Event Driven Phase-Locked Loop with Application to Atomic Force Microscopy Oscillation Non-Resonant Modes

Traditional PLLs are circuits synchronizing a generated periodic, usually harmonic, signal with a reference input signal in frequency (period) and phase. They are sophisticated, usually expensive, nonlinear control systems with difficult to achieve stability. Traditional PLLs are intensively used in various applications including RF communications, mechatronics, AFM (Atomic Force Microscopy) resonant modes, etc. However, similar synchronization for lower frequencies can be achieved by significantly simpler control methods and schematics. One such new method, called EDriPLL (Event Driven PLL), is the subject of this paper. The idea is to design a discrete event system that receives and generates events at the start of a new (maybe variable) period of the reference signal. Thus, EDriPLL design belongs to the discrete event systems field rather than electronic signal schematics. It can be applied, for instance, when synchronizing a team of robots with subordinate robots receiving sync events with transition noise from the leader; they estimate actual time of the sending events and synchronize themselves with the leader at high frequency. The application illustrated in this paper is estimation of times of events when the AFM tip (periodically) begins to move closer to the sample in the ONR (Oscillation Non-Resonant) mode. ONR mode allows mapping of force curves and related material properties of the sample in real time. High throughput LabVIEW FPGA implementation of EDriPLL and an illustration of an ONR AFM mode synchronized by EDriPLL are presented. There are many other applications of this technique in mechatronics and robotics. It may also be used in human-machine interactions.