Though PLCs and dedicated motion controllers are the backbone of motion control systems, they are increasingly being asked to perform non-control (management) functions in addition to their basic control functions. But, personal computers (PCs) that have been industrially hardened to withstand factory floor conditions are taking on an expanding role in machine control applications. Industry experts are confident that this trend will continue, but they have different perceptions of the PC role — and how it will evolve in the future of machine control.
Major shift?
After watching a gradual shift to PC based machine control over the past several years, Jeff Hanna, president of Omega Software, Cincinnati, expects to see this shift pick up momentum over the next 3 years.
An analogy between office and factory automation sets the stage for what he predicts will be a paradigm shift to PCs in machine control. Before automation, an office relied on mechanical adding machines and typewriters, and later, calculators and electric typewriters. Then came automated hardware-software systems with centralized letter storage, database capabilities, and shared printers. However, offices had to buy the entire system from a single vendor.
Introduction of the IBM PC caused a dramatic shift. Its common disk operating system (DOS) enabled many vendors to develop tools such as spreadsheets and word processors for use with a PC. Competition among these vendors drove the performance /price ratio up and gave offices more choices in DOS-compatible hardware and software.
A similar shift is occurring in the factory, says Mr. Hanna. Relays and other mechanical devices have given way to electronic control systems that contain PLCs and other control hardware. With early systems, users again had to buy all components from one vendor, but newer systems are increasingly able to link components from different vendors.
Many industrially hardened PCs have an open operating system (usually DOS or DOS/Windows) that enables engineers to link control system components from different vendors. Thus, a numerical control system for a machine tool may consist of a PC, input/output devices (I/O), programmable logic controllers (PLCs) or motion control boards, and an operator interface, all from different vendors, linked by a backplane and tied to a main computer by an Ethernet network. Here, the PC normally performs management and monitoring type functions and the PLCs handle motion control. But, advanced software developments are giving PCs the ability to perform control functions as well.
PCs gain new capabilities
Historically, three factors limited the effectiveness of PCs in factory floor machine control applications — reliability, speed, and available functions. But, these limitations are being overcome.
Reliability. In its basic form, a PC can’t tolerate the average factory environment. So, it must be converted to a factory-hardened version, typically called an industrial computer, with protection from environmental hazards, such as moisture, temperature, and vibration, as well as protection from transient electrical power conditions.
Factory hardening is achieved largely through conservative design practices: using an oversize heat sink on a microprocessor chip to limit its temperature rise, beefing up the power supply, adding a surge suppression device, and perhaps replacing a PC bus expansion card with a VME bus card that has more durable connections. The components are housed in a vibration-resistant, moisture-proof enclosure. In some cases, a solid-state diskless drive may be used to avoid vibration problems.
Speed. PCs with DOS or DOS/Windows operating systems generally operate too slowly to perform most real-time motion control functions. For example, a control application may require a scan time of 20 ms, but the PC scan time may vary from 30 to 150 ms.
But, advanced software development tools, such as Gello, FloPro, Grafcet, and Controlware, can be used to get around this limitation, often allowing PCs to run control programs at speeds faster than those of PLC-based systems. These development tools (usually called compilers) let an engineer write machine control programs in a familiar language such as relay ladder logic or graphical. The compilers also let the engineer integrate control system components from different vendors to ensure that the required speed is obtained even when the system runs several programs simultaneously.
Functions. The software supplied with proprietary control systems may not offer certain functions required for sophisticated or high-speed applications. For example, a CNC control system may require precise, fast motion, with flexibility, in several axes. Or, a user may want to customize an operator interface and link it with another vendor’s I/O. Such cases require special software packages, which are written by the user, control vendor, or a third-party software vendor.
Data processing or real-time control?
Other experts see a more limited role for PCs in control applications. Because of its speed limitations, they believe the PC is better suited for management functions or collecting and processing data rather than controlling motion in real time.
