Transcript Advanced Programming for 3D Applications CE00383-3 Data Acquisition
Advanced Programming for 3D Applications CE00383-3 Data Acquisition Lecture 10
Bob Hobbs Staffordshire university
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Definition
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Data acquisition is the process by which physical phenomena from the real world are transformed into electrical signals that are measured and converted into a digital format for processing, analysis, and storage by a computer.
data acquisition (DAQ) system is designed not only to acquire data, but to act on it as well.
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DAQ and Control
• • • Control – is the process by which such as digital control signals from the system hardware are convened to a signal format for use by control devices actuators and relays .
These devices then control a system or process.
Where a system is referred to as a data acquisition system or DAQ system, it is possible that it includes control functions as well.
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Elements of a data acquisition system
Sensors and transducers Field wiring Signal conditioning Data acquisition hardware PC (operating system) Data acquisition software
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Basic elements Sensors and transducers
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Data Acquisition and Processing
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Sensors and Transducers
• • Transducers and sensors provide the actual interface between the real world and the data acquisition system convert physical phenomena into electrical signals that the signal conditioning and/or data acquisition hardware can accept.
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wiring and communications cabling
• • Field wiring represents the physical connection from the transducers and sensors to the signal conditioning hardware and/or data acquisition hardware. When the signal conditioning and/or data acquisition hardware is remotely located from the PC, then the field wiring provides the physical link between these hardware elements and the host computer.
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Signal conditioning
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Filtering
• Amplification
• Linearization
• Isolation
• Excitation
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Filtering
• In noisy environments, it is very difficult for very small signals received from sensors such as thermocouples and strain gauges (in the order of mV), to survive without the sensor data being compromised.
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Amplification
• • Having filtered the required input signal, it must be amplified to increase the resolution.
The maximum resolution is obtained by amplifying the input signal so that the maximum voltage swing of the input signal equals the input range of the analog-to-digital converter (ADC), contained within the data acquisition hardware.
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Linearization
• • Many transducers, such as thermocouples, display a non-linear relationship to the physical quantity they are required to measure. The method of linearizing these input signals varies between signal conditioning products.
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Isolation
• • Signal conditioning equipment can also be used to provide isolation of transducer signals from the computer where there is a possibility that high voltage transients may occur within the system being monitored, either due to electrostatic discharge or electrical failure. Isolation protects expensive computer equipment
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Excitation
• • Signal conditioning products also provide excitation for some transducers. For example: – strain gauges, thermistors and RTDs – require external voltage or current excitation signals.
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Functions of Acquisition hardware
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The input, processing and conversion to digital format, using ADCs, of analog signal data measured from a system or process – the data is then transferred to a computer for display, storage and analysis The input of digital signals, The processing, conversion to analog format, using DACs, Output of digital control signals
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Hardware /Links with Computer
• Ports for data acquisition – RS232 – IEEE-488 ( GPIB (General Purpose Interface Bus) – Printer port – – – Sound Card ports Specially designed BUS Cards DAQ cards
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Software
• Application software can be a • full screen • • • • interactive panel, a dedicated input/output control program, a data logger, a communications handler, – or a combination of all of these.
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Options for software
• • • Program the registers of the data acquisition hardware directly Utilize low-level driver software, usually provided with the hardware, to develop a software application for the specific tasks required Utilize off-the-shelf application software (third party packages such as LabVIEW provide a graphical interface for programming)
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PC
• Depending on the particular application, the – microprocessor speed, – hard disk access time, – disk capacity – types of data transfer available, can all have an impact on the speed at which the computer is able to continuously acquire data.
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Classification of Signals
• • The Output signal has a relationship with the physical phenomenon.
For Example, – value of e.m.f obtained from a thermocouple, has relationship with the temperature – Voltage or current output signal from transducers has some direct relationship with the physical phenomena they are designed to measure.
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Digital signals/ binary signals
• A digital, or binary, signal can have only two possible specified levels or states ; an ‘on’ state, in which the signal is at its highest level, and an ‘off’ state, in which the signal is at its lowest level.
Examples:- the output voltage signal of a transistor-to-transistor logic (TTL), Control devices, such as relays, and indicators such as LEDs,
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Digital pulse trains
• • • a sequence of digital pulses a digital pulse can have only two defined levels or states.
