Reference signal for incremental encoders.
For each incremental encoder system the reference position is essential as a starting point for the counting.
How the reference point is determined depends on the sensor type, the magnetic tape and the
In the simplest encoder system the sensor head with sine and cosine sensors can only sample
the magnetic periods. The magnetic tape has just one track with magnetic north and
south poles. In this case the encoder system does not know the absolute position.
This is determined by the controller by adding up the counted increments. First however the reference position must be determined by making a homing move.
A sensor head with an additional reference sensor can output a reference signal as soon as it
has reached the magnetically encoded reference point on the second track of the magnetic tape. No external reference switch is necessary.
In another sensor head version a reference signal is output with each magnetic pole. The signal
repeats itself every millimeter (pole-periodic reference pulse).
The magnetic tape does not require a second track with a magnetically encoded reference
point. In this case an external reference switch must be set to the selected reference position.
The controller then precisely evaluates the reference signal when the external reference switch and the reference point signal from the sensor head are active. This means the accuracy requirements for the external reference switch are not so high.
The sensor head with an additional reference sensor can also be combined with a magnetic tape having fixed periodic reference points . Here the reference points are at integrated over the entire length in the magnetic tape at particular equal distances, e.g. every 10 cm. To determine the exact position, the homing move must be made over the entire length of the magnetic tape up to an external reference switch which selects the correct sensor head reference pulse (AND operation).
For a magnetic tape with distance coded reference points the reference points are arranged according to a mathematical algorithm.
This makes it possible to know the absolute position as soon as two reference points have
been traversed. With this type of tape no external reference switch is required.