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Rovers and SAS - how does SAS work on terrain?


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Guys, the problem isn't with input bindings. Yes, those can also create a problem, but I'm not talking about that. And I'm not talking about the reaction wheels providing input. I'm talking very specifically about SAS itself.

SAS will use the reaction wheels (etc) to maintain an orientation of pitch yaw and roll. If you are rovering around on a hill, you do not want your pitch or roll orientation to be maintained, since stable values for pitch and roll on a hill depend on which way you are facing. On flat terrain, the stable values for pitch and roll are always zero, so SAS works as desired there.

Edited by The_Rocketeer
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SAS will use the reaction wheels (etc) to maintain an orientation of pitch yaw and roll. If you are rovering around on a hill, you do not want your pitch or roll orientation to be maintained, since stable values for pitch and roll on a hill depend on which way you are facing. On flat terrain, the stable values for pitch and roll are always zero, so SAS works as desired there.

I've avoided posting in here because, for some reason, even SAS creates a divison of opinion. I'm going to start off by going out on a limb here and saying that it might also depend on your rover.

But to answer your question directly, for me...SAS works just fine on my rover. My rover only requires 1 torque wheel to flip itself over on most bodies, maybe 1.5 on Kerbin on flat terrain. However, it has 3, so I'd say there is some excessive torque available.

To get to the practical matter of SAS, with that excessive torque I do have some issues. However, the irony is that the issues are usually to do with parking or starting the rover. When stopped, there is sometimes enough torque available to keep some of the wheels in the air. When starting, if the front wheel aren't touching the ground, it won't get traction. That's because I disabled the rear motors to keep it from flipping itself under low gravity due to thr rear wheels causing a wheely, but has noting to do with SAS.

For rovering around a hill, it helps a great for me to use Stability Assist. Stability Assist is supposed to be smart enough to hold a position unless acted on by another force. That force can be engines, control surfaces, RCS, torque, or even terrain. (It doesn't exactly work that way, but that's ultimately how it behaves.) The Stability Assist SAS doesn't hold a specific attitude per se, but rather tries to hold whatever it was at when it last stopped getting pushed around.

So while rovering around a hill, the SAS is slowly pushed around and it does fight back some. But even with my jeep (and its excess torque), that slight fight back provides some extra stability instead of unusual craft orientations. The rover is usually jostled enough that it settles to a new SAS stabilized orientation and isn't forcebly pushing the rover to get back to a previous orientation.

The biggest place I notice torque helping is when making high speed turns. Ironically this is the same place I hear the most complaints. I'll leave it at that for now, since you are asking about terrain response.

Now, that is all stability assist. I have also used Hold Prograde and Hold Retrograde, but only under very special circumstances.

Hold Prograde I find useful when I catch a piece of terrain with the front wheel at high speed and it flips the rover. Up till that point, Stability Assist would be in use and, now that it's airborne, has halted the rover from completely tumbling out of control. Once airborne, it's essential to get the rover aligned with the path of motion before hitting the ground again (or it doesn't end well). Here, a click to Hold Prograde will flip the rover around the fight direction, even if I can't see it on the navball. Then while the rover is coming down, I use the torque controls to override the pitch angle to make sure the rover doesn't dive wheels first into the ground.

That's the only time I've found Hold Prograde useful for a rover. Using it for normal terrain driving doesn't seem to work out as well mostly because the rover reacts too much as it's bumping over the terrain. Unlike Stability Assist, Hold Prograde is actively seeking a direction and seems to cause the rover to dive into the ground or behave strangely when bumped off course. There also seems to be some drift, which I think might be caused by the prograde marker pointing slightly downslope, so that the rover ends up turning slightly on angled surfaces even while not experiencing any real problems.

I can say that Hold Prograde won't "stop" you from turning the rover. When inputting a turn, contact with the terrain causes the prograde marker to move also. So in that regard, it fights back to the prograde marker, but the prograde marker is also moving with the rover. It's the reactions to being disturbed by terrain that I dislike when using Hold Prograde. Although it seems to work well for boats...

