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46 SMT Magazine • February 2014 accuracy, repeatability and Specifications Now we will discuss how these concepts relate to fluid dispensers and specifications for accuracy of the system. There are many fac- tors that must be considered when generating a specification for accuracy, including the mo- tion system, the vision system, the dispensing system and the test conditions for validating the specification. Motion System When considering the motion system of a fluid dispenser, there are two main factors for consideration, the resolution of detection of the system position by the system controls and the dynamic versus static condition of the motion system during a test. Resolution, also called step size, is the small- est possible change in movement that a motion system can detect (Figure 6). The resolution is determined by capability of the motion system and feedback mechanism (encoder). It is typi- cally specified by the resolution measured at the encoder. However, this is only a measurement of the current relative positions of the encoder scale and the encoder read head. Factors such as encoder mounting location relative to the end effector, thermal expansion, frequency of en- coder reading, friction and hysteresis (backlash) are used to determine an actual position of the motion system at any given instant. The resolu- tion of a system can indicate lower boundary of repeatability and generally better resolution can result in better accuracy. But resolution by itself does not provide accurate indication of actual performance. Dynamic versus static condition of the mo- tion system becomes important when examin- ing the actual end effector (dispense valve) po- sition at the instant of fluid dispense. A system can be considered to be in a static condition when it has fully stopped moving and any re- sidual oscillations from a prior move have died out. In a static condition, the encoder read posi- tion should closely correlate to the end effector position as there should be little to no relative motion between the two positions. However, it will require some settling time to achieve such a condition and thus negatively affect produc- tivity (units per hour, UPH) of the system to achieve maximum accuracy. If the system dis- penses before any such oscillations have com- FEATUrE MaKING SeNSe OF aCCuraCy, rePeaTabILITy aND SPeCIFICaTION continues Figure 5: Difference between Cp and Cpk. Figure 6: resolution.