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Aeroengineer
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Re: Desktop PnP--some inspiration
Aeroengineer   6/23/2014 12:26:40 AM
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Two things, I think that in many ways you and I are sayign the same things, but going about it in different ways, and second I think that you think that I am much further along than I really am.  The calcs that I am doing are just some quick back of the envelope bounding calcs to get me in the order of magnitude that I shoud start my searching, and are in nowise representative of a final work.  As was ilustrated with the motor comment, you can have a motor with 9600kV, but runing on essentially 3s is not really practical, this is why I said it is in the relm of possibility but not practical.  Though if I had come up with I needed to have something with 100,000kV and 100V, then I know that I am not even in close to being in a realistic bounding box. 

With that, I really did mean to say 60 per second, not per minute, though not because I am stating that this is the goal of the project, but I am trying to bound the problem on the high side of things.  I also understand that there are other limiting factors.  One of those things is as you mention the lead nut, but more than that, it is going to be the critical speed of the shaft.  Without having looked at it, I can imagine that this will be the biggest limiting factor of all. I am going to guess (do not take as a statement of fact) that for a 3/8" leadscrew with two rigidly supported ends that the critical speed is going to be between 10k-15k rpm

In reality, I expect that the actual max munber that could be achieved would be on the order of 1-5 components per second once things such as friction, inertia, and actions which must happen in series.

Another thing to take into account is that I am writing most of this from a hotel room on an 8 inch tablet, and doing some quick checking for components here and there, though not really doing anything exhaustive, yet.  Once I get home and have access to a monitor that is better suited towards setting up a nice spreadsheet, I will do so. As for the units that I amworking in, I use IPS generally because that is what my work requires.  I even use slugs for mass.  I know what an awful system, but becaues of my location, I have more access to a larger variety of parts that are inches than metric.

This is not to say that I do not appreciate the effort that you are putting in.

 

 

 

salbayeng
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Acceleration and nozzles
salbayeng   6/23/2014 12:02:00 AM
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I tried a bit of googling, but couldn't find any actual PnP accelerations.

I did do some experiments using my hand pickup pencil (with an aquarium pump for suction)

Using a 15g luer lock nozzle (approx 1.4mm ID), I could pick and hold SMC,DPAK,tqfn32 packages vertically so 1g is definately ok , and noted
  • SMC,DPAK would fall off quite easily if gently shaken (maybe 2g)
  • the tqfn32 took a bit more to dislodge(maybe 5g)
  • and 0805 and SOD323, SOD123 nearly impossible to dislodge
  • can't pick up a D2PAK, 
  • can't pick up a 0603 (they get partly swallowed)

 

(one could repeat with an accelerometer for accurate results),

This implies that 10g should be possible with the correct nozzle. Problem with homebrew machine is that you may not have enough "correct" nozzles, so will need slower acceleration and slower speeds for some bigger parts.

A second aspect of G forces is the reaction force,  so if the gantry weighs 1/10 of the rest of the machine , then the rest of the machine will be subject to 1g forces when the gantry moves at 10g, this can shake parts out of the feeders and or move the machine around the table top.

Thirdly if the c.g. of the gantry is a long way above the rails, then there will be a strong pendulum effect trying to bend the rails and de-stabilise the servo loop. 

---------some reading--- 

Here's an interesting read on PnP dynamics (some tables about halfway through) he basically says that acceleration doesn't have much effect on throughput (as most moves are velocity limited)

http://red.cs.nott.ac.uk/~gxk//papers/mxaPhdthesis/Chapter%208.pdf

maybe something here

http://www.cree.com/xlamp_app_notes/pick_place/

more reading:

http://mdc-smt.co.jp/pdf/eng/NXHANDBOOK.pdf

a comparison:

http://www.researchgate.net/publication/222695003_A_survey_of_surface_mount_device_placement_machine_optimisation_Machine_classification/file/e0b495231cd4d09291.pdf

salbayeng
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Re: Desktop PnP--some inspiration
salbayeng   6/22/2014 11:34:59 PM
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Ok,

It's the second time you have posted "60 components per second "  instead of 60 per minute. i.e. one part placed per second. 

You need to be careful of getting scale factors mixed up in calculations, especially in spreadsheets.

60       Converting from RPM to revs per sec , also needed with fpm and ipm linear measurements.

2 x pi   Converting from "natural units" i.e. R L C & angular phase/velocity/acceleration to "human units"  specially note that torque to linear calculations almost always have a 2pi somewhere. The constant may appear as 0.159  (e.g. a motor with a 1mH + 1Ω winding will rolloff at 160Hz)

57    degrees per radian , need this for cos(angle)

32 or  9.8   Converting from gravitational to inertial units, need this to get from oz-ins to real torque. Imperial units have g embedded in odd places. Be careful when expressing accelerations in g , when you really meant to express a ratio. e.g. a block slides down a ramp at 30° implies a friction force of 0.5g , that calculation works on earth but not on the moon.

