Trends in Servomotors and motion control

01 November 2006

Compact physical sizes, direct-drive designs, stainless-steel versions, planetary gear heads—even onboard drives are among developments helping to keep servomotor technology fresh from year to year.

The engine of virtually all motion control systems is the servomotor. Just as motion control applications are varied, so are the servomotors that drive them. They come in many shapes, sizes, and configurations—from large slow-speed, direct-drive rotary torque motors to sleek, compact units with low rotor-inertia for optimum acceleration and deceleration of loads, to frameless motors, to linear motors providing high thrust force under extreme acceleration and speed. Combined with modern servo drives that incorporate advanced control algorithms, today’s servomotors offer users the highest level of motion control for many different types of applications.

Servomotor buyers are very sensitive to price: this is the most important feature, followed by motor mountings and EMI/RFI protection. The most important performance features are software for setup/tuning, high output torque (continuous), and high torque at low speed.

Tom Strigel of Rockwell Automation agrees about the importance of high motor torque output for many
applications, including material handling and packaging. Today, this capability is delivered by more compact, power-dense technology—such as Allen-Bradley MPSeries— than traditional servomotors offered.’The ability to use powerful motors in small spaces enables building machines that are compact, lighter, and more dynamic for greater productivity in a smaller footprint,’ he says.

A significant industry trend cited by Mr. Strigel is extending servo motion systems’ productivity enhancements into hostile environments with stringent cleanliness requirements, such as in food/beverage, brewing, and life sciences industries. Anticipating this demand, Rockwell Automation has
developed two product families: MPSeries Food Grade Motor and MP-Series Stainless Steel Motor.

Software is key
Beyond external environmental needs, servo-related software has been translating into easier motor setup and tuning at Baldor Electric, with autotuning, in particular, playing a big role. John Mazurkiewicz, servo product manager, claims that Baldor’s autotuning program has the unique advantage to setup loops for current, velocity, and also position. ‘These Windows-based graphical tools enable viewing of many different parameters on a ‘software oscilloscope’ to allow engineers to fine- tune response—as well as monitor
machine I/O points and other key machine parameters,’ he says. Low-pass and notch filters that allow filtering of machine resonances also are available in the setup software.

Moreover, software-based control permits variable-gain settings, that is, a number of parameters can be stored and switched easily. ‘This may be used in applications where the load is known to change; for example, in a machine tool with different [cutting] tools, or a robot arm carrying different loads. Adaptive control will become popular, with the ability to adapt itself to changing loads,’ adds Mr. Mazurkiewicz.

Trends noted at Baldor include higher line-count encoder feedback—such as SSI (Serial Synchronous Interface)—and movement toward absolute encoders. Encoders with higher resolution offer better velocity control of servo motors and greater positional accuracy. Baldor likewise mentions servo motors’ ability to handle rigors of food processing and packaging applications, typically high-pressure washdowns. The
company’s stainless-steel brushless motors meet or exceed baking industry sanitation requirements.

Flexibility offered by setup/tuning software also is highly valued. Control Techniques touts the ability of its servo drives to work with a wide range of servo motors. Its software lets users select a motor from a drop-down list or, in case of a custom motor, they can use a simple built-in autotuning procedure to determine the motor’s characteristics, explains Shane A. Beilke, strategic planning manager. ‘Our State-Space control algorithm allows our drives to operate with a wide range of inertia mismatches, greatly reducing time spent
tuning.’

Among trends in servomotors noted by Control Techniques are

....Continuation of more compact designs with higher torque densities;

....Higher speeds and multiple speed ratings in a given frame and stack length;

....Continued replacement of stepper systems as price continues to be more competitive (servo performance/quality and near-stepper prices); and

....Higher performance feedback devices; increased line densities and absolute encoders.

Smaller, more powerful units
The need for better servo tuning is increasing, as performance demands on smaller motors push their speed and acceleration ratings upward. ‘Proper servo system tuning sets the foundation for the machine’s potential performance,’ says Paul Webster, product manager in GE Fanuc’s CNC Division. ‘Specific tuning
software ensures a solid foundation before adding controller features.’

The company uses software called Fanuc Servo Guide, which combines intuitive parameter setup, test program generation, graphing functions, and servo tuning wizards into one PC-based package. This type of program, notes Mr. Webster, appeals to new and experienced users. Newcomers benefit from ability to step
through initial servo tuning without the need for lots of system knowledge, while veteran users have access to every system feature.

‘Synchronous servomotors have the ability to produce high power output at any point in the speed range while maintaining a very small footprint,’ states Mr. Webster. However, designing high power motors typically requires a trade-off between high torque and feed smoothness (for CNC applications), he explains. Fine balance is required to set magnetic field strength to gain torque output yet limit cogging. ‘Use of finiteelement- method analysis to design internal permanent magnet motors that employ high-density, rare-earth magnets strikes an excellent balance between power and precision,’ he adds.

Put the Brakes on Proprietary Controls
As North America’s largest press brake supplier, Accurpress (Surrey, B.C., Canada) has nearly 9,000 presses operating in the field, covering applications in the automotive, aerospace, and metalworking
industries. Press brakes are used for bending and forming of heavy-gauge metal. Accurpress’ top-of-the-line machine, the Accell, is driven by servo hydraulics controlled by PCs.

