The weakest point in my Sawppy V1 rover were its shaft couplers, but that bug also became an accidental feature: when the system is abused, being the weakest link meant it is a sacrificial element to break before anything more expensive would. I am now designing the servo steering mechanism for Micro Sawppy Beta 3 (MSB3) and wanted to know: do I also have an (accidental) abuse-absorbing feature on the little rover?

This was something fairly easy to test and, thanks to the low cost of SG90 plastic gear micro servos, I was willing to sacrifice one for this knowledge. I put together a MSB3 prototype steering assembly and wired the servo to power and control signal, telling to hold straight center. If I wanted to be scientific I would use a torque wrench or something to quantify the forces as I test, but I didn’t have the proper equipment so I just used my hand.

The answer is no, nothing else in this steering assembly for MSB3 will step up and absorb abuse on behalf of the steering servos. It took surprisingly little force from my wrist before I heard something snap. I disassembled the gearbox to see not one but two failures: A tooth is bent (but still attached) on the final output gear, and another tooth has broken off from the adjacent meshing gear.

Since I didn’t use an instrument to quantify the breaking force, I don’t have objective numbers to post here. Subjectively I felt the breaking point was beyond what I would reasonably expect from a little rover in normal roaming, which is good for rovers rolling along minding their own business. But I want micro Sawppy to be something a teacher can introduce to their class, and I didn’t think the breaking point was beyond the capabilities of curious/destructive children, and so the fragility worries me.

I’m sure MG90 metal gear servos would fare better, but would they be strong enough? I’ve already sacrificed a few MG90s to experimentation and I’ll want to set up proper instrumentation before I sacrifice another. And obviously a MG90 is more expensive than a SG90. Is it better to use affordable SG90 servos and replace them as needed? Or is it better to go straight to MG90 for higher durability? Answering this question definitively would require real world usage data. See how often SG90s require replacement, and whether the replacement rate is greater or less than the up front-cost increase of using MG90 servos. In order to obtain this real world usage data, I’ll have to keep working on the rest of the rover.

Getting a pair of bearings aligned with the wheel contact patch is a feature on all Sawppy rovers, so that in itself wasn’t new. But Micro Sawppy Beta 3 (MSB3) did try a new way to mount bearings, and I’m optimistic it’ll make future Sawppy designs easier to build. On top of that steering joint, I’ve also modified the servo attachment for another feature I wanted to put on my rover designs: mechanical steering trim adjustment.

Sawppy V1 had a software-based trim adjustment system, where the center point of each steering servo could be adjusted by modifying a value in a text file. I though it was something that I could set once and forget but I was wrong. The potentiometers used to sense position inside a servo motor drifts over time (and temperature, and humidity, and phase of the moon…) so in reality steering trim had to be adjusted fairly frequently.

For MSB3 the servo horn bundled with a micro servo is mounted on a 3D-printed piece with an arched hook in the front. I could then fasten it to my steering assembly with a single screw. Steering trim adjustment becomes a matter of loosening that screw, sliding to a different point within that arc, and tightening the screw back down.

Another advantage of this design is that, unlike MSB1 and MSB2, the servo is freed from handling any structural loads. It is now responsible solely for steering just as servos were for Sawppy V1. It is a feature I’m glad I could bring to a smaller scale. I thought about going one step further on ease of assembly, and tried a few clip-on servo mounting brackets for tool-less assembly and replacement. (Pictured.) But even though these little servos assert little torque, there is enough to distort 3D-printed plastic and affect steering accuracy so I returned to the concept of a screw-down servo bracket.

But the experience did remind me of one thing about Sawppy V1: I didn’t like using heat-set inserts on shaft couplings because they would slip and break. However, that slippage and breakage did have an advantage when Sawppy is stressed beyond design limits. When a child decided to break my rover, the coupler broke before that abuse was transmitted into the servo. What would happen to this little rover servo?

My little rover project Micro Sawppy Beta 2 (MSB2) turned out to have (effectively) three speeds: stopped, full speed forward, and full speed reverse. MSB2 originally intended to get its six-wheel drive from inexpensive micro servos modified for continuous rotation. This approached worked for its predecessor MSB1, but MSB2 used servos from a different manufacturer and their feedback control circuit had very poor (or poorly tuned, or nonexistent) proportional control. As a result it was nearly impossible to drive MSB2 at partial throttle.

