Smart Drives Protect Their Motors
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.
|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.
|Renee Robbins is a senior editor for Control Engineering. She can be reached at firstname.lastname@example.org|
How smart drives perform predictive maintenance
The performance of a motor is directly related to the performance of the mechanics it moves. Among other things, an intelligent drive monitors itself and the backlash between the servomotor and the axis mechanics. The drive monitors how the motor responds to the amount of play in the gears or the belt and, if it falls outside the tolerances set for that axis, the drive intelligence can do one or more things:
Generate a warning message to the controller or operator;
Modify the drive current to compensate for the change in the mechanics;
Initiate a safe shutdown before the situation gets critical, to prevent machine and product damage.
Smart drives start by using the standard, built-in measuring devices found in drives for regulation, then add software features to increase diagnostics. The good news for users who choose intelligent drives is that there are typically no additional costs for setting up predictive maintenance functions. The latest generation of drives often have built in diagnostic features, and configuration of alarms and automated reactions is done with the regular drive software.
Stan Ho, manager of low voltage drives for Rockwell Automation, says, “In previous drive models, predictive maintenance capabilities came from end users who monitored the load current for a given application. If the value of the current increased above the application’s normal level while running, the end users interpreted this to mean that the mechanical system needed maintenance. Users could then generate an external signal for the maintenance team to investigate. However, the Allen-Bradley PowerFlex 755, the newest drive from Rockwell Automation, has standard features that monitor and track information that affects the drive’s cooling fans and relay outputs. Additionally, users can program the drive to monitor the run time hours for machine or motor bearings. Software functionality is standard and doesn’t require additional software.”
Craig Nelson, product marketing manager for Siemens Energy & Automation Motion Control Business Unit, which makes the Sinamics S servo and vector drives, says, Sinamics S120 drives include built-in logic programming capabilities to decide which actions to take. A drive “can diagnose a speed feedback device going bad and switch into sensorless mode for continued operation, and do this while outputting an alarm, so the issue is addressed at the next opportune time.” Another example of smart drive diagnostics on the Sinamics S120 drives is related to the safety circuit, where a latency period can be programmed to output an alarm if the safety circuit has not shown activity over a programmed period of time.
An additional function of the Sinamics S120 drives is built-in trace buffer memory that comes standard. “This allows common measurements to be saved, even when a power failure occurs. This data can be read out in drive commissioning software to diagnose a problem or determine the order events occurred,” says Nelson. “Trace data of drive signals can also be used to diagnose problems with the process the drive is controlling, including tension regulation or regulator tuning.”
Rainer Neufeld, electronics systems manager for SEW Eurodrive, says his company’s smart drives include Movi-Drive, MoviPro, MoviFit, and MoviTrac brands. “They all have built-in intelligence, known as IPOS, which is the company’s integrated positioning and sequence control software,” Neufeld explains. “The predictive maintenance capabilities don’t cost extra; users need only the IPOS-OT version.
Neufeld says drive monitoring data interacts with the rest of the machine or production line via a MoviPLC, or other controllers like those from Rockwell Automation/Allen-Bradley or Siemens.
Drives can also be used in combination with SEW Eurodrive’s MoviPLC, “which makes those drives pretty smart. But the MoviPLC implementation would add additional cost,” says Neufeld. With the IPOS or MoviPLC and the DUO10A diagnostic unit, he says, it is possible to monitor the drive and the gearbox, to monitor “everything that has an index number, including service life of the oil, oil temperature, heat sink temperature of the drive, utilization of the drive, output current, speed, and current limits.
Ralph Maguire, principal engineer for Bosch Rexroth, says his company’s IndraDrive has three options that assist with maintenance: the Productivity Agent, the integrated IEC 61131-3 PLC, and safety technology. The IndraDrive itself detects changes in stiffness, backlash, friction, and energy consumption. Diagnostic functions are loaded to the drive using standard IndraWorks software. During production, Productivity Agent online diagnostics monitor whether the axis stays within a machine-builder-specified envelope. (There are also two offline diagnostic tools for mechanical and frequency-response analysis.) Because the Productivity Agent uses sensors already built into the IndraDrive and motor, the standard single drive hardware can be used, says Maguire.