HEMN 213 Lecture-4 PDF

Lecture-4: Mechanical Measurement Cont. Sherif H. El-Gohary , Phd Assistant Professor,Biomedical Engineering [email protected] Typical Electronic Sensor System input signal sensor data microcontroller network (measurand) Sensor...

Lecture-4: Mechanical Measurement Cont. Sherif H. El-Gohary , Phd Assistant Professor,Biomedical Engineering [email protected] Typical Electronic Sensor System input signal sensor data microcontroller network (measurand) Sensor analog/digital signal processing display communication Sherif El-Gohary, Ph.D. NIH “Bionic Man” with 14 sensor and brain controlled functions Biomedical Transducer  Review-Resistor change in metal strain gauges Mainly due to change of resistor For silicon: large RESISTIVITY FORM (geometric effect) change with stress F L L+L a a a+a R0= L/a2 a/a =- L/L =0.3 Sherif El-Gohary, Ph.D. Biomedical Transducer Force and tactile sensors Forces can be measured in many ways:  The simplest - use a strain gauge  Calibrate the output in units of force.  measuring acceleration of a mass (F=ma),  measuring the displacement of a spring under action of force (F=kx, k is the spring constant),  measuring the pressure produced by force and some variations of these basic methods.  None of these is a direct measure of force Sherif El-Gohary, Ph.D. Biomedical Transducer  Piezoelectric Sensor  Piezoelectric effect was first proven in 1880 by the French Physicist brothers Pierre and Jacques Curie  In the 1950s manufacturers begin to use the piezoelectric effect  The word “piezoelectricity” is derived from a Greek word “pizo” or “piezein” which means to squeeze or press.  Piezoelectric materials generate charge when squeezed.  The amplitude and frequency of the signal is directly proportional to the mechanical deformation of the piezoelectric material. Sherif El-Gohary, Ph.D. Biomedical Transducer  Tactile Piezoelectric sensors  Piezoelectric generate electric voltage when strain is applied over it.  Strain can be calculated from voltage.  Piezoelectric strain gauges are the most sensitive and reliable devices.  Polymeric materials that exhibit piezoelectric properties are suitable for use as a touch or tactile sensors  Quartz and some ceramics have piezoelectric properties, polymers such as polyvinylidene fluoride (PVDF) are normally used in sensors.  Operation:  The polyvinylidene fluoride (PVDF) film is sensitive to deformation.  The lower film is driven with an ac signal  It contracts and expands mechanically and periodically.  When the upper film is deformed, its signal changes from normal and the amplitude and/or phase of the output signal is now a measure of deformation (force). Biomedical Sherif El-Gohary, Ph.D. Transducer HEMN213 Operation: Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213 Capacitive Sensing Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Capacitance Transducers Recall, capacitance of a parallel plate capacitor is:  r 0 A C d – A: overlapping area of plates (m2) – d: distance between the two plates of the capacitor (m) –  0: permittivity of air or free space 8.85pF/m – r :dielectric constant Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Capacitance Transducers The following variations can be utilized to make capacitance-based sensors: Change distance between the parallel electr odes. Change the overlapping area of the parallel electrodes. Change the dielectric constant.  r 0 A C d Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Capacitance Transducers When there is difference in P1 & P2 diaphragm moves toward low pressure side and accordingly capacitance varies. So, capacitance becomes function of pressure and that pressure can be measured by using bridge.  An example of a capacitive sensor is a pressure sensor In parts a, the thin sensor diaphragm remains parallel to the fixed electrode and in part b, the diaphragm deflects under applied pressure resulting in capacitance change Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213 Inductive Sensors Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Inductive sensors Inductive position and speed sensors come in a wide variety of shapes, sizes and designs. All inductive sensors can be said to work using transformer principles and they all use a physical phenomenon based on alternating electrical currents.  This was first observed by Michael Faraday Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Inductive sensors Rely on two basic phenomena: 1. Inductance of a coil and changes of inductance due to a variety of effects (distance, materials, dimensions, etc.) 2. Induced currents in conducting materials.  Inductance is a property of a magnetic device just as capacitance is the property of an electric device  Inductance can be made to respond (change) to almost any physical property either directly or indirectly Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Movable core sensors  Thus, a simple method of changing inductance of a coil is to provide it with a movable core:  The further the movable core moves in, the smaller the reluctance of the magnetic path and the larger the change in inductance.  