kinematics.lyx

CNC 的开放码,EMC2 V2.2.8版

#LyX 1.3 created this file. For more info see http://www.lyx.org/
\lyxformat 221
\textclass book
\language english
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\layout Chapter

Kinematics in EMC2
\layout Section

Introduction
\layout Standard

When we talk about 
\begin_inset LatexCommand \index{CNC machines}

\end_inset 

CNC machines, we usually think about machines that are commanded to move
 to certain locations and perform various tasks.
 In order to have an unified view of the machine space, and to make it fit
 the human point of view over 3D space, most of the machines (if not all)
 use a common coordinate system called the Cartesian Coordinate System.
\layout Standard

The Cartesian Coordinate system is composed of 3 axes (X, Y, Z) each perpendicul
ar to the other 2.
 
\begin_inset Foot
collapsed true

\layout Standard

The word 
\begin_inset Quotes eld
\end_inset 

axes
\begin_inset Quotes erd
\end_inset 

 is also commonly (and wrongly) used when talking about CNC machines, and
 referring to the moving directions of the machine.
\end_inset 


\layout Standard

When we talk about a G-code program (RS274NGC) we talk about a number of
 commands (G0, G1, etc.) which have positions as parameters (X- Y- Z-).
 These positions refer exactly to Cartesian positions.
 Part of the EMC2 motion controller is responsible for translating those
 positions into positions which correspond to the machine 
\begin_inset LatexCommand \index{kinematics}

\end_inset 

kinematics
\begin_inset Foot
collapsed true

\layout Standard

Kinematics: a two way function to transform from Cartesian space to joint
 space
\end_inset 

.
\layout Subsection

Joints vs.
 Axes
\layout Standard

A joint of a CNC machine is a one of the physical degrees of freedom of
 the machine.
 This might be linear (leadscrews) or rotary (rotary tables, robot arm joints).
 There can be any number of joints on a certain machine.
 For example a typical robot has 6 joints, and a typical simple milling
 machine has only 3.
\layout Standard

There are certain machines where the joints are layed out to match kinematics
 axes (joint 0 along axis X, joint 1 along axis Y, joint 2 along axis Z),
 and these machines are called 
\begin_inset LatexCommand \index{Cartesian machines}

\end_inset 

Cartesian machines (or machines with 
\begin_inset LatexCommand \index{Trivial Kinematics}

\end_inset 

Trivial Kinematics).
 These are the most common machines used in milling, but are not very common
 in other domains of machine control (e.g.
 welding: puma-typed robots).
\layout Section

Trivial Kinematics
\layout Standard

As we said there is a group of machines in which each joint is placed along
 one of the Cartesian axes.
 On these machines the mapping from Cartesian space (the G-code program)
 to the joint space (the actual actuators of the machine) is trivial.
 It is a simple 1:1 mapping:
\layout LyX-Code

pos->tran.x = joints[0];
\newline 
pos->tran.y = joints[1];
\newline 
pos->tran.z = joints[2];
\newline 
pos->a = joints[3];
\newline 
pos->b = joints[4];
\newline 
pos->c = joints[5];
\layout LyX-Code

\layout Standard

In the above code snippet one can see how the mapping is done: the X position
 is identical with the joint 0, Y with joint 1 etc.
 The above refers to the direct kinematics (one way of the transformation)
 whereas the next code part refers to the inverse kinematics (or the inverse
 way of the transformation):
\layout LyX-Code

joints[0] = pos->tran.x;
\newline 
joints[1] = pos->tran.y;
\newline 
joints[2] = pos->tran.z;
\newline 
joints[3] = pos->a;
\newline 
joints[4] = pos->b;
\newline 
joints[5] = pos->c;
\layout Standard

As one can see, it's pretty straightforward to do the transformation for
 a trivial kins (or Cartesian) machine.
 It gets a bit more complicated if the machine is missing one of the axes.
\begin_inset Foot
collapsed false

\layout Standard

If a machine (e.g.
 a lathe) is set up with only the axes X,Z & A, and the EMC2 inifile holds
 only these 3 joints defined, then the above matching will be faulty.
 That is because we actually have (joint0=x, joint1=Z, joint2=A) whereas
 the above assumes joint1=Y.
 To make it easily work in EMC2 one needs to define all axes (XYZA), then
 use a simple loopback in HAL for the unused Y axis.
\end_inset 


\begin_inset Foot
collapsed false

\layout Standard

One other way of making it work, is by changing the matching code and recompilin
g the software.
\end_inset 


\layout Section

Non-trivial kinematics
\layout Standard

There can be quite a few types of machine setups (robots: puma, scara; hexapods
 etc.).
 Each of them is set up using linear and rotary joints.
 These joints don't usually match with the Cartesian coordinates, therefor
 there needs to be a kinematics function which does the conversion (actually
 2 functions: forward and inverse kinematics function).
\layout Standard

To illustrate the above, we will analyze a simple kinematics called bipod
 (a simplified version of the tripod, which is a simplified version of the
 hexapod).
 
