Automatic Generation Of PID Parameters For An Atomic Force Microscope

• US 20090062935A1
• Filed: 08/29/2007
• Published: 03/05/2009
• Est. Priority Date: 08/29/2007
• Status: Active Grant

• Current Assignee: Keysight Technologies, Inc.
• Original Assignee: Agilent Technologies, Inc.
• Inventors: Hoen, Storrs T., Workman, Richard K., Abramovitch, Daniel Y.
• Granted Patent: US 7,987,006 B2
• Time in Patent Office: Days
• US Class Current: 700/42
• CPC Class Codes: B82Y 35/00 Methods or apparatus for measurement or analysis of nanostructures
• CPC Class Codes: G01Q 10/065 Feedback mechanisms, i.e. wherein the signal for driving the probe is modified by a signal coming from the probe itself
• CPC Class Codes: G01Q 30/06 for error compensation

Abstract

Linear PID controllers have a transfer function that resembles the frequency response of a notch filter. The PID parameters, K_P, K_I, and K_D (proportional, integral, and derivative gains, respectively) can be extracted from the parameters of a linear notch filter. The linearized modes of scanning probe microscope (SPM) actuators have frequency responses that resemble those of simple second order resonance. Reasonable feedback control can be achieved by an inverse dynamics model of the resonance. A properly parameterized notch filter can cancel the dynamics of a resonance to give good closed-loop response.

Claims

1. An instrument comprising:

a scanning probe microscope;

a PID parameter generator generating PID parameters; and

a proportional-integral-derivative (PID) controller receiving the PID parameters, controlling the scanning probe microscope beyond a main system resonant frequency.

2. A method for controlling a scanning probe microscope comprising

automatically generating PID parameters;

and

applying the PID parameters to operate the scanning probe microscope beyond a main system resonant frequency.

3. A method for controlling a scanning probe microscope, as in claim 2, automatically generating PID parameters comprising:

generating a scanning probe microscope model according to its associated dynamics;

selecting filter parameters that shape selected dynamics of the model;

generating a notch filter using the filter parameters;

encoding the notch filter as PID parameters; and

implementing the PID parameters in a PID controller to control the scanning probe microscope.

4. A method for controlling a scanning probe microscope as in claim 3, where the model of the SPM dynamics is a continuous-time model.

5. A method for controlling a scanning probe microscope as in claim 3, where the model of the SPM dynamics is a sampled-data model.

6. A method as in claim 3, generating a model comprising:

measuring open loop frequency response measurement of the scanning probe microscope; and

extracting an open loop actuator model of the scanning probe microscope dynamics.

7. A method as in claim 3, generating a model comprising:

measuring a closed loop frequency response of the scanning probe microscope;

and extracting an open loop actuator model of the scanning probe microscope dynamics.

8. A method as in claim 4, wherein the notch filter and PID parameters are generated directly from the continuous-time model.

9. A method as in claim 4, comprising:

discretizing the identified continuous time model; and

generating the notch filter and PID parameters from the digital model.

10. A method as in claim 3, wherein the resultant PID controller is implemented using analog circuitry.

11. A method as in claim 3 wherein the resultant PID controller is implemented using digital logic.

12. A method as in claim 3, wherein the filter parameters correspond to a notch filter with a gain, resonant frequency, and quality factor derived from the dynamics of the model.

13. A method as in claim 3, wherein:

one of the filter parameters is gain; and

the gain is selected such that magnitude of the PID controller response is at a desired valued for a given frequency.

14. A method as in claim 3, wherein:

one of the filter parameters is gain; and

the gain is selected such that the open loop magnitude of the combined PID controller and scanning probe microscope model is unity at a desired frequency.

15. A method as in claim 2, automatically generating PID parameters comprising:

measuring the closed loop system response during operation with a nominal controller;

deriving the open loop response from the closed loop measurement;

removing the effect of the nominal controller from the open loop measurement to yield the actuator response; and

performing a curve fit on a selected portion of the actuator response to obtain a transfer function; and

generating resonance parameters from that transfer function using those parameters to design a PID controller.