Module 5: Digital Techniques and Electronic Instrument Systems

5.3 Data Conversion

Analog and digital signals

Analog signal: can take all possible

values.0 1 2 3 4 5 6 7 8 9 10

-2

-1

0

1

2

Digital Signal: can take specific values (discrete values).

0 1 2 3 4 5 6 7 8 90

1

2

3

4

0 2 4 6 8 100

2

4

6

0 2 4 6 8 10 12 140

2

4

6

2

Analog and digital circuits

3

Analog circuits: Input and output signals are analog.

Digital circuits: Input and output signals are digital.

0 1 2 3 4 5 6 7 8 9 10

-2

-1

0

1

2

0 2 4 6 8 10

-1

-0.5

0

0.5

1

1.5

-1 1 3 5 7 9 11 13 150

2

4

6

0 2 4 6 8 10 12 14 160

2

4

6

Digital circuits advantages

4

High reliability Easy design High flexibility Easy storage

Digital signals on the Aircraft Digital signals on the

aircraft: Discrete values, usually

binary values: on / off, high / low.

Examples: Is the aircraft on the ground or on

the earth? The Air Data Computer calculates

the airspeed, the altitude, the temperature etc. from data sensor inputs. These inputs are send to a digital bus (ARINC) in a series of binary numbers. These output serves as an input for cockpit instruments, such as the PFD.

Types of Computers Interactive Computers:

The plane crew interfaces with the computer. Example: Control Display Unit

(CDU).

Reference Computers: Provides information used by

other computers. Example: Inertial Reference

System. (IRS – calculates airspeed, heading, attitude etc.). Information is provides to other systems, such as Auto-throttle computer, FMC etc.

Types of Computers Storage Computers:

Store information such as flight roots, total fuel, overspeed threshold, etc.

Controlling Computers: Control a specific device.

e.g. Flap Electronic Unit (FEU). Takes input from the Flap Lever and gives input to the mechanism that extends or detracts the flaps, when the airspeed is within the allowable values.

Informational Computes:

Provide information to the crew.

e.g. ECAM (Electronic Centralized Aircraft Monitoring).

A/D & D/A Converters

Temperature is an analogue value. It must be converted to discrete value, to be displayed in a digital computer.

The user input signal to drive a flight control surface is digital. However, the signal that will drive the control surface must be analog.

D/A Converters

The circuit is called “Binary Weighted Ladder”.

3-bit Digital Input

Analog Output

A/D: Sampling Resolution of A/D

conversion depends on: Sampling rate Number of bits used

in conversion and output voltage range.

An mp3 of 44000HZ and 192kbps. Sampling rate: 44000

samples per sec. 192Kbps: Affects the

size of the discrete areas.

D/A & A/D Converters Specifications Accuracy:

The actual output vs. the converted output. E.g. if the maximum output of the converter is 10V and the error

is ±1%, then the maximum error for any voltage output is 10mV. Resolution of A/D converters:

The number of steps the input is divided into. Is expressed in power of 2. A 12-bit A/D converter divides the analog signal in 212 steps.

Sample and Hold A/D converters: Freezes the analog input at the moment a new sample is taken.

Throughput: The maximum number of bits “produced” or “consumed” per

second (the bit rate).