2011 September 23

New board designed

I’ve just sent a new set of boards for fab.  The HexMotor rev2.3 boards have several new features: LEDs for +5v and +6.25v, a reset button, 16-bit shift register instead of 8-bit, servo outputs connected to pins 13, 7, 2, 9, 10 (rather than to the pins used for PWM).  The new board should be able to do either 6 PWM motors or 4 PWM motors, 5 servos, and 2 non-modulated reversible motors.

I’m also making some breakout boards for the MPXHZ6250A pressure sensors from Freescale Semiconductor, which will require doing some SMD soldering.  At least the design uses gull-wing pins, which can (supposedly) be hand soldered.  One of the breakout boards also has a place for mounting an ADXL335 accelerometer, which may be more difficult to solder.  I don’t think I want to spend the money for a hot-air rework station, and I’m a bit dubious about my ability to solder using a toaster oven.

The pressure sensors are tiny!  My original suggestion to the robotics club was to drill a hole in the dry box and superglue the pressure sensor to the inside of the box (after the pressure sensor had been attached to the breakout board, of course).  Now I’m not so sure that there will be enough glue area to hold firmly enough.  Perhaps a dab of some sealant on the outside of the box might help, if we can keep from plugging the hole in the sensor.

The breakout board that I think that the robotics club will end up using puts a pressure sensor on one side and headers for a piggyback ADXL335 breakout board on the back.  that way there only need to be one set of wires for connecting the analog inputs and power to the sensors.

One limitation of the Arduino for use with this combination of sensors is that the accelerometer is a 3v part and the pressure sensor is a 5v part. We’ll have to set up the analog-to-digital converter on the Arduino to have a 5v range, which reduces the precision of the acceleration readings.

I’ve also bought some other sensors (not for the underwater vehicle, but for physics class and dry robotics): a couple of ultrasonic rangefinders.  More on those in a separate post, after I’ve had a chance to play with them.

2011 August 9

Board fully populated and tested

Top view of the HexMotor rev 1.3 board fully populated.

I got the board fully populated yesterday, plus I made a heat sink out of a piece of ¼” × ¾” aluminum bar stock.  The heat sink makes an enormous difference.  Before using it, running a small motor at low power for 20–30 seconds was enough to make the H-bridge uncomfortably hot.  Now running the same 12v motor stalled at full current (3A) for a full minute raises the temperature only to 100ºF.

The motor got warmer than that, and I think I burned it out, since it no longer runs and has a 400kΩ resistance.  I’m not really surprised—it was a cheap door-lock actuator, and only intended to be used with fraction of a second pulses.  Further testing will require a more robust motor.

As you can see from the photo, the screw terminal for motor 4 (second from the bottom) is a bit crooked—I’ll have to unsolder it and straighten it.  For the HexMotor 2 board, I’ll use slightly smaller holes so alignment is easier.

The thermal grease I used (Cooler Master IceFusion High Performance Thermal Compound 40G RG-ICF-CWR2-GP) was more liquid than I expected, especially since it comes with a little spatula for applying it.  I had to put on a fairly thick layer, because the aluminum extrusion was not very smooth, and when I tightened the bolts the stuff oozed out making a sticky mess. Next time, I’ll sand the aluminum smooth first and use much less thermal grease.

Closeup, showing the excess thermal grease puddling under the H-bridges, where it is very difficult to wipe off.

This closeup photo, in addition to showing the pooled excess thermal grease, shows the header pins with shorting jumpers to configure the H-bridges for either lock antiphase or sign-magnitude control.  (Because of the last-minute change from TLE-5205 to TLE-5206 chips, the silk-screen labeling of the header pins is wrong—this board is actually configured for sign-magnitude throughout not lock antiphase.

The photos also show that I did not leave room for the heat sink between the electrolytic capacitors.  The HexMotor 2.0 board will fix this problem also.

The HexMotor software now can handle 3 different boards: the HexMotor rev 1.3 board shown here, the Adafruit Motor Shield, and the HexMotor rev 2 board, which I am just about done fussing with the design for.  I’ve only tested with an Arduino Duemilanove board, but the software should work with an Arduino Mega as well.

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