Forward Kinematics of PUMA 560 Robot using DH Method

Programmable Universal Machine for Assemble (PUMA) 560 robot manipulator is a industrial robotic arm with 6 revolute joint and 6 links. The mathematical model of the forward kinematics of PUMA 560 robot manipulator has 6 degree of freedom and involves the rotation and displacement of its coordinate frames. The forward kinematic analysis of PUMA 560 robot manipulator becomes easier when we use Denavit–Hartenberg (DH) model and general homogeneous transformation matrix.

Denavit–Hartenberg (DH) model is a kinematic model that relates the rotation and displacement of a reference frame B0 to the last link or end-effector frame Bn. The 4 × 4 homogeneous transformation matrix holds the rotation and the displacement matrix. The 3 × 3 rotation matrix defines the orientation of a joint or the end-effector while the 3 × 1 displacement matrix has the position of a joint or end-effector position.

In this paper, we modelled the forward kinematic of PUMA 560 robot manipulator using the DH model. In section 2, the derivation of the DH parameters and the homogeneous transformation matrices for each frames is discussed in details. In section 2.2, we implement a MATLAB script for the forward kinematics in section 2.1. Also, we evaluate the result by checking the rotation across the waist, shoulder and elbow joint to verify the correctness of forward kinematics in section 2.

In this paper, we define the Denavit–Hartenberg (DH) model of the PUMA 560 robot manipulator. We start by setting the axes of its frames. The z-axis at each frame must be the rotation axis of the frame while the x-axis must be perpendicular to the z-axis of the frame B_n−1 and B_n. Also, the x-axis at frame B_n intersects with the z-axis at frame B_n−1. All axes at each frame must adhere to right-hand rule. The axes at each frames is shown in Figure 2.

The DH parameters relates the rotation and displacement of frames B_n to B_n−1. The parameters are the joint angles θ, twist angle α, offset r, and link lengths d. The joint angle θ is the rotation at z_n−1 to match the x_n−1 to x_n. The rotation at z_n−1 includes all angular displacement at z_n−1-axis. The joint angle at frame 1 is simply the angular displacement, θ₁. This is true to all of the frames. In contrast, the twist angle α is the rotation at the x_n to match the orientation of z_n−1 to z_n. For example, the twist angle α₁ is −90^◦ and it means that x₁ rotates at −90^◦ to match it with z₀ to z₁. All twist angles at each frames are defined by the earlier process.

Figure 2: Joint representation of PUMA 560 Robot

The offset r is the distance between the centers of n − 1 and n joints to the direction of x_n. At frame 2, the offset r₂ is equal to 431.80 mm. It denotes the distance at which the n-joint are displaced at x_n as to n−1 joint. The link lengths d is the distance between the centers of n−1 and n joints to the direction of z_n−1. In the figure, the link lengths d₂ is equal to 139.70 mm hence it is the distance between n − 1 and n joint along z₁. The DH parameters listed in Table 1 follows the earlier notation.

Frame	θ	α	r	d
1	θ 1	-90o	0	671.83 mm
2	θ 2	0o	431.80 mm	139.70 mm
3	θ 3	90o	-20.32 mm	0
4	θ 4	-90o	0	431.80 mm
5	θ 5	90o	0	0
6	θ 6	0o	0	56.50 mm

Table 1: DH Parameters of PUMA 560 robot manipulator

The Denavit–Hartenberg (DH) model

is a standard kinematic model expressing position and orientation of a fixed coordinated frame relative to the n^th−link or end-effector. The general homogeneous matrix Tⁿ⁻¹_n consist the rotation and displacement vectors for a given joint angles. We define the homogeneous forward transformation matrix at each frame as

where the rotation matrix is

and displacement matrix is

Thus, the forward kinematic of PUMA 560 robot manipulator is given by the dot product of the forward transformation matrices at each frame. This is clearly establish in DH model. The position of the end-effector is in the displacement matrix d⁰_n while its orientation is in the rotation matrix r⁰_n.

2.2 MATLAB implementation

In this section, we create a script that implements the numerical solution for the forward kinematics problem of a PUMA 560 robot manipulator. We start with initializing the DH parameters listed in Table 1.

Listing 1: Initialize DH parameters

%DH parameters
J = [theta1 theta2 theta3 theta4 theta5 theta6]; %joint angles
A =  [-90 0 90 -90 90 0] ; %twist angle
r = [0 431.80 -20.32 0 0 0]; %offset as to xn
d = [671.83 139.70 0 431.80 0 56.50]; %offset as to z(n-1)

Next, we solve for the forward transformation matrices for all frames. This can be done by iterating equation 2 across all frames. The MATLAB scripts is shown in listing 2.

