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| | | Piezo • Nano • Positioning | | |
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| | | Measuring Nanometers: Stage Metrology Selection | | |
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| | | | | | | | | | | | | | | | | | | Table 1 | | | | | | | | | | Sensor | *Sensitivity | Linearity* | * Stability / | *Band- | Metrology | Excitation | | | | Type | (Resolution) | | Repeatability | width | Type | Signal | | | | Capacitive | Best | Best | Best | Best | Direct / Non-contact | AC | | | | Strain | Better | Good | Good | Better | Inferred ** (Indirect) / Contact | DC | | | | LVDT | Good | Good | Better | Good | Direct / Noncontact | AC | | | | | | | | | | | | | | |
| | | Achieving nanometer and subna-nometer precision requires more than a piezo stage capable of making moves on this precision scale. The stage internal metrology system must also be capable of measuring motion on the nanometer scale. The five primary characteristics to consider when selecting a stage metrology system are linearity, sensitivity (resolution), stability, bandwidth, and cost. Other factors include the ability to measure the moving platform directly and contact vs. noncontact measurement. Three types of sensors are typically used in piezo nanopositioning applications—capacitive, strain, and LVDT. Table 1 summarizes the characteristics of each sensor type. PI capacitive sensors measure the gap between two plates based on electrical capacitance. These sensors can be designed to become an integral part of a nanopositioning system, with virtually no effect on size and mass (inertia). Capacitive sensors offer the highest resolution, stability, and bandwidth. They enable direct measurement of the moving platform and are noncontact. Capacitive sensors also offer the highest linearity (accuracy). PI's capacitive sensors / control electronics use a high-frequency AC excitation signal for enhanced bandwidth and drift-free measurement stability (subnanometer stability over several hours, see p. 5-6). PI's exclusive ILS linearization system further improves system linearity. If used with PI's digital controllers, digital polynomial linearization of mechanics and electronics makes possible overall system linearity of better than 0.01%. Capacitive sensors are the metrology system of choice for the most demanding applications. | | |
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| | | Nanopositioning & Scanning Systems | | |
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| | | Active Optics / Steering Mirrors Tutorial: Piezo-electrics in Positioning Piezo Drivers & Nano-positioning Controllers Hexapods/ Micropositioning Photonics Alignment Solutions Index | | |
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| | | * Note. The ratings describe the influence of the sensor on the performance of the whole nanoposi-tioning system. Resolution, linearity, repeatability, etc. specifications in the PI product data sheets indicate the performance of the complete system and include the controller, mechanics and sensor. They are verified using external nanometrology equipment (Zygo Interferometers). It is important not to confuse these figures with the theoretical performance of the sensor alone. ** Strain type sensors (metal foil, semiconductor, or piezoresistive) infer position information from strain. | | |
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| | | the guiding system of a flexure stage. It offers high resolution and bandwidth and is typically chosen for cost-sensitive applications. As a contact type sensor, it measures indirectly, in that the position of the moving platform is inferred from a measurement at the lever, flexure or stack. PI employs full-bridge implementations with multiple strain gauges per axis for enhanced thermal stability. PI's PICMA® drive technology also enables higher performance of | | actuator-applied strain gauge sensors. LVDT sensors measure magnetic energy in a coil. A magnetic core attached to the moving platform moves within a coil attached to the frame producing a change in the inductance equivalent to the position change. LVDT sensors provide noncontact, direct measurements of position. They are cost-effective and offer high stability and repeatability. | | |
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| | | Fig. 3. Response of a PI Nanopositioning stage to a square wave control signal clearly shows the true sub-nm positional stability, incremental motion and bidirectional repeatability. Measured with external capacitive gauge, 20 pm resolution. | | |
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| | | A strain gauge sensor is a resistive film bonded to a piezo stack or—for enhanced precision—to | | |
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