With any displacement measurement application, there is often a trade-off between the benefits and limitations of each particular measurement technology.
The use of non-contact displacement technologies in precision measurement is growing rapidly, particularly in production, quality control and inspection applications.
This growth in the use of non-contact displacement measurement technologies has fuelled the development of new sensor technologies, including the adaptation of existing technologies to meet these new measurement requirements, and to improve measurement accuracy and resolution. Non-contact displacement sensors come in a wide variety of shapes, sizes and measurement principles.
In practice, as well as eddy current and laser triangulation sensors, capacitive and confocal sensors are now common place in production, quality and inspection environments. It is therefore critical that engineers have a greater understanding of the strengths and limitations of each principle when selecting the most appropriate one for their application.
The eddy current measurement principle is an inductive measuring method. A coil is supplied with an alternating current, which causes a magnetic field to form around the coil. If an electrically conducting object is placed in this magnetic field, eddy currents are induced, which form an electromagnetic field, according to Faraday's Induction Law. The controller calculates the change in energy transferred from the sensor coil to the target material and converts this into a displacement measurement.
Image: The eddy current measurement principle is an inductive measuring method
EDDY AND CAPACITIVE
The advantages here are that this method can be used on all electrically conductive, ferromagnetic and non-ferromagnetic metals. The size of the sensor is relatively small, compared to other technologies, and the temperature range is high, due to the resistance measurement of the sensor and cable. The technology is high accuracy, and is immune to dirt, dust, humidity, oil, high pressures and dielectric materials in the measuring gap.
However, output and linearity depend on the electric and magnetic features of the target. Therefore, individual linearisation and calibration is required. Maximum cable length is typically 15 m and the diameter of the sensor increases as the measuring range increases.
With the capacitive principle, sensor and target operate like an ideal 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 is proportional to the distance between the capacitor electrodes. An adjustable compensating voltage is simultaneously generated in the amplifier electronics. After demodulation of both AC voltages, the difference is amplified and output as an analogue signal.
Image: With the capacitive principle, sensor and target operate like an ideal parallel plate capacitor
Since the sensor is constructed like a guard ring capacitor, almost ideal linearity and resolution against metal targets is achieved. The technology also offers high temperature stability, as changes in the conductivity of the target have no effect on the measurement. Capacitive sensors can also measure insulators, since they are sensitive to changes in the dielectric sensor gap, but operate most effectively in clean, dry applications. Cable length must be short, due to cable capacitance effect on the oscillating circuit tuning.
In laser triangulation, a 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 onto 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. The measurements are processed digitally in the integral controller, and then converted into a scaled output via analogue (I/U) and digital interface RS232, RS422 or USB. Benefits of laser triangulation sensors include: a small beam spot; very long measuring ranges are possible; the sensor operates independent of the target material; and a high reference distance between sensor and target.
Image: Back-scattered light reflected from a point is projected onto a CCD array
The method is limited by a relatively large sensor design; and a relatively clean optical path is required for the sensor to operate reliably. In addition, for direct reflecting targets, specific sensor alignment/calibration is required.
The Confocal 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 focused on the target surface or material is used for the measurement.
Image: The Confocal Principle works by focusing polychromatic white light onto the target surface
Both diffuse and specular surfaces can be measured. With transparent materials such as glass, a one-sided thickness measurement can be achieved, along with the distance measurement. Also, because the emitter and receiver are arranged in one axis, shadowing is avoided.
Confocal offers nanometre resolution and operates almost independently of the target material. A very small, constant spot size is achieved. Miniature radial and axial confocal versions are available for measuring drilled or bored holes.
Restrictions of the technology include the limited distance between the sensor and target. In addition, the beam requires a clean environment.
In production environments and metalworking machine shops, non-contact displacement measurement and inspection systems are finding an increasing number of applications. For example, Micro-Epsilon's non-contact confocal measurement system, boreCONTROL, has an integral confocal sensor that measures just 3.3 mm in diameter. This means that the sensor can now measure inside confined spaces, such as narrow cavities, surface indentations, drilled holes and bores. Machined components can be taken offline, in order to inspect their parameters for accuracy. Miniature bores and drilled holes with diameters down to 4.5 mm can now be inspected from the inside, using miniature confocal sensors.
Image: Micro-Epsilon's non-contact confocal measurement system, boreCONTROL
A typical non-contact confocal measurement system comprises a stainless steel measuring head, with integrated mechanics, a sensor probe, with integrated confocal sensor, and two controllers. Confocal technology is suitable for use with a diverse range of surfaces and does not require any reference points during measurements, which means the sensor can recognise edges and steps. Due to the length of the sensor, bore holes with depths of up to 90 mm can be measured.
In these types of systems, the sensor is rotated by an integrated servomotor, which enables a 360° inspection of the interior surface of the hole. The measuring system is typically mounted on a linear axis to enable the entire inner surface of the bore hole to be examined. This means that production engineers or machine tool operators can inspect important characteristics, such as diameter, roundness, concentricity and straightness of bores or drilled holes on machined parts. Depending on the set up, individual levels can be measured or a spiral-shaped profile of the hole can be generated. A complex structured precision bearing system ensures that the sensor is rotationally stable during measurements.
First published in Machinery, August 2011