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 .

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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 COMP322/S2000/L4 2

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 COMP322/S2000/L4 3

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.

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Classification of Robot Arms: by Specification of other Characteristics

• • • • •

Number of Axes Payload Speed Reach and Stroke Repeatability, Precision and Accuracy

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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 COMP322/S2000/L4 6

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.

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Example of a Welding Tool attached to the end of a single axis wrist COMP322/S2000/L4 8

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 - 9000 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) COMP322/S2000/L4 9

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

defined as the

total

is 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

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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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