Читать книгу Robot Modeling and Control - Mark W. Spong - Страница 57

Figure 2.5 shows a rigid object S to which a coordinate frame o₁x₁y₁z₁ is attached. Given the coordinates of the point p (in other words, given the coordinates of p with respect to the frame o₁x₁y₁z₁), we wish to determine the coordinates of p relative to a fixed reference frame o₀x₀y₀z₀. The coordinates satisfy the equation

Figure 2.5 Coordinate frame attached to a rigid body.

In a similar way, we can obtain an expression for the coordinates by projecting the point p onto the coordinate axes of the frame o₀x₀y₀z₀, giving

Combining these two equations we obtain

But the matrix in this final equation is merely the rotation matrix , which leads to

(2.9)

Thus, the rotation matrix can be used not only to represent the orientation of coordinate frame o₁x₁y₁z₁ with respect to frame o₀x₀y₀z₀, but also to transform the coordinates of a point from one frame to another. If a given point is expressed relative to o₁x₁y₁z₁ by coordinates , then represents the same point expressed relative to the frame o₀x₀y₀z₀.

We can also use rotation matrices to represent rigid motions that correspond to pure rotation. For example, in Figure 2.6(a) one corner of the block is located at the point pa in space. Figure 2.6(b) shows the same block after it has been rotated about z₀ by the angle π. The same corner of the block is now located at point pb in space. It is possible to derive the coordinates for pb given only the coordinates for pa and the rotation matrix that corresponds to the rotation about z₀. To see how this can be accomplished, imagine that a coordinate frame is rigidly attached to the block in Figure 2.6(a), such that it is coincident with the frame o₀x₀y₀z₀. After the rotation by π, the block’s coordinate frame, which is rigidly attached to the block, is also rotated by π. If we denote this rotated frame by o₁x₁y₁z₁, we obtain

In the local coordinate frame o₁x₁y₁z₁, the point pb has the coordinate representation . To obtain its coordinates with respect to frame o₀x₀y₀z₀, we merely apply the coordinate transformation Equation (2.9), giving

It is important to notice that the local coordinates of the corner of the block do not change as the block rotates, since they are defined in terms of the block’s own coordinate frame. Therefore, when the block’s frame is aligned with the reference frame o₀x₀y₀z₀ (that is, before the rotation is performed), the coordinates equals , since before the rotation is performed, the point pa is coincident with the corner of the block. Therefore, we can substitute into the previous equation to obtain

This equation shows how to use a rotation matrix to represent a rotational motion. In particular, if the point pb is obtained by rotating the point pa as defined by the rotation matrix , then the coordinates of pb with respect to the reference frame are given by

This same approach can be used to rotate vectors with respect to a coordinate frame, as the following example illustrates.

Example 2.3.

The vector v with coordinates v⁰ = (0, 1, 1) is rotated about y₀ by as shown in Figure 2.7. The resulting vector v₁ is given by

(2.10)

(2.11)

Thus, a third interpretation of a rotation matrix is as an operator acting on vectors in a fixed frame. In other words, instead of relating the coordinates of a fixed vector with respect to two different coordinate frames, Equation (2.10) can represent the coordinates in o₀x₀y₀z₀ of a vector v₁ that is obtained from a vector v by a given rotation.

Figure 2.6 The block in (b) is obtained by rotating the block in (a) by π about z₀.

Figure 2.7 Rotating a vector about axis y₀.

As we have seen, rotation matrices can serve several roles. A rotation matrix, either or , can be interpreted in three distinct ways:

1 It represents a coordinate transformation relating the coordinates of a point p in two different frames.

2 It gives the orientation of a transformed coordinate frame with respect to a fixed coordinate frame.

3 It is an operator taking a vector and rotating it to give a new vector in the same coordinate frame.

The particular interpretation of a given rotation matrix should be made clear by the context.