Smart Drives Protect Their Motors
Motion control systems get a predictive maintenance boost from intelligent drive electronics that perform like human nervous system reflexes.
Renee Robbins, Control Engineering -- Control Engineering, 5/1/2009
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Like the self-maintaining robots of science fiction, modern machines have become more intelligent, and in some ways also more human. They are incorporating more decentralized intelligence, which operates like a human’s autonomic nervous system. This makes it possible for a packaging machine, for example, to protect itself from expensive breakdowns through quick reflexes, in the form of error diagnostics and reactions handled in the drive. Without any detour through the “brain,” or control system, intelligent variable frequency drives (VFDs) can diagnose machine problems and perform predictive maintenance before failure occurs or production quality slips. Such innate intelligence supports high levels of machine reliability, availability, throughput and precision.
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| Packaging machines for pharmaceuticals and surgical instruments - products with a high dollar value - can benefit from the increased uptime provided by predictive maintenance performed in intelligent drives. Source: Bosch Rexroth |
“The constant goal of automation is to increase output and reduce maintenance costs,” says Ralph Maguire, principal engineer for Bosch Rexroth. ”If the torque of a feed chain is monitored, for example, the mechanical system can be checked for excessive friction or blockage. If the position is monitored on a ball screw, when the direction of rotation or torque is reversed, such as occurs during backlash, it can be diagnosed. Detecting these changes before a mechanism fails and causes a breakdown can prompt for a scheduled correction.”
Dan Throne, Bosch Rexroth sales and marketing manager, adds, “Based on the parameters you set, intelligence in the drive keeps you fully aware of the health of each axis, predicts if that health is degrading, and lets you apply the right 'medicine’ before the damage grows severe. If damage is developing inside the machine, similar to illnesses or injuries in people, drive-based intelligent functions detect the problem early and appropriate remedies are implemented before significant damage occurs, frequently on an automatic, or self-healing, basis.”
Drive-based predictive maintenance can monitor mechanical characteristics such as backlash, belt stiffness, tension, load variation and other conditions that are critical to a machine’s operation. “If these characteristics fall outside of the tolerance bands for that axis, the drive recognizes that something is wrong, and takes the appropriate action — the same way our reflexes automate our response if we bang our knee or touch a hot stove,” says Throne.
This kind of closed loop control can happen at the level of the controller, but response might be slower. If the human brain were responsible for the appropriate response to touching a hot stove, for example, such a delay would not be sufficient protection from burns. So humans have reflexes.
“The detection of a fatal error in the drive must lead to the proper reaction at the drive. For the electric drive itself, this is the current state of technology,” says Throne. “With [drive based] predictive maintenance, this protection is extended further to the connected axis mechanics, where additional 'reflexes’ have been included to protect the mechanical system or the entire axis.”
Drive-based predictive maintenance works best by streamlining the information exchange between the machine controller and the drive actuators and sensors. That’s why optimizing data exchange between the drive and control is given a high priority during the development of a predictive maintenance system.
This is especially valuable for synchronized multi-axis lines, says Throne. An example is a robotic pick and place application — such as placing needles in a tray — where a row of six robots on each side of a conveyor is assigned an operating zone. If one robot slips a gear tooth, an out of position situation could develop. Unless the drive knows the tolerance band for that axis, it can’t recognize that the one little gear tooth slip could jeopardize the entire line.
However, if the drive is monitoring the backlash, it “feels” when the tight coupling between the motor shaft and the gearbox shaft is malfunctioning. Then, the drive’s “reflex” is whatever the user defines: initiate an error message to the machine controller, or perform a safe shutdown to prevent damage to product or other parts of the machine.
Another example of what the drives’ sensors can monitor is heat. The drive would monitor the heat sink temperature via a thermister in the drive to ensure proper cooling. If the heat sink becomes clogged or the fan is not running properly, an alarm could be issued. If the condition persists and temperature rises, the drive can shutdown in a fault condition to prevent permanent damage.
Sometimes, a reaction in the drive can be triggered by multiple conditions. In this case, an intelligent drive can prepare the complex information and translate into a simple diagnostic for the controller. “This replaces sending all the complex information to the controller, and taxing cycle times to have the PLC do the diagnosis,” says Throne. “It’s a more efficient controls architecture, since the communication required between the control and the axes is automatically reduced.”
With intelligent drives, machines that can self-repair are another step closer to reality. Such electronic and mechanical “reflexes” help automation systems protect themselves from expensive breakdowns, and support high levels of throughput and precision.
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