DH Method DH parameter DH coordinate system

[feng...@126.com]

dear sir:
Motoman HP6 (6 axis, RRRRRR, stdDH)
+---+-----------+-----------+-----------+-----------+
| j | theta | d | a | alpha |
+---+-----------+-----------+-----------+-----------+
| 1| q1| 0| 0.15| -1.571|
| 2| q2| 0| 0.57| -3.142|
| 3| q3| 0| 0.155| -1.571|
| 4| q4| -0.635| 0| 1.571|
| 5| q5| 0| 0| -1.571|
| 6| q6| -0.095| 0| 3.142|
+---+-----------+-----------+-----------+-----------+
it is the table of mdl_MotomanHP6's DH parameters,I DON'T know why
the parameter " d",which defines the"distance", can be negative
number...additionally, According to the DH method, I establishes the
coordinate system,but my DH parameters don't equal with this.the
mdl_puma560 also have the problem.maybe i'm wrong but i have read many
papers ,the parameters they gives were also different with it.Some
the shaft Zi was built at joint i,and some was at joint i+1. so ,i
really really want to know ,,how do you establishe the coordinate
system and how do u get the DH parameters,expecially the negative“ d"
parameters. thanks a lot I need your help !

[Peter]

As I explain in the book there are two quite different conventions for
DH coordinates. This makes life very confusing...

I don't know much about the Motoman model, it was submitted by a third
party. In principle there is no problem with a negative a or d
parameter, it all just depends on the way the frames are assigned.

I'm not quite sure whether DH parameters are a help or a hindrance in
robotics, hence my interest in autogenerating them as discussed in
Chap 7.5.

peter.

[feng...@126.com]

really thanks for your answer```maybe i should read your book again
and again ,and again!！ thanks a lot again!