Growing throughput demand in high-tech equipment — machines used in semiconductor manufacturing, electronic assembly, and medical automation in particular — is a source of continual challenge for today's motion controllers. Not only does it require more integration flexibility at the system level, but also higher resolution and repeatability in the feedback signals shared among components.

Conventional control

Appropriate hardware can extend the flexibility gained with peer-to-peer control networks. On the backplane version of this motion controller, all data and power signals are accessible through two connectors.

Motion-control systems are often differentiated on the basis of network architecture. The conventional approach uses a centralized structure. Here, single or multi-axis motion controls often connect to servo amplifiers and attendant servomotors through a 10-V analog velocity command signal. The advantage of this conventional configuration is the excellent availability of many interoperable motioncontrol components from a relatively large number of vendors. The major disadvantage is an inherent susceptibility to electrical noise at the analog interface. To avoid noise-induced problems, wire runs must be kept short, making distributed control impractical based on this arrangement.

Over the years, industry has evolved various network alternatives to analog interfaces. Of these, motion control systems that stream realtime point-velocity-time data (PVT) across the network seem to be the best approach. They achieve tighter, faster, and more accurate control using less network bandwidth than alternative systems that transmit velocity and torque set points instead.

Digital networks like these can be used to implement a decentralized control structure because they allow designers to mount servo amplifiers and other components in close proximity to driven devices. But the resulting networks are still plagued by the complexity of centralized control. The problem is especially evident in multi-axis systems because central controllers must commit considerable resources to monitor and direct each axis in addition to coordinating their moves, point by point, relative to one another as well as within the process in which they are employed.

Low-level foundation

Implementing a successful network, no matter what the architecture, depends on the complexity of the signal environment. In motion systems, a growing challenge can be traced to the increasing use of analog signals. Analog encoders, in particular, are becoming more common because of the universal demand for higher precision.

Analog encoders generate sinusoidal output signals that are only one volt (peak to peak) on both A and B channels. These small signals can be electronically interpolated, resolving position far more precisely than what's possible with conventional square-wave encoders that produce 5-V TTL logic signals.

Higher encoder resolution alone does not guarantee higher positioning accuracy. Current resolution also play a role.

In a typical servo system, a servo amplifier's output current determines torque. Shaft motion, a product of torque, is thus controlled by regulating motor current. In order to leverage higher encoder resolution, a servo system must be able to move the motor shaft in correspondingly fine increments. This requires higher current resolution than most systems can achieve.

Higher current resolution is, of course, possible, and it starts with hardware that provides more bits of analog-to-digital conversion. It also requires effective filtering to separate the actual current signal from background noise. If these conditions are met, it's not unusual to achieve positioning resolutions of a few nanometers from a 14-bit microcontroller.