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.
Motion Controllers For Synchronization
In addition to closing the control loop for each axis, some motion controllers can tie the motion of one axis to that of another axis using special synchronization commands built into the motion controller. There are a couple of ways that this can be done.
One way in which two pairs of actuators are synchronized is by providing the same command at the same time to the motion controller, which updates an internal target generator per each pair of actuators to cause them to move together. In the pultrusion machine example above, the controller must also coordinate motion of the two pairs of actuators that move the clamps. The controller can use a short program (as few as 10 steps) that coordinates moving the two pairs of actuators with the opening and closing of the clamps, making sure that the speeds are matched prior to clamp closing.
It is also possible with synchronization to cause a slave axis (or axes) to move in lock step with an internal master or virtual axis. The slave axis follows that relationship precisely, varying the fluid flow from the slave valve as required. If a condition exists that would cause the slave to lag behind or race ahead of the master if it were an open-loop system, the closed-loop control algorithm will cause the valve that controls the slave to prevent that from occurring.
An example of a master/slave synchronization command is the Synch Move Relative command, which is supported by Delta Computer Systems controllers. This command initiates a ratioed synchronized move of all the axes in a particular “sync group,” such that all the axes start and stop moving simultaneously, and at any point during the move, each slave axis has completed the same percentage distance (or ratio) of its move relative to a target value, which could be the position of a master axis. Figure 3 shows a plot of the motion of a slave axis (yellow – axis 1) following that of a master axis.
Synchronized moves are also useful when several actuators are moving a rigid structure around a fulcrum. The axes do not need to start or stop at the same positions. For example, if three axes start at 0", and are to move to 2", 4", and 6", respectively, then at any point in the move, these axes’ positions will be controlled to maintain a 1:2:3 ratio.
Other types of servo controllers may have some limited synchronization capability, and could have been used as an alternative to the multi-axis motion controller in the pultrusion application, but they would not have provided as many closed-loop control options, including the ability to control the accelerations and decelerations of the pulling cylinders in order to make the motion as smooth as possible. The instruction repertoire of motion controllers allows the acceleration and deceleration of axes to be controlled with single instructions.
Closing the Loop
In order to keep track of the precise position of each of the pulling cylinders, each cylinder in the pultrusion machine was instrumented with linear magnetostrictive displacement transducers (LMDTs) that interface directly to the motion controller (see Figure 1). These devices provide absolute position information without requiring a homing step at startup, and because they work using magnetic interaction instead of physical contact with moving parts, they are also very long-lived components.
To control the cylinders, each is mounted with a proportional servo valve that is capable of an infinite number of settings, driven directly by the motion controller, so the cylinder position can be controlled with very high precision, even though very large amounts of force are being exerted.
As discussed here, synchronized motion can enable machines to work faster and more smoothly. The key is to use closed-loop control and select a controller that can coordinate multi-axis motion.
This article was written by Bruce Coons, Regional Applications Specialist at Delta Computer Systems Inc. in Battle Ground, WA. For more information, visit http://info.hotims.com/34451-331.