Made to measure

New and refined technologies are helping customers carry out more accurate measurements of often difficult surfaces, says Chris Jones

The use of non-contact displacement technologies in the field of precision measurement is growing rapidly. The main reasons for this are that customers need to measure much more accurately— to sub-micron or even nanometer resolutions — and to measure against difficult surfaces or surfaces that cannot be touched during the measurement process. These include silicon, glass, plastics, miniature electronic components, medical components and even food-based surfaces.

This rapid growth has pushed the development of new technologies and the adaptation of existing technologies to meet the new measurement requirements and improve measurement accuracies and resolutions.

So it is more important than ever to understand the strengths and limitations of each non-contact measurement principle when selecting the correct sensor technology for the task.

In practice, besides eddy current and laser triangulation sensors, capacitive and confocal sensors are proving popular with customers. But non-contact displacement sensors come in a variety of shapes, sizes and measurement principles.

The key is selecting the most appropriate sensing technology for the customer’s application.

The eddy current measurement principle is an inductive measuring method based on the extraction of energy from an oscillating circuit. This energy is required for the induction of eddy currents in electrically conductive materials.

A coil is supplied with an alternating current, which causes a magnetic field to form around it. If an electrically conducting object is placed in this magnetic field, eddy currents are induced, which form an electromagnetic field. This acts against the field of the coil, which also causes a change in the impedance of the coil. The controller calculates the impedance by considering the change in amplitude and phase position of the sensor coil.

The advantage of the eddy current is that it can be used on all electrically conductive, ferromagnetic and non-ferromagnetic metals. The size of the sensor is relatively small compared with other technologies and the temperature range is high due to the resistance measurement of the sensor and cable.

The highly accurate technology is immune to dirt, dust, humidity, oil, high pressures and dielectric materials in the measuring gap.

Yet there are restrictions. The output/input signal ratio depends on the electric and magnetic features of the target material. Therefore, individual calibration is necessary. The maximum length of the cable is 15m and the diameter of the sensor increases as the measuring range increases.

With the capacitive principle, the sensor and target operate like a parallel plate capacitor. The two plate electrodes are formed by the sensor and the opposing target.

If an AC current with constant frequency flows through the sensor capacitor, the amplitude of the AC voltage on the sensor will equal the distance between the capacitor electrodes. An adjustable compensating voltage is simultaneously generated by the sensor system’s amplifier. After demodulation of both AC voltages, the difference is amplified and output as an analogue signal.

Capacitive sensors are able to achieve equal input/output signal ratios and they have ideal sensitivity to metals. They also offer high temperature stability, as changes in the conductivity of the target have no effect on the measurement. And they can measure insulators.

However, they are only ideal in clean, dry applications and, like eddy current technology, they only have a relatively short cable length.

In the laser triangulation principle, laser diode projects a visible point of light onto the surface of the object being measured. The back scattered light reflected from this point is then projected on to a CCD array by a high-quality optical lens system. If the target changes position with respect to the sensor, the movement of the reflected light is projected on the CCD array and analysed to output the exact position of the target.

This unique measuring principle enables displacements and distances to be measured very precisely. It can even measure on diffuse and spectral surfaces.

Confocal technology offers nanometre resolution and operates almost independently of the target material. It can provide one-sided thickness measurement of transparent materials. This sort of sensing system can be offered in miniature radial and axial versions for measuring drilled or bored holes. Instead of a laser, it uses white light.

Yet the technology is limited because the sensor must be close to the target and the beam requires a clean environment.

The right sensor manufacturer will help customers choose the correct sensor for their application. In some cases hybrid technologies may be appropriate.

Chris Jones is managing director of precision sensor manufacturer Micro-Epsilon (UK)