Nano-positioning Santosh Devasia Mechanical Eng. Dept., UW http://faculty.washington.edu/devasia/
Outline of Talk 1. Why Nano-positioning 2. Sensors for Nano-positioning 3. Actuators for Nano-positioning 4. Current Efforts: Large range and high precision 5. Summary
Measuring Molecular Mechanical Properties (Pioneered by H.Gaub and J.Fernandez) Here a DNA strand is being stretched... Ref: Dynamic Force Spectroscopy of single DNA Molecule, TORSTEN STRUNZ*, KRISZTINA OROSZLAN, ROLF SCHA FER, AND HANS-JOACHIM GUNTHERODT, Proc. Natl. Acad. Sci. USA, Vol. 96, pp. 11277 11282, September 1999 Biophysics
Measuring Molecular Mechanical Properties Main idea --- find the force at different displacement (stretch levels) Force Displacement Referred to as the Force-Displacement Curve The force gradually increases and then falls abruptly because of sacrificial bonds that break. Sacrificial bonds make bone tougher
Why Force-Displacement Curves? Displacement Image from http://hansmalab.physics.ucsb.edu/forcespec.html Ex: Determine how the environment of the molecule affects its behavior The curve of bone molecules show that the forces to extend molecules found in bone increase with the presence of divalent calcium ions in the buffer. Many labs try to understand such structure/function relation in molecules. Is considered as a Molecular Signature
Obtaining the Force-Displacement Curve
Step 1: Attach Molecule Tip Molecule Cantilever Attach the molecule of interest between a cantilever and a surface (Important issues: but not the focus of this talk)
Step 2: Stretch the molecule Tip Molecule Cantilever Relative motion Move the base with respect to the cantilever to stretch the molecule (or move the cantilever up) this is the displacement X
Step 3: Measure the Force Tip Molecule Cantilever Relative motion Measure the force F (how?)
Measure the force Tip Molecule Cantilever Relative motion The cantilever is like a spring; its deflection corresponds to force Change in deflection can be measured by reflecting a laser on the cantilever
Recap: Force Displacement Curve Photo-detector Laser Tip Molecule Cantilever Fine Positioning Force Displacement
Recap: Force Displacement Curve Photo-detector Tip Molecule Laser Cantilever Find force by measuring the deflection of the cantilever. Find deflection (stretch) as the motion of the base relative to the cantilever. Fine Positioning Force Displacement
Recap: Force Displacement Curve Photo-detector Laser Find force by measuring the deflection of the cantilever. Force Tip Cantilever Molecule Fine Positioning Find deflection (stretch) as the motion of the base relative to the cantilever. This set-up is common in the atomic force microscope (AFM) Displacement
Need for Nano-positioning Photo-detector Laser Find force by measuring the deflection of the cantilever. Force Tip Cantilever Molecule Fine Positioning Find deflection (stretch) as the motion of the base relative to the cantilever. Need for positioning? Displacement is at-least the orders as the molecule size, i.e., in the nanometers --- à nanopositioning! Displacement
So, Why Nano-positioning? Photo-detector Tip Molecule Laser Cantilever 1) Single molecular studies 2) See distribution of molecules over a relatively-large area, e.g., over a biological cell (requires nanopositioning over large micron-range area) Nano-Positioning
So, Why Nano-positioning? 1) Single molecular studies 2) See distribution of molecules over a relatively-large area, e.g., over a biological cell (requires nanopositioning over large micron-range area) Image from http://www.zurich.ibm.com/st/storage/ 3) Other applications: nano-storage devices, Combinatorial AFM (but not the focus of this talk) 4) Focus of this talk 1) Nano-positioning 2) Large-range Nano-positioning
Issues in Nano-positioning 1) Sensors: would like to measure what we are doing at nano-scale otherwise we don t know if we are moving the right amount 2) Actuators: should be able to achieve the precision (Motors do not have such precision) 3) Bandwidth: How fast can we position, and measure. Don t want this to take hours (if possible). 4) Range: Want to measure properties over a large area
