Safety-related position measuring systems

THESE high requirements mainly serve to protect human beings, but material assets and the environment are also receiving more consideration.

The goal of functional safety is to minimise or even eliminate risks that can occur during normal or impaired operation of machines or facilities. This is achieved primarily with redundant systems. For example, axes that are moved in safety-related applications require redundant position information in order to perform the corresponding safety functions. Various system configurations can be realised in order to capture independent position values. One possibility is the use of two encoders per axis. In many cases, however, a more economical solution requiring only one position encoder is preferred. Until now, analogue encoders with sine and cosine signals were used for this.

HEIDENHAIN offers a purely serial, single encoder solution for safety-related position measuring systems in safety-oriented applications in accordance with EN 61 508 and EN 13 849. This means that safety applications can now take advantage of all the benefits of serial data transfer, such as optimisation of costs, diagnostic possibilities, automatic commissioning, and rapid formation of the position value.

HEIDENHAIN offers encoders that can be used in safety-related applications. These encoders operate as single-encoder systems with purely serial data transmission via EnDat 2.2. Reliable transmission of the position is based on two independently generated absolute position values and on error bits. These are then provided to the safe control.

In addition to rotary encoders with different mounting options, absolute sealed linear encoders are also available. Absolute angle encoders round out HEIDENHAIN’s product program for functional safety.

The safety strategy of the position measuring system is based on two mutually independent position values and additional error bits produced in the encoder and transmitted over the EnDat 2.2 protocol to the EnDat master. The EnDat master assumes various monitoring functions with which errors in the encoder and during transmission can be revealed. The two position values are then compared. The EnDat master then makes the data available to the safe control. The control periodically tests the safety-related position measuring system to monitor its correct operation.

The architecture of the EnDat 2.2 protocol makes it possible to process all safety relevant information and control mechanisms during unconstrained controller operation. This is possible because the safety-relevant information is saved in the additional information. According to EN 61 508, the architecture of the position measuring system is regarded as a single-channel tested system.

The intended use of position measuring systems places demands on the control, the machine designer, the installation technician, service, etc.

In order to be able to implement a position measuring system in a safety-related application, a suitable control is required. The control assumes the fundamental task of communicating with the encoder and safely evaluating the encoder data.

The architecture of the safety system and the diagnostic possibilities of the control may call for further requirements. For example, the operating instructions of the control must explicitly state whether fault exclusion is required for the loosening of the mechanical connection between the encoder and the drive. The machine designer is obliged to inform the installation technician and service technicians, for example, of the resulting requirements.