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68 SMT Magazine • May 2015 curately on the board, in an X-Y position. Fol- lowing are three methods commonly used for positioning, along with pros and cons for each. 1. Positioning with no feedback system (open loop system) 2. Positioning with rotary encoders (closed loop system) 3. Positioning with linear encoders (closed loop system) method 1: No positioning feedback loop In this system, the motor drives the part to a location on the board defined in the program by the number of steps in each X-Y axis, but there is no way to tell if it actually ends up in the right place. These systems use stepper mo- tors for positioning. Pros: Low cost. Cons: Unreliable accuracy; not recommend- ed for high quality production. method 2: Positioning with rotary encoder In this method, an encoder is mounted di- rectly on the motor shaft and delivers position feedback to the control system; however, it only reports the motor position, and not the actual position of the X-Y axis. This is dependent upon the remainder of the mechanical components that make up the machine. These machines can use stepper or servo motors (which are usually associated with cost). Pros: Low cost; this system is widely used on entry-level machines. Cons: Typical positioning accuracy of +/- .005". method 3: Positioning with linear encoder In this method, linear scales are mounted to the machine's X-Y axes table and an encoder is mounted on the traveling beam that will be car- rying the components. This method will report its actual position back to the control system and sMt quiCk tips SELECTING aN aUTOmaTIC PICk-aND-PLaCE maCHINE, PaRT 3 continues figure 2: view of a pick-and-place machine frame being welded together.

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