Nidhi > Em > Lecture_14_xe38emb-c

MEASUREMENT SYSTEMS - overview, basic principles, use, properties - measurement system GP-IB (IEEE 488, IEC625 and further) - description, principle, communication, transfer of messages and special commands, IEEE 488.2 standard (what is added), Standard commands for programmable instruments - SCPI - measurement systems based on PC plug-in boards, multifunction plug-in board (block schematic diagram, properties) - measurement systems programming

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Connecting the digital measuring instruments to a computer – an overview a) series interface RS-232 Advantages: standard interface of computers (COM1, COM2), low cost. Disadvantages: - point-to-point connection, i.e. 1 instrument to one interface (standard connection), - comparatively low transfer rate (115,2 Kb/s). b) USB 1.x Advantages: - standard interface of new computers - higher transfer rate than RS-232 (6 Mb/s). Disadvantages: - 1 instrument to one interface, (otherwise tree-structure necessary) - only new instruments. c) instrumentation interface GP-IB (IEEE 488, IEC-625) – more details see further Advantages:: -up to 14 instruments can be connected, - - higher transfer rate than RS-232 (1 MB/s), - standard for majority of medium and higher class instruments. Disadvantages - GP-IB interface is not in standard equipment of computer. CONTROL COMPUTER

MONITOR

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PRINTER

LAN

GENERATOR

CONTROLLED SOURCE

IEEE 488

OBJECT UNDER TEST

SCANNER

MULTIMETER COUNTER 2

Connecting analog signals to the computer Centralized systems (fo low distance sensor – computer) a) PC plug-in boards – details see further Advantage: low cost. Disadvantage: - EMC conditions inside PC case not standardized ↓ measurement accuracy cannot be guaranteed, - galvanic connection of inputs with PC ground, - PCI bus – low number of PC „slots“.

ANALOGOVÉ VSTUPY/VÝSTUPY

b) Modular systems (VME → VXI, industrial version of PCI → PXI) Advantages: -higher interference immunity, in VXI standardized EMC conditions inside the system, - higher number of free „slots“. - Standard for majority of medium and higher class measuring instruments. Disadvantage: high cost EXT. CONTR.COMP.

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CONTROL MODUL

INT. CONTR. COMP.

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c) Distributed systems for measurement and control Series communication channels (mostly RS-485 bus or its modifications) Industry – for large distances sensor – computer INDUSTR. PC

VME SYSTEM

WORKSTATION

LAN FIELDBUS (e.g. PROFIBUS) FIELDBUS (e.g. CAN) CAN)

ADC

DIO

DAC

REGULATOR

KEYBORD + DISPLAY

PROGRAMMABLE AUTOMAT

Cars – to simplify cabels mounting CAN – several circuits – (Š Fabia – 2 to 3) 38EMB – P14

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Measurement system GP-IB (IEEE 488, IEC625) CONTROL COMPUTER

MONITOR

PRINTER

LAN

GENERATOR

IEEE 488

OBJECT UNDER TEST

SCANNER

MULTIMETER

CONTROLLED SOURCE

COUNTER

• Maximum number of FU connected in measurement system: 15 (computer+14 instruments); • Total maximum length of bus: 20 m; • Maximum distance between two FU: 2 m; • Number of the bus wires: 24 - 8 DIO – data, addresses, multi-wire commands; 3 data transfer control („3-wire handshake“; 5 single-wire commands; 8 grounding wires); • maximum transfer rate: 1 MB/s (typical - 500 to 700 kB/s); • electrical signal levels: TTL; L (< 0,8 V); H (> 2,0 V); • logic signal levels – inverted logic: TRUE ~ L; FALSE ~ H, • standardized connector connection (2 versions), • functions: controller, talker, listener 38EMB – P14

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Data transfer control:

- DAV – data valid ( talker) - NRFD – not ready for data (listener) - NDAC – not data accepted (listener)

Single-wire messages: - IFC (interface clear) interface to initial state, - REN (remote enable) device into remote control regime - ATN (attention) defining data (T – data, F – addresses + multi-wire messages), - SRQ (service request), - EOI (end or identify) Transmitted data:- addresses – 2 identification bits + 5 bits address (01xxxxx – list.; 10xxxxx – talk.; xxxxx = 11111 - un-addressing), ATN = T, - multi-wire commands – 00 + 5 bits command (GET,GTL, DCL etc), ATN = T, - instrument messages (data), ATN = F, a) ASCII string + ending symbols, b) binary data blocks (EOI).

