Re: [SeattleRobotics] Rocker Bogies
- Sorry, David the response seems to have gone sideways. It was not intended to be negative nor generate a negative response. As such, I appologise for upsetting you or causing discomfort to your ego. For this, please accept my appology.In taking the time to read the original post, it appears that the question that was asked was not about planetary rovers, rather, the question is why they have fallen out of use, or are not widley adopted in other applications here on earth.It is further unfortunate that time was not take to read the response below in full. There is no argument stating that skid steering should be used. On the contrary, it states that it is possible but will be less efficent.I-JackFrom: David Buckley <david@...>
Sent: Wednesday, August 10, 2011 2:48 AM
Subject: Re: [SeattleRobotics] Rocker Bogies< In college ... surface.That is not quite how it is! And not relevant to slow moving planetary rovers.Apart from the tracked prototypes where it is not possible to avoid it, skidsteer should avoided for planetary rovers because it uses too much energy.
----- Original Message -----From: silverfish_21@...Sent: Wednesday, August 10, 2011 9:04 AMSubject: Re: [SeattleRobotics] Rocker Bogies
In college many Mechanical Engineers study a concept called 'sprung weight'. At a very high level this concept can be thought of as a way of suspending the mass of the moving system in an isolated spring loaded cage so that as the wheels need to displace up and over every single pebble in the roadway, this can be taken up by the springs and energy stored in the springs. It is returned to the system after the momentary displacement (pebble or whatever) has passed. This kind of efficency is needed to attain efficency at higher speeds. Rocker-Bogey suspensions do not isolate a sprung weight from the navigated surface.
Rocker bogey systems can be used in a skid steer application, however they are much less efficent at it than a hard suspension. These system (even passive ones) are designed to equalize the normal force between ground and wheel amongst all six wheels. A system with a hard suspension that has effectivly two points of contact that vibrate and slide together will break traction and spin more easily than a system with six compliant points of contact that do not move together.
- MichaelExcellent you noticed that somewhere I mentioned the Rocker pivot meaning the Bogie pivot. %-(However the bogie virtual (it moves depending on the height of the front wheel) pivot point is, for a vertical face and horizontal ground as in figure 8-5, above the initial contact point of the front wheel with the face, hence the Normal at the contact point is below that even virtual pivot point.Fig2-5 shows clearly what I mean.Even with the geometry of the bogie arms moving the virtual pivot slightly back, the Rover needs to move backwards to allow the Rocker to swing up.Since as described earlier the front wheel is effectively underneath the surface, gravity can not hold the wheel against the vertical surface. Unless something holds the wheel in place on the surface there can be no traction to enable the wheel to climb. so how is the Rocker going to swing up?Looking at Fig5-7, as the rear bogie wheels drive towards the face they impose a anticlockwise torque on the rocker-V tending to lift the front wheel and body.Infinite traction at the front bogie wheel (middle wheel) would do but then the torque from that wheel has to lift the whole Rover. Hmmm, scratch that idea.Barlas's whole aim has been to ensure the leading bogie wheel rises on a vertical path, hence the lambda geometry, enabling it to climb the step and avoiding the problems I have been describing for the front wheel, and avoiding the tuck-in problem of Fig2-7. This aim seems to have been directed by 'what if the middle wheel motor should fail'.But he has completely overlooked the fact that the problem will occur with the front wheel of the Rover which will prevent the middle wheel ever reaching the step.So once again I ask "How does the front wheel get on the step".Unless it can get on the step in the first place the lambda geometry of the bogie is useless.DAvid----- Original Message -----From: MichaelSent: Saturday, August 13, 2011 10:38 PMSubject: Re: [SeattleRobotics] Re: Rocker Bogies
DAvid,Although things may be going off track a bit, I have enjoyed lurking in on the technical discussion. In that paper you refer to figure 8-5. In reviewing some of that paper myself it seems you are referring as the point of attachment to the body (figure 6-1, pivot point "O") as the pivot for the front wheels. I understand that if the body of the rover was to be fixed in space that would be the pivot point, however in looking at the mechanics, I see the "virtual" pivot point as being somewhere near the rear axle as the front wheel climbs the curb since the body is not required to be fixed in place, but we are assuming the rear and middle wheels shouldn't be allowed to rotate in reverse to get the front wheel up the curb. In other words, using figure 6-1, pivot point "O" would move backwards for the front wheel to drive up, but the contact patch of the rear and middle wheel may continue to move forward slightly, or remain stationary applying forward force. I wouldn't mind hearing your thoughts on that possibility. It is entirely possible I am misunderstanding something as well.MichaelOn Sat, Aug 13, 2011 at 3:27 PM, David Buckley <david@...> wrote:JackI have no idea where you got all that from but it wasn't from me.You were saying how you used to play with Rovers in school in Florida and linked toI asked how the front wheel in fig 8-5 was supposed to get up the step.You accused me of being sarcastic and then went on to give links and explanations which had no bearing on the question I asked.Seemingly you have been unable to refute my analysis of the mechanics involved and now want to go on about the cost of machining aluminium wheels.I am not interested in accounts of whether this or that educational Rover could climb this or that rock. Concentrate on the question I asked. It wasn't about whether blue or gold rovers could climb better. It was simply 'how could the front wheel in Fig8-5 get up the step'. Because if it can't in the ideal mathematical world of his paper then it nullifies any of his further reasoning and brings under suspicion the correctness of the rest of his paper.Firat Barlas's paper was supposed to be about 'Design of a Mars Rover Suspension System', well at least that is the title. So unless someone can explain from where come the magic forces which allow the front wheel to climb the step I stand by my opinion that the paper is full of nonsense.
Note that this has nothing to do with the President or the Queen or the Pope.or you, unless any of you were one of his supervisors at the time.DAvid