Chapter 3 Digital Control System

Data AcquisitionConversionDistribution Systems

5. Data Acquisition, Conversion and Distribution System Signal conversion in digital control system:

1. Multiplexing and demultiplexing

2. Sample and hold

3. Analog-to-digital conversion (quantizing and encoding)

4. Digital-to-analog conversion (decoding)

2

Cont.

Block diagram of data acquisition system

Block diagram of a data distribution system

3

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

4

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)

5

Exercise 1

Name 4 parameters in process control measurement.

1. Level2. Flow3. Pressure4. Temperature

6

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

7

Cont.

Figure below illustrates the operational amplifier.

Basic op-amp

8

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

9

Cont.

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

First order low-pass filter

10

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

Schematic diagram of analog multiplexer

11

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

12

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

13

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

Sample and hold circuit 14

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

15

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

16

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

17

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

18

8-bit ADC

Below is a block diagram of ADC

8-bit Analog-to-Digital Block Diagram

19

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

20

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 data

8-bit Analog-to-Digital Block Diagram

21

Example 1

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

Solution:

m

m

FSRQ

n

6.112

2412

4

22

Example 2

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

Solution:

16

75.154

631212

36

x

x

23

Example 3

Find an output of an 8 bit ADC if the input is 5 V and the reference voltage is 10 V.

Solution:

00001000

7

2log

128log

1282

128

5.1272

2551210

58

ouputADC

n

x

x

n

24

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

25

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

26

Successive-approximation type ADC The most frequently used ADC Schematic diagram as following figure

27

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

28