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The process-control robot - new tool for flexible manufacturing.

The concept of flexible manufacturing is coming of age in the United States and Canada. The application of principles discussed in the US many years ago--but never put into practice--was pioneered by Japan and European countries. Now North American is beginning to apply those principles.

In addition, the introduction of palletization is giving transfer lines a higher degree of flexibility. They are seeing a tremendous renewed popularity in a variety of industries. This trend to automated manufacturing has forced manufacturers of coordinate measuring machines (CMMs) to develop a new class of equipment.

One of the requirements of automated flexible manufacturing is the capability of providing direct feedback of gaging data for process control. It is the independent process verifier, the measuring machine, that will yield accurate gaging data to help control the process and guarantee a successful manufacturing system.

To perform adequately, however, the measuring machine must have five main characteristic. The first of these is the capacity for electronic linkage with the control computer for the transfer line or flexible manufacturing cell (FMC) or system (FMS).

A great deal has been written about networking by means of Ethernet, DEC-NET, General Motors MAP, and other systems. It is not a simple task to link computers and have them share information. The task calls for an overall system concept, including the proper computer, operating system, and software, to make networking a reality.

Certainly, if an automatic gage is inserted in a system, but that gage cannot communicate its process data and findings to a supervisory computer or cell controller, then the entire system will fail. Planning the gaging element in any automated system is critical to that system's success.

The second main requirement is the gage's flexibility. Simply put, the gage must be as flexible as the manufacturing system in which it resides.

Third, the gage also has to be fast enough to keep up with the output of the manufacturing system. In their traditional form, CMMs do not operate with sufficient speed to keep up with today's automated manufacturing systems.

Even as CMM manufacturers have been developing increasingly accurate and sophisticated software, the machine-tool, FMS, and transfer-line manufacturing people have been producing more efficient, faster, and more highly automated systems. Thus the speed of the gage is more important than ever.

The fourth requirement is accuracy. Research indicates that users will be automating production of their most complex parts, which normally have the tightest tolerances. So, to make meaningful measurements, and to provide accurate data on offsets, the gage must be highly accurate.

The fifth and most important requirement is the need for stability in a hostile operating environment. Since ambient temperature will be changing constantly, the machine must embody as much thermal stability as is technically feasible.

Enter the PCR

To meet these five requirements, CMM manufacturers have developed a new type of measuring machine called the process control robot (PCR). Though not multifunctional as specified in the accepted US definition of a robot, the PCR (Figure 1) nonetheless can be easily reprogrammed and retooled, and in general has the design, operation, and control characteristics of a robot.

Since most parts are machined with horizontal spindles, the PCR itself features a horizontal design. This lends itself well to a variety of automatic parts-loading arrangements.

A ssucessful approach is to construct gages employing the same technology found in machine tools. These have been working in hostile environments and making parts accurately for years. This technology must be capitalized upon to further the evolution of measuring machines for flexible manufacturing.

In the PCR pictured, the robot module is mounted on the rear of the machine. The base in this configuration includes a fourth-axis rotary table. A standard robot module can be attached to a many types of specialized bases.

The robot itself is very sturdy and "shop-hardened" for hostile environments. All components are protected from damage that may be caused by the environment or mechanical factors. A high degree of flexibility is offered through use of both a rotary table and a Renishaw PH-9 two-axis, motorized probe holder. In all, the machine has six axes for performing measurements.

Applications for the PCR range from stand-alone situations in which the unit is loaded by a pick-and-place or gantry robot, to transfer lines with dual opposing (or even staggered) measuring robots, to full FMSs, with automatic guided vehicles (AGVs) docking at the robot. In FMS applications, the base of the robot becomes a pallet-shuttle and rotary-table mechanism.

Ideally, the PCR should be provided with multitasking, interactive, geometric software. Not only must the software define discrete forms of geometry (this helps the programmer define how to measure parts), but must also offer user-friendly screens that aid in programming.

In addition, system software should be fully multitasking so all functions of the machine are performed simultaneously. These functions include number crunching, motion control, communication with external computers, and communication with other devices such as pick-and-place robots, bar-code readers, and automatic fixtures.

