06 chapter 3_part_2_student (Control Systems DAE 32103)

•Data Acquisition•Conversion

•Distribution SystemsMuhammad Faizal bin Ismail

Dept. of Electrical EngineeringPPD, UTHM

[email protected]

Chapter 3Digital Control System

5. Data Acquisition, Conversion and Distribution System

• Signal conversion in digital control system:1.Multiplexing and demultiplexing2.Sample and hold3.Analog-to-digital conversion (quantizing and

encoding)4.Digital-to-analog conversion (decoding)

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Cont.

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Block diagram of data acquisition system

Block diagram of a data distribution system

Physical Variable

• Defined as an input to the system such as position, velocity, acceleration, temperature, pressure, etc.

• This parameter is converted to the voltage or current signal by transducer

• The signal represents the measured value and used in data-acquisition process

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Transducer

• Device that converts input signal (physical variable) into output signal form

• Eg. Pressure sensor that converts pressure signal into voltage output

• Classified as:1. Analog transducer (continuous function of time)2. Sampled-data transducer (periodic discrete time)3. Digital transducer (quantized discrete time)

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Exercise 1

Name 4 parameters in process control measurement.

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Amplifier

• Frequently made from operational amplifier• Amplify the voltage output of the transducer• Convert current signal into voltage signal• It also used to buffer the signal

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Cont.

• Figure below illustrates the operational amplifier.

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Basic op-amp

Low-pass filter

• Output from amplifier contains noise signal (high-frequency) which may corrupt the data

• Low-pass filter is used to reduce the noise into an analog signal

• However, periodic noise such as in power-line are filtered by notch filter

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Cont.

• Following diagram shows the first-order low-pass filter circuit.

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First order low-pass filter

Analog multiplexer

• Used when many signal need to be processed by a single digital controller

• It is actually a multiple switch operate sequentially to provide single output as the following figure

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Schematic diagram of analog multiplexer

Cont.

• Only one switch is ‘ON’ in a specific time which allows the input channel connected to the output of multiplexer

• During this short time, S/H circuit samples the analog signal and holds its value and the same time the A/D converter converts the analog signal to digital signal

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Sample-and-hold S/H

• It is actually sample-and-hold amplifier• The amplifier circuit receives an analog input

signal and holds it for a specified time• The following figure illustrates the sample-

and-hold circuit

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Cont.

• Op-amp 1 acts as input buffer with high input impedance

• Op-amp 2 acts as output amplifier that buffer the voltage on the hold capacitor

14Sample and hold circuit

Cont.

• Tracking mode (switch close) – input signal is connected

• Hold mode (switch open) – capacitor voltage holds constant for a specified time

• The operation is dictated by a periodic clock

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Analog/digital converter (ADC)

• Converts analog signal in the form of voltage and current into a digital signal which is numerically coded signal or binary number

• Comprises a single IC with some supporting components

• The hardware required some conversion time depends on type of ADC, clock frequency and number of bit

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Cont.

• ADC performs the operations of sample-and-hold, quantizing, and encoding

• amplitude quantization - a process of representing a continuous or analog signal by a finite number of discrete states

• “Quantizing" means transforming a continuous or analog signal into a set of discrete states

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Cont.

• Encoding is a process of assigning a digital word or code to each discrete state

• The quantization level Q is defined as the range between two adjacent decision points and is given by

bitofnoisnandRangeScaleFullisFSRwhere

Q

.12

FSRn

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8-bit ADC

• Below is a block diagram of ADC

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8-bit Analog-to-Digital Block Diagram

Cont.

• Vin can be any voltage between 0 V and Vref

• When Vin is 0 Vdc, the output is 00000000

• When Vin is Vref, the output is 11111111 (255 decimal)

• For input voltages between 0 and Vref, the output increases linearly with Vin

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Cont.

• A start-conversion pulse is sent to the ADC• ADC then samples the analog input and converts it to

binary• When completed, the ADC activates the data-ready

output• This signal can be used to alert the computer to read

in the binary data21

8-bit Analog-to-Digital Block Diagram

Example 1

Calculate the quantization level for a 4 bit ADC with input 0 – 24 mA.

Solution:

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Example 2

Calculate the output of decimal value of 6 bit ADC if the voltage input is 3V with maximum range 12V.

Solution:

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Example 3

Find an output of an 8 bit ADC if the input is 5 V and the reference voltage is 10 V. From the answer find the n bit of the new ADC.

Solution:

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ADC method• Among the many ADC circuits available, the following types

are used most frequently:1. Successive-approximation type2. Integrating type3. Counter type4. Parallel type• Each type has its own advantages and disadvantages• Application, the conversion speed, accuracy, size, and cost are

the main factors to be considered in choosing the type of ADC

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Counter type ADC

• Simplest type of ADC• Principle:

1. Clock pulses applied to the digital counter2. This results the output voltage of DAC which is part of feedback loop in ADC stepped up one LSB at a time3. At each pulse, the output voltage is compared to the analog input voltage4. The clock pulses stopped when output voltage is equal to the magnitude of the input voltage5. Therefore, the counter output voltage is the digital output

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Successive-approximation type ADC

• The most frequently used ADC• Schematic diagram as following figure

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Successive-approximation principle:

• Successive-approximation register turns MSB and compares it with analog input

• The comparator will determine to leave the bit on or off by comparing with analog input voltage

• The MSB is set on if the input voltage is larger

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