#robotics Logs

Dec 29 2013

06:48 SilentAssassin If I am computing for example inverse kinematics for a robot and I want to weight the movement (lets say, minimize the movement with ankles and maximize with knee) can I just multiply the components in the jacobian matrix by some constant?
08:03 ace4016 i'd probably have to look at the math again....don't remember the specifics
08:03 ace4016 but
08:04 ace4016 typically the ankle/wrist won't move much
09:00 SilentAssassin also second question, i am simulating a robot in a virtual environment, and i want to calculate his zero moment point in the end of the animation, (for example lifting foot),
09:00 SilentAssassin for calculating ZMP i need the acceleration, the problem is I don't know how to approximate acceleration from this...
09:00 SilentAssassin should I consider constant acceleration? or constant velocity
09:01 ace4016 it's calculatable
09:02 SilentAssassin and why im asking this is because the simulator accepts only angular velocities for joints..
09:02 SilentAssassin so suddenly its moving
09:03 ace4016 you should be able to sample or know how quickly something is moving, and how quickly its changing, if it's moving at all
09:03 SilentAssassin okey so I sample velocity in discrete time moments... but then what
09:05 SilentAssassin currently im calculating the acceleration from approximating the second derivative of position, ((pos(t+2) - pos(t+1)) / 2 - (pos(t+1) - pos(t)) /2) / 2
09:05 SilentAssassin not sure if its correct though
09:06 ace4016 that would be almost correct
09:07 SilentAssassin hmm
09:07 ace4016 ((pos(t+2) - pos(t+1)) / d2 - (pos(t+1) - pos(t)) /d1) / d3; where dx is the time difference between the two measured position points
09:08 ace4016 if you can get velocity directly, just take the differentiation of that (V2-V1/dt)
09:08 ace4016 that would be acceleration
09:09 SilentAssassin oh... right
09:11 SilentAssassin ace4016 by the way d3 is what
09:12 SilentAssassin oh right... nevermind
13:09 MrCurious i am finding as i get older, a new reason to dread the holidays
13:26 GargantuaSauce if i see one more car with fucking antlers i am going to lose my shit
13:28 Tom_itx scrooge
13:33 MrCurious was wanting a set of those for my minivan
13:33 MrCurious but i worried it might send the wrong message
13:35 MrCurious end of the year, ,all the news channels have is old recycled news
13:35 Tom_itx they don't want to ruin your hope of a new prosperous year
13:37 MrCurious i think the period of prolonged, unneccessary stagnation will continue until the do nothing congress gets its shit together
13:38 MrCurious looking up past friends who you went to college with to see where they are now...
13:51 MrCurious then again tom... we make stuff... we have rooms specifically configured for the fabrication of hope....
14:11 robotustra hola amigos
14:28 MrCurious whats for lunch? lamb vindaloo.... Oh what a surprise.
15:18 john888 hello how do you get the matrix from the table of denavit-hartenberg parameters?
15:19 MrCurious http://en.wikipedia.org/wiki/Denavit%E2%80%93Hartenberg_parameters
15:19 MrCurious the math there makes my eyes bleed
15:22 john888 it should be 4 x 4 homogenous
15:25 john888 give me the link for an example
15:27 MrCurious dood, math is beyond me, i just like to look up terms mentioned here to learn along with the fow
15:27 MrCurious google has to be able to give you what you want. perhaps you are asking the wrong question
15:27 john888 i know what am i asking
15:28 MrCurious i mean, switch up your search terms to match what someone who has a page would have llikely said...
15:28 john888 and who is here dexter?
15:29 MrCurious any dexter here?
15:29 MrCurious dexter? yoo hoo dexter.... come out come out
15:30 john888 dexter means an intelligent person who builds robots
15:30 john888 like that cartoon
15:31 MrCurious oh, we have lots of them here. down side, is that time zone thing... have to wait about for one ot wake up, catch up, and answer
15:32 john888 also for hartenbergs parameters?
15:33 MrCurious GargantuaSauce: rue_house rue_shop3 robotustra any of you familiar with 4 x 4 denavit-hartenberg parameters
15:33 MrCurious sometimes you just have to invoke the right names...
15:33 GargantuaSauce yes but i gotta run
15:34 MrCurious ok john888 catch up with GargantuaSauce another time for help :)
15:34 GargantuaSauce the [T]=zxzxzx is the answer to your question though
15:34 MrCurious *mind blown*
15:36 MrCurious john888: did you see the answer?
