Fix it before breakdown

The old adage, “if it ain’t broke, don’t fix it,” may have had its day—and may still apply to some situations—but is unacceptable in today’s lean manufacturing environments. Critical production processes and automated manufacturing systems can’t afford the cost of unplanned downtime amounting to thousands of dollars per hour.

02/01/2007


The old adage, “if it ain’t broke, don’t fix it,” may have had its day—and may still apply to some situations—but is unacceptable in today’s lean manufacturing environments. Critical production processes and automated manufacturing systems can’t afford the cost of unplanned downtime amounting to thousands of dollars per hour.

Prior preventive maintenance methods have attempted to plug the growing need to detect incipient failure. However, these methods were not easy to use or easily integrated with systems they served, and were limited by operating data acquisition quality and quantity. Other approaches required an add-on condition-monitoring device at the machine or motor, or brought there periodically to take measurements. Higher capability was needed.

Raising the bar on prevention

A new breed of intelligent condition monitoring (ICM) is being made available by leading automation vendors such as Bosch Rexroth and Siemens. ICM looks at trends in friction values, torque levels, and other system characteristics as an early signature of failure. Innovation comes from the servo drive (and motor) acting as a “condition sensor” of the connected mechanical system or upstream process.

Information control, software, machine control

One example of intelligent condition monitoring focuses on system torque change over time. Critical deviation from the torque tolerance band initiates immediate fault response by the drive to protect the machine or plant. Smaller changes prompt warning messages without drive deactivation.

Digital drives typically interrogate the motor to collect diagnostic and fault detection data as part of control. Adding advanced firmware and software intelligence expands capabilities to evaluation and interpretation of acquired data. This enables maintenance decisions based on more realistic conditions.

Bosch Rexroth offers ICM on its IndraDrive family, calling it “intelligent firmware functionality.” The drive executes axis-level diagnostic and maintenance tasks directly and transfers higher level tasks to a master control. It sends a warning message to the user and master control upon detection of an incipient “weak point” or unusual wear in the system. “Users can program predefined maintenance functions to suit their plant- and machine-specific needs,” says the company. To protect a user’s expertise, the drive supplier need not be involved in the maintenance process.

The servo motor and its encoder(s) are intimately involved in the sensing process, sending measured operating data (rotor speed and position, current, dc-link voltage, etc.) to the drive. Sensed values are appropriately processed to derive a time- or frequency-dependent output plus tolerance range defining a normal-running application (see diagram). Advanced data acquisition extracts the most useful signals, complemented by software algorithms and enhanced oscilloscope functions in the drive. A learning process and test cycles are involved in deploying the method.

Remote monitoring

Made practical by the Internet’s universality, remote monitoring of machine systems is another advancement for ICM. Siemens offers it on its Sinumerik CNCs (810D, 840Di, and 840D) as part of “preemptive maintenance” condition monitoring. Remote capability is deployed as a Web-based ePS Network Services utility. It makes “process-oriented service and support applications available to users on a server platform,” says Siemens. Thin-client logic is sufficient at the machine since the complex analysis algorithms reside on the ePS server. However, secure Internet access is needed.

Current ICM applications are in the machine tool sector, solving wear and dynamics problems. Specific areas cited by Siemens include tool wear, servo-axis rigidity (without using external sensors), and main spindle vibration control.

Machine builders and automation system developers need to be aware of the potential of intelligent condition monitoring to cut maintenance and lifecycle costs. Wider applications are coming; added ICM cost appears minimal.



Author Information

Frank J. Bartos, P.E., is Control Engineering consulting editor. Contact him at braunbart@sbcglobal.net .




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