walls, surface, bottom and other obstacles. Could use sonar, infrared, electric field or just microswitches on bumper wires. The electric field is the preferred sensor at this point due to difficulties implementing other systems. To be tested.
• Electronic Compass - these devices will provide a compass bearings to varying degrees of accuracy using hall effect devices to measure the orientation of the earths magnetic field. The problem is their power consumption (30mA at 5V) and their slow response time of 2.5 sec.
• GPS - Too bulky and expensive for this application.
• Infrared - Depends on water clarity.
• Sonar - Sonar uses a transducer to create a sound which travels through the water and bounces back to one or more microphones (useually other transducers). The distance is calculated from the speed of sound in water (1438 m/s @ 8 deg C, increasing with temperature) and the time taken to hear the return echo. By using more than one microphone, the physical configuration and the slight difference in arrival time of the sound will allow a direction and a distance to be calculated. Where water density varies, sound will be refracted. The best working frequency is 20 kHz and works up to 1500 m with a 50 W pulse.
• Light Levels - Light sensors will provide the ability to both follow a light and to determine if the tank is light or dark. The sensors will be CdS cells connected to 2 comparators which convert the varying levels into 2 digital inputs for the microcontroller. Comparator #1 will be a comparator with hysteresis which output a 0 when left brightness is greater and 1 when right brightness is greater than left. Comparator #2 sums both left and right inputs and outputs a 0 when the sum is less that a preset value (dark) and a 1 when the tank is light.

This robot will sense its proximity to surrounding obstacles by selectively applying an electric field gradient to the surrounding water. It will periodically apply a voltage between the head and tail of the fish and sample voltages on the top, bottom, 2 front and 2 rear fins using a sensive high impedance volt meter.

By storing the standard fin potentials, differences can be translated to objects in positions relative to the left, right, top, bottom, front or rear of the fish.

In order to conserve battery power, proximity detection will only be used when required, with the applied potential being removed between measurements.

It is noted that some electrolysis of the electrodes may decrease the effectiveness of the proximity detection process. This could be addressed by choosing appropriate contacts and by ensuring that the software compensates for slow changes.

See MC34063A voltage converter to generate proximiting sensing voltage of 28V from 7.2V cells.

Other options include the use of sonar, infrared, electric field or just microswitches on bumper wires. The electric field is the preferred sensor at this point due to difficulties implementing other systems. To be tested.

If the average density of the robotic fish can be altered to achieve neutral buoyancy, no energy must be expended maintaining depth with propulsion systems.

Most fish regulate their depth using a gas bladder. Inflating the bladder decreases the average density of the fish and causes it to ascend. Oxygen is extracted from the water into its bloodstream using the gills and is released into the gas bladder at a fairly slow rate. The bladder is deflated by reversing the process which causes the fish to descend.

The problem is that this is a positive feedback system that can rapidly become unstable (see problems with submarines). Inflating the bladder causes the fish to ascend into lower pressure water which allows the gas bladder to expand further, accelerating the ascent. The reverse also occurs when the bladder is deflated. The water presure increases with depth, compressing the bladder and lowering the density. Without fine control and/or physical swimming effort, the fish will either plumet to the bottom or shoot to the surface. If the change in depth is large, the fish is likely to suffer physical damage to internal organs.

Thus the inflation/deflation of a gas bladder must be adjusted interatively to attain a certain set depth.

Note that natures positive feedback problem is caused by the compressibility of the gas bladder.

It is important for stability that the centre of buoyancy and the centre of gravity are at the same point, otherwise the inclination will vary when ascending and descending. If these are misaligned by even a small amount, their will be a large rotation to align these 2 points vertically.

The depth setting mechanism must provide the following:

• Ability to adjust the depth of the robot by changing its average density.
• Use minimal power to adjust depth.
• Not consume any power when depth has been reached.
• Move forward when descending and backward when ascending (without tail action).
• Mimimise positive feedback problems associated with compressibility of the mechanism.
• Regulate depth automatically when external forces are applied.
• Ensure that the mechanism limits itsself to its maximum range of mechanical movement.
• Incorporate a dead-zone to limit unnecessary hunting once depth has been obtained.

