Brushless motor chipset.

Recent strides in computer and communications technology, combined with falling prices in electronics, have led to a situation where motion control systems are as varied as the applications they serve. Behind all this variation, however, are three basic motion control architectures: PC-based, embedded, and distributed.

In the past, when controllers and amplifiers used to cost more than motors, motion architectures tended to be more centralized to minimize system cost. Now there’s no such limitation, making it easier than ever to adequately solve motion problems.

PC-based, embedded, and distributed motion are the most common architectures among machine designers. These three, in one way or another, reflect the rapid improvement in the performance of digital and power electronics, as well as a reduction in their cost.

PC-based motion

A major trend in motion architectures is the increasing use of PCs as “host” processors in motion control applications. A typical PC-based motion system consists of a motion control card located inside the PC and external motors and amplifiers.

In recent years, a growing number of systems engineers have discovered the PC for use as an engine for control-oriented applications. One of the reasons for this is that the PC has become both inexpensive and more universal. Touch screens, keyboards, monitors, and a number of other off-the-shelf I/O devices make the PC an ideal platform for communicating with the machine operator.

The motion control industry has capitalized on this widespread use of the PC, and there are a broad range of motion control cards available which provide cost-effective and powerful motion features. In general, there are three levels of PC-based motion card solutions that designers can choose from.

Fully programmable. This type of card is the most common today and is available from several vendors. Thanks to onboard processors, such cards relieve the PC from time-consuming tasks. Users download complete programs in a standard language, such as “C” or in a proprietary language, and the card’s microprocessor handles the motion-related tasks.

One disadvantage, however, is that these cards may require programmers to write code in a language that is not supported by other vendors. The cards are also generally more expensive than the other types.

Semi-programmable. These cards have come on strong in recent years. A motion engine, which can perform various motion activities such as trajectory generation, servo loop closure, and commutation, is provided on the card. However, self-executing programs cannot be downloaded to the card. These card solutions are almost always based on off-theshelf motion chips which form the basic motion engine. In this approach, the PC provides the high-level sequences and serves as the “host” machine controller.

Consider a typical application where the host downloads a move command to the card such as, “Move axis #3 trapezoidally to location X.” After receiving the command, the motion card executes the entire move, performing all profile generation and servo loop control, and finally, signaling the host that the move has been completed. At this point, the host sends a new command, and the process repeats.

Semi-programmable cards are generally less expensive than fully programmable cards. And they don’t require code written in a vendor-specific language. The disadvantage is that the PC code may become more complicated, since it has greater real-time responsibilities for the system operation.

Nonprogrammable. In this approach the PC performs all motion-related activities, including profile generation, servo loop calculation, pulse generation, and real-time signal management. The card supplies quadrature encoder feedback and hardware-generated signals such as analog outputs and PWM generation. The advantage of this system is the simplicity of the hardware and its low cost. The disadvantage of this approach is that it requires a very detailed understanding of the PC’s operating system to be effective.

Embedded motion controllers

With an embedded approach to motion control, the three major elements of the machine controller (the host CPU, motion engine and amplifier) are combined onto a single embedded printed circuit card. This approach is used in situations where the machine controller must be low-cost, compact, and simple to maintain. Although some embedded motion controllers can be purchased off-theshelf, most are designed and assembled by the machine builder.

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