The stage's servo controller is another important part of the nanopositioning setup, and it can come in either analog or digital versions.
Analog controllers are capable of high speed and are very simple to use. Calibration is performed at the factory and fixed in the system; the electronics and motion device are therefore generally matched together. The position command input to the controller can be a voltage or a digital command sent via a USB, RS-232, IEEE-488 or proprietary high-speed serial interface. The best interface is not necessarily the one with the highest data transfer speed, because piezo servo controllers and motion controllers typically send and receive only a few characters at a time. Exact timing and minimal latency is more important for virtually all nanopositioning applications, particularly in time-critical tracking and scanning applications where the motions must be tightly coordinated with metrology or other processes.
In the case of an analog servocontroller with an analog voltage command input, the input voltage maps linearly to position. For analog controllers equipped with a digital communications interface, the incoming command is converted to a voltage input to the servo circuit by an internal digital-to-analog converter.
Most commonly, a digital-to-analog converter card installed in the user's PC provides the position command voltage for an analog servocontroller. For example, multifunction cards and applications software written with LabVIEW from National Instruments of Austin, Texas, are often used in optical trapping applications, and the availability of well-designed driver libraries and knowledgeable staff is an important starting point for selecting positioning equipment.
Analog interfacing is fast, with negligible latency, and available tools make it easy to program waveform generation, such as to perform force metrology for trap calibration. It is straightforward to program intelligence into the trap-control software, including tightly integrated machine vision for visualization, analysis and position tracking, and for position and force sensing. Synchronization between voltage outputs and data acquisition is easy to arrange, making it straightforward to tightly couple motion and metrology processes. Such is not always the case with commands sent via digital communications interfaces, unless they are based on a real-time protocol. On the other hand, servo controllers with the latest communications interfaces offer internal functionality, such as waveform generation plus synchronization lines, for integration with other instruments so that motion, metrology, video and other processes may be coordinated with good timing accuracy and responsiveness.
Importantly, the resolution of the internal digital-to-analog converters (DACs) that are integrated into the latest analog servocontrollers can be greater than the resolution of available PC converters and multifunction cards. Twenty-bit internal converters are increasingly common, which gains importance as increasingly longer travel piezo devices are introduced - devices with travel exceeding 1000x1000µm are now available. The "bitness" of the converter defines how small a motion can be commanded by the following formula:
Resolution (µm) = Travel (µm) / 2bits
Popular digital-to-analog converter and multifunction cards typically top out at 16 bits, or 65,536 possible addressable positions for the piezo nanopositioner. The resulting position resolution would seem to pose significant limitations for applications with nanometer sensitivities such as optical traps. Fortunately, HyperBit, a patented technology from PI (Physik Instrumente) L.P. of Auburn, Mass., provides additional subdivision of the DAC resolution to improve positioning resolution by many bits - up to two orders of magnitude - with no loss of bandwidth or accuracy and with ready compatibility with existing user programs and most converter hardware. Compatible with popular lab automation hardware and software from National Instruments and other manufacturers, this technology breathes new life into analog-interfacing setups for long-travel applications.