Design Aspects of Robot Manipulators

Transcription

1 Design Aspects of Robot Manipulators Dr. Rohan Munasinghe Dept of Electronic and Telecommunication Engineering University of Moratuwa System elements Manipulator (+ proprioceptive sensors) End-effector effector,, or end-of of-arm tool External sensors (such as vision systems), and part feeders controller Structure determines the complexity of kinematics, dynamics, and performance Existence of closed form solution for IK Simple mass distribution Simple inertia tensor Requirements to be met Workspace Load capacity Speed Repeatability (backlash, link flexibility) Accuracy

2 Designing a robot manipulator is an iterative process Though robot is meant to be a universal machine with a greater programming flexibility most robots are designed for a specific task (economic reasons) 5dof for symmetric tool applications (roll angle is immaterial) Use of external active positioning mechanism part feeders reduces #dofs# required Size, #joints, their arrangement, types of actuators, sensing systems, and control all vary on the task to be performed Large robots for handling heavy loads at slow speeds (gantry) Small robots for handling light loads at high speeds (cct( boards) Additional dofs may be required for collision avoidance (redundent arm configurations) Kinematic design is dictated by the workspace restrictions and task to be performed Load capacity depends on link sizes, power transmission system, drive system, arm position, and load being carried Speed is usually determined by the application (welding, spraying, etc..). Otherwise, higher speeds are preferable Acc/Dec phases take more time Acc/Dec capability is important (Adept One SCARA says 9000mm/s, yet performs periodic motion of 7 straight lines of 700mm in 0.9s 778mm/s in reality) Repeatability, precession, and accuracy though desired are expensive to achieve High accuracy needs good knowledge of system parameters Repeatability: ability to place the tool at the position repeatedly (backlash, and elastic bending of links due to loading cause poor repeatability) Precession: spatial resolution with which the end-effector effector can be positioned within the workspace Accuracy precession/2

3 Wrist-partitioned kinematic design Last 3 joints have intersecting axes for orientation control First 3 joints are used to position the end-effector effector in the workspace Closed-form solution exist Make the design kinematically simple Link twiats 0 or 90 Many links have zero lengths and zero joint offsets Stiffness and Deflection High stiffness is requires link cannot sag or deflect {T} is calculated from joint encoder readings and D-H D H parameters, not by direct observation Flexibility in links leads to resonance problems Cartesian Manipulator Mutually orthogonal prismatic joints IK is trivial Decoupled joint motion no singularity problem Very stiff good for large robots (gantry) Robot structure spans over the workspace not good, intrudes the workspace, limits room for other parts and fixtures Articulated Manipulator Less (or no) intrusion into the workspace Can reach confined spaces by reaching out the arm Require less space Fits for smaller workspaces SCARA First 3 joint motors don t t have to withstand weights of the manipulator or load (Mechanism supports it) Base can house actuators of the first 2 joints Low joint inertia high speeds (30ft/s abt 10 times faster than other robots)

4 Workspace volume and structural length Cartesian manipulators cost more material than articulated manipulators for a specified workspace volume Length sum Length index n L = a i 1 + d i Q L = i= 1 L / 3 workspace _ volume A good designs has a low length index more workspace for unit link Well-conditioned workspace Manipulatability W = det[ J( Θ) J( Θ) T ] Higher manipulatability is preferred Gears Torque amplification and speed reduction Spur gears (parallel axes) Bevel gears (orthogonal intersecting axes) Worm gears (skew axes) Helical gears Planetary gears (H/W) Harmonic gears (H/W) If the teeth are meshed tightly No backlash but high friction If the teeth are meshed loosely Low friction but high backlash Accurate mounting Is required Gear ration is defined as a speed reduction system

5 Lead screw transmit rotary motion to linear motion Prismatic joints supports very large loads Very high speed reduction Belts, cabals, flexible bands Transmit power while reducing speed Preloading is required to make sure that the belt/cable stay engaged on the pulley Large preloading introduces excessive friction Ideal Actuator Light weight Can be best located at the joint it drives Produces enough torque at low speeds direct drive No gears No friction/backlash Joint has the same fidelity as the actuator Real Actuator Heavy Relocation of the actuator may be appropriate to reduce joint inertia Need belts/chains to transmit power from actuator to joint Has to deal with problems of power transmission Produces low torque at high speeds cannot be directly driven Need gears friction, backlash Joint has low fidelity

6 Actuator types Hydraulic Large forces Drives directly Needs lot of equipments messy (pumps, hoses, servo valves) Not precise (friction of oil seals) Pneumatic All favourable aspects of hydraulics Clean Not precise (compressibility of air) DC brush motors Popular for small/medium sized manipulators Easily controllable, convenient interface Brushes (slow speeds) Wearing causes commutation problems Not good for clean room applications New magnetic material produce high torques (short duty cycles) DC brushless motors Better cooling (windings on the stator) Electronic commutation, High speed