Listing 2: Homogeneous Transformation Matrix

%Homogeneus Transformation
    for n = 1:6
        matT = [cosd(J(n)) -sind(J(n))*cosd(A(n)) ...
            sind(J(n))*sind(A(n)) r(n)*cosd(J(n));
            sind(J(n)) cosd(J(n))*cosd(A(n)) ...
            -cosd(J(n))*sind(A(n)) r(n)*sind(J(n));
            0 sind(A(n)) cosd(A(n)) d(n);
            0 0 0 1];
        T = [T; {matT}];
    end

Then, we solve for the orientation and positions of all joints by iterating equation 1 across all frames. We have the script as

Listing 3: Orientation and Position of Joints

%Joint Positions
    for i = 1:6
        if i == 1
            P = [P,{T{i}}];
        else 
            matP = P{i-1}*T{i};
            P = [P, {matP}];
        end
    end

We create a fPUMA function comprising all the numerical solution and iteration that leads to the forward kinematics of a 6R PUMA 560 robot manipulator. We input to the function a set of joint angles and it results to the end-effector point and a three-dimensional plot of the robot manipulator links and rotation. After which, we evaluate the derive solution in section 2.1 by substituting the initial, boundary, and random conditions. This is to check if the analytical and computational solution in section 2.1 and 2.2 yield to a correct end-effector and plot from the input joint angles. The full script for the fPUMA function is shown in listing 4.

Listing 4: fPUMA function

function fPUMA(theta1,theta2,theta3,theta4,theta5,theta6)
%input: joint angles at each frames
%ouput: end-effector position
%author: Jayson c. Jueco
format compact
format short
%DH parameters
J = [theta1 theta2 theta3 theta4 theta5 theta6]; %joint angles
A =  [-90 0 90 -90 90 0] ; %twist angle
r = [0 431.80 -20.32 0 0 0]; %offset as to xn
d = [671.83 139.70 0 431.80 0 56.50]; %offset as to z(n-1)
%forward kinematics
if J(1,1) >= -160 && J(1,1) <= 160 && J(1,2)>= -225 ...
        && J(1,2) <= 45 && J(1,3) >= -45 && J(1,3) <= 225 ...
        && J(1,4) >= -110 && J(1,4) <= 170 && J(1,5) >= -100 ...
        && J(1,5) <= 100 && J(1,6) >= -266 && J(1,6) <= 266
    T = [];
    %Homogeneus Transformation
    for n = 1:6
        matT = [cosd(J(n)) -sind(J(n))*cosd(A(n)) ...
            sind(J(n))*sind(A(n)) r(n)*cosd(J(n));
            sind(J(n)) cosd(J(n))*cosd(A(n)) ...
            -cosd(J(n))*sind(A(n)) r(n)*sind(J(n));
            0 sind(A(n)) cosd(A(n)) d(n);
            0 0 0 1];
        T = [T; {matT}];
    end
    P = [];
    %Joint Positions
    for i = 1:6
        if i == 1
            P = [P,{T{i}}];
        else 
            matP = P{i-1}*T{i};
            P = [P, {matP}];
        end
    end
    %plotting the joint positions
    x = [0 P{1}(1,4) P{1}(1,4) P{2}(1,4) ...
        P{3}(1,4) P{4}(1,4) P{5}(1,4) P{6}(1,4)];
    y = [0 P{1}(2,4) P{2}(2,4) P{2}(2,4) ...
        P{3}(2,4) P{4}(2,4) P{5}(2,4) P{6}(2,4)];
    z = [0 P{1}(3,4) P{1}(3,4) P{2}(3,4) ...
        P{3}(3,4) P{4}(3,4) P{5}(3,4) P{6}(3,4)];
    hold on
    grid on
    rotate3d on
    plot3([x(1) x(2)],[y(1) y(2)],[z(1) z(2)]...
        ,'Color','b','LineWidth',5)
    plot3([x(2) x(3)],[y(2) y(3)],[z(2) z(3)]...
        ,'Color','r','LineWidth',5)
    plot3([x(3) x(4)],[y(3) y(4)],[z(3) z(4)]...
        ,'Color','g','LineWidth',5)
    plot3([x(4) x(5)],[y(4) y(5)],[z(4) z(5)]...
        ,'Color','c','LineWidth',5)
    plot3([x(5) x(6)],[y(5) y(6)],[z(5) z(6)]...
        ,'Color','m','LineWidth',5)
    plot3([x(6) x(7)],[y(6) y(7)],[z(6) z(7)]...
        ,'Color','y','LineWidth',5)
    plot3([x(7) x(8)],[y(7) y(8)],[z(7) z(8)]...
        ,'Color','k','LineWidth',5)
    plot3(x,y,z,'o','Color','k')
    xlabel('x-axis')
    ylabel('y-axis')
    zlabel('z-axis')
    title('Forward Kinematics of PUMA 560 Manipulator (6DoF)')
    view(0,0)
    disp('The end-effector position is ')
    fprintf('at Px = %f, Py = %f, and Pz = %f'...
        ,P{6}(1,4),P{6}(2,4),P{6}(3,4));
    disp('  ')
    disp('The DH model is  ')
    disp(P{6})
else
    disp('Joint angle out of range');
end
end

3. Result

In this section, we present the result of the MATLAB implementation and validate the forward kinematics in section 2.1. Initially, we set the joint angles to zero and verify the end-effector is at the initial position shown in Figure 2.

