| | |
That's a fast servo you have there. Maybe even too fast for the task. Unless you can control the accel/deceleration of this actuator, you can end up dropping heavy parts off the nozzle.
Here's the scenario. The servo is lifting the Z-assembly up and when it stops moving at the upper position, the axis will continue to travel up (there is a play in there), then it will meet the bearing. That's going to be quite fast deceleration which could cause the part to shift on the nozzle. After that, the Z axis will drop back down a couple mm's and meet the bearing again on the way down.
See if you can add a cushion of some kind around the bearing on the servo horn or otherwise reduce the play by as much as possible.
Another way is to use a "high torque" servo that has much higher gearing ratio and is several times slower. Many good "digital" servo's can be programmed for slower speed and acceleration.
-scsi
Servo mounts are done. I was quite happy to find the photo interrupter that goes on the back of the carriage platform (black part, last picture), aligns perfectly with the plate that blocks the beam. That is for a limit switch when the Y-Axis travels to the far rear.
Hi,
I'm not sure the servo speed will me a problem. On my Yamaha P&P machine, each head is is on a pneumatic slide. It had a strong spring return. When actived the head is just pushed down until it hits something. That something is the part in the feeder or the PCB when placing the part.
The servo is a bit complicated in reality, A small pneumatic slide may be simpler and lighter. Also being a on/off control, it only requires a digital output.
Cheers,
Peter.
-------------------------------------------------
Homann Designs - http://www.homanndesigns.com/store
phomann -
I bet those pneumatic actuators on Yamaha had air speed controllers on them. I'm describing a different problem with the Z-axis hitting a hard stop while traveling down with the part hanging on the nozzle.
Another acceleration related problem to consider. I'm working on a solder paste dispenser and researching this stuff a lot. Turns out that solder paste does not like fast motion (shaking) and won't stay very long in a homogeneous suspended state when riding on a shaky Z-axis. It tends to separate - the solder spheres accumulate in one place as they are heavy and the flux cannot not hold them in suspension.
-scsi
I have looked into solder paste dispension myself, and also concerned about the separation of the paste components. The use of air pressure for dispensing the paste is also something I'm not keen about, as the amount of paste ejected has a dependency on the amount in the syringe. I have been thinking more about using a screw based actuator to plunge the paste out a measured amount, with secondary suck back. But only using a small custom vessel that will need frequent refills I otherwise use stencils, but for custom jobs or a few prototypes a small paste tank would suit. Importantly, one I can unclip and only load when required.
I'm using Mach3 for the stepper control, where acceleration rates can smooth out any rapid movements that could bump the part off the nozzle :-)
Speaking of screw based dispensers... Here's what I built today. Works surprisingly well and I can dispense 0.1uL volumes with 0.5mm nozzle while pumping thick grease. It's called auger valve. A proper one costs ~$5K : http://www.techconsystems.com/fluid-...ry-auger-valveI have been thinking more about using a screw based actuator to plunge the paste out a measured amount, with secondary suck back.
Oh very nice! How did you make the auger? Need some more pictures/details of how you have accomplished the mechanism please
The auger you can see on the desk next to the pump head. Took my Dremel and hacked the stepper motor shaft so that it resembles a flat screwdriver. Drilled couple of holes in a piece of HDPE and put some fittings here and there. Next time I will get proper luer fittings and machine the piece on my mill out of Delrin so it wont look so ugly.
The uC in the background is driving stepper by issuing step/direction commands following a simple program. Speed and pullback is adjustable via trimpots that uC is reading. Stepper driver is from Pololu. They also make cheap mylar stencils for $25+ http://www.pololu.com/catalog/product/1202
Hahaaa... now your teasing without enough detailsOriginally Posted by scsi
Literally, just two holes in a piece of HDPE not counting mounting holes. This was an ugly attempt of making something similar to the proper auger valve like Techcon ones on the picture attached. They have molded in luer fittings, which is nice. Techcon sells these Disposable Material Path valve assemblies in bags of 10 for something close to $500. Hence the attempt to make my own. It's not as easily disposable, but costs just a couple cents. The hardest part was to match the hole size of the screw well to the screw OD. I ended up making the hole slightly smaller and then used the screw itself to make it larger. Again, the screw that I'm using is the one you can see on the picture.
And another illustration - meat grinder that uses the same principle of operation. Some pressure is necessary on the inlet for the thing to work properly. Something like 5-10 PSI air is all it needs.
-scsi
Eclipze, I have been following this thread since it started, figured I'd sign up, I've been looking to either build my own pick and place, or buy one. You design looks good, if you can think of a way to add support on your floating axis I'm sure that would increase placement speed and stability, as well as accuracy. Look forward to seeing how your's turns out.
Yes. I had a really old A3 scanner (SCSI bus) by Sharp, that had a similar arrangement, but had a dolly wheel on the out-board end. The head on it was very light.
Cheers,
Peter.
-------------------------------------------------
Homann Designs - http://www.homanndesigns.com/store
Posting Permissions
| | |
LinkBack URL
About LinkBacks