For Example:- Output of level indicator, Control of speed and position of a stepper motor
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Analog signals
Analog signals contain information within the variation in the magnitude of the signal with respect to time .
information contained in the signal is dependent on whether the magnitude of the analog signal is varying slowly or quickly with respect to time.
For Example:-Temperature and Pressure measurement, control hardware like a valve actuator,
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Analog DC signals
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Analog Signals Conversion
• DAQ hardware would only be required to convert the signal level to a digital form for processing by the computer using an analog-to digital converter (ADC). • Low speed A/D boards would be capable of measuring this class of signal.
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Analog Signal
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Sensors and transducers
• • • A transducer is a device that converts one form of energy or physical quantity into another , in accordance with some defined relationship.
In data acquisition systems, transducers sense physical phenomena and provide electrical signals that the system can accept. For example, – thermocouples convert temperature into an analog voltage signal – flow transducers produce digital pulse trains whose frequency depends on the speed of flow.
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Categories of Transducers
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Active transducers
transducer.
convert non-electrical energy into an electrical output signal. They do not require external excitation to operate. Thermocouples are an example of an active
Passive transducers
change an electrical network value, such as resistance, inductance or capacitance, according to changes in the physical quantity being measured. Strain gauges (resistive change to stress) and LVDTs (inductance change to displacement) are two examples of this.
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Signal Conditioning
Filtering of signals Cut-off frequency > This is the transition frequency at which the filter takes effect. It may be the high-pass cut-off or the low-pass cut-off frequency and is usually defined as the frequency at which the normalized gain drops 3 dB below unity.
Roll-off > This is the slope of the amplitude versus the frequency graph at the region of the cut-off frequency. This characteristic distinguishes an ideal filter from a practical (non-ideal) filter. The roll-off is usually measured on a logarithmic scale in units of decibels (dB).
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Low pass filters
• Low pass filters pass low frequency components of the signal and filter out high frequency components above a specific high frequency.
Signal after filters
How Computer Takes INPUT signals
31 • • Interrupts – CPU of a computer can attend to important events such as keystrokes or characters arriving at the COM port only when they occur. – This allows the CPU to execute a program and only service such I/O devices as needed DMA – Microprocessor controls data transfers within the PC (using the IN (port) and OUT (port) instructions.
– In many I/O interfacing applications and certainly in data acquisition systems, it is often necessary to transfer data to or from an interface at data rates higher than those possible using simple programmed I/O loops.
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Interrupts
• Hardware interrupts • These are generated electrically by I/O devices that require attention from the CPU.
Software interrupts • There are 256 possible interrupt types that can be generated by software.
Processor exceptions Exceptions are generated when an illegal operation is performed in software (for example divide by zero).
Programmable interrupt controller(s)
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Computer Operations
• • • • • • Memory-read : data transfer from a memory device to the CPU Memory-write : data transfer from the CPU to a memory device I/O-read : data transfer from an I/O device to the CPU I/O-write : data transfer from the CPU to an I/O device DMA Write I/O : data transfer from a memory device to an I/O device DMA Read I/O : data transfer from an I/O device to a memory device
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Communication I/O devices
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Serial Port Parallel Port PCI Bus EISA Bus
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Serial Communication
• RS-232 • USB • Synchronous and Asynchronous
37 Serial Communication Hardware • • • UART (Universal Asynchronous Receiver Transmitter) -- Translates data between parallel and serial forms – Included in Tmote microcontroller – RX, TX, and FIFO buffers Line driver -- Converts circuit level voltages to line voltages and vice versa USB controller
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Serial Communication Parameters
• • • • • • • Baud Rate Start Bit Data Bits – 5 to 8 Parity – Error check (Even, Odd, none) Stop Bit(s) Flow Control (DTR/DSR, RTS/CTS, Xon/Xoff, none) Start and Stop bits not necessary for synchronous communication
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Serial Communication Example Parameters
• • • • • Baud rate = 115200 bps 8 Data Bits No parity 1 Stop Bit No Flow Control
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Serial Communication Signals
• Transmitted Data (TxD) • Received Data (RxD) • Ground (GND)
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Serial Communication Signals
• • • • • Request To Send (RTS) – Asserted (set to 0) by sender to – prepare receiver to receive data.
