PICduino Stepper Motor Shield

Continuing with the PICduino theme, I made a 'shield' to test some VID29 stepper motors.  These are small, inexpensive ($15 for 6), geared steppers designed for automotive instruments.  These steppers draw very little current and can be driven directly from the PIC outputs.  A single port (Port B in my case) can drive two steppers.

VID29 stepper shield
(click for larger image)

This PICduino shield has 8 LEDs (Port C), two steppers (Port D), two pots for analog-in tests, and a MAXIM MAX232CPE RS-232 driver to test some communications. 

These boards are sized to fit the Hammond Manufacturing
"Multipurpose Plastic Enclosures (1591XX "S" Series) Economical Version" enclosures ($4.95 in single unit quantities at DigiKey).  One issue with the Arduino is the cost of enclosures is about half the cost of the fully populated CPU board!

If you need a bit more board space, you could extend the board a bit to take up the full space inside the enclosure (check the enclosure datasheet for max PCB size).

I have typically been an Assembly Code hacker, and for this project I needed some more advanced math, so I tried my first C programing in 15 years.  I had some trouble getting my mind around pointers, but once that was handled, the code worked great!

This stepper has 6 phases for a complete cycle, with 3 phases making up one degree of movement (you can see where the math requirements come from).

In the short video, I have the unit move 30 phases (10 degrees) 18 times (180 degrees total). 

(NOTE: When I play back this video, the audio is out of sync, and the pointer appears to be a bit "jerky" in one-or-two places.  This is an issue with the video, the point moves beautifully smoothly).

Experiments with auto levelling and PCB routing

By their nature, PC boards are not as flat and smooth as we would like.  I have equipped my CNC with a sacrificial table (MDF) that can be leveled using set screws.  I can attach a dial caliper to my Z axis, and I can dial in the platform to within a thousandth-or-two.  Still, my PCBs are sometimes cut too shallow, forcing me to risk fingertips using an x-acto knife, or too deep, ruining the fine geometries I try to keep.

The concept of an 'auto leveler' makes great sense, and I decided to give it a try.

My setup

EDIT - see current posting 

• EMC on Linux (the version numbers escape me).  
• CadSoft Eagle 6.3.0 on Windows 7
• Testing the PCB-GCODE with the auto leveller  pcbgcode_tags_2.0-alpha1-r68.zip from the PCB GCODE forum on Yahoo
• CNC machine is homebuilt based on the BuildYourOwnCNC machine
• I have created my own 'anti backlash' nuts that allow me to place SMD devices with 50mil lead pitch
  That's about as fine as this old man can solder, so it's good enough for now.

Day one, wasted

After two half-days of getting frustrated, I turned to the forum, Art was very prompt to respond. I needed to move the autoleveller code outside of C:\Program Files(X86).  I moved them to my D:\ drive and progress was made.

Lesson 1:  Install the AutoLeveller files NOT in the Program Files directory.  (for Windows 7 at least)

Setting the initial entries into all the PCB-GCODE-SETUP fields was helped by Dan, MoDFI on the Forums. This is a good starting place

Download the new math.h if you are using Eagle 6.  Replace the old one by renaming it 'math_old.h' - don't delete anything!

How the autoleveller works

Moved to my web page: Autoleveler

Milling the board

It took me a couple of attempts to figure out the flow.  

1. Align the board and do the top etch 
2. Drill the bottom two mounting holes (0.125").  These are used when
   the board is flipped to align for back milling
3. mill bottom traces
4. mill outline of board
5. drill holes

Lessons 4 and 5:  Careful not to "spot drill" the holes too deeply.  First board was ruined as the default spot drill depth (-0.011") was too deep and wiped-out most of my vias.  Changing to -0.004" was fine.
Also, drill the two alignment holes after probing/milling - the probing/leveling process stops if you happen to probe within one of the mounting holes.

The board

I designed a board (CADsoft Eagle) modeled loosely after the "Arduino" concept - a small, basic CPU "motherboard" into which 'shields' could be plugged.  I am currently playing with the Microchip PIC processor family, so I made a modified PICduino setup.   The motherboard gives me a +5 and optional +3.3V supplies, connection for the ICSP (In Circuit Serial Programmer) a PIC 16F777 44 pin, TQFP package, optional crystal and a power indicator.  There is a LOT of space left over for whatever I can think of for Rev 01.  Traces are typically 0.016" unless there is a powerful reason to go smaller (not on this board).

Top (Click for larger image)

Bottom (Click for lager image)

CPU detail (Click for larger image)

Helpful Links

Mill PCBs 

 http://woodworkerbcncrouterproject.blogspot.com/2013/11/new-autoleveller-software.html

Experiments with auto levelling and PCB routing

By their nature, PC boards are not as flat and smooth as we would like.  I have equipped my CNC with a sacrificial table (MDF) that can be leveled using set screws.  I can attach a dial caliper to my Z axis, and I can dial in the platform to within a thousandth-or-two.  Still, my PCBs are sometimes cut too shallow, forcing me to risk fingertips using an x-acto knife, or too deep, ruining the fine geometries I try to keep.

