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P2 - Pick and Place for 3D Printers

Pick and place head providing part rotation and top/bottom machine vision in a package lighter than an E3D V6.

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This project picks up where the P1 left off.

For this version, I intend to employ a small BLDC motor driven with sinusoidal drive for part rotation. That should give me a 10.9g part rotator with ~3000 steps/revolution and will use standard 3D printer nozzles as the part picking nozzles.

The project will use a Jetson Nano and a Raspberry Pi camera for machine vision and control via OpenPNP. The single camera will perform both top/bottom vision by utilizing a flying set of mirrors.

The hardware will be controlled via a custom board providing vacuum sensing, BLDC motor sinusoidal drive, mirror (servo) control, and some additional PWM channels for use with the vacuum solenoid, lighting, and some multiplexed active part feeders.

Motion control will utilize your current motion platform and interface via serial, USB (via serial or TCP/IP), or TCP/IP.
For the prototype, the motion platform will be a BBG or PocketBeagle running Machinekit.

This project picks up where the P1 left off, but with some notable hardware improvements.

Blazing machine vision pipeline

The Jetson Nano Developer kit has the perfect blend of speed, cost, and connections to make this project something really special.  It has an onboard CSI connector which provides direct access to uncompressed HD video with extremely low latency (about 10x lower latency than a comparable USB 2.0 solution).  Combine that with the powerhouse of machine vision acceleration that this provides, and this should be a very fast platform for running OpenPNP.

Top/bottom machine vision, with one camera

The same CSI camera which provides fiducial location and calibration (top vision) will also provide part rotation/part position checking (bottom vision).  This is accomplished by the use of two tiny first surface mirrors mounted on a swingarm.  The swingarm has a hard stop and the extended position is located at the apex of the rotational travel of the servo driving it.  This provides a purely mechanical means to accurately position the mirrors at exactly the same position every time.  The mirror can deploy and retract in 0.13s.

Ultralight part rotator

Part rotation will be provided by a small BLDC motor driven open-loop with sinusoidal drive, similar to how a BLDC camera gimbal motor operates.  The torque required for part rotation is miniscule, so running it open-loop with no position feedback should be sufficient. If not, active feedback for rotational positioning can be added via a hall-effect position sensor.  The sensors and magnet required would add about $15 to the cost.

The BLDC motor I am trying first was originally designed to spin the color wheel in a DLP projector.  It was relatively painless to replace the existing 2.0mm shaft with a hollow one.  Having a hollow shaft is the simplest and most reliable method to pass the vacuum needed for picking the parts. 

Many off the shelf small gimbal motors also implement a hollow shaft, but the lightest one I've found is 30g and perpetually 'out of stock'.  I did find a couple smaller ones with a solid 2mm shaft weighing ~18g.  However, the DLP projector color wheel motor only weighs 10.9g and has 9 poles and 11 magnets, so it is also suitable for providing the rotational resolution required.

Extensible,let's use that...

OpenPNP gladly interfaces with more than one motion controller, so this seems like the logical place for where your existing machine and the additional hardware required to make it into a Pick and Place machine, can come together.  

For support of the basic electrical requirements, I've decided to put everything special required onto a single purpose-built add-on board now.  This will need a BLDC motor driver, a vacuum sensor, at least one channel of logic level PWM for servo control, and a few channels of Mosfet driven PWM for lighting and vacuum solenoid control.  I might as well include some additional PWM channels to make it easier to directly support a couple part feeders here as well.

Alternate electronics path

I have also thought about creating a 3 phase motor driver 'step-stick'.  That would allow direct replacement of one of the motion axis step-sticks on a RAMPS type board. There are other 3 phase motor drivers which would fit in this form factor and could be married to an STM32 with just enough space to spare for mounting the required support components.  Heat dissipation of the board itself would definitely be an issue if used at anywhere near the rated current of this chip of 5.5A.  The target BLDC motor would not have this issue, but of course people are going to...

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  • The real world

    Daren Schwenke06/14/2019 at 05:52 0 comments

    I've been cursed with a series of automotive issues which has consumed most of my free time for a couple weeks now.

    Vehicle one is up on jack stands with the transmission out, and will be getting a new clutch tomorrow as it suddenly disintegrated.  Prior to that the radiator and a ball joint failed.  70k miles...

    Vehicle two needs a new transmission and has the annoying tendency to randomly engage the reverse gear while in first. Prior to that, the fuel pump and the clutch slave hydraulic cylinder went out.

    With any luck, I should have the remaining issues sorted this weekend.

    There has been some progress made with our two new project members taking the reigns.  A proper introduction and the current state of the project will get posted Monday.  There is news...

  • Jetson Nano unboxing.

    Daren Schwenke05/23/2019 at 21:08 1 comment

    It's finally here...

    First off, I see 11 of the 40 pin GPIO pins have been immediately consumed by ground and power lines.  I also don't know if it is suitable to apply power to the ones labeled 5v.  I hope so.

    I'm eyeing that CSI camera connector with excitement though.  :)

    Downloading the 5.3 GB zipped disk image now and we'll see what happens next...

