I’d like to introduce the newest mjbots product, an updated revision of the power_dist, 4.3b available at mjbots.com today!

This version has a number of improvements over the previously released r3.1:

	r4.3b	r3.1
Voltage Range	10-44V	8-34V
Maximum load capacitance	4,000 uF	400 uF
Quiescent Current	300uA	5mA
Current (Continuous / Peak)	45A / 80A	unrated / 100A
Energy Monitoring	YES	NO
Switch Mode	High Side	Low Side
Dimensions	50x80mm	45x70mm
Price	$139	$79

The only real downsides are that is more expensive and slightly larger.

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.

The moteus controller uses a somewhat unique integrated position / velocity / torque controller with per-command configurable proportional and derivative gains. Through various combinations of these settings, it can emulate many different types of controllers, but one that it has struggled with until now was a pure velocity controller.

It has been minimally possible to use moteus as a purely velocity controlled since wraparound support was implemented, but that came with a caveat. Either the proportional term needed to be set to 0, in which case velocity tracking performance was poor, or if the proportional term was non-zero, an external torque would cause the position to drift arbitrarily far from the target position. Then if the external torque were released, the controller would “catch up” for all the lost ground, moving very rapidly.

To date, the pi3hat CAN channels only supported CAN properties suitable for use with moteus controllers. Given that’s what most people are using them for, that’s fine. However, there was no real constraint behind that, just laziness.

Thus, I’ve released new firmware for the pi3hat that supports configuring the bitrate, FD-ness, and other properties of all 5 CAN channels.

• https://github.com/mjbots/pi3hat/releases/tag/0.1-20210111-1

Currently only the C++ library exposes the configuration functionality, but it will be easy enough to add to python when someone needs it.

mjbots.com had a week long run where we were completely out of fdcanusbs, which meant that we were also out of all development kits too. Well, a production run just came in:

So now we’ve got everything back in stock once again!

• fdcanusb
• moteus r4.5 development kit
• qdd100 beta 2 development kit

It seems like all the posts I’m writing these days are for new products! Here’s the pi3hat r4.4:

There are two changes from the previous r4.2. First, it now supports voltage inputs up to 44V. Second, in support of future upgrades, the 5th CAN-FD port has been upgraded to support 8Mbps, but downgraded to no longer have a wide common mode voltage range.

THUS, IT IS NOT SAFE TO CONNECT THE CAN-FD PORT ON THE pi3hat r4.4 TO A power_dist r3.X BOARD.

I’d like to introduce the qdd100 beta 2!

• 

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This is the newest version of a quasi-direct-drive servo from mjbots. It has a sleek new look, and improved performance all around:

	Beta 1	Beta 2
Peak Torque	12.5 Nm	16 Nm
Backlash	+- 0.2 degrees	+- 0.1 degrees
Voltage Range	10-34V	10-44V
Mass	470g	475g

Comparison from beta 1 to the new beta 2

Additionally, the M3 mounting holes are now 3mm deep instead of the previous 2mm, which gives more flexibility when designing mounts.