**MEASUREMENT AND INSTRUMENTATIONBMCC 3743 ** PC BASED DATA-ACQUISITION SYSTEMS Mochamad Safarudin Faculty of Mechanical Engineering, UTeM 2010

**Measurement process** • Analogue signal conditioning - done • DIGITAL SIGNAL CONDITIONING Signal conditioning Recorder/display/processor Sensor/transducer measurand

**ANALOG AND DIGITAL ** • Most measurands originate in analog form • Analog signal varies smoothly in time, without discontinuty • Example: 220 V ac, 60 Hz power line voltage Example of analog signal • Digital information is transmitted and processed in form of bits • Each bit defined by one or other of two predefined “logic level” • The time interval assigned to it called bit interval • Most common two logic states is predetermined voltage levels • (say 0 and 5 V dc)

**Why digital?** • Digital electronics easier to design and fabricate • ex: IC, low cost, mass product compare to capacitor etc • Ease of data recording, storage and display • ex: digital voltmeter provides a direct numerical display • of voltage compared with analog voltage that has to be • visually interpolated if the pointer is between two scales • 3. Inherently noise resistant

**COMPUTER AS A ** MEASUREMENT SYSTEM

**Contents** • Components of computer systems • Representing numbers in computer systems • Components of data-acquisition systems • Configuration of data-acquisition systems

**Components of computer systems ** Display CPU and RAM Printer Digital input-output (ports or expansion bus) Mass storage (disk drives) Keyboard

**COMPUTERIZED DATA ACQUISITION SYSTEM**

**Typical computer components** • Central processing unit (CPU) • Program (software) • Random access memory (RAM) - ROM • Mass storage system – magnetic tape recorder, magnetic disk drive, optical disk drive • Display/monitor/screen • User input device (keyboard, mouse, joystick,etc) • Printers and plotters

**Contents** • Components of computer systems • Representing numbers in computer systems • Components of data-acquisition systems • Configuration of data-acquisition systems

**Representing numbers in computer systems** • Computers use bistableflip-flops to store information, which have only 2 possible states: on (1) or off (0) E.g. 1001 2 • 1 byte = 8 bits 4 bit binary number LSB MSB MSB:Most Significant Bit LSB: Least Significant Bit

**Examples (binary/decimal)** • Convert the 8-bit binary number 01011100 to decimal • Find the 8-bit binary number with the same value as that of the decimal number 92.

**01011100** • N10=0(27)+1(26)+0(25)+1(24)+1(23)+2(22)+0(21)+0(20) • =0+64+0+16+8+4+0+0 • =92 • By a series of divisions by 2 • remainder • 2 92 • 2 46 0 • 2 23 0 • 2 11 1 • 2 5 1 • 2 2 1 • 2 1 0 • 0 1 LSB Answer: 1011100 but we are asked for 8 bit: 01011100 MSB

**Most commonly represented using:2’s complement binary** What about negative number? Procedure: • Convert the integer to binary as if it were positive • Invert all of the bits – change 0’s to 1’s and 1’s to 0’ • Add 1 LSB to the final result e.g.convert –92 to an 8-bit 2’s complement binary number answer: from previous, 01011100 invert 10100011 +1 LSB 101000112 + 12 become 10100100 • Note that, positive numbers always have 0 as MSB and negative numbers have 1 as MSB • In a computer often a special code is used : ASCII – American Standard Code for Information Interchange, e.g. k = 011010112 = 10710

**ASCII Characters**

**Contents** • Components of computer systems • Representing numbers in computer systems • Components of data-acquisition systems • Configuration of data-acquisition systems

**Components of data-acquisition systems** • Multiplexer • Simultaneous sample-and-hold subsystem • ADCs • DACs

**Multiplexer (MUX)** • Works as an electronic switch – computer will ask MUX to select a particular channel to be read and processed, sequentially. • Can have crosstalk errors and transfer accuracy. Illustration of a multiplexer In this figure, channel 1 is connected

**Simultaneous sample-and-hold subsystem** • Need to be used to record data from different channel of MUX, precisely at the same time. e.g. Measuring tire forces using 6 component force transducers simultaneously

**Analogue-to-Digital Converters** • Converts continuous analogue waveform into discrete digital signals • Examples: audio amplifiers, TV, output voltage from transducers, etc • Output of ADCs has 2N possible values • If N , no. of possible output states , hence results more accurate

**Types of ADCs** • Unipolar single-slope integrating converter (ramp type – quite slow, not very accurate) • Successive-approximations converter (quite fast – typical 12-bit completes a conversion in 10 – 25 μs) • Parallel or flash or half-flash converter (the fastest – can be 10 ns, using lots of comparators) • Dual-slope integrating converter (used in digital voltmeter)

**Unipolar single-slope integrating converter** • A fixed reference voltage is used to charge an integrator at a constant rate • The integrator output voltage then increase linearly with time • A digital clock (counter) is started at the same time that the charging is begun • The integrator output voltage is compared continuously with the analog input voltage using a comparator • When the integrator voltage exceeds the analog input voltage, digital clock is stopped • The count of the digital clock is the digital output of the A/D converter

**Example**

**Formula to estimate A/D converter digital output** • The output of a 2’s-complement, given the analogue input voltage, is where max. positive output is (2N/2 –1) and max. negative output is (-2N/2) • The output of an offset binary or simple binary converter is given by where output will range from 0 to (2N-1) max.

