Spreading out Controls for Motion Systems
By Control Engineering Staff
R educed wiring is a prime reason for distributing controls in motion systems, along with dramatic cuts in associated installation costs and complexities of point-to-point connections it allows. This view was discussed in the main article . Other user benefits (as well as issues) arise when a 'distributed' versus a 'central' control system can be justified for a motion application.
Besides the wiring benefits, Baldor UK Ltd. (Bristol, U.K.)-a subsidiary of Baldor Electric Co .-(Fort Smith, Ark.), lists several other key benefits of distributed motion control. Distributed systems offer:
Less hardware cost (since the main controller needs no analog output or encoder input for each axis);
One operator interface that configures all the drives hooked to the network via software;
Software-based performance/configuration methods that ease the task of creating custom applications; and
Sophisticated interfaces for connecting to higher level information networks.
These capabilities and features are part of Baldor's Mint Drive product line. www.baldor.com.
Delta Computer Systems Inc . (Vancouver, Wa.), a manufacturer of motion controllers, likewise notes several reasons for the growing popularity of distributed industrial systems. With distributed control architectures, 'Highest system productivity can result when decisions and control inputs are made close to the work in process, and controllers are designed to execute complex operations responding to `high-level' instructions from upstream controllers.'
System interfaces become an important consideration because of the need to rapidly handle and share data. Delta Computer recommends that users select an interface standard where the interconnect 'best matches the rate and type of data being passed.' The principal tradeoff here is between determinism and communication throughput. Delta Computer mentions Ethernet and Profibus among several available system interface standards. 'Ethernet, for example, has become very popular because it offers high communications throughput that allows system modules to communicate blocks of data with a single operation,' says a company spokesperson.
Getting the system modules to 'talk' to each other is another story. Messages transmitted over a bus must be formatted in languages readable by the receiving devices. Yet, many industrial control manufacturers use their own proprietary languages, which has hindered the growth of open systems design. Progress is nevertheless being made to solve communications problems. One form of relief provided by some vendors is to make the devices 'multilingual.' For example, Delta Computer states that its RMC motion controllers can communicate over Ethernet in dialects spoken by Rockwell Automation/Allen-Bradley PLC/SLC/ControlLogix, Schneider Electric/Modicon Quantum, Siemens Simatic, Omron Sysmac, and Automationdirect.com DirectLogic controllers. www.deltacompsys.com.
Today and tomorrow
Distributed motion control systems of today are a far cry from what they might be in the coming decade. Some pieces are available, but not all possibilities are in place, according to Jeffrey A. Faris, product manager, General Motion Control, at Rockwell Automation (Mequon, Wis,).
'Integration of motion control systems and the information they control within the factory automation system will make this technology more useful, less complex, more powerful, and physically smaller,' states Mr. Faris. Part of that integration will be 'further refinement of the physical machinery,' allowing fewer, smaller hardware parts, as well as new form factors for actuators and transmissions. 'Machinery will seem to absorb the actuator,' he says, citing as one example, the optimal integration of a linear motor and its drive into the machine's structure.
At the same time, the forward view at Rockwell Automation is to make distributed motion control simpler. Ongoing today-with more to be seen later-is the ability to break down complex machine systems and processes into smaller segments, converting them to cost-efficient subsystems. This generally remains easier to accomplish in discrete manufacturing lines than in continuous processes. 'The idea is to deploy distributed control architectures where they make the most sense,' says Mr. Faris. 'While greater availability of capable computing power is helping, not all of the software element is in place.'
This is somewhat surprising in an era where software is thought to be capable of doing most anything. What Mr. Faris means is that the software infrastructure or the software 'services layer' is not in place, rather than the software application tools.
Mr. Faris sees control systems heavily weighted to the centralized approach for now. 'Centralized control is easier to deal with for many customers,' he explains. A fully distributed control system, along with its programming requirements, can become very complex. The system must be able define all shared information requirements and the means to handle them. This includes access to all the customer's needs for all aspects of manufacturing and documentation. 'Almost an infinite number of tasks are involved in implementing a large distributed control system,' he adds.
Few applications presently lend themselves to distributed motion control. Mr. Faris estimates 'real' distributed systems to be two to five years down the road. However, he notes various 'hybrid' implementations are in use. Here, smaller, manageable control subsystems can be distributed in hybrid fashion, within a larger overall centralized system.
To keep things moving, Rockwell Automation is pursuing object-type architectures for distributing motion control. By creating objects, the user's multitude of tasks can be uncluttered and made less complex. Also, computing power is used more efficiently in the way a given object is optimally executed by the microprocessors. Another factor that contributes to implementing distributed control is to write efficient code, expressing tasks at a lower level.
'In general, the changes in motion control, whether physical or electronic, will allow users to more quickly fit the technology to the machine and spend a greater percentage of their time on process improvement,' concludes Mr. Faris.
Rockwell Automation's Logix family of controllers is key to implementing distributed systems. Use of a multitasking, pre-emptive operating system along with a common software package that supports sequential, motion control, and drive programming and configuration, lends itself to distributed control applications. Several techniques incorporated in the Logix architecture lessen the demands on communication networks imposed by real-time multiaxis motion tasks plus the physical separation of the microcontrollers running the process. These include:
All devices in the distributed control array conform to a uniform object, which defines their attributes, ownership, and information layer.
