ZED-CAP a compact, high-precision displacement sensor

ZED-CAP: a compact, high precision displacement sensor

ZED-CAP is a non-contact displacement sensor, designed for situations in which high precision is required but power consumption and volume requirements are extremely stringent.


An image of a compact displacement sensor for cubesat useSmall displacement sensors mounted on cubesat satellite

  • Perform a fine alignment of two surfaces with a precision up to 2 nm.
  • Total volume of sensor is <1 cm³ – no additional electronics required.
  • I2C output from sensor for easy OEM integration.
  • Uses include laboratory optics alignment, QA testing, thickness measurement, aerospace applications (see case study below) as well as many other industrial and OEM applications.

ZED-CAP Performance Specification

Sensor thickness8mm
Sensor diameter10mm
Sensor travelFine
2 nm
20 nm
Measurement speedup to 90 Hz
Power consumption5 mW

ZED-CAP Mode of operation

The two parts of the ZED-Cap sensor are initally far apart (left) as motion of the machinary brings the two plates closer together the resolution increases meaning that the final part of the travel can be sensed with a precision of 2 nm.

Like many capacitive sensors, the ZED-CAP uses a change in capacitance between to capacitor plates to determine a displacement. As the two sense heads approach each other, the signal increases and the resolution goes up, leading to an initial ‘coarse mode’ with a resolution of 20 nm followed by a ‘fine mode’ with a resolution of 2 nm. This is ideal for e.g., optics alignment in which a long range may be required but high resolution is only a necessity very near the focus point.

This does impose limitations to the sensors. The two plates need to be attached to each other via a connecting wire, the high displacement precision is only over a relatively narrow distance and the sensor is not well suited for wet or dirty environments. None-the-less for applications in which high precision with low power and sensor volume requirements, there is no better choice of sensor.

Case study – HighRes: A laboratory demonstrator of satellite deployable optics

The UK ATC and their collaborators have developed a prototype nanosatellite that is has deployable optics overcoming the size limit on the optics of a small satelite

In recent years there has been a shift towards smaller and more economical satellites, known as nanosats, the most common format being the CubeSat. These satellites, with masses less than 10Kg can achieve much of what a traditional large satellite can, but cost a tiny fraction of time and money to design, build and launch. So far, a major impediment to nanosat use in capturing detailed images of earth is that their small size limits the size of the optics. Crudely speaking, the smaller the satellite, the less light collected, and therefore the blurrier the images. One way around this is design a nanosat that is compact during launch but can deploy larger primary mirrors in space, folding out like origami to collect as much light as possible.

The UK Astronomy Technology Centre (UK ATC) has developed a nanosatellite demonstrator, funded by the UK Space Agency and the Defence and Space Technology Laboratory, that validates this concept. The UK ATC, is a government funded research institute that designs, builds, develops, tests and manages major instrumentation projects in support of UK and international Astronomy. If these mirror petals are to be used to create a sharp image, then they need to be aligned into a single optical surface, with an accuracy better than 10 nm. During the planning stage for nanosatellite demonstrator it was found that there were no commercially available displacement sensors small enough, with a high enough precision to align their mirror petals and focussing by image sharpening alone was not sufficient to align the petals to until they co-phased into a single optical surface.

HighRes: the UK ATC’s laboratory demonstrator satellite

Razorbill Instrument’s gold-plated ZED-CAP sensors in use on the UK ATC HighRes satellite. Three sensors are in use to control each mirror petal to provide feedback on the three degrees of freedom: tip, tilt and piston

To fill this niche, Razorbill Instruments developed the ZED-CAP sensor. This compact sensor fulfilled the design requirements due to its high precision and extremely compact form-factor. Various coatings were investigated to improve the thermal and humidity stability and the integration with the satellite was carefully considered.

The UK ATC evaluated the ZED-CAP sensors for this application and found that their performance was similar in performance to leading commercially available displacement sensors but are a tiny fraction of the volume and weight making it an excellent choice for nanosatellite integration. A next generation of HighRes is planned to be designed and launched within the next few years with the intention of using ZED-CAP to provide displacement feedback.

Please contact Razorbill Instruments to discuss integration with your satellite project or industrial OEM application. Irespective of your budget or volume requirements we would be interested in opening a dialogue with you. We are always interested discussion and the exchange of ideas!