Today, quality control and the desire for absolute safety have contributed to the introduction of numerous sensors in our lives. Optical technologies have played an increasingly important role in this development because they combine the advantages of a small design, reduced production costs, fast measurement speeds, high precision, reliability, and non-contact measurements across large distances.
A position-sensitive detector (PSD) should be your first choice in sensors when determining the center of a beam spot. PSDs function locally analogously; they interpret the current produced by a photodiode.
A position-sensitive detector (PSD) should be your first choice in sensors when determining the center of a beam spot. PSDs function locally analogously; they interpret the current produced by a photodiode. This current is divided into one or two resistive layers. The advantages of this simple design are its stability and reliability. The electronics required to process the analog output signal are relatively simple and can be assembled inexpensively.
Unmatched Speed and Resolution
A PSD just determines the position of the center of incoming light, but it does so within nanoseconds and at a sub-nanometer resolution. It achieves a measurement tolerance of approximately 0.1%. The dynamic range of the light intensity stretches across several orders of magnitude. The accuracy can be significantly increased if reference points are saved in a value table. The optical components used together with the sensor usually introduce measurement errors; however, if appropriate corrections are incorporated into the value table, it is possible to compensate for these errors to a large extent. Because the PSD gains its position information from the photocurrents of the diodes, methods of operation used in normal photodiodes can be used here as well. This includes, for example, the modulation of light to eliminate interference by ambient light. PSDs can be manufactured in any shape. Some unusual designs include, for example, helix-shaped, circular, or spherical forms for 2D and 3D angular measurements. Entire arrays of PSDs were developed for some applications (e.g., surface tests).
Versatile Applications for PSDs
Examples of applications include alignment systems in which the position of a reference laser beam is measured relative to the PSD. This principle is used in many different areas – from bridge construction to optical benches. As PSDs can be produced to operate at very low temperatures (such as in liquid nitrogen), this alignment method is also used in infrared optics because here the infrared radiation of a PSD has to be kept to a minimum. One weakness of PSDs is their inability to differentiate between a direct beam and a reflected beam. If two beam spots are detected, the PSD provides a measurement value for the center between these two spots.
When used in connection with sophisticated signal processing methods – such as with optical filtering and synchronized detection methods – PSDs can perform measurement tasks that were previously thought impossible – such as detecting changes in the red-hot surfaces of liquid iron or measurements within a welding flame. In addition, PSDs will prove useful in applications in which the movement of mechanical parts has to be detected without contact or pressure. This includes the movement of membranes in microphones, loud speakers, and pressure sensors or of optical fibres in wind and acceleration sensors. They can also be used for fluid level measurements in fuel tanks. With a simple analog circuit you can achieve resolutions into the sub-nanometer range.