Finally, the software should be designed to add diagnostic intelligence for problem solving in untended, continuous operations. This is accomplished by automatic rechecking of apparently out-of-tolerance features.

For example, in taking three points to measure a bore diameter, the probe could encounter a burr that creates an apparent out-of-tolerance situation. Software with diagnostic intelligence would then have the probe take three different points in the bore to recheck the diameter. With this capability, the process is not constantly interrupted by meaningless information.

Modularity of design allows PCR modules to be installed at the exact locations in the process where gaging is most needed. Automatic multistation pallet-shuttle bases can be added to the PCR (Figure 2). This permits parts to be shuttled directly off a machining center or an AGV, measured, and automatically shuttled to the next AGV or station.

Modular design also makes it easy to add a robot to existing systems. For tall cylindrical parts, a trunnion base--used to rotate parts to a horizontal position--can be added to the PCR (Figure 3). Prismatic parts can still be mounted and measured in the traditional manner.

With the concept of robot modules, dual-arm PCR configurations are feasible (Figure 4). A dual-arm robot is essentially two separate modules mounted on one special base. Multitasking software makes it possible to drive both arms simultaneously with one computer and one part program. Alignments are shared back and forth between the arms, simplifying part measurement and reducing part cycle time.

Transfer-line applications will use dual-arm PCRs, primarily because of the high speed at which parts pass through the gaging station. This arrangement requires a specialized form of software that allows progressive feature measurement.

Because of time constraints, the first part past the gage will have a small percentage of its features measured. The next part through will have the next higher percentage of features measured. By the time X number of parts have passed through the gage, one complete part will have been measured.

The user therefore does not have to take a single part out of the middle of a transfer-line production run to determine whether the process is running as planned. Instead, he has accurate data spanning the entire process and run. This yields a better overall qualification of the process.

Automatic probe changing for measuring machines is now coming on the scene, and will evolve dramatically over the next few years. The probe changer (Figure 5) provides additional flexibility in that a variety of probe configurations can be stored, then used when needed in the process. As probe designs evolve, several types--vision, laser, X-ray, optical switching, nulling, and others--can be stored in the probe changer.

The probe changer also can be employed when a probe's repeatability is no longer acceptable to the process. Using a conditional branching statement to periodically check on the health of the probe, the PCR can pick up an already qualified, "fresh" touch-trigger probe and continue with the process.

Controls, accuracy, and sealing

The PCR is more than a machine; it is a highly integrated system of mechanical components, software, operating system, probes, and computers. Uniting this entire system is the robot controller.

Performance of the PCR is largely dictated by the speed and efficiency of its controller (Figure 6). The design of the controller, along with design and use of the computer's operating system and software, allows for expedient communication between dissimilar computers and controllers in an automated system.

The controller and computer must be sealed from their environment by NEMA 12 enclosures. Air cooling, heat exchanging, or air conditioning should be added according to the individual application.

Using a programmable controller adds an additional level of flexibility. This type of control (Figure 7) can be used to govern and monitor the cell's peripheral devices such as pick-and-place robots, shuttle mechanisms, peripheral rotary tables, bar-code readers, and automatic fixtures. This allows the PCR controller to be used to move the machine and manage the entire cell. The host computer can then serve strictly as a number cruncher.

It will not be uncommon to see PCRs that offer accuracies found in lab-grade bridge-style CMMs. The parts measured by these maachines will have labor-intensive, complex geometric shapes with tight tolerances.

The PCR operates in hostile environments where coolant and chips are common; this makes use of bellows covers mandatory (Figure 8). These covers are fully sealed, and for some applications--e.g., in foundries--they may have to be positively pressurized to keep out gray-iron dust or other fine particulates.

Drives and sizes

Because of the requirements for dramatic increases in the speed of measuring machines used in flexible manufacturing, drive systems traditionally found on CMMs are replaced in PCRs by positive drives such as the rack and pinion. This type of drive is both accurate and lightweight, allowing for repeatable and positive PCR motions.

