karplus4arduino

2011 August 5

Board (partially) populated and tested

My 4-up board about to be cut apart on the board shears. The adjustable railing for the back edge of the board and a railing along the bottom make it fairly easy to line up boards for repeated identical cuts (which I did not need). The bottom rail was useful for making sure that the cut was square, though.

Yesterday, I went to the basement shop where the support team for the enginineering teaching labs have their offices, and used their board shears to cut my PC board up. The shears are like a souped-up paper cutter. The noise they make when cutting through the PC board is a bit scary, though as it it sounds like you are crushing the board, rather than cutting. It makes a pretty clean cut, despite how it sounds.

HexMotor rev1.3 board with one H-bridge installed.

This morning I woke up early and decided to populate the board. I soldered in the switching power supply (and associated resistors and capacitors) first, and checked that there were no shorts and that it produced the correct output voltage. I then added the digital logic, the sockets for connecting to the Arduino, and the various headers.

It turned out that most of the headers did lock in place nicely with the Sparkfun offset-hole header design, making soldering much easier. A few of my 3-pin headers seem to have slightly smaller posts, and they did not lock in place as nicely. The hardest thing to solder in place were the 0.1″ jumper wires (which are not part of the next design). I may want to leave another 1–2 mm for the resistor, also, as I found it a bit difficult to fit the resistor I had into a 5mm spacing. Perhaps I should go with a 7mm spacing.

I only put in one H-bridge for testing, as I did not want to unsolder H-bridges if the board did not work.  If I’d been really careful, I would have waited on the switching regulator as well, since it is not really needed when there are no servos and the Arduino is powered over the USB line.

I spent most of the day writing driver code for the Arduino.  I came up with a pretty simple interface (much simpler than the AFMotor library for the Adafruit motor shield—I may want to extend the library to cover that board as well).

There is one global declaration of a HexMotorBoard to explain how the board is configured. As a minimum, you need to specify for each motor whether it is configured for Antiphase, Sign-Magnitude, or On-Off control. I suppose that if you connected the boards with ribbon cables, you could control several HexMotorBoards from a single Arduino Mega board, though with a standard Arduino there’s not  much point to having more than one HexMotorBoard, as there are only 6 PWM pins (unless you just need  full-on Forward/Backward and brake).

In the setup() procedure, you call the setup() member function of the HexMotorBoard.  To control a motor, you just use a single function: motor_name.run(speed).  The speed is always in the range -256 to 255, with a speed of 0 meaning brake. The software takes care of translating the speed into appropriate commands for the H-bridge, depending on how the H-bridge is jumpered.  (For convenience, I provided an inline function for motor_name.brake() that just expands to motor_name.run(0).)

I couldn’t come up with a much simpler user interface: declare the board, declare each motor, one setup() call for the board, and one function (run) to control each motor.  I suppose that I could get rid of the setup() function, since all it currently does is set the appropriate pins to be output pins and that could be folded into the run() function, but I’ll have to think about the value of keeping the HexMotorBoard::setup() function around to provide a place for consistency checks (like whether the configured pins are capable of PWM output).

Because I’m using the standard analogWrite() and digitalWrite() commands, and not fussing with the PWM frequency, very little code is needed to run on any Arduino.

I tested the board with its single H-bridge using a 12v door-lock actuator, and it worked in all three modes (antiphase, sign-magnitude, and on/off).  I did notice that the TLE-5206 got quite hot when the actuator was run for a few seconds, so I’m definitely going to need a heat sink.  I was planning on using a shared heat sink for the 6 H-bridges, but the electrolytic capacitors get in the way a little.  I’ll make sure to leave more clearance for heat sinks in the next revision.

I don’t have much time over the next few days, but on Sunday I hope to bring the board up to 4 or 6 H-bridges and make and  hook up the heat sink. Hmm—I don’t have any thermal grease—yet another thing to order.

 

 

 

2011 August 2

Boards arrived

The hexmotor rev 1.3 boards before cutting them apart.