  • Jetson Nano on the way!

    Daren Schwenke05/17/2019 at 22:43 0 comments

    Yay.  My target vision board for this that I ordered a month back, just shipped today.  Almost time to get busy..

  • Part placement springy thingy

    Daren Schwenke05/01/2019 at 05:20 0 comments

    When the nozzle puts the SMD parts down, the part height actually has some tolerance associated with it.  This looks to be between 0.1mm and 0.5mm of deviation I will need to account for.  I do have fine control over the Z axis with my chosen motion platform here, so that helps though.

    For commercial nozzles, the springy-ness of the nozzle is built into the nozzle itself.  I won't have that.

    The P1 handled this by having a spring loaded rotator shaft.  I think I might have a better solution here using the BLDC part rotator.

    The rotor and stator tend to stay aligned naturally in a BLDC motor, as the magnets pull on the iron stator and keep it centered.  I think I can use this to provide the required spring-ness to my static/cheap choice of nozzles.

    I got out the scale and measured 80g of force at 1mm of travel as I pushed on the shaft of the motor, which in turn pushed the stator out of alignment with the rotor.  Beyond that, I had an additional ~3mm of travel where it pretty much stayed at 80g of force.  Further still, the force required started to drop off and I'm pretty far out of alignment at that point.  1mm should be enough anyway.

    80g of force is a little low from what the OpenPNP community is saying, but it certainly feels like a lot to me.  I'm going to give it a try.  

    I'm also wondering if I could modulate the current going to the stator (for all 3 phase so I don't affect the rotational positioning) and control the placement force provided.  My gut says no as I'm both pulling and pushing on the magnets in the rotor with the energized stator and so having a stronger stator field strength would probably just cancel out.  I'll just have to measure it and find out...  :)  That would be really cool if that does work.

    I will have to flip my part rotator over and use the other end of the shaft to make this happen as I don't have enough travel with the rotor on the bottom side.  This will complicate the mounting a little, and also means fabricating something fairly accurately for my retaining nut to attach to the shaft with. 

  • Rotator runout

    Daren Schwenke04/29/2019 at 17:06 0 comments

    Runout is not perfect, but not bad.

    Sadly I may have permanently damaged this cute little motor as I pressed the shaft in slightly crooked.


  • Cheap, plentiful, accurate part nozzles.

    Daren Schwenke04/26/2019 at 22:27 0 comments

    The stainless steel 2.0mm hollow shafts for part rotator idea number one showed up today.  

    I set to work rebuilding the BLDC motor with it.  It was relatively painless.  Pressed out the old shaft, pressed in the new one.  Centering was good.  Done.

    Now on to how to mount my nozzles....

    For the P1, I used Luer lock syringe needles and a bit of machined brass for the male part they went onto.

    This worked ok, but there was basically no such thing as a syringe needle with no runout.  They all needed correction to be usable, and then crashing into the bed tended to destroy them.

    I had a little brainstorm today.  Use 3D printer nozzles.

    The thread doesn't provide any centering.  It just holds the nozzle and allows me to do automated switching by putting them in a rack and threading them on/off by rotating the part rotator.

    The centering comes from the 2.0mm hollow shaft I replaced in my BLDC motor.

    The nozzles also happen to be 2.0mm ID, so this is a nice close fit with a little drag all the way on.  Basically perfect.

    I believe they would nicely handle a head crash as well.

    Still waiting on some small value caps to start building my BLDC sinusoidal motor driver..

  • BLDC rotator plan

    Daren Schwenke04/24/2019 at 15:16 0 comments

    I'm going to use an STM32 as my microprocessor, and have that driving an L6234 as my 3 phase motor driver.

    I'm going to build it on velo-board first, but I'll be using the same SMT components I would for the finished product.

    I can do current control, but I actually don't have access to all three phases of current with that chip.  Two of the three phases have combined current sensing.  :(  That probably eliminates implementing Field Oriented Control later as I was hoping.  

    There is another chip I was looking at though, the MP6536, which does break out current sensing for all three phases...  If Field Oriented Control becomes a sticking point for me, I can switch to using that one.  The overall schematic wouldn't change much, but of course the pinouts are radically different.

    I'm using a driver chip instead of just building this with mosfets because:

    • Ultimately it's a little cheaper
    • I don't have to worry about shoot-through, 
    • No high side drivers, or most of the support components.  

    I do have to be careful to only provide logic signals after that chip is powered up though or 'bad things happen'.

    I'm still missing a few caps before I can get started.

  • Here we go again.

    Daren Schwenke04/21/2019 at 21:24 2 comments

    While I'm waiting for my Jetson to show up, I'm starting with the other stuff.

    I've been doing a lot of research into accurate open-loop positioning of BLDC motors.  If there ever was a perfect application for this, part rotation is it.  Basically zero load, and comparatively low speed requirements.

    As a first step here, I intend to program a microcontroller to drive a 3 phase motor driver IC like it was a stepper.  Step/dir interface, hard-coded microstepping, sinusoidal drive.

    I'm missing a few small value caps right now, but that's about it. 

    Working on it.

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