Incremental and Absolute Encoder Shielding
In communication systems where information flowing through a cable needs to be protected from interference from other systems, the cable has itself to be equipped with a shield and has be appropriately connected so that good encoder shielding is obtained.
In broad terms, the encoder cable shield must on the one hand be connected to the metal structures containing the encoder and on the other hand be connected to the data acquisition equipment.
Checks to be performed during installation
– the ends of the cable shield are connected on the outside of the equipment;
– for metal encoders, the shield is connected to both sides; for plastic encoders, the shield often only needs to be connected from the equipment side. If this is insufficient, the encoder must be enclosed within a metal housing to which the shield terminal is connected. In most cases it is difficult to ensure that the ground potential is the same both where the encoder is mounted and where the end of the cable shield is earthed. Hohner therefore recommends that the cable shield only be earthed on the data acquisition equipment side. This will give good encoder shielding
– as regards the connectors, their housings are made of metal or metallic plastic and the signal conductors free of the shield are as small as possible in length. The shield must be connected to the metal part of these housings;
– if cable extensions or branches are necessary, these should be enclosed in a metal box. The main and branch line shield is connected to this.
In any event, we recommend that the installer assess all the factors which might cause interference with the encoder. It is important to take every precaution to prevent the encoder from being used in critical conditions.
The primary factor is the Encoder structural type which must be assessed during the analysis.
Encoder shielding efficiency
- The shield behaves like capacitance, i.e. a high frequency filter. High-quality cable is always used, possibly using double pole wiring, especially for heavy-duty electrical applications;
- The efficiency of the shielding is affected by the earthing of the equipment and the encoder. As these are generally connected directly to the machinery, it is very important that the earthing of the system itself is very well managed. In particular, where a system of considerable size and power is involved, the long metal parts behave as impedances which can cause momentary interruptions to the earthing and thus connection inefficiency.
Note: The encoder may be equipped with an internal impedance which connects the zero-volt supply to its metal housing. This impedance prevents the internal circuit from reaching too different a potential from that of the actual housing, providing a degree of immunity from high levels of interference. This impedance performs well if the housing is at zero potential, i.e. earthed. It is obviously important to check that the entire System is correctly earthed in order to prevent any anomalies or damage to the electrical/electronic parts.
Good encoder shielding is essential for your machine to function properly without any measurement errors being introduced