Instrumentation & Measurement (ME342) Chapter 1: Fundamentals of Measurement Systems Dr. Hani Muhsen

1.1 Introduction: Measurement is a process of gathering information from a physical world and comparing this information with agreed standards

1.1 Introduction: Measurements were first needed to regulate the transfer of goods in barter trade in order to ensure that exchanges were fair. i.e. the need to quantify the amounts being exchanged. New instruments & measurement techniques have been developed to satisfy the needs of industrialized production techniques. Most of these instruments and measurement techniques had grown rapidly due to developments in electronics in general and computers in particular. The massive growth in the application of computers in industry has greatly expanded the requirement for instruments to measure, record, and control process variables.

Why are measurement systems important to us? Regulating trade by common standards and units (buying fruits and vegetables, electricity, water…) Monitoring functions to take necessary actions (check thermometer to decide to switch the heater on or off) Used as part of automatic feedback control systems.. If you can not measure it, you can not control it. (control system cannot be more accurate than the measurement system itself)

Example 1: Automatic Feedback Control system Figure 1.1 Elements of a simple closed-loop control system.

Example 2: Production Control Modern production techniques Tighter accuracy limits Economic forces/ motivation Reduced production costs The focal point of the research and development efforts of all instrument manufacturers is to create instruments to be as much accurate and inexpensive as possible.

1.2 Measurement Units The need to establish clear rules about the relative values of different quantities. Early systems of measurement were based on whatever was available as a measuring unit ( i.e. human torso was a convenient tool and gave us units of the hand, the foot, and the cubit ). The previous units are, of course, imprecise, varying as they do from one person to the next. Therefore, there has been a progressive movement toward measurement units that are defined much more accurately. The value of the physical quantity can be expressed as a simple multiple of the unit of measurement. Example: “length” is the physical quantity while “meter” is a unit of length that represents a definite predetermined length. System of Units: Imperial ( foot and Ibm), Metric ( Meter and Kilogram) an SI (International System of units).

1.2 Measurement Units Derived units: Newton [N] = [kg] [m] / [s]2 = kg.m/s2 Volt [v]= Kg.m2/A.s3

1.3 Measurement System Design Measurement is carried out with instruments that are designed and manufactured to fulfill given specifications. Instruments are developed for monitoring the conditions of physical variables and converting them into symbolic output forms. An instrument may be defined as a system which maintains a function relationship between a measured parameter and the output. The output data can then be transmitted to an “Observer” or to a Control System. The physical parameter being measured is known as the measurand.

1.3 Measurement System Design The elements of a Generalised Instrumentation System: Primary sensor Variable conversion element Signal processing element Signal transmission 5. Signal presentation/ recording

The elements of a Generalised Instrumentation System: Primary Sensing Element: Receives energy from the measured medium, and produces an output depending in some way on the measurand. The Primary Sensing Elements can have non-electrical i/p and o/p (eg) Springs, manometer etc, it could also be a filter or rectifier in Electrical Systems. The Transducer: converts the measurand energy into electrical energy. For example a strain gauge converts strain into a resistance change which can then be detected as a voltage change. Signal Conditioning . Signal conditioning is used to transform the transducer signal into a more suitable form. For example a Bridge circuit is used with the strain gauge to determine the change in resistance, and Operational Amplifiers are used to boost the voltage to a more suitable level. ADC. Analogue to Digital Conversion. In the majority of instrumentation systems the analogue data has to be transferred to a digital format. Data Transmission. If transducer is remote from the observer, telemetry or some data bus will be required.

1.3 Measurement System Design Figure 1.3 Elements of a measuring system.

Primary Sensing Element: Receives energy from the measured medium, and produces an output depending on the measurand. Liquid-in-glass thermometer measured quantity? a liquid changes its volume relative to its temperature. Bulb

Variable conversion element Needed when the output variable of the primary sensor is in an unsuitable form and has to be converted to a more suitable form. Example: Displacement measuring  resistance  voltage by “bridge circuit”

Transducers Transducers are energy-conversion devices. Transducer = primary sensor + VCE Kinetic energy  Electrical energy

Signal processing element Signal processing (conditioning) is used to transform the transducer signal into a more suitable form: Filtering: producing pure signal Amplification: used to boost the voltage to a more suitable level. Analogue to Digital Conversion (ADC): In the majority of instrumentation systems the analogue data has to be transferred to a digital format.

Signal Transmission The output measurement needs to be observed a distance away from the transducer (transducer site is inconvenient, physically inaccessible, or hazardous) Transmission methods: single or multi-core cables, fiber optics, wireless transmission.

Signal presentation/recording The point where the measured signal is utilized (for monitoring, controlling, or analyzing purposes)

Example: Displacement measurement system