Displacement Measurements PDF

Switches and Displacement Sensors Most types of non-contact sensors are actually transducers that include control circuitry that allows them to be used as switches. The circuitry changes an internal switch when the transducer output reaches a certain value. The three most common typ...

Switches and Displacement Sensors Most types of non-contact sensors are actually transducers that include control circuitry that allows them to be used as switches. The circuitry changes an internal switch when the transducer output reaches a certain value. The three most common types of non-contact sensors are: 1. Inductive proximity sensor 2. Capacitive proximity sensor 3. Optical proximity sensor Unlike a true switch, proximity sensors consume a small amount of current to operate the internal transducer circuitry. Inductive proximity sensor Detect only the presence of electrically conductive materials. Eddy currents generated in the conductive material result in increase in the impedance to the AC in the sensor ➔ drop in the internal AC current. P N P Hysteresis? Operating range and hysteresis Capacitive proximity sensors Depend on target ability to be electrically charged ➔ almost any object! When opposite plate becomes close enough to be affected by the charge in the sensor’s internal plate, it will become oppositely charged and sensor will be able to move significant current into and out of its internal plate. generates AC Schematic of internal construction of a capacitive proximity sensor  Capacitance can be evaluated using the following equation. A change in capacitance C results in a corresponding change in voltage VM as expressed by  Permanent N magnet Vo to frequency Coil pick up V S Gear teeth Induced EMF due to transient changes in flux between coil and magnet as the gear teeth passes under the magnet. Two gears concentric with the torque shaft are used to detect a change in phase between the two ends. Capacitance-type torque meter sleeve TL TL = = d 4 K 'G shaft ( )G 32 Maximum value of capacitance Where: L T = Applied torque G = Torsional modulus of elasticity T K’ = Torsional stiffness Optical proximity sensors: commonly known as light beam or (thru-beam) sensors Light generated at frequency best detected by sensor and not likely to be generated by nearby sources. Through beam Suffer from a delay between when light is received and when current is conducted. The more the current to be controlled, the slower the sensor response. Photoelectric Sensor Photoelectric triangulation sensor: (a) with light beam reflected by three different target positions, (b) reflective type, and (c) retro-reflective type. Optical proximity sensors: commonly known as light beam (retroreflective) sensors Transmitter and receiver are in the same package retroreflective t Optical a sensor r g e t Hall Effect Sensor A lever moves a magnet relative to hall effect sensor. As the magnet approaches the sensor, the current is forced toward one side of the sensor or the other. Contacts at the sides detect that the current is more concentrated at one side; a voltage across the contacts is sensed and is proportional to the closeness of the magnet to the sensor. LVDT: Linear Variable Differential Transformer Casing Insulating material Core A magnetic nickel-iron core, supported by a nonmagnetic push rod, moves axially within the cylinder in response to mechanical Primary coil displacement of the probe tip Secondary coils As long as the core remains near the center of the coil arrangement, the output is nearly linear. Electromagnetic linear-velocity transducer LVDT: Linear Variable Differential Transformer Core displacement LVDT: Linear Variable Differential Transformer Alternating input voltage is impressed in the center coil. Output voltage from the two end coils depends on the magnetic coupling between the core and the coils, which is dependent on the position of the core. In general, the frequency of the applied voltage should be 10 times the desired frequency response. The frequency response of LVDT is primarily limited by the inertia characteristics of the device. Side loads must be avoided ➔ since they will cause rubbing between the armature (core) and the LVDT body, thus reducing the unit's life and accuracy. In extreme cases, they may cause the armature to bend. Measurement of Force and Torque Fe Electrodynamic feedback load Platform cell. For weighing light objects current control Closed loop (0.1 mg to 100 g). LVDT core case The coil current is S N S Force made proportional to Coil the integral of the downward displacement. Proximity sensor application in monitoring gear speed or gear teeth condition Induced EMF due to transient changes in flux between coil and magnet as the gear teeth passes under the magnet. Two gears concentric with the torque shaft are used to detect a change in phase between the two ends. An optical coupler as a rotor speed transducer VIBRATION VELOCITY TRANSDUCER Magnetic core Stationary coil An electromagnetic linear-velocity transducer is composed of a stationary coil with a permanent-magnet core moving within the coil. The core is attached to the object whose velocity is to be measured. When the core moves, magnetic lines of the field created by the core cross the turns. An electromotive force induced in the coil is proportional to the speed of the core. Velocity Transducer A moving coil outside a stationary magnet. It is designed so that if the housing is moved, the magnet tends to remain stationary due to its inertia. The relative motion between the magnetic field and the coil induces a current that is proportional to the velocity of motion. This sensor produces an output that is directly proportional to vibration velocity. It is self-generating and needs no excitation or conditioning to operate. Disadvantages: It is relatively heavy and complex. It has poor frequency response < 1000 Hz. Velometer: is an accelerometer with a built-in electronic integrator.