Hydraulics Fill Molds Faster

ControlDesign.com

Keywords: Hydraulics, "drives and motion", "Jim Montague", "electro-hydraulic controls", "Jack May", "Iowa Fluids", VUP, "motion plot", "motion system", vibrations and "motion controllers"

Pipe Machine Uses Electro-Hydraulic Controls to Lay Down Thinner, Better-Quality Concrete Layers More Quickly

By Jim Montague, executive editor

It’s all about the layers. Whether you’re making dipped candles, Samurai swords, or concrete pipes, properly applied layers make pastries delicate and folded steel and molded concrete stronger.

Hawkeye Group and its Hawkeye Concrete Products division have been making vibration-under-pressure (VUP) machines in Mediapolis, Iowa, for more than 50 years. Its users can mold up to 12-ft diameter concrete pipe and box sections for sewers and culverts. Consisting of a central hopper, conveyor, and chute feeder, VUPs fill a form and then vibrate it. This liquefies the concrete and allows it to fill, compact, and precisely match every crevice of a form (Figure 1). This is similar to the liquification that can occur during an earthquake, when the ground vibrates, allowing it to sometimes flow like a liquid.  

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Shake It Up, Baby

Hawkeye’s VUP machines fill forms with concrete, vibrate them to compact the material, and turn out up to 12-ft diameter pipe and box sections. Photograph courtesy of Hawkeye Group

Depending on design and specification variances, Hawkeye’s VUP takes about 20 minutes to fill a form. These forms can be as wide as 12 ft sections or as small as 4x6 ft box culverts. Once compacted, the resulting concrete sections can stand unsupported, and usually cure overnight. One VUP machine typically makes 15-20 pieces per day.  

VUPs initially employed manual hydraulic levers and valves, which required operators to manually follow outlines and fill the shapes being molded. Then, about 15 years ago, Hawkeye updated its molding machines with PLCs and other automation components, which allowed users to hit a button, and let the machine track assigned shapes on its own. These previous VUPs included GE Fanuc’s 90-30 PLCs and Hawkeye’s proprietary software algorithms to run AC vector-drive motors, typically a 3-hp motor for radical functions such as main feeder pivoting and a 1-hp motor for the chute. Relatively low-horsepower motors can be used on the feeder and chute because they perform spinning functions that don’t need to overcome too much gravity, though they do need to cope with high levels of inertia.

PLC Speed Limits

Despite the benefits PLCs and electric motor drives provided, there still were some persistent drawbacks. While the PLC computed coordinate points for the motion, and drives moved the hopper and chute pivot axes, this system sometimes couldn’t handle the VUPs often heavy and varying concrete loads. The feeder mechanism alone can weigh up to 10 tons when full. When burdened with especially heavy loads, the electric motor drives would trip and shut down due to overloading. Though these stoppages happened only once or twice per day on average and it only took a couple of minutes to reset the machine, they could be problematic for users trying to produce products quickly. The system also couldn’t achieve some desired motions while staying within the motor power curve.

In addition, while the former PLC’s approximately 30-msec cycle time was relatively fast, it wasn’t quick enough to track points around the form in time to make concrete layers thinner than the usual 6 in. This was a drag on Hawkeye’s goal of reducing machine processing time without increasing its costs.

Hopper and Chute

The VUP machine’s hopper and chute must handle widely varying weights of concrete as they move hydraulically about pivot points to fill the molds. Photograph courtesy of Hawkeye Group

Reconsidering Hydraulics

During the VUP’s most recent revamp, Hawkeye was approached by long-time collaborator Jack May ofIowa Fluid Power, who proposed replacing the machine’s electric drives with a two-axis, electro-hydraulic motion controller with Profibus communication capability from Delta Computer Systems. Because the motion controller can calculate complex motion paths using its internal software, Hawkeye was able to switch VUP from its former high-performance PLC to a lower-cost PLC with more moderate, but adequate, performance. The motor on VUP’s conveyor already uses hydraulics. Delta was chosen because it already aids hydraulics on Hawkeye’s other machines. 

To make the chute’s end follow the mold’s shape, RMC70 directs the two rotational axes to follow profiles defined by user-developed mathematical formulas. Besides shaped boxes and round pipes, elliptical and arch-shaped pipes can be formed with the appropriate motion profile programs. The system incorporates an operator interface that provides key X-Y coordinates to a PLC, which passes them to the motion controller. The PLC also handles operation of the transfer belt and other control functions. The motion controller calculates additional points to complete the shape being poured, and then calculates the main pivot angle and chute angle needed to hit each point. Then the motion controller moves the axes to trace the path, and the hopper and chute move hydraulically about the pivot points to fill the molds (Figure 2).