Off-the-Shelf PLC Optimizes Motion Control Functionality

Like most blow molding original equipment manufacturers (OEMs), Saline, MI-based R&B Plastics Machinery LLC (R&B) in has to coordinate different machine motions very closely. Its machines had been using closed-loop hydraulic motions with dedicated motion control cards, which are not only expensive, but somewhat difficult to coordinate.

06/01/2008


Like most blow molding original equipment manufacturers (OEMs), Saline, MI-based R&B Plastics Machinery LLC (R&B) in has to coordinate different machine motions very closely. Its machines had been using closed-loop hydraulic motions with dedicated motion control cards, which are not only expensive, but somewhat difficult to coordinate. Recently, the company successfully integrated its blow molding machines' motion control into off-the-shelf PLCs, increasing productivity, reducing machine cycle times, and cutting costs in the process.

R&B provides solutions for plastics machinery, molds, tooling, and trimming systems. It offers programmable controllers, touch screen operator interfaces, independent parison controls, inclined or horizontal machines, free standing platens with mold face compensation, in-machine trimming, single screw extruders, and trimmers and deflashers, as well as precision-built blow mold and trim tooling products.

The company also designs and builds custom equipment for various processing requirements. In addition, it offers machinery audits, machinery installation/removal, reconditioning, and metal cutting machinery services.

Motion control via PLC

One particular R&B blow molding machine has nine axes of servo-hydraulics—three for controlling parisons (hollow tubes or spheres of material that are the first step in the blow-molding process), and six for motion control. It has two shuttles, two molds, and two blow pins. Clamp motions open and close the molds, blow pins move up and down, and carriages that hold the clamps move underneath the parison head and then out to the blow station.

“R&B is using a standard Siemens Simatic S7 PLC to control these positioning motions,” says David Chin, a technician with NDC Technologies, a Siemens strategic partner involved in the controls upgrade at R&B. “We provided a set of function blocks that gave R&B the position control with a PLC. Other machines will control motion with a motion controller; it's sort of a dedicated controller for servo-controls and positioning. We were able to customize the design of the function block to achieve the same type of functionality and performance as a stand-alone type of controller.”

A PLC is not usually associated with high precision for servo-control, says Chin; that is why a dedicated motion controller is normally used. “But here, the motion control function that is handling the positioning of various components, which are all hydraulically actuated, is controlled by a standard Siemens S-7 PLC,” he says.

The PLC is also controlling 30-plus zones of heat control from the hopper to the extruders, and through to the parison head. Twelve of these zones can heat and cool production units, and 18 are for heating only. The heat control system is critical to maintaining a temperature tolerance of

The off-the-shelf PLC uses function blocks written in standard IEC-61131 languages to control all servo-hydraulic axes. It does not require a different motion control environment. Integrating motion control capabilities into the PLC makes faster performance and more efficient processing compared to a dedicated PLC and separate motion control hardware.

Integrated parison control

The first of the motions that R&B attempted to manage through the PLC was that of the parison. Normally, blow molding machines are controlled either by a machine controller with a separate and independent parison programming unit, along with a dedicated motion control card; or by PC-based automation using specialized proprietary code to handle the motion control requirements.

R&B, however, wanted to integrate parison programming in the main machine controller, which was possible since the PLCs are fast enough to enable this. Parison control data comes directly from a position transducer as an analog input. All of the processing is done as regular PLC code, then an analog output is used to control the hydraulic valve blowing the parison without any specialized hardware.

The company then integrated the rest of the servo hydraulic axes into the PLC. The position transducer data for these axes comes into the PLC via SSI modules. If required, the position data coming from the SSI modules can be synchronized to the PLC program scan using Siemens' Profibus Isochrone mode. Again, standard analog output modules are used to control the hydraulic valves.

Another function that R&B has integrated into the PLC is camming of the blow pins with the carriages. As the carriage brings the mold toward the blowpin position, the blowpin traverses downward following a programmed profile. Since the positions of the two axes are coordinated, this process can be accomplished with minimal margin for error and without fear of crashing.

Before integrating this function into the PLC, R&B was not able to do synchronized camming because the blow pins and carriages were not able to communicate. The protocol programmed into the PLC makes this functionality possible for the first time with blow molding.

Reduced dry-cycle time

In blow molding, the time it takes to actually blow or cool a bottle is similar on all machines. But there is a difference between machines from a cycle-time standpoint — especially the dry-cycle time (when the machine is not actually blowing a bottle). Cycle time is the total time between starting to blow a bottle and being ready to blow the next bottle.

The motion control functions within the PLCs provide a superior level of coordination of the dry-cycle motions, reducing time intervals between specific actions. They enable coordinating multiple motions using position rather than time. This makes it possible to take significant time out of the dry cycle, giving R&B a cycle-time advantage. As a result, dry-cycle time has been reduced by as much as 20%.

The advantages to using a proven, off-the-shelf PLC-based automation solution for blow molding instead of dedicated hardware for motion and parison control are significant. Since all servo-functions are controlled from one function block, this means better-coordinated and faster machine cycle times, simpler and faster programming, and less equipment. Also, dedicated hardware is manufactured in small production runs, creating quality control and long-term support issues, and the need to re-engineer to fix problems, all of which require additional financial justification.

Compared to PC-based controls, proven PLC-based automation solutions also provide a more attractive controls package for the blow molder, including a more rugged unit, use of solid state memory, no dependency on third-party operating systems, longer product life cycles, and better long-term support. One of the key benefits of PLC versus PC control is the ease with which PLCs retain data when they are shut off (such as during a power loss).

“We now have one controller that is controlling our PLC functions and motion control functions,” says Jake Losee with R&B Plastics. “We no longer have the communication delays that we had before using a stand-alone card. Also, we have recipe storage of all of our axes. Previously, we had separate software packages that would require a technician who understood a particular motion card to program it. Now, users can do it all through the human-machine interface (HMI).”

Adds Losee: “One of our goals was to eliminate equipment. Additional equipment means more cost, more maintenance and more software that requires more training for our maintenance personnel. The PLCs, with their integrated motion and parison controls, have delivered a highly optimized control solution.”



Author Information

Matthias Erhardt is a plastics industry consultant with Siemens Energy and Automation. Contact him at matthias.erhardt@siemens.com .




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