15:36 john888 yes
15:37 john888 i still don't understand
15:38 john888 you've got the table of 4 parameters for each joint
15:39 GargantuaSauce you build the individual matrices using the rotation and translation of each segment
15:39 GargantuaSauce and then multiply them all together
15:39 GargantuaSauce if your rotations are specified as euler angles then there are three rotation matrices
15:39 MrCurious http://www.scribd.com/doc/173507226/Mastersds-Cours-Robot-Boimond may help
15:41 john888 what if we got 3 rotations RRR
15:42 MrCurious if i would have known robotics involved so much advanced math... i think i may have opted for a different hobby
15:43 john888 for every R joint we got a matrice
15:43 MrCurious looks like you have gone beyond simple control, and are now working on control that features grace
15:43 john888 are these matrices prebuild
15:43 john888 ?
15:45 MrCurious this helpful? http://www.learningace.com/doc/4380333/2328536965448419cac3f3becb3c4408/11-2004-10dof-lower-body-exo
15:46 john888 no
15:46 john888 where are dexters
15:47 MrCurious oddly, they have lives...
15:47 MrCurious answers are here, but it does take patience
15:52 john888 how to build the matrix from d, a, alpha, theta
16:08 robotustra what is d?
16:08 robotustra and a?
16:08 john888 hartenberg parametrs
16:11 robotustra here are matix http://en.wikipedia.org/wiki/Denavit%E2%80%93Hartenberg_parameters
16:12 john888 look you have the table of paramters
16:13 robotustra there is no A parameter in there
16:13 robotustra is it a distance?
16:13 john888 construct matrix 4x4 from those data
16:13 robotustra and what?
16:13 john888 theta, d , alpa
16:13 robotustra 4x4 matrix is rotatin + translation
16:14 robotustra do you know what is matrix and how to do matrix multiplication?
16:14 john888 absk
16:14 john888 absolutely
16:14 MrCurious dot products and cross products and shit like that?
16:15 robotustra so, what's your problem then?
16:16 john888 how to get from table of 4 paramaters to matrix
16:17 robotustra ARE YOU IDIOT?
16:17 robotustra it's WRITTEN OUT
16:18 MrCurious robotustra: want me to break out the sock puppets?
16:18 john888 rotation + translationž
16:18 john888 that's it
16:19 robotustra yes, the submatrix of rotetion iconsist of cosines ans sines
16:19 robotustra with angles
16:19 robotustra and translation matrix it the last coordinate
16:19 robotustra how old ate you?
16:20 MrCurious are
16:20 robotustra most probably you are not know university course of math
16:20 MrCurious knowing peoples ages assists in helping to target the answer in the proper level of complex/simple
16:21 john888 i am programmer
16:21 MrCurious it saves frustration on all parts...
16:21 robotustra progremmer or coder?
16:21 MrCurious you had linear algebra in school how long ago?
16:21 robotustra programmer is not a scientist
16:22 MrCurious or was it combinatorics... i forget
16:23 robotustra I would reccomend you to start with 1 shoulder robot
16:23 robotustra and work with 1 matrix
16:23 robotustra for the beggining
16:24 MrCurious john888: you got any pics of the arm or robot up on net, i am getting curious if this is academic pre-build work, or post build work
16:25 john888 let's say if I get parameter alpha, where do I put it in matrix 4x4?
16:26 robotustra john888: if I would be next to you I would do like this: http://www.youtube.com/watch?v=HcXNPI-IPPM#t=398
16:26 MrCurious see the -sin alpha sub i, i+1
16:26 MrCurious in the [Xi] =
16:26 robotustra you don't put alfa, but cos(alpha), -sin(alpha)
16:26 MrCurious would guess replace the alpha there with your alpha
16:27 MrCurious in all 4 places
16:27 MrCurious alpha is the letter that looks like a A or christian fish symbol
16:27 MrCurious a small a rather
16:29 john888 hello
16:29 MrCurious where αi,i+1 and ri,i+1 define the physical dimensions of the link in terms of the angle measured around and distance measured along the X axis.
16:30 MrCurious oh! the wikipedia page has a link to youtube explanation http://www.youtube.com/watch?v=rA9tm0gTln8
16:30 MrCurious that may shed light you need
16:31 john888 this a good video
16:31 MrCurious shame you didnt read the page when i gave it to you an hour ago :)
16:32 MrCurious you would have seen the link to this video
16:33 john888 but the problem is no examples
16:33 john888 no example
16:34 john888 where is an examplež
16:35 john888 whera are all people?
