Safe Position Measuring

Safety is becoming an increasingly important topic in machine and plant construction. This has become evident in legislation and tightened safety requirements in national and international standards designed to protect human beings, material assets, and the environment. The goal of functional safety is to minimize or even eliminate the risks that can occur during normal or impaired operation of...

08/01/2009


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Benefits of EnDat 2.2 interface
EnDat-IP available

Safety is becoming an increasingly important topic in machine and plant construction. This has become evident in legislation and tightened safety requirements in national and international standards designed to protect human beings, material assets, and the environment. The goal of functional safety is to minimize or even eliminate the risks that can occur during normal or impaired operation of machines or facilities. Here, safe position measuring systems are essential.

The increased use of processors and programmable systems led to a revision of the EN 954-1 standard, which had been in effect for machine construction since 1997 and was harmonized under the European machine directive. The EN 954-1, with its deterministic approach, was no longer adequate for the new requirements—a fact taken into account in its successor standard, EN ISO 13849. This new standard includes the reliability of components and programmable codes similar to the IEC 61508, as well as the product standards derived from it, such as IEC 62061 for electrical drives.

Particularly in mechanical and plant engineering, moving axes are a potential danger, inspiring increasing numbers of designers to integrate the mandatory safety functions directly in the drive. As a result, a technical trend has emerged toward purely digital transmission of position values from an encoder to the drive control. This means that more complex electronic systems have become necessary in both the drive control and the encoders for precise position acquisition. The functions now available in machine and manufacturing systems and the concomitant stringent safety requirements, as defined in the relevant new standards, call for position encoders with entirely new solutions for safety-related applications.

Diverse techniques sometimes come into use for producing redundant position values. Genuine dual-channel redundancy can be ensured by installing two encoders per axis. Economy, however, makes it desirable to find a solution requiring only one position encoder. Currently, analog measuring systems with sine/cosine signals are used for this purpose.

In the future, safety-related applications will require single-encoder solutions that permit redundant position value acquisition through purely serial transmission. Safety-related position measuring systems, such as those provided by Heidenhain, support single-encoder solutions for safety-related applications according to IEC 61508 and EN 13849 based on the purely serial EnDat 2.2 interface. The result is all the benefits of serial data transmission, such as optimization of costs, diagnostic capabilities, and self-configuration, coupled with the rapid formation of the position value in safety-related applications.

Basic principles

Encoders for safety-related applications are tested for compliance with IEC 61508 and ISO 13849-1 (which replaces EN 954-1). These standards describe the assessment of safety-related systems not only from the system structure, but now also based on failure probabilities of integrated components and subsystems. This modular approach helps manufacturers of safety-related systems to implement their complete systems, because they can begin with subsystems that have already been qualified. Safety-related position encoders with pure serial data transmission via EnDat 2.2 accommodate this technique. (Disclosure: Heidenhain is the originator of the EnDat master.)

In a safe drive, the safety-related position measuring system forms such a subsystem: it consists of the encoder with EnDat 2.2 transmitter component, the transmission line with EnDat 2.2 communication and cable, and the EnDat 2.2 receiver component with monitoring function (EnDat master).

The entire “safe drive” system consists in practice of the safety-related position measuring system, safety-related control (including EnDat master with monitoring functions), power stage with motor power cable and drive, and mechanical coupling of encoder and drive (for example, the connection/coupling between two shafts).

The position measuring system is integrated by a mechanical and an electrical interface into the complete system. The encoder is mechanically coupled to the drive through the shaft connection and the coupling, both of which are given by the geometry of the encoder. Including the EnDat master with monitoring functions in the safe control ensures its electrical integration with monitoring functions in the safe control. In consideration of prescribed implementation measures, the safety-related position measuring system can be incorporated as a subsystem with the corresponding failure probabilities into the safety-related evaluation of the plant’s or drive manufacturer’s total system.

The safety system of the position encoder 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 provides the two position values and mutually independent error bits to the safe control over two processor interfaces. In addition, the control monitors the functionality of the safety-related position measuring system and the EnDat master through periodically released tests (force dynamic sampling). The architecture of the EnDat 2.2 protocol makes it possible to conduct all safety-related information or control mechanisms during unconstrained controller operation. This is made possible because the safety-related information is saved in the additional information. With every sampling cycle, this additional information can be requested by the control as well as the actual position. The architecture of the position measuring system according to IEC 61508 is regarded as a single-channel tested system.

