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sensors:linear_variable_differential_transformer [2018/11/01 18:55] – external edit 127.0.0.1 | sensors:linear_variable_differential_transformer [2021/03/24 21:03] (current) – [External links & references] greg.sikora | ||
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The conditioning circuit for an LVDT is relatively complicated and plays an important part in the linearity and precision of the device (Nyce2004). Firstly, an oscillator excites the primary winding which is coupled with the secondary windings each to a degree dependent upon the position of the ferromagnetic core. If the core is in the centre position, then the secondaries will be equally coupled to the primary. If the core is displaced from its centre position, then one of the secondaries will be more strongly coupled and will echo the excitation signal to a greater degree. | The conditioning circuit for an LVDT is relatively complicated and plays an important part in the linearity and precision of the device (Nyce2004). Firstly, an oscillator excites the primary winding which is coupled with the secondary windings each to a degree dependent upon the position of the ferromagnetic core. If the core is in the centre position, then the secondaries will be equally coupled to the primary. If the core is displaced from its centre position, then one of the secondaries will be more strongly coupled and will echo the excitation signal to a greater degree. | ||
- | One of the secondary windings, being wound in opposite direction to the other two inductors, will output the excitation signal in opposite phase. This is used in order to obtain a differential voltage indicating the direction of the displacement: | + | One of the secondary windings, being wound in opposite direction to the other two inductors, will output the excitation signal in the opposite phase. This is used in order to obtain a differential voltage indicating the direction of the displacement: |
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+ | The simplest methods of demodulation involve some form of diode rectification, | ||
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+ | A simplified ratiometric signal conditioning (Szczyrbak1997): | ||
+ | {{ : | ||
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+ | The above-simplified schematic requires 5-wire LVDT (as oppose to the 4-wire LVDT appropriate for the phase-sensitive demodulation). Both secondary coils' outputs are processed independently, | ||
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+ | A fully digital approach can be applied as well, where the output of both coils is fed through ADC and processed further in the digital domain (Vemuri2017): | ||
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+ | ---- | ||
+ | Below, a __simplified__ mathematical simulation of the LVDT (MaxMSP) demodulation: | ||
+ | {{ youtube> | ||
+ | ---- | ||
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+ | ==== Signal Conditioning (Other) ==== | ||
+ | Another interesting approach for demodulation consists of the low-cost elements and is based on the sample & hold circuitry instead of the traditional rectification and low-pass. Such an approach provides better speed performance (integration every half-cycle of excitation frequency) of the LVDT and reduces circuit complexity. | ||
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+ | In (Petchmaneelumka2017) "(…) simple circuit technique to realize the LVDT signal to DC voltage converter is introduced. The technique is based on the ratio of sum and difference of the signals from two secondary windings. The proposed scheme comprises an operational amplifier (opamp) and operational transconductance amplifier (OTA) as an active circuit building block. The sum of two secondary winding signals is provided for the reference signal to generate the control signal for the SHC. The control signal for the SHC is obtained by the peak-amplitude finder (…)" | ||
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+ | The time-series below illustrate the signal path progression, | ||
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===== Devices ===== | ===== Devices ===== | ||
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* (Hoadley1936) George B Hoadley, “Telemetric System”. US Patent Application #2196809, March 17th 1936 (Patented on April 9th 1940). | * (Hoadley1936) George B Hoadley, “Telemetric System”. US Patent Application #2196809, March 17th 1936 (Patented on April 9th 1940). | ||
* (Powell2009) Mike Powell, // | * (Powell2009) Mike Powell, // | ||
+ | * [[http:// | ||
+ | * [[https:// | ||
+ | * [[https:// | ||
* eFunda, 2005, // | * eFunda, 2005, // | ||
* RDP Group, 2002, // | * RDP Group, 2002, // |