By Rami Al-Ashqar,
Bosch Rexroth Corporation – Electric Drives and Controls
Visionary architect and designer Buckminster Fuller, inventor of the geodesic dome, once gave a short explanation of his design philosophy: “Do more with less.” It’s an idea that still inspires. The effort to make technology do more – more work, more applications, and more functions – while simultaneously using less – energy, space, and resources — drives much of the innovation that shapes design.
In the world of factory automation, “doing more with less” has led to the growing use of integrated drive-motor combinations. With these units, the drive electronics are relocated out of the control cabinet and mounted directly on the motor at the machine, which conserves space and reduces equipment components and cabling.
In integrated servo drive-motors, the drive electronics are mounted directly on the motor. The casing itself is textured to increase the heat-dissipating surface area. Units like the IndraDrive Mi offer flexibilty with such features as integrated Motion Logic compliant with IEC 61131-3 (which adds drive-level motion control), and configurable I/O to support such features as integrated vision and programmable limit switches.
Some compact drive and motor combinations use 50% less space than comparable conventional servo systems, demonstrating the value of leveraging materials and electronics to combine more function into smaller, simpler units.
A typical electronic servo system consists of the electric motor that runs a specific machine axis, a power supply module (ac or dc), and the servo drive.
The drive is an electric amplifier that powers the servomotor. It receives and amplifies the command signal from a control system and transmits electric current to the motor to produce the required motion. Typically the command signal represents a desired velocity, but can also drive a specific torque or movement axis.
Conventional servo drive systems place the power units and drive control in a cabinet outside the machine enclosure. Cabling runs from the cabinet and connects the control unit to the motor, with one power unit cable and one control unit cable per motor. The control cabinet protects sensitive electronic elements from harsh factory conditions such as heat, vibration, static discharge and others.
Advances in present drive electronics, which are smaller and more rugged, have helped make integrated drive-motor units practical for use outside of the control cabinet. This placement helps reduce cabinet size, which also trims the amount of cabling, distribution boxes, and cabinet cooling devices.
Integrated units can be daisy-chained off a single power supply, dramatically reducing the cabling and associated installation costs.
Plus, these combination units speed system development and installation, and ease change for maintenance or machine upgrades, such as adding machine axes with minimal set up.
The move to integration
Several factors promoted integrated drive-motor technology. The first was the widespread change from mechanically driven production lines to electronic line shafting.
Complicated, maintenance-heavy gears, pulleys, and other mechanical components have been replaced in many applications with electronically synchronized servomotors for systems like wrappers, pouchers, and cartoners. The benefits of electronic line shafting include more accurate and flexible machine control, less downtime, lower maintenance costs, and smaller machine footprints.
Integrated drive-motor systems can reduce the Total Cost of Ownership.
Another factor was the continuing pressure to save cost and space. The control cabinet, for example, can eat up limited floor space, particularly in applications where an existing machine is being retrofitted with servos.
Then, there is the factor of distributed intelligence in factory automation. Rather than control individual axes with a PLC or industrial PC at the drive, distributed intelligence lets OEMs add scalable processing power as needed. Thanks to advances in microelectronics, intelligence can be distributed throughout a machine – to the sensors, motors, drives and other components. Combination servo drives can be quite “intelligent.”
Correcting common misconceptions
In the past, the first drive-motor products simply attached a conventional drive control to the motor frame without truly integrating the components. This architecture led to performance issues, which created misconceptions about its value and reliability. The most prevalent concern has been heat dissipation when placing drive electronics on the motor.
Thermal management is a legitimate concern. In a combined drive-motor configuration, the heat sources are the motor, the drive electronics and overall machine and factory floor conditions, which can top 100° F in some settings.
The issue, however, has been resolved in various ways. Ventilator fans or efficient heat sinks improve heat dissipation. The use of robust drive electronics will reduce heat production. These choices help ensure that the drive doesn’t add to the heat produced by the attached motor, enabling the integrated motor-drive system to work well at high output levels.
Newer drive electronics handle high-temperature, high-vibration operating environments. They are more rugged, needing less maintenance. Also, they experience fewer interruptions than earlier drive electronic components. Current generation combined drive-motor units operate on the machine itself.
One final misconception is that combined drive-motor units were designed to be underpowered to control heat production, but that limited their range of application. Current designs make it possible for motors of a given size and geometry to deliver the full measure of torque. For example, the Rexroth IndraDrive Mi is available with power/torque curve ranges from 8 to 30 Nm, making it suitable for a range of applications and industries.
Combined drive-motor configurations
Drive mounting and cabling configurations show how different suppliers have implemented integrated drive-motor packages. There are advantages and limitations to these approaches.
Some combined motor-drive units mount the control electronics on the back end of the motor. Typically, this configuration is cooled with an axial fan.
