Designing a High-Precision AC Current Source to Measure the nm-Scale Displacements in Mechanical Systems

Measuring the dynamics of a continuously moving target, at the nanoscale level, such as of a bearing raceway or any other vibrating element, requires a contactless measurement. A mission that can be easily carried out by a capacitive probe. The current flowing through the sensor has to remain constant regardless of the changes in the sensors’ output impedance. In this manner, the voltage across the probe is proportional to the distance between the probe and the target. However, the high impedance of the probe cannot be disregarded in comparison to the output impedance of the Howland source. To overcome this problem, we designed a voltage-controlled AC current source (VCCS). The new design consists of implementing two nested loops and uses two cascaded controllers, a fast but imprecise internal control loop tuned by a slow but precise outer control loop in order to obtain a high-precision AC output current. The performance of this device has been compared with the improved quad op-amp current source (IQOA). The results obtained during the numerical validation confirm the relevance of this device.