16:36 robotustra all people are prepairing the NY
16:37 robotustra I saw this video and everything is clear for me
16:38 MrCurious perhaps reading the whole page fully, and watching the video a few times and making diagrams would be the best path forward
16:38 robotustra but if you could not umderstand where to put parameters, may be you leave this task?
16:38 MrCurious then after an hour of study, revisit the conversation
16:39 MrCurious or print out the page and take a highlighter to it as you read and find gems of clue
16:40 MrCurious and being a programmer myself, i know programming is more about repeatedly banging your head against a wall to solve the problem...
16:40 MrCurious because once you do that for a few years, you will find the problems you face willl be new and novel, and ones others around you have not worked with.
16:41 john888 make the video for youtube for danavit-hartenberg parameters
16:41 john888 with examples
16:42 robotustra I think john888 is chineese
16:42 MrCurious perhaps if you googled for danavit-hartenberg example or danavit-hartenberg implementation
16:42 robotustra and he is not so sharp
16:43 MrCurious should keep in mind, this is not a paid service, but more a club of people sharing freely
16:43 MrCurious social finesse is required
16:45 robotustra so, john888 write please rotation matrix around axis Z on angle alpha
16:46 MrCurious no slovania....
16:46 john888 you got the table
16:47 john888 of four parameters
16:47 MrCurious i bet he speaks russian :)
16:47 robotustra not russian
16:47 robotustra if he sloevenian - most probably he'll understand these equations
16:48 robotustra john888: write me the matrix, line by line
16:49 robotustra ?
16:49 robotustra you told me that you know what is matrix
16:50 robotustra you know, leafs could not grow in the air
16:50 john888 link [ 1 [ 2 [3
16:50 robotustra first you have to grow the tree, brunches
16:51 robotustra and only after there will be leafs
16:51 robotustra you don't know what is matrix, so, may be first you'll study algebra?
16:51 robotustra linear algebra then
16:52 john888 i studdy robotics and mehatronics at MIT
16:52 robotustra but if you don't have visual 3D processor in your brain, sorry
16:52 robotustra I always knew that MIT has very shitty level
16:53 robotustra especial in robotics
16:53 robotustra especially
16:53 robotustra I saw DARPA competition, AMERICA HAS FAILED
16:53 robotustra it was EPIC FAIL of America in ROBOTICS
16:53 MrCurious shouldnt you have access to a professor who can explain this to you?
16:53 robotustra and it was a team from MIT
16:54 MrCurious or is this homework that you cant ask him for help on
16:54 robotustra they also failed
16:55 robotustra and now dumb student twhich was involved in Atlas programming try to understand why he has failed :))))
16:55 MrCurious john888: what is your native language may i ask
16:55 robotustra I think he is trolling
16:55 john888 let me again question, you'v got table of paramateres for each link, ho to get matrix from thisž
16:56 MrCurious yeah, i dont see the give and take of information happening. its just all take. now i ose interest
16:56 MrCurious lose
16:56 robotustra and most probably he use google translate to speak
16:56 john888 native langugage is croation
16:56 MrCurious john888: thank you
16:56 robotustra ты по русски читать умеешь?
16:57 john888 are you all from russia
16:57 MrCurious we are from all over
16:57 robotustra I'm from Canada
16:58 MrCurious but sometimes dropping to ones native llanguage can facilitate communication
16:58 robotustra don't think so
16:59 john888 answer my question and the i will leave
16:59 robotustra how stupid croatian could enter MIT if there is a queue there from the birthof kids?
17:00 MrCurious read the wikipedia page, follow alll links, read them too, then you will know the answer
17:00 john888 whaz
17:00 robotustra 1) you read internet about these parameters
17:00 robotustra 2) 10 times
17:00 robotustra 3) think 3 days
17:00 robotustra 4) if you didn't got the answer - don't come back here with this question
17:01 MrCurious if robotics was easy, everyone would be doing it just like laundry
17:01 robotustra if you could not solve this simple task yourself - it's a pity to say, you are not ready for robotics
17:01 MrCurious my robotic quest is going on 3 years, and i am not ready for that math :)
17:02 robotustra this math is simple
17:02 robotustra he didn't write a rotation matrix yet
17:02 robotustra MrCurious: could you?
17:02 robotustra Z axis, the angle = alpha
17:03 robotustra what is the rotation matrix
17:03 robotustra use google
17:04 robotustra or you can't
17:04 robotustra ?
17:04 robotustra why, only I know how the matrix will look like?
17:06 robotustra john888
17:06 john888 what?
17:06 robotustra do you know the answer?
