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.