Archive for diy

Heatsinking a Pololu A4988 / A4983 stepper driver

Posted in Arduino, DIY, Electronics Projects, Heatsink, Stepper Motors, Uncategorized with tags , , , , , , , , , , on July 13, 2012 by Nilsen

I have been using the Pololu A4983 / A4988 stepper drivers for quite some time to drive my timelapse dolly’s stepper motor. I used them due to their relative low cost and their ease of use as well as the fact that they support 1/16 microstepping mode which when using a toothed belt with the timelapse dolly is perfect for tiny movements such as ramping.

A note on the microstepping, the steps are so small that it can even be used for focus stacking etc moving just 0.2 mm per step.

The only problem I have with these stepper drivers is that the chip is so tiny and even though rated up to 2A, without proper cooling they are realistically only able to deliver about 0.7 A before overheating and shutting down.

Since the design I use for the dolly is one the “U” configuration with the belt secured on either end of the rail and the motor driver on the sledge itself (along with the camera of course) a failure or thermal shutdown of the driver could be catastrophic in terms of cameras crashing to the ground.

I use BIG stepper motors and in order to drive them at their highest capacity I would ideally need to run the driver at 2A. If I set this without any cooling it will overheat in about 15 seconds and will go into thermal protection mode, this is understandable if you consider the size of the chip:

For those of you who have not seen it before:

Pololu stepper driver front and back. (Image credit pololu)

There have been a few incarnations of my heatsink endeavors but they all rely on a the premise of using epoxy to fix a larger heat conducting material onto the chip with some thermal paste in between the chip and the larger piece of metal and then fixing an even larger heatsink to that.

The first heatsinks I built were the following:

Rudimentary but a lot of cooling area.

As you can see it relied on a large piece of aluminum with some rods coming out of it, they were pressed into holed slightly smaller than the rods to ensure they would hold. I used a piece of U-Bar aluminium to with a hole in it with which I could attach the large heatsink.

This method worked relatively successfully but it made the whole thing rather fragile because if that ever caught on anything the chip would be ripped right off the board (fun fact, epoxy gets harder when heated). The current I could put through the chip with this heatsink was about 0.4V on the ref pin, meaning about 1A of current to the motors. This is acceptable but the transfer of heat was not optimal and I was relying on the epoxy, bonded to the components on the board to hold the whole thing in place. This was the Maximim of this setup.

The calculation for current by using the sense pin (ref pin) is ref * 2.5. 0.5 * 2.5 = 1.25A

The case in which the electronics (screen, button pad and arduino) were housed also contained the stepper driver and the subsequent heatsink, this meant that the box had to be quite big and should have some sort of air flow else everything could get rather hot.

A4988 with 5V regulator, Chip heatsink and match

In order to minimize this whole setup I purchased some chip coolers meant for high performance PC components such as north bridges etc. These copper heatsinks come with thermaly conductive tape to attach to the chips however their sheer size means the amount of heat they can dissipate is only about 1A worth (VRef of 0.4). After this the chip will go into thermal runaway and shut down. Pity that nothing is ever that simple. As a side note, this was one of the methods that I found while researching ways to heatsink a pololu.

Finally I changed the general design of the controller-dolly setup. Previously I had the following setup:

  • Power cable from battery to control unit
  • Camera trigger cable from unit to camera (on the dolly slider)
  • 4 cable, high current cable from stepper unit inside control unit to stepper motor

This complexity meant when out in the field you had to worry about 3 cables, it was a pain.

I changed the design to move the stepper driver onto the sledge along with the optocoupler to fire the trigger, all I had to do was to change the 3 cables to 1 8 strand cable to send the various control signals from the arduino to the stepper driver and optocoupler circuit to drive the stepper as well as control the camera and then of course send back the 5V from the regulator to the arduino / screen and button pad.

This meant only 1 cable from arduino to dolly, and only one extra to the dolly sled in the form of the power cable.

With the added freedom of design I gained I was able to design a high power heatsink as space was no longer such an issue:

Pololu with small aluminum square tube epoxied to the chip. Chip is mounted on a breadboard which I designed for ease of connection.

Note the crocodile clip, that is the one measuring the vRef.

Closer View with thermal paste on top

Partly boxed, connecting cables soldered onto the board

Using thermal paste and a large aluminium bar I secured the bar tightly via two bolts onto the aluminium tubing.

Secured

Aluminium base with threaded holes to allow for easy bolt tightening.

Over tightening will destroy the chip.

Vref – Far too much, this would equate to 2.25A in the chip’s output.

Chip temp after adjustment of vRef to about 0.8 (2A)

Temp of heatsink, pretty damn good thermal conductivity. Use thermal paste!

I ended up turning down the pot a tiny bit as running at max all the time is no good as well when thinking of longevity.

Without any heatsink, soon after this the chip shut down.

The aim is to make this a sturdy and weather resistant device so of course I will embed it in Resin

Some copper

Mixed resin with bubbles from mixing

Bubbles being shaken out (put the resin tub into an ultrasonic cleaner)

Bubble removal… A hot topic which I will create a post on.

