Machinedesign 1469 Masterdrive Mc0899 0 0

Drives take on PLCs

Aug. 1, 2000
When your drive needs an input from a sensor, the typical link is through a programmable controller. The latest development, though, is to put that PLC into the drive.

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

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.

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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Speedy convenience

In the motion world, every movement counts. It either adds to or subtracts from the bottom line. Increasingly, though, another dimension is working its way into the equation. It's the attribute known as convenience.

In the near future, either separate drives and PLCs or drive-plcs will handle most simple, one to three-axis automation control applications. In the majority of these tasks, your choice of separate or combined systems comes down to convenience. Is it easier to wire a sensor to a PLC or drive I/O? The answer will likely depend on the sensor's location. Do you want to write a PLC program or are you more comfortable with programming drives? Would the PLC and the drive operate more efficiently if one controlled some functions and the other device handled the rest? Drive-plcs can either eliminate the need, and therefore the cost, for some PLCs, or they free these controllers for other tasks.

In addition to optimizing device operation, the choice will also involve hardware considerations. Drive-plcs eliminate duplicate power supplies and redundant I/O, as well as require far less panel space, says Casey Jones, electrical products marketing manager with Sumitomo Machinery Corp., Chesapeake, Va.

Convenience aside, drive-plcs also offer a tangible benefit, albeit for a minority of applications. They may speed up data transmissions. Say, for example, you have a drive operating a paper or textile cutto- length machine and it needs to know when the material has advanced to the next cut point. Normally, a photoswitch would send a signal to a PLC, probably located at the other end of the plant, which would then send a signal back to the drive, which would then command the motor controller to move the cutting blade.

Granted, the signals travel fairly fast, usually at serial speeds. But rather than wait for even this microsecond delay of signals moving from one end of the plant to another, a drive with the ability to execute PLC-type logic can read the sensor signal, make the decision, and command the actuator to swing the blade, shaving a few microseconds off the cycle. Removing just this fraction of time from one operation may not seem like much, but if you have tens or hundreds of cut-tolength systems, microseconds can add up to minutes, enabling perhaps several more cuts.

A peek into the future

Presently, drive-plcs offer as many PLC and motion control features as developers can stuff into the processor. The next stage will be differentiation, where you'll find a drive-plc set up only for packaging, or for winding, or conveyor control, and so on. The specialization will likely come from plug-in "personality" cards or simply by turning on and off specific software features.

And, as has happened to industrial controllers, your drive-plc may eventually become a "soft-drive-plc" - PC hardware running software that mimics drive and PLC functions.

Watch the links

Nowadays, PLCs and networks are so intertwined, you almost can't have one without the other. Most programmable controllers are linked to DeviceNet, Modbus, or Profibus on the industrial side and Ethernet on the management side.

The Internet will eventually replace these networks, even for real-time control functions. There's a Darwinian aspect to the Internet, and sooner rather than later, it will lead the pack. Some drive-plcs could help make the transition faster because they're not tied to a specific network as are PLCs and their manufacturers. Instead, drive designers have to work with all networks, and therein lies one of the incentives to move quickly to the Internet.

Probably the biggest issue engineers will face, though, is security. There are lots of security problems with the Internet. "Once you assign an address to all these motor controls and devices out there, there's no reason why anybody from anywhere with Internet access can't get to them," says Roddy Yates, drive specialist, Baldor Electric Co., Fort Smith, Ark.

And, despite firewalls, passwords, and other security procedures, there's still an area of vulnerability that will probably always exist. That area is knowledge. People with the knowledge to set up, and therefore subsequently break down, a system will be a risk to the system's uninterrupted functioning.

Here's the sort of thing that can happen. A field engineer is on the phone to a network engineer about how he wants to finish downloading program changes into a PLC on the plant floor. The network engineer, who is located about 1,000 miles away, is monitoring the process from the company intranet. The site plant manager won't stop the process for the few minutes it will take to download changes because he needs to catch up on production. So the network engineer tells the field engineer that he can cause a "break" in the process from where he is. "In a few seconds the paper web will brake and you can finish your download," says the network engineer to the field engineer. Sure enough, the web goes down, and the field engineer sends his changes while the plant engineers scramble to reset the machine controlled by the PLC.

For the forseeable future, there's no protection that can prevent communication knowledge from being used in such an unintended manner.

The next step may be here

Most available drive-plcs are drives with PLC capabilities thrown in. The next step will be to incorporate a full PLC into the drive electronics. It won't take long for this step to arrive. In fact, it should take just about six months.

Lenze GmbH introduced its Servo-PLC at the Hannover Fair in Germany last April. The U.S. branch, Lenze Corp., plans to bring it to the States sometime during the first quarter of 2000.

With a full PLC on board, it offers up to 64 inputs and outputs that are shared between the controller and drive. Using the standard PLC programming language, IEC 1131, you can develop programs in ladder logic, sequential function, graphic, or the other available styles. The controller portion is multitasking, allowing long programs with sections that don't need to execute at every scan.

Drive parameters are accessed through a CAN bus link. "It's the same as having a small PLC in your drive," says Jeff McArdle, vice president at Lenze Corp., Atlanta.

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