Universiti Malaysia Perlis ENT164 SENSORS & MEASUREMENT Lecture 5: Computerized Data-Acquisition System (DAS/DAQ) AHI Sem 2, 2011/2012

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A data acquisition (DAQ) system is a collection of add-on hardware and software components that allow the computer to receive real-world information from sensors. It consists of Sensors I/O terminal panel(s) DAQ board(s) Program/Software AHI Sem 2, 2011/2012

Example of Computer DAQ System AHI Sem 2, 2011/2012

Computer System – PC for DAQ Typical PC case Data-acquisition board with eight analog input channels and two analog output channels also showing an analog signal connection box. Manufactured by National Instruments. AHI Sem 2, 2011/2012

Computer System – Components of PC Central Processing Unit (CPU) Program (Software) Random Access Memory (RAM) Mass Storage System Display User Input Devices (keyboards, mouse, etc) Printers and Plotters AHI Sem 2, 2011/2012

Everyday number format – base 10 (decimal) Computer System – Representing Numbers in PC Everyday number format – base 10 (decimal) Computer number format – base 2 (binary) Information stored in Computers is stored in flip-flops which have two possible states (“0” or “1”) Multiple series of flip-flop required to represent a number. MSB and LSB? 2’s complement? AHI Sem 2, 2011/2012

Bit The smallest unit in digital signal is the bit, a contraction of the more descriptive phase of binary digit. A bit is a single element in digital signal, having only two possible states: on (indicating 1) or off (indicating 0). Off On 1 AHI Sem 2, 2011/2012

Bytes Bits are organized into larger units called bytes, the basic unit of information in a computer system. A basic byte contains 8 bits. The total amount of information it can convey is 28 (=256) possible combinations. Off On 1 AHI Sem 2, 2011/2012

Example 4.1: Convert the 8-bit binary numbers to 01011100 to decimal 4.2: Find the 8-bit binary number with the same value as that of the decimal value 92. 4.3: Convert the decimal -92 to an 8-bit 2’s complement binary number. AHI Sem 2, 2011/2012

Other binary codes - ASCII AHI Sem 2, 2011/2012

Sample and Hold Subsystems DAQ Components Multiplexers (MUX) Basics of A/D Practical A/D DAC Sample and Hold Subsystems AHI Sem 2, 2011/2012

DAQ Components - Multiplexers (MUX) Most computers perform instructions sequentially. Thus, computer take data from sensors one at one time using device called multiplexer (MUX) A concept of electronic switch, selecting particular channels, read and the process them. Figure 4.4 shows a mechanical analog to the DAS, switches are semiconductor devices (eg: transistors) Subject to errors: Crosstalk Transfer accuracy AHI Sem 2, 2011/2012

DAQ Components - Multiplexers (MUX) AHI Sem 2, 2011/2012

DAQ Components – Basics A/D Analog vs Digital Represent 5V in analog and digital device. How? Textbook page 89 Most transducers generate (output) an analog signal. Example: temperature sensor. Analog-to-Digital Converter (ADC @ A/D) is required. Definition (Wiki) : An analog-to-digital converter (abbreviated ADC, A/D or A to D) is a device that converts a continuous quantity to a discrete time digital representation. AHI Sem 2, 2011/2012

DAQ Components – Basics A/D Sensor output (V) Bulb 1 Bulb 2 V < 2.5 Off 2.5 ≤ V < 5.0 On 5.0 ≤ V < 7.5 V ≥ 7.5 V How this above table relate to A/D??? Refer page 90 AHI Sem 2, 2011/2012

DAQ Components – Basics A/D Characteristic of A/D Number of bits to represent the output Input range Conversion speed Number of bits: greater the bits, greater number of possible output states and high accuracy. Input range: range of analog input voltages over which the converter will produce a representative digital output. Unipolar: same sign analog input (i.e. 0 to10V) Bipolar: both positive and negative (i.e. 0 to ±5V) AHI Sem 2, 2011/2012

DAQ Components – Basics A/D Conceptual simple type of A/D: Ramp A/D converter process AHI Sem 2, 2011/2012

DAQ Components – Basics A/D Conceptual simple type of A/D: Circuit AHI Sem 2, 2011/2012

DAQ Components – Basics A/D Example 4.4 A 4-bit single slope integrating A/D converter has an input range of 0 to 10V. Compute the digital output for an analog input of 6.115V. AHI Sem 2, 2011/2012

