MECHANICAL ENGINEERING SYSTEMS LABORATORY Group 02 Asst. Prof. Dr. E. İlhan KONUKSEVEN
FUNDAMENTAL CONCEPTS IN MEASUREMENT AND EXPERIMENTATION
HOW TO MEASURE? BY MEANS OF SENSING DEVICES OFTEN CALLED: TRANSDUCERS or SENSORS used for both Sensors Actuators instead of using human senses
HUMAN SENSES LACKING IN LATITUDE (SCOPE AND EXTENT) RELATIVE LIMITED SLOW BIASED
HUMAN SENSES Our ability to sense cold and hot is only relative, with a selectivity of the order of 1 C and only over a very limited temperature range. Measuring a high electric voltage with human senses is extremely hazardous.
CLASSIFICATION OF TRANSDUCERS A transducer can be defined as a device capable of converting energy from one form into another. Transducers can be found both at the input as well as at the output stage of a measuring system. We can distinguish six different energy domains: (1) Radiant (2) Mechanical (3) Thermal (4) Electrical (5) Magnetic (6) Chemical.
CLASSIFICATION OF TRANSDUCERS The input transducer is called the sensor, because it senses the desired physical quantity and converts it into another energy form.
CLASSIFICATION OF TRANSDUCERS The output transducer is called the actuator, because it converts the energy into a form to which another independent system can react, whether it is a biological system or a technical system. For a biological system the actuator can be a numerical display or a loudspeaker to which the visual or aural senses react respectively. For a technical system the actuator could be a recorder or a laser, producing holes in a ceramic material. The results can be interpreted by humans.
TRANSDUCER TYPES : SELFGENERATING (Passive) NON-SELFGENERATING (Active)
TRANSDUCER TYPES : SELFGENERATING (Passive) A self-generating transducer is defined as a transducer which requires no auxiliary energy source to convert energy. Examples of a self-generating transducer which can be found at the input of a system are a solar cell or a thermocouple, and at the output we can find for instance a heating element converting electrical energy into thermal energy without the need of an auxiliary energy source.
TRANSDUCER TYPES : NON-SELFGENERATING (Active) Requires an auxiliary energy source to convert energy from one domain into another. A good example of a modulating transducer which is found at the input of a system is a strain gauge. This type of transducer requires an electrical energy source to become operational. The electrical current flowing in the strain gauge is modulated by a mechanical force which is converted into an electrical voltage change via a change of the resistive elements.
TRANSDUCER TYPES : Transducer Transistor Thermocouple ph meter LED display LCD display Coil Magnetoresistor Photoconductor Type description Modulating shape transducer Self-generating input transducer Self-generating input transducer Self-generating output transducer Modulating output transducer Self-generating output transducer Modulating input transducer Modulating input transducer A collection of electronic self-generating transducers with examples of 'shape' transducers on the main diagonal where no energy conversion is involved.
SENSORS : ANY OBJECT THAT HAS A PROPERTY INFLUENCED BY A VARIABLE IS POTENTIALLY A SENSOR WHICH MAY BE USED TO MEASURE THAT VARIABLE
INPUT ENVIRONMENT TRANSDUCER an interaction is necessary between Transducer and Environment in order to perform a measurement. OUTPUT
MEASURAND THE PARTICULAR SYSTEM PARAMETER / VARIABLE BEING OBSERVED BY A TRANSDUCER DURING THE MEASUREMENT PROCESS IS CALLED THE MEASURAND
INPUT - OUTPUT CHARACTERISTICS INPUT SIGNAL (MEASURED VARIABLE) IS TRANSFERRED TO THE OUTPUT SIGNAL (OBSERVED VARIABLE) BY SOME FUNCTIONAL RELATIONSHIP (INSTRUMENTATION) DESIRED INPUT F D DESIRED OUTPUT
UNDESIRED INPUTS ALL THE INPUTS OTHER THAN THE MEASURAND ACTING ONTO THE TRANSDUCER ARE CALLED UNDESIRED INPUTS OR DISTURBANCE
UNDESIRED INPUTS : INTERFERING INPUTS MODIFYING INPUTS
INTERFERING INPUTS : REPRESENT THE INPUTS TO WHICH THE INSTRUMENTATION IS UNINTENTIONALLY SENSITIVE
EXAMPLE : THE MAGNETIC FIELD DUE TO A NEARBY ELECTRIC MOTOR OR TRANSFORMER
MODIFYING INPUTS : THESE INPUTS EFFECT THE RELATION BETWEEN THE DESIRED AND/OR INTERFERING INPUT AND THE DESIRED OUTPUT
EXAMPLE : STRAIN GAGE IS A TRANSDUCER IT IS GLUED ONTO A SPECIMEN TO MEASURE STRAIN - ITS RESISTANCE CHANGES IF IT IS STRECHED ITS RESISTANCE DEPENDENT ON TEMPERATURE It changes the gage resistance due to the strain resulting from the differential expansion of the gage and the specimen since they are made out materials of different thermal expansion coefficients. TEMPERATURE IS AN INTERFERING INPUT the gage temperature is also a modifying input since the gage factor is sensitive to the temperature. TEMPERATURE IS ALSO AN MODIFYING INPUT
DESIRED INPUT F D DESIRED F MD OUTPUT MODIFYING INPUT F MI INTERFERING INPUT F D
Functional Stages of Measuring Systems: Most measuring systems consist of the following three stages: Sensor (or Transducer) is always the 1 st stage of every measuring system and it produces an output to represent the measured quantity (measurand). Intermediate Stage modifies the sensor output (transduced information) so that it becomes acceptable to the terminal stage. All processes like amplification, filtering, conversion, integration, data transmission, telemetering are performed at this stage. Terminal Stage includes all possible data storage, monitoring, processing, displaying, plotting features. SENSOR INTERMEDIATE STAGE TERMINAL STAGE Measurand Sensor Output Measurement Data (Combined single instrument: Mercury in glass thermometer)
GENERALIZED MEASUREMENT SYSTEM MEASURED VARIABLE TRANSDUCER DATA MANIPULATION DATA TRANSMISSION DATA DISPLAY OBSERVED VARIABLE
DESIGNING A MEASUREMENT SYSTEM REQUIRES 2 THINGS : DESIGN OF VARIOUS INDIVIDUAL FUNCTIONAL ELEMENTS INTERCONNECTION OF VARIOUS ELEMENTS
Perfect Measurement: There exists no such thing like Perfect Measurement due to variety of reasons like: Presence of Noise (interfering effects), The effects other than the measurand (i.e.; undesirable inputs) may produce undesirable changes in the transducer output which make the measurement susceptible to noise. Transducer Dynamics, The ideal I/O dynamics of a transducer is of with no dynamics, that is with a static characteristics or simply with an instantaneous response. - U Loading Effect, Since the transfer of information between a transducer and its environment is accompanied by an energy transfer, the presence of a sensor may well result in some structural changes in its environment (and vice versa) which in turn affect the state of the measurand. This is often referred to as Loading Effect. V + R 1 R 2 Voltmeter R m Measured Voltage V m