Grav Bots vs Walking Bots

March 29, 2011

This is an awesome picture from Traveller Art of a Zhodani Guardbot

Zhodani Guard Bot

Zhodani Guard Bot from Traveller Art

This picture is close to my mental image of a Zhodani Warbot — a grav robot that floats using its anti-gravity locomotion unit. How does that work?

First of all, the grav unit must generate enough “lift” to float the robot. That means an upward thrust capability or offsetting anti gravity force must exceed the weight of the robot in this case. Simple enough. The upward force creates the first part of the robot’s locomotion: lift.

The robot also needs to be able to move around. That requires a second force for a lateral thrust or push capability. (I’m leaving out for the moment where that force comes from but I plan to come back to that.)

So if the robot has lift and the robot has lateral thrust, it should be able to float like the one in the cool picture above.

I’ve been thinking lately about another problem which someone on the Striker newsgroup brought up. What happens when my floating robot gets hit by a character swinging a baseball bat? Or an ACR round? or a grenade blast? In other words, what happens when a sudden, unexpected force strikes my floating robot?

If the robot has only the two forces mentioned — lift and lateral thrust — then when someone hits it with a baseball bat that robot is going to move away from the bat in proportion to how much force is applied in to the bat. Or the ACR round or the grenade blast. The sudden, unexpected force striking the robot is going to make it move in a sudden and unexpected manner but one that depends on how much lateral “thrust” was applied against it.

That makes floating robots a very interesting tactical problem. What if this robot is operating with some infantry when hit? He might crash into an infantryman as a result of being it. He might lose his orientation and “tumble” into an unexpected position.

[Sidebar: I live and work in Washington DC where tourists often take a Segway tour of the sites. These “vehicles” maintain orientation by gyroscope so that usually they maintain an upright position. However on the occasion that the vehicle tips past a certain point, the force of the gyro slams the vehicle into a different (perpendicular) orientation that snaps the rider off it in a nasty way. I suspect that even gyroscopically stabilized grav bots might suffer from a similar tumble if struck suddenly in just the right direction.]

What about the surroundings? Does a floating grav bot next to a wall hit the wall when it suffers a hit itself? What happens when the guard bot is guarding a rare set of Ming vases? If hit does it crash into a vase?

I think you can see where this is going. Floaters have no “grip” against the ground and hence no inherent capacity to absorb or transfer the shock of the blow.

I’ve been pondering what that grip might be but I haven’t figured out anything yet. I have toyed with the possibility that perhaps the lateral thrust capability could be directed to offset the thrust from the incoming baseball bat. Good news on that solution is that one might prevent the bot from moving. Bad news is that the bot would need to absorb the force of two baseball bats in a head on collision. That might be a worse problem that the first strike.

That kind of mechanism assumes that the bots computer could respond to the incoming force quickly enough to offset it. My anti-lock brakes sense the braking forces in the wheel and adapt to the physics of the situation rapidly — but not as rapidly as an incoming .50 caliber round. Even given advanced computer technology, there might well be limitations on the rate of response.

What happens to the hypothetical thrust compensator when the bot is hit by multiple rounds from different directions? What happens when the incoming energy is greater than the bots lateral thrust capability? Perhaps this hypothesis is not easily workable.

I also considered the possibility that the bot might save its lateral thrust for collision avoidance. It might be better able to adapt to that situation within the mechanical limitations of its hardware and thrust capacity. Of course if the initial strike causes the bot to tumble into a bad orientation, that collision avoidance could be pretty tricky. And what about the  infantrymen operating in close proximity to it? How is he (or they) protected from collision with the floating bot?

The grip problem relates to transference of force. If a big rugby player straight dives at me  and I am facing him  perpendicular to his line of travel, he’ll knock me over since I have no way to absorb or transfer the force of impact. If I step one foot back so I am standing parallel to his line of travel, bend my knees and lean in the direction of the rugby player, I may be able to absorb the shock or transfer the incoming force through my legs into the ground. Now I have an effective grip on the ground which I can set against the incoming force. I might still get knocked down, but it takes more energy to do it when I have a grip.

I am making the case that the floating robot has no grip. Even if it could use some means of directed gravitational force, that force is unlikely to be able to transfer energy away from the strike. When hit, the bot is certainly going to have to move.

There might be an additional alternative response to being struck: spin. Tumble intentionally to dissipate the incoming force. I’ll have to think that one through, but that seems to work best in a bot that is designed to operate without any particular orientation being designed as best or most efficient. In a gun fight, a bot with a spinning defense might do a lot of defensive spinning which could harm its capacity for “normal” operations.

Grav bots are in trouble when they get hit. Even hits that don’t penetrate could stun or KO the bot. An undamaged bot that is defensively spinning could be “stunned” and lose its offensive action.

If I want my bot to stand up in a fight, it needs to have legs through which to transfer energy. It needs hips, knees, and ankles to absorb and transfer some of the energy away. A fighting bot needs to be a walker if the designer intends for it to take hits and keep fighting.

I think that Zhodani Warbots need to be walkers.

The grav bot depicted above is perfect for swift, silent manuever. It is perfect for pursuit, patrol, and observation but not primarily fighting.

Walkers fight. Floaters spy.



3 Responses to “Grav Bots vs Walking Bots”

  1. Kevin Says:

    Generally ‘Anti-gravity’ negates the gravity effects. Then all it has to do is defects in the field that cause pull and thrust in the direction of these defects. Generally a defect in the top pulls the item up, the size is a measure to the weight you wish to put into equilibrium. Then you can tilt to the direction or open a new defect to cause a pull to the direction you want to go.

    Its a pull function, not a push.

    • Kevin — You are absolutely right about the pull versus push. I was really referring to the apparent “push” that an observer might see when a defect in the field directed travel. This perceptual question is a bit like the fact that we commonly refer to “sunrise” when it is the earth that is moving and not the sun rising in fact. We perceive it as the mirror of the fact.

      As you point out though, when it comes to gravity it can only be a pull.

  2. Stephen Bankhead Says:

    Just came across this item. My take on the grav question is that it is a means of locomotion. Soldiers generally stop to shoot and generally take up a firing stance. I see grav warbots doing the same, using grav to go faster and travel easier over poor ground, but “grounding” to fire accurately. I’d say a grav warbot moving would be easier to “knock over” than one grounded in the same way that a running soldier would be easier to “knock over” than one kneeling/braced.

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