1 June 2002
Gaining Control with PC-based Soft Motion
By Bob Hirschinger, Dave Heller, and Stan Rosen
Users viewing PC-based soft motion control critically should look closer at its performance, flexibility, and cost benefits.
To PC or not to PC" used to be the question. Now we have multiple PC-based control options, including soft motion control. For many applications, soft control, which involves running motion control on a standard operating platform such as Microsoft's Windows NT or Windows 2000, can be a cost-effective alternative to using programmable logic controllers (PLCs).
By running motion control on standard operating systems without real-time extensions, you can achieve higher performance at a lower cost, thanks to hardware and software integration, easy customization, efficient information processing, and the ability to leverage open technologies.
Taking Advantage of PC Processing Power
Traditional PC-based motion systems are implemented as a bus-based, motion coprocessor card (supported bus types include ISA, PCI, and PC/104). With this system, the local host program running on the PC exchanges position, velocity, motion status, recipe information, sequencing, and other data with the motion card over the PC bus. The card then executes a local motion application program and manages all aspects of the motion.
This traditional motion coprocessor approach restricts performance, and because all the motion processing is local to the motion card, it doesn't take advantage of the PC processing power. The coprocessor approach also requires 1) complex logic to synchronize the motion card(s) and host program running on the PC; 2) a separate programming package; and 3) management of a local application program. All synchronization increases cost and hinders ease of use.
Eliminating Coprocessor Dependence
Unlike traditional PC-based systems, which perform motion functions such as command execution, trajectory planner, and servo loop closure on a dedicated motion coprocessor, the soft motion solution executes motion command and trajectory planner functions in software on the PC's CPU, into which it's loaded.
Commands are inserted directly into the logic application program to provide inherent synchronization and high performance. Logic engines and soft motion can be configured in multiple ways, with each supporting up to 32 axes. In addition, soft motion control sustains processor affinity for multiprocessor systems. This allows start/stop motion, position vs. position profile, time-based camming, gear, and other axes to be synchronized, even if they're distributed among multiple interface cards. One programming package can provide all programming and configuration support for logic, motion axes, and motion drives. Available graphical editors create complex position vs. position and position vs. time profiles, further enhancing ease of use.
Trajectory planner commands can be sent to PCI cards [either analog or SErial Real-time COmmunications System (SERCOS) interface] because soft motion supports the Windows NT/2000 operating systems. More importantly, real-time extensions aren't required. With the PCI analog card, a 200-microsecond (µsec) fine planner and servo loop closure are completed using trajectory commands from the soft trajectory planner. An analog torque or velocity command controls the attached drive. With a PCI SERCOS interface card, the trajectory commands from the soft trajectory planner are sent to one or more drives via a digital, fiber-optic connection. A 250-µsec fine planner and servo loop closure are performed on the drive.
Maximize Performance, Minimize Cost
Because the motion card is either a simple analog or digital drive interface, eliminating the need to host an application program and provide motion command execution or trajectory planner functions, soft motion has a lower cost than its traditional PC-based counterpart. The motion instructions and trajectory planner run as software functions on the PC, making full use of robust, high-level computing power available on commercial PCs and increasing operating performance. The logic and motion command executions and trajectory planner functions are tightly integrated because they run as part of a multithreaded application that shares the same process memory space.
As the speed of PCs increases, you can easily upgrade your soft motion systems to take advantage of the higher performance. Windows NT or 2000 reliability remains intact because soft motion eliminates the need for real-time modifications.
NT System Capabilities
PC-based motion controllers (e.g., Allen-Bradley's SoftLogix5800) run in the real-time priority class on Windows NT or 2000, which means they'll preempt any user-mode application—including Microsoft Office applications, relational databases, and man-machine interface applications (e.g., Rockwell Software's RSView software)—and execute both the motion planner and the user's logic on schedule. The timing for the motion planner execution has high accuracy because it's generated via hardware interrupt from motion interface card(s).
A Windows NT or 2000 system is very responsive to the user's control application, due to the bus mastering capability in PCs manufactured within the past three to four years. The bus mastering capability of the external/internal PCI bus containing the servo card and the other PC devices—such as video, Ethernet, integrated drive electronics (IDE) disks, and small computer system interface—frees the Pentium CPU from processing data transfers to/from these devices.
Because PC-based motion controllers run at real-time priority, the only potential for conflict is at the device driver level (interrupt service routines and deferred procedure calls). There's no issue with device drivers that follow the Microsoft specification for Windows NT or 2000; these are referred to as "well-behaved" drivers. Most vendors submit their drivers to Microsoft's Windows Hardware Quality Lab (www.microsoft.com/hcl); upon certification, they're guaranteed to be compliant and therefore a nonissue.
With bus mastering enabled and well-behaved device drivers, systems can operate on a standard Windows NT or 2000 system with user program execution repeatability within a 1.5-msec range. This means the path planner running under Windows NT or 2000 closes the loop with the motion interface card every 2 msec. In this example, the screen would show which updates were missed and whether each update occurred on schedule at the 2-msec interval.
With a time-based-compensation algorithm embedded in the servo card, the system can ride through a delayed coarse update, if needed, although a properly operating PC-based system won't need to use this capability because the coarse updates are reliably performed on schedule at the coarse update frequency.
With soft motion control, scan times are also exceptionally fast. Both the motion planner and the user's logic run on the Pentium processor, taking advantage of the sheer clock speed and hardware accelerated on the Pentium's CPU. Depending on the computer's Pentium processor clock speed, typical program scan times can be less than 15 µsec and max scan times less than 300 µsec. Running other applications such as MS Office and databases doesn't affect the control application's performance.
Checklist for Soft Motion
For optimum performance, run the soft motion controller on a PC that has bus mastering capability enabled for its hard disks and other IDE devices, such as CD-ROM drives. You must enable bus mastering if you're running motion applications. Disable all graphical (especially OpenGL) screen savers when running motion applications. If you're running soft control on the factory floor, you may want to consider using a rugged industrial computer. Finally, soft motion vendors recommend Microsoft-certified drivers for video cards and Ethernet network interface cards, which can be found on Microsoft's Web site at www.microsoft.com/hcl.
These are the base platform requirements for the soft motion controller:
- Intel Pentium II 300 MHz or Celeron 300A (Pentium III 600 MHz recommended)
- Windows NT 4.0 Service Pack 5 (or greater) or Windows 2000 Service Pack 1 (or greater)
- Minimum of 128 MB of RAM
- 50 MB of free hard disk space
- 800- by 600-pixel video resolution
- Any Windows-compatible pointing device
While it's true some applications still demand the real-time execution of a proprietary system, many don't. Soft control is predictable within a window of 1.5 msec. If you're casting a critical eye on PC-based soft motion control, take a closer look at its performance, flexibility, and cost benefits. Eight or nine times out of ten, a system running on standard NT will give you the reliability you need. MC
Bob Hirschinger is a motion market manager with Rockwell Automation's Industrial Motion Control Group. He graduated from the University of Milwaukee in 1981 with a BSEE and in 1992 with an MBA. Bob has more than 18 years' experience in the motion control business.
Dave Heller, of Rockwell Automation's Automation Control & Information Group, is a marketing manager with Rockwell's Logix/NetLinx business. He graduated from the University of Toledo in 1968 with a BSEE and from the University of Michigan in 1988 with an MBA.
Stan Rosen is Allen-Bradley's SoftLogix5800 project engineering manager, Logix Platforms business, Rockwell Automation.
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