PC-based motion control has evolved quite a bit since the first IBM PC. Today, the combination of faster processors, open networking standards such as IEEE-1394 (FireWire), and object-oriented motion programming is again changing the motion-control landscape.
The biggest change is the move away from traditional motion-control boards to software-based control. Although the latter has been around for a while, the computing power wasn't available to take full advantage of software. Increased processing power of PC processors and DSPs in servodrives makes software-based motion control practical.
An example of this trend comes from Ormec, Rochester, N.Y. Soft Motion, a software-based control architecture, connects servodrives directly to a standard PC via FireWire, eliminating the need for a plug-in motion-control board. The new motion-control architecture distributes computing tasks between the PC main processor and DSPs in the servodrives. Trajectory generation moves to the PC and executes in a software motion engine. And the processing-intensive loop closure moves to the DSP-based servodrives.
Open PC-based motion control uses the all-digital IEEE-1394 FireWire network to tie servodrives directly to the PC. FireWire was originally designed for PC and multimedia applications, so chips are produced in high volumes at low cost. It's ideal for industrial control because it offers deterministic isochronous as well as asynchronous communication. And because it has an established standard, it lets third-party devices such as vision systems communicate on the same network as servodrives.
Developers say the real-time determinism gives FireWire the edge over another highly touted network for industrial control; Ethernet. Developers can add real-time features such as deterministic communication to Ethernet, but FireWire has this built into the standard protocol. And the cabling is simpler because there are no hubs as with Ethernet.
The ServoWire drive network, an application protocol that runs on FireWire, also handles diagnostics and error handling. It uses the IEEE-1394 memory-mapped model where all drive setup and motion-control parameters are defined as software variables. Data passes via DMA interface in the network adapter over the FireWire bus in real-time.
Servoloops operating in the drives receive trajectory information in real-time over the bus, implementing a digital position, velocity, or torque control network for up to 16 axes. Position loops update at up to a 4-kHz rate, velocity loops at 2.5 kHz, and torque loops at 10 kHz. Software-based motion control has many advantages. For instance, the Soft Motion architecture eliminates the ±10V analog drive interface. An all-digital network also eliminates noise problems inherent with analog transmission. Manual drive adjustments are no longer necessary because parameters are software configurable.
Another consequence of a softwarebased approach is that the hardware interface gets simpler. A major drawback with motion-control boards is the interface between the servodrives and the PC. A typical servosystem has 10 to 25 connections per axis which handle such signals as position feedback, alarms, and resets. These connections are individually wired to a breakout box, which connects back to the motion-control card in the PC via ribbon cable. The hardware interface can be costly and complex, exacerbating the process of tracking down and repairing faults.
Programming is another drawback associated with motion-control boards. Though the overall machine application is written in C or C ++, software developers have historically been forced to learn a proprietary motion-control language to implement motion-control functions. Because motion control is not programmed in a single language along with HMI and logic control, some applications force programmers to maintain two sets of code.
In the Soft Motion approach, motion, logic, and HMI control are programmed in a single standard language. The standard programming and development environment uses standard C or C ++, and Microsoft Visual Studio. It comes with the MotionObjects class library providing object-oriented motion control via software building blocks. This lets existing C programs be integrated with newer programs.
BURNING UP THE WIRE
FireWire was originally developed by Apple Computer in 1986. An IEEE working group formed in 1989 christened the new protocol IEEE-1394. Designed as a high-speed serial link, it languished for want of an application. With the advent of digital video camcorders, however, FireWire found a home.
The standard FireWire cable consists of six wires; two separately shielded twisted pair transmission lines and two wires that carry power (8 to 40 V, 1.5 A max). The standard calls out a 400 Mbit/sec maximum transmission rate over 4.5 m. Longer cable runs are possible using thicker cables or by lowering the bit rate.
A new version of FireWire is due out soon that pumps up transmission speeds to a minimum of 800 Mbits/sec, with extensions to 3.2 Gbits/sec. Category 5 cable or plastic optical fiber can deliver distances up to 100 m between FireWire-equipped devices.
FireWire will be standard on most new notebook computers by the end of the year. This makes connectors inexpensive and readily available, making the cost attractive to designers.