I do layout my boards on an inch pitch, and use materials in inch thicknesses and foot lengths, BUT when it comes to any sort of mechanical calculations, I always use MKS units. So I would strongly advise you to use MKS units as the main column in your worksheet , and have columns to the side of that for other units. I shade human input cells in yellow and computed in blue ,  so e.g. distance(m) in blue = 25.4/1000* (inches in yellow). If your primary calculation column uses metres, Newtons, Kilograms, and secs and derivatives thereof then you automatically get the right answers for velocity, acceleration, watts and joules and volts and amps and heat flow and temperature rise as well. Note that the correct representation of the inertial mass of 1kg is 9.8N/m/s/s. 

You might want to double check your numbers, a 9600Kv motor on 12v spins at 115,200rpm !!! ,  (the linear speed inside the nut is 7200fpm, about 10 times the max rating of bronze of 750fpm)

 

Aeroengineer
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Re: Motion solutions
Aeroengineer   6/22/2014 9:47:14 PM
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With 10 bit total resolution (assuming 8 bit effective) on a .25" lead, this gives .001 placement tolerance measuring capability.  Though on a 24 pole 22 tooth motor, assuming only stopping on a pole, this would give about .01" placement/step.  This is not to say that it cannotbe improved by going to a lower lead, lead screw.

 

One thing that I need to look at with brushless servo motors, is how steps are performed, or if it can hold in intermediate placement between teeth.  These are some of the details that I would like to look into before writing it off.

 

As to the challenge of rolling our own, yes it may be more complex, but if one does not at least look at the idea, then you will not know if the pain is worth it, and then the product will be no better than any one else out there.

 

As a designer, I like to throw any marginally reasonable idea out there and then chase down why it will or will not work. 

 

salbayeng
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Motion solutions
salbayeng   6/22/2014 9:30:00 PM
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Trying to "roll your own" insta-spin motion solution might be turning a molehill into a mountain. I'd suggest using one of the hobbyist CNC-router 3axis motion drive packages that already included the insta-spin.  

Also noticed this in the instaspin technote re sensorless control:

"Position applications require a mechanical sensor in order to precisely identify the motor angle at zero and very low speeds. The FAST Software Encoder may provide redundancy in position control applications; this can be used as a safety feature in case the mechanical encoder fails. "

I've used magnetic encoders before, Austria Microsystems were the leader in this field about 5 years ago. You need SPI type as I2C too slow for motion control, the AS5040 is typical , 10 bits "absolute" ,  you can cascade multiple devices , so 48clocks = all 3 axes in 6bytes, (some encoders also do multiturn absolute, can save you a couple of lines of code)  . Just glue a magnet on the rear shaft (nudge it gently while slowly rotating to centre it). Austria also have ring magnet solutions too. One issue with the outrunners is the magnetic flux of the magnets might cause positioning errors. 

You can write your own trapezoidal move algorithms with 1kHz update rate on a 20MHz CPU that will give adequate positioning control. To get 9PWM's you will find PIC's may be a better option than ATMEGA's, and of course the TMS320 range have many PWM's.

When it come to encoders, you need about 4 to 16 times more resolution than your required accuracy. This is necessary to be able to accurately measure the speed right at the end of the move.

 

Aeroengineer
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Re: Oversized I think
Aeroengineer   6/22/2014 8:22:45 PM
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Thanks for the input AD.  I think that I am leaning towards a max build area of about 8"x 10" at the largest.  Perhaps even a little smaller.

 

What are people's thoughts about this size range? Still too big.   I need to go and look at what Seeed Studio is offering for their sizes.  That might give a good calibration for this.  What other popular services are people using that offer their services in certain standard board sizes?

antedeluvian
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Oversized I think
antedeluvian   6/22/2014 3:27:06 PM
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Adam

I think 11" x 17" is overkill. The widest board we make (if you are aiming this at in-house prototyping rather that hobbyist) is 100mm wide by about ~9". If you are aiming at the hobbyist, Indon't think you would need much bigger than a Arduino shield.

Aeroengineer
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Re: Build Area Size?
Aeroengineer   6/22/2014 2:11:37 PM
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I do have some familiarity of the hobby bldc motors, though the information that you post is great background for those that do not have that background.

