Instrumentasi Elektronika (Electronic Instrumentation)

Instrumentasi Elektronika (Electronic Instrumentation) BAB 3 SIGNAL CONDITIONING & CONVERSION

CAPAIAN PEMBELAJARAN Setelah mengikuti kuliah ini mahasiswa mampu: 1

Menjelaskan fungsi pengkondisi sinyal analog

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Mendesain rangkaian RC low-pass dan high pass filter untuk menghilangkan sinyal yang tidak diinginkan.

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Mendesain rangkain jembatan Wheatstone untuk mengkonversi perubahan tegangan terhadap perubahan resistansi

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Menjelaskan fungsi Op-amp sebagai pengkondisi sinyal

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Menjelaskan cara kerja sebuah penguat instrumentasi

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Mendesain sistem pengkondisi sinyal analog untuk mengkonversi berbagai masukan tegangan untuk beberapa rentang output yang diinginkan

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Merancang pengkondisi sinyal analog untuk berbagai variasi input resistansi diubah menjadi berbagai variasi tegangan yang diinginkan.

POKOK BAHASAN Rangkaian RLC (Filter LPF dan HPF)

Penguat Instrumentasi

Pengkondisi Sinyal

Operational Amplifier (Op-Amp)

Rangkaian Pembagi dan Jembatan

SISTEM AKUISISI DATA Pengkondisi Sinyal

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RANGKAIAN INTERFACE

Antarmuka atau sirkuit pengkondisi sinyal berfungsi membawa sinyal dari sensor sampai ke beban dengan format yang kompatibel

SISTEM INSTRUMENTASI PADA KENDARAAN

FUNGSI PENGKONDISI SINYAL 1. Linierisasi

5. Conversion Fyngsi Pengkondisi sinyal

4. Filtering dan Matching Impedance

2. Perubahan bias dan level sinyal

3. Efek Pembebanan

1. Linierisasi

Tujuan linierisasi adalah untuk memberikan output yang bervariasi secara linear terhadap beberapa variabel termasuk jika output sensornya tidak linier.

2. Signal Level and Bias Changes Signal Level and Bias Changes berfungsi menyesuaikan tingkat (besarnya) dan bias (nilai nol) dari beberapa tegangan yang mewakili variabel proses. Standar sinyal dalam instrumentasi • Tegangan 0V – 10V, atau 0V -5V • Arus 4 – 20 mA • Tekanan 3 – 15 psi 

Output sensor 0,2V – 0,6V, peralatan pemroses memerlukan tegangan bervariasi 0V – 5V.



Output sensor 0V – 24V, peralatan pemroses memerlukan tegangan 0V – 5V

Bagaimana solusinya:

3. Efek Pembebanan (lanjutan) Kita perlu mengetahui drop tegangan pada beban dengan label Vy. Melalui hukum Ohm tegangan Vy dapat diketahui dengan mengurangkan Vx dengan arus dikalikan Rx.

Vy = Vx – I.Rx

3. Efek Pembebanan (lanjutan) Example: An amplifier outputs a voltage that is 10 times the voltage on its input terminal. It has an input resistance of 10 kΩ. A sensor outputs a voltage proportional to temperature with a transfer function of 20mV/ °C. The sensor has an output resistance of 5.0 kΩ. If the temperature is 50 °C, find the amplifier output.

Figure 2.3

3. Efek Pembebanan (lanjutan)

4. Filtering dan Matcing Impedansi • Filtering adalah mengeliminasi sinyal yang tidak diinginkan dalam loop proses kontrol (Instrumentasi) • Impedance matching adalah impedansi internal tranduser atau impedansi line yang dapat menyebakan error dalam pengukuran variabel dinamis.

5. Conversion • Convert one type of electrical variation into another. • Signal Transmission (Voltage to current, Current to Voltage converter) • Digital Interface (ADC requires 0-5V input) Standar sinyal dalam instrumentasi • Tegangan 0V – 10V, atau 0V -5V • Arus 4 – 20 mA • Tekanan 3 – 15 psi

Rangkaian RLC, Filter LPF dan HPF

RANGKAIAN RC

Jawab: Berapa tegangan pada kapasitor setelah 35 uS inputnya diberikan input step dari 0 ke 54V, jika R = 47k-ohm dan C = 1,5uF

Time Constant pada Rangkaian RC Rangkaian RC dengana input step, maka tegangan pada kapasitor adalah:

Ec = E (1− e−t/ RC) Tegangan keti pengosongan kapasitor:

Ec = E . e−t /RC Berapakah time constant untuk rangkain RC di atas jika R= 220 kohm dan C = 2,2 uF.

RANGKAIAN RL

RANGKAIAN RCL

TEGANGAN DALAM RANGKAIAN RCL

XC = XL

Berapa arus yang mengalir pada rangkaian RCL jika R=27k-ohm, C = 2,2nF, L=33mH, dan E = 20V dengan frekuensi 35 k-Hz

RANGKAIAN RCL

(lanjutan..1)

FREKUENSI RESONANSI Merupakan frekuensi yang menyebabkan arus pada rangkaian RLC maksimum. Hal tersebut bisa dicapai ketika XL = XC.

