To obtain the desired accuracy, all the errors intrinsic to the system and the coupling with the measuring instrument must be carefully considered: this allows the right encoder resolution to be chosen and the most suitable precision ratio for the application to be obtained.
Theoretically, a precision ratio value ranging from 2 to 10 is assumed. This consists of the number of samples of the segment to be measured, provided by the measuring instrument (the greater the number of samples, the greater the precision).
Care must be taken not to measure play and flexible, mechanical movements (characteristic of all mobile systems).
The correct choice of resolution and the mechanical characteristics of the rotary encoder are obtained by ensuring that the required precision is less than the smallest flexible coupling or mechanical play, in order to reduce measurement error, defining that:
real precision ratio = units to be measured / error due to mechanical play
What is measurement error?
Measurement error is the degree of inconsistency in the value of a physical property obtained by measurement. The result of measurement is not a single value but the set of probable values within which the size may lie.
Measurement with incremental and absolute encoders
An optical or magnetic encoder is a transducer capable of proportionally translating the displacement in its rotation shaft into an electrical signal: the electrical signal, incremental or coded, is used by the data acquisition system to quantify and control movement of the moving parts.
The difference between an incremental and an absolute encoder
An incremental encoder works by providing a relative indication in relation to its previous position: this is simple to manage as the encoder resolution unit only has to be linked to the moving unit or the precision ratio.
The incremental information consists of a two-bit Gray code (phases A and B) in quadrature (50% duty cycle). The phase gap between the two bits is 90 electrical degrees and the direction of rotation must be checked.
Hohner Automazione S.r.l. assumes that the increment is positive when phase B is ahead of phase A (the rising edge of the B signal reaches 90 electrical degrees before A) and the encoder shaft rotates clockwise.
There is also a reference (Zero or Marker) which indicates the completion of a turn: this is a pulse whose size can be set as required, from 90 electrical degrees up to any other interval (in reference to the duration of the pulses).
The Absolute mode provides unambiguous position encoding signals by clearly indicating the increment direction and the value reached, avoiding extensive calculations. This technique is useful for angular applications: for example, consider the associations obtainable using a 360-degree encoder and the full circle: 1° = 1 position.
In environments where a break in the network power would make power factor correction difficult and expensive, absolute measurement is extremely advantageous: the absolute position reading is directly measured by the encoded optical disk which does not “forget” the position even if the encoder is without power as it is an integral part of the shaft.