Capacitive Position Sensors / Controllers for Nanometrology
Capacitive gauges (capacitance sensors) are the nanometrology system of choice for the most demanding precision positioning, scanning and measurement applications. PI capacitive sensors ensure highest resolution, linearity and longterm stability. These absolute-measuring, non-contact devices detect motion
at sub-nanometer levels directly (direct metrology). They provide accuracy, linearity, resolution, stability and bandwidth superior to conventional sensors
such as LVDTs, strain gauge type sensors (piezo resistive sensors), and incremental encoders (glass scale type encoders).
For the ultimate performance the D-015 - D-100 two plate sensor series are available. The new single-probe D-510 series provide similar precision and feature an integrated LEMO connector for easy mounting and replacement in the field.
New Single-Probe Capacitive Position Gauge for Nanometrology Applications
The new D-510 family of PISeca™ capacitive displacement gauges performs high-precision, non-contact measurements of geometric quantities representing displacement, separation, position, length or other linear dimension against any kind of electrically conductive target. These single-probe nanometrology sensors combine superior resolution and linearity with very high bandwidth for dynamic measurements.
Sub-Nanometer Resolution, Measuring Ranges to 500 µm
Absolute, Non-Contact Measurement of Distance / Motion / Vibration
Multi-Axis Measurements Possible
Excellent Measuring Linearity to 0.1 %
Plug & Play: Easy Setup and Integration
Very Temperature Stable
Bandwidth to 10 kHz
Guard-Ring Electrode Design for Better Sensor Linearity
ILS Linearization System in the Signal Conditioner Electronics Improves Output Signal Linearity
All Systems Factory Calibrated for Highest Possible Linearity / Accuracy
Higher Linearity through Guard-Ring Capacitor and ILS Electronics
Not all capacitive sensors are created equal. Because the sensor design has a strong influence on the linearity, PI uses a special guard-ring electrode to shield the sensor electrode from boundary effects. This ensures a homogeneous electric field in the measurement zone and results in higher measuring linearity. In addition, the E-852 sensor signal conditioning electronics are equipped with the PI proprietary ILS linearization circuit.
Easy Installation & Setup
The capacitive gauges are plug and play units. A high cable with a performance LEMO connector plugs directly into the sensor head for easy installation or replacement in the field. Installation is further facilitated by a display in the E-852 signal conditioner sensor electronics that indicates the optimum distance between probe and target.
Direct Metrology, Parallel Metrology Configurations
The capacitance sensors offered by PI are the most accurate measuring systems for nanopositioning applications
currently on the market. In contrast to high-resolution sensors measuring deformation in the drive train, like strain gauge or piezoresistive sensors, capacitive sensors are non-contact, direct-metrology devices—a fact which
gives them many advantages:
Better Phase Fidelity
Higher Bandwidth
No Periodic Error
Non-Contacting
Ideal for Parallel Metrology
Higher Linearity
Better Reproducibility
Higher Long-Term Stability
Capacitive position sensors in an ultra-high-accuracy, six-axis
nanopositioning system designed by PI for the German Institute of Standards (PTB). Application: scanning microscopy
Capacitive sensors are especially well-suited for parallel metrology configurations. In multi-axis nanopositioning
systems, parallel metrology means that the controller monitors all controlled degrees of freedom relative to "ground" (the
fixed frame) and uses each actuator to compensate the undesired off-axis motion of the others automatically (active trajectory
control). As a result, it is possible to keep deviations in the sub-nanometer and sub-microradian range.
Resolution
Resolution on the order of picometers is achievable with short-range, two-electrode capacitive position sensors.
Theoretical measurement resolution is limited only by quantum noise. In practical applications, stray radiation, electronics-induced noise
and geometric effects are the limiting factors. For example, with the 100 µm range, a D-100.00 capacitive probe and
E-509.C1A electronics, the effective noise factor is 0.02 nm/√Hz. This translates to 0.2 nm at 100 Hz bandwidth. The maximum standard bandwidth
(jumper selectable) is 3 kHz.