For most real-time applications, the PC must be linked to a high-speed peripheral control device that performs motion control functions, processing data with cycle times of 1-2 ms or less. For example, a digital signal processor (DSP) can perform math calculations for servo loops 10-20 times faster than an 80486 PC of comparable frequency and higher cost. This permits high-performance, but costeffective, multiaxis control, according to Curt Wilson, project manager for Delta Tau, Northridge, Calif.
Chuck Denton, product manager at Xycom, Saline, Mich., describes the new breed of motion control systems as a marriage between PCs and PLCs that will lead to more innovative control systems. The PC and its software provide the “power tools” (spreadsheets, databases, and networking capabilities) that perform many functions related to monitoring and managing factory floor operations. This leaves the PLC free to precisely control machine operations in real time.
PCs in the factory
Industrial PCs have been applied in many control applications including electronic circuit board assembly, machining (CNC), painting, robotics, material handling, food processing, transfer line, centerless grinding, and packaging.
Two of the largest users are the electronics and auto industries, says Mr. Hanna. Most notably, General Motors has committed to convert two manufacturing plants to PC-based control systems. One plant conversion has already been completed and the second is expected to be done by 1995. Ford and Chrysler are investigating similar conversions.
Giddings & Lewis, Fond du Lac, Wis., uses PCs extensively in their programmable industrial computer (PiC) and CNC control systems. For example, a touch control terminal is networked to PiC controls distributed throughout a machine. This 80386-based PC terminal provides operator interface and statistical process control (SPC) functions, plus maintenance and debugging of ladder diagram and function block (motion control) programs.
A typical CNC control system uses a PC as an intelligent operator station, says Bob Elliott, sales manager for Giddings & Lewis. The PC runs with third party software, such as SmartCAM, to develop part programs. And the operator uses a word processor on the shop floor to enter shift notes.
In some cases, several machine controls are tied together in a cell controller. The PC monitors machine operation, collects SPC data, part dimensions, and tool usage data. Then it processes this data and displays it to the operator or develops production status reports for management. A PLC, intelligent I/O, or motion controller linked to the PC performs machine control.
In one packaging application, R.A. Jones & Co. Inc., Cincinnati, needed a control system for adjustable pouch machines that produce 400 packages of drink mix per minute. This high production rate requires accurate control of pouch fill weights, fast execution of motion, and rapid scanning to monitor machine functions.
The company chose a PiC900 industrial controller because it integrates all aspects of machine automation through a single programming language: logic, motion, process, operator interface, data manipulations, and communications. This controller combines simplified ladder logic and function block programming for motion control. It also has touch-screen controls with animated diagnostic message guides for troubleshooting. And, it operates at up to 100 scans/sec.
In a machining application, Hydromat Inc., St. Louis, merged precision machine transfer and CNC machining functions to reduce changeover and set-up time on rotary transfer machining equipment. This enabled the company to add improved short-production run capability to machines normally used for high-production runs.
Here, a 486-based PC is used to develop CNC motion control programs for both point-to-point positioning and machining control. The PC transfers these programs (via a GE Fanuc general machine controller) to multiaxis servo controllers (12 to 16 stations with 1 or 2 axis of motion per station) over a VME bus. The servo controllers use precision linear scales for position feedback to ensure consistent accuracy and repeatability.
Study predicts bright future for PCs in machine controlAccording to a market research study, 71% of control professionals believe that industrial computers will be the core of all machine controls 10 years from now. Described in an Oct. 1993 report, “The Future of Industrial Machine Control in Factory Automation,” this study was conducted by Tom Bullock of Industrial Controls Consulting Inc., Fond du Lac, Wis. It is based on 91 replies from control vendor and user companies. The biggest reasons for this trend were cited as: • The IBM PC de facto standard in the personal computer industry. At $100 billion per year, the PC market is almost 100 times the size of the industrial machine control market and its sheer volume is bringing down hardware costs. |