Eliminating Coupled Motion
>>High Resolution Sensors
Position sensors are the most critical elements in any closed loop nanopositioning system. Mad City Labs nanopositioning systems use proprietary PicoQ® sensor networks. PicoQ® sensor are in the piezoresistive family of solid state devices that can be thought of as ultra sensitive strain sensors. They make a highly linear, ultra precise, temperature compensated position sensor when appropriately integrated into a flexure guided nanopositioning stage. Mad City Labs PicoQ® sensors have numerous advantages over traditional capacitance sensors.
The ultra sensitive response of PicoQ® sensors allows for measurement resolution equal to or better than that obtained with capacitance sensors. Sub-nanometer positioning sensitivity is typically obtained using PicoQ® sensors. In contrast to capacitance sensors, the sensitivity of our PicoQ® sensors is constant over the entire range of motion and is not effected by cable length.
Sensor electronics for PicoQ® sensors are inherently DC while sensor electronics for capacitance sensors are inherently AC. This results in some significant advantages for PicoQ® sensors - such as extremely low 1/f noise. The low impedance of piezoresistive sensors also helps to reduce position noise since the sensor's electrical noise is equal to the Johnson noise of the piezoresistor.
|Position Noise Spectrum of the Nano-HS3 Z Axis showing an ultra-low position noise floor at 1.4 picometers and high stability across the whole spectrum. Note the lack of rising 1/f noise at the left (low frequency) end of the spectrum.
For further discussion of position noise, see the Nanopositioning Accuracy page and the article "Understanding Noise at the Nanometer Scale".
Machining Errors While an ideal capacitance position sensor is linear, real-world capacitance position sensors are not. Surface roughness, imperfect parallel alignment, stray fields, and variable capacitance to ground adversely affect the linearity. None of these problems are associated with PicoQ® sensors that are integrated directly into the nanopositioning stage.
UHVPicoQ® sensors are ultra high vacuum compatible.
Coupling Capacitance sensors respond unfavorably to coupled motions (roll, pitch, and yaw) in integrated XY systems. The capacitor can not tell the difference between a small rotation and a small gap change. The capacitance sensor control electronics compensates for the coupled motion by translating the stage in an orthogonal direction, leading to positioning errors. Mad City Labs XY nanopositioning systems are designed to minimize roll, pitch, and yaw, leading to greater positioning accuracy.