Maybe that helps answer your question. I feel like I'm still spouting opinion, but personally I use SAS extensively when rovering and for me the benefits to outweigh the drawbacks. And the drawbacks are not problems with the terrain and SAS fighting it, so much as understanding how it reacts to being disturbed by user input.

Again, I also think the SAS utility will vary by user's driving style and by rover design.

Cheers,

-Claw

Edited by Claw
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That force can be engines, control surfaces, RCS, torque, or even terrain. (It doesn't exactly work that way, but that's ultimately how it behaves.) The Stability Assist SAS doesn't hold a specific attitude per se, but rather tries to hold whatever it was at when it last stopped getting pushed around.

I don't see anything wrong with anything you've written there, but I think you've missed the exact issue.

Take the situation where you're rovering along the contour line of hill (laterally across the hill face, if you like). When you turn upslope, the terrain gradient underneath you hasn't changed. That is to say, the terrain won't impart any "corrective" roll torque when you turn. Only your orientation relative to the gradient has changed. So the terrain won't force your rover into a new orientation.

But you already have some amount of roll, because you were driving laterally along a hill. As you say, SAS will try to maintain that position, which means trying to maintain the roll out of the turn. IE, instead of helping to keep you "flat" with the terrain, it will help to keep you "not flat" with the terrain, which means your CoM will be further above the fulcrum (your outer wheels), which will shift the gravity vs inertia balance further in the favour of inertia, which will make the rover more likely to flip out of the turn.

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Only your orientation relative to the gradient has changed. So the terrain won't force your rover into a new orientation.

This is exactly what I'm trying to say. After moving from one slope face to the next, SAS isn't trying to push you all the way back to the previous orientation.

As you transition across one terrain triangle to the next, yes, SAS is fighting that change. After it's been jostled into the new position, it's going to try to maintain that new position. Transition to the next triangle, and so on. It's not trying to fight all the way back to the original attitude you had when you first turned SAS on. And it isn't even fighting to get back to the last terrain triangle, once it's settled onto the new one. I'm not really sure how else to say this.

Maybe take for instance, cresting a sharp hill (like a crater lip) at high speed with Stability Assist on. The rover will launch off the top of the hill and it will indeed maintain the attitude it had when it left the ground (likely nose high). After it lands again, it will be jostled into a new position. On that new terrain face, the rover isn't fighting to get back into that wheel-high position it had when cresting the hill. It accepts the new position as the baseline. When transitioning to a new terrain triangle, it'll get jostled again into a new position.

For your example about around the hill... If you have excessive torque available, yes, the SAS will make attempts to keep the rover at the same roll angle when you come off a hill. If you have that much excess torque, it will maintain that if it isn't jostled into a new position. If you hit another bump that pushes the wheels out, then it'll settle into some new position. Even with my 3x needed torque, this really only happens when I'm in the process of crashing, or if I turn really hard and the rover starts to flip, but the SAS stops it.

Really, the thing I would recommend the most. Instead of sitting here listening to us debate and throw around personal experiences... If you want to see what the SAS is doing, go drive a rover through the exact scenario you want, and watch the trim input. The trim input shows you exactly what the SAS is inputting into the system (for any vehicle). That SAS input will be torque or RCS, but not wheel controls.

That's the only pure way to get your exact question answered, under your desired conditions, with the specific rover in question. Our SAS experiences are going to be specific to our driving style and rover design.

As an example of what I mean, here is some tests with my rover that is currently on Duna. It's crashed a few times, so currently its down to one torque wheel (what I consider bare minimum for this rover).

Javascript is disabled. View full album

I didn't want to overload the pictures here, but you can see the reactions from the SAS via the trim input in the bottom left. It rolls INTO turns, and yaws AWAY from turns on slope sides, just as it does on flatlands. This is why rovers in my experience, "dig in" and flip over, due to the yaw.

Cheers,

~Claw

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If you have excessive torque available, yes, the SAS will make attempts to keep the rover at the same roll angle when you come off a hill.

