14. Measurement Systems

Transcript

14. 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 AE2B38EMB – L14 1 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 PRINTER LAN GENERATOR CONTROLLED SOURCE AE2B38EMB – L14 GPIB, IEEE 488 OBJECT UNDER TEST SCANNER MULTIMETER COUNTER 2 Connecting analog signals to the computer Centralized systems (for 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“. ANALOG INPUTS / OUTPUTS 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 CONTROL MODUL EXT. CONTR.COMP. INT. CONTR. COMP. AE2B38EMB – L14 3 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 cabel mounting CAN – several circuits – (Skoda Fabia – 2 to 3) AE2B38EMB – L14 4 Measurement system GP-IB (IEEE 488, IEC625) CONTROL COMPUTER MONITOR PRINTER LAN GENERATOR CONTROLLED SOURCE          IEEE 488 OBJECT UNDER TEST SCANNER MULTIMETER 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 AE2B38EMB – L14 5 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 (F – data, T – 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). AE2B38EMB – L14 6 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$,N,EM) number of the used interface board address of F.U. (decimal value) the sent string ending symbol of string string length Similarly for points 5 and 6 RECEIVE(D,ADR2,MDATA$,N,EM) number of the used interface board address of F.U. (decimal value)) data received from FU AE2B38EMB – L14 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 separation by „:“ or „;“ parameters by space Several levels: MEAS:VOLT:AC? AE2B38EMB – L14 “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 bitů) 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 AE2B38EMB – L14 9 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 isolated 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. isolation to up 1500 V; I/O terminals protection up to 240 V; error < 0,05 % of range, non-linearity < 0,02 % of range. type. 30 31 32 34 35 36 37 AE2B38EMB – L14 description mV-input -narrowband V-input - narrowband Current input thermometers 100 Pt (2-, 3-wire connection) thermometrs100 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 10 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 AE2B38EMB – L14 11 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. AE2B38EMB – L14 12