The printed circuit boards for hexmotor rev1.3 came today from 4pcb.com. I ordered the boards on July 21, so the total turnaround time from order to delivery was only 12 days. No wonder the company is so popular for student projects—low prices and fast service will do that!

The boards looked pretty good, so the first thing I did was to check some crucial dimensions:

  • I put headers into the holes for the connection to the Arduino board and checked that it would plug in.  No problem!
  • I checked that I could spread the pins of the TLE-5206-2S chips to fit the holes where they were supposed to plug in.  No problem!  In fact, it was surprisingly easy to get the right spacing, by using the board itself as a tool for bending the pins.
  • I checked that I could put in both the MTR2 TLE-5206-2 chip and capacitor C5 without conflict (as I’d noticed that the silk screen outlines overlapped only after I’d sent in the order).  This also turned out not to be a problem
  • I checked that the mounting holes on the board lined up with those on the Arduino.  Only 2 out of 3 do.  What is going on???I got the hole placement from the Adafruit library for Eagle, and it has 3 holes, but they are not in the same places as the Arduino board.  It turns out that two of the holes correspond to holes on the Arduino board, and two to holes on the Arduino Mega board, which has holes in different locations.  Neither pair corresponds to the holes on the  Adafruit motor shield, which has only 2 diagonally opposite holes, the two holes that are actually in common between the Arduino and the Arduino Mega footprints.  For the next revision of the boards, I have to decide whether to use the 3-hole design of this board, or to go with the pair of diagonally opposite holes.  Or put in 4 holes, so that either Arduino or Arduino Mega can be securely attached with 3 screws. I think that there is room for the fourth hole, if I move one of the electrolytic capacitors.
  • I also did some continuity checks on the power lines.  The power and ground lines connect to the right pins on the TLE-5206-2 footprints, and they aren’t shorted.  The 6.25v and 5v lines also seem fine.
  • I had to make some Eagle library components for the screw terminal blocks I was using, and they seem to be fine, except for two little problems: I put a silkscreen mark for the bump on the side of the 5mm block on the wrong side, and the holes specified by the manufacturer seem a bit too big for the pins.  I’ll specify a smaller drill hole (closer to the spec for the pin size) in the next revision.  Neither of these little problems will interfere with functionality.
  • I checked out the Sparkfun header pattern, with the slightly staggered holes to hold the headers while soldering.  The holes must be a bit bigger than Sparkfun gets from their manufacturer, or my headers smaller, because the headers were not held by the staggered holes.  They fit fine, but the holes are loose enough that they aren’t held.

Tonight, I’ll fix up my Eagle library to have a 4-hole Arduino template and to have a better 5mm screw terminal component.

Here's a 12" board shear that costs $379 from Circuit Specialists.

Tomorrow I’ll go up to campus and see if I can cut the boards apart with the board shears.

I took a look at  printed circuit board shears on the web (you need to include “printed circuit” if you search, or you get book-binding equipment), and they range in price from $125 to $500, depending on size and quality.  I don’t plan to buy one for myself, but it seems like the sort of low-price, special-purpose tool that a place like Makers Factory ought to get.

2011 July 31

More design errors in hexmotor rev1.3

I should have known better than to send the PC board to fab before I wrote the software.  That’s precisely the sort of optimistic thinking I chide my students for.  There are all sorts of design errors that can be uncovered when you work out the details of how to use the hardware.

In my case, the motor functionality of the board all looks ok (so far), but the extra servo functionality is somewhat messed up, because I had not really understood how servos are controlled and how the Arduino servo library uses pins.  I “knew” that servos used pulse width to control position, but I had not bothered to look into the details or think about exactly what that meant—I just assumed that you had to use the PWM pins for the purpose.  Wrong!  If I’d ever actually used a servo with the Arduino, I would have known better.

