Reviewing the Myriad of Motion Control Choices

Synopsis: Today''s machine designer is in the fortunate position of having a wide range of motion control options to select from. However, the choice of the best controller for a specific application is often confusing. This article will review the various choices for motion control and present the pros and cons of each.

Single-Axis, "Intelligent Drives"

"Intelligent Drives" 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-axis applications or 1 ½-axis applications where coordination is not required. 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 multiaxis, distributed control applications where each amplifier is placed in close proximity to each motor. The advantage of the distributed approach is that the designer gets the flexibility of matching specific drives to individual axes and can eliminate the long wires that connect to a multiaxis, central controller.

The major disadvantage of using single-axis intelligent drives for multiaxis applications is that the host is burdened with the complex task of synchronizing amplifiers for precise motion coordination. This also puts the burden of coordinated motion control on the system designer. Another disadvantage is the large communication burden placed on the host to talk to the various drives.

Single-Axis, "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 intelligent 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 seamlessly work together. The disadvantage of using one supplier, however, is that users cannot mix-and-match components from different vendors and, therefore, cannot pick the very best-in-class for each component.

Also, the intelligent motor solution becomes more costly and complex for multiaxis applications requiring motion coordination. As with intelligent drives, the intelligent motor solution puts the burden of axis synchronization and coordination on the host computer.

Bus-based Multiaxis, "Intelligent Controllers"

Intelligent, bus-based motion controllers fit inside a computer I/O slot such as the PCI bus. They are typically available in multiaxis formats where the control is centralized onto a single card. For example, a single 8-axis controller card can command eight separate drives and motors.

Unlike distributed, single-axis solutions, centralized controllers relieve the host from the intense burden of motion coordination. Also, the communication burden between the host and controller is reduced. Additionally, there is a significant cost savings with a multiaxis controller as compared to purchasing multiple single-axis controllers. This is 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 them to various drives and motors. For example, a computer that contains the central control card cannot reside close to the motors and drives, resulting in long wires. Also, 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 that the format could possibly become obsolete, like the ISA bus.

Serial Multiaxis, "Intelligent Controllers"

Serial, multiaxis controllers are designed with an RS232, USB, or Ethernet serial link, and they 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, numerous devices are available for this commonly used, low-cost network.

Like other central controllers, a serial, multiaxis controller will relieve the host computer and programmer of 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 his 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.

Centralized Multiaxis, "Intelligent Controller/Drive" Combinations

Galil''s DMC-21x3 Ethernet controller series is an example of an intelligent and centralized multiaxis controller that is sandwiched with a multiaxis drive. As with other central controllers, the DMC-21x3 accomplishes all motion coordination tasks. Some of the wiring complexity typical with other central controllers is eliminated because multiaxis amplifiers are attached directly to the controller without any cabling. Also, multiaxis amplifier boards are more economical than purchasing separate single-axis drives, resulting in a significant cost savings.

However, multiaxis amplifiers are less flexible for applications that require a different drive style for each axis. This new breed of multiaxis, intelligent controller/drive combinations provide the best advantages of both distributed and central control schemes by providing the designer with a cost-effective solution with reduced wiring and minimal burden on the host.

Distributed Multiaxis, "Intelligent Controller/Drive" Combinations

Another approach for motion control is to construct a distributed control system using multiaxis controller/drive combinations. Unlike traditional distributed control systems that use single-axis controller/drives, a distributed control system using multiaxis controller/drives take the burdensome 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 designer might solve a 16-axis project 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 multiaxis controller/drives are 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.