Eight things you need to know about confocal displacement sensors

When specifying a non-contact confocal displacement sensor, several factors need to be considered including measuring speed, resolution, type of material and surface, the operating environment and whether thickness measurements are required, says Glenn Wedgbrow

Are you familiar with confocal measurement technology? Are you aware of the benefits this particular measuring principle can offer for both displacement and multi-layer thickness measurement of transparent surfaces?

The confocal measurement principle is designed for high accuracy, non-contact displacement and position measurement against any surface: solid, transparent, polished mirrored surfaces, low reflective matt surfaces and even liquids. If a surface is transparent, a one-sided thickness measurement can be achieved with inbuilt correction for the material refractive index. Confocal sensors are often selected when laser triangulation or other optical sensors are not accurate enough or stable enough on the surface being measured.

Almost all industry sectors can benefit from this measurement principle as it can be used in both R&D and in-process measurements. In particular, the semiconductor, micro lens, automotive parts, medical, glass and MEMS (micro-electromechanical systems) industries use this sensor technology in many ways.

How it works

The confocal chromatic measurement principle works by focusing polychromatic white light onto the target surface using a multi-lens optical system. The lenses are arranged in such a way that the white light is dispersed into a monochromatic light by controlled chromatic deviation. A certain deviation is assigned to each wavelength by a factory calibration. Only the wavelength that is exactly focussed on the target surface or material is used for the measurement. An optical arrangement images the light reflected onto a light sensitive sensor element on which the corresponding spectral colour is detected and evaluated. In the case of multi-peak measurements, several distance points are evaluated accordingly.

When selecting a confocal sensor, a number of factors need to be considered.

(1) Measurement speed

If you need to measure the displacement, thickness or surface topography of a target material at high speeds, confocal technology is the most suitable optical displacement measurement technology. The latest confocal sensors and controllers from Micro-Epsilon, for example, achieve measurement rates of up to 30kHz using an LED light source integrated into the controller. With high measuring rates, it is important to adapt the exposure to the respective surface. This means the controller should dynamically regulate the exposure of the CCD line. This exposure control compensates for colour and reflectivity changes of the measuring object in order to increase the measurement accuracy at high measuring rates. Various interface options are available with these controllers, including Ethernet, EtherCAT, Profinet, EtherNet/IP, RS422 and analogue output, allowing the measurement data to be shared in real time with production and quality control systems.

In order to generate stable measurement signals on curved and structured surfaces, confocal sensors should tolerate a large measuring angle. Confocal sensors from Micro-Epsilon can tolerate a large tilt angle of up to 48 degrees.

(2) Resolution and spot size

The confocal principle operates using a very small, constant spot size (typically 3-25 micron) through the measuring range, resulting in very stable, nanometre resolution measurements. On polished or highly reflective surfaces and even transparent materials, confocal sensors offer greater stability than on dark, diffuse materials.

(3) Thermal stability

In terms of thermal stability, confocal sensors are more stable than laser triangulation or eddy current sensors. This is due to the design of the sensors, which typically comprises a cylindrical tube with a series of optical lenses. The sensor is considered “passive”, as the controller and electronics are housed separately and so can be located further away from the target object, which normally means they can be mounted in a more controlled temperature environment.

(4) Type of material or surface

The confocal principle is considered surface-independent, enabling measurements on any type of surface, both diffuse and specular – from dark, diffuse materials to highly reflective, shiny, mirrored or even transparent surfaces.

With translucent or transparent materials such as glass, a one-sided thickness measurement can be achieved using a single confocal sensor, along with the distance measurement. Also, because the emitter and receiver on a confocal sensor are arranged in one axis (i.e. the sensor measures vertically down and the beam is reflected back vertically up from the target), shadowing is avoided.

If you need to measure inside restricted spaces such as drilled holes, bored holes, cavities or recesses, miniature radial and axial confocal versions are now available. Some confocal sensor suppliers can offer miniature versions with diameters of just 4mm, allowing the sensor to be inserted into very tight or narrow gaps and cavities. Some confocal sensor suppliers offer 90-degree sensor versions that can measure the finest of interior contours.

(5) Environment

As confocal sensors are considered “passive”, i.e. do not contain any electrical components, they are suitable for vacuum applications in semiconductor and microelectronics production. In clean rooms or vacuum environments, specific confocal sensors can be provided to suit either a low level vacuum or an ultra-high vacuum with zero outgassing. As there are no electronic components inside the sensor, this means the sensors do not emit any heat radiation during operation, which in turn prevents mechanical expansion of parts inside the sensor or the target being measured. The result is a much more stable, accurate sensor.

Confocal sensors are connected to their controller (which houses all the conditioning electronics) via fibre optic cable. Cable runs can be long (up to 30m) with no degradation of the signal. And for vacuum environments, a vacuum feed-through connector is supplied.

Confocal measurement systems are now available where the sensor and controller (electronics) are combined in one compact housing. This offers greater flexibility for system integrators and machine builders, as there is no longer any need to route or connect fibre optic cables. It also saves space and simplifies installation in production lines and machines.

(6) Shape, size and surface topography of MEMS

Confocal chromatic sensors provide significant advantages when it comes to inspecting the shape, size and surface topography of MEMS structures during or post-production. These benefits include extremely high sensitivity and sub-micrometer resolution. The sensors can also be integrated to linear X-Y stages, machine tools or special purpose inspection systems with closed loop feedback control. Confocal sensors are capable of measuring the surface and groove depth of difficult materials, ranging from highly reflective, mirrored surfaces to dark, diffuse surfaces. Confocal controllers from Micro-Epsilon are already prepared to receive encoder input signals to synchronise measurements from stage position signals.

(7) Compensating for difficult surfaces

Traditionally, most confocal controllers perform poorly when trying to compensate for difficult and changing surface conditions, particularly in high speed surface scanning tasks. However, the latest confocal controllers, IFC2465 and IFC2466, also feature double the light intensity to maintain high speeds, even on dark surfaces, as well as using intelligent software algorithms based on the company’s experience in the design of optical laser sensors. These algorithms enable the controller to compensate in real time for surface reflectivity, enabling users to scan surfaces very rapidly at high resolution. The controllers also provide high speed triggering that allows them to be synchronised with encoders and other motion control devices. The result is a controller that provides more stable, higher accuracy measurements, down to nanometre resolution if required.

(8) Thickness of multi-layered materials

Laminated safety glass, solar panels, flat screens and smartphone displays comprise multiple layers of different transparent materials. Measuring the exact thickness of these individual layers, as well as any air gaps between these layers, during production is a critical but physically complex process and a challenge for measurement technology.

For quality inspection and process control of manufactured transparent multi-layer materials, confocal sensors and controllers can together provide multi-peak measurement capabilities for multi-layered materials such as glass. The result is improved product quality and increased production yields for manufacturers. These confocal sensors and controllers offer faster measuring rates, improved signal-to-noise ratios, and real time surface compensation for difficult-to-measure surfaces, including mirrored surfaces. Some suppliers such as Micro-Epsilon provide software that can evaluate up to five layers by evaluating six measurement values on the boundary areas. In order to accurately determine the thickness of each layer, the controller retrieves the refractive index of each material layer from a database. Each refractive index is corrected depending on the wavelength.

Glenn Wedgbrow is Business Development Manager at Micro-Epsilon UK.