Servo control is based on the actual on-time of the pulses, not the ratio of on-time to off-time (as is the case for controlling brushed DC motors).  You do have to give up one of the timers used for PWM in order to control the pulse width, but you can provide the pulses on any output pin.  The Arduino Servo documentation clearly says

The Servo library supports up to 12 motors on most Arduino boards and 48 on the Arduino Mega. On boards other than the Mega, use of the library disables analogWrite() (PWM) functionality on pins 9 and 10, whether or not there is a Servo on those pins. On the Mega, up to 12 servos can be used without interfering with PWM functionality; use of 12 to 23 motors will disable PWM on pins 11 and 12.

So putting servo control on the PWM pins is precisely the wrong choice of pins for maximum flexibility.  The hexmotor rev1.3 board has more or less the capabilities I thought it had (I can do up to 6 servos or up to 6 motors, and I can trade off servos for PWM control of motors, though I always have to give up PWM control of motors 4 and 5 to get any servos).  The initial application of the hexmotor board called for 4 fully PWM-controlled motors and 2 extra controls that could be motors or servos.  We can still do that, though we have a choice of 2 PWM-controlled motors or 2 servos and 2 forward/backward/brake (not PWM control), and can’t mix a servo with a 5th PWM-control motor.

But there is no reason to limit the board the way I did.  With slightly different wiring I could have simultaneously allowed 4 PWM-controlled motors, 2 forward/backward/brake motors, and 5 servos, with the main limitation being the power to the servos (due to the 1.5A limit of the switching regulator).  Three extra servo outputs could be really handy!

I also did not think about making the board compatible with the Arduino Mega board.  The current design does not use the Arduino Mega’s extra pins, which is no big deal, but it also blocks access to those pins, which could be a problem.  Furthermore, I did not look at what pins the Arduino Mega provides PWM on, nor which it gives up for the Servo library.

Incidentally, I was frustrated in looking on the Arduino site for the pin mapping of the Arduino Mega boards—they’d hidden that essential information behind a password (for no discernible reason).  I finally found the Mega pinout info on the forums, where someone frustrated by the inaccessibility of the pin mapping page had re-created it from other documentation.  It turns out that all the digital pins 2 through 13 can be used for PWM on the Mega, so any redesign I do on the board would have at least as much functionality on the Mega as on the standard Arduino.  I will have to be careful to put in conditionals for the pins in the software, though, as some of the pins are mapped from different ports on the Mega.

Redesigning the board to pass through the Arduino Mega pins would be a pain, as the pins take up three sides of the 1.825″×4″ footprint of the Mega board, and the remaining side is where the USB and power connectors are, so through-hole components can’t be mounted there.  I don’t think I can fit in all the TLE-5206-2 chips and screw terminals for the 12v power and motors and stay below the Eagle 8cm×10cm board size limit if I have to put in connectors for all the Arduino Mega pins.

I won’t be able to stack boards above the motor board anyway, as the electrolytic capacitors I’ve designed in to clean up the power are very tall (2cm), as are the TLE-2506-2 chips: particularly if I put on heatsinks.  I’ve bought a strip of 1/8″ thick, 3/4″ wide aluminum bar stock for a heatsink—I plan to drill 6 holes in it to bolt the TLE-5206-2 chips to.  It won’t matter if the bar connects the chips electrically, as they all have a common ground connection anyway.

I think that I’ll populate one rev 1.3 board with only 4 TLE-5206-2 chips, the minimum for the robotics club to be able to experiment with their underwater vehicle, and see how it works before making a new set of boards with full functionality.

So, for rev 2.1 I’m definitely going to change the way that the servo outputs are wired and make sure the software can work with the Arduino Mega boards, but I won’t be trying to connect to the Arduino Mega’s other pins. (Hmmm—I just had an idea for how I could pack everything in, with a massive rearrangement of components.  I might try that as separate branch of the design, to see whether it really is feasible.)

2011 July 30

Makers Factory Meetup

Filed under: Makers' Factory — gasstationwithoutpumps @ 23:18
Tags: ,

On Friday 12 Aug 2011, the Santa Cruz New Tech Meetup is gathering at 6 p.m. at
Cruzio & Ecology Action Green Building

877 Cedar Street, Santa Cruz, CA
to discuss the Makers Factory that is starting in Santa Cruz in October.  (I blogged about the Makers Factory already.)