But this was not always the case. As long ago as 2001, the R&D team began an initiative to upgrade the Accell proprietary control system. Its performance was considered acceptable, but suffered from high cost
and limited programming flexibility, significant obstacles to maintaining a competitive edge.

‘Most of the competition Accurpress sees in the high-end area comes from Europe,’ R&D team leader, Alex
Kvyatkovski said. ‘That led us to continue to look closely at European controls providers and do thorough
research on emerging trends that fit the Accurpress vision.’

At the time, no press brake manufacturers were using PC-based controls, so they had no ‘proper point of reference’ where to begin. Representatives from Beckhoff Automation convinced the R&D team to try industrial PCs for controllers. Mr. Kvyatkovski says he liked how one CPU could do everything—the entire
machine's motion control and the HMI. This was much cleaner and more costeffective than the multiple-hardware PLC route. And the system was significantly less expensive than the original Accell controller,’ he said.

Flexibility adds features
The Accell design features a Beckhoff C6240 Control Cabinet PC and TwinCat IEC 61131-3 software for point-to-point axis positioning. The positioning is done with an advanced algorithm in which profiles are generated with jerk limitation and pre-control of speed and acceleration to minimise errors. Accell features Beckhoff Control Panels for the HMI hardware, with flat-screen displays and housings milled from solid blocks of aluminium.

Accell machines had originally used Beckhoff Lightbus as the fieldbus network, but with a custom application that required faster response times than conventional fieldbus technology Accurpress moved to EtherCat. ‘Since EtherCat uses offthe- shelf, standard Ethernet hardware and cables, we’re confident this approach will be the most cost-effective. It will also help make our Accell machines even more repeatable by
significantly reducing cycle times.’ EtherCat can process 1,000 distributed I/O signals in 30 ìsec, or 100 axes in 100 ìsec, while giving the option of using standard CAT5e cable or fibreoptic cabling.

A recently added feature is the Sheet Follower system. This device, mounted at the front of the machine, handles materials too heavy for an operator to lift. Via an electronic axis, TwinCat synchronises motion with the press and allows Sheet Follower to lift and lower heavy materials safely without slamming into stationary supports.

Accell also features an active angle measurement system, which measures material spring-back in the press brakes while the press operates.

CPU centralisation is said to save Accurpress money and provide a clean and flexible architecture. Mr.
Kvyatkovski says, ‘We’re able to customise according to almost anything a customer asks of us, something a lot of our competitors may not want to do.’

Today, Accurpress quickly, easily, and cost-efficiently develops custom features for customers using TwinCat software, often at the same time the company sells them. Shorter lead time (as little as two to three months for major customisation) tops its competitors, Accurpress says. ‘With an off-the-shelf controller from a third-party vendor, this simply would not be possible and several lucrative machine orders would be lost as a result,’ Mr. Kvyatkovski says.

‘I know we made the right decision in the move to PC-based controls,’ he says; ‘We find that press brake users are seeking synchronised presses like ours, while traditional PLCs and proprietary designs are quickly losing favour.’

Decentralised motion control
SEW-Eurodrive has introduced Movifit (Field Integrated Technology), which it calls the ‘next generation in intelligent decentralised control.’ Movifit combines the advantages of decentralised control with the latest in
drive application and communication technologies for automotive, food and beverage, logistics, and other
sophisticated material handling applications.

Movifit is designed to move control out of the cabinet and close to the drive motors, simplifying the overall architecture of the system. It is designed to connect with special hybrid cables, which include both power and control signals, to SEWEurodrive’s Movimot gearmotors. The Movimot motors have their own integrated inverters.

The modular construction, in standard IP65 and hygienic IP69K ratings, provides a highly scalable platform for plantwide decentralised control in a wide range of wet and dry environments. Preconfigured control routines for horizontal and vertical conveyor applications provide ready-to-use functions that simplify programming and start-up. Using standard conveyor modules, drive parameters can be set simply without configuration tools using DIP switches or programmed over a fieldbus network.

Real-time diagnostics for greater equipment availability is enabled through the common industrial fieldbuses, simplifying integration with existing plant systems.

‘In our field trials,’ says Dave Ballard, ‘the unique capabilities of Movifit enabled customers to reduce the cost of design, installation and startup, compared to a centralised control system, by up to 30%.’

‘Intelligent’ bearings monitor status of motion system
SKF’s Sensor-Bearing Units are mechatronic machine components covering the fields of both sensor and
bearing engineering. The sensor body, impulse ring and bearing are mechanically attached to each other, forming an integral ready-to-mount unit.

Working as incremental encoders, the SBUs are used to verify the precise motion status of rotary or axial components in motion control systems. They can record the number of revolutions, speed, direction of rotation, relative position/counting, and acceleration or deceleration. They are sealed for protection so that relubrication is not necessary during the lifetime of the product.

The bearing performs all dynamic functions associated with ball bearings. The active sensor consists of three parts: a magnetised impulse ring attached to the bearing's inner ring or race; sensor body with Hall-effect sensors carried by the outer ring; and connecting cable. Shielding is provided against external
electromagnetic effects.

The operating principle of the bearing unit is the generation of a magnetic field of changing polarity as the impulse ring—and bearing inner race—rotates past the outer stationary sensor ring. Based on the number of polarity changes per second, an output pulse is produced by the sensor, which is transmitted via the connecting cable to generate the required application-specific information.


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