It's not a question of excessive torque. SAS will resist change to the roll angle. But you want to roll... in the example where you turn left to go uphill, you want to roll left. Yes, SAS will also fight the tendency to roll right, but the point is that SAS is trying to keep the CoM in a rolled state, which contributes to intertia's attempt to flip the rover. If SAS were off, then as the rover turned left the *only* thing attempting to lift the left wheels off the terrain would be the torque around the CoM. In other words, SAS gives that inertial force a head start over gravity that it would not otherwise have. The degree to which it contributes depends upon the amount of SAS torque you have, but that it contributes does not.

Really, the thing I would recommend the most. Instead of sitting here listening to us debate and throw around personal experiences... If you want to see what the SAS is doing, go drive a rover through the exact scenario you want, and watch the trim input.

As I said earlier, I tested this with various rovers with various amounts of torque available to SAS, I think it was 8 vehicles in total.

It rolls INTO turns, and yaws AWAY from turns on slope sides, just as it does on flatlands. This is why rovers in my experience, "dig in" and flip over, due to the yaw.

It is providing left roll in that example because inertia is significantly overpowering gravity, thus you have already gone past your previous degree of roll, and SAS is now trying to resist. In other words, SAS is trying to keep you up on your two right wheels. It is not trying to get your left wheels down.

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It's not a question of excessive torque. SAS will resist change to the roll angle.

Yes, unless or until, as Claw says, it's 'jostled'.

But you want to roll... in the example where you turn left to go uphill, you want to roll left. Yes, SAS will also fight the tendency to roll right, but the point is that SAS is trying to keep the CoM in a rolled state, which contributes to intertia's attempt to flip the rover.

Well, no. SAS nulls out some of (sometimes all of, given enough torque) gravity's attempt to level the rover through any axis, not just roll. Inertia is a separate 'force' in the original forward direction, and it's proportional to how much you're changing yaw orientation that it becomes in a lateral force. It's then only converted to a roll force by friction of the wheels on the surface.

If SAS were off, then as the rover turned left the *only* thing attempting to lift the left wheels off the terrain would be the torque around the CoM. In other words, SAS gives that inertial force a head start over gravity that it would not otherwise have. The degree to which it contributes depends upon the amount of SAS torque you have, but that it contributes does not.

I disagree with your last statement here. Yes, SAS does give a 'head-start' to the mounting roll effect of inertia relative to gravity, but what it doesn't do is contribute further to that roll. In fact, SAS will immediately resist the inertial roll torque in this situation, for the same reasons that it resists gravity - it's trying to pull the vehicle off its original axial alignment.

As I said earlier, I tested this with various rovers with various amounts of torque available to SAS, I think it was 8 vehicles in total.

Well, a nice report would have helped us to analyse your findings. I'm not sure whether you're really asking a question that you want help with, or if this is a quasi-tutorial. You certainly seem somewhat resistant to most advice offered.

It is providing left roll in that example because inertia is significantly overpowering gravity, thus you have already gone past your previous degree of roll, and SAS is now trying to resist. In other words, SAS is trying to keep you up on your two right wheels. It is not trying to get your left wheels down.

No, as above. SAS doesn't care if it's a gravity force or an inertial force, anything that pulls you off the orientation it's determined to fix will be resisted. The fact that inertia has overpowered gravity is neither really here nor there, the issue is that SAS orientation is 'roll left' from it's current attitude. This is what I meant in the italicised above.

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Yes, unless or until, as Claw says, it's 'jostled'.

Which isn't going to happen here.

Well, no. SAS nulls out some of (sometimes all of, given enough torque) gravity's attempt to level the rover through any axis, not just roll. Inertia is a separate 'force' in the original forward direction, and it's proportional to how much you're changing yaw orientation that it becomes in a lateral force. It's then only converted to a roll force by friction of the wheels on the surface.

Actually, yes. I realize SAS will try to resist the change in orientation... I've said that at least twice in this thread. But it's not going to be able to null out the change in pitch, the terrain will *force* that to happen. SAS will resist it in a way you wouldn't really want it to, digging the front wheels into the ground for a moment, but ultimately the planet is going to win that fight. But the terrain isn't doing anything to force the roll to level out.

and it's proportional to how much you're changing yaw orientation that it becomes in a lateral force. It's then only converted to a roll force by friction of the wheels on the surface.

