RoverSat Electronics
Colorado SpaceGrant
Jesse Wilson
Summer, 2003
1. Design
The rover's controller board is a small custom-designed PC board. Its two purposes are to
support the CPU and connect the CPU to the rover's devices. While many teams that used a
microcontroller used an OEM development board, we chose to construct our own. This
resulted in a board that is smaller, weighs less, and is more suited to our purposes. While
most OEM development boards are designed to survive a garage workbench at worst, we
could also ensure that all the components we used would be rated for the expected operating
environment, reaching temperatures as low as -40 C. We used the software package CadSoft
EAGLE to design the controller board. When it came time to build the PC board, the same
program was used to generate CAM files that were sent to a prototyping house. About a
week later, we received our board in the mail, ready to solder all the components onto.
2. Components
a. Core Components
At the core of the controller board is an Atmel MegaAVR processor. It is an 8-bit
RISC processor with 32 KB of flash RAM on the chip. Running at 8 MHz, it is
capable of providing more than adequate performance for controlling the rover. We
selected a low-power, industrial-rated version of the ATMEGA-32L, capable of
operating at temperatures as low as -50 C.
Two Energizer lithium photo batteries supply power for the rover. Running in
parallel, they can provide power to the rover for about 5 hours-- plenty to fill the
camera's memory with pictures after landing. According to Energizer, these batteries
will supply power even at -40 C. Thus we can avoid the extra weight of heating
elements and still be sure the rover will function the entire flight.
Power from the batteries is fed through a +5V regulator before feeding into the
processor. Another +5V regulator sits on the camera, supplying the sensor array. A
+3V regulator supplies power to the camera. All servos are fed the raw +6V from the
batteries, routed through a Toshiba Darlington pair driver IC first. The only two
components not rated to function in the low temperature environment are the +3V
camera regulator and the driver IC, since we do not wish to operate the camera or the
servos until the rover has landed and warmed up.
b. Electromechanical Components
The rover uses four servos: two to drive around, one to move the camera, and fourth
to operate the release mechanism. The camera servo is a lightweight servo
manufactured by Grand Wing Servo (GWS). Its typical application is for moving
control surfaces on model RC airplanes. The other three servos are stronger and
heavier, with metal gears instead of plastic. Since servos are designed for a limited
range of motion, the drive servos had to be modified. The procedure involved
removing a metal tab in the gears and calibrating a position-sensing potentiometer
inside. The release servo also needed to be modified for continuous rotation. But the
release servo only needs to be turned on for a second or so, whereas the drive servos
require more precise operation. Thus the drive servo was also modified so that the
control electronics were bypassed, providing simpler control for the CPU.
c. Sensors
The rover has several sensors to provide it with information about its environment. A
small thermistor on the controller board provides information on temperature. On the
camera is mounted an array of photo-resistors that can be used as a makeshift
compass on the ground, figuring direction from the sun's position in the sky. An
infrared emitter and detector on the top of the camera facing forward allows for
detection of obstacles in front of the rover.
The most important sensor is the camera. We selected an Aiptek PenCam, available
in consumer electronics stores for about $80. While it is not that great of a digital
camera for consumer use, it is small, lightweight, inexpensive, and easy to modify for
control. The camera was disassembled, leaving only the circuit board and lens
assembly. Power is fed through a +3V regulator, controlled by the driver IC. This
driver allows us to turn the camera on only when it is being used, saving batteries.
This was most useful, given that the camera turned out to be one of the largest
consumers of power, in addition to having a bad habit of spontaneously developing a
short from time to time. Our power control technique prevented this from consuming
the rover's batteries. The camera also had to be modified so the shutter could be
controlled with an electrical signal instead of a mechanical switch. So the switch was
removed, and a wire leading to the controller board was soldered in its place.
3. Controller Board
Figure 1 shows the board schematic, while figure 2 shows the layout. The most difficult
design challenge was to figure out how to lay out so many connections to peripheral
components in such a small space. The connection headers are described in figure 3.
Figure 1
Figure 2
Camera/Sensors
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Sensor array ground
Sensor array +6V
Forward light sensor
Infrared emitter
Rear-left light sensor
Infrared detector
Rear-right light sensor
Camera/USB ground
Camera/USB –Data
Camera/USB +Data
Camera/USB +5V
Camera shutter control
Camera power
Unused
1
2
3
4
5
6
7
8
9
10
11
12
Release servo +6V
Release servo ground
Unused
Left drive servo +6V
Left drive servo ground
Left drive servo signal
Right drive servo +6V
Right drive servo ground
Right drive servo signal
Camera servo +6V
Camera servo ground
Camera servo signal
1
2
3
4
5
6
Status light ground
Status light signal
Status light 2 ground
Status light 2 signal
Launch switch ground
Launch switch signal
Servos
Misc. Devices
USB Connector
1
2
3
4
Figure 3
Ground
-Data
+Data
+5V
In-System Programmer
1
2
3
4
5
6
MISO
+5V
SCK
MOSI
~RESET
Ground
Parallel Port
1
ACK
2
Data 0
3
Data 1
4
Data 2
5
Data 3
6
Data 4
7
Data 5
8
Data 6
9
Data 7
10 STRB
11 Ground
Figure 3 (continued)