Machine operators often need access inside the guarding when setting up work pieces or clearing jams and other malfunctions. Instead of a complete machine shutdown, safety considerations can now be made where motion is controlled at safe limits when guards are open to prevent injury and damage to equipment. Given the speed of industrial equipment, safety devices must react quickly to safety sensors, either bringing equipment to a stop or to safe operating speeds. High axis acceleration on the order of 1 m/s2 means that full speed will be realized in a fraction of a second. Therefore, emergency devices must react in milliseconds.As machines become more sophisticated, distributing intelligence to components such as servo drives is the next step in fast, reliable, and cost-effective machine safety. Today’s intelligent drives can integrate the safety function directly into the drive, thereby offering more functionality in safety protection. The PLC or motion control is not required; each drive has the intelligence to act autonomously in responding to a fault. Unlike conventional approaches to safety, this drive-based technology requires no contactors on the main power or motor power lines, needs no additional external speed-monitoring devices, and operates independent of any supervisory control system. The drive prevents unintended axis movement and creates safe motion that reduces the risk of operator injury by allowing continued but limited axis movement. Safe motion is the most important type of safety because it directly affects the movement of the machine. Safe motion in the drive allows several different types of safety functions to be implemented. While many solutions offer “safe stop,” the key to safe operation is a controlled stop. Rather than simply removing power to stop the motion, safe motion technology in the drive controls the motion so that stopping is both expeditious and orderly. Depending on the need, the drive can decelerate an axis at the best possible speed, or at the fastest speed. Drives can be de-energized to remove all torque, or they can be held in position under energy to allow jogging at safely-reduced speeds for machine maintenance, troubleshooting, repair, or setup. If axes carrying heavy weights are located in the access area, dropping is an additional danger. After a request for access, the drive tests the corresponding stopping brake function before enabling the protective door to be held open. While someone is underneath the axes, the safe operation stop and the tested brake both operate in parallel so that redundant stopping systems prevent uncontrolled falling of the axes.
With a drive-based safe motion solution, setup and workpiece inspection can be done with axes held at a safe standstill while other axes are safely jogged at low speeds by an operator inside the work envelope.
Joe Biondo is a strategic marketing manager at Bosch-Rexroth. His responsibilities include product and marketing management for the electric drives and controls technology group.