Start
•
Mecahnical Assembly •
GPS - Digital
Compass •
Proportional Servo •
Circuit Description •
Software •
Test Modes •
Crossing the Atlantic •
Sources
Circuit
Description
The
electronic circuit receives power from the 12-V battery
through the main switch SW1. When SW1 is on, the green
led (D1) lights to show that the power is on.
Twelve
volts are applied directly to the electric motor,
under control of IC4-PTC2, through the buffer IC3f
and the electronic switch TR1. C11 helps reduce the
noise generated by the motor when in use.
The
rest of the circuit is powered with 5 V through the
voltage regulators IC1–C2 and IC2–C3. The
reason for using two identical regulators is to isolate
the servo so the noise spikes that it generates are
not passed to the rest of the circuit. So, IC2 powers
just the servo and IC1 powers the rest of the circuit.
The
M68HC908GP20 (IC4) is the core of the circuit. The
Roboat’s controller works with a frequency of
4.9152 MHz generated by the oscillator X1, IC3a, R3,
C7, C8.
C4
and C5 are mounted close to the power-supply pins
VDD and VDDA, to prevent noise problems. A stronger
filter for the pin VDDAD is provided by R4 and C6,
in order to prevent noise that might affect the A/D
conversion.
SW2
and C7 provide the manual reset to IC4. The pins IRQ,
RST, PTA0,7, and PTC0,1,3 of IC4 are connected to
the MON08 connector, which provides a link (via a
16-wire flat cable) with the ICS08GP20 board. This
arrangement enabled me to control the Roboat directly
from a PC for testing purposes and whenever new waypoint
coordinates need to be loaded.
When
the link isn’t in use (i.e., when the Roboat
is sailing autonomously), a proper socket must be
fitted to the connector instead of the flat cable.
IC4-PTD4 pin generates the PWM signal that controls
the servo and is connected to the control input of
the servo through the buffer IC3b.
The
IC4-PTE1 pin receives the serial signal from the GPS
after the signal is inverted by IC3d. IC4-PTC4 pin
controls the GPS powerdown mode through the buffer
IC3e. IC4-PTD6 controls the red LED (D2) through the
buffer IC3c and lets it flash every time the GPS receives
a valid position.
The
digital compass analog outputs, as well as R8 (the
trimmer for centering the rudder), are connected to
the A/D converter inputs of IC4 (pins AD0, AD1, AD2)
through the low-pass filters R5-C10, R6-C11, R7-C12.
The
construction of the PCB has some peculiarities. First
off, IC4 is mounted on a custom socket that converts
the 44-pin QFP geometry into a 22 + 22 pin DIL, with
the more practical pin spacing distance of 0.1?.
The
socket is built with a small PCB (32 × 57 mm), single
sided, with the copper traces facing upwards. IC4
is soldered on top of the PCB and two rows of 22 pins
are soldered at its sides, facing downwards. This
assembly is mounted on the main PCB, in the two rows
of 22 + 22 SIL sockets.
The
GPS is mounted on the PCB main board with four screws,
bolts, and spacers. The GPS standard output is made
of 12 individual wires, only 10 of which are actually
used. To obtain a cleaner assembly, the 10 wires were
assembled into a 5 + 5 flat-cable socket. The socket
is finally connected with the 10-pin connector (K4)
on the main PCB.
The
soldering instructions provided with the digital compass
warn to take special care in soldering the device,
because overheating will damage the sensor. To avoid
any risk, the digital compass used in this project
was not soldered, but rather it was mounted into a
couple of 3 + 3 SIL sockets previously soldered in
the main PCB.
When
the Roboat works autonomously and isn’t linked
to the ICS08GP20 board, suitable jumpers must be fitted
on the MON08 connector instead of the flat-cable connector.
This is more easily accomplished by a double row of
16-way sockets soldered on a tiny PCB 20 × 5 mm.
The
potentiometer R8 is fixed to the PCB with washers
and its terminals are soldered directly to the PCB
copper tracks.