The inertia is created by the mass of the vehicle wanting to continue moving in a straight line. If you were drawing it on paper, the line to represent this would come out of the CoM of the rover. Since the wheels are creating friction against the ground, this becomes a torque. The amout of torque depends upon the length of the lever arm. This is why a low CoM makes for a stable vehicle.

Think about what would happen if you could magically put the CoM 1m underground. Now, compare that to what would happen if the CoM was 1m above the rover. Now think about what happens when the rover is already starting the left turn with right roll. If you want to learn more about this, study this image. The physics of this are not up for debate.

I disagree with your last statement here. Yes, SAS does give a 'head-start' to the mounting roll effect of inertia relative to gravity, but what it doesn't do is contribute further to that roll. In fact, SAS will immediately resist the inertial roll torque in this situation, for the same reasons that it resists gravity - it's trying to pull the vehicle off its original axial alignment.

If a vehicle starts a left turn already rolled to the right, then inertia has a head start over gravity, and is therefore more likely to win. It therefore absolutely does contribute further to the roll, because it is weakening gravity's ability to resist it. If the CoM of the rover was high enough to begin with, then starting a left turn already rolled to the right will mean that both gravity and inertia will be trying to flip the vehicle out of the turn.

Well, a nice report would have helped us to analyse your findings. I'm not sure whether you're really asking a question that you want help with, or if this is a quasi-tutorial. You certainly seem somewhat resistant to most advice offered.

My question was about what SAS is expected to do in the scenario, not about the physics. People seem to be trying to "explain" the physics to me (and are wrong about said physics). It is not impolite for me to be extremely resistant to this mistaken understanding of the physics of the situation.

Edited by allmhuran
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I've found SAS useful on a small rover on Minmus. Due to the low gravity, the four wheels didn't have much traction which made it difficult to accelerate/brake. Using the reaction wheel torque, I could lift the rover onto its rear wheels and disable the front two. There was then double the traction on the remaining wheels to make progress segway-style.

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My question was about what SAS is expected to do in the scenario, not about the physics. People seem to be trying to "explain" the physics to me (and are wrong about said physics). It is not impolite for me to be extremely resistant to this mistaken understanding of the physics of the situation.

All you've demonstrated to me is that you know how to design rovers that handle badly. I'm not seeing any test results, I'm seeing a tale from a guy who has no details, much less images, craft files, or anything of substance. If you're going to claim that someone who's driven completely around multiple planets doesn't know what he's talking about in a surface vehicle, you really should bring something to the table to back that up.

It's not that you're being directly impolite, it's that you're telling people with literally hundreds of hours of testing and photo albums to prove it, along with detailed reports on their own tests, that your small scale test that nobody's seen is the only valid one. Really guy, if you know more than Claw about how rovers work, write a tutorial. With all the griping that goes on about how hard rovers are to drive I'm sure a lot of folks would really appreciate it.

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Which isn't going to happen here.

Actually, yes. I realize SAS will try to resist the change in orientation... I've said that at least twice in this thread. But it's not going to be able to null out the change in pitch, the terrain will *force* that to happen. SAS will resist it in a way you wouldn't really want it to, digging the front wheels into the ground for a moment, but ultimately the planet is going to win that fight. But the terrain isn't doing anything to force the roll to level out.

The inertia is created by the mass of the vehicle wanting to continue moving in a straight line. If you were drawing it on paper, the line to represent this would come out of the CoM of the rover. Since the wheels are creating friction against the ground, this becomes a torque. The amout of torque depends upon the length of the lever arm. This is why a low CoM makes for a stable vehicle.

Think about what would happen if you could magically put the CoM 1m underground. Now, compare that to what would happen if the CoM was 1m above the rover. Now think about what happens when the rover is already starting the left turn with right roll. If you want to learn more about this, study this image. The physics of this are not up for debate.

If a vehicle starts a left turn already rolled to the right, then inertia has a head start over gravity, and is therefore more likely to win. It therefore absolutely does contribute further to the roll, because it is weakening gravity's ability to resist it. If the CoM of the rover was high enough to begin with, then starting a left turn already rolled to the right will mean that both gravity and inertia will be trying to flip the vehicle out of the turn.

