Motion is described through the kinematic variables acceleration, speed and distance. As far as potential hazards are concerned, torques and forces also play a key role. The above variables are covered by the safety functions listed in the standard EN/IEC 61800-5-2. The implementation of safety-related monitoring is heavily dependent on the sensor technology used within the system. The sensor technology used within the drive technology is generally not safety-related and must be monitored for errors. For example, a critical status would occur if the rotary encoder was unable to supply a signal due to a defect, while power is applied to the motor and it is accelerating.
Moved axes in safety-related applications need redundant positional information in order to carry out relevant safety functions. There are various ways to obtain independent position values: One possibility is to detect the defect through a second encoder. In this case, a safe component would have to monitor both encoders and guarantee that the plant is switched to a safe condition if an error occurs. Sometimes the advantage of this solution is that the two encoder systems detect the movement at different points on the machine and so can detect defective mechanical transmission elements.
Rotary encoders generally have several signal tracks, enabling them to detect direction or defined positions within a revolution, for example. These signals can also be consulted for feasibility tests, so that a second encoder system is not required. However, this is not a universal dual-channel structure as the movement is recorded from a shaft or lens. Dual encoder systems are also now available on the market. Such systems are suitable for functions such as safe absolute position. With a strict, diverse, dual-channel design it is even possible to achieve SIL 3 in accordance with EN/IEC 61508. In addition to an optical system a magnetic sensing system may also be used, for example. In terms of costs, however, an increase by a factor of two to three is to be expected compared with a non-safety-related encoder system.
Multi-turn encoders offer a more economical solution; they set their separate multi-turn and single-turn tracks in proportion and can therefore detect errors. In this case, safety-related pre-processing takes place within the encoder system itself. Another option is to use motor signals: by recording voltages and/or currents, calculations can be used to indicate the mechanical movement of the motor. A comparison with the encoder signals will uncover any dangerous failures.
Moved axes in safety-related applications need redundant positional information in order to carry out relevant safety functions. There are various ways to obtain independent position values: One possibility is to detect the defect through a second encoder. In this case, a safe component would have to monitor both encoders and guarantee that the plant is switched to a safe condition if an error occurs. Sometimes the advantage of this solution is that the two encoder systems detect the movement at different points on the machine and so can detect defective mechanical transmission elements.
Rotary encoders generally have several signal tracks, enabling them to detect direction or defined positions within a revolution, for example. These signals can also be consulted for feasibility tests, so that a second encoder system is not required. However, this is not a universal dual-channel structure as the movement is recorded from a shaft or lens. Dual encoder systems are also now available on the market. Such systems are suitable for functions such as safe absolute position. With a strict, diverse, dual-channel design it is even possible to achieve SIL 3 in accordance with EN/IEC 61508. In addition to an optical system a magnetic sensing system may also be used, for example. In terms of costs, however, an increase by a factor of two to three is to be expected compared with a non-safety-related encoder system.
Multi-turn encoders offer a more economical solution; they set their separate multi-turn and single-turn tracks in proportion and can therefore detect errors. In this case, safety-related pre-processing takes place within the encoder system itself. Another option is to use motor signals: by recording voltages and/or currents, calculations can be used to indicate the mechanical movement of the motor. A comparison with the encoder signals will uncover any dangerous failures.
No comments:
Post a Comment