Application

Safety-related position measuring systems are designed so that they can be used as single-encoder systems in applications with control category SIL-2 (in accordance with IEC 61508). This corresponds to performance level “d” of ISO 13849, or category 3 according to EN 954-1. Also, the functions of the safety-related position measuring system can be used for the following safety functions in the total system (see also IEC 61800-5-2): safe stopping, safe controlled stop, safely limited jog increment, safely limited speed, safely limited acceleration, safely reduced speed, safely limited position, safe torque off.

Certain constraints described by the standards are to be kept in mind when integrating the safety-related position measuring system into the safety system of a drive or a machine. For example, the safe control must support the two EnDat master interfaces (interfaces 1 and 2) over two independent interfaces (dual-channel structure). These could be two different microprocessors, for example. In the event of error, the control switches to a safe condition.

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 (Position 1 and Position 2). The efficiency tests (test interval & 8 h) in the EnDat master and in the encoder also are to be started by the control. Error bits are subjected to forced dynamic sampling, and their reactions are evaluated.

In the event of error, the control must go into a safe condition. The reaction that the control triggers when an error is revealed depends on the application, and is thus a part of the strategy required of the drive or control manufacturer.



Author Information

Thilo Schlicksbier is product manager for Dr. Johannes Heidenhain GmbH, a privately owned enterprise in Traunreut, Germany, that manufactures linear and angle encoders for use in automated machines and systems, particularly in machine tools. Heidenhain is the first manufacturer to present safety-related position measuring systems based on purely serial data transmission over the EnDat 2.2 interface for use in safety-related applications. The described technique with EnDat 2.2 will support not only the now available rotary encoders, but also absolute linear and absolute angle encoders in the future, according to the company.

Author Information

Thilo Schlicksbier is product manager for Dr. Johannes Heidenhain GmbH, a privately owned enterprise in Traunreut, Germany, that manufactures linear and angle encoders for use in automated machines and systems, particularly in machine tools. Heidenhain is the first manufacturer to present safety-related position measuring systems based on purely serial data transmission over the EnDat 2.2 interface for use in safety-related applications. The described technique with EnDat 2.2 will support not only the now available rotary encoders, but also absolute linear and absolute angle encoders in the future, according to the company.


Benefits of EnDat 2.2 interface

The EnDat interface, developed by Heidenhain, is a fast, purely serial and fully digital interface for high-response drives. It yields significant savings in connection technology for the entire system, and reduces required installation space in the machine. Since the analog scanning signals are digitized and subdivided directly at the place of measurement, characteristics of servo motors such as positioning accuracy and speed stability can also be improved. Along with the automatic self-configuration of encoders and drives in automation systems, the transmission of additional information to the subsequent electronics is possible without additional wires. More information is available online at www.endat.de

Benefits of EnDat 2.2 interface

The EnDat interface, developed by Heidenhain, is a fast, purely serial and fully digital interface for high-response drives. It yields significant savings in connection technology for the entire system, and reduces required installation space in the machine. Since the analog scanning signals are digitized and subdivided directly at the place of measurement, characteristics of servo motors such as positioning accuracy and speed stability can also be improved. Along with the automatic self-configuration of encoders and drives in automation systems, the transmission of additional information to the subsequent electronics is possible without additional wires. More information is available online at www.endat.de

EnDat-IP available

According to an FPGA Blog, MAZeT announced the EnDat-IP for customized applications in March 2009. The IP core has been certified by Heidenhain for implementation in FPGAs and ASICs. The serial communication interface for rotary and linear distance measuring systems was developed by MAZeT. The EnDat-IP has been used to date in a diverse range of technologies and application-specific modifications including safety-relevant applications.

fpgablog.com/posts/heidenhain/

www.mazet.de

EnDat-IP available

According to an FPGA Blog, MAZeT announced the EnDat-IP for customized applications in March 2009. The IP core has been certified by Heidenhain for implementation in FPGAs and ASICs. The serial communication interface for rotary and linear distance measuring systems was developed by MAZeT. The EnDat-IP has been used to date in a diverse range of technologies and application-specific modifications including safety-relevant applications.

fpgablog.com/posts/heidenhain/

www.mazet.de