However, the entire drive-motor package is longer, potentially reducing the motor’s effective length, and its power rating. The length may also affect the package’s ability to fit into machines with limited space, or to be used as a retrofit option for existing installed equipment.
In addition, mounting the drive this way limits the drive casing’s heat dissipation efficiency, which is why an axial fan is typically added to ensure sufficient cooling. The fan itself, though, represents an additional moving part subject to wear, breakdown and replacement.
An alternative approach is to mount the drive horizontally, on the long axis of the motor. Here, the motor casing becomes a heat sink. It is textured to increase its surface area, improving thermal dissipation, while the horizontal mounting provides a longer, better radiating surface than vertical mounting on the end of the drive.
Multiple parts are eliminated with this approach, such as fans, electrolytic capacitors or relays that can wear out over time. Some drive-motor designs exhibit a slim form factor and tight sizing without sacrificing motor torque or power.
The motion needs of pick and place gantry robots suit the capabilities of Integrated drive-motor systems.
Combined drive-motor units offer the opportunity to use new cabling arrangements between the control cabinet and the drives. Since the drive is no longer located in the cabinet, there is no need to run separate power and control cables from each motor back to the cabinet.
Suppliers have adopted several cabling approaches – some are more efficient than others. One approach still uses separate power and control cables; however, instead of connecting each motor individually to a control system, each motor on a machine is connected to the next in sequence. While this approach cuts down on the cable lengths, it does not reduce the number of cables needed, and therefore offers no cost advantage.
A second approach combines drive power and communications into one cable. The cable connects to a distribution box, which can link up to five drive-motor units (one cable per drive). While this option reduces the aggregate number of cables, it adds a distribution box to the cabling architecture – adding components. The box must be mounted in a convenient location for drive operation; plus, if an application requires more than five axes, an additional box is needed, which can add substantial cost to the project.
A third approach is shown in Rexroth’s IndraDrive Mi. Itprovides a separate, efficient cabling arrangement. A single cable carries power and Sercos communications. Up to 20 units can be daisy-chained off a single power supply, reducing cabling and the associated installation and maintenance costs. The cable connections use pre-fabricated components, and are rated to protection class IP65 for longer usage life and reliable operation.
Suited for broad application
The features of many integrated drive-motor units make them well-suited for most machines calling for high-performance servos. From food and packaging to metal cutting to woodworking, integrated drive-motors fit two general scenarios.
The first is constant synchronized motion, or electronic line shafting – when all the motors stop and start simultaneously and turn at a constant, synchronized speed. Example applications include machines processing a web of material such as coated paper for magazines or carton stock for packaging.
These applications call for continuous current with little variation in the peak currents on the drives. The primary role for drive control is to keep all axes spinning synchronously, and make small, pre-defined adjustments to the phase angle of a given axis for smooth, stable handling of the processed material.
The other use is step-by-step applications, such as a gantry robot in a pick-and-place operation. The motion profile here calls for lifting and moving a mass in space, which means greater acceleration and decelerating, moving varying loads with intermittent motion.
Reducing machine costs
Integrated drive-motor units deliver more than improved design and more efficient automation systems. Ultimately, they offer opportunities to reduce costs – component cost, development costs, and Total Cost of Ownership (TCO).
The more efficient the cabling layout the greater the potential savings, which comes in two ways: reduction in the numbers of cables used, and reduction in cable lengths.
Consider a typical eight-axis cartoner, with motors mounted in line with the axes of the machine. The first motor is mounted three meters from the control box, with each subsequent motor mounted three meters apart thereafter.
Using a conventional servo unit, with the drives sitting in the control cabinet, each motor has two cables running back to the cabinet. For the first motor, six meters of cable (2 cables x 3 meters) must be run; to the second motor, 12 meters of cable (2 cables x 6 meters); to the third motor, 18 meters of cabling (2 cables x 9 meters) etc. The final cable run, from the cabinet to the eighth motor, is 48 meters of cabling (2 cables x 24 meters). The total cable need is 216 meters!
For the same application, consider using one cable daisy-chained from the control cabinet to all eight drive-motor units. Eight cables at three meters per cable equals 24 meters total – a potential 89% reduction in cabling.
When you add in the cost of cable pathways, cable installation, testing and long-term maintenance, the integrated drive-motor units offers dramatic savings to factory automation machine developers.
The most significant advantage integrated drive-motor systems offer, though, is modularity. You can develop machines with tighter footprints. You can implement complex multi-axis systems faster and have room to modify and scale up automation applications with greater ease.
While some may argue that the initial cost of combo drive-motor solutions can be higher than conventional servos, the overall system costs are lower when you consider the savings of reduced cabling, installation, testing and maintenance.
Bosch Rexroth Corp.
:: Design World ::
Filed Under: Motion control • motor controls, PCs
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