Given this setback opportunity to try other ideas, I started modifying all six wheel drive servos. I unsoldered the flawed control circuit board and soldered wires directly to the motor input terminals, intending to route those wires to a motor control H-bridge circuit of my own choosing. This would mean I’m only using the mechanical portions of the micro servos, effectively turning it into a DC gearmotor.

But I only got halfway. Unsoldering the board and soldering motor wires is well within my comfort zone, but wrangling with wires got very tedious and frustrating. In order to replace the three-wire continuous rotation servo wires with two-wire motor control, I have to struggle with the intricate wiring channels I’ve designed into suspension components. In fact I have to struggle with it twice: once to remove the wire, and once to add the replacement.

That’s when I realized MSB2 has failed some very important project objectives: making the rover easy to build, easy to diagnose, and easy to repair. For the three wheels I managed to convert, it took far more time to wrestle with the wires than it did to perform soldering, and that’s just wrong. I want a design where, if a servo dies, it can be quickly replaced to get the rover back up and running. The wiring channels may give us a tidy looking rover, but not one easy to work with. As a result I’ve decided MSB2 is going down the wrong path and stopped work on this iteration. But I am thankful for the cute little rover, it taught me the tradeoff for tidy wiring and gave me motivation to examine using DC gear motors to drive a little rover’s wheels.

Once I built up my little rover Micro Sawppy Beta 2 (MSB2) I observed that its steering servos were acting differently from the batch I used for MSB1. Beyond the clearly visible physical differences, these servos also had different control logic that can get into a never-ending oscillation.

A contributing factor here (if not the main factor) is that this micro servo’s motor control logic is far too eager in its corrections. It applies too much power and, if that power is not absorbed by damping, we have over-correction which repeats the cycle. In theory the motor controller uses power in proportion to the error in position. If the target position is close, it should use less power than if the target was far away. This particular servo control board has poor (or at least poorly tuned) proportion control. It seems to use the same amount of power whether the target position is close or far away.

The occasional oscillation is moderately annoying for rover steering, but at least it is usually damped out when the rover is sitting on its wheels. I didn’t experience the full impact of control algorithm used in these micro servos until I converted them to continuous rotation for rover’s six wheel drive. Now the poor proportional response manifests in a rover that I can’t run at partial power. I could make MSB1 crawl slowly up obstacles, but MSB2 didn’t want to crawl slowly like a real Mars rover would. It is either stopped or booking across the landscape at full speed.

In this scenario I suppose I could also apply damping by increasing the workload. Which means either adding friction to the drivetrain or making the rover heavier. This is assuming the motor will exhibit proportional behavior at higher load levels, which is possible but not yet proven. But despite the light weight causing other problems, I didn’t like the idea of adding friction or weight purely for the sake of ballast. So I am going to explore a different venue by removing the flawed servo control board. I’ll have to find some other motor controller and use only the mechanical portions of these micro servos as a DC gearmotor assembly.

The robot arm I built as a test mockup for my little rover Micro Sawppy Beta 2 (MSB2) was a simple manually-posed affair. I wanted to get a better feel of how such a thing would articulate before I put in the work to motorize it, and I had no suitable actuators on hand anyway. In fact, I had no idea what I would use for such a project, since I found problems with the micro servos I’m already using in this project.

There are a few robot arm designs built around the same small affordable micro servos. For example, the MeArm is a super minimalist robot arm designed to be assembled from laser-cut pieces. I’ve seen a few MeArm in person and based on how they moved, I could tell there are some challenges to use them effectively. One problem I witnessed was that, under certain circumstances, a particular servo would get into an oscillation bouncing on either side of the target position. Unable to ever reach that position and hold it steadily.

I built and ran MSB1 without seeing such behavior and had been feeling pretty good about things, but MSB2 used a different batch of micro servos from a different manufacturer. With this batch, I saw the same oscillation I saw on a few MeArm affecting MSB2 steering and I think it’s coming from certain servo control boards. Or at least I don’t see how it could be caused by my mechanical design changes between MSB1 and MSB2.

Mechanically loose play + simplistic control algorithm = tendency for self-perpetuating oscillations. Ah, the joys of working with inexpensive micro servos. pic.twitter.com/2q6Y3DYuey

— Roger Cheng (@Regorlas) December 23, 2020

The comments on my tweet support my hypothesis: this batch of micro servos have some shortcomings in its feedback control logic to cause this behavior. I think it is very likely a deliberate engineering tradeoff decision dropping some feature to make it cheaper, given that these micro servos were built to minimize cost. There were multiple suggestions to add damping which does indeed mitigate the problem. For MSB2, this oscillation only occurs when I hold the steering mechanism up in the air. Once I set down the wheel, friction between wheel and ground adds enough damping to stop the oscillation.

