Hydraulics Make the Cut

New hydraulic motion control system increases sawmill productivity

Machine builders often have a choice of motive power sources for their applications, including electric, pneumatic, or hydraulic.

In fact, many machine builders find the optimum design results from some combination of these power sources.

The challenge of selecting and controlling the right power source was recently faced by lumber mill operator Mueller Brothers Timber, Old Monroe, Mo., in the design of a new debarking machine. Mueller Brothers engineers surveyed available debarking equipment from a range of forest product machinery suppliers and decided, when it came right down to it, they could build a better machine themselves.

The Mueller Brothers debarker in operation, with hydraulic motion axes controlling the grinding head, the bull wheels that rotate the log in the cradle, and the traverse carriage that moves the grinding wheel lengthwise along the log.

The company signed up C&L Steel Fabricators of Union City, Ind., to fabricate the hardware, but decided to develop the motion control system themselves, with help from hydraulics distributor JH Foster, St. Louis, and electronic control system designer Paxton Electric of Elk Grove, Calif.

Precise Motion Control Presents Challenges
The Mueller Brothers sawmill produces high-end, furniture-grade hardwood lumber. Profitability can be dramatically impacted by maximizing the yield of the valuable timber inputs. A debarker grinds the bark off a log with a rotating grinding head. The key to maximizing yield is minimizing the amount of usable wood fiber that is removed from a log along with the bark.

The basic motion elements of a debarking system consist of an electric motor that turns the grinding head, which in turn is moved into contact with the log by a motion actuator as the log is moved and turned beneath it. As the log rotates at approximately 2 ft. per second, the grinding head must react to the shape of the log and maintain controlled pressure on the log.

If the motion of the grinding head is too "stiff," the teeth on the head can dig into the wood at undesired locations. If the motion is too "soft," the head can bounce and cause an irregular log surface.

Historically, positioning of the grinding head above the log was done pneumatically-,the head was lowered by a pneumatic actuator until it came into contact with the log and was left to rest on the log under its own weight as the bark was ground off. Since the grinding head tended to bounce off or dig into the log, good wood fiber was typically lost in the process of removing the bark. Mueller Brothers machine designers needed a better solution.

Hydraulics Is the Answer
Investigation revealed hydraulic power was a better choice than pneumatic or electric. Hydraulics offered the strength and flexibility of fluid power with the ability to exercise very precise motion control.

As mentioned in the sidebar, "Electric vs. Pneumatics vs. Hydraulics," hydraulics has the advantage of being able to provide high torque at very low operating speeds. The designers rejected electric drives because they tend to generate excess heat when run slowly. Designing a cooler electric motor-based solution would have been much more expensive.

The catch was that the engineers at the mill did not have experience designing precision hydraulic control systems. An initial attempt used a PLC to control the hydraulics directly, but the engineers found the PLC wasn't able to respond quickly enough to pressure feedback from the grinding head on the log.

In order to achieve precise control of the grinding head, the machine needed a high-performance motion controller that could interface directly to a servo valve to exert automatic, proportional control of the hydraulics. Continuos automatic control was needed to avoiding the imprecise "bang-bang" operation of manual on/off hydraulic valve control.

The motion controller also needed to interface directly to pressure and position transducers. Pressure transducers are placed in the cylinder on either side of the piston. These allow the motion controller to interpret the difference between the pressures as force is applied on the grinding head by the actuator.

Since each log has a different diameter, the holding pressure needs to change from one log to another and according to varying diameters down the length of a single log.

The diameter is determined dynamically using position information from a magnetostrictive displacement transducer (MDT). An MDT was selected because it needs no calibration and has a long operating life as compared to systems that use resistive sensors or limit switches.

Because hydraulics was chosen for the grinder, it also made sense to use hydraulics to power the two other motion axes in the machine: the bull wheels that rotate the log in the cradle, and the traverse carriage that moves the grinding head lengthwise along the log.

There are two sets of six bull wheels that rotate the log under the grinding head. Each set of wheels is powered by a hydraulic motor. Hydraulics is an ideal power source here because the bull wheels rotate at a relatively low speed and need high torque.

