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EXM7
Experimental methods E181101
Data acquisition
Protocols
A/D conversion
Labview
Rudolf Žitný, Ústav procesní a
zpracovatelské techniky ČVUT FS 2010
EXM7
Data transfer
Typical arrangement of measuring chain consists in probes (Pt100, thermocouples, strain
gauges…), each driven by its own module (e.g. current source, Wheatstone bridge). Analog signals
(voltages 0-10V, current 4-20 mA) are connected to combined multiplexer (switching channels) and
analog/digital converter 12-18 bits. This device is controlled via serial or parallel bus
USB,RS232,IEEE488 to a PC controller.
interface
IAE232,425
IEE488
Pt100
bridge
Pt100
bridge
PC
Multimeter
MUX+A/D
converter
DIGITAL PART – transmission
Pt100
SERIAL/PARALLEL, by wires, optical fibres,
IR, elmag.waves (Bluetooths)
bridge
ANALOG PART current loop 4-20 mA, voltage 10V
Instrument
mass balances
pH meter
power meter
thermostat
Rudolf Žitný, Ústav procesní a
zpracovatelské techniky ČVUT FS 2010
interface
IAE232,425
IEE488
PC
EXM7
Multiplexer MUX
I0
out
I1
sel
MUX selects one of many analog input signals and forwards the selected input
into a single line. A multiplexer of 2n inputs has n select lines, which are used to
select required connection.
EMR (electro mechanical relays) or SSR (solid state relays –e.g. using MOSFET
transistors) are used for switching of input/output connections
SSR exhibits a longer operational life (EMR typically only 106 cycles, SST 1010).
SSR will almost always exhibit higher input to output isolation voltages than an EMR. Important e.g. in telecom
applications design.
Package Dimensions are less for SSR.
EMR have resistance in the range of 100 milliohms, whereas SSRs have an On Resistance in the range of 10
Ohms. This is the reason why EMR allows for greater load current capability and less signal attenuation.
EMR have an output capacitance of less than 1 picoFarad, whereas SSRs typically have a capacitance of greater
than 20 picoFarads. EMRs are therefore a better option for HF(high frequency) applications.
EXM7
A/D conversion
IN
ADC
OUT
Analog signal (voltage in the nominal range 10V) is converted to
integer number represented by 8 to 24 bits. Number of bits
determines resolution of A/D converter (for 10V range):
12bits
14bits
16bits
18bits
resolution 2.4 mV
resolution 0.61 mV
resolution 0.15 mV
resolution 0.04 mV
There are several ways how to realize this conversion: for example using a discharged capacitor with
continuously decreasing voltage that is compared with measured voltage at precise time clocks.
Number of clocks corresponding to equality of voltages is the result of conversion.
Example: NI USB-6281 18 bit
16 analog inputs (18-bit), 625 kS/s single-channel (500 kS/s aggregate)
2 analog outputs (16-bit, 2.8 MS/s); 24 digital I/O (8 clocked); 32-bit counters
Accuracy 1 mV at +-10 V range
Accuracy 0.028 mV at +-10 mV range.
EXM7
Data transfer
Serial data transfer
8bits (character) are converted to sequence
RS-232 is a specification for serial communication with scientific instruments
(multimeters, weights, analyzers,…) and peripherals such as printers using only 3-5
wires (transmit, receive, ground). It is possible to connect and control only one device
at a time. Slow interface with typical data rates of less than 20 kbytes/s.
The USB connects peripheral devices, such as keyboards to PCs. The USB is a Plug
and Play bus that can handle up to 127 devices on one port, and has a theoretical
maximum throughput of 480 Mb/s. Only 4 wires (supply 5V, +data,-data, ground),
cable length restricted. Like RS-232, USB is useful for applications in a laboratory
setting that do not require a rugged bus connection.
Parallel data transfer (8 wires for data, another wires for periphery addressing)
The General Purpose Interface Bus (GPIB) is an IEEE-488 (a standard created by
the Institute of Electrical and Electronics Engineers) standard parallel interface used
for attaching sensors and programmable instruments to a computer. GPIB is a digital
8-bit parallel communications interface capable of achieving data transfers of more
than 8 Mbytes/s. It allows daisy-chaining up to 14 instruments to a system controller
using a 24-pin connector. It is one of the most common I/O interfaces present in
instruments and is designed specifically for instrument control applications.
by hardware
EXM7
Labview
Hopper
EXM7
Labview
Variety of transmission protocols and broad variation of hardware complicate
development of software that should be user friendly and portable.
1. International standards represent systems of hierarchically arranged
standardized layers
OSI Open System Interconnection model (International Organization for Standardisation) sub-divides a
communications system into smaller parts called layers. A layer is a collection of conceptually similar
functions that provide services to the layer above it and receives services from the layer below it. The
lowest physical layer describes specific protocols (RS 232…), hardware connections (voltage, pins,
cables…). The highest application layer represents interface to user (example is TCP/IP).
2. There exist proprietary tools (not standardized by international committees)
making use programming of instrument control quite easy for beginners.
Example is LabVIEW (Laboratory Virtual Instrumentation Engineering Workbench) - a visual
programming language from National Instruments. LabVIEW is commonly used for data acquisition,
instrument control, and industrial automation on a variety of platforms including Microsoft Windows,
UNIX, Linux. Problems are described by the structure of a graphical block diagram (the LV-source
code) on which the programmer connects different function-nodes by drawing wires. These wires
propagate variables and any node can execute as soon as all its input data become available
(Labview is an interpret but the executable code can be generated).
EXM7
Labview example
Hardware configuration
NI USB-6281
NI
USB connector
Measured voltage
EXM7
Labview example (1/7)
Create new VI
(Virtual Instrument)
project
Select driver for active
instrument NI 6281 (it will
be automatically
identified)
EXM7
Labview example (2/7)
Specification of
signal (voltage,
resistance…)
Icon of Digital
Acquisition Unit
appears
Select channel
(16 possible
channels)
EXM7
Labview example (3/7)
Range and units
of measured
voltage
Specify frequency
of sampling
Other option is
CONTINUOUS
sampling
EXM7
Labview example (4/7)
Right click on
the DATA item of
the DAQ icon
DATA output will
be redirected to
the GRAPH icon
EXM7
Labview example (5/7)
RUN/ABORT
buttons
Front view panel
with representation of
GRAPH and STOP button
Block Diagram for
graphical
programming
EXM7
Labview example (6/7)
Actual value is
indicated here
A SPOOL appears at the
position of locator on the
connection wire. CREATE
e.g. numeric indicator
Move the DAQ Icon to the
right: the connecting WIRE
appears
EXM7
Labview example (7/7)
If you need to
delete something
use Edit Ctrl X
CREATE CONTROL
to change the
sampling rate
Hint: Right Click
when outside of the block
diagram submits menu of
express functions
CREATE box
(express function) for
signal processing
EXM7
Labview project syringe
Rliquid
R
V3 V2 V1
V3-voltage drop on
fixed resistor
V2-voltage drop in
liquid
V1-output of pressure
transducer
EXM7
Labview project syringe
Comments are written to
data file that complicates
MATLAB processing
Rudolf Žitný, Ústav procesní a
zpracovatelské techniky ČVUT FS 2010