Details about the event are on the meetup web page—I’ve already committed to going, despite the $5 cost.  So far, it looks like they will not be getting the sort of expensive shop equipment that places like TechShop (over the hill) have, but lower-priced stuff suitable for a home shop.  I hope this means that they’ll be a lot cheaper than TechShop, which is $100 a month.

2011 July 26

Time to start doing software

The PC board for the hexmotor H-bridge is ordered and supposedly going to arrive soon, and I’ve ordered all the components from Digi-Key. The total coast of the project is looking like about $180 (including the PC board and shipping) for 2 fully populated boards (not counting an enormous amount of my time). I’ve already started designing a revised board (rev 2.0) that will be designed around the TLE-5206-2 from the beginning, and will have more versatility. I’m thinking that I’ll populate one rev 1.3 board for testing, finalize the rev 2.0 design, and send it out for fab, rather than populating a second rev 1.3 board.

Here is a preview of what my panel of 4 boards should look like.

In addition to the 6 TLE-5206-2 H-bridges, both boards have 74HC594 serial-to-parallel converters for control signals and 74HC86 XOR gates. The serial-to-parallel conversion is similar to that of the Adafruit Industries motor shield (which I have, but which does not have enough current capacity as it uses L293D H-bridges, which only support 0.6A per channel, and I need at least 2.5A per channel).

Copying the choices on the Adafruit board, I made the pin mapping LATCH=Arduino pin 12, DATA=Arduino pin 8, CLOCK=Arduino pin 4. I was wondering at first why they did not use the built-in SPI interface (which would simplify the programming of the serial output), but then realized that the output bit of the SPI interface (MOSI=pin 11) is also OC2A, one of the PWM outputs. Since there are only 6 PWM outputs (one per motor), and the serial interface does not need to be very fast, using software control of other pins is a good idea.

I’ve also brought out all the PWM signals to 3-wire servo connections, so that I can use each PWM signal as either a servo control or a motor speed control. All the connections to the Arduino digital pins are jumpered, so I can reconfigure the board if needed. For example, if I just want forward/backward/brake control of a motor, without speed control, I can rewire the input that would normally be to a PWM bit to be to a different digital pin, and free up that PWM bit for servo control.

On the hexmotor rev 2.0 board I added another 74HC594 chip (in series) so that I have 16 control bits available. Six of them are used for motor control, as on the rev 1.3 board. Another 6 are brought out to header pins adjacent to the speed control pins, so that those pins can be jumpered either to the Arduino PWM pins or to the shift register outputs, allowing 6 motors and 6 servos to be controlled by the board, as long as only 6 continuous values are needed, and the servos don’t exceed the 1.5A capacity of the switching regulator. The remaining 4 of the 16 parallel bits of the rev 2.0 will be brought out to individual header pins, so that they can be used as arbitrary digital outputs. (Also, the hexmotor 2.0 board will have an LED to indicate that 5v power is properly being provided—if the board is used as intended, with the Arduino powered from the switching regulator on the hexmotor board, this tests that all the power connections are made and the Arduino board plugged in.

I will have to start writing software for the hexmotor board soon. I’ll probably base some of the software on the Adafruit AFMotor package, to make it easier for the robotics club to switch between the Adafruit motor shield and the hexmotor board, but there will be some important differences. Since the hexmotor board has several jumpers that can customize it, but no way for the Arduino to read that configuration, I need an easy way to tell the software how the board is configured. I want software that will work with either revision of the hexmotor board. Since the rev 2.0 board has more possibilities for configuration than the rev 1.3 board (thanks to the 8 extra output bits), I’ll design the software for that, making sure that the rev 1.3 board is covered as a special case.

