I/O Breakout Board
Description:
The I/O breakout board (IOB) will be used to house vehicle-specific sensors and actuator circuits.
This module will be unique for each vehicle application, and only needs to connect to the I/O Controller
(IOC) through two external headers (Analog and Digital). A typical IOB may include connections for
analog sensors, headers for PWM signal outputs, inertial/spatial sensors on an SPI or I2C bus, etc, and
route these signals to the IOC / IOB header.
Although normally it will be up to the vehicle engineers to make this board, an IOB will be
designed and manufactured so that the entire system can be tested and verified to be working properly. In
the case of the UAV, the IOB will contain the sensors used for the acquisition of flight parameters such as
altitude, pressure, temperature, and position. The UAV IOB will also break out PWM I/O in order to
control the UAV control servos.
Metric
Units
Target
I/O Peripherals connect externally
Provide interface to telemetry
Yes/No
Yes/No
voltage
range
#
(list)
#
Yes/No
Yes/No
Yes/No
Yes
Yes
Voltage ranges of analog sensor outputs
Number of analog channels
Protocols Supported
Number of Concurrent Digital Peripherals
Peripherals are easily detached or disconnected
Manual override exists
External Payload interface specification and interface exists
Diff 1.8-4.5V
3
min set
2
Yes
Yes
Yes
Figure 1 – Engineering specs associated with I/O breakout board
When considering P10236 – Configurable Control Platform for Unmanned Vehicles - the
engineering metrics in Figure 1 can been be used to outline IOB specifications. The controller should be
compatible with the previous MAV 2 project sensors and the servo motors in the concurrent Airframe C
project. Therefore all the sensors used by MAV 2 must be included on the IOB in order to test/meet
engineering specs. This basic set of sensors is also the minimum set of components that P10236 will
support and can be viewed below.
DEVICE LIST FOR UAV IOB
Analog Devices IMU
Tyco Electronics GPS
Airspeed Differential Pressure Sensor
Altimeter Absolute Pressure Sensor
Thermistor
Figure 2 – List of Sensors from MAV 2 to be included.
The P10236 system will need to be installed in a vehicle which may need its sensors permanently
mounted. As a result, it is necessary to allow the IOB to be easily and readily disconnected from the
sensors and actuators. No meet this need, the servo outputs and receiver inputs will connect via header
pins on the sides of the IOB box. The pressure sensor tubes will protrude from the sides of the box and
connect to the sensor tubes with a coupler. This configuration allows the IOB to be completely
disconnected and removed from the vehicle without the need to remove the peripherals.
Analog Peripherals:
The I/O Controller provides up to eight, 16-bit, differential ADC channels. The full-scale reference
voltage can be set for each input using a programmable 8-bit DAC, which drives the full-scale reference
input of the ADC. The ADC channels therefore have the following interface requirements:
Diff V_IN
Abs( V+ - V-) +/- 3.1 V
V_IN max. V+ or V-
5.1 V
V_IN min.
- 0.1V
V+ or V-
In order to meet the input requirements, any analog output whose voltage may exceed 5.1V must be
divided down before connection to the ADC. The absolute minimum voltage of -0.1V will be met by
using the analog ground provided by the IOC as the negative-most potential of the sensor ICs.
In order to take full advantage of the digital range of the ADC (both + and – differential voltages), the Vinput will be supplied with the median output voltage (the voltage in the middle of the dynamic output
range of the sensor). This reference is generated (in this implementation) through the use of a voltage
divider network.
For example. An analog sensor with an output range of 3-5.5V must first divide the output by 1.1 in
order to meet the absolute voltage limit of the ADC. The output full-scale range then becomes 2.72 – 5V,
with the median output being (5 + 2.72)/2 = 3.86V. A 3.86V reference signal would be generated by a
voltage divider network from a 5V source and fet to the V- input of the ADC. With a differential range of
+/- 1.14V relative to V-, the DAC would be programmed provide 1.14V as the full-scale differential
reference voltage to the ADC, therefore maximizing the accuracy of the digital conversion.