My question was about what SAS is expected to do in the scenario, not about the physics. People seem to be trying to "explain" the physics to me (and are wrong about said physics). It is not impolite for me to be extremely resistant to this mistaken understanding of the physics of the situation.

I'm not debating the physics - surprisingly you seem to have restated (in slightly greater detail maybe) exactly what I said. I'm also in no confusion about what inertia is or how relates to CoM or affects the motion of the vehicle. I was simply attempting to highlight the relevant mechanical details. You are using vague language in saying that 'inertia overpowers gravity', or that SAS 'contributes to the roll'. Based on the way your present this, I can only assume that your understanding of how SAS works is wrong.

This seems obvious actually, considering the way you restate your OP question. However, if you're not prepared to be lectured a little on which forces are acting on the vehicle and which SAS resists and how, I doubt your understanding will improve.

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All you've demonstrated to me is that you know how to design rovers that handle badly.

I'll gratiously not respond in kind to this obviously inflammatory remark, mostly because the design of the rover has literally nothing to do with anything relevant to this particular thread. Yes, there are good and bad ways to design rovers, but you can't design a rover in a way that changes how SAS behaves. Most people seem to be under the impression that the amount of torque is somehow relevant to that. It is not, and a little extra thought would make this clear. If a lot of SAS torque has a large effect, then a little bit of SAS torque has a little effect, but It doesn't change the way in which it contributes, only the degree to which it contributes.

I'm not seeing any test results, I'm seeing a tale from a guy who has no details, much less images, craft files, or anything of substance. If you're going to claim that someone who's driven completely around multiple planets doesn't know what he's talking about in a surface vehicle, you really should bring something to the table to back that up.

Nowhere did I say anyone "didn't know what they were talking about". If you read my original post, you'll see that I had a question about how SAS worked, and a suggestion based on how I believed it worked. Further testing, as discussed later in the thread, confirmed that this belief was correct, and several other people reported the same. Given that everyone seems to agree that SAS does perform in that manner then, notwithstanding those who are arguing that somehow the amount of SAS torque changes the logic, the physics are the only thing people can possibly be taking as a point of disagreement. But the physics of this is not some unsolved question, it's all well understood principles.

It's not that you're being directly impolite, it's that you're telling people with literally hundreds of hours of testing and photo albums to prove it, along with detailed reports on their own tests, that your small scale test that nobody's seen is the only valid one.

The test is, for the purpose of this thread, irrelevant, since (per the previous paragraph) nobody seems to disagree regarding the way in which SAS operates. What people seem to be resisting is what I have described as the consequence of that. This consequence doesn't need to be empirically verified, it is mathematically guaranteed.

Edited by allmhuran
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However, if you're not prepared to be lectured a little on which forces are acting on the vehicle and which SAS resists and how, I doubt your understanding will improve.

Given that, as per my response to Hagen, there does not seem to be any disgreement regarding the way in which SAS works (again, other than from those who believe that the amount of SAS force changes the principle, which it can't, it can only change the degree of contribution), there is no "lecturing about SAS" to be done.

You are using vague language in saying that 'inertia overpowers gravity', or that SAS 'contributes to the roll'

If you consider that vague, read it as "the contribution of inertia towards rolling the vehicle out of the turn is less than the contribution of gravity towards preventing that". I thought this was obvious but here it is stated explicitly, just to be sure.

Edited by allmhuran
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Hello, trying to figure out what I can contribute here (spent many many hours with rovers, including a rover circumnavigation of Kerbin, link in sig), but I'm a little confused.

I'm seeing a lot of back-and-forth about "SAS does this," "No, SAS does that," "No, you're wrong," etc, but not a lot of actual discussion. Would you mind stating your question again, just so I can be sure I'm responding to the right thing? :) It's mainly a question of how the SAS and reaction wheels respond to differently sloped terrain, right?

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Well then let's make this very clear. SAS:

1. neither resists nor contributes to any external forces whatsoever. Those forces are linear/planar, whereas SAS is rotational/axial.

1a. does resist any axial/rotational effect arising from those forces if they would alter the vehicle's attitude.

2. applies reaction wheel torque or steers control surfaces (but not wheels) to maintain the vehicle's attitude in a fixed position.

