SAFETY-RELATED POSITION MEASURING SYSTEMS

HEIDENHAIN offers safety-related position measuring systems that are based on pure serial data transfer, via EnDat 2.2. A safety-related position measuring system can be used as a single-encoder system in conjunction with a safe control in applications with control category SIL-2 or performance level "d". The safe transmission of the position is based on two position values generated independently from each other and on error bits. These are then all provided to the safe control.

Safety is becoming increasingly important in machine and plant construction. Proof of this can be seen in new legislation and in the heightened safety regulations of national and international standards. These high requirements serve to protect human beings, but material assets and the environment are also receiving more consideration.

The goal of functional safety is to minimize or reduce the risks that can occur during normal or impaired operation of machines or facilities. The first step of this is achieved with redundant systems. For example, axes that are moved in safety-oriented applications require redundant position information in order to perform the corresponding safety functions. Various system configurations can be realized in order to capture independent position values. One possibility is using two encoders per axis, but due to cost considerations, a solution with only one position encoder is aspired to in most cases. Until now, analog encoders with sine and cosine signals were used for this.

HEIDENHAIN is now the first manufacturer to offer a purely serial, single-encoder solution for safety-related position measuring systems in safety-oriented applications, according to IEC 61 508. This means that safety applications can now take advantage of all the benefits of serial data transfer, such as optimization of costs, diagnostic possiblities, automatic commissioning and rapid formation of the position value.

BASIC PRICIPLE
The HEIDENHAIN measuring systems for safety-related applications are inspected for compliance with ISO 13849-1 and IEC 61 508. These standards describe the assessment of safety-related systems, for example on the basis of failure probabilities of integrated components and subsystems.

This modular approach helps manufacturers implement their complete systems, because they can begin with prequalified subsystems. Safety-related position encoders with pure serial data transfer via EnDat 2.2 accommodate this technique.

In a safe drive, the safety-related position measuring system presents such a subsystem. The safety-related position measuring system consists of:

• Encoder with EnDat 2.2 transmission component
• Data transfer line with EnDat 2.2 communication and HEIDENHAIN cable
• EnDat 2.2 receiver component with monitoring function

practice, the complete "safe servo drive" system consists of:

• Safety-related position measuring system
• Safety-oriented control (including EnDat master with monitoring functions)
• Power stage with motor power cable and drive
• Physical connection between encoder and drive

INTEGRATION OF THE POSITION MEASURING SYSTEM
The position measuring system is integrated, via a physical and an electrical interface, into the complete system. The physical coupling of the encoder to the drive is determined by the encoder's geometry, and including the EnDat master in the safe control ensures its electrical integration. Certain measures are to be taken for the integration and evaluation of the EnDat master in the control in order to use the safety system of the position encoder.

Complete safe servo drive system.

TRANSMISSION OF THE POSITION VALUE
Scanning in the encoder simultaneously produces two independent position values that are transmitted using 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 provides the two position values and independent error bits to the safe control over two processor interfaces. The control monitors the correct operation of the safety-related position measuring system with periodic tests. The architecture of the position measuring system according to IEC 61 508 is regarded as a single-channel tested system.

FIELD OF APPLICATION
Safety-related position measuring systems from HEIDENHAIN are designed so that they can be used as single-encoder systems in applications with control category SIL-2 (in accordance with IEC 61 508). This corresponds to performance level "d" of ISO 13849 or category 3 of EN 954-1. Also, the functions of the safety-related position measuring system can be used for the following safety functions in the complete system:

• Safe switch-off
• Safe stop
• Safe controlled stop
• Safe reduced linear or rotational speed
• Safe limited job increment
• Safe limited absolute position
• Safe limit of torque/power

Safety-related measuring system.

REQUIREMENTS TO BE FULFILLED BY THE SAFETY-ORIENTED CONTROL
A majority of the important tasks of the safe control for setting up a safe axis using a safety-related position measuring system are detailed below.

• The encoder and the EnDat master must be subjected to a switch-on test when they are switched on.
• The position values and error bits are to be evaluated by the safe control during operation. Depending on the safety function required, this includes monitoring of the servo lag, standstill monitoring and the comparison of the two position values.
• If an error should occur, the control must assume a safe condition. The reaction that the control triggers when an error is revealed depends on the application, and is therefore part of the strategy of the drive or control manufacturer.
• The control is to perform efficiency tests in the encoder and in the EnDat master. As part of this, the error bits are subjected to forced dynamic sampling, and their reactions are evaluated.
• The following mechanical defects must be detected by the safe control:
• Breakage of shaft or coupling
• Static shaft misalignment
• Dynamic shaft slip

HEINENHAIN's latest encoders.

REQUIREMENTS FOR DATA TRANSFER FROM THE ENDAT MASTER TO THE FURTHER COMPONENTS OF THE SAFE CONTROL

The EnDat master makes the two processor interfaces 1 and 2 available. Position 1, Position 2 and the error bits must reliably be transmitted to the safe control over these two interfaces.

The EnDat master and the encoder must each be supplied with protective low voltage according to EN 60 204-1 with overcurrent protection, and overvoltage protection if necessary, from the subsequent electronics.

Acceptance testing must be defined when the machine is first put into operation, or when changes are made that influence the safety functions of the machine.

HEIDENHAIN offers encoders for safety-related position measuring systems with purely serial data transfer. The two measured values are already formed independently of each other in the encoder, and are transmitted to the safe control via the universal EnDat interface. This saves on the cost of expensive connecting cables, eliminating unnecessary wires.

There are currently two rotary encoders available in both singleturn and multiturn versions as "safe" position encoders. Further absolute linear and angle encoders are in preparation.