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ELEC3201W1
ROBOTIC SYSTEMS DURATION 1440 MINS (24 Hours) This paper contains 4 questions Answer All questions An outline marking scheme is shown in brackets to the right of each ques- tion. This examination contributes 100% of the marks for the module. This version of the exam paper has been prepared with student revi- sion for the kinematics topicin mind. Answers are given to the kine- matics questions only Please note the solutions are indicative of what would be expected A foreign language dictionary is permitted ONLY IF it is a paper version of a direct Word to Word translation dictionary AND it contains no notes, additions or annotations. 16 page examination paper. An sw er s ELEC3201W1 Question 1. (a) The Denavit Hartenberg (DH) Method is a commonly used approach for obtaining the forward kinematic equations of robot manipulators. Which statement(s) is/are correct? (i) The homogenous transformation matrix derived using the DH Method is unique (ii) The DH Method uses left-hand coordinate frames (iii) The DH Method is not applicable to manipulators with only prismatic joints (iv) In the DH Method, only two axes need to be chosen for every joint. [2 marks] (b) Give two reasons why it is beneficial to derive a robot’s forward kinematic equations using the DH Method. [2 marks] (c) In the DH Method, the coordinate frame F0 is attached to the first joint. What does the frame F1 represent? What do frames F2,F3, and so on, represent (if present)? [2 marks] (d) Figure 1 depicts two joints of a robot manipulator. The coordinate frame attached to the first joint is shown, along with three different possible coordinate frames for the second joint. In the DH Method, which of these coordinate frames are valid? Which is preferable? [2 marks] (e) Consider the diagram of the robot manipulator in Figure 2 (i) Determine the DH parameters of this manipulator, tabulate them, and deduce three homogenous transformation matrices associated with each joint. Marks will be awarded for the accuracy and clarity of your working. [6 marks]
An sw er s ELEC3201W1 Joint 1 Joint 2 FIGURE 1: Robot joints with possible reference frames Joint 1 Joint 2 Joint 3 FIGURE 2: Robotic manipulator with three joints, with joint variables θ1, d2 and θ3.
TURN OVER Page 3 of 16 An sw er s ELEC3201W1 (ii) Use these matrices to derive the homogenous transformation matrix from base frame to end-effector. Marks will only be awarded for an accurate expression and clear working. [2 marks] (iii) In the final homogenous transformation matrix, identify the rotation matrix and the vector describing the (x, y, z) positions of the end effector with respect to the base. Deduce the orientation of the end- effector w.r.t. the base frame using the Euler angle convention. [2 marks] (iv) With this robot is it possible to independently control the (x, y) posi- tions? Explain your answer. [2 marks] (f) The forward kinematics of an experimental robot are described by the equation x1x2 x3 = l1 cos q1 + l2 cos q1 cos q2l1 sin q1 + l2 sin q1 cos q2 q3 − l2 sin q2 where qi are the joint variables, xi are the end-effector positions and li the link lengths. Derive an equation describing the inverse kinematics of the robot. Marks will be awarded for the accuracy and succinctness of the final expression. [5 marks]
TURN OVER Page 7 of 16 An sw er s ELEC3201W1 [25 marks]
An sw er s ELEC3201W1 Question 2. (a) Why would an engineer use the joint-space for trajectory planning? Ex- plain your answer. [2 marks] (b) A robot has six joints and is required to move from a certain initial po- sition to a final position, while passing through 5 intermediate points. Assuming cubic splines are to be used to generate the trajectory, calcu- late how many parameters will need to be calculated to enable this. [1 mark] (c) A robot is given a repetitive task to perform and a trajectory for a single joint needs to be generated. The starting position of the joint is θ(0) = 45◦ and the starting velocity is θ˙(0) = 0. The end-point and velocity is identical to that at the start. The robot must pass through a way point of θ(tw) = 90 ◦ with continuous velocity and acceleration of the joint. The time to execute the task is 6 seconds with the way point to be reached at 3 seconds. (i) Set up a matrix equation which needs to be solved for the parame- ters of the cubic functions. Marks will be awarded for accuracy and clarity. You do not need to solve the equation. [5 marks] (ii) Could a “smaller” matrix equation be solved instead? Explain your answer. [1 marks] (iii) Assume now that the robot joint should stop at the way-point and not travel to the end-point. An engineer wants to check the maximum joint velocity over this portion of the trajectory. Derive the time of interest which s/he should check. [4 marks] (d) The forward kinematics of a robotic manipulator are given bypxpy pz = d1 cos θ2 cos θ3d1 cos θ2 sin θ3 −d1 sin θ2
TURN OVER Page 9 of 16 An sw er s ELEC3201W1 where the joint positions are d1, θ2 and θ3. Calculate the analytic Jaco- bian of this manipulator and identify any limitations in mobility [4 marks] (e) A roboticist is developing a new mobile robot and needs to choose ac- tuators and sensors for the system. (i) Compare and contrast hydraulic and electric actuators. Give rea- sons why each may be suitable for the system under consideration. [4 marks] (ii) The mobile robot features a manipulator with a gripper at the end. The gripper needs to carry delicate objects. Explain the sensing capabilities such a gripper would ideally be equipped with. [4 marks]
An sw er s ELEC3201W1 Indicative Solution for Question 2. (a) Planning of trajectories in the task space involves choosing the end effector path directly in task space and then determining the joint positions via inverse kinematics at each time-step. Planning of trajectories in joint space involves choosing the start, end and possibly intermediate points in task space. The joint positions corresponding to these points are then determined, but the trajectory of the joints, from one point to another, is chosen in joint space. The advantages of task-space trajectory planning is that more control over end-effector position is attained, but the drawback is that the inverse kinematic equations have to be solved at each time instant to generate the trajectories in joint space. Joint space removes these computational problems, but less control over the end effector position is the drawback. (Other answers possible) (b) In total there are seven points, so 6 cubic functions are needed. Each cubic function has four parameters to determine, so 24 parameters per joint and hence 6× 24 = 144 parameters in total. (c) (i) First assign the two cubic polynomials, the first for the portion of the trajectory between t ∈ [0, tw] and the second for t ∈ [tw, tf ]:- θ(t) = a10 + a11t+ a12t 2 + a13t 3 θ(t) = a20 + a21(t− tw) + a22(t− t2)2 + a23(t− tw)3 Joint velocity is then given by θ˙(t) = a11 + 2a12t+ 3a13t 2 θ˙(t) = a21 + 2a22(t− t2) + 3a23(t− tw)2 and acceleration by θ¨(t) = 2a12 + 6a13t θ¨(t) = 2a22 + 6a23(t− tw) Using the information about the start position and velocity - t = 0 - we have 45 = a10 (4)