Data Acquisition Card (DAC)
The DAC is a PC/104 compatible card that resides between the between the PC1/04 and Wireless Card. The DAC is model MM-32-AT made by Diamond Systems. The DAC has two sets of output pins on opposite sides. The larger of the array is the analog output, and the smaller is the digital output. The DAC card is essential to several components of the AUV, as it provides a means of analog communication between the PC/104 and non-standard systems. The digital portion of the DAC is used less frequently, and is usually used to trigger relays.
There are 4 analog output pins on the DAC and 32 analog inputs. The inputs can be configured (using jumpers) as 32 single-ended, 16 differential, or 16 SE + 8 DI with 16-bit resolution. A single-ended input is a single-wire input that is measured with reference to the board’s analog ground. A differential input is a two-wire input that is measured by subtracting the low input from the high input. This type of connection offers two advantages: It allows for greater noise immunity, and it allows for the signal to float away from the board’s ground. Both the input and output pins can be programmed in unipolar or bipolar mode set to input/output ranges of 0V-5V, 0V-10V, +-5V, and +-10V. For our purposes the board is set to operate in bipolar +-5V, with all inputs set to differential.
The 4 output channels on the DAC control the four Tecnadyne 250 thrusters onboard the AUV. The signal voltage from the DAC controls the amount of power (and thus thrust) to each thruster. The built in control circuitry on the Tecnadyne thrusters recognizes -5V as full reverse and +5V as full forward. Thus, by sending the thrusters a range of control voltage in the +-5V range, the computer is able to precisely control the thruster movement.
The input channels of the DAC are utilized for several purposes. Pins 31-34
are used in differential mode to read in the voltages of two lithium polymer
power systems (thruster power and electronics power). A voltage divider circuit
(see power board), is used to scale the output of the batteries to a readable
range (between 0-5V). This is sent into the DAC where the value is scaled
back in software to restore the original battery voltage. The li-po voltages
are important values since they tell how much battery life remains, as well
as the voltage levels supplied to the thrusters. Since this value changes
over time, thruster control algorithms can be tweaked to stay stable as the
thruster supply voltages drop over the course of the competition (from roughly
The input channels (29-30) are also used for the pressure sensor input. The pressure sensor delivers a voltage between +0.5V and 4.5V, which is read through the DAC and converted in software to correlate with the AUV’s depth (in meters) under water.
Since all analog outputs of the DAC are utilized by the thrusters, the digital
outputs of the DAC are used to trip any relays necessary in the competition.
Currently, the Marker Dropper device is activated using this method. When
the computer decides to drop a marker, a +5V signal is sent through the digital
output, where it trips a Crydom relay. The relay allows raw voltage (from
the thruster supply) to cause the droppers to retract and drop the steel balls
into the targets.
Data Acquisition Card
Wireless Ethernet Card
Power Management Board
Doppler Velocity Logger