Stepper Drivers
The BeBoPr can either control external stepper drivers connected to connector J5, or use Pololu compatible modules that plug directly into the board. At first I've been using the BeBoPr with a separate TB6550 board (see first sign of live video) connected via J5. As the plug-in modules have become more powerful, I've started using these instead. Below is an overview of the modules that I've used and found to be compatible with the BeBoPr.
The older modules use Allegro A498x driver chips, of which the A4988 is the most popular. These chips can drive most of the smaller motors. The latest modules have a more sophisticated board design and use the Texas Instruments TI8825 driver chip. Better cooling and lower on-chip losses allow for higher currents with these devices, if properly cooled. Most modules come without a heat-sink, or with a very small one that attaches on top of the driver chip. This only allows for a moderate motor current, as the heat being generated is roughly proportional to the current squared. So twice the amount of current will generate four times the amount of heat! That's why the newer modules take special measures to get the heat out, often from the bottom of the chip that has an exposed die or heat transfer pad.
The Allegro based devices give you up to 1:8 or 1:16 micro stepping, depending on the chip being used. The Texas Instruments TI8825 gives up to 1:32 micro-stepping and up to 2.5A continuous motor current.
As the TI8825 bases modules are only slightly more expensive than the older A498x based ones, the choice may be obvious. This currently leaves two competitors: The Pololu #2133 (md20b) module and the Panucatt SD8825, each with its own pros and cons:
pros:
- plug-in compatible with older A498x modules, no software change needed
- broad availability
cons:
- very coarse Vref adjustment because only the first 35% of the adjustment range is used
- Vref probe pad no accessible at bottom of board (instead, measure very carefully on wiper of R1)
- comes without heat-sink and tape
- needs soldering of the header pins to the board
measurements: TBD
pros:
- has a pad to probe Vref voltage when adjusting the motor current
- fine tuning of Vref, using full range of the trimmer
- includes heatsink and tape
- no soldering required, has header pins already assembled
- attaches heatsink to the PCB instead of to the chip, for better cooling
- can use extra pins with decay mode jumper on TAKE-5 carrier.
cons:
- has reversed motor direction compared to all (most?) other modules, may need a direction change in the software. Or change the wiring (reverse one coil) to change the motor direction.
dissipation:
- The Panucatt modules were tested with Vref set to 0.75V for 1.5A maximum motor current. Natural convection was used for cooling. After one hour of continous operation, the temperature of the heatsink reached a very acceptable 60-65 degrees Celsius. About the same temperature was measured at the top of the chip. This corresponds with between 1.2 and 1.5 Watt of generated heat, which matches the calculated power dissipation in the chip.
Thus far the Panucatt module seems to have only one drawback: The reversed motor direction. But that's a problem only if you want to swap modules often (as I tend to do). Further measurements must show if the difference in design between the two TI8825 modules will affect performance. TBC