Joost Jansma
Joost is Consulting Engineer at 3T and Competence Officer for Sensor Interface technology. He blogs about technology in his field of expertise.

Active damping

Capacitieve Sensor

Figure 1: A typical capacitive position sensor (1)

In the chip manufacturing industry, nanometer positioning accuracy is of crucial importance. One of our customers has a table in its machines which can be moved in horizontal (X and Y) direction and must be positioned very accurately. The table reads its position by observing the fine grid on the fixed plate suspended above it. In order to speed-up production, the customer wants to let the table move faster. Despite the very stiff suspension of the plate, it was found that due to air movement the plate now moves a few nanometers up and down, disturbing the horizontal positioning system. Because of the very low-frequency nature of the vibrations (order 0.2Hz), mechanical dampers are not practical and it was decided to use active dampers. This particular active damper system has the following components: a sensor / actuator unit which forms the electro-mechanical interface, an interface unit for analog / digital conversion (AD for the sensor and DA for the actuator), and a remote digital motion controller for implementing the control loop.
3T has developed the interface unit, which, because of specific requirements for dissipation at the sensor / actuator, is equipped with an external sensor preamplifier.

Capacitive sensor

A capacitive sensor has been chosen for position (distance) measurement. This is basically a flat plate capacitor, measuring the distance between both plates. Without additional measures, such a sensor would show strong non-linear behavior. By guarding (3) the sensor, it can remain compact and well-linear.

Formule afstandberekening Capacitieve Sensor

Figuur 2: Formula for distance calculation (2)

This application requires high resolution. Of interest for this are: a strong signal, a low-noise amplifier and a short cable (up to half a meter) between sensor and amplifier. However, the electronics could not be placed closer than 5 meters. Together with the customer a workable solution was found in a small amplifier that could be placed at 0.5m from the sensor. A critical additional requirement for the amplifier was very low and stable dissipation of at most 25mW.
Low dissipation typically conflicts with high resolution (a strong signal and low noise). 3T has invented a smart topology based on active adjustment so only the small variations in position are passed by the amplifier. This allows a low supply voltage and consequently a low dissipation.

Preamp performance

Dimensions: 9 cm2 (30mm x 30mm x 10mm)
Dissipation: 24 mW
Resolution (noise density): 0.030 ppm/√Hz
Resolution (in 100 Hz, 1st order low-noise pass): 0.37 ppm

PreAmp actieve demping

Results

The result of this project is that the customer has seen a clear improvement in positioning accuracy.

(1,2) Source: MTI instruments, http://www.mtiinstruments.com/technology/Capacitance.aspx

(3) Guarding: Avoiding an undesired current from going through in order to keep the environment at the same potential. A common technique when measuring and testing. Examples: in-circuit test (ICT), reduce cable capacitance eg medical electrode cables, reducing leakage current across the PCB at picoampere meters such as in pH sensor electronics.