Linear encoder

Linear encoders are used in metrology instruments, motion systems, inkjet printers and high precision machining tools ranging from digital calipers and coordinate measuring machines to stages, CNC mills, manufacturing gantry tables and semiconductor steppers.

[3] US Patent 3820110, "Eddy current type digital encoder and position reference", gives an example of this type of encoder, which 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.

Servo controlled motion systems employ linear encoder so as to provide accurate, high-speed movement.

The industry standard analog output for linear encoders is sine and cosine quadrature signals.

Compared to digital transmission, the analog signals' lower bandwidth helps to minimise EMC emissions.

Quadrature sine/cosine signals can be monitored easily by using an oscilloscope in XY mode to display a circular Lissajous figure.

Highest accuracy signals are obtained if the Lissajous figure is circular (no gain or phase error) and perfectly centred.

In the latter case, the interpolation process effectively sub-divides the scale period and thereby achieves higher measurement resolution.

The major advantages of linear incremental encoders are improved noise immunity, high measurement accuracy, and low-latency reporting of position changes.

Most incremental, linear encoders can produce an index or reference mark pulse providing a datum position along the scale for use at power-up or following a loss of power.

Multiple, equally spaced reference marks may also be placed onto the scale such that following installation, the desired marker can either be selected - usually via a magnet or optically or unwanted ones deselected using labels or by being painted over.

Many of these protocols are proprietary (e.g., Fanuc, Mitsubishi, FeeDat (Fagor Automation), Heidenhain EnDat, DriveCliq, Panasonic, Yaskawa) but open standards such as BiSS[9] are now appearing, which avoid tying users to a particular supplier.

Two limit switches are frequently included such that on power-up the controller can determine if the encoder is at an end-of-travel and in which direction to drive the axis.

Linear encoders may use transmissive (glass) or reflective scales, employing Ronchi or phase gratings.