17:07 john888 no
17:08 robotustra the matrix will be:
17:08 robotustra | cos(alpha) sin(alpha) 0 |
17:08 robotustra |-sin(alpha) cos(alpha) 0|
17:08 robotustra | 0 0 1|
17:09 robotustra do you know what "cos" and "sin" mean?
17:09 john888 yes
17:09 robotustra great
17:09 john888 so this matrix is precomputed
17:10 robotustra if you have fixed angles - yes
17:10 robotustra and you probably have
17:10 robotustra for every particular robot
17:10 john888 so i plug alpha in
17:10 john888 and that
17:10 john888 is it
17:11 robotustra it's only exaple of simple rotation around axis Z on angle alpha
17:11 robotustra if you do rotation for example around axis Y the matrix will be different
17:11 john888 ok this is for alpha
17:12 john888 what about for a
17:12 john888 and d
17:12 robotustra this martix is not applicable for your case
17:13 robotustra do you know how translation matrix looks like?
17:13 john888 4 x 4
17:14 robotustra for instance you have translation along axis Z on D
17:14 robotustra how the matrix will look like?
17:15 robotustra do you know the answer?
17:15 john888 you add in another matrix 4x4 and mulitply with original
17:15 robotustra write out this matrix for me here
17:15 robotustra for instance you have translation along axis Z on D
17:16 robotustra do you know the answer?
17:16 john888 [ 0 0 1 0 ];
17:17 john888 [0 0 d 0];
17:17 robotustra it's a vector
17:17 robotustra that's it?
17:17 robotustra no, the matrix will be:
17:18 robotustra | 1 0 0 0 |
17:18 robotustra | 0 1 0 0 |
17:18 robotustra | 0 0 1 D |
17:18 robotustra | 0 0 0 1 |
17:19 john888 where do i plug those parameters?
17:19 robotustra now if you want to do rotation along Z at angle alpha and translation D along Z
17:19 robotustra I would say.... in your ....
17:20 robotustra equation
17:21 john888 there are any equations
17:21 john888 matrices and table
17:22 robotustra I would better watch youtube about karma
17:25 robotustra read this article http://www4.cs.umanitoba.ca/~jacky/Robotics/Papers/spong_kinematics.pdf
20:19 MrCurious wonder if john88888 (mister mit in croatia programmer) figured matricies out
20:38 MrCurious anyone planning on making any robotic new years revolutions?
20:38 Jak_o_Shadows I might tidy the shed
20:38 Jak_o_Shadows does that count?
20:39 MrCurious does it involve hoards of robots taking over the world, or your local region of it?
20:39 MrCurious note revolutions, not resolutions
20:39 MrCurious but i think you are onto something with the idea of tidying up the HOPE fabrication lab
21:16 Jak_o_Shadows Now to find those new drillits
21:58 Jak_o_Shadows too much stuff, not enough shed
22:08 MrCurious what was it the tardis used to be bigger on the inside than on the outside...
22:08 MrCurious work spaces need that
22:08 Jak_o_Shadows Yeah.
22:08 Jak_o_Shadows I moved some stuff around, can get to that stuff easier.
22:08 Jak_o_Shadows also have more floor space.
22:08 Jak_o_Shadows It's a pity about the stuff sitting outside
22:17 MrCurious i have been reorging mine too. half way done, then life called and demanded attention
22:20 rue_house MrCurious, no
22:20 MrCurious its cool rue_house robustra solved it by sending him back to understand matrix math
22:21 robotustra it was evident that he is weak in the math
22:22 Jak_o_Shadows It frustrates me that numpy defaults to elementwise artihmetic.
22:24 MrCurious one would think MIT would demand skill in that area from its students before making them do robotics...
22:24 robotustra he is not from MIT
22:24 robotustra definetly
22:27 robotustra I go sleep
22:27 robotustra good night
22:28 Jak_o_Shadows night
23:05 rue_shop3 I find no place for matrix math in a real, even half optimized world
23:08 Triffid_Hunter rue_shop3: don't get into IK, kalman filters or 3d rendering then :P
23:09 Jak_o_Shadows Matrices are just a convenient way to hold information, and to do common operations on them.
23:37 Jak_o_Shadows I now have more floorspace. The workbench is a damn mess, but it's getting there
23:57 rue_shop3 I have NO proble3ms doing the math without matrixies
23:58 rue_shop3 matrixes are a convienient way to convolude a simpel process or to optimize a process that always happens the same way
23:59 rue_shop3 I have no problems passing data sets to functions that dont do pointless math on half the numbers that dont need to be changed