PC-based data acquisition I
FYS3240
PC-based instrumentation and microcontrollers
PC-based data acquisition I
Spring 2015 – Lecture #8
Bekkeng, 29.1.2015
General-purpose computer
• With a Personal Computer (PC) we mean a general-purpose
computer. Such systems are easy to expand with more
memory, and more I/O ports etc.
• A PC is designed to be able to run all kind of application
programs that you can buy or intend to develop. A generalpurpose computer need to be ready for new device drivers and
software to run hardware it doesn't know about yet, like new
printers or hard drives, and it need to run different application
programs.
• The PC usually need to run several programs at the same time
on the CPU by sharing CPU time between the different
applications (multitasking), or by running different applications
in parallel on different CPUs or different CPU cores. The typical
PC today have two or four CPU cores, but can have up to 20
cores with ten CPU cores on two CPUs
General Purpose Operating Systems (OS)
• Windows, Linux, MacOS, Unix
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Processor time shared between programs
OS can preempt high priority threads
Service interrupts –keyboard, mouse, Ethernet…
Cannot ensure that code finish within specified time limits!
Data acquisition (DAQ)
•
Data acquisition involves measuring signals (from a real-world
physical system) from different sensors, and digitizing the
signals for storage, analysis and presentation.
•
Analog input channels can vary in number from one to several
hundred or even thousands
Computer-based DAQ system:
NI SC DAQ
Overview of a PC-based Data
acquisition (DAQ) system
A DAQ system consists of:
• Sensors (transducers)
• Signal Conditioning
• Cables
• DAQ hardware
• Drivers
• Software
www.ni.com
DAQ Device
– Most DAQ devices have:
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Analog Input
Analog Output
Digital I/O
DAQ Device
Counters
– Frequency measurements (digital edge counting)
– Angular measurements from angular encoders
(quadrature encoders)
– Connects to the bus of your computer
• PCI, PXI, PCIe, PXIe, or USB
Computer
FPGAs in PC-based DAQ-systems
• DAQ-cards with a programmable FPGA
– Multi-rate sampling
• Allows different sampling frequencies on the I/O channels
– For comparison, when using an “ordinary” DAQ-card
(without a user reconfigurable FPGA) all channels must
have the same sampling frequency
• User defined processing in the FPGA
• FPGA-based hardware timing/synchronization
– Remember that without an external driver/buffer the current output
(source) from an FPGA output pin might not be able to driver your
external electronics!
Common Applications for FPGAs in
DAQ and control systems
Video
Camera
• High-speed control
• Hardware programmable DAQ-cards
• Onboard processing and data reduction
Host PC
Video Frame grabber
FPGA
– e.g. video processing
Image processing etc.
• Co-processing
– offload the CPU
FFT
Hybrid DAQ & signal processing
architecture examples
•
Numerical Computing
– multicore computer (CPU)
– GPU computing system
•
DAQ
– multicore computer (CPU)
– FPGA-based data acquisition
– FPGA-based co-processor
PXI
• PXI = PCI eXtensions for Instrumentation.
• PXI is a high-performance PC-based platform for
measurement and automation systems.
• PXI was developed in 1997 and launched in 1998.
• Today, PXI is governed by the PXI Systems Alliance (PXISA), a
group of more than 70 companies chartered to promote the PXI
standard, ensure interoperability, and maintain the PXI
specification.
PXI
• PXI systems are composed of three basic components:
– Chassis
– Controller
– Peripheral modules
PXI chassis
• The PXI chassis contains the backplane for the plug-in DAQ
cards
• The chassis provides power, cooling, and communication
buses for the PXI controller and modules.
• Chassis are available both with PCI and PCI Express
• 4 – 18 slots chassis are common
PXI controllers
• PXI Embedded Controller
– Can run Windows or/and real-time OS
• Laptop Control of PXI
– Using e.g. ExpressCard serial bus
• Desktop PC Control of PXI
> 800 MB/s possible (MXI bus)
PXI-based DAQ systems
• PXI-based data acquisition systems include a more rugged
packaging suitable for industrial applications.
• PXI systems offer a modular architecture
– Possible to expand the DAQ system far beyond the capacity of a
desktop computer.
PXI triggering and timing
•
•
One of the key advantages of a PXI system is the integrated timing and
synchronization.
The PXI chassis includes reference clocks, triggering buses and slot-toslot local bus.
– Any module in the system can set a trigger that can be seen from any
other module.
– The local bus provides a means to establish dedicated
communication between adjacent modules.
VISA
• VISA = Virtual Instrument Software Architecture.
• NI-VISA is the NI implementation of the VISA standard.
• LabVIEW instrument drivers are based on the VISA standard,
which makes them bus- and platform-independent.
• Supports communication with instruments via:
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GPIB
Serial
Ethernet
USB
PXI
NI-DAQmx
– NI-DAQmx (multithreaded driver) software provides ease of
use, flexibility, and performance in multiple programming
environments
– Driver level software
• DLL that makes direct calls to your DAQ device
– Supports the following software:
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NI LabVIEW
NI LabWindows CVI
C/C++
C#
Visual Basic .NET.
NI Measurement & Automation
Explorer (MAX)
Icon on your
Desktop
• All NI-DAQmx devices include MAX, a configuration and test utility
• You can use MAX to
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Configure and test NI-DAQmx hardware with interactive test panels
Perform self-test sequences
Create simulated devices
Reference wiring diagrams and documentation
Save, import, and export configuration files
Create NI-DAQmx virtual channels that can be referenced in any
programming language
MAX Example
LabVIEW Express: DAQ assistant
Using the the DAQ
assistant is the easy
way to configure and
read from a DAQ card!