The concept of an 'auto leveler' makes great sense, and I decided to give it a try.

My setup

EDIT - see current posting 

• EMC on Linux (the version numbers escape me).  
• CadSoft Eagle 6.3.0 on Windows 7
• Testing the PCB-GCODE with the auto leveller  pcbgcode_tags_2.0-alpha1-r68.zip from the PCB GCODE forum on Yahoo
• CNC machine is homebuilt based on the BuildYourOwnCNC machine
• I have created my own 'anti backlash' nuts that allow me to place SMD devices with 50mil lead pitch
  That's about as fine as this old man can solder, so it's good enough for now.

Day one, wasted

After two half-days of getting frustrated, I turned to the forum, Art was very prompt to respond. I needed to move the autoleveller code outside of C:\Program Files(X86).  I moved them to my D:\ drive and progress was made.

Lesson 1:  Install the AutoLeveller files NOT in the Program Files directory.  (for Windows 7 at least)

Setting the initial entries into all the PCB-GCODE-SETUP fields was helped by Dan, MoDFI on the Forums. This is a good starting place

Download the new math.h if you are using Eagle 6.  Replace the old one by renaming it 'math_old.h' - don't delete anything!

How the autoleveller works

From the novice perspective.  Gcode allows for the saving and retrieval of data during the milling process. 
Here are some snippets from the Gcode produced for the calibration board:

Variables are defined:

   #101=0.5000    (clearance height)
   #102=0.0400    (traverse height)
   #103=-0.0200   (probe depth)
   #104=30.00     (traverse speed)
   #105=1.00      (probe speed)
   #106=-0.5000   (tool probe depth max)
 
The variables above come directly from the settings in PCB-GCODE-SETUP:

       

This bit of GCode repeats throughout the beginning of the .tap file

1.  G00 Z[#102]
2.  #2000 = #5063
3.  (PROBE[1,0])
4.  G00 X-0.4585 Y0.2170 Z[#102] F[#104]
5.  G38.2 Z[#103] F[#105]

Line 1  move the Z axis to '#102', where #102 is the traverse
           height (0.0400")
Line 2  I have no idea
Line 3  not too sure, I'm guessing this is setting the 
           storage location for the first probe
Line 4  moves to the X  &  Y coordinates for the measurement 
           (based on the Probe Grid Size setting) with Z = the Traverse
           Height and the movement speed (F) of 30 ips (Probe 
           Transverse Feed Speed)
Line 5  'G38.2' is the actual "probe" command


Lesson 2:  Set up the 'probe in' signal on your CNC machine 

To avoid punching holes in your PCB, you need to wire-up and configure the "Probe In" line using the StepConfig Wizard (EMC).  I already had all my inputs brought to an opto isolation board so wiring up and configuring this line took only a few minutes.  For my installation, I needed to check the "invert signal" checkbox for the Probe In line.  For the actual probe, I use the same bit I will be using to mill.  I do not move the bit at all between probing and milling, I just disconnect the wire and turn on the spindle.

Lesson 3:  Calibrate your setup.

The copper on a 1oz PCB is 35 µm or about 1.4 mils (.0014") thick.  That isn't much if the surface of the PCB is 'waving around' by 3-4 thousandths or more!  I recommend playing around with this calibration board and 'futzing' with your settings until you have the best results possible.

Milling the board

It took me a couple of attempts to figure out the flow.  

1. Align the board and do the top etch 
2. Drill the bottom two mounting holes (0.125").  These are used when
   the board is flipped to align for back milling
3. mill bottom traces
4. mill outline of board
5. drill holes

Lessons 4 and 5:  Careful not to "spot drill" the holes too deeply.  First board was ruined as the default spot drill depth (-0.011") was too deep and wiped-out most of my vias.  Changing to -0.004" was fine.
Also, drill the two alignment holes after probing/milling - the probing/leveling process stops if you happen to probe within one of the mounting holes.

The board

I designed a board (CADsoft Eagle) modeled loosely after the "Arduino" concept - a small, basic CPU "motherboard" into which 'shields' could be plugged.  I am currently playing with the Microchip PIC processor family, so I made a modified PICduino setup.   The motherboard gives me a +5 and optional +3.3V supplies, connection for the ICSP (In Circuit Serial Programmer) a PIC 16F777 44 pin, TQFP package, optional crystal and a power indicator.  There is a LOT of space left over for whatever I can think of for Rev 01.  Traces are typically 0.016" unless there is a powerful reason to go smaller (not on this board).