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Basic algorithm: 1. Function units into remote control 2. Addressing chosen unit as listener 3. Sending control string - older multimeter HP 3455A - "F1R3M3" function – DC volt.

range 10 V math. off

- new multimeters (SCPI) - "VOLT:DC:RANG 10;CALC:STAT OFF" voltage

DC

range 10 V

math. off

4. Start measuring (GET, or send control string, e.g. „E“ or „FETCh:VOLT:DC“ 5. Addressing chosen unit as talker 6. Sending measured values, e.g. string „V ±X.XXXXXE±YY“ or binary data block. Points 1 – 3 and/or. 4 realized usually by one function of interface driver in higher-level language, e.g. for interface boards National Instruments: function unit addressed as „listener“, message for it sent by the command: SEND(D,ADR1,A$,N,EM) number of the used interface board address of F.U. (decadic value)

the sent string

ending symbol of string string length

Similarly for points 5 and 6 RECEIVE(D,ADR2,MDATA$,N,EM) number of the used interface board address of F.U. (decadic value)) data received from FU 38EMB – P14

ending symbol of string length of data 7

IEEE 488.2 (IEC 625-2) - data format and syntax of messages; - extended state model of equipment; - set of general commands and queries; - instrument message protocols. SCPI – Standard Commands for Programable Instruments • Group of 9 most important producers of measuring instruments; • Universal „language“ – independent of type and manufacturer –not bound to GP-IB bus • general commands and queries from IEEE 488.2, extended state registers • Tree command structure, based on English terminology (abbreviated), Voltmeter setting:

Generator output setting:

CONF VOLT AC 0.5,0.005

“CONFigure:VOLTage:AC 0.5,0.005“

INP IMP 50

COUP DC

FILT ON

“INP:IMP 50;COUP AC;FILT ON“ “INP:IMP 50“ “INP: COUP AC“ “INP: FILT ON“

Commands sepatation by „:“ or „;“ parameters by space Several levels:MEAS:VOLT:AC?

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“CONFigure:VOLTage:AC 0.5,0.005“ “INP:IMP 50; FILT ON“ “INIT:IMM“ “FETCh:VOLT:AC? “ 8

PC and notebooks plug-in boards ANALOG INPUTS

MUX

S/H

ADC

FIFO

CONTROL REG.

f/T

ANALOG

COUNTER/TIMER DAC

OUTPUTS

I N T E R F A C E

B U S

DAC fO DIO

DIO

Multifunction module:Analog inputs (8 - 32), analog outputs (1 – 4), dig. I./O (4 - 16 bitu) Limitation: - galvanic connection to PC ground, - basic range ± 5V, ranges < 1 V – strong interference, - limited f S (ADC, bus), - more-channel measurements (n channels): f CH = f S/n; time shift 38EMB – P14

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Single-function modules for measurement and control High sampling-rate modules („oscilloscope“ boards) up to 2 GSa/s, internal data memory (tens MSa); data transferred to PC memory after the end of measurement. Modules with several analog outputs

Modules with galvanically separatetd digital I/O

Counter-timer modules

Modules with analog filters

Standardized modules for signal preprocessing 5Bxx (or MBxx or DT5xx, xx specifies the module) Basic standard parameters: galv. Separation to up 1500 V;

I/O terminals protection up to 240 V;

error < 0,05 % of range,

nonlinearity < 0,02 % of range.

type. 30 31 32 34 35 36 37

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description mV-input -narrowband V-input - narrowband Current input thermometers 100Ω Pt (2-, 3-wire connection) thermometrs100Ω Pt (4-wire connection) Potentiometric sensors thermocouples

type. 38 39 40

description Strain gages Current output mV-input –broadband

41

V-input – broadband

45

Frequency meter

47

Linearized thermocouples

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ANALOG INPUTS

SENSOR

controlled object

ACTUA TOR

POWERLINE SWITCHES

MULTIFUNCTION GALVANIC SEPARATION

MEASURING MODULE

ANALOG OUTPUTS GALVANIC SEPARATION DIG. I/O

DIO

INDUSTRIAL PC VERSION

3x380/220 V

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Programming of measurement systems 1. At higher-level languages using produces-supplied drivers (usually in C, C++). Advantages:

optimized memory and speed.

Disadvantages: knowledge of programming requested, time -consuming. 2. Using graphical development environments Advantages: knowledge of algorithm is sufficient, easier (sub-programs as icons connected according to data flow and control signals) Disadvantages: larger memory needed, lower speed, limited selection of drivers. 3. Combination of both possibilities – combines advantages and suppresses disadvantages of both.

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