\layout Standard


\begin_inset Float figure
wide false
collapsed false

\layout Caption

Bipod setup
\begin_inset LatexCommand \label{cap:Bipod-setup}

\end_inset 


\layout Standard


\begin_inset Graphics
	filename bipod.png
	scale 80

\end_inset 


\end_inset 


\layout Standard

The Bipod we are talking about is a device that consists of 2 motors placed
 on a wall, from which a device is hanged using some wire.
 The joints in this case are the distances from the motors to the device
 (named AD and BD in figure 
\begin_inset LatexCommand \ref{cap:Bipod-setup}

\end_inset 

).
\layout Standard

The position of the motors is fixed by convention.
 Motor A is in (0,0), which means that its X coordinate is 0, and its Y
 coordinate is also 0.
 Motor B is placed in (Bx, 0), which means that its X coordinate is Bx.
\layout Standard

Our tooltip will be in point D which gets defined by the distances AD and
 BD, and by the Cartesian coordinates Dx, Dy.
\layout Standard

The job of the kinematics is to transform from joint lengths (AD, BD) to
 Cartesian coordinates (Dx, Dy) and vice-versa.
\layout Subsection

Forward transformation
\begin_inset LatexCommand \label{sub:Forward-transformation}

\end_inset 


\layout Standard

To transform from joint space into Cartesian space we will use some trigonometry
 rules (the right triangles determined by the points (0,0), (Dx,0), (Dx,Dy)
 and the triangle (Dx,0), (Bx,0) and (Dx,Dy).
\layout Standard

we can easily see that 
\begin_inset Formula $AD^{2}=x^{2}+y^{2}$
\end_inset 

, likewise 
\begin_inset Formula $BD^{2}=(Bx-x)^{2}+y^{2}$
\end_inset 

.
\layout Standard

If we subtract one from the other we will get:
\layout Standard


\begin_inset Formula \[
AD^{2}-BD^{2}=x^{2}+y^{2}-x^{2}+2*x*Bx-Bx^{2}-y^{2}\]

\end_inset 


\layout Standard

and therefore:
\layout Standard


\begin_inset Formula \[
x=\frac{AD^{2}-BD^{2}+Bx^{2}}{2*Bx}\]

\end_inset 


\layout Standard

From there we calculate:
\layout Standard


\begin_inset Formula \[
y=\sqrt{AD^{2}-x^{2}}\]

\end_inset 


\layout Standard

Note that the calculation for y involves the square root of a difference,
 which may not result in a real number.
 If there is no single Cartesian coordinate for this joint position, then
 the position is said to be a singularity.
 In this case, the forward kinematics return -1.
\layout Standard

Translated to actual code:
\layout LyX-Code

double AD2 = joints[0] * joints[0];
\newline 
double BD2 = joints[1] * joints[1];
\newline 
double x = (AD2 - BD2 + Bx * Bx) / (2 * Bx);
\newline 
double y2 = AD2 - x * x;
\newline 
if(y2 < 0) return -1;
\newline 
pos->tran.x = x;
\newline 
pos->tran.y = sqrt(y2);
\layout LyX-Code

return 0;
\layout LyX-Code

\layout Subsection

Inverse transformation
\begin_inset LatexCommand \label{sub:Inverse-transformation}

\end_inset 


\layout Standard

The inverse kinematics is lots easier in our example, as we can write it
 directly:
\layout Standard


\begin_inset Formula \[
AD=\sqrt{x^{2}+y^{2}}\]

\end_inset 


\layout Standard


\begin_inset Formula \[
BD=\sqrt{(Bx-x)^{2}+y^{2}}\]

\end_inset 


\layout Standard

or translated to actual code:
\layout LyX-Code

double x2 = pos->tran.x * pos->tran.x;
\newline 
double y2 = pos->tran.y * pos->tran.y;
\newline 
joints[0] = sqrt(x2 + y2);
\newline 
joints[1] = sqrt((Bx - pos->tran.x)*(Bx - pos->tran.x) + y2);
\newline 
return 0;
\layout Section

Implementation details
\layout Standard

A kinematics module is implemented as a HAL component, and is permitted
 to export pins and parameters.
 It consists of several functions:
\layout Itemize


\family typewriter 
int kinematicsForward(const double *joint, EmcPose *world, const KINEMATICS_FORW
ARD_FLAGS *fflags, KINEMATICS_INVERSE_FLAGS *iflags)
\begin_deeper 
\layout Standard

Implements the forward kinematics function as described in section 
\begin_inset LatexCommand \ref{sub:Forward-transformation}

\end_inset 

.
\end_deeper 
\layout Itemize


\family typewriter 
extern int kinematicsInverse(const EmcPose * world, double *joints, const
 KINEMATICS_INVERSE_FLAGS *iflags, KINEMATICS_FORWARD_FLAGS *fflags)
\begin_deeper 
\layout Standard

Implements the inverse kinematics function as described in section 
\begin_inset LatexCommand \ref{sub:Inverse-transformation}

\end_inset 

.
\end_deeper 
\layout Itemize


\family typewriter 
extern KINEMATICS_TYPE kinematicsType(void)
\begin_deeper 
\layout Standard

Returns the kinematics type identifier.
\end_deeper 
\layout Itemize


\family typewriter 
int kinematicsHome(EmcPose *world, double *joint, KINEMATICS_FORWARD_FLAGS
 *fflags, KINEMATICS_INVERSE_FLAGS *iflags)
\begin_deeper 
\layout Standard

The home kinematics function sets all its arguments to their proper values
 at the known home position.
 When called, these should be set, when known, to initial values, e.g., from
 an INI file.
 If the home kinematics can accept arbitrary starting points, these initial
 values should be used.
 
\end_deeper 
\layout Itemize

int rtapi_app_main(void)
\layout Itemize

void rtapi_app_exit(void)
\begin_deeper 
\layout Standard

These are the standard setup and tear-down functions of RTAPI modules.
\the_end