In a previous SensorTech post, we discussed improving the accuracy of linear motion systems while lowering total system cost by employing external linear position encoders as secondary feedback. The secondary feedback supplements the primary feedback provided by a rotary encoder mounted to the drive motor.
Now Clint Hayes, Sales and Product Manager for Linear Technologies at Bosch Rexroth, has written an excellent “How To” article for Machine Design magazine entitled “Six Keys to More-Precise Linear Motion.” Mr. Hayes identifies precision as a combination of accuracy and repeatability, where accuracy is the discrepancy between target and actual position, and repeatability is the ability of a motion control system to return to a given position when repeatedly approaching that position from the same direction He discusses the important effects of various mechanical design elements and operating conditions for linear guides that can influence these important motion control system specifications.
One of the important specifications discussed in the article is Positioning Accuracy. Mr. Hayes points out that positioning accuracy is dependent on the capabilities and tolerances of the mechanical drive mechanism. He also highlights the technique of implementing electronic position correction to compensate for rising mechanically-induced deviation as travel distance increases.
The reference measurement for this electronic correction can be derived from an externally mounted linear scale encoder. The external encoder provides actual load position data that the motion controller uses to calculate the required amount of correction needed to compensate for the non-linear mechanical deviation over distance.
If you’d like to know more about the benefits of external position feedback, there’s a White Paper available called “Motion Control Primer: Direct load position sensing with secondary feedback encoders”.