3. can have its target attitude reset by external 'jostling' or manual resetting.

That's really all there is to be said.

Edited by The_Rocketeer
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In 0.90, with my rover, yes, if it were on two wheels as described, SAS would keep it there. If I were moving forward, and rolling on two wheels (let's say both right wheels) it will in fact be more unstable, but SAS, with enough reaction wheels and torque behind it, can keep it balanced (until it hits something and starts flipping through the air). So generally, depending on your amount of reaction wheel torque, you may have to manually adjust to the angle of the new section of terrain. A well-built rover will usually be able to somewhat correct for this itself (CoM, wheelbase, camber, etc.) but not in every situation.

So! With the question being: "will SAS try to fight the pitch caused by the terrain incline?", the answer would be yes, at first, but it is generally very easy to correct for. Often, though, the rover may "settle" onto the slope. (Assuming it's built with that in mind)

Edited by Slam_Jones
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Well then let's make this very clear. SAS:

1. neither resists nor contributes to any external forces whatsoever. Those forces are linear/planar, whereas SAS is rotational/axial.

This is false, as the force component diagram in my previous post should make clear. Both inertia and gravitational forces have a torque component which I have drawn through the CoM and around the contact patch, ie, a "roll force", which SAS will attempt to resist.

- - - Updated - - -

So generally, depending on your amount of reaction wheel torque, you may have to manually adjust to the angle of the new section of terrain.

Yep, or as others have said, have a button which toggles the reaction wheels on and off, allowing the rover to fall back down to a stable equilibrium position.

Edited by allmhuran
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^^ Right. I believe you can press "F" to temporarily invert the SAS setting. Meaning, if you have SAS on, then holding down "F" will disable it, until "F" is released. Or tap "T" twice I guess, if you're averse to holding down buttons.

But I agree, a stability mode designed specifically for rovers would be awesome. Planes can get their "hold horizon" (hopefully) and rovers can get "hold terrain" or something like that. In a perfect world, that is...

Edited by Slam_Jones
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This is false, as the force component diagram in my previous post should make clear. Both inertia and gravitational forces have a torque component through the CoM and around the contact patch, ie, a "roll force", which SAS will attempt to resist.

Actually it's not false, but your interpretation could be. The 'roll force' you describe is covered by point 2, but just to be clear I've amended my post with a point 1a.

I think I see the issue with your understanding of SAS on rovers now though. You seem to be thinking that this is the case:

Gravitation torque (Gt) = 2 (positive)

Inertial torque (It) = 5 (negative)

So, SAS torque (St) = 2 (negative to counter Gt)

So, total torque = 5 (negative)

But actually, it should be:

Gt - It = 2 (positive) minus 5 (negative) = 3 (negative)

So, St = 3 (positive)

So, total roll torque = 0.

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Actually it's not false, but your interpretation could be. The 'roll force' you describe is covered by point 2, but just to be clear I've amended my post with a point 1a.

I took "neither resists nor contributes to any external forces whatsoever" to mean "neither resists nor contributes to any external forces whatsoever". Your amendment directly contradicts this, but is correct.

I think I see the issue with your understanding of SAS on rovers now though. You seem to be thinking that this is the case:

No, I don't think it works the way described here. That would be a really weird way for it to work, acting against gravity but not inertia.

But actually, it should be:

Gt - It = 2 (positive) minus 5 (negative) = 3 (negative)

So, St = 3 (positive)

So, total roll torque = 0.

That is the situation covered in the picture by the "best case scenario text". Ie, you have proposed a situation where SAS has enough torque to overcome the differential. In your example here, at least 3. There is thus no change in roll relative to the horizon, and the rover goes up on two wheels as it turns uphill (which is still not an equilibrium position and is therefore by definition not stable).

However, what if SAS was only able to put out a maximum of 2? Then it may be able to fight gravity's attempt to pull the rover down when you begin the turn (where gravity is strongest relative to inertia), and as you turn your roll angle relative to the surface will increase (because you are turning on a gradient), potentially to a point where the inertial force is greater than 4 (as per your example, where it is 5), leaving SAS with a deficit of -1, and the rover will flip. QED. This is the "worst case scenario" in the picture.

