RG Engineering, Virginia Beach, VA, makes 10-color, high-speed printing machines. The company prides itself on both speed and a low rate of registration error—a mishap that puts some of the color outside the boundaries of an image. (The effect is similar to looking at a 3D picture without putting on 3D glasses.) RG had always favored analog drives as having faster response times than digital drives can provide. But when the company wanted to interface their drives with high-precision position sensors, it was time for a switch to digital. The company called in Danaher Motion.
The problem with digital ampifiers is that they sample the feedback loop at intervals, causing a system delay that limits loop gains. Analog amps run the loop continuously. The solution lay in adding an "observer," a software algorithm that runs inside the drive and puts a model of the motion system side by side with the real system. By comparing the real system output to the model, the algorithm forces the model to follow the actual. The observer can thus calculate what's happening in the motor as soon as current is applied, rather than having to wait for the position feedback device. This speeds up the servo loops, allowing higher servo gains and ultimately better performance.
David Ellingsworth, RG Engineering's vice president of engineering, comments, "This drive [has] the best performance we've seen from a digital drive to date, and the only digital drive that meets the performance needs of our application."