Top (Click for larger image)

Bottom (Click for lager image)

CPU detail (Click for larger image)

Helpful Links

Mill PCBs 

 http://woodworkerbcncrouterproject.blogspot.com/2013/11/new-autoleveller-software.html

PCB Alignment for double-sided milling

I wanted to be able to mill the top, flip the board, mill the bottom, drill the bottom, all without having to re-set the X and Y home position.

First off, I use the non-commercial CADsoft Eagle license, which limits me to a 3x4" board.  I decided to use board mounting holes to do "double duty" - mounting holes and  aligning the boards for milling.  I pre-drilled a bunch of PCB 'raw stock' with two precisely placed mounting holes (more on that later).

Alignment pins need to be placed on the top side of the sacrificial board.  To do this, mount the X-Y home block and home the X & Y axis.  Then drill four 0.125" holes 0.3" deep into the top of the sacrificial board at:

1. X =  3.80, Y = 0.15
2. X =  0.15, Y = 0.15
3. X = -0.15, Y = 0.15
4. X = -3.80, Y = 0.15

I cut-off some old, broken drill bit shanks to 0.4", chamfered the ends a bit to ensure no burs and epoxied them into the holes.

Sacrificial board, top view.
(click for larger image)

The image shows the sacrificial board, mounted on the milling table.  I could extend the life by drilling half of my boards via the top, and half via the bottom.  In the image, you can see all the drilling so far is via the bottom of the board.  No particular reason.

PCB mounts to the left two pins (negative X coordinates) for bottom etching/drilling and flips over to the right two pins (positive X coordinates) for top etching/drilling.  Thin double-sided tape is used to hold the PCB securely to the sacrificial board.

Setting the tool home point quickly

Home Block Top
(click for larger view)

Now that we can quickly and accurately mount and re-mount the sacrificial board to the milling table, let's turn our attention to the top side of the sacrificial table.  First, we need a method to quickly, accurately and repeatedly 'nail' the X=0 and Y=0 points, without having to "eyeball it".

To do this, I made an "L" shaped jig, with conductive brass edges that pins very snugly onto the top of the sacrificial board.  The conductive strips are wired into the X-home and Y-home inputs of my optical interface board (see earlier post on Option Isolation Board).

The idea is to set the zero point one time, good for etching and drilling both sides of the board without having to reset anything.

Home Block Bottom
(Click for larger view)

The alignment pins are 1/4" aluminum rod, epoxied into place and are on a 3.0 inch center-to-center.

The square brass bar is set into small dados so they are sticking out just a little bit proud of the wood surface. This ensures the tool will touch the bar before it touches any wood.

This block fits snugly into the holes drilled in the sacrificial table in the previous step.

Home Block mounted to
sacrificial board (click for lager image)

Preparation for double-sided PCB - sacrificial surface

A replaceable backing system is necessary since the drilling will damage whatever is behind the PCB.
To hold the PCB, I use a 3/4" MDF (Medium Density Fiberboard).  The PCB backing board needs:

• flat and true
• inexpensive
• easy to replace
• fast alignment
• fast leveling

MDF clearly meets the first three requirements.  To meet the last two, I decided to use three 1/4" registration pins and "T" nuts with set screws for leveling.

0.250" Registration holes in milling table
(Click for larger image)

I drilled 3 holes into my machine table. I made an "L" shaped layout.  The pattern (in inches) for the three holes are:

• 0,0
• 12,0
• 0,8.5

These holes are small and do not interfere with any operations when I'm milling large stock.

---

The actual sacrificial table is 3/4" MDF, 10" x 14" and is routed from the back to accommodate:

Underside of sacrificial table
(Click for larger image)

• Three alignment pins - the reverse of the dimensions above (1/4" aluminum rod stock)
• Three "T" nuts with inset hex head set screws for leveling
• Two alignment holes for homing the tool (setting x=0 and y=0)

This part is milled from the back side (as shown in the image the above) - this allows all holes to be milled at one time.

Close-up of alignment pin and
leveling screw
(Click for larger image)

Here is a close up of one of the alignment pins (epoxied into place) and one of the "T" nuts with the leveling set screw visible.

Double-Sided PCB Isolation Routing

I've been thinking about this for a very long time.  My design requirements:

• Easy-to-setup
• Easy to get the PCB 'flat'
• Accurate and fast
• Automatic setting of home
• Jigged to make it super-easy to mill both sides without changing my X & Y zero points

Click to enlarge

I am excited to say I think I got it.  My recent test had trace widths down to 0.012 inches and they were razor sharp. The image to the right (20x) shows four of the 0.012" traces ending in pads for a 25pin D connector.  The board only needed very light sanding and the isolation paths were excellent.

Front-back alignment was very good (some issues during the test may have contributed to a less-than-perfect alignment).

I will document the enhancements I made.  I added an anti-backlash feature - it is both easy and, based on my recent results - razor straight 0.012" traces, quite effective.  I have a table/jig that makes flipping the PCB a zero-time exercise - without having to reset my X & Y home points!.  I made a simple jig to nail homing my X and Y axis