Acquisition of experimental data

Transcription

1 Otto-von-Guericke-Univ. Magdeburg Vorlesung «Messtechnik» Acquisition of experimental data Dominique Thévenin, Katja Zähringer Lehrstuhl für Strömungsmechanik und Strömungstechnik (LSS)

2 Position of this lecture The main part of the lecture «Messtechnik» describes basic properties and methods, usable sensors, what could in principle be measured and how to do it. Today: first example to illustrate how it is possible to acquire experimental data in practice (basic examples only...) See also later: How can we process efficiently the data after acquisition (filtering, correlations...)? More complex measurement techniques, in particular optical methods Applications (starting this afternoon)! 2

3 Acquisition of data: purpose h(t) Purpose: Get the value h (one- or multi-dimensional variable) of a signal, usually at a regular time interval (sampling period= dt, sampling frequency = /dt ) dt N t 3

4 Three limit cases (/2) ) Control measurements: basic process control, quality check, security... Usually point-wise measurements, fixed sensors, standalone systems, low acquisition frequency Not very complex: buy it off-the-shelf! 3) High-end measurements: to support fundamental research and development in science and high-end engineering Typically multi-dimensional, high to very high acquisition frequency Very complex and expensive systems (optics, laser, intensified cameras...) You will only see a little bit of it at the end (LDA...) 4

5 Example: flow instabilities in pump 5

6 6 Three limit cases (2/2) 2) Standard measurements: gain some information, e.g. from a fluid, to solve observed problems, propose improvements for the process or dynamically control complex process conditions Often one-dimensional, possibly multi-dimensional Almost always quantitative (issue of precision) Intermediate acquisition frequency Typically the purpose of this teaching unit! Will be extensively realized during the hands-on Is therefore considered for all following illustrations

7 7 Three system families ) «All integrated»: the acquisition system is fully standalone, like a multi-meter... 2) «PC free»: the acquisition system is more complex, contains usually several elements connected by specific cables, but does not require a dedicated computer 3) «Acquisition chain»: a complex, multi-component system involving at least a computer 2) and 3): typically the purpose of this teaching unit! Will be extensively realized during the hands-on Is therefore considered for all following illustrations

8 Typical acquisition chain Experiment sensor amplifier analog-digital converter actuator, step motor,... results, pictures,... acquisition card digital-analog converter PC differed processing (acquisition) card control instructions real-time analysis storage (file) 8

9 9 Analog-digital converter (/2) Allows to discretize the continuous, analog signal Sampling of the continuous signal with a period dt timer (clock) sensor A-D conversion acquisition card

10 Analog-digital converter (2/2) A-D converter Typically delivers a value between - and + with precision P P = 2(=range) / 2^number of significant bits Example : 2 bit-card P = 2/496 6 bit-card P = 2/65536 timer The timer is directly controlled by the clock, usually located on the acquisition card (or external). At an acquisition time t, the card can typically acquire a (nearly simultaneous) dataset (i.e. several values).

11 Data-set Example with a single clock: nearly simultaneous acquisition timer (from clock) dt Acquire channel Acquire channel 2... Acquire channel n dt dt dt with dt << dt

12 The first solution will be retained for the hands-on 2 Acquisition card for data Interface card/user The interfacing system is usually delivered with the acquisition card. There are three main possibilities: The interface relies on an existing, commercial, userfriendly software (Labview, Matlab,...): much easier to use, but more expensive (one full Labview license for a Windows PC: 3 ) Or in-house routines, usually written in C or C++, allow to communicate directly with the acquisition card: less expensive, but requires development by the user Or independent but simple standalone programs are delivered with the card.

13 3 Acquisition card Can be external or internal (integrated in PC) Example: National Instrument PCI 625 E ( )

14 Parameters of data acquisition Acquisition card typically connected to a PCI (Peripheral Component Interconnect) bus For the considered system: 6 analog inputs in common modes, 8 analog inputs in differential mode single clock Clock frequency up to 2 khz (single input) Input resolution 2 bits Input range +/- V Input gains,, 2 analog 2 bits 8 digital inputs/outputs 4

15 Data acquisition Example for A/D and D/A system Analog input (BNC) Digital input/output 5

16 Digital inputs/outputs Digital inputs/outputs are usually simply coded at bit level with s and s! = =5 =7 =255 6