Rocklin, CA —No one knows exactly how the proliferation of Ethernet, web servers, and embedded web server technology will impact motion control, and no one knows what role the Web will finally play on the factory floor. But if the E-Series Motion Controllers from Galil Motion Control Inc. are any indication, the line between central and decentralized control is going to get blurred.
Often, the first choice an engineer makes when he needs to control several motors is between centralized and decentralized control architecture—two fundamentally different techniques. The biggest advantage of a centralized approach is simplified programming on tightly coordinated multi-axis applications, because the host computer only needs to communicate with a single device. However, wiring everything back to the host is the biggest drawback. Distributed systems eliminate such wiring complexity by placing a single-axis controller near each motor, and connecting them to the host using a variety of protocols such as Sercos, Fieldbus, or DeviceNet. The result is relatively short wires. However, the task of coordinating between the various controllers makes the host computer program more complex.
To realize the shorter wiring of a distributed control system, and the low cost and simple programming of centralized control, Galil's E-Series controllers are designed specifically for Ethernet-based distributed control systems. E-Series firmware allows the controller to be configured as a master or a slave. As a master, one controller commands all the other slaves via Ethernet, and acts like a "virtual multi-axis" controller to the host PC.
According to Galil, benefits include:
Simpler code. It off-loads programming from the host to the controller.
Efficient communication. The host needs to communicate with only one controller instead of many.
Lower cost. Two-axis capability allows engineers to group motors in pairs for coordinating motion instead of using two separate single-axis controllers.
One DMC-3425, designated master in its firmware, communicates with the remaining slave controllers on behalf of the central controller through an Ethernet LAN hub.
Often, the application size influences an engineer's choice between central and distributed control. In smaller applications, such as tabletop machines, long wires aren't as big an issue. So engineers typically use a centralized approach to control. Centrally controlled systems commonly use one embedded ISA-, PCI-, or PC 104-bus based multi-axis motion card within the host. However, a central control may also be configured with a stand-alone controller that communicates serially via RS-232 or something similar. Either way, explains Galil President Jacob Tal, by using one motion controller to control all of the motors, the advantages are: simple communication because the host only needs to communicate with a single device; easy programming because information on all the motors is present in one controller; and low controller cost per axis. The main disadvantage, he adds, is wiring complexity because data must be transported from all the various I/O devices, such as amplifiers, encoders, and limiters, back to the central controller. "Extra wires imply added system cost," Tal says. "And this can produce noise levels that may degrade performance, especially in larger systems."
That's why distributed systems are used. With a single-axis controller near each motor, the wires are shorter. "However," explains Tal, "because each controller deals with a single motor, communication is complicated, motion programming is complex, network components increase cost, and the cost of controller per axis is higher."
To meet the objective of short localized wiring, simplified programming, low-cost components, and efficient communications, Galil developed a new design approach that Tal calls the E-Servo. "The technology combines all the benefits of the two approaches," Tal says. To work, the motion controllers and the host communicate through an Ethernet LAN hub. Controllers placed near the motors and the I/O signals reduce wiring. But Galil went an extra step to simplify the programming. By making one controller a master that communicates with the other controllers, it becomes a virtual central controller to the host. "This means the host need communicate with only one controller," says Tal.
In an effort to reduce the cost, Galil allows each controller to control two motors. "Because coordinating axes require motors operate in pairs," explains Tal, "using a single controller to control both motors can significantly reduce the cost, and the wire length is not increased much."
Contact Lisa Wade, Galil Motion Control Inc., 3750 Atherton Rd., Rocklin, CA 95765;
Tel: (800) 377-6329; Fax: (916) 626-0102;