While continuing to bring up the r4.1 moteus controller I reached a mini-milestone where it spun a motor for the first time!
After having produced the first functional demonstration of the moteus servo mk2, my next step was to decrease the weight. While I was at it, I made two other changes:
- Axial connections: I switched to a design with entirely axial connectors, which removes the need for 4th axis machining when producing the parts.
- Planet Input Bearing: I switched the planet input bearing to be inserted from the rotor side. This way, the bearing is captured between the planet input and the rotor, rather than between the planet input and the gears. That also improves the ability to assemble and disassemble the unit.
Slimming down the housings drops around 100g from the total weight. I may still try to take out some weight from the planet input and output, but they don’t weigh much to begin with so the potential gains there are small.
Next up I’ll manufacture and assemble this, then get it running.
Another step in my plan for the next revision of the moteus servo mk2, is an updated controller board. As mentioned in my roadmap, I wanted to revise this board to make improvements in a number of domains:
- Communications: Now instead of RS485, the primary communications interface is FD-CAN. This supports data rates of up to 8 Mbit and packet lengths up to 64 bytes. The header is nominally at the original CAN bit rate, but I have no need to be standards compliant and am running very short busses so I may run everything at the higher rate.
- Connectors: Now there exist power connectors, in the form of XT30 right angle connectors and they are also daisy chainable like the data connectors. Additionally, all the connectors exit from the bottom of the board to make routing easier in configurations like the full rotation leg.
- Controller: This uses the relatively new STM32G4 controller series. It is lower power than the STM32F4, supports FD-CAN, and also supports closely coupled memory, which may allow me to improve the speed of the primary control loop execution by 3 times.
- Voltage range: This board now has 40V main FETS, with all other components at 50V rating or higher. Thus it should be safe with inputs up to 8S (34V or so).
It still maintains a number of the capabilities of the moteus r3.1 controller:
- Integrated FOC encoder: An AS5048 encoder is mounted in the center of the back, which allows direct mounting above the rotor for FOC control.
- Form factor: The entire board is 45x54mm, with M2.5 mounting holes. It is smaller than a 60mm BLDC motor and much smaller than an 80mm one.
- Integrated current sensing: It uses a DRV8323 to drive the FETS, which includes current sensing for closed loop current control.
My first attempt at this, “r4”, came back from fabrication in an nonredeemable state. I used the digikey supplied footprint for the STM32G4 UQFN part, which looked mostly correct on the surface. However, while the footprint was good, the pinout was for the TQFP variant! This resulted in me shorting out several power pins to ground right next to the exposed pad in a way I couldn’t easily rework.
r4.1 seems to be in better shape so far. It powers up, and I now have blinking lights!
Next up is actually porting the control software to the new controller and communications interface.
The back housing is the final piece of the moteus mk2 servo that I wanted to prototype. (The planet output is identical to the mk1, so I could use extra stock I had of it for the prototypes). It is large, and only mates directly to 4 other things, which makes it a little less complex than the front housing.
I had initially designed the back housing to mate to the as-yet-unannounced new version of the moteus controller, the r4.x series. Unfortunately, I don’t have any of those working yet, so I tweaked the design to temporarily fit a r3.1 controller, which looks like this:
It mates to the rotor bearing in the center, the outer housing around the perimeter, provides support, mounting and heatsinking to the controller, and provides a mounting interface for the controller cover.
The only design intent change I made in the mk2 version here was to decouple the outer housing interface axially from the rotor bearing interface. In mk1, the back housing was a flat plate. Now in mk2, it is a very shallow cone, to slightly reduce the axial length of the large diameter outer housing and eventually make it possible to reduce the weight more because of it.
Like the front housing, this was challenging to produce on the Pocket NC. It required even more steps than the front housing, since it had no final flat surface which also had threaded holes. Thus I ended up using a manual machining stage, followed by 3 operations on the Pocket NC.
The manual machining merely took 4in round stock that was rough cut to 5/8″ and faced it to approximately 15mm, then drilled a center 1/2″ hole.
For the first Pocket NC operation, I used a 3d printed bracket to clamp the stock down, and then the four M2.5 holes used to secure the back cover were drilled and threaded. These will be used in the next operation.
In operation 2, those holes were used to bolt the stock to another 3d printed fixture, which was then bolted to the same 3d printed assembly as used in operation 1 (and for the front housing).
The final step of operation 2 drilled and threaded 4x M3 holes on the back surface purely for fixturing purposes. They are used to bolt the in-progress work to yet another fixture, so that the rest of the back can be machined.
