Give developers a device with more capability or capacity than they need and what happens? They find a way to put that surplus to work.

Microcomputers in particular have been a stimulus for on-going product improvements, and in the case of drive design, today's faster and more powerful chips have finally led to a drive that not only runs motors, but also executes functions usually found on nano and micro PLCs.

A logical step up

A new family of drives, from Baldor Electric Co., lets you select the drive-plc functionality you need. FlexDrive includes limited PLC functions in addition to an optional CANbus interface. The Flex+Drive adds point-to-point position. The MINTDrive, with an integrated MINT motion control card, eliminates the need for a separate external motion controller or PLC. Simple words are used to program electronic cam, gearing, and flying shear functions.

Contrary to what you might think, the code needed to program a microcomputer for motor control doesn't take up much space. "Programmers have learned to use a small amount of memory and processor functions to do some of the "gee-whiz" motor control features that have come out in the last few years," says Roddy Yates, drive specialist, Baldor Electric Co., Fort Smith, Ark. "Which means, as chips improve, there's more memory for developers to take advantage of, and they're using that memory to add programmable functions, more elaborate I/O features, and special software that enables greater machine control."

Drive-plcs differ slightly from other drives. They are usually general- purpose ac inverters (although you'll find among them all drive types including servos and vector drives) with additional digital and analog inputs and outputs and a few counters and timers.

Because drives typically don't use all of their inputs and outputs in an application, anywhere from one to ten are available for other uses. With optional plug-in cards they can link up to 30 I/O points. At current chip speeds, any more degrades the microcontroller's ability to execute drive functions.

A new family of drives, from Baldor Electric Co., lets you select the drive-plc functionality you need. FlexDrive includes limited PLC functions in addition to an optional CANbus interface. The Flex+Drive adds point-to-point position. The MINTDrive, with an integrated MINT motion control card, eliminates the need for a separate external motion controller or PLC. Simple words are used to program electronic cam, gearing, and flying shear functions

Drive-plcs handle sequential functions. You program them just as you would an ordinary drive through an attached keypad or a PC.

The microcontroller includes operating system software that uses multipliers, adders, subtractors, and flip-flops to execute PLC-type instructions. Each drive manufacturer uses its own operating system.

PLC programming knowledge, however, is not needed. Most systems either prompt you to enter parameters or select among menu choices, linking functions as you go along. A few versions take a different approach, requiring you to program in IF-THEN statements.

Typically, the maximum number of instructions you can link in a sequence is 32. A few systems offer more complex programming in that they let you put in conditional branching.

In the coming months, the programming language will likely become an issue. Right now, there is no standard language for coding driveplcs. Most drive manufacturers agree such a language is needed; they just don't agree on which one it should be.

Expanding flexibility

By incorporating programmable controller functions, a drive-plc can take several actions. For example, it can read data from a machine, calculate, and then directly apply the results to command actions in the drive controller. It's your option whether you also want the drive to report the results to an upper level operator interface and control system to indicate it has taken an action. On the other hand, it could send the data to a higher level PLC for analysis and wait for instructions, as might be the case with separate drives and controllers.

"Drive-plcs are not PLCs yet," says Joe Zoll, marketing manager, Siemens Energy and Automation, Alpharetta, Ga. "But, because of the number of I/O and their instruction set, there's a definite overlap with nano and micro PLCs." Relatively simple control needs that have been met by smaller PLCs are doable by drives with plc capability instead. These needs include start and stop functions (even E-stop), motions or machine actions based on simple sensor input, and some positioning functions, such as those used in flying shears, cut-to-length, and robot control. And at least one manufacturer is including PLC capability into its high-end drives. Thus, drives can now offer motion and PLC capability which PLCs don't because they lack the scan speed and position feedback interfaces.

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