On the mjbots discord, people are often looking for the cheapest possible way to command and monitor a moteus controller. One possible solution that comes up over and over again is the CANBed-FD board, as sold by Seeed Studio (and others). I decided to get one of these in house to see if I could make it work:

The first thing I figured out was that the DB-9 connector used a non-standard pinout for CAN_L and CAN_H. I just switched to the terminal block connections instead of the DB-9 to get around that. For the software, I decided to use the acan2517FD Arduino library, as it was quite a bit more robust and featureful than the one provided by Longan Labs.

For context, see part 1, part 2, or part 3.

r4.0

My first attempt at an r4 design was based on the TI LM5066 hot swap controller. It is one of the more full featured controllers, since it supports built in energy monitoring over a SPI bus with no additional components. This first iteration was actually surprisingly close to being workable. There were two factors that it performed poorly on, quiescent current and energy monitoring. The quiescent current was similar to the r3.1 version. Energy monitoring was present, but at the full scale range necessary for power_dist, it was almost unusably inaccurate. With a design set for a peak of 100A, and also the lower 25mV current sense range, the current noise was measured in multiple amps.

This is one of a series covering the new mjbots power_dist board. See part 1 and part 2 for more context.

As mentioned previously, hot swap controllers are primarily used to allow a card to be inserted live into a server backplane, while minimizing disruption to the primary power bus while doing so. Additionally, they often implement protection features like over-current and short-circuit protection, and some support energy monitoring.

Typical topology

A typical hot-swap topology looks like:

Last time I covered the limitations of the power_dist r3.1, here I’ll cover some iterations of the design process.

My initial design goals for this version are based largely around improving the major limitations identified before:

• Positive side switching: By switching the positive rail, a whole class of use failures is removed, as most people expect ground to be common throughout a system.
• Increased voltage range: moteus r4.5 and the pi3hat both support 44V, so any new power_dist board should support at least that.
• Lower quiescent current: Ideally, the quiescent current would be measured in microamps, or at least at a level that it does not confuse BMS systems.
• Energy monitoring: Often in the development of the quad A1, I wanted to have a system level power and energy monitoring solution so as to identify the energy cost of various maneuvers and gaits. Tracking that at the power_dist level seems like a logical place.
• Wider load envelope: The 3.1 version had a relatively limited maximum downstream capacitance and turn-on current draw. It was enough to power on 12 moteus controllers and a small computer, but not much else.

To achieve these goals, I decided to try using what is known as a “hot swap controller”. These are integrated circuits that are intended for use in cards that plug into server backplanes. Given that any given card could potentially have a large decoupling capacitance, inserting it live into a backplane could cause arcing, and high currents that cause the overall bus voltage to drop outside of tolerable limits.

The current iteration of the mjbots power_dist board released back in the summer of 2020 is pretty useful. It pre-charges the input, provides a soft switch, and gives you a bunch of output connectors to make wiring easier.

r3.1 Limitations

However, this version did have some limitations and potential problems. The first is that the pre-charge method it uses, a simple on/off pre-charge resistor, is unable to support a wide range of supply voltages. Either the resistor has a low value, in which case large input voltages will cause thermal failure, or for larger values, it isn’t able to actually pre-charge the bus sufficiently before engaging the primary MOSFET.

While testing some variants and new versions of the power_dist board, I wanted to be able to simulate the types of loads that it experiences with a fully loaded robot. Some things are easy, like this capacitor attached to an XT30 connector:

I also have giant power resistors in a similar form factor:

However, a dumb load resistor isn’t a particularly representative load. Most likely, the loads that a power_dist will drive are active loads with switching regulators. When the output voltage is lower, the current will be correspondingly higher. That is especially important when validating pre-charge behavior, because it means that the current is much higher during the initial pre-charge window than it would be for a pure resistive load.

I’ve been using a relatively inexpensive microscope for SMD soldering work for some time, connected via HDMI to a 24" monitor.

For the price, I’m definitely happy with it, but as I’ve been doing more soldering work, I’ve become less happy with the mounting stand. The arm it mounts to often does not reach far enough to get the optics over the part of the board in question, or the base is too tall or wide to fit under it. If you want to examine something from the side, you have to tip the entire base over. I have resorted to spinning the microscope around and counterbalancing the base with a large weight, which works for some definition of “works” but only improves the reach by a little bit.

Various users have been trying to use lower-cost Raspberry Pi CAN-FD adapters for the moteus controller for some time (like this one from Seeed), but have had problems getting communication to work. I buckled down and went to debug the problem, discovering that the root of the issue was that the linux kernel socketcan subsystem calculates very sub-optimal CAN timings for the 5Mbps bitrate that moteus uses. This results in the adapters being unable to receive frames sent at the actual 5Mbps rate, but instead only slightly slower.

In the spirit of “most of your users are yet to come”, I’ve gone through a moderate re-organization of some of the mjbots github repositories to make them more usable and sensible.

The biggest change is that mjbots/moteus has had a history rewrite and a directory structure re-organization. The current development (and default branch) is now “main”. The biggest changes:

• Proprietary pdfs have been removed
• All hardware which isn’t related to the moteus PCB has been removed
• The firmware source has moved from “moteus/” to “fw/”
• The client side utilities moteus_tool and tview have moved to the “utils/” directory
• There is now a “lib/” directory which houses client side libraries

Second, mjbots/power_dist is a new public repository that houses the power_dist board firmware and software.

I’ve muddled along for a long time soldering with a little Weller WES50. I’ve done a lot of work with it, but given how many SMD boards I’m doing with big ground planes and tiny components, I needed something a bit more capable. Enter the Pace ADS200 from tequipment:

Made in the USA, with 120W of power and a wide range of tip selection it has been an incredible upgrade. All those soldering jobs which were painful before are so much easier, and I don’t even have all the tips I wanted for different jobs yet. I also have the MiniTweez, but don’t yet have the tips that will let me show that off. I’ll try to post some soldering videos in the not too distant future.