Breaking News
Blog

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

NO RATINGS
1 saves
View Comments: Newest First | Oldest First | Threaded View
<<   <   Page 9 / 15   >   >>
Aeroengineer
User Rank
Blogger
Re: Motion solutions: motors- a mammoth post
Aeroengineer   6/23/2014 10:40:35 AM
NO RATINGS
Good info. I will read this in more detail when I get back to the hotel tonight.  I will see what I can digest and then ask a few more questions.

Aeroengineer
User Rank
Blogger
Re: Oversized I think
Aeroengineer   6/23/2014 10:39:07 AM
NO RATINGS
Correct, this was the overall footprint of the device, not the build area.

Aeroengineer
User Rank
Blogger
Re: Trading speed for size and accuracy
Aeroengineer   6/23/2014 10:38:22 AM
NO RATINGS
What size work area are you thinking of Max?  It seems that we are talking as low as 4"x 6" up to 8"x 10".

Max The Magnificent
User Rank
Blogger
Trading speed for size and accuracy
Max The Magnificent   6/23/2014 10:29:02 AM
NO RATINGS
Personally, I'd be happy to trade speed for increased PCB size and increased placement accuracy.

FYI I've already cleared a corner of my desk in anticipation of this unit :-)

salbayeng
User Rank
Rookie
Re: Oversized I think
salbayeng   6/23/2014 5:16:35 AM
NO RATINGS
Hi antedeluvian  ,

I think the 11" x 17" refers to the size of the machine sitting on the desktop. 

The actual max pcb might only be 4" x 6" . (I think all the hobbyist proto shop sizes run in multiples of 50mm)

The amount of travel needed to get to the PCB and the parts, depends on how the parts are picked up , could be double the PCB area.

 

salbayeng
User Rank
Rookie
Counterweights ??
salbayeng   6/23/2014 5:05:03 AM
NO RATINGS
some crazy ideas with counterweights:

This only works on the Y axis: have a two head machine, going in opposite directions on either side of the gantry (could be a paste head one side, placing on the other) or camera on one side?, This probably needs a long "fast" counterbalanced Y axis and a slower X axis, so you would put components on a pickup grid above and below the PCB. Maybe 4"PCB + 4"grid top + 4"grid bottom = 12" Y travel, and 6" X travel = PCB width. 

Or you could go all out, and put the X motor underneath the workpiece (PCB+platen+parts+motor), have the heavy workpiece move sideways 1", while the light gantry moves 5" other way for 6" total . 

Also for the  Z axis , have you considered using bail bars (like the old fashioned pen lift on chart recorders) the "high position" has no accuracy requirement, while the "low position" will have a spring anyway, this would be adequate for placing, but might need better Z control for pasting.

You might consider making "double wishbone" suspensions for the placing nozzles from laser cut shim, this gives parallel motion and a soft spring (wouldn't be that hard to make it 4 nozzles spaced 1/2" apart  either) Need some thought how the component aligning fingers work with multi-head. 

salbayeng
User Rank
Rookie
Re: Acceleration and nozzles
salbayeng   6/23/2014 3:26:34 AM
NO RATINGS
Ok the counterweights are a clever touch,  and should work (marginal with stepper motors maybe), if the trajectories are mostly constant velocity, then lower accelerations have lesser effect on transit time. Also if the motor is selected for dynamic stiffness, it should have excess torque available, and if the acceleration is limited by stability issues, then adding a counterweight with damping (i.e.attached with sorbothane) will allow more acceleration, and less "ringing".

Have you considered using a stainless wire and drum drive? (instead of belts/leadscrews), it's a bit old fashioned, but you can move the Y motor off the gantry, and attach a counterweight on a dead leg. 

Wow impressive history of achievements, I was once lucky enough to visit Moog in Aurora, you can't really appreciate the power of the shuttle engine controls until you stand next to an actuator- Awesome.  I once built an aircraft simulator (payload) shaker for Lockheed Martin, impressive and scary at the same time!

Have you had a look at radio telescope drive systems?You would appreciate them, Monstrously slow and heavy, but insanely accurate, they use two huge motors and high reduction boxes, with torque control operating in a tug-of-war mode to reduce backlash to zero, some still have selsyn transducers.

Tough working on a laptop in a hotel room,  I try to carry a "lab in a suitcase", but I sure do miss the dual 24" monitors when I'm on the road!

 

 

 

salbayeng
User Rank
Rookie
Re: Motion solutions: motors- a mammoth post
salbayeng   6/23/2014 2:12:20 AM
NO RATINGS
re "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. "

All motors are, from an academic perspective, identical and can be represented in an equivalent d-q model (direct and quadrature). First consider a motor frame with two poles consisting of a north magnet and a south magnet, and the rotor is a steel bar with a coil of wire wrapped around it. Obviously when you put current in the coil , it will point either up or down (i.e. zero or 180degrees = direct) , and the available torque will be zero. When you rotate the coil to left or right (with current applied) the torque is maximum (i.e. 90 or 270degrees=quadrature) at this point the torque is proportional to current, the torque is sinusiodal with rotation.

If you rotate the rotor with power off it will detent twice per revolution. This effect is known as "saliency" or "cogging", it is deliberately enhanced with stepper motors due to the poles being tooth shaped, a good servo motor on the other hand has negligible cogging (using a skewed rotor or stator) , the average BLDC (not designed for servo work) will have some slight cogging. 

