Pneumatic solutions in machine control design

For machine designers, the introduction of servo-pneumatic systems has meant new applications for pneumatics that were, not too long ago, reserved strictly for electric servos driving optimized mechanisms.

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Superior Air PowerBy Loren Shaum

 

LONG BEFORE the current world of sophisticated mechatronic solutions for machine automation emerged, many industrial OEMS embraced pneumatic solutions. In certain machine segments, designers had few cost-effective mechatronic solutions, and were more comfortable with the simplicity of pneumatic valves, cylinders, actuators, motors, etc., and the plumbing associated with their operation. They incorporated them into every design they could.

 

But as machine performance requirements evolved, builders had to adopt alternative solutions to be competitive. For more sophisticated applications, the move to mechatronic solutions became necessary. Today, for more complex and flexible machines, mechatronics dominate, especially where quick changeover to another part/assembly production, higher precision, and more speed are essential.

However, even on these more precise, higher-speed machines, many pneumatic components are incorporated into the design—creating a hybrid solution of sorts. For automating machines with fixed or limited variability in output or product, pneumatic components often dominate.

Large segments of manufacturing operations today have a huge installed base of pneumatically operated equipment. According to several companies that sell mechatronic and fluid-power solutions, the legacy installed base is so large that pneumatic component sales for maintenance purposes far exceed mechatronic equipment sales in many parts of the country. About the only segment of pneumatic control that’s become virtually extinct is pneumatic logic control. Even so, pneumatic logic still is used in some explosion-proof applications.

An argument against hybrid machine solutions might come from designers who prefer centralizing their pneumatic power source (pumps and compressors). Non-fluid-power solutions most often require power sources integrated at or nearby the point of machine action. However, with the advent of multiple interfacing capabilities integrated into pneumatic components and equipment, such as the various fieldbus interfaces, the ability to centralize controls for pneumatic equipment becomes significantly easier.

Advances in Pneumatic Control
The introduction of servo-pneumatic systems has meant new applications for pneumatics that were, not too long ago, reserved strictly for electric servos driving optimized mechanisms. One noteworthy application is controlling press rolls on a lumber mill machine (See Figure 1 below), in this case, built by Pacific Fluid Systems, Portland, Ore.

The controller positions the press rolls that hold logs, cants, or pre-cuts as they transfer through the saw in a mill. Controlling the press rolls means holding the logs tightly without damaging them, based on a previously defined profile.

 

     FIGURE 1: HYBRID PNEUMATIC SOLUTION
  Hybrid Pneumatic Solution
 

The sensor-controller-actuator combination positions press rolls that hold logs, cants, or pre-cuts as they transfer through a lumber mill saw. The control of the press rolls has to hold the logs tightly without damaging them, based on a previously defined profile.


 

Figure 2 below diagrams a press-roll station configuration designed and built by Pacific. These configurations typically are used on planers, edgers, and band mills. Lineal wood-feed speeds can be as fast as 2,000 board-ft/min. The press rolls have to sustain a force of 1,200-1,800 lbs, depending on log type and speed, to keep the logs from moving as they pass though the saw. Cylinders are usually 6-in. bore.

“Pre-programming desired cylinder position and forces are critical to this application,” says Tom Wells, Pacific’s engineering manager. “It allows us to operate in a higher-bandwidth, proportional-control mode to get the cylinder to the position where it can decelerate to the final position. Once there, the controller moves to a controlled stop, then switches to a force mode to sustain the proper pressure. This reduces our setup to hours, rather than weeks.”

A typical application is a machine called an edger. For this machine, there is one station of dual press rolls at the input to the saw and another at the output. This application demonstrates the advantage of auto-tuning of the active damping algorithms.

 

FIGURE 2: INSIDE THE PRESS ROLL CONTROL LOOP     
Press Roll Control Loop  
Pre-programming the cylinder position and forces are critical to the press roll application, operating in a higher-bandwidth, proportional control mode to get the cylinder to where it can decelerate to the final position. Then the controller moves to a controlled stop, and switches to a force mode to sustain pressure.

 

“With these advances, it’s common to reduce the time required to tune a system by an order of magnitude,” claims Steve Nylund, CEO of Delta Computer Systems, Vancouver, Wash., a provider of electro-pneumatic motion control solutions. “For the machine builder and customer, reduced time to machine commissioning is highly important. The use of various standard communication interfaces that allow the machine control designer to implement a best-in-class solution also is a big plus.”

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