Changes to safety legislation have increased the complexity and cost of machine building. However, Lenze believes that its 9400 series servo drives offer major cost savings in machine construction by incorporating machine safety into the drive. These cost savings arise from the advantages of reducing the number of external control components, which, in turn, reduces the panel size. In addition, the resulting simpler control system speeds up machine build times and the time needed for commissioning. Safety in the drive also means faster response to safety signals, which can result in reduced downtime and smaller footprints.
The simplest safety option that can be achieved in the drive is Safe Torque Off. On receiving a safety stop signal, power to the motor is safely disconnected and the machine coasts to a stop under the influence of the dynamic inertial and frictional conditions. The safe power disconnection means that there is no danger of the motor being accidentally restarted and causing dangerous machine movements.
Lenze's safety module for Safe Torque Off is certified to EN 954-1, Category 4, and IEC 61508, SIL 4. As the power isolation of the motor is achieved within the drive, the power cables between the drive and the motor need not be interrupted by a contactor. This removes a potentially troublesome focal point for EMC interference, as well as saving the cost of the contactor, the panel space required and the time needed to wire it in. Normally safety isolation is achieved by a further contactor on the input to the drive. This means that on restart a delay of tens of seconds is required while the drive capacitors recharge. With the Lenze plug-in safety modules, this contactor is also unnecessary, so restarts can be instantaneous - thereby saving in machine process times.
The above mentioned Safe Torque Off means an uncontrolled stop with the machine coasting to a halt, which may not be suitable for many types of high-speed machines. Such machines stop quicker under a powered stop, where the drives and motors act as brakes, regenerating energy. This type of powered stop can also be done safely and is known as Safe Stop. The Lenze 9400 servo drives achieve this with a simple plug-in module that meets EN 954-1 Category 3. After the powered stop has brought the machine to rest, there is a choice of two safely controlled states. The power to the motor can be removed so that the machine can be manually turned (Safe Stop 1) or the motor supply can be maintained so that the drive and motor hold the machine in a fixed position (Safe Stop 2). Both options are possible by programming the safety module and, even if Safe Stop 2 is chosen with the motor remaining under power, uncontrolled motion is safely prevented.
The Lenze concept for safety in the drive can achieve much more than Safe Stops. For example, a machine such as an oven might need to continue operating at a safe low creep speed after a safety signal has been received. This is possible with a function known as Safety Limited Speed.
Many machines need the facility to jog during set-up. The same safety module plugged into the drive can be programmed for the maximum permissible jog speed and the maximum duration time of the jog, making the jog a safe controlled process. The function is also known as Safe Tip. In other types of machine it may be necessary to run backwards after a safety triggered stop. This function is called Safe Direction and it also can be programmed into the module. Multiple functions can be combined; for example, a Safe Stop 2 can operate with a Safe Tip and even a Safe Limited Maximum Speed.
Provision is made for connection of active safety sensors that monitor their own condition, also passive safety sensors that need external monitoring. Therefore a wide range of sensors - such as light guards, proximity switches and emergency stop switches - can be connected directly into the drive. Connection of safe fieldbuses such as PROFIsafe is also possible, with the potential benefit that a single safe fieldbus can be used for complete machine control without a second general duty bus.
Building safety into the drive does not take away the responsibility for overall machine safety, which lies with the machine manufacturer. However the Lenze 9400 servo system with optional plug-in safety modules gives the machine designer great flexibility to increase performance whil at the same time saving money; he can select and pay for only the functionality he needs. The simplicity of the Lenze safety concept means saving in time and money for the machine builder, plus enhanced safety performance for the end user.
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