I’m feeling particularly irritated at this servo behavior, because it turned my beloved feature into a bug: the smooth low friction rotation enabled by ball bearings is a contributing factor for this oscillation. If I didn’t use ball bearings and had high friction steering, this oscillation would probably not happen. But annoying as this was for steering, it became a disaster for driving.

The main objective of MSB1 was to establish that a scaled-down rocker-bogie suspension system could nominally function and expose problems along the way. That objective did not require much of a rover body, so MSB1 only had a minimalist “Scarecrow” box. The main objective of MSB2 was to build upon its established suspension system and add a body with some resemblance to Mars rovers Curiosity and Perseverance.

On top of that body is not a realistic model of a rover camera array. Instead, it has a smiling face, which follows the precedence set by rover illustration JPL used for Mars 2020 “Name the Rover” contest (resulting in Perseverance) and that of ESA’s ExoMy rover. I had contemplated various ideas on how the face can change, maybe small OLED panels or an e-ink display. For this first trial I just drew a face with black marker. (There was also a layer of cellophane tape to block marker ink from capillary action.) I did this during a virtual meet of makers and I was surprised that a simply drawn smiling face made a hugely positive difference on how people perceived my little rover project. Though I probably shouldn’t be surprised: there was a similar change in perception when I added googly eyes to Sawppy V1 after seeing it done by another Sawppy builder. A tiny change makes people smile.

For electromechanical actuation, MSB2 used MG90S metal gear servos from a different manufacturer so I can compare how they perform against the ones used in MSB1. This particular batch were the MG90S that had a mild case of false advertising: only the final output gear was metal (visible as silvery metal) and the remainder were all black plastic. The two batches were bought from the same Amazon vendor on the same product page (*) but they are obviously distinctly different products. One of the differences had a serious impact on MSB2 and changed the direction of the project, more details in a future post.

I originally intended for MSB2 to be up and running in time for launch of Perseverance rover, but as is usually the case, other things happened. All I could do was a short video where I manually moved MSB2’s poseable arm waving a flag cheering Perseverance on. But it became a part of JPL’s official launch tweet, and I’m quite happy with that accomplishment even as I work through MSB2 design problems.

Baby rover will be cheering for @NASAPersevere #CountdownToMars pic.twitter.com/L4SUa205WJ

— Sawppy (@SawppyRover) July 20, 2020

---

(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.

Continuing the tour of micro Sawppy beta 1 (MSB1) from the ground up, the rear steering knuckles are attached to the suspension bogie. In Sawppy V1, the bogie’s attachment joint to suspension rocker had a mechanical design oversight: there were no mechanical limiters to rotation angle. I didn’t realize it was even a problem until Sawppy over-rotated during a climbing demo and flipped the bogie assembly, pointing mid and rear wheels skyward. Not only does it render Sawppy immobile, but it also puts a ton of stress on the wiring harness. I designed and installed a crude limiter to prevent wire breakage, but solving the problem more elegantly was on my to-do list. MSB1 represented the first attempt.

The joint itself is an adaptation of Sawppy V1’s design. A pair of bearings are embedded in the printed joint. On Sawppy a length of 8mm metal shaft is used as rotational axle, and the bearings are held in place with E-clips cut into the shaft. I wanted to move away from those clips so here I’m trying Ameer’s idea of using fasteners directly. In the case of MSB1, M3 screws.

A washer is used as spacer to give the bogie a little room to rotate without rubbing against the rocker arm, this aspect was the same as on Sawppy V1. (After I took this picture I realized the washer is shown installed on the wrong side, they’re supposed to be on the far side of the camera.)

I was wary of bearings damaging the M3 thread, but hoped the rover is light weight enough that it would not become a huge issue. That turned out to be secondary to a bigger problem: this design is very sensitive to how tightly the fastener was torqued down. In Sawppy V1 this wasn’t a problem because the e-clips stay in their slots. But as this screw is tightened, it presses down along bearing center axis. Too much pressure, and the bearing could no longer rotate smoothly. Too little pressure, and the joint rattles. This is a problem shared with a few other designs like the 4tronix rover, which mentioned this specific issue in their online assembly instructions.