I’m going to have to dive into both the ATMega328 data sheet and the Arduino code to find out how to get access to all 6 PWM signals.  I already know that the Arduino initialization messes with the PWM frequencies, so the AFMotor.h control for the PWM frequencies doesn’t work.  It may turn out that using OC1A and OC2A (which the Adafruit board does not use, but which I use for motors 4 and 5) interacts in some unpleasant way with other Arduino functions.  I may have to put up with some constraints on PWM frequency, as I want to retain clock and analog input capabilities.

2011 July 23

Possibly salvage the boards

My son and I were looking together about whether we could salvage the board design by using the TLE-5206-2 parts that are available.

The pinout is identical, but the functionality is different.

In1 In2 5205 out1/out2 5206 out1/out2
0 0 1/0 0/0
0 1 0/1 0/1
1 0 0/0 1/0
1 1 Z/Z 1/1

The arrangements of signals that I have available to jumper to were S, PWM, and S+PWM.  The easy jumper arrangements I had were (IN1=S, IN2=PWM) and (IN1=PWM, IN2=S+PWM), which with the TLE-5205 were supposed to give me lock anti-phase drive and sign-magnitude drive.

With the 5206, lock anti-phase would require (IN1=PWM, IN2=not PWM), which I don’t have available, but the jumper setting (IN1=S, IN2=PWM) now almost makes sense as active collapse (run/brake) mode (except that it is not quite sign magnitude, since the interpretation of the pulse width is flipped depending which way the motor is supposed to turn).  The other jumper setting makes no sense.

So now I have choice:

  1. Populate a board with TLE-5206 chips and have only the run/brake mode (I could solder in jumpers, since there are no choices).  I could also omit the now-useless OR gates.
  2. Design a new board with the TLE-5206 chips in mind, with jumpering between two modes:
    • (IN1=S, IN2=S xor PWM)   run/brake with consistent magnitude interpretation
    • (IN1=PWM, IN2=S xor PWM)  lock anti-phase if S=1, brake if S=0

Wait a second!  I don’t need a new board for that! If I replace the 74HC32 quad nor-gate with a 74HC86 quad Xor, then the 5-pin headers will be

S IN1 PWM IN2 S xor PWM

I can solder the IN2 pin to S xor PWM, and just have a 3-pin header to connect IN1 to either S (for run/brake) or PWM (for lock anti-phase). The labeling on the silk screen will be wrong, but the board will be fully usable!

The interpretation of the pulse width modulation if  IN1 is set to PWM is that 0 means always on and 255 means always braking, which is kind of weird, but easily compensated for in the software. S determines the direction the motor turns.

If IN1 is set to S, then S is interpreted as RUN if it is high, STOP if it is low, and PWM gives the direction and magnitude when running (with 0 being full on in one direction and 255 being full on in the opposite direction).  PWM should probably be set to constant 0 when braking.

I think the board can be saved, and still have all the functions we needed, though we switched from sign-magnitude to active collapse as one of the modes.  The next revision of the board (if I need to make more) would fix the silk screen and replace the 5-pin header with a 3-pin header for each chip.

H-bridge not available

I’ll be getting a decorative but useless PC board next week.  It is a good thing it only cost me $50.

It turns out that the H-bridge that was recommended to me, Infineon’s TLE-5205, is made of unobtanium.  I thought I had checked Digi-Key’s stock before starting my design, but I must have mistyped the part number, as Digi-Key customer service just called me to say that they haven’t had any since Sept 2010 and don’t expect any in the next 18 weeks.  They do have a few  TLE-5206-2 still in stock, probably because it is a less useful part, not having the Hi-Z output state. I must have checked that part instead of the TLE-5205-2.  Probably no other distributor has any TLE-5205-2’s either, if Infineon has been unable to make any since last September.

I didn’t know that Digi-Key had customer service reps working at 8p.m. on a Saturday night—it is good to know that a supplier cares enough to keep engineer hours.

In any event, I’m going to have to completely redesign the board. The TLE-5205 is unavailable even in the surface mount packages, so I can’t just change packages.  I’ll have to find and use a different H-bridge, which will be a major pain, as the TLE-5205 came close to being ideal for this application. I suspect that since the 5206 is still available, that the problem might be with the freewheeling diodes.  Since the 5206 always has either the high-side or low-side on, it doesn’t need Schottky freewheeling diodes.  Unfortunately, most of the other H-bridges I’ve looked at expect external diodes—the TLE-5205 was attractive because it had built-in diodes, saving a lot of board space.

