Difference between variable reluctance transducer and lvdt displacement
It can be linear or angular rotational motion. With the help of displacement transducer, many other quantities, such as force, stress, pressure, velocity, and acceleration can be found. In case of linear displacement, the magnitude of measurement may range from a few micrometers to a few centimetres.
A majority of displacement transducers detect the static or dynamic displacement by means of suitable mechanical links coupled to the point or body whose displacement is to be measured. The main electrical displacement transducers work on the principle of Variable resistance: transducer is strain gauge. Variable inductance: transducer is linear variable differential transformer Variable capacitance: transducer is parallel plate capacitor with variable gap Synchros and resolvers: used to measure angular displacement.
The LVDT is, by definition, a transformer and requires an oscillating primary coil input. The carrier frequency is generally selected to be at least 10 times greater than the highest expected frequency of the core motion. The external housing of the LVDT is fabricated of material having a high-magnetic permeability therefore desensitizing the device from the effects of external magnetic fields.
No sensing spring element exists within an LVDT and therefore, the output of the sensor is hysteresis-free. Some LVDT displacement measuring sensors are, however, provided with internal armature return springs to allow profile measurement. When there exists no direct contact with the moving armature is allowed no mechanical wear results. The provision of linear bearings to prevent armature to coil structure contact and to limit wear can greatly extend LVDT operating life expectancies.
The strong relationship between core position and output voltage yields a sensor design that shows excellent resolution, limited more by the associated circuitry than the sensing method. The internal core of the LVDT is generally constructed of an annealed nickel iron alloy with the high-temperature limitations of the device limited to the curie point of the core and the winding insulations used. The thermal response characteristics of the LVDT are excellent for static and quasi-static thermal environments due to the physical and electrical symmetry of these devices.
The physical symmetry also contributes to excellent zero repeatability over time and temperature. Most thermal-sensitivity shift errors result from the significant thermal coefficient of resistance TCR of the copper transformer windings. With increasing temperature, the primary coil resistance will increase causing a decrease of the primary current in the constant-voltage-excited case and therefore decreasing the magnetic flux generated and voltage output correspondingly.
The use of constant-current excitation will ensure a constant primary flux regardless of the coil resistance. Since the equivalent circuit of the constant-current source is a voltage source with an infinite series resistance, the use of a low-TCR resistance, in series with the primary, will function in much the same manner as the piezoresistive span-compensation resistor by causing the primary voltage to increase as a function of temperature thus offsetting the TCR-induced losses.
The use of the series low-TCR resistor in the primary circuit allows the constant-voltage source to appear to the LVDT as a constant-current source. Other thermally-active methods may also be used to compensate for the primary winding TCR by causing the primary voltage to increase, with rising temperature, in proportion to the increase in the primary coil resistance.
The temperature coefficient of magnetic permeability is another contributor to the thermal-sensitivity shift and is compensated out as a net effect by the means described above. Within approximately 2 seconds of power application the LVDT oscillator and demodulator circuitry will stabilize sufficiently for dynamic measurement. Due to self-heating of the primary coil, warm-up times for high precision static measurement are comparable to strain gaged sensors and are dependent upon the thermal stability of the measuring environment.
Important factors for the specification of Linear Position Sensors. Determine the displacement The length of displacement that needs to be measured will most likely determine the type or range of sensors available rod, slide or cable operated. Consider the mounting of the sensor Can the sensor be mounted close to the movement, integrated within the equipment, or will it need to be situated away from the moving part?
Consider the aftachment method The attachment between the sensor and the moving part can either be a fixed mechanical interface or a spring biased probe that follows the moving surface. Vibration conditions Careful consideration needs to be given to the impact of vibration on the sensor, and whether this can be detrimental to operation and life.
This factor may determine the type of sensing element to select - contacting or non-contact. Shock conditions High levels of shock can seriously affect the operation of a sensor, either permanently damaging the device or degrading the output, so careful selection of a device that can withstand this treatment is important. Temperature variation or extremes Extremes of temperature hot or cold need to be considered, and whether the sensor will be required to operate within its specification at these extremes or just survive under storage conditions.
Some sensor technologies are particularly susceptible to changes in temperature, resulting in drifting output signals, which could be mistaken by a control system as a valid movement of a machine part. Resistance to ingress of particles and liquids Environmental protection of the sensor may be required where it is operating in harsh conditions, to stop the ingress of harmful particles or liquids that may damage the sensor.
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