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# Desktop Pick-&-Place Machine: An EETimes Community Project

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Re: Desktop PnP--some inspiration
6/22/2014 4:41:45 AM
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Mechanics:

The piezo motor sounds like an "inchworm" drive, are these available at sensible prices nowadays? (they used to be ~ \$1000 each)

Ok 60 parts per MINUTE might be manageable for a low budget machine.

That's 7.5" there and back per second = 0.42mph , it's much better to do all the mechanics calculations in metric units (=200mm/sec avg), you will eventually come unstuck persisting with furlong-fortnights , to the order of 32 or worse.

Lets consider  1 cycle/sec, if we limit the ramp to 1g , then we have 250ms to accelerate, 250ms to decelerate,250ms to accelerate, 250ms to decelerate,  so the maximum distance traversed for the ramp of  250ms,1g = 300mm or 600mm = 2ft point to point. So you would be ramping ~ 10% of the total trip.

It takes time to pick up the part and place it. So a  full cycle might be 100ms build up suction. 80ms Zacc , 80ms Zdec,100ms X acc, 100ms X dec,  80ms Zacc , 80ms Zdec , 100ms to release vacuum,  then 80+80+100+100 +80+80 to get to next part.  = 1.24secs / cycle. These numbers give 48 cycles/sec. for 4" point to point move, the actual distance doesn't change the calcs much if you have a servo drive, (8" point to point is 1.44secs, 16" is 1.64secs)

Acceleration is important too, and given you may not have the large variety of nozzles, and suction variability as commercial machines, you will need to limit the acceleration, particularly with larger parts. Stepper motors can be jerkier than DC servos too.

Stepper motor will need a coarse leadscrew to get any reasonable speed.

Rotation speed will be limited to ~ 1000rpm for metal nuts, to prevent galling.

A double start 5mm pitch leadscrew with acetal nuts would be something I might consider.

Backlash can be fixed by using two nuts with a spring between them (and anti-backlash positioning algorithm)

Torque vs speed is a tradeoff with motor type and leadscrew vs pulley. So to get 8inch per sec , you need 2440rpm with a 5mm leadscrew or 338rpm with a 1/2" diam pulley. 2440rpm with a stepper motor generally requires a very high voltage drive (e.g. a typical "5v 6A" stepper motor might need 100v to get 1200rpm). Unlike a servo motor, the available torque at full speed is severely reduced in a stepper motor, (typically you need to use half the rated speed and half the rated torque in your calcs)

Be careful in your calculations from linear values into rotational (motor)  values , there is almost always a 2 x pi in there somewhere.

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Re: Build Area Size?
6/22/2014 1:00:35 AM
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Yes agreed that there are some tradeoffs as you suggested between a gantry system and a moving bed/workpiece system.  I currently prefer the gantry system for mounting the head to for both solder paste and the vacuum head.  I am not stuck on it, but for the reasons that you mentioned, I think that trying to have a moving bed may end up being more complex and producing an inferior result, including solder bridges on find pitched parts.

Now for the area which contains the unplaced parts.  Here I think that a moving bed concept of some sort is the way to go.  The reason for this is that it will both simplify the operation, while reducing the foot print of the device.  As you mentioned, there will have to be some general indexing to know where to go and hunt for the next component, but then again a regular feeder needs to know how far to index the tape as well, so I do not really view this as an added complication.

The other thing (I am not sure that I mentioned it yet) is that I am in contact with one of the guys at TI in their C2000 group that does motor control.  I am looking at what the trade would be to do servo motors vs stepper motors.  The one thing that this would open up is that hobby brushless motors could be used in conjunction with a standard type of encoder.  This also would play to the fact that this type of machine does not need hi holding force, more that it needs to traverse quickly from one point to another.  This is now starting to play into the strengths of a servo motor over a stepper motor.  This is not even close to being decided, but I do want to do some investigations into it as this might be a way to get some higher speed out of the device that might not normally be there for this type of a machine.

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Re: Build Area Size?
6/22/2014 12:46:41 AM
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Ok.

Let's make the distinction between "grid pickup" and "feeders"

(a) Feeder: always picks up at the same X, Y coordinate for any component

(b) Grid pickup: picks up at the "next" coordinate

So you are considering grid pickup (this entails some extra software, as it has to maintain a database of "next" for every strip of components, and these may be on 2 or more plates)

The second piece of the puzzle is the axes arrangement, there are basically two approaches

(a) keep the work area stationary and move the head around in X,Y,Z

(b) move the work area in X , and the head in Y (much like those large plan plotters where the paper goes in and out and the pen goes side to side.