I put that on my list of new problems to think about, and moved on to the old problem of over-rotation. I 3D-printed several ideas to interrupt rotation in some way and eventually settled on this design:

A little C-shaped piece of plastic that can clip into place over the M3 screw. One end goes over the head of the screw, and the other end covers the nut on the opposite side. This keeps its center portion in place between the rocker and bogie suspension components, preventing over-rotation. This draft is bulkier than it needs to be, I will thin it down for future revisions so it looks less like a big wart on this bogie’s connection to suspension rocker.

I had decided to use micro servos, converted to continuous rotation, to drive the wheels of my micro Sawppy beta 1. (MSB1) The next challenge was to design a way to steer the four corner wheels.

Given my fixation on using ball bearings to shoulder workloads in rover designs, it shouldn’t be a huge surprise that the first thing I worked on was a method to incorporate a ball bearing into my design.

Mounted at the bottom of a suspension arm with a single M3 screw into plastic, it will bear the weight of the rover and define this corner’s axis of rotation. It will also become the fulcrum for any lateral wheel movements pushing against the steering servo.

In consideration of these forces, the servo horn sits at the top of the steering assembly to maximize length of lever arm and minimize stress on servo gearbox.

I oriented the servo such that the wires are pointing towards the body. This seemed like the obvious choice for wire path management. And given that servo wires exit at varying locations from one micro servo manufacturer to the next, I had to design a “funnel” to accept a range of wire positions.

A consequence of this decision is that the steering knuckle is quite wide in order to give enough clearance to the body of the servo. This consumes precious space in such a tiny rover. At the rear of the rover, I wonder if this will clear the body.

At the front of the rover, it limits the amount of space available for a rover robot arm. This front view picture shows the steering knuckle is taking up almost a third of the width between steering servos.

Fortunately, I could shape the structure to minimize impact on wheel ground clearance. These wheels are quite happy to climb over obstacles with little risk of collision with the steering knuckle body. Which is one less thing to worry about as I moved on to designing the suspension bogie for MSB1.

By default, remote control hobby servos are tasked to hold a particular rotational angle specified by its given control signal. However, a common modification is to turn them into “continuous rotation servo” where the control signal commands the motor forward or backwards without regard for position. This is useful for tasks like driving a little rover’s wheels.

Performing this modification is done by disconnecting the potentiometer from the output shaft, and replace it with resistors that will result in an unchanging resistance value. Such a servo will think the control shaft is always at center. So when given the command to move to a position off center, its control circuit will fruitlessly spin the motor trying to get to a position it will never reach, giving us our forward/back motor control.

Studying a few micro servos batches sold by different vendors, I’ve established there’s a fair amount of variety among generic micro servos. So these pictures will probably not exactly match whatever might be on hand for your project, but the general concepts should still hold.

This conversion starts with a standard servo with two resistors. The precise value isn’t very important, but they do need to be at least a few hundred ohms (so very little current flows through them) and they need to be as close to identical to each other as practical. I used two 1 kΩ resistors.

In reality, the two resistances will not be equal, and servos at this price range wouldn’t be very precise about voltage measurement anyway. So this conversion method should only be used when we have means to adjust throttle trim (a.k.a. defining the center point) in the control system. If our control system doesn’t have such adjustments, then the potentiometer needs to be preserved to allow physical adjustment. In the case of this project, I have software adjustment, so I could proceed.

Hopefully your micro servos are not glued shut and can be opened up by removing a few screws. In this particular type, removing two screws allow access to both the gearbox and control electronics.

Even though MG90S micro servos are commonly called metal gear servos, I’ve found that some of them don’t actually have all metal gears. They might have metal gear only on the final output shaft or maybe one or two intermediate stages behind that. This particular one is actually all metal, but of course there’s no guarantee on how strong the metal might be. What’s important right now is whether the final output gear has something that prevents continuous rotation. Not all of these servos have one. But if present, it needs to be removed. Hold that output gear securely…

And remove the stop.

Now the servo is mechanically able to rotate continuously, and we can proceed to electronics modification to the position-sensing potentiometer.

Hold the control circuit board securely.

Unsolder the three legs of the potentiometer.

Use the two resistors to build a voltage divider that evenly divides the voltage across either side into an average value on the center pin. This is what the potentiometer used to do at its center position, and now it will read as that position forever.

