Fig. 1

Fig. 2
The voltage signal of the inductive sensors (LVDTs) of the ZHN is recorded with AD converters of 24 bit resolution and a frequency of 156 kHz. The effective digital resolution for a maximum acquisition rate of 10 kHz is 28 bit. This corresponds to a resolution of the displacement measurement of better than
0.002 nm.

The standard deviation of the noise floor for an acquisition rate of 16 Hz is typically less than 0.3 nm for the displacement measurement and less than 3 µN for a force measurement with the 2 N head. This is the best what could be reached for such a sensor up to now. Smaller acquisition rates will further reduce the noise level. The level is slightly higher at the end of a measurement range.

The big advantages of LVDTs against capacitive sensors consist in a significant larger measurement range of up to 1 mm (for the used sensor type) and a much larger overload protection. Because there are no limit stops like for capacitive sensors, it cannot be destroyed easily during overloading. This makes the ZHN heads so robust.

Fig. 1 is showing the noise level for the maximum force of 2 N over a period of 200 s. In contrast to many other instruments the force signal is measured with a second sensor, completely independent on voltage or current of the actor. The standard deviation of the difference to a fit curve (red) is 3.2 µN. The slight curvature of the curve comes from the feedback control which is quite week for such small differences from the target value.

Fig. 2 is showing the displacement signal during a hold period of one minute for the determination of thermal drift. For a smaller data rate the standard deviation of the displacement noise floor is only 0.15 nm. The thermal drift was -0.068 nm/s.