Steve Reese
Parker Hannifin Corp.
Automation Group

Almost all motion controllers on the market today can synchronize two or more axes. But too many times, this merely means that the controller has the ability to effectively parallel command signals to two or more drives, or follow a master encoder. While in the strictest sense this can be described as synchronous motion, often it is not enough command to coordinate multiple axes that must work together to produce usable products on machines.

To determine what level of control is needed for synchronous applications, let's look at two typical cases.

Gantry systems
On gantry systems, it's very common for two motors to work as a pair on parallel axes running the length of one Cartesian plane. In this case, it would seem logical to command the X and X motors, for example, from the same signal, but it's usually not the answer.

Merely sharing a command signal can cause real synchronization issues. Axis X may lead if X is highly loaded, because X will be unaware of the impedance to travel. This application-induced error is eliminated by controls that set a reference point.

Assume we have a dual-axis press and, as is often the case, one of the rails is loaded more heavily than the other. Under these circumstances, commanding both motors with a common signal will likely cause the less heavily loaded motor to lead the other.

One way to prevent this application-induced error is with a control approach that establishes a 0° angular difference across the (X and X) rails as a reference point. Maintaining the reference point keeps the two motors in line, holding the machine in synch even if the load shifts.

Implementing this type of "lock" control is no easy task. A lock command measures the offset between axes caused by the load, scales it, and then cascades it (with the drive command), producing a "locked-axis signal" for each motor amplifier. In addition, the system's integrity is actively managed by individually adjustable gains for each motor, preventing skew between the two synchronized axes. This attention to detail is required, however, and is often the difference between a working machine and one that functions but is incapable of producing repeatable results.

Computer-aided manufacturing
Another class of motion benefiting from full synchronization is the widely employed camming function. Cams have been used for years to establish complex timing relationships, especially in web processing applications. More frequently, however, teams of servomotors coordinated by cam-emulating algorithms do the same job faster and more reliably — and can be reprogrammed on the fly.

A good example of a discrete motion-centric manufacturing process that relies on electronic camming can be found in any modern plant that makes shotgun shells. A 12guage shotgun shell costs less than a quarter, but the liability associated with a faulty product could bankrupt many cities. To keep hunters happy and lawyers at bay, ammunition factories must produce highquality products in high quantity, while maintaining high repeatability despite plant temperature variations, humidity levels, time of day, or how long machines have been running.