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Driving improvement: Advances in drive technology meet today’s demanding application requirements


June 15, 2006
By Jon Simons

Topics

The last decade has witnessed remarkable transformations in variable frequency drive technology, including AC vector drive performance; expanded communications capabilities; smaller, more compact designs; and integrated control capability driven by more powerful and faster microprocessors. Users can now optimize drive performance to meet a wide range of application requirements without the added cost of customization or complex programming.

Vector control
Perhaps the biggest breakthrough over the past 10 years was the addition of vector control to AC drives. At the centre of this technology is the use of a field-oriented control scheme that replaces the volts-per-hertz regulator core–found in most AC drives–with a high-bandwidth current regulator that allows independent speed and torque control, while adapting to motor and load changes. The ability to identify flux and torque separately allows the drive to continuously regulate those same quantities in the motor, resulting in more precise and improved overall motor performance.

Drive manufacturers continue to improve the performance capabilities of vector control AC drives, taking advantage of advances in microprocessor technology. Improved field-oriented control algorithms help optimize drive performance to meet the speed or torque requirements of the application. This enables high-powered AC drives to outperform DC drives, including delivering torque independent of motor speed, with a quick reaction to shock loads. As a result, field-oriented control of AC drives combines the advantages of DC drives–constant torque down to zero speed ó with the benefits of AC drives–simplified installation, tuning and maintenance.

In hoist and crane applications, tight demands on torque response require the drive to deliver high torque levels without losing control of the load and the ability to stop the load accurately and precisely. Likewise, applications such as mixers, centrifuges and extruders require a drive capable of delivering full torque immediately upon startup, constant torque throughout the operating speed range, and full torque down to zero speed.

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Integrated control capabilities
Other advances in drive technology centre around logic control capabilities. These control features allow users to optimize general-purpose drives for specific applications without having to order a special drive or write additional code in a separate processor. Because the embedded control is tightly coupled, users achieve higher speed and throughput. For example, users may be able to position-synchronize a printing press to boost throughput, or more quickly and precisely move items within a work cell, such as lifting and moving vehicle parts on an automotive assembly line.

By embedding the logic right into the drive, users can also reduce the size of the drive-control package, and improve reliability by minimizing the number of required connections. While applications with large I/O requirements may still require a separate controller, many simple processes can benefit from the improved speed and efficiency of a standalone drive unit.

Drives and positioning applications
In the past, AC drives were not designed to place products and materials with the accuracy required to be effective. Although high-performance drives can sometimes match the accuracy of servo motion technology, more commonly, todayís drives are replacing older mechanical systems, such as those on assembly lines.

For example, the Ford Motor Companyís plant in St. Paul, Minn., recently replaced hydraulic and mechanical transfer systems on the assembly line producing the Ford Ranger with drives that contain an embedded controller. This enables the drive to meet the exact positioning requirements of the assembly line. The drives direct the truck frames to the exact point needed on the assembly line, streamlining production and reducing maintenance associated with complex mechanical systems.

Ease of use takes prominence
As microprocessors get faster, more powerful and less expensive, drive manufacturers continue to look for ways to reduce the size, cost and complexity of AC drives, while enhancing performance. New techniques allow higher bandwidths of control, even when faced with machine resonances. Still, while some end-users demand more application versatility and flexibility, others need ease-of-use features, rather than technological advances.

One of the biggest areas of technology innovation is occurring in the area of drive programming and configuration, where simplification and ease of use drive the developments. The use of programming wizards for drive startups is becoming an area of focus. These tools operate much like the setup programs in new PCs, where after a few prompts, the wizard automatically installs the software and required drivers, sets all the parameters and recognizes the hardware devices that are plugged into the PC.

For example, new drive programming wizards will prompt the user for information about the application and size of the motor. It will then automatically set up all the parameters to meet the defined application and hardware requirements.

For more complex installations, like a centrifuge or lifting processes, the wizard might request additional parameters, such as the type of application, the speed of the process and the weight of the load. It will then automatically adjust the parameters to optimize the drive for that application. These tools can dramatically reduce drive startup and commissioning time, and improve setup accuracy by eliminating a significant amount of manual configuration.

One result of increased connectivity is that individual devices will no longer be viewed and managed as isolated components, but rather as part of an integrated system. Thatís because users will be able to program, control and troubleshoot drives, controllers, relays, I/O and motion devices from a common interface using a single software package. This includes the ability to add logic using common software tools with the same look and feel, regardless of whether the application involves a small drive in a standalone process, or a high-performance drive as part of a large PLC-based, integrated and networked system.

In addition to reduced setup and operational costs, a key benefit of this integrated environment is that users are able to save their drive parameters and control logic in a single database.

The proliferation of high-speed Ethernet and wireless networks on the plant floor enables users to continually increase the ability to monitor and control drives and share information. New technologies such as time-synchronized services in Ethernet provide even higher levels of control. The drive can then become an “information window” to the production process.

Improved diagnostics is another feature of newer drives. For example, with current technology, users typically have a few minutes until the drive will trip as a result of an overload condition. Enhanced thermal regulators extend this time by optimizing the insulated gate bipolar transistor (IGBT) switching patterns during periods of thermal stress. New diagnostic tools allow users to perform a trend analysis over longer periods of time to show that a drive may be drawing more current than normal to achieve the same speed.

With access to more detailed information over longer periods of time, users are able to potentially predict problems and prevent catastrophic failures. Moreover, the improved quality and availability of data enable maintenance personnel to be better positioned to troubleshoot, helping to reduce costs and improve uptime.

Advances in AC drive technology are helping manufacturers increase productivity and save energy throughout their facilities. New technology delivers more precise speed and torque, as well as enhanced communications capabilities. It reduces downtime, increases throughput and gives a more accurate picture of the manufacturing environment.

Jon Simons is technology leader, drives business, for Rockwell Automation.