2011 July 22

Makers Factory in Santa Cruz

Filed under: Makers' Factory — gasstationwithoutpumps @ 01:42
Tags: , ,

I’ve long been envious of people on the other side of the hill who have access to maker spaces equipped with fancy tools and offering classes in how to use them.  I only see the stuff at Maker Faire and on the web—I don’t get to play with it.

Now it seems like Santa Cruz is getting one: Makers Factory — A new way to learn :: A new world to learn.

An image from the Makers Factory home page, showing how they envision the space.

I’m wondering if there is a way I could help out: teaching Arduino programming, for example, or PC board design (which I’ve just been learning myself—see the series about printed circuit boards).  It would be great if the high-school robotics club could get some cheap access also, perhaps in return for volunteer hours.

I’ll have to get in touch with them soon and see what opportunities are available.

2011 July 21

Board panelized and ordered

I used Gerbmerge today to make a larger board out of four of my little boards, since 4 of the maximum-size Eagle freeware boards (8cm × 10cm) will fit in the 60 sq-in area limit of the $33 board from 4pcb.com’s student program.

Here is a preview of what my panel of 4 boards should look like.

It turned out to be much more difficult than I expect to install Gerbmerge.

First I had to install the SimpleParse package for Python.  That was straightforward, once I realized that the big “Looking for the latest version? Download SimpleParse-2.0.0.zip (301.5 KB) ” button on http://sourceforge.net/projects/simpleparse/files/ actually downloaded an old version, and you had to click through the directories to find version 2.1.1.a2, which was needed to get the sublibraries.  Running setup.py from the downloaded directory installed SimpleParse correctly.

Then I downloaded the gerbmerge-1.8 directory and tried installing it.  That was not so much fun, as the setup.py file was badly written.

  • First, I had to edit it to remove the MS-DOS extraneous carriage returns, which Python 2.7 does not seem to like on a Mac—that is no big deal.
  • Second, it failed because it was looking for distutils.sysconfig.get_config_var('LIBPYTHON'), which is a configuration variable that does not exist on my system.  I eventually figured out that what it should have been doing there is distutils.sysconfig.get_python_lib() and got setup.py to run.
  • Third, I still could not get gerbmerge to run, because setup.py put it in the wrong place.  I had to edit 
    fid.write( \
    r"""#!/bin/sh
    python %s/site-packages/gerbmerge/gerbmerge.py $*
    """ % DestLib)

    to   
    fid.write( \
    r"""#!/bin/sh
    python %s/gerbmerge.py $*
    """ % DestDir)

    and remove os.path.walk(os.path.join(DestLib, 'site-packages/gerbmerge'), fixperms, 1)

After all that fussing, I finally got Gerbmerge to run without crashing.  I then had to put together a configuration file to explain how I wanted the panelization job done.  The configuration file is not intuitive, but it is well-documented with an excellent sample to modify.

The statistics reported by Gerbmerge did alert me to one minor problem: the vias used by the auto-router used a different size hole from the ones I had placed by hand.  I figured out how to change the auto-router via size (it grabs the smallest legal size from the design-rule checker), ripped up the auto-routed stuff, and rerouted it.  That introduced some design rule violations, where newly placed vias were overlapped by some of the silkscreen, but I fixed that my manually moving the offending vias and rerouting to them.

I checked the combined board both with FreeDFM from 4pcb.com and BatchPCB, and both accepted it with no complaints.  BatchPCB charges by the square inch, so it would cost $138.57 for the 4-up board from them, more than the $124 for the boards already properly cut apart.

I ordered one of the 4-up boards from 4pcb.com for $33 plus shipping, for a total of about $50.  I only found out afterwards that they have a $50 surcharge if you have multiple copies of the same design on the board.  I hope that they don’t charge me that surcharge!  If they ask, I’ll cancel the order and redo with 4 slightly different designs (maybe customizing the silkscreen for the robotics club).

