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.