 

The thing that I love about the hobby brushless motors is that there is such a large varity at very cheap prices.  The other fact is that their power output per volume is much higher than the steppers.  Though their downfall, as you mentioned is that they would need to be controlled in a closed loop fashion.  The great thing about this, though is that you can reduce the size of the motor as you are not as concerned with slipping at high torque if you have the right motor control solution that can handle that. 

 

The standard hobby controllers would be out of the question, but I have been speaking with TI about their Instaspin Motion control solution.  I am not saying that this is the direction to go, but it does provide an itneresting option that may be more ideal for this type of setup.

 

As to the actual motors that I would be looking at, I am actually thinking that outrunners are going to be the desired option if this was the path to go down.  They can be found in small sizes at lower kV (in the 1200-2500 range) and would not be that difficult to adapt for this type of operation. 

 

On the topic of encoders, I would like to go with perhaps a hall based, non contact encoder.  the biggest issue is going to be update rate, but there may be some solutions out there that could deal with this.  It is something that I really need to iinvestigate much more.  Standard contact encoders are rather large, but as you said, there is not a large need for high line count.  I am sure that there is a technical solution, but is it cost effective?  That will be the question.  This is something that I would like to have play out in a post as it will provide some valuable learning.  I think that this application will be right on the cusp of both solutions.  I think that there will be value found in both and that both could be made to work.  It will then turn to what is the cost of each solution, and do they make technical sense from an engineering perspective, not just a that is a cool thing perspective. 

Aeroengineer
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Re: Desktop PnP--some inspiration
Aeroengineer   6/22/2014 1:55:53 PM
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You are correct that my calculation was rather simplistic.  The calculation that I did was more than anything to bound the problem on the high end.  Removing all other aspects including acceleration (I recognize that this is a futile assumption), what would be the bound on the high end.  60 components per second with an average of 8 inches of travel would require a .25 inch lead, lead screw and a 9600kV motor at 12V.  All of these things are in the relm of possiblity.  Are they practical, no, and are there other factors that I need to look into, yes, just have not had time to do that yet.

 

I am going to take the information that you presented and put it into a spreadsheet so that we can play with accel rates and other fixed time variables to generate a more realistic bound for the speed of the machine. 

 

I also think that a 1g acceleration is a little low, without having given it any great thought, I do not see right away why up to a 10g acceleration cannot be achieved.  Some of the details that would have to be looked into would be power/torque capacity of the motor, structural concerns (though I do not think that this will be a major concern), vacuum capacity to hold the part (friction), and positional accuracy

salbayeng
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Re: Build Area Size?
salbayeng   6/22/2014 5:30:18 AM
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The postings have crossed in the ether!

I've half addressed the motor tradefoffs just now.

I've also looked at using the hobby BLDC motors for machine type applications, I've collected a few motors and a few ESC's  but got too busy to play with them.

Some notes re hobby BLDC motors:

(a) Most have the rotor on the OUTSIDE , known as an "outrunner", can be a bit awkward mechanically, so check which one you have.

(aa) Likewise most don't have two shafts so mounting an encoder on the back might be difficult.

(b) They spin very fast, denoted by the "k" number , so if k=3000rpm/volt , you get 30,000rpm for 10v. For our purposes a LOW k is desirable.  so Kv=500 is about 10,000 rpm at 20v , close to our requirements.  Note that the torque constant is the inverse of Kv, so halving the Kv , doubles the available torque per amp,  

(c) Some motors (designed for helicopters) have an integral gearboxes , so combined with a finer leadscrew, you could drive the motor in vector mode and get stepper type encoder-less positioning

(d) The motors are designed for sensorless operation,  making it difficult to generate much torque at near standstill.

(e) almost all BLDC's are 3 phase, so if you want to retrofit an encoder for commutation or vector control, it needs to have <some multiple of 3> lines per revolution (most are multiples of 4 or 25) 

 

Accuracy / resolution is another issue, consider you probably need 0.05mm to place a 0.65 pitch part, so with a 5mm pitch leadscrew you need to locate within 1/100 of a rotation, requiring perhaps a 100 line encoder = 400quad counts. 

A 200step stepper with a 5mm pitch leadscrew gets you 0.025 without microstepping. Same 200step motor with a 1/2" diam pulley gets you 0.18mm without microstepping, 4:1 microstepping is achievable so 0.05mm resolution is then possible.

Apart from the gear backlash , a 38mm standard "robotic" gearmotor, with 3:1 gearbox and 100 line encoder is in the right speed/resolution range with a 5mm leadscrew (but might fall apart from all the load cycling) 

 

 

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