LabVIEW - Sequential DAQ design
• Configure
• Acquire data
• Analyze data
• Visualize data
• Save data
LabVIEW: Low-speed DAQ
• Sequential architecture
• DAQ assistant Express VI used in the block diagram
• Data written to file using the Write to Measurement File
Express VI
DAQ Assistant -> standard VIs
LabVIEW: Medium-speed DAQ
• Example: Cont Acq&Graph Voltage -To File (Binary).vi
• Sequential architecture
• Standard VIs used, and data written to a binary file
Create
header
Create file
Write
Create
analog
input
channel
Set
sample
rate
Start
acquisition
Read data
Close file
Stop
acquisition
High-speed DAQ
• Based on the producer-consumer architecture
– Parallel programming architecture
Producer loop
Hardware timing, since no Wait
function is used in the producer
loop. The producer loop rate is
given by the DAQ-card setup
𝑓
𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑟 =
Consumer loop
𝑠𝑎𝑚𝑝𝑙𝑒 𝑟𝑎𝑡𝑒 (𝐻𝑧)
𝐷𝐴𝑄 𝑐𝑎𝑟𝑑 𝑏𝑢𝑓𝑓𝑒𝑟 𝑠𝑖𝑧𝑒
Producer – consumer DAQ Example
When we have multiple tasks that run at different speeds and cannot
afford to be slowed down.
Hardware timing; no Wait function is used in
the producer loop. The producer loop rate is
given by the DAQ-card setup:
𝑓
𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑟 =
𝑠𝑎𝑚𝑝𝑙𝑒 𝑟𝑎𝑡𝑒 (𝐻𝑧)
𝐷𝐴𝑄 𝑐𝑎𝑟𝑑 𝑏𝑢𝑓𝑓𝑒𝑟 𝑠𝑖𝑧𝑒
Transferring Data from DAQ-card to
hard drive
• Acquired data are stored in the hardware's first-in first-out
(FIFO) buffer.
• Data is transferred from the DAQ-card FIFO buffer to PC RAM
using interrupts or DMA, across a dedicated PCI/PCI Express
link, and across the computer I/O bus.
• The samples are then transferred from RAM to hard drive via
the computer I/O bus and PCI/PCIe.
Data acquisition card
Analog signals
ADC
sclk
FIFO
buffer
MB
DMA/Interrupt transfer
Digital Data
RAM
GB
PC
Hard drive
TB
LabVIEW DAQ - hardware setup
• When the sample clock (DAQmx Timing.vi) is
configured, DAQmx configures the board for
hardwared-timed I/O
– DAQ card sample clock or external sample clock
• By enabling continuous sampling DAQmx
automatically sets up a circular buffer in RAM
• DMA is the default method of data transfer for
DAQ devices that support DMA
Data acquisition card
Data
From sensor
ADC
sclk
FIFO
buffer
DMA transfer
Data
To PC buffer
Circular
buffer
RAM (in the PC)
PC
buffer
Application
memory
DAQ data overwrite and overflow
• An overwrite error indicates that information is lost and occurs
when the software program does not read data from the PC
buffer quickly enough. Samples that are written to the circular
PC buffer are overwritten before they are read into the
application memory.
– Solution: use Producer-Consumer architecture.
• An overflow error indicate that information has been lost
earlier in the data acquisition process. Overflow errors indicate
that the First In First Out (FIFO) memory buffer onboard the
data acquisition card has reached its maximum capacity for
storing acquired samples and can no longer accept new
samples. An overflow error is symptomatic of a bus transfer
rate that falls short of the requested data input rate.
– Solution: use a Direct Memory Access (DMA) transfer mechanism.
How Is Buffer Size Determined in
LabVIEW DAQmx ?
• If the acquisition is continuous (sample mode in DAQmx Timing.vi
set to Continuous Samples), NI-DAQmx allocates a PC buffer equal
in size to the value of the samples per channel (gives the number
of samples to acquire) property, unless that value is less than the
value listed in the following table. If the value of the samples per
channel property is less than the value in the table below, NIDAQmx uses the value in the table.
Sample Rate
PC Buffer Size
No rate specified
10 kS
0–100 S/s
1 kS
101–10,000 S/s
10 kS
10,001–1,000,000 S/s
100 kS
>1,000,000 S/s
1 MS
• You can override the default buffer size using the function DAQmx
Configure Input Buffer.vi
Advanced DAQ with multiple while loops
A DAQ program usually have several
while loops running in parallel, and data
(and messages) should be distributed
between the loops using queues
Black
Blue
Red
: Messages transfer (using queue)
: Data transfer using Queue (NB: queues have memory – no data is lost)
: Data transfer using Notifier (NB: notifiers do not have memory/FIFO)
Sample project in LabVIEW – queued message handler
Remote control and data distribution
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Remote Control
– Enabling another computer to connect to the experiment and
control that experiment remotely.
•
Distributed Execution
– A system architecture that shares the acquisition and analysis of
the test among several computers.
Distributed DAQ examples
PC
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Remote DAQ
– Transfer data from a remote DAQ device to a
single PC for display and storage
Networked (distributed) DAQ
– Distribute measurement data to several clients
connected to a network
– Enable a central computer to acquire all of the
data from several machines and then process
or store that data
Ethernet
uC/FPGA
& ADC
Clients
data
Noise reduction techniques
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Correct amplifier gain, more ADC bits, oversampling
Position noise sources (e.g. motors and power lines) away from data
acquisition device, cable, and sensor if possible
Place data acquisition device as close to sensor as possible to prevent
noise from entering the system
Twisted pairs, coax cable, shielding
Software Filtering (e.g. averaging)
Coax cable
Examples: Cameras and machine vision
Thermal images
Tracking
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