A producer-consumer concept helps minimize overhead and traffic.
Motion tasks function as direct in-line commands within the application program. These motion objects eliminate network delays and avoid the need for complex synchronization logic.
`Talking' to system elements
Efficient, practical communication among system elements is crucial for the success of distributed motion control. Several bus and networking methods have been applied, but very few of them have been developed with motion control in mind. SERCOS (SErial Real-time Communication System) has digital motion system origins and is one networking method often associated with motion control. It is also an international standard (IEC 61491).
Though developed for computers and consumer electronics, Firewire (IEEE 1394) is another network interface recently making waves in the motion control community. Its high-speed data transmission capability-up to 400 Mbit/sec-is a chief attraction.
Both Firewire and SERCOS have circles of advocates.
Nyquist Industrial Control (Eindhoven, Netherlands) confirms that until recently there has been a holdback for distributed motion control due to few choices for the communication method. 'Perhaps the most important block has been the unavailability of high-speed, yet cost effective communications,' says the company. Nyquist has adopted Firewire as the communication bus for its PC-based motion control products. '[Firewire] will speed the acceptance of distributed control in the machine industry.' www.nyquist.com.
Kollmorgen/Seidel (Düsseldorf, Germany), a Danaher Corp. business unit, has allied with Nyquist to integrate their respective servo drive and PC-based motion control technologies. Shown at the SPS/IPC/Drives 2000 Exhibition in Germany in late November, Nyquist has developed KS3000, a FireWire Servo Drive connection module that links the two product lines into one. KS3000 plugs into a Kollmorgen ServoStar 6000 digital servo amplifier to create a 'smart drive,' while the motion control loop and set-point generator run in a Nyquist NYCe3000 motion controller. The connection module contains the necessary intelligence for real-time, high-speed synchronization and interpolation tasks of multi-axis control. A Microsoft Windows NT host handles I/O signal processing, machine sequencing, operator interface graphics, and trajectory generation.
'This architecture optimizes use of PC technology, while significantly reducing machine cabling,' according to Nyquist. Initial target for the integrated motion control solution will be European OEM customers. www.kollmorgen-seidel.de.
Beckhoff Automation (Minneapolis, Minn.; Verl, Germany) supplies software automation systems for various industries. It also manufactures industrial PCs, and fieldbus-independent distributed I/O systems. Beckhoff's control structures-including those for motion control-are software based and rely on PC-compatible real-time platforms using Microsoft Windows NT and Embedded NT. These techniques lend themselves to implementing distributed control systems for automation.
The company's mainline product, TwinCAT, adds hard real-time capability to Windows NT operating systems. Use of PC processors allows TwinCAT to remain independent from proprietary hardware. Motion capabilities of TwinCAT include control of up to 256 axes, cam profiling, programmable limit switch, flying shears, point-to-point positioning, G-code interpolation in 3D, and support for a variety of drives (servo, variable-frequency, stepper, etc.). www.beckhoff.com
The architecture of distributed motion control must include connectivity with all system peripherals, such as sensors and transducers. Delta Computer Systems cites a trend to provide direct transducer interfaces from the motion controller. Its controllers offer direct interfaces to position and pressure sensors. The new SSI (synchronous serial interface) standard is presently the 'hot' connection between controllers and sensors/transducers. SSI is said to support very high resolution of position information (up to 2 microns [0.00008 in.]) and provide greater noise immunity than digital or analog interfaces.
In the same connectivity vein, the Orion motion controller family from Ormec (Rochester, N.Y.) uses Ethernet TCP/IP (in multiple independent networks) to streamline communications with system elements. Orion's architecture allows location of two Ethernet cards within the controller to separate critical network traffic going to and from I/O modules-for example-from colliding with less time-sensitive operator interface and enterprise system tasks.
The second card can also serve the needs of a development computer. Ormec has integrated support for Opto 22 (Temecula, Calif.) Snap Ethernet I/O products into Orion motion controllers to simplify communications and to easily implement a modular I/O rack system.
In a typical Orion system, a ServoWire Drive Network (IEEE 1394-based) that 'talks' to a bank of distributed servo motor drives is likewise separated from the Ethernet I/O network. This fully isolates commands controlling the servo drives from those accessing the I/O network.
Configuration and setup data for Ethernet I/O points are incorporated directly into Ormec's MotionDesk development environment and stored in the motion controller's Flash ROM. A properties 'wizard' in MotionDesk helps to streamline configuration of I/O properties and select specific Snap Ethernet I/O modules. 'The Project Navigator is used to enter, edit, and display the configuration of hardware and software components for I/O and servo motor control,' says Ormec. www.ormec.com. www.opto22.com.
The full promise of distributed motion control architectures still lies ahead. According to Rockwell Automation, companies with a 'total enterprise' approach to factory automation and machine/process information management will deliver the new motion control capabilities. The overall requirements are too critical to be otherwise. They include real-time global communications, management of complex logistical issues, and focused expert engineering services supplied locally as needed.