The sensor is made of a simple coil with a ferrite core. Together with a resistor this forms an R-L network, the time constant of which is measured by a TDC. Placing a vane of the compressor wheel in front of the coil will change the time constant of the R-L network. This is caused by the influence of the vane metal on the parameters of the coil inductance L. A DSP-unit following the TDC measurement unit processes the time data, identifies the passing of each single vane and calculates the rotational speed.
The time measurement is done with very high precision. The resolution is about 125 ps. The time constant of the R-L network can be very small, allowing high sampling rates in the range of 1 to 3 million samples per second.
The physical effect of the vane metal on L varies, depending on the metal. The PICOTURN system is currently optimized for aluminum wheels. The system is basically suitable for other metals, too. However, the measurement sensitivity, and correspondingly the required distance between sensor and wheel, will vary depending on material and vane thickness.
For aluminum, the maximum usable distance between sensor and wheel varies typically from 0.8 mm (32 mm wheels) to 1.6 mm (90 mm wheels). Thanks to the ratio measurement no trimming of the sensor coil is required. The length of the cable between sensor and controller may be up to a few meters. This allows separation of the electronics from the sensing components, reducing thermal and vibrational influences on the electronics. For longer distances, it is recommended to employ the PT2G-system, featuring increased noise immunity through integrated signal processing and extension cables only on the digital side of the system.
This special kind of measurement fully relies on magnetic fields. It is thus completely immune against distortion or sensitivity degradation from dirt or other factors which would influence optical or electric field detection. It detects the wheel itself and requires no modification of rotating parts.
