Amplification. Most common signal conditioning

Transcription

1 1. Labview basics virtual instruments, data flow, palettes 2. Structures for, while, case,... editing techniques 3. Controls&Indicators arrays, clusters, charts, graphs 4. Additional lecture State machines, SubVIs, MainCluster 5. Modular programming + recording SubVIs File I/O Analysis Signal processing Communication between loops 6. Instrument control DAQ, Data collection, GPIB, Serial 7. Additional lecture Data Acquisition, Instrument control Course contents

2

3

4

5 Amplification Most common signal conditioning

6 Signal Sources Grounded Signal referenced to system ground (earth, building ground) example: devices that plug into building ground through wall outlets (e.g. signal generator) be aware of ground loops: Two independently grounded signal sources are generally not at the same potential

7 Signal Sources Floating signal not referenced to any common ground for example batteries, thermocouples

8 Measurement systems Differential measurement measuring with respect to floating ground neither of the inputs tied to fixed reference (building ground)

9 Measurement system Referenced single-ended measurement with respect to building ground

10 Measurement system Nonreferenced single-ended all measurement with respect to a common reference

11 What system to use? In general, differential measurement system is preferable Differential measurement rejects ground loops and noise from the environment Single-ended measurements allow twice as many channels as differential Use single-ended only if you have all of the following: high-level signals (normally, greater than 1V, so that the induced errors are lower than the required accuracy) short or properly-shielded cabling (normally, less than 3 m) all signals can share commmon reference signal at the source Do not use referenced single-ended connections with groundreferenced signal sources (ground loops!)

12 What system to use? The noise rejection with non-referenced single-ended mode is better than referenced single-ended Differential is better than non-referenced single-ended mode (AISENSE connection is shared with all channels)

13 Connections See the user manuals for more information e.g. USB f/manuals/371931f.pdf

14 Differential Referenced single-ended Non-referenced single-ended

15 Multichannel scanning considerations Multiplexer switches from one AI channel to the next Instrumentation amplifier has to settle to the new input range Settling time: time it takes the amplifier to amplify the input signal to the desired accuracy before it is sampled

16 For fast settling times: Use low impedance sources accumulated charge in multiplexer capasitor leaks through from previous to the next channel when switching between channels (ghosting) Carefully choose the scanning order avoid switching from large to small input range scan grounded channel between signal channels: improves settling time even with the same input range selected, if you know the expected signal levels, group the similar expected ranges together in your scanning list If it s not necessary to switch between channels, scan for example 100 samples from the first channel and only then switch to second channel and scan 100 samples

17 For fast settling times: Avoid scanning faster than necessary more time to settle example: You need to scan 10 channels over a period of 20 ms average the result. Even if scanning with 250 ks/s gives more samples and therefore improves the standard error of the mean, scanning with 125 ks/s gives more settling time and can in some cases give more accurate results.

18 Analog input circuitry

19 Analog-to-Digital Converter (ADC) Resolution number of bits in your ADC Example: 3-bit ADC divides the measurement range to 2 3 = 8 divisions With 16-bits you have divisions

20 Analog-to-Digital Converter (ADC) Device Range minimum and maximum analog signal levels the device can digitize

21 Analog-to-Digital Converter (ADC) Code Width smallest detectable change in the signal, i.e. resolution device range code width resolution(bits) 2 for example: 16-bit resolution, range from -10 to +10V code width = 20 V/2^16 = 305 µv Nominal resolution is worse due to the calibration method of the device

22 Sampling rate How often A/D conversion takes place Aliasing is a result of too low sampling rate Nyquist theorem sampling rate has to be at least twice the measured frequency to accurately represent the signal Nyquist frequency = Sampling frequency/2

23 Sampling rate Example: Sampling rate 100 S/s; signal at 25 Hz is measured correctly but signals at 70 Hz, 160 Hz and 510 Hz are aliased to 30 Hz, 40 Hz, and 10 Hz

24 Hardware vs Software timing Timing source can be on hardware or on software on hardware a clock on the device determines the timing on software the program loop determines the timing Hardware timing is more accurate and faster

25 Analog output Digital-to-Analog conversion generate analog signal from computer Single point update software timed generation change the output value everytime the program calls the VI Buffered analog output hardware timed generation upload a waveform to the device and set the update rate of the device to go through the points

26 Digital I/O Two states: high and low Control digital or finite state devices switches, LEDs Program devices or communicate between devices Example: Digital frequency generator takes 30-bit control word which defines the generated frequency use digital output ports of a DAQdevice to generate this word

27 USB-6008 Wirings

28 Instrument Control GPIB Serial port Image Acquisition USB Ethernet Parallel port

29 GPIB General Purpose Interface Bus (GPIB) a.k.a HP-IB, IEEE 488 GPIB is usually used in stand alone bench top instruments to control measurements and communicate data supported by many instrument manufacturers Digital, 24-conductor, 8-bit parallel communication interface 16 signal lines, 8 ground return lines 8 data lines: data sended in bytes 3 handshake lines: control the transfer of messages 5 interface management lines Data transfer rate typically 1Mbyte/s IEEE and define standards for GPIB

30 GPIB GPIB configurations you can have multiple devices connected to the same computer Device groups Talker Listener Controller

31 GPIB GPIB has one (active) controller that controls the bus usually this is the computer it connects the talkers to listeners Physical requirements maximum separation between two devices 4 m (for high-speed use only 1 m) maximum total cable length 20 m maximum of 15 devices on a bus (at least 2/3 turned on)

32 Serial Port Communication Communicate with only one device No need to buy additional hardware like with GPIB (although modern computers don t always have RS-232 port anymore) Send data one bit at a time you can have long distance between devices data transfer rate is low

33 Serial Port Communication Before communication you need to define baud rate number of data bits for a character parity bit number of stop bits Two voltage stages positive > 3V negative < -3V area between +3V and -3V is designed to absorb noise

34 Instrument Drivers Software to control a particular instrument VISA Virtual Instrument Software Architecture library for controlling GPIB, serial, Ethernet, USB, or VXI instruments Example: Agilent Digital Multimeter

35 Instrument Drivers Download from ni.com Help >> Find Instrument Drivers requires login

36 Instrument Drivers After installation the drivers can be found from functions palette

37 Links User manual for M-series USB-621x Labview data-aquisition manual Labview Measurement Manual Understanding Instrument Specifications Ghosting in multichannel sampling 028DDF?OpenDocument