Stephen J. Prosser, Lucas Control Systems, Schaevitz Sensors

The world market for proximity, displacement, and position sensors is a large industrial sector. Because most of these products are mature, the market has displayed moderate growth at best. However, some market segments, such as Hall effect and photoelectric sensors, are experiencing high annual growth rates.

The first devices to appear on the market were mechanical switches with electromechanical LVDT and potentiometric devices designed for displacement and position sensing. Innovations in this technology have mirrored developments in the electronics industry. As the electronics have improved, sensors based on electromagnetic effects have emerged in the form of inductive and capacitive devices.

Over the past 20 years, a number of sources have predicted the demise of the LVDT "within five years." This view of the future, though, has not come to pass. Today, the need for LVDTs is higher than it has ever been. However, in some markets—notably measurement and inspection—noncontact sensing technologies are gaining support. The laser-based sensor is one of these.

Photo 1. Because of their durability and longevity, noncontact sensing technologies, such as laser-based devices, are well suited for rugged and safety-critical applications. Schaevitz's DistanceStar is appropriate for such applications as distance measurement, vibration signature analysis, and alignment/positioning.

Laser-based devices are finding their way into many different markets, in particular medical, communication, transportation, and home applications (see Photo 1). For example, in the early 1990s, Lucas Aerospace successfully demonstrated aircraft tail rudder control using an optoelectronic link. Lucas has also developed an electric power­assisted automotive steering system that uses an optical torque sensor. Laser sensors have traditionally had a higher price tag than contact sensors, but prices are coming down rapidly for particular applications (e.g., for process control and measurement).

The DistanceStar measures with a resolution of 10 µm at an averaged frequency of 9 Hz (sampling frequency of 3 kHz). The sensor emits a visible light beam (for ease of alignment) from a laser diode. The beam strikes

Figure 1. The DistanceStar's laser emits a beam that strikes the target. The beam is reflected back onto the position-sensitive detector (PSD). Displacing the object changes the angle of incidence, and hence position on the PSD.

an object's surface, which reflects a spot onto a position-sensitive detector (PSD). Signal processing electronics translate the PSD output currents into a voltage proportional to displacement (see Figure 1). The sensor has an operating distance (i.e., the distance between the laser and the target) of 80 mm and is available in models offering four measurement ranges: ±5 mm, ±10 mm, ±15 mm, and ±20 mm. The DistanceStar is appropriate for measuring distances, profiling, vibration signature analysis, alignment/positioning, and eccentricity. Differential measurement by two sensors allows online thickness and profile measurements to be made at high rates.

The TwinStar uses optical triangulation to sense surface variations as fine as 0.007 µm when signal averaging to 100 Hz is used and can deliver measurement rates up to 100 kHz. TwinStar's laser diode emits a laser beam, which strikes the object's surface. The beam is reflected back and passes through TwinStar's twin imaging optics onto two PSDs. The twin optics provide

Figure 2. The TwinStar's twin optics effectively increase the amount of light captured by the system, which in turn gives better object contour tracking.

better contour tracking than other measurement options when sharp increases in object height are present. If one PSD is obscured by a change in an object's surface, the other PSD still sees it (see Figure 2).

The sensor's proprietary optical triangulation technology allows the user to measure under difficult surface and lighting conditions. The user doesn't have to modulate the laser beam because of the high intensity of the beam in comparison to the background light. The operating distance is 15 mm, with models that provide three measurement ranges: ± 1.5 mm, ± 3 mm, and ± 6 mm. The TwinStar is suitable for a variety of applications, including inspection of circuit boards and measurement of roughness, coating thickness, and integrated circuit pin height.

The BeamStar sensor provides precise measurement up to 1 m from the target at a sampling frequency of 3 kHz, using a separate emitter and receiver. Projected through an aperture that shapes the visible beam to 1 mm by 10 mm, 1 mm by 15 mm, or 1 mm by 25 mm, passing objects block all or part of the beam. The portion of the beam that reaches the receiver is determined by the size or position of the object; the smallest detectable object is 0.25 mm. Offering a PLC-compatible DC output, the BeamStar is suitable for measuring width/ thickness and particulates in fluids, profiling edges, and measuring roller gaps.

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