Air-be-gone

Completed and filled (the two copper sinks were thrown in for good measure, really not necessary but giving a more serious look)

So That is my method of heatsinking the tiny A4988 stepper driver, it is now able to drive up to it’s maximum potential and is also waterproof and nicely packaged.

Open Source!

Cheers!

*Update* – Mounted driver unit

Mounted driver module – front looking back

Control cable and stepper motor cable

The mounted module with the cabling attached

 

 

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Mold Making

Posted in DIY, Electronics Projects, molds with tags , , , , , on June 9, 2012 by Nilsen

After the “pain” I had when creating the wonderfully clear and smooth finish, the crazy sanding with 60 grit belt sander, then 200, then 600 water paper then 1200 water paper then hours of polishing it just was not feasible to make these things more often but since I had a shape I wanted I ordered some mold making silicone.

It comes in a tin with a catalyst which you obviously mix together, stir vigorously and pour around your item which you have encased in some sort of box.This produces a negative of the item which you can then easily use to replicate as you just pour in your resin and it does not stick to the mold. Brilliant.

I used some acrylic and a hot glue gun to quickly make this box.

Acrylic is perfect as it’s non porous meaning the silicone mold will easily pop right out when I remove the walls.

Box with device hotglued into it.

 

Unfortunately as can be seen in the first picture, when mixing the silicone many bubbles are introduced through the process of stirring so to remove them I could use a vacuum pump but I don’t have one so I found the “bombs away” method on Youtube. This method relies on the fact that the silicone is so viscous that if you let it stretch thin enough no bubbles can survive the thin end of the stretch. Maybe better explained in the below pictures.

Here my silicone in a cup with a hole made in the bottom covered with sticky tape.

Look at it go!

 

Here you see the silicone stretched extremely thin by gravity and the viscosity of the liquid

 

Make sure to land next to the object, not on it else you could introduce more bubbles, let the liquid flow over the object from the bottom of the mold box.

 

Nearly done.

 

The bottom of the mold box.

 

Here the completed thing.

 

Completed.

 

Now I will add another completed circuit and pour in some resin. Lets see.

Results!

Posted in DIY, Electronics Projects with tags , , , , on May 19, 2012 by Nilsen

Here they are! The cast products.

Firstly I am a little disappointed that I have not yet been able to buff the initial test cast to a clear glass like finish. It just won’t buff up even though it is so smooth now and I have tried various polishing compounds including a brasso equivalent (or maybe not) and a metal polish for chrome.

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Orange tinge is due to the cardboard box leaking some colour

I sanded the piece with 600, 800 then 1200 water paper and subsequently started to polish it, unfortunately it is not reacting the way I had hoped. I even attached a polishing wheel to my drill press but even that only left the resin dull. I know it is crystal clear as before I sanded the top it was like water.

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Another View

Here the trigger before sanding the cardboard mold away.

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In it’s little conffin.

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Coffin Removed!

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With backlight, my work needs a little work 🙂

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Bottom

And here some other projects I have on the go, please welcome the Nilsen Slider – Control box.

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Controller being assembled.

More to come in a separate post.

Results with Resin

Posted in Electronics Projects with tags , , , , on May 10, 2012 by Nilsen

A quick update on the test with resin:

Notes, this is polyester resin, not epoxy resin, thus it is hardly useful for presentation purposes unless you want to drop in an insect and fake a fossil, nevertheless here are the results of my casting attempt.

I cast my 555 flashing LED timer into the “Amber” aaaaaaaand… it cracked due to too much catalyst. I have not seen catalyst like this, it’s a sort of paste so measuring 2-4% by volume is near impossible.

Here’s a picture to show the little waste of time.

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Here some components which weathered the storm of cracks…

The crack on the LED and some resistors, amazingly they survived.

Here’s a pic of my top secret project!

This is the circuit (properly made of course) to go into the clear resin, notice the simple components…

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More to come soon…

A test with resin

Posted in Electronics Projects with tags , , , , on May 6, 2012 by Nilsen

I had a lazy Sunday ahead and this was the perfect opportunity to try my hand at resin casting, especially casting electronics into resin.
The only resin I found was at a local hardware store and it looks like amber…
To be honest this whole thing started with blog I saw about a pre-amp which was cast in a clear resin cast and it looks absolutely amazing.
I have a lot of projects which I want to make and sell and I think this kind presentation sets it apart from the rest.
Here’s a link to the site:
http://runawaybrainz.blogspot.com/2012/04/audio-crystal-cmoy-freeform-headphone.html

I wanted to build a simple project which could benefit from the resin casting but also as a proof of concept for casting electronics into resin. I built a freeform 555 timer circuit which flashes a red and green LED, I used a circuit off a google search but didn’t have a 1uF cap, I used a 10uF instead so it switches every 7 seconds.

It is powered off USB so it can be one of those useless PC peripherals 🙂

I will create a new one and cast in clear resin once I get it but I will also post the details of how it goes!

Here some details of 555 timer circuits, just added another LED to the positive rail to pin 3.

http://bit.ly/JfOrx2

Anyway its a start, I have subsequently ordered some clear resin, pics of the project coming soon.