`Safe` drives could ease the shutdown blues

A movement is underway to develop "safe drives" that could recover rapidly after safety-related shutdowns, as well as being easier to set up and maintain. The effort is being led by the SafetyBus p Club International which has set up a drives group that has held workshops with drives suppliers to develop the concept.

At present, opening a safety gate on a machine usually leads to the production line or cell being shut down completely and the power supply being isolated. This disrupts production severely. It would be preferable if the movement was allowed to continue at a much slower speed, or if the motion was confined to a smaller working envelope. For this to happen, the drives will have to become part of the safety control system.

A draft "daughter standard" to the safety standard IEC 61508 focusing on variable speed drives will be available soon. The standard (IEC 61800-5-2) will pave the way for the development of safe drives.

In line with the draft standard, the SafetyBus p Club is suggesting allowing a master PLC to retain direct control of the drive, but for the safety system to monitor the output of the drive. As long as the drive continues to operate as the safety system expects, the PLC will stay in control, but if there is any deviation, the safety system will take control and shut down the drive.

If a safety gate is opened, the safety system will instruct the PLC to run the drive in a "safe" mode and instruct the monitoring system to check that this is being done.

This could be achieved either by using an external monitoring device connected to the drive, or by using a special drive with built-in safety monitoring functions. For either approach, the system will need to monitor: the standstill condition; the reduced speed operation; the drive`s position; the synchronisation of two or more positions or speeds; and communications with the safety controller.

The German manufacturer Steuerungstechnik has already developed a monitoring system that can be used with fixed or variable speed drives (including hydraulic drives). The company`s SafetyPilot SP100 uses absolute or incremental signals to monitor the motor position or speed via an encoder interface. The data is compared with expected figures and the results are processed on-board or forwarded via a SafetyBus p network for processing by another safety-related controller.