THE LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
The Linear Variable Differential Transformer (LVDT) is a displacement measuring instrument and is not a strain-based sensor.
The LVDT models closely the ideal Zeroth-order displacement sensor structure at low frequency, where the output is a direct and linear function of the input.
The LVDT is a variable-reluctance device, where a primary center coil establishes a magnetic flux that is coupled through a mobile armature to a symmetrically-wound secondary coil on either side of the primary.
Two components comprise the LVDT: the mobile armature and the outer transformer windings. The secondary coils are series-opposed; wound in series but in opposite directions.
When the moving armature is centered between the two series-opposed secondaries, equal magnetic flux couples into both secondaries and the voltage induced in one half of the secondary winding is balanced and 180 degrees out-of-phase with, the voltage induced in the other half of the secondary winding.
The balanced condition provides total cancellation of secondary voltages and therefore zero voltage output. When the moveable armature is displaced from the balanced condition, more magnetic flux will couple into one half of the secondary than into the other producing an imbalance voltage output at the primary coil excitation frequency. The output voltage of the LVDT is therefore a direct function of the displacement of the mobile magnetic armature. The LVDT is, by definition, a transformer and requires an oscillating primary coil input.
The DC LVDT is provided with onboard oscillator, carrier amplifier, and demodulator circuitry. The AC LVDT requires these components externally. Due to the presence of internal circuitry, the DC LVDT is temperature limited operating from typically -40 C to +120 C.
The AC LVDT is able to tolerate the extreme variations in operating temperature that the internal circuitry of the DC LVDT could not tolerate. Typically, LVDTs will be excited by a primary carrier voltage oscillating at between 50 hertz and 25 Kilohertz with 2.5 Kilohertz as a nominal value. 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.
(This extract is taken from 'The Art of Practical & Precise Strain Based Measurement' by Jim Pierson.
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.
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.
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. Protection to lP68 can be specified in some specialist designs, but IP66 is normally readily available as an option on standard models.
Protection from the effects of corrosive materials may be required. A sensor that has been manufactured using corrosion-resistant materials (such as stainless steels or engineering polymers) will be necessary in these applications.
If the application is in an area where explosive gases are present, then consideration must be given to selecting a sensor that has been specially designed, tested and approved to be safe to operate in this environment.
The duty cycle of the application being measured is important when selecting the type of sensor to use. A typical benchmark for linear potentiometers is 200 million operations, but a really heavy-duty cycle may be better suited to a sensor that uses technology operating on a non-contacting principle, although this can have an impact on cost.
The accuracy of the sensor is determined by a combination of the output signal conformity ('linearity' or 'non-linearity') and the temperature coefficient of the sensor. Overall system accuracy should be considered over the operating temperature range of the equipment.
The resolution of a sensor is the smallest measurable change in the output signal. Most linear position sensors now use technologies that provide virtually infinite resolution; this is normally stated in sensor manufacturers' technical data.
The ability of the sensor to provide repeatable signals is of paramount importance. Sensor manufacturers will quote a figure for the deviation in indicated position when a point along a stroke length is approached repeatedly from the same direction. This factor is often confused with the sensor resolution.
This is the difference in indicated position for the same point when reached from opposing directions. This may be an important factor to consider but most linear position sensors have minimal or negligible values.
Power supply available
An important factor to consider is the supply requirement to the sensor. Most operate on values within the range of 5VDC to 3OVDC.
Output signal required
The output from the sensor can vary, but can be DCV, ACV, DCmA or a range of digital signals (such as TTL, R5232 or CAN). The control interface to the sensor will usually determine the type of signal required to be specified.
The ability of a sensor to withstand operation in electrically noisy environments has become more important since the introduction of European regulations on EMC/EMI. CE marks ensure testing and compliance with regulations.
Cost of ownership
A factor often overlooked when selecting a position sensor is the cost of ownership over a period of time. Selecting a sensor on price alone may compromise the reliability of a system, particularly if constant failure involves service costs, downtime and lost production.
Sensors that are readily available from stock or manufactured within days of ordering can provide a considerable advantage to project development times. Additionally, holding spare parts to support after-sales is virtually eliminated.
Do not underestimate the value of asking suppliers about their experience.
(Specification selection article provided by Penny & Giles Controls Ltd. - www.pgcontrols.com)
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