Posted: 11/9/2007 3:56:24 PM EDT
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I got some questions and I'm horrible at navigating websites and searching the net... I've tried. In the past in NASA threads there have been some posters here that seemed like they either knew a lot or knew how to find it... So... The recent shuttle visit to the ISS and the events that went on got me thinking. During the robotics operations this occurred: The station arm (Canada arm 2) handed something over to the shuttle arm (Canada arm) Then Canada arm 2 ran down its track to a new position. And then Canada arm handed the item back to Canada arm 2. Why couldn't Canada arm 2 move along its track while carrying a payload? This handing off happened several times. and slowed things down a lot. The arms are also 100% human controlled. Why? Why have a human move one joint at a time? Why can't you tell the computer to take this module from this position to this position and let it go? Why isn't all the technical measurements programed in so the computer can compute a safe path? Why do we need astronauts outside to watch and give instructions to the guy inside on how much space is left and which way to move the module? It just seems so inefficient. We have industrial robots making cars that could move what ever to where ever quickly and accurately. Why aren't we using this technology? And my last question ... Why does the arm move so excruciatingly slow? |
| What happens if an industrial robot breaks or screws up? A part might be damaged. What happens if a robot damages the ISS or shuttle? At best millions in losses at worst people and equipment are lost. I would be humans controll it and have it move slow on purposes so they can stop something before it breaks. |
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Because you cant afford a miscalculation. A module given its own momentum could cause irreversible damage to the ISS if a simple HUMAN error were made in entering calculations or in programming the way the onboard computer handles them. As for the speed you cannot generate too much momentum because you can negate it at the end of the track. You cant just jerk suddenly to a halt because that will then translate to the structure holding the arm/item. Start slow, move slow, and end slow... |
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My guess is its just a matter of controlling the amount of mass of the P6 array. They probably could move it all with the station arm since I remember reading years ago that the station arm was designed to grab the shuttle and help it dock, but they realized that they didn't need that later on. So its sized for it, but since they had the shuttle there they could just stage the movement via the mulitple arms and not have to worry about moving all that mass with the moble base system and control it. ETA: Here is a link that shows some facts, including how much mass the arm can hold. it confirms what I understood, the arm can hold the shuttle if needed. ETA2: Found more details on the arm, it certainly can hold the mass. I think its just a convervative approach more than anything. check it out here |
I work with industrial robots and can answer some of these questions. As to the moving so slow, it probably has something to do with the amount of inertia the arm can create by moving fast. These things weigh a lot and if they were moving around in space as fast as we move them here you're going to have to have an extremely strong base that it connects to. We have them mounted to big heavy steel pedistals that are bolted into concrete. I'd guess that they are using something a little more lightweight and not as durable, hence they would need to move the robot slower. Also, the momentum could be enough to move stuff around up there since the station itself is just "floating", though this is just hypothetical on my part. In reference to why there's a human driving it.....that's probably the easiest way to do it. Sure we've got robots building cars and everything else but all of those motions are repetitive. The robot is doing the same thing over and over and over. In order to get the robot to do that, each point has to be taugh individually by a person. Since the robots on the station are making different moves each time to assemble the pieces in different places this makes it more complicated. Yeah it could be "taught" to do it automatically but it would have to be done once first to teach it the moves. If we were building 100's of stations then that would be the way to go but just doing everything once, doing it manually is easier. Not to mention, if a robot is online running a program and it makes a mistake, it doesn't always stop until it crashes into something. The only way to be 100% sure that it doesn't crash is with a human doing it manually and slowly. As far as why the two arms were handing stuff back and forth, I have no idea. |
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Thanks! Knew there was someone who knew where to look. The second link seems to indicate that they can set it to do automatic movement... I wonder exactly how much. Slow does make a certain amount of sense... I suppose. I didn't see any info on how much the mobile base system can hold... |
Look on the 3rd line of the box/table and it shows how much mass each part can hold. |
The center of mass of all the parts, taken as a whole, will stay in the same orbit as the parts get shifted around. They could accelerate the module/arm for half the trip (up to the arm's design limit), and decelerate for the other half, but why go fast when, if something breaks, you can't just take a walk over to Ace Hardware? I wouldn't be surprised if the handoffs are just to get practice. Might have been to clear some obstacles along the way, though. Unfortunately I didn't see it. 
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| I could understand if it for some reason had to go from one side of the shuttle to the other. IE there was something the arm had to go "under". But I don't think that was the case. They seemed to not want anything but the arm on the mobile base while it was moving. |
Probably because they want to keep the stresses on the mobile base to a minimum. Even in a weightless environment (well, freefall anyway), there's still mass to deal with. The force needed to start and stop movement will obviously increase when mass increases. That's likely the same reason things moved slowly--less momentum, less stress on the parts. If something breaks, it's not like they can go down to the hardware, buy a replacement, and be back up and running in a few minutes. Remember too that here on the surface, we've got gravity to help stop movement. In orbit, once something starts moving, there's really nothing to stop it. So you're not going to want to impart a lot of momentum to an object, because you're going to have to apply more force to get it to stop moving. |