Connecting an LVDT (linear-variable-differential transformer) to a microcontroller can prove challenging because an LVDT requires ac-input excitation and measurement of ac outputs to determine its movable core's position (Reference 1). Most microcontrollers lack dedicated ac-signal-generation and -processing capabilities and thus require external circuitry to generate harmonic-free, amplitude- and frequency-stable sine-wave signals. Conversion of an LVDT's output signals' amplitude and phase into a form compatible with a microcontroller's internal ADC usually requires additional external circuitry.
In contrast with conventional microcontrollers, Cypress Semiconductor Corp's PSoC microcontrollers include user-configurable logic and analog blocks that simplify generation and measurement of ac signals. PSoC devices have the unusual feature of being able to generate analog signals without demanding continuous CPU attention. The PSoC's flexible analog and digital blocks can drive an LVDT and measure its outputs without requiring any external circuitry. Figure 1 shows the complete circuit of the LVDT interface, and Figure 2 shows the PSoC microcontroller's internal circuit blocks.
The PSoC uses pairs of user-configurable switched-capacitor blocks to implement both bandpass and lowpass filters. You can create a high-quality sine wave by generating a square wave and applying it to a PSoC switched-capacitor filter through a modulator built into the first switched-capacitor block. Passing the square wave through a narrow bandpass filter centered on the square wave's fundamental frequency removes most of the harmonics.
To obtain the highest fidelity sine waveform from a PSoC switched-capacitor bandpass filter, use the highest possible oversampling rate—a factor of approximately 33—or 33 steps per sine-wave cycle. The resultant sine wave is smooth enough to drive an LVDT, which attenuates any residual higher order harmonics. Scaling the PSoC's internal voltage reference with a programmable-gain amplifier provides coarse control over the square wave's amplitude before it undergoes filtering. To compensate for the waveform's dc-offset voltage, an amplifier buffers the 2.6V internal analog-ground reference and drives an output pin that serves as the LVDT's analog-ground return.
The LVDT's output consists of a variable-amplitude sine-wave voltage whose phase angle with respect to the sine-wave excitation voltage undergoes a significant and variable shift that sometimes exceeds 180°. A signal from the LVDT drives one of the PSoC's programmable-gain amplifiers, whose output feeds a switched-capacitor lowpass filter followed by a modulator for synchronous rectification. The rectified signal drives an output pin and one of the PSoC's switched-capacitor ADCs.
Applying the LVDT's output to a synchronous rectifier followed by a lowpass filter produces a dc voltage that can feed an ADC or directly drive an analog feedback-control system. In a PSoC microcontroller, a lowpass switched-capacitor filter connected to an ADC requires that the same sample clock drive both circuits, resulting in a conversion rate for the PSoC's 11-bit delta-sigma ADC that's approximately one-half of the lowpass filter's corner frequency. Synchronous rectification produces a ripple frequency twice that of the excitation frequency and thus is easier to remove with a lowpass filter. Relocating the lowpass filter's corner frequency to one-third of the excitation frequency allows measurements of the LVDT's output to 11-bit resolution with a standard deviation of 1 LSB (least significant bit) or less.
Dividing the PSoC's 24-MHz internal system clock with logic blocks configured as counter chains generates all of the digital clock signals the switched-capacitor analog-circuit blocks require. After power application or a reset, the PSoC's CPU configures all the configured analog and digital blocks and starts their operation. From then on, the hardware excites the LVDT and measures its output at 500 samples/sec without further intervention by the CPU. With the PSoC's CPU running at 12 MHz, processing the ADC's housekeeping activities and interrupts consumes less than 3% of the CPU's resources.
Plenty of the PSoC's resources remain available for calculating the LVDT's position and for displaying the results in text format on an LCD module. Four analog blocks, five logic blocks, and many I/O pins remain available to support a more demanding application. Figure 3 shows configurable blocks that are available for adding features.
Illustrations:
Figure 1
