Press Roll Case Study

New Press Roll Machine Features Smooth Motion Via Advanced Pneumatic Control

Pacific Fluid Systems (PFS) of Portland, Oregon, has developed a new press roll controller that offers higher throughput and lower maintenance costs for sawmill operators. The controller uses anadaptive pneumatic control system that features an easy to program motion controller from Delta Computer Systems, Inc. of Vancouver, WA.

The job of a press roll is to hold a cut log (called a "cant" in forest products industry parlance) down against the conveyor chain, limiting the cant's horizontal and vertical movement as it is fed into the saw (see Fig. 1). The challenges in controlling a press roll include the need to carefully bring the roll into contact with the cant as it arrives at the roll's position and to insure that the roll follows the irregular surface of the cant as it passes under the roll. Considerable finesse is required in order to avoid banging the roll against the cant (which can damage the wood and over time break the machine), while maximizing throughput of lumber into the saw.

Most press rolls support only two roll positions during operation: open (full up) or closed (full down on the cant). The new PFS press roll machine supports three different roll positions: closed, open and an infinitely-variable midpoint. The system senses the cant as it moves under the rolls, and based on the width of the cant, it dynamically varies the force that is applied so as not to damage the cant. When the load has moved out from under the roll, the roll automatically returns to the midpoint (or hover) position.

The smooth operation of the system causes less wear and tear on the machine's pins, bushings, etc. Machine owners can eliminate periodic re-welding to repair shock and vibration damage, a maintenance step that can cost thousands of dollars per incident. The new system is automatically adjusted under program control, cutting maintenance time and improving reliability; Older two-position ("bang-bang") systems are often manually adjusted and prone to adjustment problems, which can also cause machine breakage.

Smooth motion is also key to the high throughput that PFS' system offers, and its Delta Computer Systems motion controller employs a combination of open-loop and closed-loop motion steps. To achieve fast accelerations and maintain control (at times such as when the roll is being raised to the hover position), the system uses an "open-loop move." At other times, such as when the roll is closely following the contour of the cant, a closed-loop control profile is used.

All control inputs are applied via a single proportional pneumatic valve, a feature unique to the PFS system. The choice of pneumatic versus hydraulic control was made because of the compliant nature of pneumatic force (air in a pneumatic system can be compressed whereas hydraulic oil can not). This choice provided added assurance that damage to the cant and wear and tear on the machine would be minimized.

In order to maximize throughput and accuracy under closed-loop control, the PFS machine designers found that the control system must be able to execute control algorithms that have a predictive element, a function called feedforward. Most controllers implement simple PID loops and aren't capable of feedforwad operation. Some controllers offer feedforward, but use the same feedforward parameters for both extending and retracting motions. This would not be optimal control for the press roll application because the roll behaves differently when it is being lifted against the pull of gravity than when it is extended with the pull of gravity. Lag times could result. Using its advanced pneumatic controls with different feedforwards for extending and retracting, the PFS system is capable of moving a 330 pound roller at approximately 40 inches per second and can position to within .015 inches of a target position.

To meet the requirements of adaptive control with multiple feedforward parameters, the PFS designers chose the RMC100 motion controller from Delta Computer Systems. Delta controllers are among the few motion controllers that support this function with the additional advantage of allowing the motion parameters to be changed "on the fly." This allows the use of extreme amounts of feedforward in certain portions of the motion where the mechanical advantage is lower. With the RMC, all the parameters can be changed every scan cycle (1 ms if doing force control, 2 ms if not). The PFS engineers tested four other motion controllers and found that they could not match this level of responsiveness, even when running simpler non-feedforward control algorithms (PID loops).

The PFS engineers also found the Delta controller to be easy to program and tune. Setting up motion steps is as easy as loading parameters into a spreadsheet, and tuning to optimize motion profiles is facilitated by graphical tools that are also provided by Delta. In addition, the Delta controller provides a direct analog interface to drive the pneumatic valve, simplifying system design (see system schematic - Fig. 2).

"As a tribute to the ease of using the RMC, we were 85% there with only 12 hours of testing," said Tom Wells, PFS engineering manager. "Delta's onscreen help software, specifically the graphing functions they support, are truly what makes the problem understandable and solvable."

Figure 1. Pneumatic Positioning System

Figure 2. Pneumatic Control System Schematic

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