At initial condition, the end-effector point is located at P_x = 411.480000, P_y = 139.700000, and P_z = 1160.130000. Figure 3 shows the plot indicating the orientation and position of each frames of the robot manipulator. The orientation of the forearm of robot is the same to the basis of the DH model in section 2.

(a): 2D View (x to z plane)

(b): 3D view
Figure 3: fPUMA resulting plot for initial condition

Next, we checked the rotation of the elbow and the shoulder joints by setting its respective angles to 45^◦. These joints must have the same plane rotation to have a correct response. When we set joint angle θ₂ to 45^◦, we observed that it rotates along the x-z plane, as shown in Figure 4a. The end-effector is located at P_x = 636.240540, P_y = 139.700000, and P_z = 726.149943 for joint angle θ₂ at 45^◦ while the other joints at zero. For joint angle θ₃, we set it at 45^◦ while all other joint to zero. This result to an end-effector point located at P_x= 636.240540, P_y = 139.700000, and P_z = 726.149943. In Figure 4b, we observed that the elbow joint rotate in the x-z plane.

As stated earlier, the shoulder and elbow joint have the same rotation plane to yield a correct end-effector. Figures 4a and 4b indicate that the shoulder and elbow joint rotate in x-z plane. The results verifies the correctness of the kinematic equation in section 2. Furthermore, when we set the shoulder and elbow joint to 45◦ and other joint angles to zero, we observed the same plane rotation as shown in Figures 4a and 4b.

(a): Joint angle θ₂ set to 45^◦ (Shoulder)

(b): Joint angle θ₃ set to 45^◦ (Elbow)

(c): Joint angle θ₂ and θ₃ set to 45^◦

(d): Joint angle θ₁ set to 90^◦ (waist)

Then, we check the rotation of the waist joint. We set the joint angle (waist) θ₁ to 90^◦ and the other joint to zero. We observed that the waist rotate correctly, as shown in Figure 4d, in reference to the initial condition in Figure 3b. For a given set of joint rotation, we correctly locate the end-effector which was clear all throughout the plots in this section.

4. Conclusion

In this paper, we modelled the forward kinematics of PUMA 560 robot manipulator by using DH model. We established the DH parameters from the axes and joint representation for PUMA 560 robot manipulator as shown in Figure 2. We formed the homogeneous transformation matrices for each frames using the equations in section 2.1. The forward kinematic equations was implemented in MATLAB and evaluate the rotation across the joints. In section 3, we observed that the shoulder joint (joint1) and elbow joint (joint 2) rotate in the same plane. The result in section 3 verifies that the joint has correct rotation and response. Aside from checking the joint rotation, the end-effector is also verified. At initial condition (all joint angle θ equal to zero), the end-effector is at correct location as observed from the basis configuration in section 2. Thus, the kinematic equation taken from the DH model correctly gives the end-effector position and orientation.

5. References

[1] Lynch, K. and F. Park. “Modern Robotics: Mechanics, Planning, and Control.” (2017).

[2] John J. Craig. 1989. Introduction to Robotics: Mechanics and Control (2nd. ed.). Addison-Wesley Longman Publishing Co., Inc., USA.

[3] Merat, Frank. (1987). Introduction to robotics: Mechanics and control. Robotics and Automation, IEEE Journal of. 3. 166 - 166. 10.1109/JRA.1987.1087086

[4] M. Aghajarian and K. Kiani, "Inverse Kinematics solution of PUMA 560 robot arm using ANFIS," 2011 8th International Conference on Ubiquitous Robots and Ambient Intelligence (URAI), Incheon, 2011, pp. 574-578, doi: 10.1109/URAI.2011.6145885.

[5] Singh, T. P., Suresh, P., & Chandan, S. (2017). Forward and inverse kinematic analysis of robotic manipulators. International Research Journal of Engineering and Technology (IRJET), 4(2), 1459-1468.

[6] Singh, G., Banga, V. K., & Yingthawornsuk, T. (2019, November). Inverse Kinematics Solution of Programmable Universal Machine for Assembly (PUMA) Robot. In 2019 15th International Conference on Signal-Image Technology & Internet-Based Systems (SITIS) (pp. 518-524). IEEE.

[7] Hayat, Abdullah & Chittawadigi, Rajeevlochan & Udai, Arun & Saha, Subir. (2013). Identification of Denavit-Hartenberg Parameters of an Industrial Robot. ACM International Conference Proceeding Series. 1-6. 10.1145/2506095.2506121.

(Note: All images in the text are created by the author except for the 3D model in Figure 1. The 3D model is created by Gonzalo Loredo Neri and is free to download at grabcad.com)