Graphical motion controller design

Nov 30, 2007 10:44 AM

Sometimes, off-the-shelf controllers aren't suitable for an application. Graphic development platforms simplify custom controller design.

Rahul Kulkarni
National Instruments Corp.
Product Manager
Austin, Tex.

Off-the-shelf motion controllers are a convenient and economical solution for most automation needs. But when applications demand more than the average offtheshelf (OTS) controller can provide — faster servo update rates, better adaptability to changeovers, intelligent fault handling, and advanced feedback techniques — it may be necessary to design a specialized controller. To do so, one must recognize the need for it, understand how a motion controller works, and then design it.

Why design a motion controller?

Until the sound barrier was broken, airplanes and their various components were designed for a top speed of 330 m/sec. Then along came new engine technology, requiring a redesign of almost every aerospace part.

In a typical system, software sends commands, such as position, to a controller. The controller converts these commands, such as voltage, into signals for the drive. Drives then generate current to move the motor. Here, a motor converts current into torque, causing mechanical parts to do work. Finally, feedback devices report motor position back to the motion controller.

Similarly, until piezo actuators were invented, motion PID control loops were designed to run at about 1 kHz. Today, however, piezo actuators are common in semiconductor and biomedical applications, requiring controllers with PID update rates approaching 50 kHz.

Speed isn't the only challenge facing today's motion systems. Most machines adapt to frequent changeovers, and accordingly, motion controllers should be able to adapt to different loads. Otherwise, designers would have to retune PID parameters each time, for instance, a machine handled a box of different weight. This too calls for new (custom) control functions such as adaptive algorithms that can handle highly non-linear dynamics.

Responsiveness is another area where OTS controllers fall short. Consider a laser guidance system where the actual trajectory must be modified within microseconds of issuing a new command. Or an automated coordinate-measuring machine that must activate an emergency shutdown sequence within microseconds of detecting a fault. In either case, the challenge is the same: transmitting a command from software to the actuator within microseconds, which ultimately requires a custom controller.

A motion controller consists of three main loops — supervisory control, trajectory generation, and a control loop. Together these loops convert high-level user commands into command signals that drives use to move actuators.

Interface issues also beg for customization. It's not uncommon, for example, to encounter feedback sources with non-standard interfaces. This may include serial peripheral lines for a custom image sensor and a proprietary serial interface to an encoder — neither of which are well served by OTS controllers. Customized motion controllers may also be better at coordinating multiple feedback sources, such as force and position, in the same control loop. And, OEMs building machines may prefer creating their own controller for a broader choice in vendors.

A spline engine function using a cubic spline algorithm interpolates between two positions from the trajectory generator. The result is a refined profile that the control loop uses.

The inner workings

The need for customization is now clear. Understanding how a motion controller works, as well as the components interacting with it, is the next critical step toward designing one. This requires a look at each component's job.

Application software sends commands such as target positions and motion control profiles to the controller.

The motion controller, the center of a typical motion system, oversees trajectory generation, control loops, and supervisory control. It converts high-level user commands (from software) into signals for drives. Additionally, it monitors the entire system for error conditions, faults, asynchronous events that can cause unplanned starts/stops, and speed and direction changes.