Chapter 1: Introduction to Electronic Instrumentation

1.1 Instrumentation; characteristics and types of instrumentation.

1.2 Standard units, 1.3 Error measurements1.4 Precision1.5 Types of error1.6 Error limits

1.1 Instrumentation; characteristics and types of instrumentation.

Definition…Instrument is a device or mechanism used to determine the present value of the quantity under measurement

Instrumentation is an application of instrument that is used for physical quantity measurement and control.

Electronic Instrumentation – the use of electronic instrument.

Classification of Instrument

1. Indicating InstrumentHumidity and Temperature SensorsPlain humidity sensing only Plain humidity and temperature sensing only Humidity or RH/T with LCD Display Humidity or RH/T with LCD Display, Temp Set Point,

RH Accuracies Available 5%, 2% RH

2. Recording InstrumentRecording charts perform the dual function of providing a graphic display at the point of data collection and a permanent record of these data or events for future validation. The display and recording of data and events pertinent to tracking operations or conditions such as process control, temperature, humidity, pressure, voltage, current, natural gas flow, motor truck operation and many other parameters is an important activity.

Graphic displays take several forms. These includeink writing, heat sensitive, pressure sensitive and electrosensitive.

3. Integrating Instrument Hybrid systems combine test and measurement components from modular instrumentation platforms such as PXI and VXI and stand-alone instruments that connect externally across GPIB, USB, and Ethernet/LAN. The hybrid system diagram of Figure 1 gives one example of a hybrid topology, but any number of combinations is possible. In the diagram, the nerve center of the system is a PC - either a standard PC or an embedded PXI controller. The instruments themselves include PXI and PCI plug-in devices as well as traditional instruments connecting to the PC over GPIB, USB and Ethernet/LAN/LXI

Physical and Signal VariablesPhysical Variables

TemperatureLengthPressureResistanceVelocityEtc

Signal VariablesCurrentForceVoltageLightFrequencyDisplacementPressure

Operational Mode of InstrumentDeflection Type

Only one source of input required.Output reading is base on the deflection from the initial condition of the instrument.The measured value of the quantity depends on the calibration of the instrument.E.g : Pressure gauge - Value of the quantity being measured is displayed in terms of the amount of movement of a pointer.

Contd…Null Type

Require two inputs; the measurand and balance input.Must have feedback operation (iterative balancing) that compare the measurand with the standard value.More accurate and sensitive than deflection type instrument.

Null Type Instrument Agilent 86100AInfiniium DCAHigh Bandwidth Oscilloscope to measure I patent and bit error rate (BER)

Null Type Instrument

A general scheme of pH-meters using a null-recorder method.

Contd…

Analog Mode

Produce the signals that vary in continuous way.Infinite number of values in any given range.

Digital Mode

Produce the signals that vary in discrete steps.Finite different values in a given range.

Contd…

An analog oscilloscope works by directly applying a voltage being measured to an electron beam moving across the oscilloscope screen. The voltage deflects the beam up and down proportionally, tracing the waveform on the screen. This gives an immediate picture of the waveform.

A digital oscilloscope samples the waveform and uses an analog-to-digital converter (or ADC) to convert the voltage being measured into digital information. It then uses this digital information to reconstruct the waveform on the screen.

Instrument Model

Contd…

Important element is sensor which can convert the physical variable into signal variable.Signal variable can be displayed, recorded or integrated in the secondary instrument system.Signal variable may also be used as an input signal of a control system.

Elements of Electronic Instrumentation

Elements Contd…Transducers

Device that converts a change in physical quantity into a change of a electric signal quantity.

Power SupplyProvide energy to drive the transducers.

Signal Conditioning CircuitsElectronic circuits that manipulate, convert the output from transducers into more usable electrical signal.

Eg. Filters, compensators, modulators, demodulators, integrators and differentiators

Elements Contd…Amplifiers

Amplify low voltage signal from transducers or signal conditioning circuit.

RecordersUsed to display the measurement for easy reading and interpretation.

Recorders can be analog or digital. The voltage from amplifier is an analog signal that is the input to recorder. Analog recorders, such as Oscilloscopes or magnetic tape display or store the analog signal. Digital recorders accept an analog input and convert this signal to a digital code that is then displayed in a numerical array or stored on magnetic media.

Data ProcessorsCan be a microprocessor or microcontroller.

Incorporate analog-to-digital converters (A/D) and provide the output signal representing the measurement in digital code.

Elements Contd…Process Controllers

Used to monitor and adjust any quantity at the specified level or value.

Signal from instrumentation system is compared with a command signal that reflects the required value of the quantity in the process.The process controller accepts both the command signal and the measured signal and forms the difference to give an error signal. The error signal is automatically adjust the process.

Command GeneratorProvide control voltage that represents the difference of the parameter in a given process.

Eg. The time-temperature profile for an oven must be controlled in curing plastics. The command generator provides a voltage signal that varies with time in exact proportion to the time-temperature profile required of the curing oven.

Areas of ApplicationEngineering Analysis

To validate new design of structure, component or system by theoretical and experimental approach.

Process ControlMonitoring process – provide on-line (real-time) data that allow operator to response and make adjustments to control the process.Automatic process – provide on-line (real-time) operating data that are used as feedback signal in closed-loop control systems to continuously control the process.

Basic MeasurementMeasurement …definition

Determination of magnitude of a given quantity by comparison between quantity and standard value.

Measurement is a process of translating the physical parameters into numerical value.Numerical must accompanied with unit to identify the property and the characteristics of measurand.

Measurement Methods

Indirect MethodMeasurand is determined by measuring other functionally related quantities and calculating the desired quantity.

Direct MethodMeasurand is directly compared against a standard.Most widely used in engineering practice.

Analog Oscilloscope Block Diagram

Output from Measurement

Signal travels directly to the vertical deflection plates of the cathode ray tube (CRT). Voltage applied to these deflection plates causes a glowing dot to move. (An electron beam hitting phosphor inside the CRT creates the glowing dot.)

Digital Oscilloscope Block Diagram

Digital oscilloscopes use interpolation to display signals that are so fast that the oscilloscope can only collect a few sample points.

Interpolation "connects the dots."