To Bus Or Not To Bus

Aug. 23, 2007
Q. What kind of information typically does a servo send out over a digital bus network?

Q. What kind of information typically does a servo send out over a digital bus network? There seems to be different schools of thought about how to use different bus schemes, or even when an analog interface is more appropriate.
A. Anywhere from 25 to 40% of the servo applications we see are still analog. That said, most analog connections today tend to be found between inexpensive controllers made by one manufacturer and servodrives made by another. The two may support digital bus schemes, but perhaps not the same digital bus schemes. So the easiest connection between the two is a ±10-V analog command signal to the drive and a digital pulse-position signal from the drive to the controller.

But machine controllers have no access to detailed servodrive information that could help diagnose problems unless the two communicate over some kind of digital-bus network. For example, consider the case of a malfunctioning motor-position feedback device. The drive would fault and generate a general trouble signal to the machine controller in the form of a digital output.

A controller that just gets a digital signal from the drive would only know that a fault happened somewhere. There are no specifics that could help solve the problem and quickly get back online. It is usually necessary to open the control cabinet, read the fault code on the drive display, then find the manual and locate details on the fault code, or call the drive vendor.

In contrast, servodrives on such digital-bus networks as SynqNet, CANopen, Profibus, DeviceNet, EtherCat or Sercos let the controller query over the network for fault information in the event of a problem. The controller can then display specific details about the difficulty to the machine operator — as well as recommend a course of action via software in the controller.

Additionally, information available from the drive through the digital-bus network can warn the controller of potential future problems. For example, a rising demand for motor current often indicates excessive machine wear that could eventually damage the machine and reduce product quality. Through a digital network the operator can be prompted to schedule maintenance or take other action to keep the machine performing properly.

Many digital-bus networks also let the controller store the drive's application parameters and then download them to the servodrive. This is particularly helpful in the case of a drive swap out. It lets the controller send application parameters to the new drive quickly and easily.

It is also sometimes convenient to let the controller change the drive parameters on the fly. An example is tuning gain in a system that must cut through a work piece slowly and accurately, but travel as fast as possible between the end of one cut and the beginning of the next. Such systems may be optimized by setting one PI gain when cutting, a different one when traversing.

Carroll Wontrop is a senior system engineer with Danaher Motion Corp. Got a question about motion control or mechatronics? Ask Carroll via e-mail at [email protected]

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