High-speed, multi-tasking ability claimed for new 'transputer' controls.

The latest generation of controls introduced by Battenfeld GmbH in Germany last September is said to offer new possibilities in parallel control of simultaneous functions on injection and blow molding machines (see PT, Nov. '91, p. 11 for initial announcement). One key to the new Unilog TC-40 control system is that it is based on so-called "transputers," which are billed as high-performance microprocessor systems particularly suited for applications in which a high number of open- and closed-loop tasks must be processed simultaneously. The TC-40 will not replace Battenfeld's present Unilog 4000 or Unilog 9000 controls, but is being marketed as a high-end control for complex processing tasks. It will be shown for the first time in the U.S. next month at the Plastics Fair in Cleveland.

RISC ARCHITECTURE

Battenfeld claims that transputers are distinguished from typical multiprocessor control systems in two ways: First, transputers are based on RISC (reduced instruction set computer) architecture, which makes possible rapid processing and simultaneous handling of multiple tasks. Second, each transputer is equipped with four hard-wired serial interfaces or links, which reportedly have very high data-transmission rates of up to 20 million bits of data per second.

The communication channels made possible by these high-speed links, which are actually RS-485 connectors, are a key strength of the TC-40, according to Siegfried Kohler, Battenfeld's head of electronics development. Data transfer to and from I/O boards is still achieved via a system bus, as with multiprocessor systems. The advantage of the links is in the communication between the transputer chips. Links are point-to-point connections--i.e., they only affect two communicating partners. Therefore, messages can be exchanged between transputers without slowing down other processors in the system, as is unfortunately often the case in multiprocessor systems, in which all data exchange between microprocessors is via a common bus.

One important advantage of the transputer links is the possibility of a linear increase in data-processing capacity as more processor boards are added to the system. By contrast, says Kohler, typical multiprocessor control systems can only be increased to a certain limit before experiencing a drop-off in effective computing power, as the workload for communication management increases disproportionately.

TANGIBLE BENEFITS

Multi-tasking capability of transputers makes it possible to carry out several program sequences in parallel simultaneously, and to define separately the speed at which individual program tasks are carried out. Battenfeld claims that its new controllers were especially developed for closed-loop control systems, and that these controllers have a very high scanning frequency, which provides high repeatability of the molding process. According to Kohler, the theoretical scan rate of the controller is 0.5 millisec. However, this would demand one transputer for each process to be controlled. In order to keep the cost down, the closed-loop pressure controller is limited to 1-millisec and the velocity controller to 2-millisec scan time. In comparison, the scan rate of the Unilog 9000B, a high-end system, is 2.5 millisec for all controllers.

One claimed improvement in the area of closed-loop controls is the ability to use "dumb" hardware actuators with intelligent software controllers. Functioning software controllers are now available for speed, pressure, position, and temperature control.

For example, in hydraulics, higher accuracy and smoother movement can be achieved by replacing the internal electronics of the valve with the TC-40's software controller, according to Rudi Loehl, marketing manager of Battenfeld of America, West Warwick, R.I. In this configuration, an integrated power section and position controller is interfaced with the TC-40's software controller. The TC-40 can calculate the ramp for each movement, which results in less deviation between actual and set values.

Transputer-based control is also said to be useful in eliminating inconsistency in position of the check valve during injection, which affects the critical switch from injection to holding pressure and the repeatability of the shot size. One solution to this problem is "active locking" of the check valve. In this system, a pressure transducer is placed at the nozzle to measure peak pressure, and the injection stroke is timed from that value to determine the switchover from injection to holding pressure. Loehl claims that the rapid scan time of the TC-40 permits an accurate reading of the pressure.