Edited by allmhuran
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If a lot of SAS torque has a large effect, then a little bit of SAS torque has a little effect, but It doesn't change the way in which it contributes, only the degree to which it contributes

SAS torque doesn't exist. SAS is a system that can act on forces you make available to it. You build the craft, you decide what SAS can and can't do.

I also don't know what you mean with this nonsense about rover design is irrelevant, tests are irrelevant, etc. You made a thread titled "how does SAS work on terrain" in a rover. You were told by people who have tested a whole bunch of rovers that it depends on rover design, and you should test it. You directly contradicted those assertions, because physics, which you proved with an irrelevant test (your words). So what's wrong with this picture, guy?

Maybe you don't mean to, but you come across as extremely patronizing. You can choose to be offended by that, but in the end it only hurts yourself! Either way, good luck with your rovers. ;)

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You build the craft, you decide what SAS can and can't do.

I don't understand what you mean by this, so please elaborate. You can change your CoM with your rover design, you can change how wide and long your wheelbase is, you can change how much reaction wheel force, or rcs, or control surfaces, SAS has available to use. All of these are irrelevant to the topic of this thread, which it's possible you haven't grasped yet. More and more, I'm of the opinion that this is the issue: people aren't doing the mental geometry with respect to what happens when you turn on a gradient. But, in any case, I'm quite certain you cannot change the manner in which SAS operates.

I also don't know what you mean with this nonsense about rover design is irrelevant, tests are irrelevant

The rover design is irrelevant to this topic, which I can elaborate upon if you haven't grasped it yet. The tests were regarding the way in which SAS works, which was the original question, which everyone seems to agree on. The test therefore has no relevance with respect to the disagreements that are arising.

Maybe you don't mean to, but you come across as extremely patronizing

I haven't accused you or anyone else of building bad rovers, or called you "guy", or "patronizing", or described what you've written as "nonsense". I'm simply stating the facts.

Edited by allmhuran
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I took "neither resists nor contributes to any external forces whatsoever" to mean "neither resists nor contributes to any external forces whatsoever". Your amendment directly contradicts this, but is correct.

Ok, first of all a force is not an effect of a force. In the case of gravity or inertia, SAS resists the torque effect of a linear force, absolutely NOT the force itself. This is precisely why it's absolutely wrong to say that SAS contributes to a force such as inertia! Is that in enough detail for you to see why you're wrong?

I'm really not interested in such petty nonsense. I'm perfectly happy to help develop the community's understanding of the effect of SAS on rovers, but I'm afraid your insulting and patronising tone is completely putting me off further involvement in this particular discussion.

If that's not what you think SAS is doing (and I agree it's pretty stupid to think so, but then everyone's a bit stupid sometimes) perhaps you can make clear exactly what you think it is doing and why you think this justifies having a forum thread to discuss it? I mean, if you've nailed it already and we're all wrong, why did you post this at all?

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Ok, first of all a force is not an effect of a force. In the case of gravity or inertia, SAS resists the torque effect of a linear force, absolutely NOT the force itself. This is precisely why it's absolutely wrong to say that SAS contributes to a force such as inertia! Is that in enough detail for you to see why you're wrong?

This may clear up your understanding of torque.

Edit: Perhaps you think I've said somewhere that SAS is increasing the gravitational force itself, or the inertia itself? If so, that's not what I've said.

perhaps you can make clear exactly what you think it is doing and why you think this justifies having a forum thread to discuss it?

It's all in the OP, but to say it all again: The initial question was about how SAS would respond to orentation changes caused due to changes in the terrain under a rover (or changes in the rover's orientation with respect to the terrain). Again per the OP and subsequent posts (and confirmation via testing), we all seem to agree that SAS resists orientation changes (relative to the horizon), whether caused by terrain or otherwise. The original post also included a suggestion, as was predicted in the first line of the OP.

The disagreement which is continuing seems to arise from a misunderstanding of what "resisting orientation changes" implies in certain terrain situations, such as the example in the OP where you turn upslope. I can explain this further if needed.

Edited by allmhuran
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