Issues in Nano-positioning Details in A Survey of Control Issues in Nanopositioning by S. Devasia, E. Eleftheriou, and R. Moheimani. IEEE Transactions On Control Systems Technology, 15(5), pp. 802-823, 2007 Will cover the main concepts in this talk
Outline of Talk 1. Why Nano-positioning 2. Sensors for Nano-positioning 3. Actuators for Nano-positioning 4. Current Efforts: Large range and high precision 5. Summary
Types of Nano-positioning Sensors 1) Inductive Sensors 2) Capacitive Sensors 3) Piezo Sensors 4) Thermal Sensors 5) Optical Sensors Review the operating principle of a couple of them (Details in A Survey of Control Issues in Nano-positioning)
Ex 1: Inductive Sensor Image From: http://newton.ex.ac.uk/teaching/cdhw/sensors/ 1. Primary Coil on Left 2. Secondary Coil on Right 3. Coupling depend on the material between them 4. Movement of ferromagnetic material changes 5. magnetic coupling between the two coils (different voltage level is induced in secondary coil) 6. Induced Voltage signal can be used to measure the motion 7. Advantage: Non-contact (no friction issues) 8. Resolution: Nanometers
Ex 2: Capacitive Sensor http://www.analog.com/library/analogdialogue/archives/40-10/ad40-10_01.jpg http://images.apple.com/ipad/ Variation of the capacitance-sensors used in touch panels, as in your i-pad
Ex 2: Capacitive Sensor http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imgele/ppcap.gif 1. C is inversely proportional to distance d 2. Motion changes the distance d between the plates. 3. The resultant change in capacitance C is measured to determine d
Outline of Talk 1. Why Nano-positioning 2. Sensors for Nano-positioning 3. Actuators for Nano-positioning 4. Current Efforts: Large range and high precision 5. Summary
Types of Nano-positioning Actuators 1) MEMS-based Electrostatic Actuators 2) Piezo-electric Actuators 3) Magneto-strictive Actuators 4) MEMS-based Electromagnetic Actuators 4) MEMS-based Thermal Actuators Review the operating principle of a couple of them (Details in A Survey of Control Issues in Nano-positioning)
Ex 1. Electrostatic Actuation Voltage is applied (say + to Stator and to shuttle); the shuttle moves to reduce the distance between the electrically charged plates.
Ex 2: Piezo-based Actuators Will discus this actuator in more detail Reason 1) I work in these actuators Reason 2) Commonly used in AFM (used to measure Molecular Mechanics)
Operating Principle of Piezos Piezo-effect first discovered by the Curie brothers (1880) Squeeze certain materials --- you get an electric charge (can be a sensor) http://www.aip.org/history/curie/images/photos/0901-matte.jpg Pierre Curie (upper right) with his brother Jacques and parents
Converse Piezo Effect Apply a voltage. The material will change shape (fine displacement with sub-nano precision)
3-axis nano-positioner--- tube piezos Apply voltage +x expands x contracts to get bending Similarly +y and y for bending in other direction If you expand all of them you get elongation
Issues in Nano-positioning 1) Sensors: would like to measure what we are doing at nano-scale otherwise we don t know if we are moving the right amount 2) Actuators: should be able to achieve the precision (Motors do not have such precision) 3) Bandwidth: How fast can we position, and measure. Don t want this to take hours (if possible). 4) Range: Want to measure properties over a large area
Bandwidth --- High Speed Positioning If you try to position faster you start getting vibrations in the position (Causes errors in the force-displacement curves)
Goal: High Speed Nano-Positioning But without vibrations in the displacement
The Research Problem in high-speed nano-positioning Find the input voltage to piezo that achieves a desired displacement (stretch) y d --- we use inversion approach Desired Stretch Time (t)
What is Inversion-Based Control? Input Output Consider a System --- My Nephew Let the desired output be, say, eat dinner!