DAQ Components – Basics A/D In reality, there are simple ways to estimate the digital form of an analog output. Page 93 and table 4.2 for explanation. Some formula could also be used, as in Figure 4.7 Example 4.5 Example 4.6 Example 4.7 – input resolution error Errors in A/D: Systematic error: linearity, zero and sensitivity errors Thermal-stability error: can effect zero, sensitivity and linearity. AHI Sem 2, 2011/2012

Formulas to estimate A/D converter digital output. Figure 4.7: Formulas to estimate A/D converter digital output. AHI Sem 2, 2011/2012

DAQ Components – Practical A/D Commonly used A/D in DAQ – successive-approximations converter employs an interval-halving technique (iterative bisections calculations). A series of known analog voltages are created and compared to the analog input voltage, before halved to approximate its digital representation. Repeated iteratively. AHI Sem 2, 2011/2012

DAQ Components – Practical A/D Figure 4.9 Block diagram of a successive-approximations A/D converter. Copyright 1990 Instrument Society of America. From Computer-based Data Acquisition Systems by J.Taylor. AHI Sem 2, 2011/2012

DAQ Components – Practical A/D Example 4.8 An 8-bit A/D unipolar successive-approximations converter has an input range of 0 to 5V. An analog voltage of 3.15V is applied to the input. Determine the digital output. Answer: 0101 0100 0011 AHI Sem 2, 2011/2012

Solution for Ex. 4.8 Trial digital output (Do) Do X ∆V Pass/Fail Actual Digital Output 1000 0000 (128) 2.5 Pass 1000 0000 1100 0000 (192) 3.75 Fail 1010 0000 (160) 3.125 1010 0000 1011 0000 (176) 3.438 1010 1000 (168) 3.281 1010 0100 (164) 3.203 1010 0010 (162) 3.164 1010 0001 (161) 3.144 End 1010 0001 AHI Sem 2, 2011/2012

DAQ Components – Practical A/D Type and Advantage of Practical A/D Parallel or Flash A/D Fastest conversion Completed algorithms in one step As fast as 10ns Block diagram in Figure 4.10 Half-Flash Converter two flash converters Complete 12-bit conversion in about 100ns Dual-slope integrating converter Used in digital voltmeters Most accurate type of A/D Noise is averaging out (eliminate) Conversion time slower compared to previous two. Figure 4.11(a0 and (b) AHI Sem 2, 2011/2012

DAQ Components – Practical A/D Figure 4.10 Block diagram of a parallel A/D converter. Based on S. Franco, Design with Operational Amplifiers and Analog Integrated Circuits, McGraw-Hill, 1988. Used with permission of McGraw-Hill Inc. AHI Sem 2, 2011/2012

DAQ Components – Practical A/D Figure 4.11 Block diagram of a dual-slope integrating A/D converter: (a) block diagram; (b) integration process. Copyright 1990 Instrument Society of America. From Computer-based Data Acquisition Systems by J.Taylor. AHI Sem 2, 2011/2012

DAQ Components – D/A Even though purpose of measurement is to collect data or display them, but sometime the result of measurement could serve as control function. Most devices used to control are analog device, eg solenoids, heaters, valves, etc etc. Remember again computer are digital, and input TO computer from DAQ is converted to digital using A/D. Thus, for control function, the computer is required to convert them back to analog. Such device are known as Digital-to-Analog Converter, D/A or DAC. Schematic is shown in Figure 4.12, 4 bit and 4 switches. Generally, electrical power from D/A are small, and insufficient to control any control devices (solenoids, valves, etc). Normally it is amplified. AHI Sem 2, 2011/2012

DAQ Components – D/A AHI Sem 2, 2011/2012

DAQ Components – Simultaneous Sample & Hold Subsystems When using a multiplexer. Recall principle of multiplexer. Recording all channels or its subset in precisely same time could be done by using Simultaneous Sample & Hold Subsystems Figure 4.13 shows the subsystem. AHI Sem 2, 2011/2012

DAQ Components – Sample & Hold Subsystems AHI Sem 2, 2011/2012

DAQ Configuration Software for DAQ Internal single board plug-in system External system Virtual instrument Digital storage oscilloscope Data loggers Software for DAQ Commercial Software Package Example?? AHI Sem 2, 2011/2012

Some of DAQ boards available in market Some of DAQ boards available in market. > Keithley products > National Instrument (NI-DAQ) > Agilent > etc etc AHI Sem 2, 2011/2012

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What kind of DAQ did u used for your lab? What kind of software? THANK YOU AHI Sem 2, 2011/2012