Here’s a video showing all the machining steps:
The front housing is the most complex machined piece in the moteus servo mk2, as it was in the mk1. It is relatively large and mates with many other components with the associated tight tolerance surfaces. For mk2, the front housing is even larger in diameter, but otherwise has the same basic features.
Building a prototype of this was a real challenge given the tools I have available to me now. For mk1, I didn’t even try and just had Xometry build my prototypes, and was lucky enough that the first ones worked. My only CNC currently is the Pocket NC v2-50, which is just barely big enough to deal with this part, and has no convenient workholding that can be used for the stock. Also, it has a low material removal rate, such that starting from stock here would be prohibitively time consuming.
My approach was similar to that of the outer housing, in that I prepared the materials on the Artisan’s Asylum manual machines, then did the remainder of the machining on the Pocket NC.
I started with 4″ diameter round stock cut to 1″ +0/0.125:
Then, on the mill, I faced the parts to the correct 22mm height:
Then I drilled a 5/8″ hole close to the center:
At this point, I switched to the lathe and performed some roughing operations, removing some of the OD and ID to reduce the amount of cutting the Pocket NC needed to do. These were done with some 3D printed spacers to help align the stock in the lathe’s 4 jaw chuck.
Now that the part was roughed out, I first mounted it to the Pocket NC using a machined aluminum plug bolted to a custom 3d printed plate.
My first attempt used a 3D printed plug, which just delaminated and failed, thus I re-drilled some more holes for this first prototype offset from the original 8. This operation threaded the holes on the front side, which are used to secure the piece for the second operation.
In the second operation, a second custom 3d printed fixture is bolted to the newly drilled front holes, and then bolted to a plate that is mounted on the B axis. This enables the Pocket NC to reach the full back side and around the perimeter. The bracket needed to be slightly offset from the center of the B-plate, otherwise the mill couldn’t reach all the way around.
Datron 4mm endmill
This was the first part for which I used a Datron 4mm endmill on the V2-50. It is definitely a good tool, although it has its shortcomings. The biggest win is that it can remove material at more than double the rate of anything else I’ve got. Granted, this isn’t exactly fast, but it is fast for a Pocket NC and makes parts like this front housing even remotely feasible. It also produces a nicer surface finish, and has less deflection so straight walls are straighter. For simple geometries I used 47k rpm, 0.25mm optimal load and 4mm stepdown and then slowed those down when dealing with the more complex shapes.
There are some downsides though. One, it uses a 4mm collet, which negates much of the value in having the convenient tool changing handle. It takes a longish time to switch collets so I have to arrange toolpaths to do all the 4mm collet things together. Second, it produces more chips than just about any other tool I’ve used on the Pocket NC. So much so, that I had to schedule breaks every 45-60 minutes in the program just to clear out chips. Otherwise the chip tray became too full to use effectively. For some geometries, such as when clearing pockets, the chips are thrown onto the X ways and require even more frequent clearing, otherwise the mill can’t move to the full X extent as it squishes the chips against the front side of the enclosure.
Here’s a video of the third one I made, which used finalized fixtures for all operations.
And here’s one of the front housings with all the various things mated in place.
The outer housing for the moteus servo mk2 is just a precision round tube with some mounting holes drilled peripherally. Still, manufacturing it was slightly annoying, mostly because of my available machining resources.
I started off with round tube stock with some extra margin on the inside and outside:
Then I went and used the manual lathe at Artisan’s Asylum to get the correct ID, OD and length:
At this point, I loaded it into the Pocket NC with Sherline 4 jaw chuck, using a 3d printed bracket to align the assembly with the base of the chuck.
Now, I could use the B axis as an indexer, and drill and countersink all the holes.
As the first part of the new moteus servo mk2, and continuing in my series of learning about CNC by building parts for the quadruped, next up I machined the input to the planet gears on my Pocket NC V2-50. This was a part, that for my quad A0 build, I used a 3d printed part in PETG as it is probably the least stringent part in the gearbox in terms of tolerances and load, although I still expect the plastic ones will likely wear and fail after some time with heavy use.
In the gearbox, this planet input interfaces to a number of different sub-assemblies:
The planet output inserts into its studs, the planet shafts insert into recessed holes in the face, and the planet input bearing fits into its center. Bolts fit through the back to pull the planet output towards the input. There is also an indexing cutaway on the outside for an eventual absolute position system.
I made these from round stock, 1.75in in diameter cut to 1″. The machining was done in two setups, each using the Sherline 4 jaw chuck mounted to the B axis of the Pocket NC.
In the first setup, the back side was roughed out, the center hole was cut out, and each of the holes for the bolts was bored along with a countersunk region.