Reassembling all parts completes the conversion. We are left with a extraneous piece of mechanical stop, and a potentiometer that is no longer used to sense position. Now this motor can be controlled like a small gearmotor assembly with built-in H-bridge, perfect for mounting inside a micro rover wheel steering knuckle.

Micro Sawppy Beta 1 (MSB1) is the test chassis I built to verify I could miniaturize concepts of my Sawppy rover down to a smaller simpler version built around micro servos. This tour of MSB1 starts at the bottom where the rubber plastic meets the road dirt. These wheels were a simplified version of Sawppy wheels, keeping the 48 grousers inspired by Perseverance rover’s wheels. The six wheels spokes don’t conform to the shape of the real thing, because that required multiple pieces made with 5-axis CNC machining and I wanted a design that is 3D-printable in one piece. But I did keep the curved spiral that also contributes to shock absorption, something also present in bigger Sawppy wheels.

I tried to carry over Sawppy’s wheel axle design, where a metal shaft is responsible for shouldering all the weight of the rover, helped by a few ball bearings. This freed the LX-16A serial bus servos from structural load duty, letting them focus on their job of turning the wheel to drive the rover forward/back. Unfortunately I couldn’t figure out a good way to scale down that design without inheriting all the problems.

Thus these wheels were mounted directly to the servo horn via some self-tapping M1.2 screws. Those fasteners were from an assortment kit of small self-tapping metric screws I found on Amazon (*) but I’m worried whether they are commonly available worldwide.

Fastening the wheels directly to servo horns meant the micro servo gearbox will have to shoulder weight in addition to their responsibility for driving the wheels. This force will be applied perpendicular to its axis of rotation and the micro servo gearbox isn’t optimized to handle that type of force long term. As an attempt to mitigate this, I decided to mount the wheel on the inside surface of the horn, shortening the lever arm by a few precious millimeters.

Despite my misgivings about this design, I decided to forge onward. Perhaps a micro rover is lightweight enough that gearbox wear would not be an issue, and building this prototype will tell me if there were other unforeseen problems with this approach. Before I can drive a rover with these, though, I’ll need to convert them from position control actuators into continuous rotation servos.

---

(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.

With mechanical foundations established, it took a few weeks of experimentation to get to Micro Sawppy Beta 1. (MSB1) My first running test of a small Sawppy rover, this prototype verified I could make a rocker-bogie rover chassis using micro servos for all electromechanical articulation. It was also my first time veering away from mechanical scale fidelity, altering proportions towards a cute baby rover.

Since the focus was on rocker-bogie suspension design, I didn’t put much effort into the rover body for this chassis, it’s just a minimalist box. This follows the precedence of NASA JPL’s “Scarecrow” rover which was likewise a test chassis for Curiosity rover’s rocker-bogie suspension and has no body to speak of. It also has no onboard computer processing, and this lack of electronic brain is where its “Scarecrow” name came from.

MSB1 similarly has no onboard autonomy, but that matches my Sawppy V1 rover which never got software beyond turning it into a remote control vehicle. In fact, like my Sawppy, MSB1 is also running the software I wrote for SGVHAK rover. Shortly before its public premiere, SGVHAK rover needed software support for a steering hack with remote control hobby servo, and I reused that code base for this micro servo rover. What was slapped together for a single steering servo was expanded to cover servo control for four steering servos and wheels driven by six continuous-rotation servos.

Using SGVHAK rover code, running on a Raspberry Pi 3 with the Adafruit 16-channel PWM/Servo HAT, was the most expedient way to get MSB1 up and running. Following ExoMy’s lead, I had put some effort into wire management, but the end result is a tangled mess because I made a miscalculation somewhere in the scarecrow body box for this rover. It was too small to hold a Pi 3 with the servo HAT, so those two circuit boards were forced to dangle outside the box and now everything is a mess. Ah well, that’s why we do prototypes.

So let’s take a little tour of MSB1 from the ground up, starting with its wheels.

Baby #Sawppy rover takes first steps! This is an early prototype to make sure wheel and suspension parts work together as designed. Real rovers @MarsCuriosity and @NASAPersevere had a similar test, read more about @NASAJPL "Scarecrow" here: https://t.co/thAfoPwnC3 pic.twitter.com/QmX7xKyqcK

— Sawppy (@SawppyRover) July 4, 2020