Incremental rotary encoders simplify machine and plant safety implementation

In any application where such systems are in use and moving machinery parts are driven by high-performance systems, there is the potential for danger to the operating personnel.

According to the European Machinery Directive and relevant safety regulations for health and safety in the workplace, even brief lapses in attention must not result in shearing, crushing and impact injuries – or worse. The preferred use of rotary encoders in such applications is to monitor rotating or swiveling components in drive systems, making them a core safety consideration in drives.

In order to keep the expense of safety measures within limits, rational safety concepts will increase in importance in the future. Consequently, in this context the functional safety of individual system components, such as rotary encoders, assumes new significance. This transpires against the background of a changing situation with regard to standards, since the standard EN 954-1, which has been binding since 1996, remains valid only up to 29.12.2009.

The deterministic approach of EN 954-1, of countering the deduced risk categories by means of an appropriate system behavior, will now be superseded by the more effective standards EN ISO 13849-1 and EN 62061. With their consideration of failure probabilities (probabilistic approach) these are also more in keeping with the requirements of modern programmable electronics, which has long been subject to the considerations of safety technology. The IEC 61508 serves as the higher-level standard for functional safety.

Since the architecture determines a significant proportion of safety costs, smart ideas can reduce these costs while even increasing the efficiency of sensor-system safety. The use of mechanisms featuring two-channel operation, without completely duplicated execution of the functional units, is an effective approach.

However, even systems with two rotary encoders or a dual encoder do not necessarily have to be classified according to the new standard. In order for a sensor to receive a high safety evaluation, it must be ensured that the Safe Failure Fraction (SFF) and the degree of Diagnostic Coverage (DC) are appropriate to comply with the minimum requirements. Self-diagnosis implies that the device is able to detect its own fault conditions and then reliably communicate the same. In this way machines and control systems are able to immediately carry out a defined stoppage, before subsequent failures of any kind can lead to a dangerous situation.

To address these issues, Pepperl+Fuchs has introduced RVS58S Incremental Rotary Encoders with integrated safety technology.

RVS58S encoders with self-diagnostics are specifically designed for use in safety aligned systems up to SIL3 in accordance with IEC 61508, performance level “e” in accordance with IEC 13849, and Category 4 in accordance with DIN EN 954-1. They are also suitable as a motor feedback system for safe drives in accordance with IEC 61800-5-2.

“In view of the increasing level of automation in applications involving machinery and plant construction, plant manufacturers, system integrators, and suppliers see themselves obligated to increase functional safety,” said David Rubinski, P+F product manager.

“Specific framework conditions create new and extended safety standards that take into account the requirements of modern electronic safety systems. Against this backdrop, rotary encoders with functional safety take on a new significance. These encoders simplify integration and at the same time comply with all safety requirements.”

In response to these needs, P+F developed the RVS58S rotary encoder with functional safety that enables economical system integration. This new concept is said to provide easier connection methods to the control system and allows it to be used in high safety category systems.

Technically speaking, RVS58S incremental encoders have a sin/cos interface, 1024 or 2048 signal periods, and are thermally stabilized for high resolution interpolation. Units operate at 5 Vdc ±5% and deliver a maximum output frequency of 200 kHz in operating temperatures ranging from -20° to 80°C. They are rated to handle axial shaft loads of 40 N at maximum 6000 rpm or 10 N at maximum 12,000 rpm, and radial shaft loads of 60 N at maximum 6000 rpm or 20 N at maximum 12,000 rpm.

“When it comes to plant safety, specifying safe system components is vital. Sensors such as safe rotary encoders, which stand right at the start of the information processing chain as quasi sense organs, undertake important functions. Certified rotary encoders with simple integration concepts not only render safety solutions easier to implement, they are also more economical,” added Rubinski.

“Simple integration of components is necessary to help machine manufacturers meet the safety requirements of the new standards, as it is for end users with retrofit installations. The safe encoders now available from Pepperl+Fuchs feature both technical updating of the architecture and a simple user interface that needs no additional cables, interface cards or safety modules.

The sensors are mechanically compatible with standard components and use existing communication paths – this is the key to simple integration,” explained Rubinski.
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