There are some significant advantages to moving the carriage in only one direction, as it can get pretty bulky by the time you add a camera, pickup nozzle, alignment plates, glue dispenser. You can also combine the two, so the carriage can move say 12" in Y and only 4" in X, but the workpiece (PCB + parts) is on a X conveyer so could be quite long.

You can make good use of the Z dimension too (which I think it what you are suggesting, when I hand place with tweezers, I use a cantilevered parts tray that sits about 1/4" above the pcb, less likely to bump already placed parts.)

If you are moving the workpiece (the grid and/or PCB) you need to be mindful of the X accelerations ,  the smaller parts are OK, but electrolytics, inductors and SM connectors have low paste area to mass+height ratio, so may need to limit acceleration to 0.2g at the end when the big bits get placed.

When you are thinking about plates with grids of parts , (I will call this a "grid plate" from now on, ) ensure the grid plates have sufficient weight to limit the acceleration induced during normal handling, e.g. the plates should be 1/8" minimum thickness aluminium. Or if made from PCB stock, glue a 1/16" sheet of steel on the bottom.

Even if using "grid pickup" you should add a vibro feeder, these are simple and will take all the SO-xx & Dpak parts in tubes.

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Re: Desktop PnP--some inspiration
6/21/2014 11:08:36 PM
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As I have been looking into the pneumatic systems, most have a popoff valve to help with the "inertial" effect.  I actually would prefer to use a squiggle motor for this type of system.  I need to see if I can convince them to help us out with a motor to use as a prototype.  This is great as it has both the motor and the leadscrew already integrated.  They are piezo electric systems.  Pretty neat little devices that have very high force in a well concentrated device.  By actually making it a closed loop system based upon displacement, then I think that very fine control can be had in a dispensing effort.  Thankfully this will come as a later addition to the machine, but the basic framework needs to be laid out from the beginning so as to be able to integrate it later.

As a side note, I was working out some basic calcs for speed of the machine if we were to want to have it place 60 components per second.  If the average travel distance were 7.5" per component, then that would mean that the average head speed would be about 50mph.  This seems like it might be managable, but will have to look into the wear characteristics of the components.  By going with a smaller build area, this would allow for leadscrews to be used and compete in cost with other components for a belt system.  Leadscrews will allow for better positioning of components, but one of the maintenance items will have to be the lead nuts.  Anti backlash nuts could be used, but are pricy.  Might be able to use two nuts and make our own anti backlash nuts for cheaper.  Will have to look into it as things get closer.

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Re: Desktop PnP--some inspiration
6/21/2014 10:57:38 PM
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Re Pneumatic pasting;

All of the concerns about length and size of plumbing are irrelevant (provided they are not excessively long or large) the pressure builds up in milliseconds, and more crucially the plumbing is constant during the production run, so it merely adds a calibration constant if anything.

It's all done with TIME so 200ms = 0805 pad, 400ms=1810 pad. you manually dial up pressure to suit the nozzle / viscosity. Some machines do a suckback after a dispense pulse to avoid after-dribble.

With bigger pads you might consider going diagonally across the pad (as you would when hand squeezing.

For hand squeezing paste I use a 20g or 22g tapered plastic nozzle on a 2ml syringe. I have tweaked the 22g nozzle by slicing the end at ~ 30deg, this makes it easier to lay down a sausage.

A colleague has a hand placement machine with pneumatics, that uses a 5ml syringe, and typically a 18g or 20g straight nozzles. It has an adjustable timer, every pedal press gets you a dot of paste hold pedal down & it goes dot..dot..dot.. with a bit of practice you can do ~ 100pads/minute, your machine would about the same.

Note if you are an infrequent hand user, then the tapered nozzles are a better choice (harder to clog, easier to clean, less pressure)

If you want to do fine pitch or 0402 pads, you will need a small nozzle (22g?) and you have a lot of clogging issues if not used everyday, and you will need ~ 100psi so you need a real compressor (You can get some really nice, quiet shoebox size units designed for airbrush use) Note if using pneumatics you can use any diameter syringe, (as the pressure in nozzle=air pressure)

For a hobbyist type application, I'd lean toward re-using the same 2ml syringes you use for hand squeezing in the machine, this keeps the paste fresher, and then use a leadscrew with a tiny DC motor to push on the plunger, and just use a timer to control dispense volume. The leadscrew/motor will be heavier and larger so will slow down the motion system. It's kind of a tradeoff, (more versatility + slower) vs (more dots per second + hrs wasted unclogging nozzles).

It's probably preferred to <design> for a motor/leadscrew, as this can be swapped to pneumatic in a matter of seconds, much harder to retrofit the motor assembly.

