Engineering Experimentation & Measurements [ EM312 ]

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Chapter #01

The General Instrumentation and Measurement Systems

Measurement System in Daily Life

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Measurement System in UCSI

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• PURPOSE: To present an observer with numerical value corresponding to the variable being measured.

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Purpose and Performance of Measurement

Systems

• The purpose of the measurement system is also to link the observer to the process via the obtained data (in Table 1.1) from the measurement of the process

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• A process is a system which generates information such as chemical reactor, a jet fighter, a car, a human heart, and a weather system.

• The observer as a person who needs this information from the process such as a car driver, the plant operator or the nurse.

• The input to the measurement system is the true value of the variable; the system output is the measured value of the variable. In reality, the measured value does not equal the true value.

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• The purpose of the measurement system is to link the observer to the process via the obtained data (in Table 1.1) from the measurement of the process

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Accuracy of the System • Can be defined as the closeness of the measured value to the

true value.

• A perfectly accurate system is a theoretical ideal.

• The accuracy of a real system is quantified using measurement system error E, where

• Error is the main performance indicator for a measurement system.

• Ex: Measured value of the flow rate of gas in a pipe is 11.0 m3/h and the true value is 11.2 m3/h, then the error E = −0.2 m3/h.

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• The general measurement system consists of several elements or blocks such as

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Structure of Measurement Systems

1. Temperature Measurement System

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Examples of Measurement Systems

Thermocouple

• Thermocouple = A sensor to measure the temperature sensing element; this gives a millivolt output.

• Signal conditioning consists of a circuit to compensate for changes in reference junction temperature, and an amplifier.

• Analogue-to-digital converter = Convert the voltage signal into digital form.

• Computer = Corrects for sensor non-linearity, and the measured value is displayed on a VDU display.

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Temperature Measurement System

2. Speed Measurement System • Use an electromechanical generator to produce electrical

power from mechanical energy, usually the turning of a shaft.

• Schmitt trigger = An active circuit which converts an analog input signal to a digital output signal (a sharp pulse). (Digital Tachometer)

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Schmitt trigger

• Electromagnetic Tachogenerator = Sense the speed of rotation of an engine which gives an a.c. output signal with frequency proportional to speed.

• The Schmitt trigger = Convert the sine wave into sharp-edged pulses.

• Counter = Count the sharp-edged pulses over a fixed time interval. Then, the digital count is transferred to a computer.

• Computer = Calculates frequency and speed.

• Digital display = Present the speed in digital form.

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Speed Measurement System

3. Flow Rate Measurement System

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Orifice Plate

• Orifice plate sensing element = A device used for measuring flow rate ; this gives a differential pressure output.

• Differential pressure transmitter converts this differential pressure output into a current signal and therefore combines both sensing and signal conditioning stages.

• The ADC converts the current into digital form and

• The computer calculates the flow rate, which is obtained as a permanent record on a chart recorder.

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Flow Rate Measurement System

4. Weight Measurement System

• Cantilever = Load placement with weight

• Strain Gauge = A device used to measure strain on an object

• Wheatstone Bridge = An electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit

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• Cantilever is a primary sensing element which converts weight into strain.

• Strain gauge converts strain into a change in electrical resistance and acts as a secondary sensor.

• There are two signal conditioning elements: the deflection bridge converts the resistance change into millivolts and the amplifier converts millivolts into volts.

• The computer corrects for non-linearity in the cantilever and the weight is presented on a digital display.

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Weight Measurement System

Wheatstone Bridge •The strain gauge is connected into a Wheatstone Bridge circuit with a combination of four active gauges (full bridge), two gauges (half bridge), or, less commonly, a single gauge (quarter bridge) • As stress is applied to the bonded strain gauge, a resistive changes takes place and unbalances the Wheatstone Bridge. •This results in a signal output, related to the stress value. As the signal value is small, (typically a few millivolts) the signal conditioning electronics provides amplification to increase the signal level to 5 to 10 volts, a suitable level for application to external data collection systems such as recorders or PC Data Acquistion and Analysis Systems.

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Measurement Applications categories:

1. Monitoring of processes and operations 2. Control of Process and operations 3. Experimental engineering analysis

Applications in Manufacturing

1. Monitoring of processes and operations: • Measuring devices are being used to keep track

of some quantity E.g.: thermometers, barometers, radars, etc.

• To indicate the environment condition, and the reading does not serve any control functions in the ordinary sense.

• In automotive industry speedometer, fuel gauge,

outdoor temperature sensor and compass belongs to this class of operation.

Applications in Manufacturing

2. Control of processes and operation:

• Automatic feedback control systems.

• Sensors are used as feedback systems.

• Every feedback control system have at least one measuring device as a vital component

Applications in Manufacturing

Applications in Manufacturing

3. Experimental Engineering Analysis:

• All the engineering design, development and research lies on laboratory testing

• Solutions

– Theory ~

– Experimentation ~

Applications in Manufacturing

Mohamadariff Othman

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Static Characteristics of

Measurement System Elements

Chapter #01

Characteristics of Measurement System Elements

Discuss on characteristic that typical elements may posses and give effect on the overall performance of the system.

I O Element

- Systematic Characteristics

- Statistical Characteristics

- Generalized Model

Characteristics of Measurement System Elements

Element Characteristics

Static Characteristics

(Steady-state Characteristics)

Dynamic Characteristics

(Transient Characteristics)

Generalized Model

(Block Diagram or Mathematical forms)

Systematic Characteristics

(Graphical or Mathematical forms)

Identification of Static Characteristics

(Statistical Characteristics)

- Range

- Span

- Linear

- Non-Linearity

- Sensitivity

- Hysteresis

- Resolution

- Error Band

- Environmental Effects:

+ Modifying Env. Inputs

+ Interfering Env. Inputs

- Repeatability

- Tolerance

- Calibration:

+ Standards

+ SI Units

+ Experimental: O vs I (Im=Ii=0)

O vs Im, Ii (I=Constant)

Repeatability Test

• Static/Steady-State Characteristic

The relationship between input I (constant value or changing slowly) and output O.

• It has three kind of characteristic:

– Systematic Characteristic = Those can be exactly quantified by mathematical or graphical means

– Statistical Characteristic = Identification of static characteristic.

– Generalized Model = To represent static and dynamic characteristic in term of block diagram and mathematical form.

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Characteristics of Measurement System Elements

• Dynamic Characteristic

The ways in which an element responds to sudden input changes.

– These are most conveniently summarized using a transfer function G(s).

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Characteristics of Measurement System Elements