This type of sensor is called a linear variable inductance sensor. (Linear here means that the motion is linear).  LVDT - Linear Variable Differential Transformer A better displacement sensor is a sensor based on the idea of the transformer.  Inductance is a measure of the position of the core The same may be used to measure force, pressure, or anything else that can produce linear displacement. Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  LVDT - Linear Variable Differential Transformer  Based on the coupling coefficient between the two coils is varied by physically moving the core while the two coils are fixed.  One primary coil, two secondary coils connected in opposition  It is a passive transducer.  It measures force in terms of displacement of ferromagnetic core of a transformer.  It is based on the principle of electro-magnetic induction.  Motion to either side changes the output Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Structure and equivalent circuit of an LVDT The output is zero for core centered Motion to the right or left changes the output in different directions (polarity) This is a variable reluctance transformer Detects both distance and direction of change Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213 Accelerometer Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Accelerometers By virtue of Newton’s second law (F = ma) a sensor may be made to sense acceleration by simply measuring the force on a mass. At rest, acceleration is zero and the force on the mass is zero. At any acceleration a, the force on the mass is directly proportional given a fixed mass. Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Accelerometers - principles The mass can only move in one direction (along the horizontal axis), Newton’s second law may be written as: ma =  kx  bdx dt Assumes that the mass has moved a distance x under the influence of acceleration, k is the restoring (spring) constant and b is the damping coefficient. Given the mass m and the constants k and b, a measurement of x gives an indication of the acceleration a. Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213 Type-1: Strain gauge accelerometers The mass is suspended on a cantilever beam. Strain gauge senses the bending of the beam. Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Type-2: Variable inductance accelerometers A rod connected and moving with the mass links to a coil. The inductance of the coil is proportional to the position of the mass. An LVDT may be used Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Type-3:Accelerometers - Capacitive  One plate of a small capacitor is fixed and connected physically to the body of the sensor.  A second plate serves as the inertial mass of the sensor is free to move and connected to a restoring spring. ›Acceleration either increases the capacitance or decreases it, depending on the direction of motion. ›Some kind of damping mechanism must be added to prevent the springs or the beam Sherif El-Gohary, Ph.D. from oscillating Biomedical Transducer HEMN213 Tilt Sensors Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Introduction Tilt sensors are devices that produce an electrical signal that varies with an angular movement. Tilt sensors allow you to detect orientation or inclination. These sensors are used to measure slope and tilt within a limited range of motion. Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Tilt Sensor Working Principle  These sensors consist of a rolling ball with a conductive plate beneath them.  When the sensor gets power, the rolling ball falls to the bottom of the sensor to form an electrical connection. ›When the sensor is tilted, the rolling ball doesn’t fall to the bottom so that the current cannot flow the two end terminals of the sensor. Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Testing a Tilt Sensor When pointing down, the switch is When pointing up, the switch is Open Loop (no continuity). closed (low resistance / continuous). Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Electrolytic tilt sensors Electrolytic tilt sensors are non-signal conditioned sensing elements. The function of an electrolytic tilt sensor is to measure an angle or a null or level position with reference to gravity. The angle may be expressed in anyone of the following:  degrees  arc minutes(1/60th of a degree)  arc seconds(1/60th of an arc minute). Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Structure and operation  The tilt sensor functions like a liquid potentiometer.  The electrically conductive fluid creates a variable resistance between the electrodes.  When the sensor is in the null or balanced position, the resistances between the center electrode to each outside electrode are equal. Sherif El-Gohary, Ph.D. Biomedical Transducer HEMN213  Thank you Please Are still magic words