Linear encoder is a sensor, transducer or readhead paired with a scale that encodes linear position. The linear encoder sensor reads the scale in order to convert the encoded position into an analog or digital signal, which can then be decoded into position by a digital readout (DRO) or motion controller. The linear encoder can be either incremental or absolute. Motion can be determined by change in position over time. Linear encoder technologies include optical, magnetic, inductive, capacitive and eddy current. Optical technologies include shadow, self imaging and interferometric. Linear encoders are used in metrology instruments, motion controller systems and high precision machining tools ranging from digital calipers to coordinate measuring machines.

Types of Linear Encoder Sensors

  • Optical Linear Encoder Sensor – Optical linear encoders dominate the high resolution market and may employ shuttering / Moiré, diffraction or holographic principles. Typical incremental scale periods vary from hundreds down to a few micrometres and following interpolation can provide resolutions as fine as a nanometre. Light sources used include infrared LEDs, visible LEDs, miniature light-bulbs and laser diodes.
  • Magnetic Linear Encoder Sensor – Magnetic linear encoders employ either active (magnetized) or passive (variable reluctance) scales and position may be sensed using sense-coils, Hall Effect or magnetoresistive readheads. With coarser scale periods than optical encoders (typically a few hundred micrometers to several millimeters) resolutions in the order of a micrometer are the normal.
  • Inductive Linear Encoder Sensor – A popular application of the inductive measuring principle is the Inductosyn. In effect it is a resolver unwound into a linear system. The Spherosyn encoder is based on the principle of electromagnetic induction and uses coils to sense nickel-chrome ball-bearings mounted within a tube.
  • Capacitive Linear Encoder Sensor – Capacitive linear encoders work by sensing the capacitance between a reader and scale. Typical applications are digital calipers. One of the disadvantage is the sensitivity to uneven dirt, which can locally change the relative permittivity.
  • Eddy Current Linear Encoder Sensor – Eddy current type digital encoder uses a scale coded with high and low permeability, non-magnetic materials, which is detected and decoded by monitoring changes in inductance of an AC circuit that includes an inductive coil sensor.

There are two main application areas for linear encoders: linear position measurement and motion control systems. Measurement application include coordinate-measuring machines (CMM), laser scanners, calipers, gear measurement, tension testers and Digital read outs (DROs).

Servo controlled motion systems employ linear encoder so as to provide accurate, high-speed movement. Typical applications include robotics, machine tools, pick-and-place PCB assembly equipment; semiconductors handling and test equipment, wire bonders, printers and digital presses.