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Re: Build Area Size?
6/21/2014 10:19:46 PM
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This is good information.  Before I bias the result, I will let a few more respond to the size question, but you have given a pretty good argument for the sizes you recommend.

As to the issue of feeders, I am heavily biased against them for this type of design, and prefer something perhaps a bit of an upgrade to what you were saying with having an area off to the side.  I am thinking at a minimum a moving plate that you can place rows of cut tape on (indexed to an edge) and then the plate would move to the next row of components as it switched from one component type to another.  I also have a concept that is a bit of an upgrade from that, which would allow for more components to be stored (twice as many) and would not increase the footprint of the overall machine.  It would almost be a conveyor style that would bring a row of components into place right under the picking head, and then it would index to the next row once complete with that component type.

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Re: Build Area Size?
6/21/2014 10:08:11 PM
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G'day,

There seems to be a popular panel size in use by PCB manufacturers of about 10.5 x 16" ,  So anything bigger would be a "special".

All of my bigger PCB's would fit 2, 3, 4, 5, or 6 onto a panel.

Note if you are using a stencil, you try to make a "set" of PCBs approximately the size of a sheet of paper i.e. 12" x 10"  to make the stencil economical.

But for the machine you are thinking of with paste dispensing, you can do a single PCB at a time (this possibly reduces the niumber of feeders in use too)

A quick measure of what "big" PCB's I have lying around the lab I have 5x6  2.5x4 3x6 5x7 3x4(many) 1.5x5  all of these are revenue PCB's , So I could load 100% of my PCB's over the last 3 decades with a 5 x 7 work area.  It should also be borne in mind that any PCB longer than ~8" is likely to be flexed sufficiently to crack MLCC capacitors. This puts an upper reliability limit on PCB size.

The exception to the size rule would be LED illumination strips (so maybe have a provision where you can have the pcb hanging outside the work area by removing some feeders)

So I would recommend an absolute minimum of a 6 x 8  or a more reasonable minimum of 8 x 10.

What will happen in practice is the dimensions will be limited by the feeders, (not the PCB size) (e.g a 8mm feeder might be 1/2" wide, so 20 of them for 10")

From a "number of feeders" perspective, it makes more sense to be able to pickup from the work area , i.e. use double sided tape to stick down 8" lengths of tape with bigger IC's , just next to the PCB. This requires some software support as you can't gaurantee you have stuck the tape down straight. Real PnP nachines do this anyway so they can pick from waffle packs. On the basis of using half of the work area for grid based pickup, I'd make the work area something like 12x10 or 16x10.

You can refill the "grid" really quickly using a hand pickup pencil (made from a 1ml syringe)

Just my ideas..

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Re: Positioning / verification
6/21/2014 3:09:37 PM
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It is a whole lot easier to reply to comments when I am not working 12-14 hours a day.

I wanted to elaborate a bit on some of the things that you have brought up in this comment.  The alignment of components will be critical to be able to push into the smaller component sizes and pad spacing.

I see this as a two pronged effort.  The first effort is to ensure that the electromechanical design can actually provide the accuracy and precission that is required for the task.  I have seen a lot of systems that are not designed for this, and they suffer.  Even in the 3D printer world, many of these groups only speak of the min step size, but not about the repetability of their process.  I think that this causes a lot of confusion and is perhaps borderline false advertising.

The other prong of this effort is to look at how to integrate a camera vision system to help in the alignment of the component.

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Re: Desktop PnP--some inspiration
6/21/2014 1:54:37 PM
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I have a bit more time now that I can elaborate on my previous comment.  One of the reasons that I think that they were having some issues for paste dispensing is that they were using a pneumatic method.  While this is common, there are issues of compressability and the fact that you need to control the built up pressure in the system in a way similar to how you would control an inertial device.  Added to this, there is a lot of flex in the tubing and such that connects the pump to the syringe.  This is greatly influenced by temperature.  This means that efforts to calibrate it are going to be difficult.  I can also imagine, but I have not actually looked at in depth, that there are differences in viscosity in the solder pastes and that these too can change with respect to temperature.

To combate these, I am thinking of one of two things.  The first, if going with the airpump method, there is a need to reduce the amount of line between it and the syringe.  I would look at doing that by mounting the pump right to the head of the divice.  There are a lot of small pumps that would be suitable for this.  The other option is to go to a mechanical system.  The mechanical system is going to be the most precise as it does not suffer from the "inertial" effects of the pressure system, nor the variability.

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Build Area Size?
6/21/2014 1:47:59 PM
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I am wondering, I have not heard too many comments about the build area size that people would be interested in for this type of machine.  I would love to hear your thoughts and why you think that it should be that size.

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