The only constant in the world of motion control seems to be change. One of the most crucial aspects of motion control is the feedback mechanism. After all, if we can’t measure where we are, we cannot control it.
The first order of business is to estimate what feedback resolution will be needed for the application. More resolution than needed can create serious instability issues in the control system. High resolution feedback also requires greater bandwidth in the controller, which will cost extra. This makes it worth your while to be sure what that you are buying just what you need for the application.
A subtle aspect of the feedback device is the minimum pulse width of the square wave. When you get to 250 kilohertz feedback frequency, the minimum pulse width of the signal is 4 microseconds. Which is kind of quick. So another parameter to check on the controller is the minimum pulse width of the input so you don’t end up in missing pulses accidentally when a certain speed is reached.
Most applications depend on the feedback attached to the servo motor as the reference for position. This is not always the best case. Even precision gearboxes have some backlash in them. So resolution requirements should dictate where the feedback is applied physically. Many rotary applications require a second feedback device to be attached directly to the load. Linear applications get a real boost from the use of a linear feedback sensor that can provide an independent reference for the linear axis. Again, this requires the “axis and a half” input to be supported on the controller.
But best of all, manufacturers are proliferating some excellent choices in the feedback arena. Optical encoders have been the staple technology in the past. But now there are Hall effect magnetic feedback systems from numerous vendors. In addition there are now capacitive feedback devices from companies like CUI Stack and others which offer programmable feedback features and superior environmental resistance.
Linear feedback devices are proliferating as well. Linear feedback devices were primarily available for high resolution semiconductor applications. The high resolution millionth of an inch feedback, required by the semiconductor industry was simply overkill in most industrial situations. And controller that could handle the 20 megahertz data rates that were generated by high speed linear motors were rare and again, expensive.
Recent product introductions from vendors such as Turck make linear tape scale feedback devices available with resolutions from .004″ to .0001″. This is ideal because the predominant feedback requirement in general purpose motion control is in this exact range.
So progress, while sometimes slow, has been made. There are a lot more choices available to the controls and mechatronics engineer. Let’s take advantage of that and try to make the best choices for the best solutions we can come up with.