The visioning system of a modern placement machine can account for a degree of misalignment of the component from the nozzle centre at the point of picking. However, it cannot yet account for any further displacement from that point onwards. As the machine head traverses the tool path, involving multiple axis accelerations, forces are imparted on the component which if unbalanced, may destabilise or displace the component on the nozzle tip, resulting in errors in placement such as component skew, as seen in figure 1.

Figure 1: Component skew, due to drag forces

We have already discussed how aerodynamic drag forces can be determined by CFD analysis of the airflow around the component, and many other forces act at the nozzle tip/component interface. Some are stabilising forces and must be maximised, others are destabilising forces and must be minimised, for successful nozzle placement:

Destabilising forces:
Inertia, proportional to the component mass and the tool path acceleration Aerodynamic drag, proportional to the square of the tool path speed.

Stabilising forces:
Friction, proportional to the vacuum pressure, and the interface friction coefficient. If the tool path and the mechanical properties of the component and nozzle are known, then the equation of motion of the component relative to the nozzle tip can be solved to determine the locus at the nozzle tip – component interface, providing useful information on the net displacement error.

Figure 2: Sowing external drag on nozzle during placement cycle. Red areas show high drag, blue areas show low drag

Development engineers at Metro use specialised proprietary software capable of relating the displacement locus to the tool path for any nozzle/component combination. This analysis tool enables them to minimise net displacement error for a given tool path. This is achieved by selecting the optimum nozzle/component combination capable of delivering reliable, accurate component placement for the minimum vacuum pressure, which is good for business and good for the environment.