Each pole may have more than one slot to place wire in, so by rearranging the wire in the slots you can achieve a non-constant magnetic field across the pole face. This affects cogging, and also the back emf. Back-emf is caused by moving the coil through the magnetic field , so the waveform of the back emf  (imagine the motor is a generator) could be sinusoidal (synchronous AC, most servomotors) or trapezoidal (most BLDC, some servomotors, most brushed motors) . Like wise the torque vs angle curve can have differing wave shapes. A trapezoidal back-EMF suits simple motor drivers, where you only have to turn transistors on or off. Note that the difference between the back-EMF, and the voltage you apply to the motor creates torque, so if the waveform is wrong it creates torque ripple (A stepper motor has really high torque ripple)

The minimum requirement to cause a motor to turn is two coils at  right angles, so as to create a magnetic field alignment between 0 and 360degrees (it's just sine and cosine) you can do the same with 3 phases, 4 phases, 5 phases etc in the end all you have is a magnetic field strength and an angle.  So the term "FOC", field orientated control, merely means you are focussed on controlling the field direction (aka "vector") (and amplitude). 

Voltage vector control means you apply voltages to the windings, and hope the motor aligns with the rotor, a stepper motor is an obvious application of this, you apply 5v to any of 4 windings and it points there, the vector space for this motor only has 4 points per electrical rotation (but there are 200 electrical rotations per mechanical one) . More sophisticated VFD's change the voltage with frequency. The vector space for BLDC motors is 12 points per revolution. The vector space for sinusoidally wound motors is essentially infinite.

Most servo controllers use current mode control, they force a current in the winding to generate a restoring torque (to drive position error to zero), note that the phase angle of the current is 90deg for maximum torque (unlike zero degrees for  voltage control). Note that the power dissipated in the windings varies with load,  so you can "borrow" upto 10 times rated torque, and let the motor cool down later at constant speed (can't really do this with voltage control).

All of the above have effects on the positioning accuracy of a motion system, typically the positioning error is the sum of {saliency effects} and  {effective stiffness x (friction + static)force}.  
  • Stepper motor (with microstepping) high saliency means that applying 25% + 75% winding currents won't put you at 25% of the pole spacing , and the stiffness is highest on a pole and softer between.
  • BLDC motor (trapezoidal winding) with microstep, also won't be linear between poles , but better than a stepper
  • A servomotor with Voltage control , has no angle dependant accuracy, and the stiffness is constant (but softer than an equivalent stepper)
  • A servomotor (closed loop current control) has the highest stiffness and best accuracy.

So getting back to your original query, the "steps" are actually commands to the drive, and it works out what to do from there, for example the drive might be set up for 1024steps per mechanical revolution, so:
  • stepper motor this means each of the 200 poles is carved up to ~5 steps , but really only accurate to +/-2 steps. (slow , fairly accurate)
  • BLDC, 6 pole pairs , each of the 12staes withing each pole are carved up (interpolated) to ~14steps, maybe accurate to +/-5steps. (very fast, but sloppy)
  • Servo (closed loop) 4 pole pairs + 4096qc encoder , each of the 4 poles carved up to 256 points, accuracy +/- 0.5step (fast and accurate) 

So a BLDC does not have "teeth", but it does have "poles", while "steps" are commands to a drive. All (magnet) motor types will "hold" at intermediate positions, but the accuracy and stiffness while holding varies across types. Closed loop control dramatically improves holding accuracy and stiffness. Closed loop control can double available accelerations, but no effect on limiting speed.

 

 

Aeroengineer
User Rank
Blogger
Re: Desktop PnP--some inspiration
Aeroengineer   6/23/2014 12:55:47 AM
NO RATINGS
Just for reference, the all knowing wikipedia seems to indicate that older pnp chip shooting machines were doing around 15 components per second on a single head, and that current machines can do around 40, though it is ambiguous as to if that is with a single head, or with multiple heads. 

 

So 60 as a high bound was a good guess to get in the ballpark, and I think that the low end may be around 1 per second with a max reality no higher than 5 per second, and even that may be dicey once we get looking into it in more depth. 

Aeroengineer
User Rank
Blogger
Re: Acceleration and nozzles
Aeroengineer   6/23/2014 12:43:10 AM
NO RATINGS
Ah, and now you get to the heart of an idea that I am contemplating, but not sure the best way to make it work, and that is using counter weights.  It would slow the actuall acceleration of the machine, but also make it so the thing is not jumping like popcorn in a hot frying pan.

 

I will take a look at the links that you posted.

 

If you want to get an idea of the sorts of mechanical designs that I am capable of, you can go to my twitter page and look at the background pic.  That wind tunnel model had a span of almost 40feet and had two installed jet engines.  I designed all the wing structure, which included a blowing slot and flaps, the engine intakes, systems placement/attachment, thrust reverser, and even designed a rotary actuator for this thing as we had a flap that hung out into the exhaust and was on a 12" moment arm.  Nothing like designing an actuator that had to work in the heat with 4000lbs of thrust upon it and had to hold position to within .05 degree.  The final design used a hypocycloidal gearbox to a low speed ac motor.  It was capable of 9000 ft lbs of holding torque, and had an inline set of strain gauges to measure the torque.  The part had to be designedso that it could handle the expansion due to heat, which was on the magnitude of .25" from nominal room temperature.

 

So have confidence that right now, I am doing very, very high level calcs to understand how big the design space is, and I hope that this week I will be able to sit down and really spend some time putting things together in a much more refined and detailed fashion.  These will take into account someof the realities that I have currently been ignoring up till now, though you are good to point these facts out ;)

<<   <   Page 9 / 15   >   >>
Flash Poll
Radio
LATEST ARCHIVED BROADCAST
Join our online Radio Show on Friday 11th July starting at 2:00pm Eastern, when EETimes editor of all things fun and interesting, Max Maxfield, and embedded systems expert, Jack Ganssle, will debate as to just what is, and is not, and embedded system.
Like Us on Facebook

Datasheets.com Parts Search

185 million searchable parts
(please enter a part number or hit search to begin)
EE Times on Twitter
EE Times Twitter Feed
Top Comments of the Week