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COMP322/S2000/L41 Classification of Robot Arms:by Control Method The Control unit is the brain of the robot. It contains the instructions that direct the.

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Presentation on theme: "COMP322/S2000/L41 Classification of Robot Arms:by Control Method The Control unit is the brain of the robot. It contains the instructions that direct the."— Presentation transcript:

1 COMP322/S2000/L41 Classification of Robot Arms:by Control Method The Control unit is the brain of the robot. It contains the instructions that direct the manipulator to perform various movements in the proper manner and at the proper time to accomplish a particular task. Feedback is the process of receiving information on how well a task or positioning of the end-effector is progressing. Closed-loop servo-controlled systems use continuous feedback information. Open-loop nonservo-controlled systems do not have feedback information. A Servo mechanism (simply referred to as Servo) is a technique for knowing the exact position of a moving object at any time. This is accomplished by feedback information.

2 COMP322/S2000/L42 Classification of Robot Arms: by Control Method Open-loop nonservo-controlled Systems No way of knowing whether the arm has moved as indicated Controlled entirely by on/off switches or by simple speed adjustments, i.e. no feedback information. Techniques have been developed to have adequate ‘feedback’ information to function adequately, e.g. –Have a physical stop (or switch) to indicate the end of a move or a limit is reached; or –Use a very reliable drive system to ensure the movement is as indicated. Relative high speed due to small size of arm & full power can be applied to the axis actuators Low cost and easy to maintain and operate Extremely reliable devices, ==> good repeatability Have limited flexibility w.r.t. positioning

3 COMP322/S2000/L43 Classification of Robot Arms: by Control Method Closed-loop servo-controlled Systems Information about the position and velocity of the links are continuously monitored and feedback to the control system Allow the arm to move and stop anywhere within the limits of travel of the individual links The controller (computer) has ‘large’ memory capacity Have at least 6 components: –A sensing device to provide the feedback: e.g. force, touch. –A command or input signal specifies a desired position, speed, etc. –A feedback signal: indicates how well the arm is carrying out the command (from the sensing device) –A ‘comparer’: compares the input signal and the feedback signal. –An amplifier: to raise the comparer signal to a power level capable of activating the output device –An output device: usually the motor to be controlled

4 COMP322/S2000/L44 Classification of Robot Arms: by Control Method The Control unit can also be classified according to the operating method: Pick-and-Place Control Units –generally small and pneumatic powered –no positional information feedback => open-loop nonservo-controlled Point-to-Point Control Units –able to reach any point within its work envelope –can have as many points in its work sequence as a particular task may require (limited only by the size of its memory) Continuous-path Control Units –able to reach any point within its work envelope, (similar to point-to- point) –can map the exact path to take between points –able to perform semiskilled tasks, e.g. arc-welding, spray-painting.

5 COMP322/S2000/L45 Classification of Robot Arms: by Specification of other Characteristics Number of Axes Payload Speed Reach and Stroke Repeatability, Precision and Accuracy

6 COMP322/S2000/L46 Classification of Robot Arms:by Number of Axes Degrees of freedom 3 major axes, 1-3, (from the base) to determine the configuration of the wrist => gross work envelope 3 minor axes, 4-6, (the wrist) Additional ones (7-n) are called redundant ones: to provide extra flexibility to reach ‘obscure’ space, get around corners, avoid obastacles, grasp special types of objects

7 COMP322/S2000/L47 Classification of Robot Arms: by Number of Axes The ‘Wrist’ 3 degrees of freedom: Yaw, Pitch and Roll (YPR) determines the possible orientations that the tool (hand) can assume.

8 COMP322/S2000/L48 Example of a Welding Tool attached to the end of a single axis wrist

9 COMP322/S2000/L49 Classification of Robot Arms: Payload (kg) Load carrying capacity, i.e. the maximum weight of the load that the robot can carry Speed (mm/sec) Closely related to payload Maximum speed: no load Cycle time: defined as the time required to perform a periodic motion similar to a simple pick-and-place operation. Examples: –Westinghouse robot - 92 mm/sec (no load) –Adept robot mm/sec (no load) –Adept robot: carrying a 2.2 kg along a 700 mm path consisting of 6 straight line segment has a cycle time of 0.9 sec. => average speed is approx. 778 mm/sec (<< 9000 mm/sec)

10 COMP322/S2000/L410 Classification of Robot Arms: by Reach and Stroke A rough measure of the size of the work envelope Horizontal Reach is defined as the maximum radial distance the wrist can be positioned from the vertical axis about which the robot rotates. Horizontal Stroke is defined as the total radial distance that the wrist can travel Reach > Stroke Minimum distance the wrist can be positioned from the base axis ? => (Reach - Stroke) Vertical Reach/Stroke

11 COMP322/S2000/L411 Classification of Robot Arms: Repeatability, Precision & Accuracy Repeatability vs Accuracy Precision is a measure of spatial resolution with which the end-effector can be positioned within the work envelope.


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