Air-bearing technology, long the basis of accuracy and repeatability for bridge-style measuring machines, has yielded in PCRs to solid roller and ball bearings like those employed in machine tools. Use of these bearings precludes settling under extreme acceleration and deceleration, and speeds up the entire process of measurement. The resulting accuracy and repeatability are identical to those possible with conventional CMMs.

The PCRs pictured in this article weight up to 16,000 lb each. This contrasts markedly with the 3000- to 4000-lb weight of a bridge-type measuring machine. As with machine tools, mass relates to stability during operation.

The new controllers required for PCR applications must have velocities equal to at least 20 ips, with primary attention given to acceleration. Since most measurements are taken during acceleration, the PCR's high level of acceleration (up to 0.3 G) enables the machine to measure parts rapidly. In some situations, parts can be measured 8 to 10 times faster than is possible on the fastest of bridge-style CMMs.

Research has indicated that the typical size of the PCR should be about 0.5 cubic meter, since 80 percent of all parts manufactured in the world fit within this cube. The next major size category is 0.5 to 1.0 cubic meter.

PCRs are offered in sizes that accommodate these two major part-size categories. Since the PCR has a modular base that can be customized, the X axis can be as long as the application dictates.

Programming and communications

The "factory of the future" typically includes an FMS host computer, machine-tool controllers, machine tools, and a PCR. Each part number is sent down to the PCR as the part arrives at the measuring machine's base. The PCR measures the part, and may provide actual offset information to either the machine-tool controller or the FMS host computer. There is, however, one major element mission: A CAD/CAM system.

Since most PCRs will be integrated into automated, untended, in-line systems, there is not enough time to program parts on the PCR as is normally done on CMMs. Off-line part measurement and editing are therefore mandatory.

The easiest way to effect this, and at the same time take advantage of existing design data, is to program the PCR on a CAD/CAM system. Many major CAD/CAM manufacturers have worked directly with CMM manufacturers to develop measuring modules that reside within the CAD/CAM system. With these specialized postprocessors in place, CAD data files can be accessed and used by the PCR.

CMM and CAD/CAM manufacturers that have implemented or plan to implement this type of interface include GE's Calma, IBM's CADAM, Prime Computer, GM's CGS, Computervision, Applicon, and Catia. Many other companies likely will join the list.

Not only can design people use the CAD system to design parts, but manufacturing and quality-control people can use the same system and data to generate their parts programs. The user's design database can be written in APT, Fortran, or Pascal. Through use of either CAM programs or computer-aided inspection modules, programs can be created to drive both the measuring machines and the machining centers. The result is a complete closed-loop system.

All of the design data, programming, and information relating to the process are stored in the host computer. This means that the three key organizations (QC, manufacturing engineering, and design engineering) speak the same language using the same CAD/CAM system.

As Figure 9 shows, the FMS host is the supervisor of the entire system. The PCR sends process data to the host, or sends offset changes to the machine-tool controllers. Database models can be created on the PCR and sent to the host, and process data from the PCR can be formulated into the user's quality-information system.

An important aspect of this scheme is that if the PCR and its ability to communicate information to the FMS host are eliminated, all the communications lines to the major elements in the system are also eliminated. This creates "islands" within a manufacturing system that have no relationship to one another.

An in-line, independent process verifier--i.e., a PCR-is required to complete the system. Touch-trigger probes wielded by the machine-tool spindles may always be useful in these systems. At the same time, though, the machine tool's primary job is to make parts, not to take measurements. Further, employing a machine tool to monitor its own accuracy is a questionable practice.

One way or another, the majority of manufacturers in the world will come to agree that an in-line, independent process verifier is needed to help qualify the manufacturing process. It is the level of attention and planning given to the gaging element of an automated system that will determine its ultimate success.

Even though the gaging element of a system constitutes a small portion of the system's overall price, history tells us that, if there is no gage, or if the gage has been improperly specified, the entire system will have serious shortcomings.

For details from Brown & Sharpe on their process control robots and systems, circle 533.
COPYRIGHT 1985 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1985 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Genest, David H.
Publication:Tooling & Production
Date:Dec 1, 1985
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