WAM Arm – User’s Manual
www.barrett.com
© 2008 Barrett Technology®, Inc.
Document: D1001, Version: AH.00
77 of 80
Denavit-Hartenberg Parameters -
The
D-H parameters
form an
n
x 4 matrix of parameters that
define the kinematic relationship between coordinate frames that are attached to links in a robot.
Knowledge of the D-H parameters allows immediate construction of the transformation matrices.
Multiplying the chain of transformation matrices enables the final tool frame to be described
relative to the user’s robot-base or room coordinates.
Encoders -
Encoders
come in two main types: incremental and absolute. The sensor types are
usually optical, but may also be magnetic or electrical. In absolute encoders, when power is
booted, the encoder knows its absolute location before there is any motion. Incremental encoders
rely on A and B pulse trains in quadrature to report relative distances and direction. An optional
index pulse on a third track allows the incremental encoder to calibrate and begin reporting
absolute position only after the rotor rotates far enough (up to 1 full turn) to locate it.
Ethernet Communications -
Ethernet
is a high-speed non-deterministic serial communications
protocol. There are versions for 2-, 4-, and 8-wire variants and optical physical layers, with
bandwidth highest in the 8-wire and optical versions.
Firmware -
Software that is embedded in a hardware device that allows reading and executing the
software, but does not allow modification, e.g., writing or deleting data by an end user.
Gravity Compensation -
Gravity compensation
is a specialized type of feedforward control used
in backdrivable robotic arms. The kinematics, relative orientation of the gravity vector, and
instantaneous pose must be known or knowable. The link and payload masses must be known a
priori or measured (in a process called identification). In these circumstances, only a few sines and
cosines must be evaluated to determine the unique set of joint torques required to balance gravity
precisely and these balancing torque change only gradually as the arm moves or is pushed around
the workspace.
Haptic Device -
An articulated machine designed to come in physical contact with people, usually
through a handle that a person grasps, and to have the capability to control the forces that the
person experiences.
Haptic Object-
A
haptic object
is a virtual object that is suspended (and can move and morph)
within the workspace of a haptic device. A person moving the handle, stylus, or thimble of the
device senses resistance forces generated by the device to simulate collision with the membrane of
the haptic object.
Haptics -
Haptics
is the study of the perception of touch in exploring and controlling objects and
fluids that we can touch.
Home Position
- The default
home position
for the WAM (specified in the Quick-Start Guide) is
J2 folded about 2 rad back against its large rubber stop, and J4 folded in against the inner link (+pi
rad). It looks like the WAM is trying to touch its shoulder. In order to change the default home
position, you must change the defined home joint positions in wam.conf. The normal joint
readings for home positions are as follows:
J1 J2 J3 J4 J5 J6 J7
0 -2 0
3.14
0 0 0
Jacobian -
The
Jacobian
is a mathematical matrix representing the first derivative of kinematics
and is very important in robotics. There are two interpretations of the Jacobian: one using the
Jacobian-Inverse, and the other using the Jacobian-Transpose. The latter is only used in advanced
robotics and is more powerful.
