In the Denavit-Hartenberg convention, the four parameters per link are used to describe the relation between adjacent frames. Which set of parameters is correct?

• A. a_i, alpha_i, d_i, theta_i
• B. l_i, w_i, h_i, t_i
• C. p_i, q_i, r_i, s_i
• D. x_i, y_i, z_i, w_i

Answer: (A)

In DH convention, each link is described by four parameters that capture both the distance and the orientation between consecutive joint axes: a_i (link length), alpha_i (link twist), d_i (link offset), and theta_i (joint angle). The pair a_i and alpha_i describe how the z-axes are arranged relative to each other, while d_i and theta_i describe how far and how much to rotate to bring frame i into alignment with frame i-1. Concretely, the transformation from frame i-1 to frame i is built by rotating about Z_{i-1} by theta_i, translating along Z_{i-1} by d_i, translating along X_i by a_i, and rotating about X_i by alpha_i. This set of four parameters—link length, link twist, link offset, and joint angle—best matches the standard DH formulation. The other sets do not correspond to the DH parameter names.