In the near future, the TC-40 may tie new pmT (pressure-mass-temperature) software for optimizing the holding-pressure phase directly into its open-loop control. The advantage of the pmT process, which is under development at the German Plastics Processing Institute, or IKV, in Aachen, is that it does not depend on specific materials data, which is the case of its alternative, pvT (pressure-volume-temperature) control. The pmT process is now being tested on machines of up to 200 tons clamp force by Battenfeld Austria, and may be demonstrated at K'92 in Dusseldorf, Germany, in November.

The TC-40 includes a VGA-standard color LCD flat screen with a resolution of 640 x 480 pixels, said to be well suited for graphics display. The operator can input data either through a block of number keys or with a graphical cursor, which makes it possible to enter small setpoint changes quickly. There are separate logs for changes in setpoints and for alarm indications. The TC-40 has the ability to enter up to 30 injection-speed profile points and holding-pressure values, compared with only five steps for Battenfeld's basic Unilog 4000 control.

Cost of the Unilog TC-40 is about $16,000 more than Battenfeld's basic Unilog 4000 control. (CIRCLE 29)

A 'NATURAL EVOLUTION'?

Despite the capabilities of the TC-40, some controls experts see transputers as only one of several possible approaches to parallel processing that are becoming available as the price of hardware continues to drop. Louie Faillace, controls consultant at Davis-Standard Div. of Crompton & Knowles Corp., Pawcatuck, Conn., points out that the idea behind parallel processing, in which individual intelligent modules that handle assigned tasks communicate with each other independently, is not new.

Standard communication buses, (typical parallel interfaces), have limitations in distance, and elements in a control system usually have to be placed in proximity to avoid degradation of the signal. This is why multifunctional process controllers are arranged in a card or modular rack assembly. In contrast, high-speed serial interfaces--of which transputer links are one type--can transmit data at very high rates over long distances. An analogy can be made with modern telephone communication in which fiber optics, another form of high-speed serial link, is providing higher signal volume and improved quality. Still, these interfaces have been an available, if expensive, option for years, says Faillace, and their applications may be limited on a molding line. On the other hand, as the cost of computing power continues to decline, more powerful hardware options will naturally work their way into plastics applications, he adds.

A number of other plastics machinery suppliers say they have looked at transputer technology, and a few point out that there are other ways to achieve very fast scan times and high repeatability in critical machine functions.

For example, Cincinnati Milacron Plastics Machinery in Batavia, Ohio, uses an approach that allows the system to focus processing power where it is needed, according to Ronald Sparer, supervisor of controls development. The control performs supervisory functions at a 20-millisec scan rate, triggers critical transfer from filling the mold to packing the part at 2 millisec, and performs critical closed loops, such as injection pressure and velocity, at 0.5 millisec. The system uses dedicated digital signal-processing boards to take the signals from the machine and runs "firmware" (hardware-embedded software) that closes the loops and performs real-time tasks.

Similarly, Husky Injection Molding Systems in Bolton, Ontario, subdivides its control program so that the main program will be scanned at a 15-20 millisec rate, but critical machine functions are scanned at 3 millisec. Also, new technology to increase cpu speed, providing faster scan rates, is rapidly becoming available. For example, according to Vincent Butera, sales manager of the machine division, Husky is adopting a "2X module" that will double its cpu speed; and in 1993, Husky plans to increase its cpu clock speed from 6 to 32 MHz.

Ironically, some experts point out that extremely fast scan rates may be pushing controllers to a point of diminishing returns. Husky's Butera asks how much machine repeatability can be achieved before other factors, such as hardware component accuracy or material viscosity variations, begin to play a role. Battenfeld's Kohler states, "Scan times of under 1 millisec might not be required because the inertia of hydraulic components currently used in injection molding machines is too high to respond to such fast scan times. However, a scan time of 1 millisec vs. 2.5 millisec is still a significant improvement."

All of which is not to say that transputers may not find wider use in machine controls in the future. Other suppliers say they are evaluating transputers, and at least one machine-tool manufacturer, Maho Machine Tool Corp. of Naugatuck, Conn., is using transputers in a digitizer that traces models for machining into molds.