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PR 502 Robot Dynamics & Control 11/9/2006
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PR 502: Robot Dynamics & Control
Robot Kinematics:Articulated Robots
Asanga RatnaweeraDepartment of Mechanical Engieering
9 November 2006 Asanga Ratnaweera, Department of Mechanical Engineering
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Articulated Robots
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9 November 2006 Asanga Ratnaweera, Department of Mechanical Engineering
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Denavit-Hartenberg (DH) Representation
Developed by Denavit and Hartenberg in 1955 for kinematic modeling of lower pairsHas become a standard way of representing robots and modeling their motions.However, direct modeling techniques learned before are faster and straight forwardQuite useful for Articulated robot modeling
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Links, Joints and Their ParametersMechanical manipulator consists of sequence of rigid bodies (links) connected either revolute or prismatic joint.Each joint-link pair constitutes one degree of freedom (dof).Hence, for n dof system has n number of links.Usually the first link (link 0) is attached to a supporting base and last link is attached with the tool.
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DH Coordinate frames
Establishing base coordinate system:A right handed Cartesian coordinate system XYZ or xo, yo, zo is assigned to the base of the manipulator with the zo axis lying along the axis of motion for the 1st link (joint 1) and pointing towards the shoulder of the robot.
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DH Coordinate frames
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DH Coordinate framesEstablishing the joint axis:
All joints without exception are represented by a Z axis.If the joint is revolute the Z axis is the axis of rotation.If the joint is prismatic, Z axis is along the direction of the linear motion.
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DH Coordinate framesEstablishing the joint axis:
Defining X axisAssign the X axis along the common normal between two Z axes.If two z axes are parallel then assign X axis along the common normal to the previous jointIf two Z axes are intersecting each other, assign the x axis along a line perpendicular to the plane formed by the two Z axes
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DH Coordinate frames
Common normal between two z axes of joint 1 and 2
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DH Coordinate frames
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DH Coordinate frames
Z1X1
Y1
X2Z2
Y2
Z3
X3
Y3
1 2 3
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DH Coordinate frames
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Z(i - 1)
X(i -1)
Y(i -1)
α( i - 1)
a(i - 1 )
Z i Y i
X i a i d i
i
Z(i - 1)
X(i -1)
Y(i -1)
α( i - 1)
a(i - 1 )
Z i Y i
X i a i d i
i
Denavit-HartenbergParametersConsider any link i
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DH ParameterFour parameters (a,α,d,θ) are associated with each link of a manipulator.
a : the perpendicular distance between the adjacent joint axes (Z axes).
ex: ai-1 is the perpendicular distance between Z(i) and Z(i-1) axes.
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DH Parameters
ai-1
αi-1
αi-1
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DH Parameterα : Amount of rotation around the
common perpendicular so that the joint axes are parallel.
Ex: αi -1 is how much you have to rotate Z(i-1)about X(i-1) axis so that the Z(i-1) is pointing in the same direction as the Z(i) axis.Positive rotation follows the right hand rule.
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DH Coordinate frames
ai-1
αi-1
αi-1
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DH Parameterdi : The displacement along the Zi-1 axis needed to align the ai-1 common perpendicular to the ai common perpendicular. In other words, displacement along the Zi to align the Xi-1and Xi axes.Note: a, α are called link parameters and d, θ are called joint parameters
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Denavit-HartenbergParameters
Z(i - 1)
X(i -1)
Y(i -1)
α( i - 1)
a(i - 1 )
Z i Y i
X i a i d i
i
Z(i - 1)
X(i -1)
Y(i -1)
α( i - 1)
a(i - 1 )
Z i Y i
X i a i d i
i
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Denavit-HartenbergTransformation
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Denavit-HartenbergTransformation
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The Denavit-HartenbergMatrix
Just like the Homogeneous Matrix, the Denavit-Hartenberg Matrix is a transformation matrix from one coordinate frame to the next. Using a series of D-H Matrix multiplications and the D-H Parameter table, the final result is a transformation matrix from some frame to your initial frame.