• Variable Capacity Cylinder - This is a combination of the submarine balast task and the gas bladder into what would appear a better and simpler mechanism. A large piston (60ml syringe) is linked to a pressure cylinder via a tube in a closed air system. A reversible electric motor is placed inside the air piston and drives a screw thread to vary to volume of air enclosed. The piston will not compress and so is not subject to positive feedback problems, whilst the motor may be stopped and the volume will not vary. The control circuit is linked to a pressure transducer so that the controller only needs to set the depth. Also see Electric Geared Pump and Piston ballast tank
• Let us assume that no part of the mechanical fish is compressible because the tail assembly is solid and surrounded by water and the controller is enclosed in a water tight incompressible case. This means that the density of the fish will be the [mass (kg)] divided by the [volume (m cubed) + variable capacity cylinder volume].
• Water pressure varies with depth, but so does water density to a much smaller degree, and our aim is to set the density of the fish to exactly match the density of water at the same depth.
• This is a combination of both the above options into what would appear a better and simpler mechanism. A large piston (ceringe) is linked to a pressure cylinder via a tube in a closed air system. A reversible electric motor is placed inside the air piston and drives a screw thread to vary to volume of air enclosed. The piston will not compress and so is not subject to positive feedback problems, whilst the motor may be stopped and the volume will not vary. The control circuit is linked to a pressure transducer so that the controller only needs to set the depth.
• In order to limit the travel of the piston inside the cylinder, limits in eiter software or using limit switches is required. This project will start with a software solution first and resort to limit switches if this does not work.
• In order to determine the size of cylinder required for this application, we need to specify the maximum depth that is required.
• Motor - It is important to first select the motor that will be used. It needs to turn slowly so that the volume of the cylinder is easily controlled. The physical dimensions (including gearbox) should allow it to fit inside the cylinder that will be used so that it remains protected from the environmental water. It is important to have no power to the motor when the depth is not being regulated. The operational current should be minimised to conserve battery power. The motor torque should be sufficient to operate the screw thread without stalling. Think tiny!
• Motor Choice - Stepper Motor A stepper motor rotates through a known angle for each set of pulses received. It has the advantage that we can get away without limit switches if we count pulses. It is probably better to have single limit switch so that the controller can calibrate itself at say the minimum cylinder air volume (maximim depth setting). There is a very small bipolar DELTA 15N20S stepper motor (or equivalent) inside every old CD-ROM drive, however these are 4 wire bipolar types that require 2 sets of complex H-Bridge driver circuits for each motor. We could use a single ED E1204 Bipolar Stepper Motor Controller ID. Bipolar creates 40% more torque for the same current. The L297 or L6506 (step controllers) and L298N or L293 (drivers) used together with flywheel diodes can drive a bipolar motor.
• Motor Choice - Brushed DC Motor A conventional brushed DC motor with gearbox ...
• Driver Circuit - There are a number of options for driving the motor from the controller digital outputs. Since we need forward and reverse and off control, we need to use at least 2 bits to control the motor. We need to source and sink about 30mA of motor current and drive this from low power digital outputs (1mA). The motor must run at full speed in forward or reverse to minimise power loss in control circuits and improve responsiveness.
• Driver Circuit 1 - Darlington Array - a darlington array such as the UNL2074B provides 4 darlington pairs with isolated pinouts. Each darlington pair is capable of sinking 500ma when driven by a digital input inside the 16 pin package. External clamping diodes will be required for the inductive motor load to protect the device from back EMF. No inverter is required as we can use one bit for ascend and one bit for descend. If both bits were turned on however there would be a supply short circuit through all 4 darlington pairs. On-time is 1usec and off leakage expected to be < 10uA. No pullup resistors on the inputs are required as these are built into the controller output lines. A 1sec timer will interrupt the CPU to adjust depth. Preferred for simplicity and cost.
• Driver Circuit 1a - Line Driver - The 74367 hex line driver can sink 32mA and source 5mA.
• Driver Circuit 1b - Line Driver - The 74244 or the 74HC244 octal line driver can sink 24mA and source 15mA for 130mW total.
• Driver Circuit 1c - Line Driver - preferred - The 20 pin 74HC240 octal tri-state line driver can sink 20mA and source 30mA for a total of 700mW. This option will require protection diodes.
• Driver Circuit 2 - Relays - The use of 2 x DPDT relays and 4 solid state relats was considered, but discounted due to the current demands of the coils, board space and additional inverters and other logic.
• Driver Circuit 3 - Servo Controller - The 51660L Servo Motor Controller can drive a motor in both directions using PWM input.
• Driver Circuit 4 - Stepper Motor Controller Or we could build a circuit from transistors and inverters or one out of the ULN2003 chip.
• Motor Control Circuit The depth setting (0-255) in the controller should cause the motor to be driven in the appropriate direction to ascend or descend until the pressure transducer indicates that the required depth has been reached. There should be a dead band so that the motor stops if the depth is within a set tollerance. The motor should operate automatically to counter any externally applied depth change.
• Circuit 1 - capture the pressure as an analogue input to the controller and programmatically operate the ascend/descend bits. This uses the minimum of hardware but requires periodic interuption of the main program to check and adjust depth.
• Circuit 2 - Preferred - output the required depth using 8 output lines to an 8 bit latch feeding a DAC. The output voltage is fed to a comparator circuit which also receives voltage from the pressure transducer. This circuit provides the ascend/descend bits and the dead-zone. This option uses 8 I/O lines but allows a set-and-forget approach to depth regulation. It also requires a number of hardware components including comparators.

• Gas Bladder - An option is to use a Gas Bladder with a negative damped feedback circuit to stabilise depth changes automatically. The microcontroller outputs a voltage between 0V and 5V to the depth control mechanism. A small air cylinder is connected via a reversible pump to a gas bladder (balloon) in a closed air system. The circuit runs the pump in the appropriate direction until the presure transducer indicates the required depth has been reached. Thus the depth is changed and set just by setting the control voltage. The problem with this option is the compressibility of the gas bladder, air leaks due to attempting to maintain pressure differentials and if depth changes are force on the fish, valves will tend not to operate correctly as the outside pressure may be higher than the set bladder pressure.
• Ballast Tank - Submarines pump water into and out of balast tanks to adjust the buoyancy. This could be a better solution because reversible hydraulic pumps are easier to come by than pneumatic pumps.