Figure 1 shows a D-015, 15 µm capacitive position sensor and an interferometer,
both measuring nanometer-range actuator cycles. The graphs clearly show the superior resolution of the capacitive position sensing technique.
D-050 Capacitance Sensor Probes.
Fig 1. Piezo
nanopositioning system making 0.3 nm steps, measured with PI capacitive sensor (lower curve) and with a highly precise laser interferometer. The capacitive sensor
provides significantly higher resolution than the interferometer
Stability and Linearity of PI Capacitive Position Sensors
PI capacitive position sensor electronics incorporate a proprietary design providing
superior linearity, low sensitivity to cable capacitance, low background noise and low drift.
The Integrated Linearization System (ILS) compensates for influences caused by errors, such as non-parallelism of the plates.
When used with PI digital controllers (which add polynomial linearization techniques) a positioning linearity of up to 0.003 % is achievable.
Figure 2 shows the linearity of a P-752.11C piezo flexure nanopositioning stage with integrated capacitive position sensor operated in
closed-loop mode with an analog controller. All errors contributed by the mechanics, PZT drive, sensors and electronics are included in the resulting linearity of better than 0.02 %.
Even higher linearity is achievable with PI digital controllers, see the E-710.
The exceptional long-term stability of the PI capacitive position sensor and electronics design is shown in Figure 3.
Fig 2. Linearity of a P-752.11C, 15 µm piezo nanopositioning stage operated with E-500/E-509.C1 control electronics.
The travel range is 15 µm, the gain 1.5 µm/V. Linearity is better than 0.02 %; even higher linearity is achievable with PI digital controllers
Fig. 2
Fig 3. Measurement stability of an E-509.C1 capacitive position sensor control board with 10 pF reference capacitor over
3.5 hours (after controller warm-up).
When measuring distance by detection of capacitance changes, fluctuations in the cable capacitance can
have an adverse effect on accuracy. This is why most capacitive measurement systems only provide satisfactory results with short, well-defined cable lengths.
PI Systems use a special design which eliminates cable influences, permitting use of cable lengths of up to 3 m without difficulty. For optimum results, we
recommend calibration of the sensor-actuator system in the PI Metrology Lab.
Signal Paths to 15 Meters
Longer distances between sensor and electronics can be spanned with special, loss-free, digital transmission protocols.
A remote sensor interface box is available for the E-710 digital nanopositioning controller.
Features & Advantages, Applications
Two-plate capacitive sensor working principle.
Quality control and long-term stability testing of capacitive displacement sensors at PI.
Custom, 7-channel, capacitive position sensor electronics.
Working principle of STM (scanning tunneling microscope) with
integrated capacitive position sensors
Properties of PI Sensors
Highest resolution (0.01 nanometer) of all commercially available position sensors
Measuring ranges of up to 1 mm
Ideal for parallel metrology applications
Linearity to 0.003%
Extremely high long-term stability (better than 0.1 nm / 3 hours)
Vacuum compatible
Bandwidth to 10 kHz
Invar versions for highest temperature stability (5 x 10-6 / K)
Multi-channel digital electronics available
Compatible with PI nanopositioning system servo-controllers
Two-plate and single-plate sensors
Custom models
Reasons for Choosing PI
Over 30 years experience in designing ultra-high-precision mechanics
In-house design & manufacture of sensors and electronics
State-of-the-art equipment for simulation, production and testing
State-of-the-art metrology lab with multiple thermal, acoustic and
seismic isolation for meaningful sub-nanometer measurements
In-house controller development
PI has the most-experienced nanopositioning systems development and production teams in the field
Dual Plate. Hightest Precision. Sub-Nanometer-resolution. Other materials on request
*Custom dimensions, sensors, designs for volume buyers.
We manufacture a variety of capacitance sensors specifically for use in our nanopositioning systems (see selection guide below)
Selection Guide: Piezo Stages with Integrated Capacitance Sensors
* Ask about custom sizes, sensors or special designs.
Capacitive and LVDT sensors are direct metrology devices.
Capacitive sensors provide the highest accuracy, bandwidth and linearity.