**Example:** From example before, estimate the digital output for 6.115 V analog input to A/D converter Answer: Since this is a simple binary devices the second equation is applicable:

**Quantisation error** • Resolution uncertainty (or treated as random error, analogous to the reading error of a digital display) due to output of ADC with discrete steps, given by Input resolution error = • The quantisation error is thus ±0.5 LSB

**Successive-approximations converter (most common type)** • A series of known analog voltages are created and compared to the analog input voltage • In the first trial, a voltage interval of one-half the input span is compared with the input voltage • If the input voltage is in the upper half of the range, the MSB is set to1; otherwise it is set to zero • This process is repeated with an interval half the width of the interval used in the first trail to determine the second MSB and so forth until LSB is determined Successive aproximation method for 4 bit A/D converter

**Example:**

**Example:** A 12-bit A/D converter has an input range of -10 to +10 V. Find the resolution error of the converter for the analog input. Answer: Using above equation The resolution uncertainty of ±0.00244 is the best that can be achieved Comment: if input voltage=0.1 V (low end of input range), The quantization error would represent 2.5% of the reading, which is probably not acceptable. The input signal should be amplified probably before the signal enters the converter

**Digital-to-Analogue Converters** • Converts discrete digital signals into continuous analogue waveform • Examples: To operate heaters or valves under computer control • Similar specs as ADCs, i.e. depends on no. of input bits, analogue output range and conversion speed.

**4 bit D/A converter** • Rn=2nRf • When the switched is closed, in flows to the summing bus • The op-amp converts the currents to voltages

**Example:** A digital code 1011 (equivalent to 11) for the circuit above with Rf= 5 kW and vs=-10 V. then i1=-1 mA i2=0 i3=-1/4 mA i4=-1/8 mA Summing these currents and multiplying by Rf gives Vo=6.875 V which is 11/16 of the full scale (ref) voltage

**Contents** • Components of computer systems • Representing numbers in computer systems • Components of data-acquisition systems • Configuration of data-acquisition systems

**Configuration of data-acquisition systems** Several configuration of DAS • Internal single board plug in system (PCI slot) • External system (USB, IEEE1394, Ethernet, RS-232, GPIB) • Virtual Instruments (modular) ex: Labview from NI • PC turns into digital storage oscilloscope • Data loggers (simpler and specialized) ex: flight data recorder in airplane

**INTERNAL SINGLE BOARD PLUG IN SYSTEM** PCI DAQ

**EXTERNAL SYSTEM** USB DAQ from NI

**VIRTUAL INSTRUMENTS** LABVIEW from NI

**DATA LOGGER**

**EXAMPLE:** How many bits are required for a digital device to represent the decimal number 27541 in simple binary? How many bits for 2’s complement binary? Solution: The following table presents the maximum decimal number versus the number of bits for simple binary: No. Bits Max. Dec. No. Simple Binary 12 212 -1 = 4095 13 213 -1 = 8191 14 214 -1 = 16383 15 215 -1 = 32767 16 216 -1 = 65535 Consequently, 15 bits are needed to represent 27541 in simple binary. For a two's complement binary number, the MSB will be zero so 16 bits will be required.

**An A/D converter is to operate with a full-scale voltage of** 10V. How many bits should be employed to obtain a resolution of 0.01 %?

**A 12-bit A/D converter has an input range of ± 8 V, and** the output code is • offset binary. Find the output (in decimal) if the input is: • 4.2 V • -5.7 V • 10.9 V • -8.5

**N = 12** Vru = 8V Vrl = -8V Vin = input voltage (a) By second equation (b) By second equation : (c) Since 10.9V falls outside the input range, Do will have the maximum output: Solution: (d) Since -8.5V falls outside the input range, D0 will take the minimum value:

**A 12 bit A/D converter has an input range of ± 10V and an** amplifier at the • Input with a gain of 10. The output of the A/D converter is in 2’s complement • Format. Find the output of the A/D converter if the input to the amplifier is: • 1.5 V • 0.8 V • -1.5 V • -0.8 V

**We need the first equation to solve this problem** (a) When the 1.5V signal is amplified with a gain of 10, it becomes 15V which exceeds the input range of the A/D converter (it is saturated). According to the formula, the maximum output is 2N/2-1 = 212/2-1 = 2047 (b) With the gain of 10, the input becomes 8V. The output, in decimal, is then: (c) When amplified, -1.5V results in an input to the A/D converter which is below the input range (it is saturated). The largest negative output is –2N/2 = -2048 (d)With the amplifier, this voltage results in an input to the A/D of –8V. The output is then:

**A 3.29 V signal is input to a 12 bit successive** approximations converter with an input range of 0 to 10 V and simple binary output. Simulate the successive approximation process to determine the simple binary output

**The reference voltage increment is:** The output is 010101000011 or 1347 in decimal.

**Thank You**