What is Inversion-Based Control? Input Output = Y d Let the desired output be, say, eat dinner! Question: What input should you apply? (negotiate, encourage,???)
What is Inversion-Based Control? Input Output = Y d Let the desired output be, say, eat dinner! Question: What input should you apply? (negotiate, encourage, bribe always works for me!)
The Inversion-Problem Input =? Invert System Model Desired Output Prior Knowledge Invert the known system model (G 0 ) to find input. Input = G 0-1 [ Desired Output]
The Inversion-Problem Input =? Invert System Model Desired Output Prior Knowledge Invert the known system model (G 0 ) to find input. Input = G 0-1 [ Desired Output] (His Mom know s how --- she has a reasonable model)
The Control method using Inversion Desired Output G 0-1 Invert System Model Input System G Output Prior Knowledge Actual System Use Inverse input as the feedforward input to system
Feedforward is Common in Human Systems Desired Output G 0-1 Invert System Model Input System G Output Prior Knowledge Actual System Examples: Walking, Playing Baseball, Driving a Car
Problem --- model uncertainty Desired Output G 0-1 Invert System Model Input System G Output Prior Knowledge Actual System Is Desired output = Output? Yes if we know the model perfectly! But, we rarely know a system perfectly (G 0 G, G 0-1 G -1 )
Solution: Addition of Feedback Desired Output G 0-1 Invert System Model + Input + System G Output K Prior Knowledge + - Actual System Observation Exploit knowledge of the system through feedforward input Account for errors (uncertainties, perturbations) using feedback
Re-Cap Key Idea: Feedforward Input is found using System Inversion Desired Output Invert System Model Input Output G 0-1 G (1) Feedforward input uses system knowledge to control the output (2) Feedforward should be integrated with feedback
Use in Piezo Nanopositioners Desired Output G 0-1 Input G Output System inverse is used to find input voltages, u a, which compensate for positioner dynamics and achieve the desired nano-scale displacement, i.e. y = y d
Key Results Sub-Angstrom Level Positioning in STM At low speeds one can get good images Lattice pattern of carbon atoms in graphite (HOPG) Positioning is achieved at subangstrom level
Key Results Sub-Angstrom Level Positioning in STM At high-scan frequency 445Hz, distortions appear in the image Our Lab was the first to demonstrate that dynamics induced vibrations in SPM can be compensated by using feedforward
Without Feedforward Image at 445 Hz with distortions!
Results: With Feedforward Feedforward Input compensates for the positioning errors [1] Croft and Devasia Vibration Compensation for High Speed Scanning Tunneling Microscopy, Review of Scientific, Vol. 70 (12), pp. 4600-4605, 1999. [2] Croft, Mcallister and Devasia High-Speed Scanning of Piezo-Probes for Nanofabrication, J. of Manufac. Sci. and Engin., Vol. 120 (3), pp. 617-622, 1998.
Issues in Nano-positioning 1) Sensors: would like to measure what we are doing at nano-scale otherwise we don t know if we are moving the right amount 2) Actuators: should be able to achieve the precision (Motors do not have such precision) 3) Bandwidth: How fast can we position, and measure. Don t want this to take hours (if possible). 4) Range: Want to measure properties over a large area
Increasing Range Piezos have great precision --- But small range How can we increase the range? Solution: Make Multiple Steps! Just like Walking
Experimental Nanostepper System Piezo Actuators (small range)
Experimental Nanostepper System Current challenges --- vibrations during each step needs to be reduced --- optimizing the design (number of legs and motion of each leg)
Summary of Nano-positioning 1. Important to Molecular Studies 2. Issues in Choice of Sensors and Actuators 3. High-speed Positioning can cause Vibrations 4. Control Methods can increase speed while suppressing vibrations 5. Large Range Nano-positioning is still a challenge 6. Nano-steppers might be a solution (on-going work)