Servo drives simplify machine safety issues

A company has developed a new drive with built-in safety functions that offer benefits to machine builders, systems integrators and end users alike. Dean Palmer reports Due to recent legislation, safety of machinery and equipment is now a major concern for design engineers, machine builders, systems integrators and end users. Controlling potential hazards from moving parts on high speed, precision machinery is often solved by using servo drives. The technology now exists to implement drive-based safety, especially when it comes to servo drives. Intelligence can now be built into decentralised drives, enabling them to perform many of the functions traditionally handled by motion controllers and/or PLCs. And, with drive-based safety, the drive is also able to perform safety functions that would otherwise require multiple safety relays, monitoring units and a dedicated position/speed sensor. A new servo drive launched last month, Lenze's L-force 9400 series, provides a shining example of how to achieve drive-based safety. The 9400 has a simple plug-in module that delivers safety functionality. There are currently two types of module available. The SM 100 provides a 'safe torque off' function, which is activated via a single emergency stop switch or other passive sensor. It is suitable for use with safety related control systems conforming to the requirements of BS EN 954-1 Category 4 or IEC/EN 61508 SIL 3. There's also a single channel output on the SM 100 for signalling to a PLC, plus there are two diagnostic LEDs. The other safety module, the SM 300, is far more capable and is approved for use in safety related control systems meeting the requirements of BS EN 954-1 Category 3 or IEC/EN 61508 SIL 3. The module provides a Type 1 safe stop, in which the motor is ramped down to a controlled stop and the torque is then removed. There's also a Type 2 stop in which the motor is ramped down to a controlled stop and the torque is then used to actively maintain zero speed, which also enables immediate resumption of an interrupted operation. Other safety functions of the SM 300 include a 'safely limited speed' operation, a 'safe tip' (hold-to-run) mode, a 'safe direction' mode (the motor is permitted to run in one direction only), and a 'safely limited increment' mode, in which the motor moves in response to an input signal, but only for a limited increment before a safe stop (Type 1 or 2) is applied. After the pre-defined incremental move has been completed, another input signal is required before the next pre-defined incremental move is performed. The fact that Daimler-Chrysler Germany, one of Lenze's first customers of the 9400 servo drive and one of the automotive leaders in terms of production-based safety products, is proof that Lenze's new drive is worth a closer look. As well as safety functions and an Ethernet Powerlink module, the 9400 also boasts a unique backplane installation concept. The backplane handles all the high voltage connections and is separate from the drive. Installers will mount the backplane into the control panel, connecting the power supply, the DC bus if required and connections to the motor. Set up is made even easier by having an integrated RFI filter and optional plug-in braking module. Once all the connections have been made, the drive itself can be manually plugged onto the backplane and locked into position, a process that takes only seconds. The backplane concept leads to further time savings for maintenance and there will be less production downtime should a failure occur. Currently, Lenze is offering 0.37kW up to 11kW versions of the drive, but by the end of 2006, aims to offer models rated up to 400kW. The servo drives operate with suitable motors from any manufacturer, the safety concept is equally applicable to machine upgrades and new builds, and there is also scope for users to introduce more safety modules with extra or alternative functions in the future. The 9400 is the leading product in Lenze's new L-force drives and automation products and services portfolio, which includes motors, geared motors, servo drives and field devices such as I/O and remote maintenance modules. Software also plays a key role in L-force for logic and motion control, with drive-based and PC-based solutions. There are now customer-friendly programmes for parameter setting, commissioning and diagnosis, and pre-configured technology functions that reduce programming time.