2.8.3
Position Control
The Hexapod implements the PID-based controller described in Section 3.2 in the
PID
subsystem block, shown
in Figure 2.12. The control force, i.e., effort required to attain the desired position, is converted to current for the
amplifier in the
Hexapod IO
subsystem.
The joint velocities used in the PID controller can be generated through hardware or software. Use the manual switch
in the
Hexapod IO API
subsystem shown in Figure 2.14 to select between the two sources for the joint velocity:
1.
Hardware-based velocity
obtained directly from the DAQ board using HIL Read Timebase block (see the
Hexapod User Manual for channel details).
This is the default
.
2.
Software-based velocity
obtained by passing the joint values through a second-order high-pass filter.
2.8.4
Kinematics
The forward and inverse kinematic block are available in the
Hexapod Kinematics
block shown in Figure 2.17. For-
ward kinematics are used to find the world-based end-effector position (X, Y, Z, Roll, Pitch, Yaw) from the six joint
positions,
q
, and the leg lengths. It also the rotation matrix,
R
.
Inverse kinematics calculates the six joint positions from the world coordinate of the end-effector position. The
Inverse
Kinematics
block is used in the Hexapod_World_Control.mdl model to find the joint-level commands required to attain
the desired end-effector position. But it is also used in Hexapod_Joint_Control.mdl to compare it again the actual,
measure joint positions.
Figure 2.17: Forward and inverse kinematic blocks for the Hexapod
2.8.5
Performance Tracking
The
Performance Tracking
subsystem, depicted in Figure 2.18, includes various scopes to monitor the position and
speed of the Hexapod.
•
Joint Position Tracking
: The yellow trace displayed in this plot is the measured position of the joint and the
purple plot line is the commanded or desired position. The joint to be displayed on the scope can be selected
using the
Joint Index
block (values between 0 and 5).
•
Joint Speed Tracking
: Similarly as
Joint Position Tracking
scope, but displays the desired and measured
joint-level velocities instead.
HEXAPOD Laboratory Guide
v 1.3
