Control Engineering Online Update for June 10, 2005


Control Engineering Online Update - Technology

June 10, 2005


Sponsored by FCI

Choosing the best machine controller for an application can be a confusing process. Knowing the pros and cons of key motion control products—such as intelligent drives, intelligent motors, and intelligent bus-based motion controllers, as well as serial, centralized, and decentralized options—can help you make the best decision.

Motion Control Options

Manufacturers and machine designers having a range of motion control options to select from. However, choosing the best controller for a specific application is often confusing. The following reviews the various choices for motion control and presents the pros and cons of each.

Intelligent drives
Intelligent dives are single-axis, digital amplifiers that contain a basic position controller that closes the servo loop and accepts position data in the form of a contour data stream. These drives interface with the host computer via RS232, CanOpen, Ethernet, or an alternative communication protocol. Intelligent drives are typically used for single or 11/ 2 -axis applications that do not require coordination.

They offer cost and space savings for single-axis projects by often eliminating the need for a separate controller. Intelligent drives can also be used for multi-axis, distributed control applications where each amplifier is placed in close proximity to each motor. The advantages of the distributed approach is that the designer has the flexibility to match specific drives to individual axes and eliminate the long wires that connect to a multi-axis, central controller.

Single-axis intelligent drive architecture

Intelligent motors
When a motor, encoder, drive, and controller are combined in a single package, it is often called an intelligent motor. Like intelligent drives, these motors are ideal for single-axis applications and they provide a significant savings in cost, space and wiring.

Many users find the intelligent motor solution convenient since all components come from one supplier and work together seamlessly. The disadvantage of using one supplier, however, is that users cannot mix-and-match components from different vendors and, therefore, cannot pick the best-in-class for each component. Also, the intelligent motor solution becomes more costly and complex for multi-axis applications requiring motion coordination. As with intelligent drives, the intelligent motor solution puts the burden of axis synchronization and coordination on the host computer.

Single-axis, intelligent motor architecture

Intelligent controllers
Intelligent bus-based motion controllers fit inside a computer I/O slot, such as the PCI bus. They are typically available in multi-axis formats where the control is centralized onto a single card. For example, a single eight-axis controller card can command eight separate drives and motors. Unlike distributed, single-axis solutions, centralized controllers relieve the host of motion coordination. Also, the communication burden between the host and controller is reduced. Additionally, significant cost savings can be realized with a multi-axis controller because the incremental price per axis drops with each axis added to a central controller.

The main disadvantage of bus-based, central controllers is the connectivity and wiring complexity required for interfacing to various drives and motors. For example, the computer that contains the central control card may not reside close to the motors and drives. This results in long wires. Also, the bus architectures are at the mercy of PC vendors, as there may be a limited number of bus slots available. There is also the issue of the format perhaps becoming obsolete, like the ISA bus.

Multi-axis, bus-based intelligent controller architecture

Serial multi-axis intelligent controllers
Designed with an RS232, USB, or Ethernet serial link, serial, multi-axis controllers do not have to reside inside a PC bus slot. This gives the designer more flexibility in controller placement and less dependency on PC manufacturers. Also, with Ethernet readily available on most PCs, there are numerous devices available for this commonly used, low-cost network.

Like other central controllers, a serial, multi-axis controller will relieve the host computer and programmer from the time-intensive and complex task of motion coordination. Also, the designer can mix-and-match motor types and choose the best drives and motors for the application. Unlike distributed control systems, central controllers tend to have more complex and longer wires going from the central controller to the various drives and motors.

Multi-axis, serial intelligent controller

Centralized intelligent controller/drive
Centralized controllers can be used to accomplish all motion coordination tasks. In some centralized controllers, some of the wiring complexity is eliminated through the attachment of multi-axis amplifiers directly to the controller without any cabling. Also, multi-axis amplifier boards are more economical than purchasing separate single-axis drives, which results in a significant cost savings.

However, multi-axis amplifiers are less flexible for applications that require a different drive style for each axis.

Multi-axis, centralized intelligent controller/drive

Distributed intelligent controller/drive
Another approach for motion control is to construct a distributed control system using multi-axis controller/drive combinations. Unlike traditional distributed control systems that use single-axis controller/drives, a distributed control system using multi-axis controller/drives takes the tasks of motion coordination off the host computer. Also, the designer has more flexibility by being able to choose the number of controller axis combinations best suited to the application.

For example, the user might design a 16-axis system using four 4-axis controllers, two 8-axis controllers or eight 2-axis controllers. These are then distributed on a single network such as Ethernet. One controller can even be designated as the master over the other controllers. The master would receive commands from the host computer and then distribute appropriate commands to each controller in the network.

This provides the advantage of reducing the communication burden for the host computer. Another advantage is that multi-axis controller/drives are typically more economical than multiple single-axis products. Further cost and complexity reductions are achieved because the controller and drive are combined as a single unit.

Multi-axis, distributed intelligent controller/drive (hardware)

Multi-axis, distributed intelligent controller/drive (communication structure)

This article was supplied by Galil Motion Control Inc.,

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