Figure 3 – Reference Voltage Generation Divider
Figure 3 is the designed voltage divider that produces Vin- which is determined as follows for each sensor:
((Vo,max + Vo,min) / 2) = VThe above equation yields the median output voltage (a value halfway between max and min output
voltages). This value will be the negative input voltage to create a differential input for the ADC.
Airspeed Sensor:
(4.7V + .2V)/2 = 2.45V
Air Pressure Sensor:
( 4.5V + .5V)/2 = 2.5V
Temperature:
The output voltage values for temperature need to be determined before calculating V in-. These
maximum and minimum voltages were calculated by finding the thermistor resistance at the two
extreme operating temperatures and then calculating the output voltage of the thermistor voltage
divider circuit. A formula for determining thermistor resistor is given in the components
datasheet. Assume a 5V input and a 10kΩ resistor for the voltage divider (the recommended
setup).
Temperature(⁰C)
-80
120
Resistance
3.58MΩ
481Ω
Vout
4.986 V
0.229 V
Figure 4 – Operating temperature characteristics
These values lead to the following value for Vin-.
(4.986V + .229V) / 2 = 2.493V
For circuit simplicity a voltage of 2.5V will be used for Vin- for all sensors.
Next the full-scale differential reference voltage must be calculated. V ref is calculated by halving the full
scale span of each sensor (from data sheets). The full scale span is the difference between maximum
output voltage and minimum output voltage. Each sensor will have its own reference voltage for A/D
conversion, set by the DAC onboard the IOC when conversion is taking place.
Sensor
Altimeter
Air Pressure
Temperature
Vref
2.25V
2.0V
2.38V
Figure 5 - Vref for each sensor (calculated by (Full Scale Span)/2)
Number of Analog Channels ,Protocols Supported and Number of Concurrent Digital Peripherals:
Digital Interface
The UAV IOB will contain two digital peripherals, the GPS and IMU (inertial measurement unit). The
protocols required are SPI and UART (NMEA). Since the IOC supports these protocols by design, the
IOB only need route the digital signals to the corresponding pins on the Digital I/O header to the IOC.
Provide Interface for Telemetry:
The telemetry module will be connected as another digital I/O peripheral. The requirement for this
interface is that the telemetry system use SPI, I2C, or a UART protocol in order to transfer data between
itself and the IOC. The UAV IOB will provide a DB9 header outside the case, which will be routed to
one of the digital I/O blocks.
Manual Override:
A manual override of the control system will be necessary during flight in case of control system failure
or some other need for manual control of the vehicle. This override will be controlled through the use of
two, quad 2-to-1 MUX ICs, switched by one of the spare receiver channels. This puts the manual override
under sole control of the human pilot, regardless of the control system state. The MUX is powered by the
RX system, making it impervious to control system power failures.
External Interface:
The CCP/IOC/IOB boxes will be standard aluminum (or similar) project boxes. An area inside the plane
will be fitted with a bottom plate. The project boxes and plate can be drilled in any desired configuration,
and attached via nut and bolt. This provides mounting flexibility to the aircraft for CG adjustments, while
still providing a firm mount in the aircraft.
NOTE: DB-9 CONNECTOR AND ROUTED MUX SWITCH SIGNAL NOT SHOWN
Packaging:
The IOB case in will be an off the shelf aluminum project box. The appropriate holes will be cut to fit the
selected connectors and sensors. This will allow for greater flexibility in IOB layout rather than selecting
a box with predetermined connector locations and other such requirements. All connectors have not yet
been specified but distributors have been bookmarked and lead times have been researched so any risk
due to this selection has been mitigated. The connector which will be used for the hobby servo
connection as well as receiver pack has been selected. These will both be 3 row, 24 position right angle
Molex connectors. Each of the eight columns will contain one ground, one power and one signal line.
The pressure sensors will have couplings on the ends in order to allow connection without disassembling
the box. The tubes connected to the differential airspeed sensor will go to a pitot static sensor mounted on
the wing of the plane which reads dynamic and static pressure values. The altitude pressure sensor may
have one tube for static pressure measurement inside the plane.