斯坦福大学网络视频课程之机器人学（英文版）.doc

斯坦福大学网络视频课程之机器人学Instructor (Oussama Khatib):Okay, lets get started. So todays movie segment isabout a special actuator. Probably you saw this on the first lecture, but we will see somemore details. So this is a flexible actuator that comes from Toshiba, and this wasdeveloped in the early 90s. It was presented at 1991 video proceedings.Video:This new actuator is made of fiber re-enforced rubber and is driven pneumatically orhydraulically. It has 3 degrees of freedom; pitch, inaudible, and stretch, which areadequate for robot mechanisms such as fingers, arms or legs.The actuator has three internal chambers, and the pressure in each can be controlledindependently though flexible tubes. The rubber is circularly re-enforced with fiber toreduce deformation in the radial direction.The actuator can be flexed in every direction by controlling the pressure in each chamber.Inaudible developed actuators ranging in size from 1 millimeter to 20 millimeters indiameter. This is the formula meter actuator. The design is easily miniaturized because ofits simple structure.This is a modified version. The rubber is reinforced spirally with fiber so that rotationalmovement is possible. We can apply these flexible micro-actuators to miniature robotmanipulators. By connecting them serially, we get an arm with many degrees of freedomand snake like movements.This is a prototype consisting of two actuators and a mini gripper. It has 7 degrees offreedom including the gripper. It can accomplish delicate tasks, which could be handledonly with great difficulty by conventional robots.Constructing miniature robot manipulators is easy because the actuators also act as therobot structure. On the other had, combining the actuators in parallel results in a multifingeredrobot hand. They form a dexterous hand with a delicate touch. This prototypeconsists of four actuators, each 12 millimeters in diameter, and it has 12 degrees offreedom.Its able to handle fragile and complicated work with ease, because the actuatorsdeformed to suit the shape of the work piece itself. The bolt is easily tightened with onlyrough settings of the position and orientation of the hand because the actuators have suchgood compliance.Miniature robots with a soft touch and no conventional links can be created using theseactuators. People see the use of flexible micro-actuators Instructor (Oussama Khatib):So, what do you think? What would be the advantages ofusing pneumatic in this way? Yes?Student:Probably safer for a lot more objects.Instructor (Oussama Khatib):Safer, you said? So safety is a very, very important aspectof the design of a robot, especially if the robot is going to interact with humans, and youreally dont want this robot to just go crazy and hit and make a large impact. So softactuation using pneumatic is very good because it basically its compliant, right.Now another implication of the fact that you are using pneumatic is the structure of therobot is going to be lighter, because if you think about operating these fingers with themotors or inaudible I think about an arm with motors. You need to carry the motors,you need to put gears, you need a lot of structure to handle it, so definitely this is lighter,safer, more flexible, compliant, all of that.Any disadvantage you can see? Yes?Student:Harder to control?Instructor (Oussama Khatib):Yeah. Basically well, I mean, it depends what you wantto achieve, but you cannot expect to achieve the tasks. What kind of tasks you cannotcontrol with this type of actuation?Student:Inaudible motion?Instructor (Oussama Khatib):Inaudible motion or fast dynamics if you want tochange directions because? What is the problem with that inaudible?Student:InaudibleInstructor (Oussama Khatib):So yeah, I mean basically the response of this is going tobe slow because you are using air pressure and you cannot push the air pressure to a pointwhere you can really get fast dynamics. Well later on in the quarter we will see a conceptthat combines this idea of using pneumatic, which is light to carry, and would result intoa nice light structure, combine it with other type of actuation to bring hybrid actuation ina way that combines both the advantages of the light structure and the fast dynamics thatwe need to achieve all the different tasks that would require the robot to respond quickly.Yes?Student:Is there any good way to inaudible that, like, you dont actually know whereyoure inaudible?Instructor (Oussama Khatib):Well that inaudible, you would see the robot was liketurning. Okay, anyone would like to answer this question? Im sure you have an idea, but,this is really not fundamental to the robot design. It is more on the fact that we have noexternal feedback or no touch, or we are not using the information about the touch torealize that we already left that contact.But this is actually something that you can add on top of the design to like would What kind of sensory you would use?Student:Pressure sensor?Instructor (Oussama Khatib):Touch sensor, you mean like you want to know if you aretouching or not?Student:Pressure would be inaudible to know exactly like how far you are, but withsomething like that, inaudible. Like inaudible put something that exactly tells you theposition of the end.Instructor (Oussama Khatib):So we can put a sensor at the end that is localized at thetip of the finger and then we can feel whether the sensor is on or off. That is just touch,but if you want more information about the pressure, you need a sensor, inaudiblesensor that would measure.But then the problem I mean, now we come into a much harder problem, which is thefact if youre holding something lets imagine with your two fingers youre holdingsomething. And there is always slip, so you need to measure the slip so that you canapply larger pressure.And to do that you need the sort of dynamic inaudible. So there has been a lot of workactually in the group of Mark Cokoskey. A lot of research on inaudible, dynamicinaudible and also the idea of using pressure is a very good idea because in fact, bymeasuring the pressure and the control pressure there is a difference between what youare expecting and what you see.You will be able to deduce some information about contact. There was a comment there?Student:Inaudible sensors on the surface and use those to find the position?Instructor (Oussama Khatib):Um hm. Now if you instrument the environment,obviously you will be able to get a lot of information about the environment, but that iscostly. So you want more to put the sensors on the robot. But there is another type ofsensor that will give you more information about the environment that especially aboutwhether you are in com I mean, close or not, like to localize and see where things are.To see where things are, what do we call?Student:Inaudible Instruction:Vision. Put couple of cameras and you would see where you are with respect to theworld. And so you have a mechanism, you have the controllers, but really you need toclose the loop. But to close the loop you need perception, and perception could usesensors in the environment, sensor external that are monitoring the environment orinstrumentation on the robot itself.Anyway, this cute design actually