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A Smarter Way To Keep Danger At Bay

Tue, 11/28/2006 - 4:00am
Joe Biondo

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.

The benefit of safe motion in the drive versus the controller is seen in reaction times. Drive-based safety has reaction times that are up to 400 times faster than a controller-based solution that uses contactors to produce a safe stop. By the time an operator in a protected zone has managed to respond to an error with contact-based verification, a linear axis with a ball screw can move 100 to 200 mm and linear motors as much as 800 mm. The integrated safety system in the Bosch Rexroth IndraDrive, for example, can detect the error within 2 msec and limits the axis movement to just 2 mm.

A machine can be set for safe operation via the dual channels directly at the drive by changing the position of the operating mode switch from normal to special operating mode. As soon as all the selected drives have switched independently to safe status, a drive that has been defined as the master enables holding the protective door open so that the operator can enter the protected area without danger for testing or inspection. Thus, the machine builder saves himself the implementation of power protection in the network or motor feed areas. For the use of robots or palletizers with this drive, it is no longer necessary for an operator to move to a particular safe position before accessing the work area. He or she can switch to the special operating mode from any position. The drives remain in position-monitoring mode.

With access request signals, which are connected redundantly to the drives, multiple safe velocities or movement increments can be made. Monitoring of the limits using redundant software and hardware modules occurs inside the drive, so no additional rotary velocity measuring system is needed. The operator does not have to use external measuring systems and monitoring equipment, which are only appropriate for certain highly dynamic movements because of their switching delays. The safely reduced maximum velocity can be permanently activated as a protection for a machine.

The push for productivity means that machine downtime and setup time must be minimized. One advantage of drive-integrated safety is that it allows the machine to be returned to normal production in the shortest possible time. Because power does not have to be removed from the drive, the drive can return to work without waiting for capacitors to recharge. And since there is no loss of position, machines do not have to “home” before restarting production.

Safe motion in the drive offers fast response times, autonomous operation, a range of safety functions, and efficient return to production. In addition, the use of freely programmable safety logic greatly increases the flexibility of a safety system.

Economically, drive-based safety can cut costs by removing complexity from the design, speed the time required to integrate safety functionality, and reduce the number of components needed.

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.

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