Electric DC drives or geared AC motors also would have worked, but would have added costs to the system because a hydraulic power source was already required to power the grinding head positioning arm. An AC motor could have been used for the traverse, but the availability of a power source also made hydraulics the lowest-cost solution for this axis.

Special-Purpose Controller Needed
The designers chose to separate the grinding-head control from the rest of the machine control operations. This allowed the engineers to use the PLC to support control functions that respond to direct operator inputs, and to employ the dedicated hydraulic motion controller to position the grinding head.

The traverse carriage and bull wheels are controlled digitally by a machine operator. The operator issues commands via a human machine interface (HMI) connected to a PLC. The PLC interfaces via analog output cards to drive the proportional hydraulic valves.

To control the hydraulic motion, an electronic motion controller from Delta Computer Systems (www.deltamotion.com) was selected. The advanced control capabilities built into the controller allow the system to transition smoothly from position to pressure control as the grinding head comes into contact with the log.

Coarse positioning of the grinding head is done using position control. The motion controller is given a target position that is calculated to just bring the grinding head into contact with the log.

As the head touches the log, the motion controller detects an increase in hydraulic pressure and switches control algorithms to maintain a precise target force on the log while the bark is ground off. Maintaining a constant force ensures that the grinding head doesn't bounce off the log or cut too deeply.

The controller is programmed by writing high-level motion commands in a function table inside the controller using Delta's RMCWin programming software. Up to 255 commands can be loaded into the controller in a single operation. After these commands are loaded, an entire motion sequence can be specified by the PLC by selecting desired commands via a serial interface. The PLC is then free to handle other control functions while the motion controller manages the motion.

The software package provides a motion-graphing tool to help tune the system. This was particularly useful for setting the optimal pressure set points for positioning the grinding head. Without this tool, mapping the pressure would have been a process of trial and error.

Using the motion-graphing tool, the actual motion profile parameters and corresponding target values are displayed versus time. This makes it easy to see where even small positioning errors occur, and this greatly facilitates the optimization process.

Electronic Control Adds Benefits
Delta Computer Systems engineers provided support during the design of the control system and also made recommendations that provided the machine with new capabilities beyond the scope of Mueller Brothers' initial machine design goals.

Many debarking machines have trouble dealing with log knots. Knots contribute to irregular log surfaces that can cause the log to bounce and be cut unevenly when it comes into contact with the grinding head. A special operating mode in the controller, called the pressure-limiting mode, provided a solution.

In the pressure-limiting mode, two PID control loops run simultaneously. One loop controls the position of the actuator and the other loop controls hydraulic force on the log. The motion controller simultaneously reads inputs from the position sensor and the pressure transducers and attempts to hold position, but backs off the grinding head when the pressure reaches a limit.

The advantage is that the grinding head can progressively back off from the log when it hits a knot. Without the ability to relax the position of the grinding head, the head might bind on the knot and bounce the log around as it attempts to cut off the knot. With the pressure-limiting feature enabled, the knot is carefully ground down after multiple turns of the log, resulting in a smooth log surface.

Hydraulics Increases Yield and Lowers Costs
In addition to grinding knots smooth, the hydraulic controller enabled the debarker to remove flares (wider irregularly shaped bottom ends) from logs prior to passing them on to other saws in the mill. This makes the downstream task of sawing the logs into boards much easier because saws don't need to deal with the irregularities.

The result is improved machine throughput and increased yields of board lumber because subsequent sawing stages no longer make complex decisions on how to cut irregular logs. Mueller Brothers estimates the productivity improvement at 5% for this capability alone.

The mill operator estimates that the increase in log yields due to the new hydraulics system saves hundreds of thousands of dollars yearly, enough to pay for the cost of upgrading the control system in a matter of months.

Mueller Brothers expected to pay for the machine by decreased fiber loss alone, but found they could further increase mill production by grinding flare irregularities with the debarker. The electronic-hydraulic motion controller also provides smooth hydraulic operation, which limits hydraulic shock and contributes to longer machine life and lower life-cycle maintenance costs.

Rick Paxton is an engineer with Paxton Electric, Elk Grove, Calif.

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