Inductive linear position sensors are non-contact devices that measure a target object’s distance and movement. Using similar means, inductive proximity switches determine whether the object has approached within a set distance.  These sensors measure the change in impedance (effective resistance) of a coil through which an alternating current flows generating a magnetic field. An object made of ferrous metal (or non-ferrous metal at higher frequencies) enters and disrupts the field.  

Types

There are a number of different types of inductive linear position sensors and switches. They include:

LVITs (linear-variable-inductance transducers) are simple and economic. They may be sources of and susceptible to electromagnetic intereference. Ranges up to 40 in (1 m).

LVDTs (linear-variable-differential transducers) have high outputs with reasonable temperature performance. Ranges between 0.01 and 10 in (0.25 to 250 mm).

LVRTs (linear-variable-reluctance transducers) are less expensive than LVDTs but have lower outputs and may require support electronics. Ranges under 0.4 in (10 mm).

Specifications and Features

Inductive linear position sensors and switches can be described by a number of different specifications and features.

• Sensors measurement range varies (e.g. 0.0394 to 0.0787 inches). Some have minimum ranges.
• Repeatability can vary (e.g. better than 2.46E-5 mils to better than 2 mils).
• Accuracy ranges (e.g. better than 0.01 ±%FS to better than 2 ±%FS).
• Devices can tolerate certain temperature ranges (e.g., -13 to 158°F).
• Targets can be magnetic/nonmagnetic, ferrous/nonferrous, and may have minimum diameters.
• Devices may be field adjustable.
• Output can be analog (current, voltage, or frequency), digital (serial, parallel, or other), an alarm, or a change in state of switches.
• Display options include analog, digital, and/or video.
• Power supply may use AC or DC voltages.
• Connections may be fixed cable (often include bare or stripped leads), integral (attached to existing systems) or terminals (clamp down or screw into place).
• Sensors can be shielded (protected against EMI and RFI), weld field immune, short circuit protected, intrinsically safe (cannot cause ignition of atmospheric mixtures), and/or temperature compensation (ignore fluctuating ambient temperatures).

Applications

Inductive sensors are contact-free, inherently robust, and have infinite resolution with high repeatability. They are often used where long-term reliability is important, particularly in harsh and hostile environments.

References

Gill Sensors Blade Principles

Inductive Linear-Position Sensor/Limit-Sensor Units

Linear Position Sensor

Image credit:

Baumer Ltd.