WayOutWest wrote:Curious, what's the "pressure" bottle for?
Is this so that when the pickup solenoid cuts off the vacuum, it (or some other secondary solenoid) also cuts in a slight positive pressure?
Adam, you are exactly right:
I have connected the tube from the pickup nozzle to two solenoid valves via a T-piece.
Of these two separate solenoid valves, one connects to negative and one to positive pressure
reservoir.
A small electronic circuit switches those solenoids with configurable timing.
Whenever a components is placed, I let a small "puff" of compressed air into the tube so the component is released immediately.
By doing this, I more or less elminiated the pneumatic "capacitance" of the long tube running to the nozzle.
The same circuit also controls the pump in such a way that the pressure in the reservoirs is almost constant - within a small hysteresis window.
In practice, it works quite well
But to be honest: It was much effort for little gain. If I build the pneumatic system again, I would limit myself to the
vacuum reservoir only. I would keep a controller for the pump though to have almost constant negative pressure - makes things more reliable.
Instead of positive pressure, I would mount a threeway solenoid to the head just like in Juha's original design. I believe with a bit of dwell time that should work almost as good and the impact on overall placement speed should be negligible.
I originally thought that having positive pressure might also come in handy for paste dispensing or pneumatic actuators, e.g. to activiate ratchet feeders. I am not sure I will use it for these purposes though as stencils are much better and any actuator could also work electrically.
By the way: Having a pressure sensor like Reza suggested directly connected to the nozzle is also very beneficial - this could not only be used to verify component pickup but also to dynamically adjust dwell time (if pressure in tube equals outside pressure, nozzle can safely be removed from part).
