# Forward Differential Kinematics

In this series of tutorials, we’ll go over the basics of manipulator robot kinematics. So far, we’ve discussed how to parameterize coordinate frames in a kinematic chain via the Denavit-Hartenberg parameters and compute the pose of the endeffector as function of the joint variables and robot geometry via forward direct kinematics. Now, we’ll present forward differential kinematics which describes how joint velocities combine through manipulator geometry to result in the endeffector velocity.

# Forward Differential Kinematics

In order to compute derivatives of the link poses - in geometric terms, link twists - from derivatives of the joint variables, we use forward differential kinematics. This mapping is described by a matrix called the Jacobian.

There are two ways to arrive at such a mapping - the first is called the geometric Jacobian and the second is called the analytical Jacobian.

The geometric Jacobian is derived in a manner similar to that of the direct kinematics function in which one sums up the contributions of each individual joint velocity to the total end-effector velocity; this mapping is configuration-dependent.

The analytical Jacobian results from differentiating the direct kinematics function (when this function describes the pose with reference to a minimal representation in operational space) with respect to the joint variables.

## Geometric Jacobian

For an n-DOF manipulator we have the direct kinematics equation

It is desired to express the end-effector linear velocity $\mathbf{\dot{p}}_{e}$ and the end-effector angular velocity $\mathbf{\omega}_{e}$ in terms of the joint velocities $\mathbf{\dot{q}}$. It will be shown that these relations are both linear in the joint velocities and are given by

where $J_{P}\in R^{3\times n}$ and $J_{O}\in R^{3\times n}$ are the Jacobian matrices relating the contributions of the joint velocities to the end-effector linear and angular velocities, respectively. We can write this in compact form as the differential kinematics equation

where

is the manipulator geometric Jacobian which is, in general, a function of the manipulator configuration. This matrix is derived as follows.

First, consider the time derivative of a rotation matrix $R=R(t)$. Since such a matrix is orthogonal,

Differentiating this expression with respect to time yields

Defining $S(t) = \dot{R}(t)R(t)^{T}$ we have

which implies that the matrix $S(t)$ must be skew-symmetric since the sum of it and its transpose equals the zero matrix.

Since $R^{-1}(t) = R(t)$ we can solve our expression for $S(t)$ to yield

which is the differential equation relating the rotation matrix to its derivative via the skew-symmetric operator $S$.

Consider a constant vector $\mathbf{p'}$ in a rotating reference frame described by $R(t)$ and its image $\mathbf{p(t)} = R(t)\mathbf{p'}$ in the fixed frame in which $R(t)$ is defined. Taking the derivative of $\mathbf{p(t)}$ yields

which, using the definition of the derivative of a rotation matrix, can be written as

From mechanics, however, we know that this is simply given by $\mathbf{\dot{p}}(t) = \omega(t)\times \mathbf{p}(t) = \omega(t)\times R(t)\mathbf{p'}$ where $\mathbf{\omega}(t)=[\omega_{x}, \omega_{y}, \omega{z}]^{T}$ is the angular velocity of the rotating frame with respect to the reference frame at time $t$.

This means that we must have

We can thus write $\dot{R} = S(\mathbf{\omega})R$. Further, for a rotation matrix we have the property

Now consider the coordinate transformation of a point $P$ from Frame 1 to Frame 0 given by

Differentiating this expression with respect to time yields

where $\mathbf{r}_{1}^{0} = R_{1}^{0}\mathbf{p}^{1}$ represents the point $P$ after it has been rotated into Frame 0 but not translated, ie $r_{1}^{0} = p^{0} - o_{1}^{0}$. This is known as the velocity composition rule. If the point $P$ is fixed in Frame 1 then this reduces to

Now consider deriving the relationships between the linear and angular velocities of successive frames. Using the same DH convention for choosing link frames, it can be shown that

gives the linear velocity of Link $i$ as a function of the translational and rotational velocities of Link $i-1$. Note that all vectors are expressed with respect to a fixed Frame 0 and that $\mathbf{v}_{i-1,i}$ denotes the velocity of the origin of Frame $i$ with respect to the origin of Frame $i-1$ as expressed in terms of Frame 0. In addition,

gives the angular velocity of Link $i$ as a function of the angular velocities of Link $i-1$ and of Link $i$ with respect to Link $i-1$ ($\mathbf{\omega}_{i-1,i}$).

Using these general results, we have that for a prismatic joint

since the orientation of Frame $i$ with respect to $i-1$ does not change when Joint $i$ is moved and thus $\mathbf{\omega}_{i-1,i}=0$. For the linear velocity we have

since this joint is articulated in the direction of axis $\mathbf{z}_{i}$.

For a revolute joint we have

and

## Jacobian Computation

Consider the expression $\mathbf{\dot{p}}_{e}(\mathbf{q})$ relating the end-effector position to the joint variables. Differentiating this yields

due to the chain rule. Thus, the linear velocity of the end-effector can be obtained as the sum of $n$ terms, each of which represents the contribution of a single joint to the end-effector linear velocity when all other joints are still.

Thus, we have ${J_{P}}_{i} = \mathbf{z}_{i-1}$ for a prismatic joint and ${J_{P}}_{i} = \mathbf{z}_{i-1} \times (\mathbf{p}_{e} - \mathbf{p}_{i-1})$ for a revolute joint.

The angular velocity of the end-effector is given by

and thus for a prismatic joint ${J_{O}}_{i} = 0$ and for a revolute joint ${J_{O}}_{i} = \mathbf{z}_{i-1}$.

In summary, the full Jacobian is formed from $3\times 1$ vectors ${J_{P}}_{i}$ and ${J_{O}}_{i}$ as

where we have

The vectors on which the Jacobian depends are functions of the joint variables and can be computed from the direct kinematics relations as follows.

• $\mathbf{z}_{i-1}$ is given by the third column of the rotation matrix $R_{i-1}^{0}$; if $z_{0}=[0, 0, 1]^{T}$ then $\mathbf{z}_{i-1} = R_{1}^{0}(q_{1})\cdots R_{i-1}^{i-2}(q_{i-1})\mathbf{z}_{0}$.
• $\mathbf{p}_{e}$ is given by the first three elements of the fourth coumn of the homogeneous transformation matrix $T_{e}^{0} = A_{1}^{0}(q_{1})\cdots A_{n}^{n-1}(q_{n})$.
• $\mathbf{p}_{i-1}$ is given by the first three elements of the fourth column of the homogeneous transformation matrix $T_{i-1}^{0} = A_{1}^{0}(q_{1})\cdots A_{i-1}^{i-2}(q_{i-1})$

Finally, note that the Jacobian has been developed here to describe the end-effector velocities with respect to the base frame. If it is desired to represent the Jacobian with respect to a different Frame u then the relation is

# Wrapping up

In this tutorial, we introduced the basics for defining manipulator link frames via Denavit-Hartenberg parameters, as well as computing the incremental homogeneous transformations between successive links. We worked through the specifics for a three-link planar arm and a more complex anthropomorphic arm. Next, we’ll implement classes to define general manipulator kinematics based on this theory.