Rangkaian RLC seri

Rangkaian RLC paralel

FILTER RC Jenis-jenis Filter RC:

• High-pass. Meneruskan frekuensi tinggi dan menahan frekuensi rendah; • Low-pass. Meneruskan frekuensi rendah menahan frekuensi tinggi; • Band-pass. Menersukan frekuensi dengan range tertentu; • Band reject. Menahan range frekuensi tertentu; • Twin – T. Filter Band Reject, tetapi dengan karakteristik respon yng lebih sempit/tajam. To eliminate unwanted noise signals from measurements, it is often necessary to use circuits that block certain frequencies or bands of frequencies.

LOW PASS FILTER (LPF)

First order and second order low-pass filters,

Gain vs frequency characteristics of first order and second order filters.

Designing the anti-aliasing filter

• Note ω is in radians ω = 2πf

• Exercise: Find an R+C pair so that the half-power point occurs at 30 Hz

FILTER RC

Highh Pass High

(a) High-pass filters, (b) band-pass filters, (c) twin-T band reject filters, and (d) their frequency characteristics.

FIGURE 2.16. Response of the high-pass RC filter as a function of frequency ratio.

Rangkaian Pembagi Tegangan dan Jembatan

RANGKAIAN PEMBAGI TEGANGAN

Figure 2.4

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EKT314/4 - Electronic Instrumentation

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RANGKAIAN JEMBATAN DC Bridge circuits are used to convert impedance variations into voltage variations

Figure 2.5

RANGKAIAN JEMBATAN DC (lanjutan)

if Zero difference and zero voltage across the detector


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*-ve result means that Vb larger Va.


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FIGURE 2.6 When a galvanometer is used for a null detector, it is convenient to use the Thévenin equivalent circuit of the bridge.


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Bridge resolution: A function of the resolution of the detector used to determine the bridge offset.

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FIGURE 2.7

For remote sensor applications, this compensation system is used to avoid errors from lead resistance.

LEAD COMPENSATION -When bridge circuit may be located at considerable distance from the sensor whose resistance changes are to be measured. -Problem many effect that change the resistance. -any changes in lead resistance are introduced equally into both arms of the bridge circuit, thus causing no effective change in bridge offset

FIGURE 2.8

The current balance bridge.

CURRENT BALANCE BRIDGE -this method uses a current to null the bridge

Figure. 2.8


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Potential measurement using bridges: A bridge circuit is useful for measuring small potentials at very high impedance using either a Wheatstone bridge or current balance bridge. Performs by placing the potential to be measured in series with the detector.

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FIGURE 2.9 Using the basic Wheatstone bridge for potential measurement. Using Wheatstone bridge

Using Current balance bridge

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ac bridges

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ac bridges

FIGURE 2.10

A general ac bridge circuit.

FIGURE 2.11

The ac bridge circuit and components for Example 2.10.

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FIGURE 2.12 (a) Bridge off-null voltage is clearly nonlinear for large-scale changes in resistance. (b) However, for small ranges of resistance change, the off-null voltage is nearly linear.

Primary application of bridge circuits -To Convert variations of resistance into Variations of voltage

If the range of resistance variation is Small and centered about the null value Then then nonlinearity of voltage Resistance is small.

Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Rangkaian Operational Amplifier (Op-Amp)

∆Vin = V1-V2

-∆Vin = V1-V2

KONFIGURASI OP-AMP Voltage follower

Differensial Amplifier

Inverting Amplifier

Op-Amp

Summing Amplifier

NonInverting Amplifier

The op amp voltage follower. This circuit has unity gain but very high input impedance. High impedance Low impedance

The op amp inverting amplifier.

FIGURE 2.27 (continued)

Nonideal characteristics of an op amp include finite gain, finite impedance, and offsets.

FIGURE 2.28

Some op amps provide connections for an input offset compensation trimmer resistor.

FIGURE 2.29

Input offset can also be compensated using external connections and trimmer resistors.

Curtis Johnson Process Control Instrumentation Technology, 8e]


A noninverting amplifier.



The op amp summing amplifier.

The basic differential amplifier configuration.

An instrumentation amplifier includes voltage followers for input isolation.

PENGUAT INSTRUMENTASI

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FIGURE 2.39

A voltage-to-current converter using an op amp.

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FIGURE 2.40 A current-to-voltage converter using an op amp. Care must be taken that the current output capability of the op amp is not exceeded.



FIGURE 2.41

An integrator circuit using an op amp.



FIGURE 2.42

This circuit takes the time derivative of the input voltage.



FIGURE 2.43

A nonlinear amplifier uses a nonlinear feedback element.



Instrumentasi Elektronika (Electronic Instrumentation)

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