Capacitive Position Metrology
Overview

Properties of PI Sensors
  • Measurement Ranges from 10 up to 500 µm and More
  • Sub-Nanometer Position Resolution
  • Non-Contact Absolute Measurement of Displacement / Motion / Vibration
  • Immune to Wear and Tear
  • Ideal for Multi-Axis Applications
  • Improved Linearity with ILS Signal Electronics
  • High Bandwidth up to 10 kHz
  • Measures Position of the Moved Interface (Direct Metrology)
  • High Temperature and Long-Term Stability (<0.1 nm/3 h)
  • Vacuum Compatible
  • Compact 1- and 2-Electrode Sensors, Custom Designs
  • Guard-Ring Electrode Eliminates Boundary Effects
  • Invar Versions for Highest Temperature Stability (5 x 10-6/K)
One- and Two-plate Sensors
Capacitive sensors perform noncontact measurements of geometric quantities representing distance, displacement, separation, position, length or other linear dimensions with subnanometer accuracy. PI offers capacitive sensors for the integration in user applications in two-plate-capacitor versions for highest performance and as PISeca™ single-electrode sensors, for more flexibility and easier integration.

Measurement Principle
The measurement principle in both cases is the same: two conductive surfaces set up a homogenous electric field; the change in displacement of the two plates is proportional to the signal conditioner output. Dual-plate sensors measure the distance between two welldefined sensor plates with carefully aligned surfaces which generate the most accurate electric field and hence provide optimal results. Singleplate capacitive sensors measure the capacitance against electrically conductive references, such as metallic plates, and are very convenient to install and connect.

Nanopositioning and Nanometrology
PI offers the widest range of high-dynamics and high-resolution nanopositioning systems worldwide. The precision and repeatability achieved would not be possible without highest-resolution measuring devices. Capacitive sensors are the metrology system of choice for the most demanding nanopositioning applications. The sensors and the equally important excitation and readout electronics are developed and manufactured at PI by expert teams with longstanding experience.

Test and Calibration
PI’s nanometrology calibration laboratories are seismically, electromagnetically and thermally isolated, and conform to modern international standards.

PI calibrates every capacitive measurement system individually, optimizing the performance for the customer’s application. Such precision is the basis of all PI products, standard and customized, and assures optimum results in the most varied of applications.
E-852 signal conditioner electronics with PISeca™ D-510.020 1-plate capacitive sensor


Standard D-015, D-050, D-100 2-plate sensors (front from left) and a selection of custom sensors
 
Function, Properties, Advantages

Accuracy
Accuracy, linearity, resolution, stability and bandwidth are far better than with conventional nanometrology sensors like LVDT or strain gauge sensors.

Non-contact operation means no parasitic forces influencing the application and results in measurement free of friction and hysteresis.

Guard-Ring Design for Improved Linearity
Sensor design has a strong influence on linearity. The superior PI design uses a guard-ring electrode that eliminates sensor electrode boundary effects. This ensures a homogenous field in the measurement zone and results in higher measuring linearity.

Single- and Multi-Channel Electronics
PI’s signal conditioner electronics are specially designed for high bandwidth, linearity and ultra-low noise and are perfectly matched to the various PI sensor probes. PI offers signal conditioner electronics and controllers for one to three channels. The E-509 multichannel modules plug into the modular E-500 / E-501 controller chassis. Bandwidth and measurement range can be factory- set to meet the specific needs of each application. The E-852 one-channel signal conditioner electronics for PISeca™ single-plate sensors are designed as stand-alone systems with user-selectable bandwidth and range setting and can be synchronized to operate in multi-channel applications.

Higher Linearity through ILS Electronics
All of PI’s signal conditioning electronics are equipped with the PI proprietary ILS linearization circuit that minimizes nonparallelism errors.

Easy Handling and Integration
PISeca™ single-electrode sensors are particularly easy to install in a measurement system. On the single-channel electronics, an LED-bar indicates the optimum probe-totarget gap for the different measurement range settings. The multi-channel electronics come optionally with displays and/or a PC interface on a module in the same housing.

Ideal for Closed-Loop Piezo Nanopositioning
Closed-loop nanopositioning systems may be controlled by sensor / servo-controller modules of PI’s E-500 series. Such modules are available for connecting up to three position sensors, either stand-alone or integrated into the motion system. Closed-loop operation eliminates the drift and hysteresis that otherwise affect piezo actuators.

For nanopositioning tasks with the most stringent accuracy requirements PI offers highend digital controllers.


Two-plate capacitive sensors D-100 (2 pairs), D-050 and D-015


D-510.050 with LEMO connector for easy handling


The P-752.11C piezo nanopositioning system with integrated capacitive sensors provides position resolution down to 0.1 nm

Applications for Capacitive Position Sensors
Measuring Displacement with Nanometer Precision Nanopositioning / Closed-Loop Systems Parallel Metrology / High-Precision Multi-Axis Measurements
Capacitive displacement sensors measure the shortest ofdistances with highest reliability. The quantity measured is the change of capacitance between sensor plate and the target surface using a homogenous electric field. Accuracies in the sub-nanometer range are regularly achieved. Absolute measurement is possible with a well-adjusted, calibrated system. One application of high-resolution displacement measure-ment is for nanopositioning. Two-plate capacitive sensors can measure distance, and hence position, of a moving object with excellent precision. The high sensor bandwidth allows closed-loop control in high-dynamics applications. Closed-loop, multi-axis nano-positioning tasks are realized with high-performance positioners that make use of direct metrology and parallel kinematics. This allows measuring all degrees of freedom at the same time, which compensatesguiding errors (Active Trajectory Control concept). Here, capacitance gauges are the most precise measuring systems available, and give the best position resolution results.

Tip / Tilt Measurement and Compensation Layer Thickness with Sub-Micron Accuracy Out-of-Plane Measurement /Constant-Height Scans / Out-of-Round Measurement
Integrating capacitive sensors in a system is a good way to measure tip/tilt motion precisely. The moved object's tip angle is measured differentially, and, if required, compensated out. Measuring the thickness of a film or layer of non-conducting material on a moving, conductive, surface (e.g. a rotating drum) is an ideal job for capacitive sensors due to their non-contact operation and their high dynamic performance. Compensation of undulating and oscillating motion, e.g. inconstant height scans or in white-light interferometry, are applications for which capacitive sensors are especially well-suited.

Measuring Straightness and Flatness / Active Cross-Talk Compensation Force Sensors with Micronewton Sensitivity Vibration, Flatness, Thickness
Excellent resolution in straightness and flatness measurements over long travel ranges is achieved with capacitive single electrode sensors. One application is measuring cross-talk in nanopositioning. Crosstalk, off-axis motion from one actuator in the motion direction of another, is detected immediately and actively compensated out by the servo-loops. The high sensor bandwidth provides excellent dynamic performance. Single-electrode capacitive sensors, which measure sub-nanometer displacement from a distance with no contact, are frequently used as high-resolution force sensors. In a system having suitably well-defined stiffness, the measured displacements translate to forces with resolutions in the micro-newton range, all without influencing the process being measured. The high dynamics of the PISeca™ capacitive gauge system even allows measurements of vibrations and oscillations with excellent resolution. Flatness of a rotating work-piece or differences in thickness in the nanometer rangecan be detected. One field of application is in the production of disk drives or in active compensation of vibration.