Position Probes

Position probes measure the position of a target’s surface, usually with a high degree of accuracy. Suppliers divide products into several categories. Sensor elements or chips are simple devices such as thermocouples or bare dies that do not include packaging. Sensors or transducers are more complete products that include packaging and/or signal conditioning. They are powered by an external source and provide outputs in volts, direct current (DC) or current loops. Meters and instruments are self-contained units that provide an output at or near the position probe. They usually include signal processing and conditioning. Gauges and indicators feature an analog display, but do not provide electronic outputs. Recorders and totalizers record values, accumulate totals, and may include statistical functions. Measurement range and accuracy are the most important specifications to consider when selecting position probes. Operating temperature, shock rating, and vibration rating are additional considerations. 

Position probes use many contact and non-contact sensing technologies. Linear variable differential transformers (LVDT) produce electrical outputs proportional to the position of a solid, cylindrical core within a hollow shaft. Linear encoders sense and digitize linear position changes and provide feedback to control systems. Linear potentiometers produce a resistance output proportional to the displacement or position. The resistance element is excited by DC or AC voltage, and the output voltage is usually a linear function of the input displacement. Capacitive sensors can be used with conductive or non-conductive target materials, but are sensitive to environmental parameters that change the dielectric constant of the medium. Eddy current devices contain two coils: an active coil that is influenced by a conducting target, and a secondary coil that completes a bridge circuit and provides temperature compensation. Fiber optic elements are flexible strands of glass or plastic which transmit light along their length. Inductive position sensors determine an object’s coordinates with respect to a reference. Optical time of flight devices determine displacement and distance by measuring the time that light travels to a target and back again. Optical triangulation sensors use reflected waves while ultrasonic products use reflected acoustical signals. Magnetostrictive and magnetoresistive position probes are also available.

Position probes differ in terms of electrical outputs. Analog current levels such as 4 – 20 mA are suitable for sending signals over long distances. Feedback is used to provide the appropriate current regardless of line noise and impedance. Analog voltages are simple, usually linear functions of the measurement. Modulated analog output signals are encoded, but still analog in nature. Examples include sine wave, pulse wave, amplitude modulation (AM), and frequency modulation (FM) signals. Several types of digital outputs are available. RS232, RS422, and RS485 are common serial, digital protocols. Popular parallel protocols include the general-purpose interface bus (GPIB), a standard which is also known as IEEE 488. Position probes with outputs that change the state of a switch or alarm are also available.

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Part Numbers for Position Probes