Clear To Send (CTS) – Asserted by receiver to acknowledge RTS and allow transmission.
Data Terminal Ready (DTR) – Asserted by device to indicate that it is ready to be connected. If the device is a modem, this may "wake up" the modem, bringing it out of a power saving mode. Data Set Ready (DSR) – Asserted by host to indicate an active connection. Data Carrier Detect (DCD) – Asserted by host when a connection has been established with remote equipment.
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Flow Control
• • Flow control avoids overflow Can Eliminate the need for flow control by… – Regulating speeds – Packet size smaller than buffers
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Signal Processing
• • • Data acquired can be in a variety of forms DA hardware has to process the signal Device driver manages conformed signal
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Analog & Digital Signal
• Analog signal must be converted into Digital form (Discrete) before DSP techniques can be applied. The analog signal is basically denoted as x[t] or x
a
[ t ] because it varied by sinusoid (sine or cosine wave).
• The Analog signal is digitized by using Integrated Electronic Circuit device called an Analog-to-Digital Converter (ADC). The output of ADC will be in the form of binary number that represents the analog signal such as electrical voltage.
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Analog & Digital Signal
• The analog signal are always come with noise. Thus the noise filtering is needed before the signal goes to ADC. The filtering can be done by using DSP techniques.
• The special purpose microprocessors are designed to carry out application of DSP. It is named as Digital Signal Processors (DSPs) and used in real time application.
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Digital Signal Processing
DSPs are programmable devices and capable of carrying out millions of instruction per second.
It is vital to know how Digital Signal Processing work before we go to DSPs (The diagram of the process is shown in Figure 1 and 2).
The signals and systems must come together. The systems are needed to operate the signals. For example, we need to use Thermometer to measure Temperature, Microphone to carry out analog signal (human voice) and convert it to electrical signal, Charge-Couple Device (CCD) used in in Camera or Digital Camera to convert image to picture and so on. In general, the system is characterized by the type of operation that it performs on the signal.
Discrete Signals
47 Figure 1 : Digitized process of signal
48 Figure 2 : Complete Process of Digital Signal
49 • From the diagram, it can be seen that ADC and DAC are 2 vital devices used in signal processing to convert the signal from analog to discrete (digital) and vice versa.
• ADC is basically consists of Sampler, Quantizer and Coder. All this elements are built up by CMOS Switched-Capacitor (for Sampling), Op Amp (Signal Amplification) & Comparator (Quantizer).
50 • Quantization is the conversion of discrete time continuous-valued to discrete-time discrete- valued (digital) signal. The difference of this is called Quantization Error .
• The coder in ADC will convert the output of the Quantizer to b-bit binary sequence that can be read by DSPs (Digital Signal Processors).
• The DAC, will perform a reverse operation of ADC in order to generate back analog signal.
51 • DSP CAN BE APPLIED IN THE FOLLOWING FIELDS : => COMMUNICATION SYSTEMS (MOBILE PHONE , SATELLITE, RADAR, SONAR, INTERNET) => ENTERTAINMENT ELECTRONICS (RADIO, TV, Hi-Fi, CD/VCD/DVD PLAYER,MP3), => MULTIMEDIA ( ACOUSTICS, IMAGE, SPEECH RECOGNIZATION & VIDEO CODING, DIGITAL CAMERA),
52 => MEDICAL INSTRUMENT (ECG – Electrocardiogram – provides information about the condition of patient’s heart), => GEOPHYSICS (Seismology) ( Apparatus used to measure earth movement ) => INSTRUMENTATION (ELECTRONIC TESTER Such as Digital Multimeter, Oscilloscope) => IC TECHNOLOGY => DATA COMPRESSION
Signal and noise
53 Signals convey relevant information… but what is relevant can be very relative!
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Continuous and discrete signals
Continous signal x a (t) Discrete signal (sequence) x[n] x[n] = x a (n T ) f s T = 1/ T : sampling period : sampling rate
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Basic sequences
Unit impulse Unit step Sinusoidal Exponential Periodic Random
Examples of digital signals 56 Periodic .
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Random
Speech signals… 57 Time
one more…
58 Time
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more examples…
more examples… 60 Earthquake prediction???