I previously discussed a lot of issues I experienced with my first robot build that stemmed from a lack of planning. To avoid those pitfalls this time around, I decided that a good place to restart things would be a detailed wiring diagram for the robot.
Between 2014 and several months ago, I managed to fry the Sabertooth 2X32. I’m not entirely sure how it happened, but the excellent support folks at Dimension Engineering informed me that moving the robot manually can generate enough back current to fry it.
On my previous build I had a switch between one of the positive battery terminals and the Sabertooth B+ contact. The Sabertooth is a regenerative motor controller, meaning any current that enters from the motor leads get fed back into the batteries as I understand it.
Well when that switch is off, current generated by the motors has nowhere to go. Wheeling the robot by hand into the garage generates current (it’s basically a DC generator at that point) and that could have created enough current to blow the Sabertooth I guess.
Alternatively, the geared motors on the robot have a lever that can be used to put the gearbox in “neutral” so that it’s easier to push and so that wheel rotation doesn’t rotate the armature of the motor. If I were smart I would have just flipped the lever.
Dimension Engineering kindly offered to replace my blown Sabertooth 2X32, but I decided to upgrade to the Sabertooth 2X60 instead. For an extra $65 you get a cooling fan (more on that later) and twice the current carrying capacity. Definitely overkill, but one the things I learned on my previous build was to not skimp on quality and robustness.
Notice the two 30A fuses on the M1A and M2A motor leads. If you do get large back current, the fuses should blow before you damage the Sabertooth motor controller. Spending $5 is cheap insurance to protect a $200 part.
Mauch BEC and Current Sensor
Because the Sabertooth can’t output enough current at 5V to fully power the Pixhawk and it’s accessories, I had to find an alternative way to power them. However, because my power supply is two 12V 35AH SLA batteries, I was hesitant to use the stock BEC that comes with the Pixhawk. And in any case, I couldn’t use it to control my motors with, so I thought it would be worth it to find an alternative solution.
That’s when I found Christian Mauch’s line of BECs and current sensors. The wiring for these guys is a little weird, at least to me, but it makes sense once you examine it.
- The BEC is connected to the 24V battery supply, but after the current sensor.
- The BEC steps down the 24V supply to 5.30 +/- 0.05V at 3A and feeds it into the current sensor.
- The current sensor has a 10 gage wire that sits between the positive terminal of the 24V battery supply and everything else, so it can measure current consumption.
- The current sensor then has a 6 wire output that is fed straight into the power port on the Pixhawk.
Got all of that? Here’s the diagram from Christian’s website.
That 5.30 +/- 0.05V is written intentionally. The Pixhawk decides which power supply to use by selecting the supply with the highest voltage. If the servo rail provided, say, 5.2V and the power port on the Pixhawk was supplied with 4.9V, the Pixhawk would use the supply on the servo rail. Christian designed his BEC to output on the high side to make sure the Pixhawk selected his BEC with a clean output for power.
According to the documentation, the HS-200-HV is capable of measuring up to 200A of current. Yep, that’s 200A. And you thought the Sabertooth 2X60 was overkill.
The reason for this choice isn’t completely unfounded. Down the road I may want to have an electric motor for the cutting blade on the mower. I’m not sure how much current that will draw, but it won’t be chump change, I imagine. Hopefully this will provide flexibility in the future.
New Three Position Switch
To charge the SLA batteries on my first setup, I had to remove the two terminal lug screws and attach a 12V trickle charger to the battery. I had to do this twice, once for each battery. Kind of tedious.
On this new configuration, I intend to have a three position switch: one position for on, a neutral position where nothing is connected to the battery, and a charge position, where the batteries will be connected to an XLR port for a 24V XLR charger.
The idea is that you flip the switch up for operating the robot, and down for charging the batteries. For storage you have the neutral middle position so that the charging port isn’t energized by the batteries.
Hopefully these new features will correct some of the problems I experienced on my first build. This wiring diagram will also prove invaluable for the next step of planning.