Synchronizing Axes

In motion control, the term synchronization is a very general term. In the most general sense, the RMC does everything synchronously, that is, it does things at specific times and responds to events within a certain amount of time. This topic describes specific types of synchronization of axes and explains how to achieve them in the RMC.

Identical Motion (Synchronized Identical Positions)

In this type of synchronization, the motion of the axes are identical. For example, two cylinders on a press always move to the same positions at the same time, and must always be at the same position during the moves.

To achieve this type of synchronization, issue identical commands (such as the Move Absolute (20) command) to each axis simultaneously. To issue the commands simultaneously, create a user program and put the commands in the same step. Or, from a PLC issue identical commands to all axes in the same write.

Keeping the axes in sync:

To keep the axes from getting out of sync, do the following:

Put all Axes in the Same Halt Group
Put all the axes in the same Halt Group. If one axis in the halt group halts, then all axes in the group will halt.
Set the Position Error Tolerance
Set the Position Error Tolerance for each axis. This will cause the Following Error bit to turn on when the Actual Position comes too far away from the Target Position. When the error bit turns on, its Auto Stop setting determines what type of halt occurs.
Make sure the Following Error Auto Stop is set identically for each axis. Also make sure the Open Loop Halt Ramp (or Closed Loop Halt Deceleration, depending on your halt type) parameter is identical for each axis.

With these parameters set correctly, when you simultaneously issue identical motion commands to each axis, the Target Position will be identical for each axis at all times. If the Position Error on one axis becomes too large, the following error bit will turn on, and all the axes will halt at the same time. Notice that the maximum allowable skew is twice the Position Error Tolerance.

Ratioed Motion (Synchronized Unequal Travel Distances)

In this type of synchronization, the motion of the axes is synchronized such that all the axes start and stop moving simultaneously, and at any point during the move, each axis has completed the same percentage distance (or ratio) of its move. For example, if three axes are at 0 inches, and are to move to 2, 4, and 6 inches, respectively, then at any point in the move, the axes' positions will be at a 1:2:3 ratio.

To achieve ratioed motion, use the Sync move commands:

Gearing

In this case, the motion of one axis must move at a rate proportional to the position or speed of another axis or reference input. See the Gearing topic for more details.

Synchronizing RMCs

An RMC can synchronize all its axes. To synchronize more axes, multiple RMCs are needed. There are several ways to synchronize axes across RMCs.

Synchronize all axes to a single axis
This applies to applications such as gearing multiple axes to a single feed chain. A transducer provides the master positions, to which all the other axes are synchronized to. A quadrature encoder signal can be daisy-chained to multiple RMC quadrature inputs. Or, the Universal I/O Module can be used to daisy-chain SSI position data to multiple RMCs. An analog position signal can also be connected to multiple RMCs.
Start identical motion simultaneously
If the axes need only do identical motion, then the motion can be started simultaneously using a discrete output to trigger each RMC.
Via communications
If the axes need only do identical motion, and do not require very tight synchronization, then the motion can be started simultaneously via the communication channel. The communication channels do exhibit a certain amount of latency and jitter.
UI/O Inter-Module Communciation
The Universal I/O Module can exchange data between RMC150s each loop time. Therefore, this is suitable for tight synchronization of motion between controllers, such as gearing. See the Universal I/O Module for details.