Strange output using symbols in a matrix

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Jason Gauthier
Jason Gauthier 2018-10-13
评论： Jason Gauthier 2018-10-13
I always seem to have some weird results when using symbols in matrices. I wrote a simple function that generates a transformation matrix based on arguments.
function rot = dh_t (a,alpha, d, theta)
rot = [cos(theta) -sin(theta) 0 a;...
sin(theta)*cos(alpha) cos(theta)*cos(alpha) -sin(alpha) -d*sin(alpha);...
sin(theta)*sin(alpha) cos(theta)*sin(alpha) cos(alpha) d*cos(alpha);...
0 0 0 1]
end
When I call this function:
dh_t(0,pi/2,L1,t2)
I get an ugly output.
[ cos(t2), -sin(t2), 0, 0]
[ (4967757600021511*sin(t2))/81129638414606681695789005144064, (4967757600021511*cos(t2))/81129638414606681695789005144064, -1, -L1]
[ sin(t2), cos(t2), 4967757600021511/81129638414606681695789005144064, (4967757600021511*L1)/81129638414606681695789005144064]
[ 0, 0, 0, 1]
I stick this in my calculator where I wrote an equivalent function:
[cos(t2) −sin(t2) 0 0]
[sin(t2) cos(t2) 0 0]
[0 0 1 l1]
[0 0 0 1]
Suggestions are welcome on how to handle this. Thanks!
  2 个评论
madhan ravi
madhan ravi 2018-10-13
I stick this in my calculator where I wrote an equivalent function:
What do you mean by this? can you elaborate more?
Jason Gauthier
Jason Gauthier 2018-10-13
Yes. It doesn't happen with my Ti-Inspire calculator.

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回答（2 个）

Walter Roberson
Walter Roberson 2018-10-13
dh_t(0,sym(pi)/2,L1,t2)
Star Strider
Star Strider 2018-10-13
Use the vpa (link) function.
Result = vpa(dh_t(0,pi/2,L1,t2), 3)
  3 个评论
显示 1更早的评论
Jason Gauthier
Jason Gauthier 2018-10-13
although that appears more to be an issue with cos/sin inaccuracies.
Star Strider
Star Strider 2018-10-13
I suspect that the TI-inspire is not using the extended-precision representation that the Symbolic Math Toolbox does. It may instead be using the IEEE 754 standard, and considers 6.12e-17 as essentially 0.

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