Many hydraulically operated machines perform adequately with on/off “bang-bang” valves, but some need special controls to avoid maintenance problems and deliver quality production output. This is particularly true when multiple hydraulic axes need to be synchronized. In these cases, designers should use an electro-hydraulic motion controller with multi-axis synchronization capability.
When hydraulic cylinders need to push or pull together, big problems can happen if their operation isn’t coordinated using closed-loop control. Even when it looks to the observer like the cylinders are moving together, minute differences in their rate of travel — due to environmental factors, subtle differences in the controlling components, vibrations, or inconsistencies in how the work material responds to each actuator — can cause jamming or racking of the machine’s frame structure and headaches for operators and the maintenance team. Closed-loop control of each cylinder can ensure that the factors and effects that would cause the cylinders to get out of synch are prevented.
Consider pultrusion applications, where hydraulic cylinders are used to pull a stream of carbon or glass fibers impregnated with epoxy through a heated die that shapes and hardens them into a continuous structure, which is then cut to length. Pultrusion machines that produce components of high-volume commodity products such as ladders or tool handles typically use only one hydraulic cylinder to pull the fibers. Single-cylinder pultrusion machines, by the nature of their design, must pull the part along an axis that is off-center with respect to the part. The larger the parts being pulled, the larger the alignment problems with the single-cylinder method, and this method is not satisfactory for machines that need to control the shape, strength, and composition of the parts precisely, such as those required for aerospace and military applications. In order to control part composition very precisely, the speed of the pultrusion process must be tightly controlled, with the fiber structure perfectly smooth and in-line. For these applications, two cylinders working together, but controlled independently, are used.
One might ask why the multiple cylinders can’t be operated by the same valve in order to move with synchrony. The basic answer is that all cylinders are different and exhibit different mechanical characteristics. They don’t respond precisely the same way to pressure changes coming from the valve. In order to ensure that they move precisely the same way, they need to be controlled separately, with each one’s motion being sensed independently with the appropriate position, velocity, or force feedback being used by the controller’s control loop algorithm for that cylinder.
A Typical Application
Figure 1 shows the arrangement of cylinders in a pultrusion machine (Figure 2) that was developed for KaZaK Composites of Woburn, MA for its Hudson, NH plant by James L. Gallagher, Inc., a composites engineering firm in Mattapoisett, MA. The machine was designed to pull a square-shaped part with dimensions approximately 18" on a side, but is flexible enough to handle manufactured parts up to 36 × 36". Because of the large size of the pultrusion being manufactured, the press needed to be able to exert 100,000 pounds of force by the main pulling cylinders. The machine actually uses two pairs of cylinders, each pair operating a separate gripper device.
The sequence of operation is as follows:
- Grippers 1 and 2 move all the way to the left.
- Gripper 1 clamps the pultrusion part and moves to the right.
- Before gripper 1 reaches the end of its travel (i.e., before cylinders 1 and 2 are fully retracted), gripper 2 clamps the part and begins moving to the right.
- Gripper 1 unclamps and moves rapidly to the left as cylinders 1 and 2 extend.
- Gripper 2 reaches the end of its stroke (cylinders 3 and 4 fully retracted) and gripper 2 unclamps, at which time gripper 1 re-clamps the part and begins pulling.
The solution for controlling all four pulling cylinders in the KaZaK system is to control each hydraulic cylinder independently, while making sure that the resulting motion is synchronized. This requires a multi-axis motion controller.