The second setup, flipped over, first roughed, then proceeded to finish each of the necessary surfaces. There were two more interesting bits here. First, I made an alignment fixture so that I could get the holes from the back and front half to align. That consisted of a cylindrical shell that fit into the mounting pattern on the B table of the pocket NC and a thin plate that fit under the part. The thin plate just stays in place during the machining operation, where the shell pulls out.
The other interesting part was that I ended up clearing away a bit of the inside of each stud so that my bit could reach down to finish the bearing mounting surface. That way I could get away with just using an 11mm flute, which already gave a terrible enough surface finish that going longer would have only made worse.
As I’ve observed in the past, I had yet another problem with tool pullout during this part. Here, the problem was very similar to my past incident, when Fusion left a thin wall, then tried to punch through it. My fix from then was doing the right thing, however the wall was just too thick I believe, causing the toolholder to lose grip. In this clip, you can see that after it breaks through the wall, the mill cuts through some stock as it repositions over. The slip was only about 0.3mm, but that’s enough to mess things up.
However, this time I think I figured out an even better solution. Simply lie to Fusion 360 about the diameter of the cutting tool and say it is slightly undersized. That results in fusion leaving the adaptive passes closer together, and thus no thin walls or foils are left behind. It would be nice if that were just an option in the adaptive settings. I suppose you could override it in the “Compare and Edit” window, but creating a faux “tool” just for roughing makes it easier for me to see that I’ve applied the override correctly.
Here’s a video showing the different tool paths for the finished part:
The stock cut of 1″ is oversized in this part and adds a bit more than an hour to the cycle time over a minimum sized piece of stock. I need to get a setup for cutting stock smaller than an inch here soon.
Also, I’ve discovered that ekramer3 has been testing the 4mm single flute Datron endmill, which should be able to nearly double the MRR for the roughing passes. I’ll give that a try on the next part I make.
As described in my roadmap, making a new revision of the moteus servo is up there on my list of things to do. The initial servos were a work of art, yes, but also pretty fragile, very labor intensive, and still not all that robust. My goals this time around are:
- Manufacturability: The servo mk1 took about 2 or 3 man-days of manufacturing time per servo once all the steps were factored in. I’d like to get that down to an hour or two at most per servo.
- Robustness: The planet input, outer housing, back housing, and controller cover of the mk1 servo were 3d printed, mostly to save cost and time. This necessitated adding reinforcing rings on the outer housing, as it is nearly impossible to 3d print something with the required material properties in a single print. At this point, all of these components should just be made of aluminum like the others.
- Repairability: Once the mk1 was assembled, there was no way to disassemble it, as installing the stator interfered with the ability to remove the outer housing, and the outer housing in place interfered with the ability to remove the stator.
- Convenience: The mk1 servo used the r3.1 moteus controller, which had RS485 connectors sticking straight out the back, and bare power wires coming out the back. That orientation for connecting things was not terribly convenient in the full rotation leg design, and required making extension cables. The newer moteus controller has the connectors sticking out the bottom, so the servo needs to accommodate that.
Changes in moteus servo mk2
The current design for the moteus servo mk2, as the exploded video above shows, has a number of changes.
First, the outer housing has been changed to be purely cylindrical. This allows it to be machined out of round tube stock, and also assembled and removed in any order. Thus the front housing now has a slightly larger outer diameter, and has threaded holes around the perimeter and 8 primary mounting holes instead of 6.
The rotor is custom machined, so that the sun gear holder assembly is no longer required. A not shown in the explosing mini 3d printed adapter will hold the magnet and fit inside the rotor bearing on the back.
The planet input now has a small indexing slot to eventually register a magnet holding assembly that can be used to sense the position of the output stage, and a position sense board is installed in the front housing to sense it.
The back housing has been updated to mount a newer moteus controller, provide heatsinking to it, and also be slightly slimmer due to being manufactured from aluminum.
The overall dimensions are approximately the same as the mk1, with the depth increasing by a few millimeters (largely because of the connectors on the new moteus controller), and the outer diameter decreasing by a few millimeters. I believe I should be able to get the weight to be about the same as the mk1, around 430-450 grams.
First, I’ll make a functional prototype to verify that all the parts fit together and work. Then I’ll work to get the weight back down to closer to the mk1, after which I’ll start producing enough of these to make more robots.
Some time ago I put in orders for all the long lead time items on a second version of the moteus servo. This is primarily aimed at improving the manufacturability and reliability, along with some minor performance improvements. I’ve now got at least samples of all the long lead time parts in house!
Coming up soon I’ll post a more detailed design update on the servo.