In fact, I just realized that I could probably define a few new “customization” layers in Eagle, and automatically generate 4 slightly different designs from a single .brd file.  If I have to go that route, I will, and I’ll certainly not make the mistake of sending 4 identical designs to 4pcb.com.  If they want to put in arbitrary rules for their student designs, I’ll play rule lawyer with them.

More design feedback

My friend, Steve, who has done a lot of PC board design, looked at my design and provided some detailed feedback and some questions for me.

First, he noticed that one of my ICs did not have a bypass capacitor, and that one of the other bypass capacitors was badly routed.  I’d been worried about that myself, so having it be his first comment told me I really had to fix it.

He also suggested that all the logic GND signals have only a single point of contact with the motor GND signal, to avoid ground loops.  I’d not thought about ground loops, and realized that I had a redundant contact make a ground loop that ran through the Arduino board!  I removed the extra ground routing and re-routed the logic ground wires to run to connect to the end of the motor ground so that they were as separated as possible from the noise injected by the motors.

He asked me several questions about heat dissipation, also, but I was ready for those.  The TLE-5205 chips I’m using have only 400 milli-ohms of on-resistance (for high and low legs added), so at 2.5A (where I expect to use them) they should be dissipating 1W.  With a junction-to-ambient thermal resistance of 65ºK/W, I should see a temperature rise of no more than 65º, up to 90ºC.  That is pretty hot, but well below the 150ºC limit.  If I add a chunk of aluminum as a heat sink, the temperature rise should be much less.

Steve also noticed a problem with the 74*32 and 74*594 parts on the board.  One was given as 74HCT32 and the other as 74LS594.  He recommended that both be 74HC series, which is really what I intended, but the 74xx-us Eagle library had not included that technology for those parts.  I went into the library and added the HC and HCT technologies (just a button click!), so that the parts are now properly labeled.

My other friend, Gabriel, suggested that if I went with the $33 4pcb.com board, I could perhaps put multiple boards on the panel and cut them apart with board shears at work.  I’ve never seen board shears, but I understand that the Jack Baskin School of Engineer has some that I could use.  I picture them as looking like a heavy-duty paper guillotine, making one cut across the board at right angles to the edge.  (Gabriel says that is indeed what it looks like.)

Of course, since I’m at the maximum design size (8cm×10cm) for the freeware version of Eagle, I’ll have to find a tool to panelize the design. Gabriel suggested Gerbtool to do the panelizing (the same tool that Steve was using to look at the designs, and the one that they teach students to use in computer engineering and electrical engineering courses), but that is a Windows-only, license-fee tool. I wanted to do this task with free tools as much as possible (not only because I’m cheap, but so that other hobbyists could try the same things), and I have a strong aversion to Windows.

I looked around for a free tool (now that I knew that the keywords were “Gerber panelization”) and found GerbMerge, which is free, open-source software written in pure Python 2.4, which should be installable on any Mac OS X, Linux, or Windows machine.  I’ll try downloading it tomorrow and see whether 4pcb.com will accept the output. Since my design is only 8cm×10cm, I could put 4 boards down in 17cm×21cm, which at 55.34 sq-in is still under their 60 sq-in limit.

If I can get 4 boards for $33+shipping from a high-quality fab like 4pcb.com, I’ll be happy.  At some later date, I’ll probably release the design through BatchPCB, which would allow others to make copies for about $31 each, but probably not until I’ve tested the board myself and gotten the robotics club to solder one up successfully.  I need to make up a bill of materials for populating the board and put in an order with Digi-Key.  The expensive parts are the switching regulator and the H-bridges, which will come to about $70, I think.  I’ll probably buy a set of parts for myself, but ask Infineon and TI if they will donate samples for the high-school robotics club.  I’ll be happy if we come out of this project with two fully populated working boards for under $200 in parts (counting my time as freely donated volunteer time).

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