was pursued for a couple of years. They built even abig robot that is walking with those legs, and Im not sure if we will see it. Then thisproject just I mean, they didnt go any further.It is like many of the designs that make use of air pressure, only you end up with reallylimitations, a lot of limitations on the use of on the ability of the robot to form tasks.And in fact there is a lot of work today in this area that is with artificial muscles to createfaster muscles that use air pressure.And there are many different solutions that will push this little further, but still you havelimitations, and as I said, we will discuss little bit more about those issues of design,especially in the context of safety because safety really, really is becoming a veryimportant aspect in robot design because we have been working with robot withinaudible robots.So inaudible robots are working alone or working with parts and objects you dontreally worry too much if there is an accident just between well, the robot and thatenvironment. But if you are going to work with humans, you really have to make surethat there is no danger to the human, and that is really a challenging problem, so we willcome back to this later.Any other comment about this? Okay. All right, so lets go back to the lecture. So lasttime we saw this tool we call homogeneous transform, and the homogeneous transformreally has several interpretations, or can fulfill several functions.And the first one of them is the fact that transformation like this allows us to describe theframe, so frame B is described with the respect of frame A given this transformation. Soif I know the homogeneous transformation between B and A, that is this four by four TMetricsthat describes A B in with a relation to A.Then I have description of this frame B, and this description contains the rotation of theaxis of frame B with respect to A, and the location of the origin of frame B with respectto frame A. Now we saw also that there is another role this homogeneous transformationcan play.And this second role third role, whatever, do you have an idea what can we use thistransformation for? What can we what can this transformation help us do?Student:Operations?Instructor (Oussama Khatib):Operations, that is we can inaudible thetransformation as an operator that is acting on a vector and changing this vector, rotatingthe vector or rotating and translating that vector. So this is a second interpretation of thetransformation as an operator. Or? One more?So if you have a vector in space, describe with respect to some frame, B, and you want itsdescription in a different frame, A, can you use this transformation? So this is themapping, what we call mapping. That is, we take the description of vector P in a frame well, B, and we map it to a description in frame A.So the vector B, this green vector over there is now the red vector that is this one,describing this point in frame A.And you can see here we have two different vectors. If there was no translation, thenbasically it will be the same vector with two different set of components. And as yousaid, we have also the description of homogeneous transformation as an operator.That is we take a vector P1 and change it to a vector P2. So the vector P1 is now P2 afterthis translation. So these different roles of the homogeneous transformation use the samemathematics, but the application, the interpretation is going to be different and we have topay attention to the way we apply the definition.So the next question that we have to address is how we now use. So if you remember,when I talked about homogeneous transformation, I said by building this metrics, four byfour metrics in a higher dimensional space, we are able to have a homogeneous relationbetween vectors.So this property is going to help us propagate and go from one frame to another and havedescriptions that are related by the individual transformations between frames. So here isan example; you have this camera monitoring the environment and here in fact you havea robot, a mobile manipulator, this is Romeo, that is did I introduce Romeo and Juliet toyou? No? Not yet?Okay, well maybe you will have a chance later. So Romeo is essentially a mobileplatform, a holonomic mobile platform with an arm, and it allows you to move in theenvironment and manipulate the environment. But because of the platform, this is doneeverywhere in the world, not like when you have just one arm fixed on a table where youbring material to be processed.Here you are able to explore the human environment. And, in fact, the robot is movingand its location is always difficult. So the question is how can we, for instance, locate thisrobot? So you need to find, actually, the transformation between the base frame of therobot with respect to the camera.This is little bit difficult unless you are able to find elements and different things. Sosuppose the camera is monitoring the endofactor. So you have the endofactor here andyou can see it. So if you have this relation that is in a given frame, you are able to see andidentify the end of factor, which is, lets say this frame. Then through the other path thatis going from here to the base through those transformations to the endofactor, you haveanother path.You know this and you know this then you can compute this. And we need to be able topropagate and resolve this transform equation. Now this comes everywhere, yeah, well,Romeo is capable inaudible. I will show you maybe later if we have a chance, little timein the lecture.So again, where you are inaudible, where is the base and you have this loop in theenvironment if you are observing the environment, and you need to be able to go throughthe transformations. From the base to the endofactor, you have always the forwardkinematics through all the transformations between links.But obviously with the ground you have slippage and you cannot determine exactly therelationship between a fixed camera and the location of the base.So first of all, how we combine transformations; lets consider two frames, and considerthat now were going to introduce a third frame, C, so we have a transformation from Cto B, we have a transformation from B to A, and obviously Im interested in thetransformation from C to A, the total transformation.I would like to combine these two transformations. So if we have a vector, vectordescribed in C, that is, we have a point P, describe in frame C. The question is what is thedescription of this vector in frame A?Well the result is obvious; I mean probably you already get it. We are going to multiplythese two transformations. And to prove it, lets first compute what is the transformationto frame B. P and B is simply going to be obtained by the homogeneous transformationfrom C to B, right?Now if we write the same thing for this point in B, we can go to A through thehomogeneous transformation, B to A. Now is we substitute this with the expression thatuses C, we will obtain this relation; that is, the vector in C is transformed into thedescription in frame A using this form, which mean that essentially we are going fromdescription in C to A. That is, from C to A. And that means essentially, thetransformation corresponding to these two