Linear motion is particularly impacted by the choice of feedback. And for most systems the use of feedback is not an option. Linear motors, for example, cannot be operated without a feedback device. And because of the linear motor’s roots in semiconductor manufacturing, the feedback is usually a high resolution linear tape scale.
How much feedback resolution is enough? Most of the time more resolution is better. But there is an element of control theory that says if the feedback resolution is ten times greater than the position accuracy that you are trying to measure, the control system can become unstable. The other side effect of extremely high resolution feedback is the tendency to “jitter” because it is responding to tiny variations in the real world, which the control system will then have to contend with. So spending extra money for high resolution feedback may cause other problems.
Where should the resolution be put? Obviously, if you are using a rotary servo motor, just use the feedback on the motor as the linear position reference. This works when the required resolution is not very high because in all mechanically linked systems, there is lost motion called backlash between the motor and load. But most motion controllers and many indexing drives contain dual feedback loops, so using an external feedback sensor will produce great benefit in accuracy and repeatability.
The big benefit in using linear feedback is the elimination of mechanical error as part of the control system. On a project I did a few years ago we were evaluating a special grinding machine that had a 13 foot long lead screw in it. The customer know the lead screw had wear and error in it, and that was part of the problem that needed to be addressed in rehabilitating the machine. Instead of replacing or re machining the lead screw, we specified an external linear tape scale feedback. The results were fantastic. Accuracy and repeatability were phenomenal and combined with an integrated servomotor system, led to a 300% increase inthroughput for the customer. Backlash? What Backlash?
How much distance do we need to sense? Some linear motors like piezo-electrics and voice coil motors have very limited stroke lengths. Similarly, different feedback technologies have scalability parameters such as sensing airgap and length requirements are considered. Some feedbacks work in the range of 2 to 6 inches in overall stroke length, some are capable of 3 feet, some up to hundreds of meters.
The exception is the stepping motor and leadscrew combination which can be operated without feedback on the assumption that the load is not varying dramatically. But even the leadscrew and stepping motor needs feedback when the load varies. Current detection can be used to determine if the motor has stalled, but doesn’t necessarily give you the opportunity to recover position without an external source. So the extra cost of external feedback is a judgement call based on the accuracy requirement and how “robust” the system needs to be.
The variety of types of linear feedback are equally challenging, and as with most things, must be considered based on cost and performance. The most popular feedbacks are linear tapescale systems that use reflected infrared beams that are interpolated to achieve very high accuracy. The classic linear feedback from the machine tool era is the glass scale which uses through beam optics and a grating embedded in glass to product the linear position information. Check out companies like Renishaw, Heidenhahn and others for details. Information on Heidenhahn’s latest innovation is featured on the Project Mechatronics website.
Over the last few years there have been a number of magnetic solutions where a magnetized linear scale is interpolated by taking the sinusoidal waveforms produced by Hall sensors or inductors, and digitizing the results. Integrated circuitry combining Hall effect arrays and functional support to linearize output are now the prevailing state of the art. Check out NewScale Technologies Tracker product for details on their new offering.