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Deciding Between Open-Loop and Closed-Loop Piezo Stages

The term “closed-loop” in nanopositioning indicates that a sensor is used to monitor the position of the stage in real-time. These readings feed back to the controller to provide error correction for the system. Closed-loop control ensures that the nanopositioner reaches the desired commanded position. nPoint uses interferometric calibration for each stage to confirm position throughout a movement or scan. Digital electronics allow for calibration features that are not available in simplistic analog control. This ensures the accuracy of scans not available with other non-calibrated systems on the market. Closed-loop control provides fast, accurate positioning with minimal piezo drift.

Closed-loop also allows for faster control movements such as in step and settle applications.  The following is an example of a long travel nPFocus400 stage with a 220g microscope objective. Data is taken directly from nPControl GUI, standard with every controller. PID control and notch filters are used to achieve the settling time in closed-loop. More information on setting these parameters are available on the nPControl Software Page.

Open-loop settling time of 984 ms. nPFocus400 with a 220g load
Open-loop settling time of 984 ms. nPFocus400 with a 220g load
Open-loop settling time of 984 ms. nPFocus400 with a 220g load
Closed-loop settling time of 39 ms. nPFocus400 with a 220g load

Linearity deviation is the error between the actual position and the first order best fit line. Linearity deviation in open-loop will typically be 7% while the deviation of a calibrated system using capacitive sensors will be lower than 0.01%. As an example, on a 100µm stage the actual position of the stage can have a deviation up to 7% over the commanded position at any given location within the scan. This could result in a location deviation of 7µm. A commanded position of 50µm could be within 43µm and 57µm. A similar closed-loop system with a linearity deviation of 0.01% would be within 50.01um and 49.99um for a commanded position of 50µm.

Closed-loop control also reduces drift effects of the piezo actuator. During the first minute from when a signal is sent to the actuator drift will be approximately 0.5% of the travel range. The second minute of operation will have approximately 0.1% drift. After this time, the percentage of drift will steadily decrease to less than 0.02% per minute. When used in closed-loop control, piezo drift is dramatically reduced to maintain position as long as the system is under power.

Data taken with an interferometer exhibits
open-loop drift over a 10 minute period.

Open-loop systems have lower noise (and resolution) than a closed-loop system because the sensor noise and driver compensation is eliminated from the system. nPoint’s piezo stages are listed for resolution in closed-loop. Electronics’ resolution and noise will often be the limiting factors in open-loop resolution. nPoint electronics have 20-bit resolution.

Closed-loop control is important for most positioning applications. Certain applications allow a user to close their own loop with external feedback methods (interferometric etc.) that can interface directly to nPoint controllers. Open-loop control is an economical choice for applications where known position with internal sensors is not necessary or where step and settle speed is not as important.

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