Motion control applications – ranging from factory automation to control systems for mobile machines – require precise, real-time information about the physical location of mechanical equipment.

Absolute rotary encoders can provide precise and unambiguous measurements without losing track of their position due to a temporary loss of instrument power.

An alternative approach is to use special “safety cerified” encoders. This type of encoder has two measurement modules installed in a single housing, sharing the same input shaft. A signal processing chip compares outputs from the two modules and – for most devices
of this type – shuts down measurements and issues an alarm signal if a discrepancy is detected. Redundancy, in this case, is built into the encoder. Encoders with these characteristics can be designed to comply with Safety Integrity Level (SIL) or Performance Level
(PL) standards. (See sidebar for a summary of safety standards.)

An advantage of safety certified encoders is that they can simplify the development of safety-critical systems.
The control system will receive either reliable position data, or a clear signal that the encoder has developed a fault.
However, this approach can be inflexible when handling failure situations: if the sensors simply switch transition the machinery to a safe state.

Diverse-Redundant Encoders
A new type of encoder introduced by POSITAL provides a middle ground between complex duplicate encoder installations and expensive safety certified devices.
Diverse-redundant encoders have two measurement modules built into a single housing, sharing a common shaft.